Resumé en français

La concentration atmosphérique de CO2 a doublé au cours du dernier siècle à cause de la surexploitation des combustibles fossiles. La méthode la plus développée à ce jour pour éviter l'émission de ce gaz à effet serre dans l'atmosphère est le procédé de lavage aux amines, déjà appliqué à l'échelle industrielle pour purifier des gaz issus de la combustion. Malgré sa maturité, l'application du procédé est ralentie par son cout élevé. Dans le but d'augmenter l'efficacité du lavage aux amines, beaucoup d'efforts ont été faits dans la recherche pour développer des agents de captage du CO2 innovants.

Les amines capturent le CO2 par les réactions de carbamatation et carbonatation, spontanées et exothermiques. Grâce à la réversibilité des liaisons N-C et O-C, les solutions de carbamates et carbonates sont sujets au contrôle thermodynamique et elles répondent aux principes de la Chimie Combinatoire Dynamique. Notre approche à l'étude des systèmes de captage consiste à identifier et quantifier les membres des librairies dynamiques amine-CO2 en solution et à corréler leur concentration avec l'augmentation du taux de charge en CO2. Une étude spectroscopique (RMN, IR) de plusieurs polyamines commerciales et synthétiques avec le CO2 a été mené pour comprendre les paramètres qui règlent la formation des polycarbamates. L'amplification des états pluri chargés en solution pourrait induire un mécanisme de captage coopératif et une meilleure efficacité du procédé de lavage aux amines. L'étude moléculaire des solutions a permis d'identifier un mécanisme de captage coopératif avec la Diéthylènetriamine (DETA) dans le méthanol, du à un binding non-covalent entre deux espèces ammonium-carbamate en solution. Une relation directe a aussi pu être établie entre la longueur de chaine des diamines et la concentration de bicarbamate en solution. La capacité cyclique des différents systèmes de captage, quantifiant les moles totales de CO2 capturées pendant un cycle complet d'absorption-désorption, a été mesurée pour évaluer l'efficacité thermodynamique. Pour estimer la performance cinétique, la vitesse d'absorption a été mesurée par spectroscopie infrarouge couplée avec RMN quantitatif. L'introduction dans une librairie combinatoire dynamique d'une molécule cible (« Template ») peut modifier l'équilibre thermodynamique et induire l'amplification de l'adduit qui a la plus grande affinité pour la cible.

Dans le but d'induire la coopérativité du captage du CO2 par les polyamines, une série de complexes de Bis(Diethylenetriamine)Co(III) ont étés synthétisés et caractérisés en tant que « templates » pour les polycarbamates (eq.( 4), Fig. 2a). La force du binding non covalent entre les complexes et les polycarboxylates a été mesuré par titration UV-Vis et RMN. Une étude spectroélectrochimique a montré notamment que grâce aux interactions supramoléculaires avec les polycarbamates, la formation in-situ du complexe dans des solutions de DETA-CO2 contenant du chlorure de Cobalt(II) est accélérée.

L'hydratation du CO2, menant à la production de bicarbonate (HCO3 -) et carbonate (CO3 2-), est la voie de captage la plus commune en nature et elle est moins exothermique que la carbamatation. Pourtant, la carbonatation est plus lente que la carbamatation. Pour déplacer la sélectivité du captage dans les solutions de polyamines vers la carbonatation, nous avons développé des « Templates » pour le bicarbonate. Plusieurs sels de polyguanidinium ont été synthétisés et exploités comme additif aux solutions de polyamines dans l'eau. L'étude spectroscopique (RMN, IR) des solutions de captage montre qu'en présence de ces templates organiques la production de bicarbonate est amplifiée. La présence de ces cibles organiques favorise la production du HCO3 -du à la géométrie complémentaire avec les fonctions guanidinium, permettant la formation de ponts salins qui stabilisent le bicarbonate. Ces systèmes de captage innovants présentent une capacité cyclique accrue tout en gardant la même efficacité cinétique que les polyamines seules.

Cette thèse a aussi été dédiée à l'étude de la réaction aérobie spontanée entre la Diéthylènetriamine et le méthanol en présence de Chlorure de Fer(III). En conditions aérobies, le Fer(III) initie un mécanisme réactionnel comprenant oxydation des amines et transimination inter et intramoléculaire, qui génère finalement un mélange de produits cycliques oxydées (Fig. 3a). L'étude spectroscopique du système a permis d'identifier les produits principaux de la réaction, qui contiennent des unités imidazole, parmi d'autres structures macrocycliques en quantité inférieure.

Du point de vue synthétique, malgré le faible rendement (environ 20%), ce système représente une remarquable alternative « verte » comparé aux méthodes synthétiques classiques pour les imidazoles. D'une autre perspective, l'obtention d'un mélange complexe et riche en espèces oxydés et polycycliques à partir d'une simple amine aliphatique, par action du Fer et du dioxygène, rappelle des mécanismes de chimie prébiotique. L'utilisation du CO2 ou sa transformation en produits commerciaux pourrait compenser une partie des couts du captage. Le développement d'une méthode de conversion pouvant être appliquée directement sur les solutions d'amines chargées en CO2 représenterait un avantage économique pour le procédé, car les étapes de désorption, compression et transport du CO2 seraient évitées.

Le CO2 peut être réduit en monoxyde de carbone, formate, méthanol et autres produits par réduction électrochimique en présence d'un catalyseur. Nous avons étudié la réduction du CO2 catalysé par des complexes organométalliques de Manganèse en présence de polyamines dans le méthanol. Pour augmenter l'efficacité et le temps de vie des catalyseurs, le catalyseur a été fonctionnalisé sur un support conducteur (Carbon Cloth) qui a été utilisé comme cathode pendant l'électrolyse. Des rendements élevés en produits de réduction gazeux (CO) et solubles (HCO2 -) ont été obtenus par électrolyse du CO2 en présence de PMDETA (Pentaméthyldiéthylènetriamine) en concentration millimolaire.

Un grand remerciement va à mon directeur de thèse Julien Leclaire pour avoir accepté de m'accompagner dans la recherche à partir du jour où, il y a maintenant 5 ans, je l'ai contacté depuis l'Italie pour demander la possibilité de faire un stage dans son laboratoire. Pendant ces dernières années on a pu échanger innombrables fois, pendant lesquelles il m'a accompagné dans le passage d'étudiante à chimiste. Merci pour avoir toléré mes fautes et apprécié mes réussites, et merci pour m'avoir inspiré à aller chercher toujours un peu plus loin.

Merci à l'ensemble de l'équipe CSAp qui a animé mes journées au laboratoire. Merci à Florent Perret, Laurent Vial, Isabelle Bonnamour et Natalie Perol pour les nombreuses discussions autour de la chimie ou des enseignements.

J'ai eu la chance d'être accompagnée tout le long de ma thèse par de collègues formidables. La première crew de doctorant.e.s sera toujours dans mon coeur, Clara, Alex, Benji et Fanny: merci pour avoir créé une ambiance exceptionnelle pendant ma première année de thèse. Merci à Maxime, Marc, Nathan, Marion et Laure pour les derniers mois dans l'équipe, c'était vraiment cool de vous rencontrer et merci pour avoir écouté mes plaintes pendant la rédaction. Bon courage pour la suite de votre travail dans la science.

Tout le long du stage et de la thèse, j'ai été accompagnée par l'amie, collègue et soeur que dont je n'aurai même pas pu rêver, Clara. Merci d'avoir été toujours présente pour moi, tu as été essentielle.

Merci à tous les membres du ICBMS, c'était enrichissant de travailler dans un institut si vivant et de vous croiser pendant les conférences, les pauses café et dans les couloirs. Un grand remerciement au personnel du centres commun de spectrométrie de masse (CCSM, Alexandra, Elodie, Antoine, Christian) et du centre commun de RMN (CCRMN, Anne, Emmanuel, Christophe) pour avoir collaboré avec moi, malgré mes analyses complexes et nombreuses.

Aurevoir à l'équipe d'enseignants UCBL dans laquelle j'ai travaillé en tant que ACE. Un remerciement spécial à Michele Fiore, sans lequel mon début dans l'enseignement aurait été 1000 fois plus dur.

Merci au groupe de recherche de Turin, qui m'a accueillie chaleureusement à plusieurs reprises et qui a énormément travaillé pour consolider mes résultats.

Il n'y a pas d'ordre dans les remerciements qui vont suivre, parce que chacun.e des personnes que je vais citer a contribué énormément à mon bien être et à ma santé mentale dans les dernières années et à me pousser jusqu'à la fin de cette thèse.

Kevin a été le pilier sur lequel je me suis appuyé chaque jour en rentrant chez moi. Merci avoir toujours été là, malgré mes humeurs noires. Merci à Caro pour m'avoir accompagnée dans plein d'aventures. Merci à Matilde, je suis contente qu'on se soit re-rencontrées des années après notre première rencontre. Merci à Paolo pour les nombreuses discussions en rentrant en voiture à Turin et pour les cafés à CPE.

Merci à tous les thésards ICBMS avec lequel.le.s j'ai eu le plaisir de discuter pendant les pauses café, les bières, les repas et les pots de thèse.

Merci à mes amis à Turin, Olli, Luca, Fra, Como, Nena, Dani, Pippo, Silvia et les autres, car ils étaient toujours disponibles pour se rencontrer quand je rentrais en Italie. Merci même à ceux qui ne sont jamais venus à Lyon ! Merci à mes parents et à mon frère pour être venus plusieurs fois à me visiter et pour m'avoir soutenue à distance. Merci aussi à ma tante, à mon grand-père, à mes arrière-tantes et au reste de ma famille pour avoir été des présences constantes malgré les nombreuses fois où je ne répondais pas au téléphone.

Merci à Pierre pour avoir été dans ma vie dans les derniers 2 ans. Merci pour m'avoir soutenue et motivée à pousser mes limites pendant l'écriture de la thèse, et merci pour m'avoir écoutée à chaque fois que je craquais.

Merci aux bars, aux quais, au parc de la tête d'or, au parc de la Feyssine, au Rhone, au 1 rue Emile Duniere, au papi du marché, au pire boulanger de Villeurbanne, à Idriss, aux vélos, aux ateliers, aux pates, aux pizzas pas bonnes, au tiramisù, à Domuss et à sa tarte au citron. Merci à Curien, à Raulin, à Lederer, aux déchets chimiques, aux bidons de solvant, aux tiols. Merci à l'azote liquide à chercher à l'autre bout du monde, aux livraisons, aux plusieurs déménagements. Merci aux avions, aux trains, aux bus, aux covoits. Merci à la Chine, à l'Ardeche, à Montpellier, à Toulouse, à Paris, à Marseille et à Bordeaux.

Merci à moi, pour ne pas avoir lâché, pour avoir eu confiance en moi et pour être arrivée encore une fois exactement où j'avais prévu d'arriver, quand je voulais y arriver.

Abbreviations

General Introduction

This thesis explores the use of supramolecular chemistry to improve the efficiency of homogeneous amine sorbents for CO2 capture.

Post-combustion CO2 capture by Amine scrubbing is the most mature technology proposed to limit CO2 emissions in order to ease the transition from a fossil fuel-based to a renewable energy-based economy. This process, though well understood from its thermodynamic and kinetic point of view, suffers from strong economic and energetic burdens which hamper its worldwide application. To improve the efficiency of amine scrubbing, abundant research has been dedicated over the past decades to the design of the ideal amine sorbent.

In Chapter I the State-of-the-art of the Carbon Capture, Utilization and Storage (CCUS) technologies will be presented, with a focus on carbamation and carbonation, the fundamental reactions regulating CO2 capture by amines in water.

Our innovative approach to the study of CO2 capture systems is based on the qualitative and quantitative analysis of the components of the dynamic amine-CO2 libraries. Chapter II will present the study of a series of polyfunctional amine sorbents, whose capture properties will be characterized spectroscopically. This chapter is dedicated to the analysis of the parameters which drive polyfunctional capture agents towards the generation of polyvalent CO2-bound products. The amplification of polyanionic states is relevant for the development of cooperative capture mechanisms, that could increase the thermodynamic efficiency of the amine scrubbing process.

Thanks to the reversibility of carbamate (N-CO2 -) and carbonate (O-CO2 -) bonds, the amine-CO2 solutions can be investigated through Dynamic Combinatorial Chemistry (DCC) principles. [1][2][3] This approach is based on the displacement of non-covalent or covalent reversible binding events by the application of an external stimuli, such as an electrical or magnetic field, a temperature change or the introduction of a new chemical. In the last case, if the additive displays a preferential affinity for one component of the Dynamic Combinatorial Library (DCL), it can favor its generation among other potential products, thereby behaving as a "template".

In Chapter III, we will present the development of template species able to influence the distribution of the capture products in polyamine-CO2 dynamic libraries. Coordination complexes containing Cobalt(III), synthetized by procedures either reported in the literature or developed by us, will be presented as templates for polycarbamates, unstable species presenting multiple CO2 bound on the same polyamine molecule. The supramolecular assembly of polycarbamates with the complexes in their second coordination sphere will be confirmed by spectroscopical and spectroelectrochemical evidence in Chapter IIIC.

CO2 hydration, resulting in carbonic acid and/or bicarbonate and carbonate formation, is the most thermodynamic efficient capture pathway and the most common one in nature. However, its exploitation in amine scrubbing systems is hampered by an inferior kinetic performance compared to carbamation. In order to shift the chemoselectivity of CO2 capture in polyamine solutions from carbamates to bicarbonate, we synthetized a panel of polyguanidinium salts. The experimental evidence, reported in Chapter IIIA, will prove that preformed polyguanidinium salts act as bicarbonate templates in polyamine solutions, stabilizing this CO2 capture product through supramolecular interactions (salt bridges). The templated capture systems display an increased thermodynamic and kinetic capture performance compared to the untemplated libraries, proving the advantages of supramolecular chemistry implementation to CO2 capture systems.

Chapter IV will present the study of the inedited reactivity between Diethylenetriamine and FeCl3 in methanol in aerobic conditions, which we encountered during some experiments devoted to CO2 capture agents' development. Spectroscopical analyses will lead to the identification of a reactive pathway initiated by the oxidation of amine and alcohol moieties, thanks to the catalytic properties of the FeCl3-O2 couple. The reaction starts from linear, aliphatic polyamines and generates structurally complex products such as imidazoles and polycyclic structures, through a spontaneous aerobic process which may be reminiscent of some prebiotic mechanisms. Chapter V will discuss the results obtained in collaboration with the University of Turin concerning the development of an integrated CO2 capture and recycling (CCR) process. The direct application of a transformation process on a loaded amine solution would represent an energetic and economic benefit to amine scrubbing, as the expensive steps of desorption, compression and transportation of removed CO2 would be avoided. We will present the efficient catalytic system that was obtained by performing CO2 electroreduction, catalyzed by a Manganese-bipyridyl complex grafted on a conductive support, in methanol in the presence of amines.

This thesis has delved into several branches of chemistry, encompassing organic chemistry, physicalchemistry, supramolecular chemistry, analytical chemistry, organometallic chemistry and electrochemistry. To ease the transition between each part, a brief bibliographic study is presented at the beginning of each Chapter, to introduce the most meaningful evidence for the following Results and Discussion section.

The most relevant results are presented and discussed in the main text. Operational procedures, synthetic methods and calculations are reported in the Experimental Section, which will be abbreviated as "Exp" in the main text. Additional spectra and graphs are reported in the Annexes.

Chapter I: Carbon Capture Utilization and Storage I. CO 2 emissions: a global emergency requiring an urgent solution During the 35 th International Geological Congress in 2016, it was declared that our planet has effectively entered a new era, called Anthropocene. 4 This name refers to the fact that humans have dramatically altered the Earth's climate due to the over-exploitation of the planet's resources, first of all fossil fuels.

The average atmospheric CO2 concentration has reached 420 ppm in 2021, a value about 200 ppm higher than at the beginning of the first Industrial Revolution and the highest of the past 800000 years. 5 At the present day, humans have emitted more than 70% of the proposed 2900 Gton global carbon budget 5,6 , a parameter indicating the maximal amount of CO2 that can be emitted by anthropogenic activities without exceeding the 2 °C average global temperature rise above pre-industrial levels fixed by the Paris Agreement. [START_REF]COP21 Paris Agreement[END_REF] If emissions continue at the current rate (around 35 Gton/Year) the carbon budget will be largely exceeded in 2050, exposing the planet and human life to serious and irreversible consequences due to global warming.

The main contribution to CO2 emissions from anthropogenic activities comes from the exploitation of fossil fuels for energy generation and from land use change (i.e. the human exploitation of land for agriculture and farming, Fig. 1). To achieve an equilibrated ecosystem, the quantity of CO2 emitted should be compensated by natural ocean and land sink (i.e. atmospheric CO2 absorption by plants and soil), and ideally by CO2 capture and storage in geological reservoirs (CCS).

Covid-19 provided an interesting perspective on the extent of the CO2 problem. Because of the sudden brake that was put on the world's economy in the first quarter of 2020, the annual CO2 anthropogenic emission were reduced by 6.4% compared to 2019 (2.3 Gton reduction). [START_REF] Quéré | [END_REF] However, experts calculated that to limit global warming to +2 °C before 2050, a 7.6% annual reduction of CO2 emissions would be required. This means that the tremendous impact of Covid-19 on the world's economy was not even sufficient to put us on the good track to respect the Paris agreement. [START_REF] Iea | IEA[END_REF] These key figures convey the message that our life-style should be completely re-considered in order to meet the urgent demand for a solution to global warming. [START_REF] Quéré | [END_REF]

II. CCUS technologies -State of the art

Industrial emissions account for 40% of the 35 Gton/Year global CO2 emissions. The flue gases emitted from coal and natural gasfired power plants are amongst the most concentrated outputs of CO2. Experts agree in stating that the only feasible way to reduce CO2 emissions is the implementation of a CO2 capture process on the currently existing and operating power plants. [START_REF] Rochelle | [END_REF] The CCUS acronym (Carbon Capture Utilization and Storage) refers to the technologies applied to cover the complete life cycle of CO2, from its removal from flue gases to its transportation and subsequent storage or utilization. 11 The most mature CO2 capture technology to date is Amine Scrubbing. Despite its maturity, this process has seen a slow development since its design in 1930. [START_REF] Bottoms | Process for Separating Acidic Gases[END_REF] In fact, it suffers from significant operational (OPEX) and capital expenditures (CAPEX) which have not yet been compensated by a marketable utilization of removed CO2.

The scientific community has recently started to treat CCUS as a whole value chain, by integrating the capture process to a Utilization or Storage pathway that could inject some added value and compensate the costs of CO2 capture. The design of an economically viable integrated CCUS project has been identified as one of the "Holy Grails" in chemistry for humankind development in the future. 13 Amine scrubbing will herein be discussed in detail, from the processual to the mechanistic point of view.

1. Post Combustion Capture by Amine scrubbing: the industrial process More than 30 power plants currently apply amine scrubbing to treat flue gas derived from natural gas or coal combustion (Post Combustion Capture, PCC). 14,15 The industrial plants, such as the Petra Nova PCC plant (Fig. 2), usually present a cooler unit, to bring the flue gas flow to 40 °C, an absorber tank, where CO2 is removed from the gas stream by reaction with amines, and a desorption tank, where the CO2-rich solution is heated to release CO2 and regenerate the sorbent. Released CO2 is then pressurized (CO2 compressor, Fig. 2) in order to be stored and/or transported for subsequent utilization, while the regenerated sorbent is reinjected in the absorber for subsequent absorption/desorption steps.

The most relevant steps of the process are detailed hereafter.

Absorption step

Flue gas derived from coal combustion contains in average 15 % CO2 in combination with other inert (N2) or reactive (SOx, NOx) molecules. The influence of SOX and NOx can be minimized by introducing an alkaline scrubber prior to the absorber. [START_REF] Rochelle | [END_REF] The absorber usually contains a concentrated aqueous solution of one or two amines. Upon mass transfer from the gas to the liquid phase 16 , CO2 can react with amines through carbamation and carbonation, the former being the predominant reaction in concentrated primary and secondary amine solutions. As both reactions are reversible, the capture process is thermodynamically regulated, and the amine solution will capture CO2 up to a saturation point.

To capture one CO2 molecule as a carbamate, two amine groups are required, one acting as a nucleophile and one as a base (eq.1, Fig. 3), resulting in an ammonium carbamate salt. To obtain the product, one N-H  bond and one C=O  bond must be broken while a new N-C  bond and a N-H  bond are formed. Consequently, the standard enthalpy of reaction (HR 0 ) for carbamation is negative, due to increased stability of the products resulting from the replacement of one  and one  bond with two  bonds. On the other hand, the standard entropy of carbamation (SR 0 ) is negative, as the capture of CO2 from the gas to the liquid phase involves a reduction of the number of independent molecules. The enthalpic contribution to eq.1 outweighs the entropic one at 40 °C, so CO2 absorption by carbamation is a spontaneous, exothermic process. In the industrial PCC setup, heat exchangers are usually exploited to recycle as much of the heat dissipated during CO2 absorption as possible.

In the presence of water, CO2 can get hydrated to carbonic acid (H2CO3) and its conjugate bases (bicarbonate H2CO3 -, carbonate CO3 2-) are also obtained (eq.2, Fig. 3). This CO2 capture reaction is catalyzed by amines which therein act as Bronsted bases. It can compete with carbamation, even if the latter is the kinetically and thermodynamically favored capture reaction in concentrated primary and secondary amine solutions. The competition between these two antiparallel capture pathways will be discussed in section 3.3 of this chapter.

Primary and secondary amines, due to their nucleophile character, usually react with CO2 via eq.1, thereby generating ammonium carbamates. On the other hand, tertiary amines react with CO2 exclusively in the presence of water, by acting as Bronsted bases and catalyzing eq.2, leading to bicarbonate/carbonate formation.

Desorption step

Once the amine solution is loaded with CO2 (rich state), it is transferred into a desorption tank (Regenerator, Fig. 2), where it is heated to release CO2 and to regenerate the absorbent.

From a thermodynamic point of view, the release of captured CO2 is enthalpically unfavored, as it requires the cleavage of the stable N-C and N-H  bonds and the regeneration of a  (N-H) and  (C=O) bond (Fig. 3), but it is entropically favored due to the release of gaseous CO2. The energy required to reverse eq.1 and eq.2 is usually provided by heating the solutions up to 100-120 °C and stripping with water vapor (generated in the reboiler, Fig. 2). This treatment enables to release CO2 as a pure stream, while the amine solution in the lean state, i.e. unloaded, is re-injected into the absorber tank to perform further absorption-desorption cycles. Some relevant parameters used to assess the performances of CO2 capture agents are reported hereafter. 1.3. Parameters used to assess CO 2 capture performances

CO2 loading

The loading () quantifies the mol of CO2 captured per mol of amine molecule and it is often calculated as in eq.3.

𝛼 =

𝑚𝑜𝑙 𝐶𝑂 2 𝑏𝑜𝑢𝑛𝑑 𝑚𝑜𝑙 𝑎𝑚𝑖𝑛𝑒

eq.3

As carbamation is a thermodynamic controlled equilibrium, the system will reach a  at saturation proportional to the partial pressure of CO2 (pCO2), following Henry's law (eq.4).

𝐶 = 𝑘𝑝 𝐶𝑂2

eq.4

C= concentration of CO2 in solution [mol/l] k= constant dependent on the gas, solvent, temperature [mol/atm•l] pCO2= partial pressure of the gas in the headspace above the solution [atm] In this document, the theoretical loading at saturation for an amine solution reacted with pure CO2 will be indicated as max th , while the experimental loading at saturation, which often differs from the theoretical one, will be indicated as max.

For monofunctional primary and secondary amines, max th is equal to 0.5, as two equivalents of amine are required to bind one CO2 as ammonium carbamate (eq.1, Fig. 3). If carbonation takes place to a significant proportion in the capture solutions containing water, the experimental loading at saturation (max) can be higher than the nominal value as only one amine group is required to yield bicarbonate (HCO3 -, eq.2, Fig. 3).

Polyfunctional amines can reach a max th higher than 0.5, as they can generate zwitterionic capture products, i.e. presenting ammonium and carbamate groups on the same molecule (Fig. 4 for the monocarbamate of piperazine, 1-PZ). In this text, monocarbamates, i.e. capture adducts presenting one CO2 bound on an amine molecule, will be indicated as 1-followed by the abbreviation of the correspondent amine. Dicarbamates, i.e. species presenting two CO2 bound on the same amine molecule, will be indicates as 2-followed by the amine abbreviation (3-for tricarbamates and so on).

The normalization of the loading with the number of binding sites on the amine sorbent (nN) enables the true comparison of the capture performances of mono and polyfunctional amines (N, eq.5). This calculation will be exploited for most of the CO2-absoprtion systems presented in this document.

𝛼 𝑁 =

𝑚𝑜𝑙 𝐶𝑂 2 𝑏𝑜𝑢𝑛𝑑 𝑚𝑜𝑙 𝑎𝑚𝑖𝑛𝑒 * 𝑛 𝑁 eq.5

The industrial amine scrubbing process does not operate over the full loading range of a sorbent. In fact, the flue gas derived from coal combustion contains in average 15% CO2, corresponding to a pCO2= 0.15 atm if the gas is injected at atmospheric pressure. The operational loading in industrial PCC (max op ) is consequently inferior to the max th that would be obtained by injecting pure CO2.

Another reason why full loading of the sorbent solution is not attained in practice in PCC, is that the absorption rate decreases with increasing CO2 loading. The growing concern for CO2 atmospheric emissions has lowered the tolerance threshold for CO2 emissions from power plants, so the full capacity of the sorbent is generally not exploited in order to respect these requirements. For example, even if the Nmax th of PZ is 0.5 (Fig. 4), the operating range of the industrial process relying on concentrated PZ (8 M) is comprised between 0.31< N op <0.41. 15

Cyclic capacity (CC)

After desorption, the unloaded amine solution usually presents some residual CO2: the loading (respectively normalized) of a sorbent solution after desorption will be abbreviated as min (respectively min).

The Cyclic Capacity (CC) quantifies the mol of CO2 captured and released over one thermal swing cycle per mol of amine sorbent. This parameter truly assesses the performance of a capture agent as it takes in consideration both the mol of CO2 captured and desorbed. CC can be calculated (eq.6) as the difference in loading between a sorbent in the rich state (max) and the same sorbent in the lean state (min, after thermal desorption).

𝐶𝐶 = 𝛼 𝑚𝑎𝑥 -𝛼 𝑚𝑖𝑛 eq.6

The utilization of the normalized loading (Nmax, Nmin) in eq.6 delivers a normalized CC (CCN), a better suited value to compare the efficiencies of amines with a different multiplicity.

Reboiler and regeneration duty

The reboiler duty (Qreb, eq.7) quantifies the heat required for solvent regeneration. 17 The three main contributions to Qreb are: 17

• Reaction enthalpy: energy required to reverse the capture equilibria (fCO2HCO2).

• Sensible heat: energy required to heat the aqueous amine solution up to 120 °C. This contribution depends on the solvent flow rate in the desorber tank and on the temperature difference between the solvent entering and leaving the stripper (m̄solvCp(Tin-Tout)). • Heat of vaporization: energy required to generate water vapor for solvent stripping in the reboiler (nvap,H2OHvap,H2O).

The regeneration duty is calculated by dividing Qreb with the CO2 stripping molar flow rate fCO2 (eq.8).

Qreg = Qreb/fCO2 eq.8

After desorption, CO2 is collected as a pure stream and compressed to 20 bar: in these conditions, CO2 is in the supercritical state, enabling an easier transportation and/or storage compared to a gas. [START_REF] Rochelle | [END_REF]

OPEX

The thermodynamic efficiency, i.e. ratio of work output to energy input, of the currently operational PCC plants is around 50%. [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF][START_REF] Lin | [END_REF] More than half of the Operational Expenditures (OPEX) of the process are due to the energetic requirement for desorption from the loaded sorbent (Fig. 5b, next page). The reduction of the power supply of a power plant due to the implementation of a PCC unit is defined "parasitic power", and it represent on average 20% of the plant's production.

The CO2 compression step after desorption also weighs on the energy duty of the process, as it contributes to 30% of the OPEX (Fig. 5b). 20 The benchmark amine sorbent, 30 % w/w Monoethanolamine (MEA, Fig. 5c) in water displays a Qreg= 3.5 MJ/kg CO2 captured. With the current, optimized operational conditions, CO2 capture relying on MEA costs 50 $ per ton of CO2 captured. 17

CAPEX

The installation of an absorption unit to a medium-sized 550 MW coal-fired power plant costs around 500 M US $ in investment. The significant Capital Expenditures (CAPEX) of the PCC process mostly depend on the requirement to install stainless-steel absorber and desorber columns, heat exchangers, pipes and pumps, which should resist to the corrosiveness of the concentrated aqueous solutions exploited to capture CO2. 20 Recent economic assessments revealed that the absorption/desorption columns and the compression units account for 46% and 40% of the capital cost for a PCC unit respectively. 17 To accelerate the worldwide deployment of PCC, the CAPEX and OPEX would have to be compensated by an economic benefit to CO2 removal, but this is not the case at the present day.

The current amine scrubbing process requires to be counter-balanced by a profitable utilization either of the loaded amine sorbent or of captured and purified CO2, to justify the energetic and capital demand of the process itself.

Carbon Utilization and Storage

Different technologies were developed over the last decades to either store or use the CO2 removed from flue gases. Any realistic storage or utilization option should meet as many criteria as possible from the following list: [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF]22 • Cost: it should be beneficial from the financial and environmental perspective. Ideally, it should imply a marketable value for CO2 that could compensate at least part of the costs of the capture process itself. • Scale: it should be applicable on a large scale, required to deal with the 35 Gton of CO2 emitted every year by anthropogenic activities. • Duration: CO2 should be sequestered for a long time, ideally years to centuries.

• Maturity: it should be technologically mature and ready to be implemented on the operational power plants, to comply with the emergency of the need.

The most mature storage technologies are presented hereafter.

Storage technologies

In terms of scale and duration, CCS (Carbon Capture and Storage) is considered the only viable solution to deal with the extent of CO2 that would be captured if PCC was massively deployed in the near future. 23 The most common storage approaches can be separated into in-situ and ex-situ CCS.

a. In-situ: Geological Storage

Geological storage (Fig. 7) consists in injecting compressed CO2 in geological formations, such as depleted oil reservoirs, saline aquifers, or coal bed formations, at depths of 800-1000 m underground, wherein CO2 can be stored in gas, liquid or supercritical state, depending on the conditions of the site. 11 Geological storage meets considerable resistance from the population that would be leaving near the sequestering-sites, even if recent risk assessment studies estimated that a well-regulated storage has a 50% probability to limit leakage below 0.0008%/year, and that 98% of CO2 would be retained over 10000 years. 24 In the presence of Mg and Ca oxides or silicates in the geological reservoir, CO2 can eventually undergo mineralization through reactions (9a-d), yielding Mg and Ca carbonate minerals. These reactions can take place in one or several steps, depending on the pressure and the overall conditions of the storage site. Mineral carbonation is the only thermodynamically downhill pathway involving CO2, but it usually requires geological time scales to be completed.

This partly depends on the formation of superficial layers of silica or carbonate on the reactive minerals, that slow down the reaction with CO2. [START_REF] Mazzotti | IPCC Special Report on Carbon Dioxide Capture and Storage[END_REF] Oxides: MO + CO2 → MCO3 + heat (9a)

Silicates: Olivine: Mg2SiO4 + 2CO2 → 2 MgCO3 + SiO2 H= -89 kJ/mol CO2 (9b) Serpentine: Mg3Si2O5(OH)4 + 3CO2 → 3 MgCO3 + 2SiO2 + 2H2O H= -64 kJ/mol CO2 (9c) Wollastonite: CaSiO3 + CO2 → CaCO3 + SiO2 H= -90 kJ/mol CO2 (9d)
The conversion of CO2 to carbonate minerals represents a long-time storage option due to the stability of these materials. The best scenario to deploy in-situ CCS is the availability of a geological reservoir near to the PCC plant, otherwise cost-intensive transportation of CO2 would be required.

b. Ex-situ: Mineral Carbonation

A combined storage-utilization option is the industrial conversion of CO2 into mineral carbonates. 24 The advantages of ex-situ mineral carbonation are that the conversion of CO2 to long-term storage materials is accelerated in comparison to the natural process, leakage is avoided, and marketable products are generated, as Mg and Ca carbonates can be used as additives to cement or filling materials. In addition, the process can be applied on sites which are far from geological reservoirs. [START_REF] Snaebjörnsdóttir | [END_REF]27 The disadvantage is mainly energetic, as the solid reactants must be mined and mechanically pretreated to accelerate mineral carbonation reactions (9a-d), two energy-demanding steps which may consume up to 30-50% of a power plant production. 28 Alkaline industrial wastes such as combustion ashes, alkaline paper mill waste and mining wastes can be used as substitutes to alkaline rocks. This approach has the advantage of combining two waste materials to generate products with a marketable value, but the contribution of these feedstocks to the CCUS challenge would cover only a small part of CO2 emissions. Enhanced Oil Recovery (EOR) combines CO2 Storage with Utilization, by exploiting CO2 in supercritical conditions to extract crude oil or natural gas from hardly accessible deposits (Fig. 7).

Supercritical CO2 mixes well with oil trapped underground, decreasing its viscosity, and favoring its extraction. 31 EOR can extract 30 -60 % of the oil originally present in the well, compared to traditional extraction which recovers 20 -40 %.

EOR is currently exploited in 28 large-scale plants over the globe, most of which are based in the USA (Fig. 6, page 11). Worldwide, over 4000 kilometers of pipelines transport CO2 to oil exploration sites to perform EOR. 22 At the present day, this is the most mature CCUS technology which relies on the unique extracting properties of captured CO2 and has the potential to deal with the great extent of CO2 emissions.

Utilization technologies

Recent analyses showed that CCS is being developed 100 times too slow to meet the requirements of the Paris agreement. 23 As a complement to storage, many efforts were deployed to develop sustainable CO2 utilization processes. The main objective of Carbon Capture and Utilization (CCU) is to deliver a utilization or transformation of CO2 that meets a commercial demand, hence, give an added value to the waste CO2 and compensate part of the costs related to capture. 11 The CO2-sourced product should:

• Be competitive in comparison to the fossil-fuel based production.

• Meet a substantial market demand.

• Have a low environmental footprint. The total amount of CO2 emissions avoided during production and utilization of the product should be higher than the quantity that would have been emitted without the transformation. 20 • The scale of production should be large and the utilization process mature.

CCU routes envisaged to date are unlikely to contribute to greenhouse gas (GHG) mitigation by more than 1%, considering their potential scale of deployment. [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF] Examples of industrially mature utilizations, which can pay the price for upstream capture and remain competitive with respect to oil-based pathways, are scarce.

The most common CO2 utilization value chains, currently applied on the industrial scale (Fig. 8) are presented hereafter.

a. Food and beverages

CO2 is used in commercial processes such as the production of food and beverages, or in its supercritical state for the extraction of caffeine or other active principles. 32,33 However, these applications require CO2 of high purity/grade which is usually issued from other sources than PCC. 11 b. Recycled fuels/Chemicals CO2 is used in some well-known industrial processes, such as the synthesis of urea 34 (0.11 Gton; 75% share market), salicylic acid (30 kt; 40% share market) and cyclic carbonates (40 kt, 50% market share) [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF] : the related scales of production and CO2 consumption are reported in Table 1.

CO2 can also be converted to carbon monoxide, formic acid or MeOH that can be further transformed into high added value chemicals or fuels. 35 Unfortunately, most of these processes currently rely on CO2 issued from cheaper sources than PCC.

The conversion of CO2 to fuels and chemicals inevitably suffers from an energy penalty because of the intrinsic stability of the starting molecule. 11,36 Many efforts were made in the scientific community to develop catalysts able to lower the energetic requirement of CO2 conversion. Some examples of catalyzed CO2 transformation processes will be presented further on in this document.

c. Microalgae

Microalgae can purify flue gas by fixing CO2 and using it as nutrient. Once fully grown, the biomass can be harvested, dried, and converted into biofuel. Cultivation takes place in ponds that usually require large land area, large volumes of water and produce aqueous wastes. The expenses of this CO2 capture and utilization process are consistent, for an overall limited productivity. 37

Integrated CCUS

An innovative approach, defined integrated CCUS, proposes to entirely revisit the CO2 capture process by looking for potential applications of the loaded solutions prior to desorption. [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF] In the integrated CCUS scheme, CO2 would be eventually re-emitted and would require a long-term storage solution, but the integration of a utilization brick after capture and before storage may compensate part of the economic burden of the capture process itself.

Thanks to the reversibility of the CO2-binding process, the amine solutions can be exploited as switchable polarity solvents 38 , ligands 36,39 , membranes and materials. 40 Amines can bind to molecular partners through interactions that can be tuned by introducing or stripping CO2. Carbon dioxide has thus the potential to be used as molecular trigger for green separation processes, such as the purification of complex metal mixtures developed in our group which will be presented in section 6 of this Chapter. 3 Some interesting yet early stage (in terms of maturity) examples of integrated CCUS processes are reported hereafter.

a. Integrated Absorption -Mineralization (IAM)

A recent integrated CCUS scheme consists in the implementation of CO2 mineralization with capture by reacting loaded amine solutions with metal oxides (e.g. Calcium oxide, CaO) or silicates, a strategy defined Integrated Absorption Mineralization (IAM). 41 The reactions involved in the IAM process with Calcium oxide (CaO) are presented in Fig. 9.

Once loaded with CO2, the amine solution is transferred in a carbonation sink containing CaO. Calcium oxide dissolves in water as Ca 2+ and OH -ions (1-10). The generation of OH -increases the pH of the loaded solution and induces the hydrolysis of carbamates into bicarbonate (HCO3 -, (2-10)), which is then deprotonated to generate CO3 2-(3-10). Carbonate co-precipitates with Ca 2+ as CaCO3 (4-10) while the amine is chemically regenerated. The pH-swing imparted by the reaction of carbamates with CaO enables concomitant CO2 sequestration as CaCO3 and amine recycling, completely avoiding the thermal desorption step.

b. Integrated Absorption -Hydrogenation

Methanol is the CO2-sourced product with the largest deployment capacity, due to its growing application as fuels precursor, with a current production of 70 Mt/Y. Rezayee et al. were able to integrate the catalytic transformation of CO2 into methanol with capture by dimethylamine (DMA, Scheme 1). 42 Thanks to a Ruthenium-based catalyst a conversion higher than 95% could be achieved.

DMA plays a dual role:

1. It binds CO2 as an ammonium carbamate (1-DMA, Scheme 1). Under the catalytic conditions CO2 is released and hydrogenated to MeOH, regenerating the lean amine.

2. It reacts with formic acid (H2CO2), generated as an intermediate of CO2 hydrogenation, which brings to dimethylformamide (DMF) formation.

Another example of integrated CCU was presented by the group of the late professor George Olah. A robust catalytic system, based on the Ru-MACHO-BH catalyst (Scheme 2), enabled one-pot CO2 capture and conversion to MeOH in the presence of pentaethylenehexamine (PEHA). 43 Scheme 2: Integrated CO2 capture (PEHA) and hydrogenation to MeOH in the presence of Ru-MACHO-BH catalyst. Reaction developed by Kothandaraman et al. 43 The system operates in relatively harsh conditions, at 125-165 °C under 75 bar of a CO2/H2 mixture in 1/3 ratio, under which CO2 is probably released from 1-PEHA. Methanol can be separated from the catalytic system by distillation, enabling the direct recycling of the catalytic system for further absorption-conversion cycles.

The production of MeOH from CO2 is relevant in the "methanol economy" context, which aims at developing an anthropogenic carbon cycle similar to photosynthesis. A CO2 to MeOH transformation process, based on heterogeneous catalysis and geothermal energy, is currently applied in the George Olah Renewable Methanol plant in Iceland. 44

Reactions of amines with CO 2 in water

The following paragraphs will present the mechanistic details of the carbamation and carbonation reactions, together with some considerations about the competition between the two processes in aqueous solutions of amines.

The first step involved in every capture reaction is the mass transfer of CO2 from the gas to the liquid phase (eq.11, Scheme 3). For simplicity, the (l) indicating the solubilized state of CO2 (CO2(l)) will be neglected in this document.

Carbamation

CO2 is a stable, neutral, linear molecule presenting a quadrupole moment (3.00*10 -26 esu cm 2 ). 36 The presence of two electronegative oxygens doubly bound to the carbon atom imparts a partial positive charge to the central carbon, making it reactive towards nucleophiles.

Scheme 4: Carbamation mechanism of primary/secondary unhindered amines with CO2. B=Base.

The capture of CO2 requires one species acting as a nucleophile and one acting as a base (NuH and B, eq.12, Scheme 3). Amines and alcohols can react with the carbon of CO2 through this Lewis and Bronsted acid-base equilibrium delivering oxyanionprotonated base ion pairs. 45,46 The reaction mechanism of unhindered primary and secondary amines with CO2 is presented in Scheme 4.

The first step is the nucleophilic attack of an amine on CO2 (1-12a) which generates a zwitterionic capture adduct. This unstable species can undergo intramolecular proton abstraction (3-12a), generating a carbamic acid. This is a transient species, as it will directly react with a base (B= R-NH2, R-OH -) (4-12a) yielding the final product, a carbamate-protonated base (BH + ) ion pair. Alternatively, the zwitterion can undergo intermolecular proton abstraction (2-12a), leading directly to the final product without carbamic acid generation.

If the base (B) is a second amine equivalent, the capture product is an alkylammonium alkylcarbamate salt. These products are extremely stable, with negative formation enthalpies (HF 0 ) between -60/-80 kJ/mol CO2. 47 Unhindered primary amines (such as MEA) are the most reactive CO2 capture agents and they generate more stable ammonium carbamate products compared to secondary amines. 36 Carbon dioxide dissolution in water with the generation of carbonic acid is a fundamental reaction regulating many environmental and biological processes. All equilibria regulating carbonic acid generation and its dissociation are summarized in Scheme 5.

Carbonation

CO2 hydration by neutral water is a slightly exothermic process (H= -19.7 kJ/mol for eq.11 + eq.13a, K13a=2.7*10 -5 ). 36 When the pH is higher than 8.5, CO2 will react preferentially with OH -(eq.13c), thereby generating bicarbonate (K13c=3.02*10 7 ). 48 From a thermodynamic perspective, the reactants of neutral water hydration (eq.13a) are more stable than the products, while the hydroxide CO2 capture pathway (eq.13c) is strongly displaced towards the product.

From a kinetic point of view, the reaction rate of CO2 capture by hydroxide (k13c=12.1*10 3 M -1 s -1 ) is 10 7 times higher than CO2 hydration by neutral water (k13a= 37*10 -3 M -1 s -1 ). 48 As previously mentioned, tertiary amines or hindered primary and secondary amines do not yield carbamates as capture products but they catalyze CO2 hydration by hydroxides thanks to their (Bronsted) basic character. The reaction of these species with CO2 corresponds to eq.12b (Scheme 3, page 18), where the amine is the basic species (B) which abstracts a proton from water, so OH -is the nucleophile (Nu) that binds CO2 as bicarbonate. Due to the fact that one amine and one water molecule are required to capture CO2 as HCO3 -(eq.12b), tertiary amines can reach a higher nominal loading (max th =1 for a monofunctional tertiary amine) compared to an equimolar solution of a primary or secondary amine (max th =0.5 for a monofunctional primary or secondary amine).

Carbamation versus Carbonation: antiparallel competitive capture pathways

Scheme 6: Representation of antiparallel competitive carbamation (eq.12a) and carbonation (eq.12b) CO2-capture pathways in aqueous solutions of amines (primary, secondary).

In water, carbamation and carbonation are competitive CO2 capture pathways, as they share the same building blocks (amine, CO2) and take place in the same conditions. The ratio between carbamates and carbonates in amine aqueous solutions influences the thermodynamics of the capture process, as carbonation is less exothermic than carbamation by 15-30 kJ/mol. 49,50 The predominance of one pathway over the other depends on the nature of the amine, on its molar concentration and on external parameters such as temperature, pH and solvent nature.

Scheme 6 shows the competing carbamation and carbonation equilibria. Primary/secondary amines could react with CO2 either by acting either as nucleophiles and generating ammonium carbamate (eq.12a) or as Broensted bases, in the presence of water, generating ammonium bicarbonate (eq.12b).

Depending on the experimental conditions and on the presence of water, carbamates can be hydrolyzed to ammonium bicarbonate (eq.14, Scheme 6). However, this reaction is usually unfavored due to the stronger nucleophilic character of amines with respect to water, which results in the higher stability of the ammonium carbamate compared to the ammonium bicarbonate couple. 51 Carbamate hydrolysis usually takes place with highly unstable carbamates, such as the dicarbamate of piperazine (2-PZ), which bears two negative charges on the same amine molecule (Scheme 7, eq.14-2PZ).

Scheme 7: Carbamation reaction of Piperazine (PZ) with the generation of 1-PZ (zwitterionic monocarbamate) and 2-PZ (dicarbamate). Dicarbamate hydrolysis with generation of bicarbonate (HCO3 -), regulated by the equilibrium constant K14-2PZ.

For 2-PZ, the equilibrium constant of carbamate hydrolysis (K14-2PZ=4.6*10 -1 ) is 1.8 times higher than the equilibrium constant of the second carbamation (K12a-2PZ=2.6*10 -1 ).

Aqueous homogenous sorbents for amine scrubbing

First generation sorbents

Figure 10 : Structures of some common amines for amine scrubbing with the corresponding abbreviations. [START_REF]was obtained by performing the same anion exchange procedure described in the previous paragraph (i) on a solution of diastereomerically pure mer-CoDCl2PF6. The product could be isolated in 99% yield by recrystallization in water and methanol[END_REF] a. Monoethanolamine (MEA)

Monoethanolamine (MEA) in 20-30% w/w concentration in water is the most exploited sorbent for amine scrubbing at the present day. [START_REF] Rochelle | [END_REF] The reaction of MEA with CO2 follows eq.12a (Scheme 3, page 18) and results in the generation of a stable ammonium carbamate couple (1-MEA, carbamation reaction depicted in Fig. 5, page 10) with a Habs= -88.91 kJ/mol CO2. 53 This sorbent displays a CC=0. 83 (CCN=0.42) upon absorption at 40 °C and desorption at 120 °C. 53 MEA is a cheap chemical (42€/Kg) synthetized industrially by combining NH3 and ethylene oxide. Some disadvantages connected with its utilization as CO2 capture agent are its sensitivity to thermal and oxidative degradation and corrosiveness when used in high concentration (30 % w/w equal to 4.9 mol/l). 54 However, the biggest disadvantage of this sorbent is the high energy requirement for desorption. An economic study on a 450 MW operating coal fired power plant showed that the implementation of PCC with a 30% w/w MEA aqueous solution required 0.37 MW/h to remove CO2 from the flue gas stream, resulting into a cost of 52 $/ton of CO2 captured. 55 The implementation of the current MEA-based capture process to a coal-fired power plant decreases the thermal efficiency by 25-40% and increases the price of electricity by 70-100%. 17 Despite the abundant research dedicated to develop a more efficient capture agent than MEA, performing efficient absorption while requiring less energy for desorption, it still represents the benchmark sorbent for amine scrubbing. 54

b. Piperazine (PZ)

The adoption of a sorbent with a higher capture capacity than MEA, which is limited to a max th =0.5, could increase the efficiency of the amine scrubbing process by minimizing the heat losses from heating and cooling the solution.

Piperazine (PZ) is a strong alternative candidate to MEA. The presence of two amine groups on the molecule results in a max th =1 thanks to the possibility of intramolecular proton abstraction, with the generation of a zwitterionic ammonium carbamate (1-PZ, Scheme 7, page 20).

The generation of the less stable dicarbamate (2-PZ), eventually accompanied by one equivalent of diammonium, can take place either by a second carbamation on 1-PZ (eq.12a-2PZ, Scheme 7, page 20), or by isomerization of two 1-PZ equivalents (eq.15-2PZ, Scheme 8). The CO2 capture properties of polyamines and the thermodynamics of the carbamate isomerization equilibria will be discussed in detail in Chapter II. At the pilot-plant scale, 8 M PZ displayed 75% better capacity and 2-3 times faster absorption rate then 5 M MEA. 56 PZ is also more resistant to thermal and oxidative degradation, less volatile and less corrosive. It can be heated up to 150 °C without degradation, which accounts for a better energy performance throughout the thermal swing cycle. 15,57

Second generation sorbents

Another alternative to MEA are the so-called "second generation" sorbents, based on mixtures of differently saturated amines in water.

Aminomethyl propanol (AMP, Fig. 10, page 21) is a hindered primary amine characterized by a high capture capacity, as it predominantly forms carbonates/bicarbonates (eq.12b, Scheme 3, page 18), but it suffers from a slow absorption rate, as most carbonation agents. The combination of AMP with PZ in 28/17 wt% ratio in water resulted in a sorbent with a high capacity, granted by AMP, and fast absorption rate, granted by PZ. 58 The mixture was tested at the pilot plant scale, where it granted a 10% Qreg reduction compared to MEA. Idem et al. presented in 2006 a pilot-plant study with the application of a 4:1 mixture of MEA/MDEA (Fig. 10, page 21), which showed a remarkable decrease in heat duty compared to MEA. 59 Rochelle et al. also proved that a mixture of MDEA and PZ offered 15-22% energy savings with respect to MEA. 60 More than 10 operational plants use a proprietary hindered amine, KS-1, to capture CO2 from flue gases produced by the combustion of clean fuels. In general, advanced amines such as KS-1 are more resistant to degradation than common amines but they are more expensive and require complex gas pretreating to avoid economic losses from the effects of SOx, NOx and fly ash. [START_REF] Rochelle | [END_REF]

Water-lean solvents

Aqueous amine solutions are considered the most sustainable option for large-scale PCC, despite the almost double heat capacity of water compared to most organic solvents. The adoption of organic solvents for amine scrubbing has been disregarded due to their volatility and toxicity. In addition, alkanolamines and amines are generally soluble in most organic solvents and are often liquid themselves, but CO2 capture induces a strong polarity change which can bring to the precipitation of the CO2-bound species from solvents less polar than water.

The most suitable option for PCC at the present day is the development of a homogenous process, that could be easily implemented on the existing power plants. In fact, the generation of a solid upon CO2 absorption would require additional phase separation steps that would increase the complexity and the cost of the process.

Water lean solvents are CO2 sorbents containing few percentages of water, developed to lower the sensible heat contribution to Qreg. [START_REF]was obtained by performing the same anion exchange procedure described in the previous paragraph (i) on a solution of diastereomerically pure mer-CoDCl2PF6. The product could be isolated in 99% yield by recrystallization in water and methanol[END_REF] Some examples are presented hereafter.

a. Organic solvents

The option to substitute water with organic cosolvents such as methanol (MeOH) or ethylene glycol (EG) was explored in the 1990s to increase the physical solubility of CO2 and thus the capacity of the sorbents. 61,62 Indeed, the addition of EG or polyethylene glycol (PEG) to MEA with a small water percentage was found to increase the CO2 solubility and capture capacity of the amine. 63 However, organic solvents were discarded since then due to the increasing concern for the environment and the consequent requirement to develop environmental-friendly industrial processes. Sterically hindered alkanolamines such as TBAE (2-[(1,1-dimethylethyl)amino]ethanol) can capture CO2 in the absence of a solvent (Scheme 9). The alcohol binds CO2 as it is more nucleophile than the hindered amine group, which in turn acts as a base. The zwitterionic capture products (1-TBAE) are often solid, but can be homogenized by the addition of cosolvents such as EG or MeOH. 64

b. Neat sorbents

c. CO2 BOLs

A remarkable example of water-lean solvents are CO2 BOLs, mixtures of aliphatic alcohols with superbase promoters (amidines or guanidines), that are liquid both in the lean and rich state. 65 These sorbents can capture CO2 in anhydrous conditions by generating alkylcarbonate salts. Desorption can be performed upon stripping with an inert gas at mild temperatures (from room temperature to 50 °C). Due to the absence of water and to their inferior heat capacity compared to water, CO2 BOLs display a 50% reduction of Qreg compared to MEA in water. These water-lean sorbents will be discussed in detail in the bibliographic section of Chapter III.

d. Ionic liquids

Scheme 10: TSIL for CO2 Capture by Bates et al. 66 Ionic liquids are often presented as viable alternatives to classic aqueous amine sorbents. The main advantage of these molten salts is their negligible volatility up to their decomposition temperature, usually around 300 °C. 67 The main disadvantage of ILs is their cost, which as of today is not competitive with alkanolamines.

Their properties can be modulated by tethering appropriate molecular functions on the cationic or anionic part, generating the so-called Task Specific Ionic Liquids (TSILs). The first example of TSIL adapted to CO2 capture was presented in 2002 by Bates et al. 66 An amine moiety was branched on the imidazolium cation, coupled with a BF4 -counterion (Scheme 10). The loaded sorbent could be regenerated by heating at 80-100 °C during several hours without degradation or evaporative loss. Dynamic Combinatorial Chemistry (DCC) is a synthetic strategy which enables to obtain complex molecular architectures from simple molecules, that spontaneously associate through reversible covalent and non-covalent interactions. 68,69 A mixture of building blocks subject to one or multiple reversible binding events, called a Dynamic Combinatorial Library (DCL), can evolve spontaneously through intra and/or intermolecular bond formation and breaking until thermodynamic equilibrium is attained. At equilibrium, the system spontaneously selects the most stable aggregates available for the given set of starting conditions (DCL at equilibrium, Fig. 11). 1 DCL can respond to external stimuli, such as a temperature or pressure change, the application of an electric or magnetic field, or the introduction of a new chemical, if they influence the relative stability of the components. 70 A "template" is a species able to influence the distribution of the library members at equilibrium due to its preferential affinity for one (or multiple) member (members) of the library. The introduction of a template in a DCL starts a series of bond breaking/formation in solution, which brings to the selective formation or amplification of the library member with the highest affinity for the template (Fig. 11, New Thermodynamic Equilibrium). 71 The modification of DCLs has for the most part been mediated via supramolecular interactions, but it can also be based on covalent reversible processes. 72,[START_REF] Saur | Chemical Communications[END_REF] Mixtures of molecules subject to reversible covalent bond formation, such as carbamation, carbonation, condensation, transamination and transimination respond to DCC principles. 2 The following bibliographic section will present some results obtained in our group by the application of DCC to amine solutions for CO2 capture.

Dynamic Combinatorial Chemistry and CO 2 : Equilibrium displacement by covalent or supramolecular means

Agonism and antagonism in CO 2 capture Scheme 11: Scheme of carbamation equilibrium displacement via the introduction of antagonist (Ant) or agonist (Ago) molecular partner.

Thanks to the reversible nature of carbamation and carbonation, the CO2 capture systems can be regulated with a Dynamic Combinatorial strategy.

For example, the capture equilibria can be displaced towards the reactants by the reaction with a molecular partner with a specific affinity for the lean amine (eq. 16, Scheme 11). This species acts as an "antagonist" (Ant) to CO2 capture, as it can favor CO2 desorption by stabilizing the lean state of the amine. The equilibria could also be displaced in the opposite direction, towards the rich state, by the introduction of a species with a preferential affinity for the CO2-loaded products: the additive would then behave as an "agonist" (Ago, eq.17, Scheme 11) towards CO2 capture, by stabilizing the carbamate or carbonate products. Some examples of agonists and antagonists to CO2 capture are presented hereafter. [START_REF] Septavaux | [END_REF]

a. Carbamation Antagonism by amine-aldehyde condensation

Aldehyde and ketones can react reversibly with free amines through condensation, generating imines, oxazolines or aminals. Therefore, they can play the role of antagonists to carbamation, by competing with CO2 for the reaction with amine groups. Both processes are exothermic and in a close enthalpy range. 75 The concept was demonstrated on a mixture of α,β-amino alcohols and α,γ-diamines (diam, Scheme 12, following page) in water. Upon CO2 introduction, carbamate formation was reduced at low loading due to the sequestration of some amine moieties by condensation with the antagonist (Scheme 12, eq.16a). Nonetheless, even if CO2 absorption was initially inhibited, at high CO2 loading the condensation products were progressively hydrolyzed (reverse of eq.16a) and the free amine participated in bicarbonate generation (eq.12b-d).

Interestingly, the extent of desorption was higher in the presence than in the absence of aldehydes (Ant), due to the stabilization of the lean amine state, which resulted in a 30% increase of the Cyclic capacity compared to the Ant-free library. 2 Scheme 12: Representation of the binding antagonism between aldehydes (R'-CHO, Ant: antagonists) and CO2 toward -diamines (diam).

b. Carbamation Agonism by Supramolecular self-assembly

An agonistic system was identified by combining a ternary aldehyde (1,3,5-triformylphenol) with DETA (Diethylenetriamine, Fig. 10, page 21), which resulted in a spontaneous aggregation phenomenon upon CO2 introduction (Scheme 13).

Scheme 13: Representation of the binding agonism between 1,3,4-triformylphenol and DETA, with the generation of a three-component CO2-loaded supramolecular solid (green background). 2 Solid-state analyses suggested that the aldehyde had condensed with three amine equivalents, converting into a triimine. The primary amines of DETA condensed with two separate aldehyde molecules, so that the amine stood as a bridge between each aromatic ring. The secondary amine of DETA bound CO2 as a carbamic acid, stabilized through hydrogen bonding (Scheme 13, green background). The formation of the solid was driven by the complementarity of the building blocks, whose H-bond donor and acceptor sites were completely saturated within the lattice.

The precipitation of the three-component product acted as a driving force towards CO2 capture, as the progressive decrease of building block concentration in solution continuously shifted the capture equilibrium (eq.12a-D + eq.17a, Scheme 13) towards the generation of the product.

The solid completely released CO2 upon heating at 65 °C: this desorption temperature, significantly lower than those required for alkanolamine-CO2 solutions regeneration (100 -120 °C), was sufficient to start the chain rupture mechanism of the supramolecular aggregate into smaller, soluble, binary products. This 2010 DCC-based study was the first example of cooperative CO2 capture, which will be discussed further in Chapter II.

c. Metal salts as Transcarbamation triggers

Scheme 14: Scheme of the differential affinity of amines with soft metal cations (Ant), and carbamates with hard metal cations (Ago).

Further studies by our group showed that metal salts can play the role of agonists or antagonists to CO2 capture.

Polyamines, either lean or carbamated, are efficient ligands for metal cations. 36 In methanol and in the presence of a polyamine, rare earth metals (Y 3+ , La 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Tb 3+ , Dy 3+ ) tend to precipitate upon CO2 introduction while most alkaline, alkaline earth and transition metals (Li + , Na + , K + , Cs + , Mg 2+ , Ba 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Pd 2+ ) remain soluble. 39 The differences can be justified with the Hard and Soft Acids and Bases (HSAB) theory: lanthanides are "hard" cations which will preferentially bind to negatively charged, oxygenated ligands (such as carbamates, eq.17b, Scheme 14) than to lean amines, generating CO2-rich, solid, supramolecular aggregates. Transition metals on the other hand bind to lean amines as soluble coordination compounds (eq.16b, Scheme 14).

The metal separation properties of Diethylenetriamine-CO2 solutions were applied to the treatment of metal mixtures issued from LaCoNi batteries. 3 DETA was chosen as model amine for this study for its efficient CO2 binding and metal coordinating character, due to the presence of three amine groups. The study of the complex library of carbamates obtained from CO2 capture with DETA will be detailed further up in this document. In methanol, two monocarbamates (1a-DETA and 1s-DETA) and one dicarbamate (2s-DETA) were detected by NMR (Fig. 12a). At any given loading (), the members of the polyamine-CO2 libraries can interconvert between one another through isoenergetic transcarbamation equilibria (green arrows in Fig. 12a). 39,40 The influence of the metal salts on amine-CO2 libraries was evaluated by quantifying the change in concentration of the carbamates in the solutions containing the metal templates.

Upon CO2 introduction in a DETA solution containing a LaCl3, a precipitate forms. The solid is a neutral complex containing La 3+ coordinated by two dicarbamate (2s-DETA) strands (Fig. 12b). Due to the precipitation with La 3+ , the total quantity of 2s-DETA generated in the presence of the metal salt is higher than in its absence. In the presence of the metal the transcarbamation of 1a-DETA into 2s-DETA (eq.15-DLa, Fig. 12b) is favored, due to the precipitation of the dicarbamate with the metal. Thereby, the metal acts as an agonist to CO2 capture and a transcarbamation trigger.

The selective precipitation of La 3+ upon CO2 capture with DETA enabled the separation of 99.4% of the initial metal in 99.81% purity from a mixture issued from the acidic dissolution of a La2CoNi9 alloy. Further treatment and separation procedures with DETA and CO2 resulted in the isolation from the mixture of Nickel in 98.3% purity, 95.3% yield and of Cobalt with 55.2% yield and 97% purity. 3 This innovative method to separate metals from complex mixtures represents at the same time a sustainable metal recycling process and an economic switch for PCC, because of the substantial added value it implies for the CO2-loaded amine solutions. In fact, the commercial value of Cobalt is currently US $40,000/ton, while the cost of CO2 capture by amine scrubbing amounts to US $40/ton of CO2 removed. 3 The goal of this thesis was the development of innovative polyfunctional CO2 capture agents for amine scrubbing by exploiting the principles of supramolecular chemistry.

III. Main objectives of this thesis

Chapter II will present the characterization of a series of polyamines, either commercial or synthetic, as CO2 capture agents. The Dynamic Combinatorial Libraries (DCL) will be studied from the molecular perspective, i.e. by qualitatively and quantitatively analyzing the library members, by NMR and FTIR spectroscopies. The objective of this study was to understand the parameters which drive polyfunctional amines towards the generation of the different CO2-bound species (monocarbamates, polycarbamates, bicarbonate) in solution. A thorough study of monocarbamate disproportionation leading to dicarbamate generation (eq.15, Fig. 13) was conducted for diamines.

As an application of Dynamic Combinatorial Chemistry, Chapter III will present the development of some template species, designed to influence the equilibria of the polyamine-CO2 libraries presented in the second chapter.

As a follow-up of previous work from our group 3 , a series of Bis(diethylenetriamine)Co(III)X3 complexes (CoDX3; X=Cl -, ClO4 -, PF6 -, BPh4 -) were synthetized and characterized as templates for polycarbamates (eq.15, Fig. 13). The interactions of polyoxoanions with the complexes by H-bonding in the second coordination sphere were studied by 1 H NMR and UV-Vis spectroscopy. A spectroelectrochemical study was conducted to evaluate the influence of the supramolecular carbamate-complex binding on the rate of the spontaneous aerobic Co(II)/Co(III) oxidation. These results are discussed in Chapter III Part C.

Taking inspiration from the wide literature concerning guanidines as oxyanion receptors, we characterized some bisguanidinium salts as water-soluble templates for bicarbonate (eq.14, Fig. 13). The systems were studied by NMR, which highlighted an amplification of bicarbonate due to the stabilizing supramolecular interactions with the guanidinium cations. The switch from the carbamate to the carbonate capture chemistry induced by these templates led to a capture system of increased thermodynamic efficiency and unchanged absorption kinetics. These results are presented in Chapter IIIB.

During some parallel tests aimed at finding a cheaper and more sustainable alternative to Cobalt (as polycarbamate template), we encountered an unexpected reactivity between FeCl3 and DETA in methanol in the presence of atmospheric levels of dioxygen. The spectroscopical study of the reaction will lead to the identification of a novel synthetic method to access imidazoles and heterocyclic compounds. The details of this previously unexplored reactivity, which may be reminiscent of some prebiotic mechanisms, will be discussed in Chapter IV.

Finally, we aimed at contributing to the integrated CCUS field by combining the electrochemical transformation of CO2 with its capture with amines. The results, presented in Chapter V, were obtained during a collaboration with the University of Turin. The application of an electrolytic CO2 reduction process, catalyzed by a heterogeneous Manganese-bipyridyl catalyst, on amine solutions in methanol, led to development of an efficient CO2 reduction process.

Polyamines as CO 2 capture agents

Over the years, polyamines were often studied as CO2 capture agents. The reaction of polyamines with CO2 follows eq.12a (Scheme 3, page 18) as for monofunctional amines, but with the possibility of generating zwitterionic ammonium carbamates via intramolecular proton abstraction. The carbamation reaction is made explicit in Scheme 15 for -diamines (diam, next page), i.e. aliphatic diamines presenting two amine groups on the terminal ends of the carbon chain. Due to intramolecular salt bridging, zwitterionic ammonium carbamates (1-diam) can be expected to be more stable than intermolecular homologues, hence more difficult to convert back to free amine and CO2. 76 Dicarbamates (2-diam) can be obtained from the disproportionation of two monocarbamates (eq. 15-d, Scheme 15, next page). The thermodynamics of this isomerization equilibrium, regulated by equilibrium constant K15-d, will be discussed in the Results and Discussion section of this chapter.

Scheme 15: Carbamation reaction of -diamines (diam) with CO2 with the generation of zwitterionic monocarbamates (1-diam), and/or diammonium (damm) dicarbamates (2-diam).

This dissertation will show that polyammonium polycarbamate salts are unstable species which are hardly generated in the polyamine-CO2 libraries.

Dicarbamates were identified and quantified for the first time in 1951 in Ethylenediamine (EDA, Fig. 14, page 33) and 1.6-diaminohexane solutions loaded with CO2. 77 The quantification was obtained by reaction with diazomethane, a methylation agent which converted mono/dicarbamates into mono/dicarbomethoxy derivatives. An approximate 1:1 ratio between mono and dicarbamates was obtained for both amines, but the reaction likely affected the distribution of the capture adducts, therefore the quantification probably did not reflect the composition in solution.

The complexity of the polyamine-CO2 libraries increases with the number of amine groups, as both CO2 and protons can bind to multiple sites. The characterization and quantification of the members of these complex systems is a hazardous task that requires a suitable technique to discriminate between different species.

The qualitative and quantitative analysis of the components of amine-CO2 libraries upon increasing loading ( or ) is called a "speciation" and can be performed by NMR 78 , potentiometric 15 or conductimetric 16 methods.

Speciation by quantitative NMR (qNMR), which relies on the integration of the signals displayed by each species with respect to an internal reference, is the method of choice in our group for the study of CO2 capture libraries. Given the complexity of the systems and the similarity between different carbamated species 13 C quantitative NMR is usually preferred to 1 H NMR. 3,79 1.1 CO 2 capture performance comparison between polyamines and MEA

The values reported in Table 2 (following page) were taken from three separate works 47,76,53 to enable a comparison between some relevant kinetic and thermodynamic performance evaluators for polyamines and MEA. Kim et al. reported in 2014 a performance comparison study of MEA, EDA, DETA, TETA and TEPA (molecular structures in Fig. 14, page 33). 76 max was measured for 30% w/w solutions of amines upon injection of CO2 (30% with N2 70%). As expected, the experimental loading max of polyamines was higher than MEA. However, the normalization of the loading with the number of binding sites on each amine molecule (Nmax, eq.5, page 8) shows that MEA displays a higher atom efficiency compared to polyamines (Nmax MEA=0.55 > Nmax EDA, DETA, TETA, TEPA).

In a later work, the same authors performed a screening of triamines such as P1, P2 and P3 (Fig. 14, page 33), MEA and PZ in 30% w/w concentration in water, providing an extended list of thermodynamic and kinetic parameters for the evaluation of the sorbent's performances. Amongst other parameters (absorption rate, viscosity, degradation), they measured the CC as the loading difference between an amine solution in the rich state (equilibrated at pCO2=5 kPa at 40 °C) and in the lean state (equilibrated at pCO2=0.05 kPa at 40 °C). In comparison with MEA, the CC of polyamines was respectively inferior for P1 and P2, slightly superior for P3 and two times higher for PZ (Table 2). The initial absorption rate values in Table 2 show that polyamines generally capture CO2 faster than MEA, with the exception of EDA and DETA. 80,81 On the other hand, the initial desorption rate of DETA is 1.9 times higher than MEA, and a similar desorption acceleration can be observed for TETA and TEPA.

In general, this performance comparison highlights the exceptional properties of PZ as capture agent as, in addition to higher thermal resistance, inferior degradability and corrosivity than MEA, it displays a two times higher CC, a fast absorption rate and a low Habs. 15 Open chain polyamines generally display an absorption enthalpy close to MEA, a higher max and for some amines (P1, P2, P3, TEPA) a faster initial absorption rate, but the positive effects are counterbalanced by an inferior CC, which implies an inferior efficiency of the capture process.

Structurereactivity studies

The molecular structure and the number of binding sites on a polyamine molecule influence its CO2 capture properties. This statement is illustrated by the work of Zhang et al., which This trend was attributed to the different stability of short-chain zwitterionic monocarbamates, characterized by stronger ammonium-carbamate intramolecular salt bridges (Fig. 15, 1-1.2-diam R-N-CO2 -… 3 + HN-R, 1.58 Å) with respect to longer diamines, whose monocarbamates profit from weaker salt bridging (Fig. 15, 1-1.3-diam R-N-CO2 -…+ 3HN-R, 1.99 Å). The least stable monocarbamates are more easily hydrolyzed to generate HCO3 -(eq.14, Scheme 6, page 20), thereby the corresponding diamines display an increased capacity compared to shorter homologues. 82 Only few studies have identified and quantified the different capture products generated upon CO2 absorption and desorption with polyamines. The analysis of the carbamation pattern, i.e. the distribution of carbamated states in a polyamine library, is essential to understand the relative stability of the different adducts and the capture properties of the system. N=molCO2/2*molPZ). Reproduced from Hilliard et al. [START_REF] Hilliard | A predictive thermodynamic model for an aqueous blend of potassium carbonate, piperazine, and Monoethanolamine for carbon dioxide capture from flue gas[END_REF] Hilliard et al. obtained the speciation of PZ (5 M) in water at 40 °C from calculations (Fig. 16). 15,[START_REF] Hilliard | A predictive thermodynamic model for an aqueous blend of potassium carbonate, piperazine, and Monoethanolamine for carbon dioxide capture from flue gas[END_REF] The speciation graph consists in a plot of the molar fraction/concentration of each library component (ispecies, y-axis) upon increasing loading ( or N).

For PZ, at low loading (N < 0.25), the bimolecular ammoniumcarbamate couple was the most abundant species in solution. The zwitterionic state became more stable at higher CO2 loading (N > 0.30) and it was indeed the dominant species at Nmax=0.5. Dicarbamate (2-PZ) was detected in the system, though it remained below 5% of the population.

The 2016 study by Kim et al. coupled the measurement of Habs and absorption rate with the speciation of MEA, DEA, MAPA, DETA, TETA and TEPA. [START_REF] Kim | [END_REF] For all the polyamines studied, primary amines resulted more reactive than secondary ones. Primary-secondary dicarbamates were detected for all the polyamines tested, but they were the first to release CO2 upon heating, a clear indication of their instability with respect to monocarbamates. A correlation was highlighted in this study between the decrease of Habs and the increase of secondary amine groups on the sorbent molecules.

Hartono et al. 78 performed the qualitative speciation of DETA by 13 C NMR, whose results agreed with our study of the same amine by quantitative NMR. 3 The speciation of DETA will be detailed in Section 2.2 of Results and Discussion.

The few examples of polyamine speciations reported in the literature all point out that primary amines are the most reactive groups, as primary carbamates are the most abundant species observed at low loading, while secondary carbamates and polycarbamates, if present, are generated at high loading. 47,78,85

Drawbacks of polyamine sorbents

Table 3: Degradation % of amines. ( 1) amine saturated with CO2 at 4 (a) or 2.5 (b) M concentration 86 ,T=140 °C,p=2 MPa,15 days;(2) amine saturated with CO2, 30% w/w concentration, T=120 °C for 3 days. 53 Despite their attractive properties as capture agents, polyamines are mostly used as additives to monoamines rather than exclusive capture agents. The main drawbacks of these CO2 sorbents are their higher price and faster degradation rate compared to MEA. The study by Lepaumier et al. focused on methylated amines such as TMEDA, TMPDA, TMBDA, PMDETA (molecular structures in Fig. 14, page 33), which are more stable than the non-methylated homologues, as demethylation is required to initiate the degradation pathway. 86 However, despite the methylation, these polyamines were subject to a higher degradation rate than MEA, which increased upon chain elongation and with the number of amine groups.

A similar consideration can be applied to P2 and P3, which despite the presence of some methyl substituents, underwent 15-20 times more degradation than MEA (Table 3). P1, structurally close to DETA, was subject to consistent degradation, as 40% of the amine was lost after 3 days at 120 °C, thereby displaying a 10 times higher degradation rate than MEA.

Depending on their structure, amines can degrade via addition, ring closure or opening, oxidation, elimination or transalkylation. Some of the most common oxidative and thermal degradation products for polyamines and MEA are represented in Fig. 17. 86,87 Linear polyamines such as EDA and DETA, but also MEA, tend to deliver cyclic urea compounds or oxazolidinone as degradation products with CO2. Alternatively, polyamines can oligomerize to deliver longer chain open polyamines, such as for EDA which generates DETA as degradation product. [START_REF] Davis | Thermal degradation of aqueous amines used for carbon dioxide capture[END_REF] Figure 17: Some of the most common degradation products observed for polyamines and MEA. 86,87 2. Taking inspiration from nature: cooperative CO 2 capture

Thermodynamic model

Cooperativity, in molecular systems presenting multiple binding sites, arises when multiple interactions interplay and induce properties that differ from the sum of the individual contributions. Depending on whether one interaction favors or disfavors the other, cooperativity can be positive or negative respectively.

A typical example found in nature is the allosteric binding of O2 by Hemoglobin, wherein the binding of the first O2 molecule induces a structural modification on the globular protein which facilitates the capture of three additional dioxygen molecules.

Cooperativity can be described by a mathematic model based on the statistical distribution of occupied states in a multi-site receptor. [START_REF] Hunter | [END_REF] Scheme 16: Representation of binding for a monofunctional ligand (B) to a) a monofunctional receptor (A) and b) a polyfunctional receptor (AA). (i) = interaction parameter.

A typical binding event between a monofunctional receptor and a monofunctional ligand ((18a-1), Scheme 16a) is described by a constant, K18a-1, calculated as in eq.18a-1.

𝐾 18𝑎 = [𝐴𝐵] [𝐴][𝐵] 𝒆𝒒. 𝟏𝟖𝒂 -𝟏
If the receptor presents multiple binding sites (AA, Scheme 16b), upon association with a monofunctional ligand (B) it could present three different macroscopic states: fully unbound (AA + B), partially bound (AA-B), fully bound (AA-BB) (Scheme 16b). The AA-B state presents two degenerate microscopic states, that cannot be distinguished experimentally but must be considered in the calculations as they influence the entropy of the system.

In this case, the association will be described by two binding constants, K18b-1 and K18b-2, calculated as in eq.18b-1 and eq.18b-2. The 2-fold statistical factor for K18b-1 accounts for the degeneracy of the partially bound state AA-B.

2𝐾 18𝑏-1 = [𝐴𝐴 -𝐵] [𝐴𝐴][𝐵] 𝒆𝒒. 𝟏𝟖𝒃 -𝟏 1 2 𝐾 18𝑏-2 = [𝐴𝐴 -𝐵𝐵] [𝐴𝐴𝐵][𝐵] 𝒆𝒒. 𝟏𝟖𝒃 -𝟐 𝛼 (𝑖) = 𝐾 18𝑏-2 𝐾 18𝑏-1 𝒆𝒒. 𝟏𝟗
The presence of cooperativity in a binding event can be assessed with the cooperativity factor (i), calculated as the K18b-2/K18b-1 ratio (eq. 19), which will be equal to 1 for a non-cooperative, higher than one for a cooperative, inferior to 1 for an anticooperative system. [START_REF] Hunter | [END_REF] At the molecular level, positive cooperativity () corresponds to a concentration of intermediate states (AA-B) lower than the statistical prediction. Systems characterized by positive cooperativity tend toward an "all-or-nothing behavior", wherein the system ideally shifts directly from the unbound (AA) to the completely bound (AA-BB) state.

Cooperativity can be observed by the shape of the binding plots, which show the variation of the concentration of bound/unbound states (y axis) upon a change in conditions (x axis), such as the introduction of a molecular partner. The binding plots of cooperative systems usually present a sigmoidal shape, such as the ones presented in Fig. 18, representing the sharp shift from the totally unbound to the totally bound state (and vice versa for negative cooperativity) when the conditions reach the threshold value for the transition.

Cooperative CO 2 capture systems

The implementation of a cooperative capture process to amine scrubbing would result in an increased thermodynamic efficiency. 90 The advantages of cooperative CO2 sorbents with respect to classic ones, such as MEA in water, can be deduced from the comparison of their (ideal) absorption plots, presented in Fig. 19. 90 For a gas capture process (O2 capture by hemoglobin, CO2 capture by amines), the binding plot corresponds to an isothermal absorption plot. For CO2 capture, the plot shows the increase of CO2 loading ( or , y axis in Fig. 19) of the sorbent upon increasing CO2 partial pressure (pCO2, x axis, Fig. 19).

The ideal cooperative CO2 sorbent should be able to shift directly from the lean to the rich state when contacted with the flue gas entering the absorbent (Fig. 19b, ideal isothermal absorption plot for cooperative sorbents at three different temperatures Tlow, Tmedium, Thigh). In average, Tlow= 40 °C is the temperature of the flue gas after cooling, and Pabs= 0.15 atm, the partial pressure of CO2 in the flue gas when it is not pressurized (15% of CO2). The step of the sigmoidal absorption plot at Tlow should consequently be centered before the 0.15 atm threshold. The working capacity defines the grams of CO2 removed/100 g sorbent, which in Fig. 19 corresponds to the area delimited by Pabs, Pdes, max and min. For a classic sorbent (Fig. 19a), the steepness of the isothermal plot decreases progressively upon a temperature increase (from Tlow, blue line, to Thigh, red line), so great temperature shifts are necessary to achieve an adequate working capacity. On the other hand, for the sigmoidal (cooperative) isotherm the plot shifts remarkably upon a small increase in temperature (Fig. 19b, Tlow vs Tmedium), which results in a higher working capacity for a smaller temperature increase with respect to a classic sorbent. 90 In 2015, a diamine-appended MOF displaying cooperative CO2 adsorption was presented by Long et al. 90 This solid adsorbent displayed a high capacity of 2.7 mmol CO2/g sorbent (MEA 30 % w/w 2.5 mmol/g) and a high selectivity for CO2. The regeneration energies were in the 2.2 -2.3 MJ/kg CO2 range, significantly lower than with classic sorbents (MEA 30% w/w 3.5 MJ/Kg).

The mechanism elaborated by the authors, based on X-ray diffraction and modeling evidence, starts with a carbamation on a metal-bound amine, followed by a proton abstraction by an amine borne by an adjacent site ((1-20), Scheme 17). The generation of an ammonium destabilizes the metalbound amine on the same molecule, that becomes more reactive towards CO2. Carbon dioxide is postulated to insert more easily in the N-metal bond than when the neighboring binding site is empty. This insertion triggers a chain mechanism which, in a true cooperative behavior, brings the amine-based material to a fully loaded state (2-20).

Scheme 17: Cooperative CO2 Capture mechanism of MMEN-M n+ -dobpdc (MOF). 90 The exploitation of solid sorbents for amine scrubbing presents some disadvantages, due to their water sensitivity and to the significant capital costs connected with the implementation of an adsorption process to the currently existing power plants.

Another cooperative capture system was developed by Esser-Kahn et al. by coupling a bisguanidine (1,1′-(propane-1,3-diyl)bis (1,3,3-trimethylguanidine)) with 1,5-pentanediol with diglyme (bis(2methoxyethyl)ether) as co-solvent (Scheme 18). 91 Scheme 18: Cooperative absorption system based on a diol and a bisguanidine in Diglyme. 91 The capture mechanism involves the binding of CO2 by one of the alcohols, with the generation of an alkyl carbonate, assisted by proton abstraction from the guanidine ((1-21), Scheme 18). The first binding event brings the remaining active sites in proximity and facilitates the capture of a second CO2 equivalent, resulting in a solid bis alkylcarbonate-bisguanidinium salt ((2-21), Scheme 18). The precipitation of the fully bound product favors cooperativity because the equilibria ((1-21), (2-21)) are continuously shifted towards the right side.

This cooperative binding would not be easily implemented on the industrial process due to the generation of a solid capture product, which would require a separation step, and to the exotic nature of the solvent. Moreover, this system it is highly sensitive to moisture which can induce the hydrolysis of alkylcarbonates ((3-21), Scheme 18) to bicarbonate.

Few more examples of cooperative CO2 capture were claimed over the years, but the fully loaded adducts were generally insufficiently characterized (if not at all). In addition, the applicability remains limited because the system either precipitates upon capture or operates in fully anhydrous/anaerobic conditions. 91,92 The stabilization of polyanionic states in homogenous conditions is a tedious task that no research group has been able to fulfill at the present day. The ideal cooperative absorbent should remain homogenous throughout the process, afford a fast and selective CO2 capture at Tabs≈ 40 °C and pCO2=0.15 bar, and an effective desorption at Tdes<100 °C while being unexpensive and based on sustainable and "green" chemicals.

A part of this thesis was dedicated to study the intra and intermolecular interactions which can drive polyfunctional amines towards the generation of polyammonium polycarbamates, with the ultimate goal of improving the thermodynamic efficiency of amine scrubbing solutions.

Objectives

Chapter II presents the study of a series of polyfunctional amine solutions for CO2 capture. The objective of this screening was to gain insight into the parameters which drive polyfunctional capture agents towards the generation of polyanionic CO2-bound species.

Achievement

All experiments were performed in solution (either H2O or MeOH). The polyamine-CO2 libraries were analyzed by quantitative 1 H and 13 C NMR speciation. The concentration of each library component was calculated by integrating the corresponding NMR peaks with respect to an internal reference. Molar fractions were calculated for each species with respect to amine concentration, and they were plotted against normalized loading () to obtain the final speciation graphs.

A detailed experimental (NMR speciation) and theoretical (virtual thermodynamic cycle) analysis of the isomerization of zwitterionic ammonium carbamates into diammonium dicarbamates (eq.15-d, Scheme 15, page 34) was performed for -diamines of increasing chain length.

In-situ Fourier Transform Infrared (FTIR) spectroscopy with an immersion probe was exploited to monitor the increase of loading upon time of CO2 injection for different polyamines. For Diethylenetriamine (DETA, Fig. 14, page 33) an additional FTIR-qNMR calibration was performed in order to calculate the initial absorption rate.

Cyclic Capacity (CC) was measured by a combination of in-situ FTIR, for qualitative evaluation of the extent of loading, and quantitative NMR, for the accurate quantification of the loading. Five consecutive absorption (25 °C) and desorption (100 °C) cycles were performed on the different capture solutions, delivering CC and CCN mean value and rmsd.

Most polyamines were studied in water. DETA was studied in water and in methanol, as in the latter solvent a cooperative capture behavior was highlighted previously in the group. 3 The cooperativity was confirmed by the obtention of two sigmoidal isothermal absorption plots, by gravimetry and FTIR.

Finally, the CO2 capture properties of a tetraazacyclophane (3,7,11,15-tetraaza-1,5,9,13(1,3)tetrabenzenacyclohexadecaphane), obtained through a synthetic procedure reported in the literature 93 , were studied in methanol.

1. Assessing monocarbamate disproportionation for a -diamines The bibliographic study highlighted the unstable nature of polycarbamates, that were minority products in the speciations of polyamines. The amplification of polyanionic states requires strong stabilizing agents, such as MOFs or polyguanidinium arrays.

Statistical distribution of microstates

In the following paragraph, we present a detailed theoretical and experimental study of the disproportionation equilibrium (eq.15-d, Scheme 15, page 34) of zwitterionic ammonium monocarbamates (1-diam) into diammonium dicarbamates (damm, 2-diam) for a series of diamines (diam) of increasing chain length.

A prediction of the diamine-carbamate library composition purely based on statistics is presented in Fig. 20. From the carbamate point of view, the diamine-CO2 libraries present three macroscopic states with an occupation factor equal to 0 for diam, 1 for 1-diam and 2 for 2-diam and damm. At this stage, it has to be noted that damm, which accompanies the dicarbamate as counter-ion (Scheme 15, page 34), will be experimentally quantified as the CO2-free subsystem.

Each macroscopic state corresponds, on the microscopic level, to a series of microstates. Even if the microstates cannot be distinguished experimentally, their degeneracy influences the entropy of the systems, which will favor the generation of the carbamate with the most microstates (most disordered state). At full loading, the system presents four microstates for 1-diam and two for 2diam (Fig. 20). This indicates that, from an entropic perspective, 1-diam is favored over 2-diam. At any lower loading, the system can be depicted as a mixture of fully unloaded (100 % diam) and fully loaded state (Fig. 20).

This statistical analysis provides a first insight on the entropic side of eq.15-d and enables to define the cooperativity factor for a -diamine based CO2-capture system. The statistical (2-diam)/(1diam) ratio of 1/3, independent on the loading of the system, defines this threshold: the system will be cooperative if the experimental distribution of 2-diam is higher than the statistical one, i.e. more than 1/3 of the carbamate species.

1.2. Exploiting a virtual thermodynamic cycle based on pK a s to assess monocarbamate isomerization constants We examined the structural and electrostatic parameters regulating eq.15-d for a series of -diamines with a number of -CH2-spacers comprised between 2 and 5 (n=1, 2, 3, 4 in Fig. 21).

From an enthalpic point of view, 1-diam is stabilized by an intramolecular ammonium-carbamate salt bridge. Conversely, 2-diam should be destabilized, to a different extent, from repulsive electronic effects from the two negative charges. In both the reactant and product states, these attractive and repulsive effects should propagate both through the covalent backbone and through space.

This situation is strongly similar to Bronsted organic polyacids and bases, wherein charged states may be stabilized or destabilized by neighboring electronic effects. Therefore, we tried to determine whether the global electrostatic effects governing the reaction enthalpy of eq.15-d could be evaluated exploiting the pKa values of organic Bronsted acids and bases.

We demonstrate in the following discussion that the equilibrium constant of the isomerization (K15d) can be expressed as a linear combination of tabulated pKa values of simple aliphatic carboxylic acids and amines, as ammonium carbamates derivatives. Eq.15-d can be decomposed into multiple derived equilibrium through a virtual thermodynamic cycle approach, based on Hess law which states that the enthalpy change of a process depends on the initial and final state but not on the pathway and steps in between. As the same bond substitution process was performed on both states (reactants and products), it can be hypothesized that the resulting equilibrium constant of the second equilibrium is, at first order, identical to the carbamate disproportionation constant (K15-d).

1-diam

At this stage, it should be noted that the potential 2-diam intermolecular salt bridge interaction with damm is preserved in the carbon-based system (dac -damm). At first order, the nitrogen to carbon substitution should affect this noncovalent interaction to a similar extent in the reactant state (zwitterionic) than in the product state (doubly charged ions).

An additional virtual transformation is required on both aa and dac + dam states to easily calculate K15-d from tabulated pKa constants.

a. Left side of the disproportionation equilibrium

In 1-diam and in its model compound aa, the stabilization arises from the presence of an intramolecular salt bridge. In physical organic chemistry, it is well known that the mutual stabilization between carboxylic and ammonium moieties borne by the same backbone can be evaluated by comparing the pKa of the amphoteric species with those of the corresponding monoacid and monobase.

Herein, the first pKa of the aminoacid bearing n+2 methylene units (dissociation of the carboxylic acid moiety) is to be compared with the pKa of the monocarboxylic acid bearing n+3 carbon atoms (pKac, Figure 23, next page). In addition, the second pKa of aa (dissociation of ammonium moiety) shall be compared to the corresponding monoammonium bearing n+3 carbon atoms (pKam).

The sum of these two pKa ratios enables to assess the stabilizing influence of the carboxylate on the ammonium and of the ammonium on the carboxylate, both through space and through bonds. The resulting constant K1a-22, calculated through eq.1a-22, is a measurement of the enthalpic zwitterionic stabilization with respect to the reference state, corresponding to singly charged ions.

LogK1a-22 = Log K am /K aa 2 + Log K aa 1/K ac = = (Log K am -Log K aa 2) + (Log K aa 1 -Log K ac ) = = (pK aa2 -pK am ) + (pK ac -pK aa1 )= = pKam + pKac eq.1a-22
b. Right side of the disproportionation equilibrium Following the same pathway, it can be assessed that the dicarbamate, and thus the model compound, dicarboxylate (dac), suffers from an enthalpic destabilization which can be evaluated by comparing the pKa of the second ionization process with respect to a monofunctional carboxylate.

This enthalpic factor corresponds to the ratio between the second dissociation constant of dac (K2 dac ) with the pKa of a monofunctional carboxylic acid with an additional carbon atom (ac, K ac , Fig. 24). The pKdac represents a measurement of the dicarbamate enthalpic destabilization.

The same calculation can account for the inferior stability of the diammonium (dam, K1 dam ) compared to a monofunctional ammonium with an additional carbon atom (am, K am ).

The sum of the two ratios delivers K2a-22 (eq.2a-22), the second contribution that will be used to calculate K15-d.

LogK2a-22= Log K dac 2/K ac + Log K am /K dam 1 = = (Log K dac 2 -Log K ac ) + (Log K am -Log K dam 1 ) =
= (pK dac 2 -pK ac ) + (pK am -pK dam 1)= =pk dac + pK dam eq.2a-22

c. Calculation of the different (de)stabilization factors and of K15-d

At this stage, it should be noted that the model shall be valid providing that the monofunctional system (i.e. mixture of singly charged organic ions) is similar, and thus has the similar enthalpy on both sides. This hypothesis seems to be fulfilled as the reference state for the zwitterion (left side of the disproportionation equilibrium) is the mixture of the n+3 C alkylammonium and the n+3 C alkylcarboxylate while the reference state for the doubly charged ions (right side of the disproportionation equilibrium) is the mixture of the n+2 C alkylammonium and the n+4 C alkylcarboxylate.

This calculation enables to apprehend semi-quantitatively the enthalpic (de)stabilization of each molecular entity involved in eq. 15-d, and to determine which species between damm or 2-diam suffers from the main enthalpic destabilization. It may finally indicate to the supramolecular chemist which species has to be stabilized by non-covalent interactions with an additional partner, in order to displace this unfavored equilibrium.

LogK15-d can be calculated with LogK1a-22 and LogK2a-22 (eq.23).

LogK15-d= LogK1a-22 -LogK2a-22 eq.23

We calculated LogK15-d with the tabulated pKa of aa, ac, am, dac, dam model compounds for four -diamines of increasingly longer carbon chain. The values of LogK15-d, LogK1a-22, LogK2a-22, pka dac and pka dam (used to calculate LogK2a-22) are reported in Table 4 (following page).

All LogK15-d values resulted positive, confirming that the disproportionation equilibrium is unfavored for -diamines.

A chain length increase positively influences the stability of the dicarbamates, as LogK15-d decreases along the series (from n=1 to n=4). The lowest value is the one relative to 1.4-diam (n=3) with a LogK15-d=1.72. Interestingly, the evolution of the pKas with n (chain length) suggests that the most destabilizing contribution to eq.15-d comes from the electrostatic repulsion on the diammonium ion. In fact, pKa dac , i.e. the contribution to K15-d due to the electrostatic repulsion on the dicarboxylic acid, is inferior to pKa dam , i.e. the term indicating the destabilization of the diammonium due to electrostatic repulsion, for all the diamines studied.

Amine

The difference is the most remarkable for n=2, which presents a pK dam = -3.45, the strongest in absolute value among the series. The strong destabilization of 1.2-damm can be easily justified by the electrostatic repulsion between the two cationic groups in this short-chain diammonium. The destabilization decreases with increasing chain length, but pK dam is always higher (in absolute value) than pK dac , indicating that the electrostatic repulsion between ammonium cations on 2damm is stronger than the electrostatic repulsion between carbamate anions on 2-diam.

The decomposition of eq.15-d in individual contributions performed in this section represents a simple non-computational method to assess the relative stability of doubly charged species with respect to zwitterionic ones, based on tabulated pKa values.

The same diamines were analyzed by NMR speciation, which enabled the measure of the experimental concentration of each carbamate species upon increasing CO2 loading, and consequently the calculation of LogK15-d from experimental data. These results will be presented in paragraph 1.4 of this Chapter.

The next paragraph will describe the study of the capture properties of EDA (1.2-diam), the first member of the -diamine series (n=1). The species generated upon CO2 capture in a 1 M solution of Ethylenediamine (EDA, 6% wt) in water were characterized and quantified by quantitative 1 H and 13 C NMR. The equilibria involved in the dynamic carbamate/carbonate EDA library are presented in Scheme 19.

The operational procedure of a speciation is detailed in the Experimental Section and briefly described hereafter. A speciation experiment consists in injecting increasing quantities of CO2 in individual samples (1 ml) taken from the same mother solution of amine. The samples are loaded with pure CO2, which is injected at a fixed flow rate (5 -10 ml/min) through a mass-flow controller.

After loading, each sample is analyzed by 13 C (and when the spectra are sufficiently resolved, by 1 H) quantitative NMR in the presence of an internal reference.

The CO2 loading of each sample can be accurately evaluated by gravimetry (Exp) for amine concentrations higher than 1 N. For more diluted samples, a calculation of  or  with the concentrations of CO2-bound adducts obtained by qNMR is advisable. In fact, the internal reference enables the accurate measure of the effective loading regardless of the potential solvent evaporation induced by CO2 injection.

A stack of the 1 H and 13 C NMR spectra (Fig. 25 for EDA, following page) upon increasing normalized loading (N) enables the attribution of the signals to the corresponding species, despite a variation of the chemical shift. The concentration of each species is calculated by integrating the related NMR signals with respect to the internal reference.

In the final speciation graph (Fig. 26 for EDA, page 53), the molar fraction xi (with respect to the initial amine concentration) of each library component is plotted against N. The experimental points are fitted with polynomial functions of degree 3 -5 for the comfort of the eye.

The speciation of EDA (Fig. 26) expectedly reveals that monocarbamate 1-EDA is the most stable species within the library: its concentration increases almost linearly with N and it is the most abundant species (xi=0.83) at full loading (Nmax=0.57). 2-EDA, though present, remains a minority product (xi max = 0.06) and it is absent at full loading. This species is obtained from the disproportionation of two equivalents of 1-EDA (eq. 15-dE, Scheme 19), an unfavored equilibrium for this short-chain amine, as demonstrated in the previous paragraph (LogK15-d E =3.15).

Bicarbonate (HCO3 -) appears in solution around =0.4 and increases rapidly until reaching the final value of xi=0.3 at full loading. From the shapes of the different curves, it may be hypothesized that dicarbamate hydrolysis (eq.14-2E, Scheme 19) is a highly contributive pathway to bicarbonate formation, as 100 % of the dicarbamate reservoir gets hydrolyzed at full loading.

At full loading (max=0.57) 82% of the amine is carbamated. The remaining 18% EDA should be almost entirely protonated to ensure the charge neutrality with HCO3 -(xHCO3-= 0.3 at max) and may interact intermolecularly through salt bridges with bicarbonate.

The generation of bicarbonate at high loading explains the slightly higher experimental loading at saturation with respect to the theoretical max th =0.5 for an exclusively carbamated system. It also experimentally confirms the structure-reactivity relationship established at the previous paragraph: the proximity of the two binding sites (the primary ammoniums) strongly affects the chemo selectivity, favoring 1-EDA and bicarbonate formation instead of diammonium dicarbamate.

b. Rate of absorption

Table 5: IR vibrational frequencies (cm -1 ) of carbamates and carbonates from literature [94][95][96] and those detected and exploited in this work.

The absorption rate of polyamine solutions was evaluated in real-time by FTIR with an immersion probe. This method enabled the detection of carbamate and carbonate formation and the monitoring of the extent of absorption and desorption with time. In a typical absorption rate experiment (procedure detailed in Exp), a stock solution of amine was prepared and transferred into a flask containing a magnetic stirrer, the FTIR probe and the gas inlet. The entire setup was immersed into a thermostated water bath (25 °C for absorption).

Vibration mode

This work (cm -1 ) Literature (cm -1 ) CO3 Pure CO2 was injected at a fixed flow rate through to a mass-flow controller, while FTIR spectra were recorded with a 1 second time interval.

Carbamate and bicarbonate vibrations were assigned by comparison with values reported in literature (Table 5). The N-CO2 -stretching at 1320 cm -1 and the HO-CO2 -stretching at 1360 cm -1 were selected as characteristic vibrations for carbamates and bicarbonate respectively.

The plot of IR intensities upon time of CO2 injection for EDA (Fig. 27) shows that in the selected experimental conditions (EDA 0.5 M, T=25 °C, p= 1 atm, CO2 100%, CO2 flow rate= 200 ml/min for 100 ml of amine solution) the solution saturated after 1 hours and 30 minutes of CO2 injection. Absorption is faster in the beginning of the experiment due to the reaction of CO2 with primary amines, then it slows down after 20 minutes, as bicarbonate is generated while the system approaches saturation. The increase of FTIR intensity at 1360 cm -1 at the beginning of absorption is due to the overlap with adjacent carbamate vibrations, but HCO3 -is absent at the beginning of absorption, as previously assessed by NMR speciation.

To obtain a precise measurement of the absorption rate, indicating the mol of CO2 captured per time unit, the FTIR intensities should be correlated with the corresponding molar fractions (xi) of CO2-bound species, which would consequently enable the calculation of the N corresponding to the points in Fig. 27. Two FTIR-NMR calibration curves enabling absorption rate calculation were obtained for DETA, the polyamine which was most studied during this thesis, but they should be replicated for EDA as the absorption coefficients for carbamate and bicarbonate are expected to vary with the nature of the amine.

c. Cyclic Capacity

CYCLE

identical to those of an absorption rate evaluation, but at the end of absorption, an aliquot of the loaded solution was transferred into an NMR tube containing an internal reference in D2O and it was analyzed by 13 C qNMR to quantify the loading. The remaining solution was heated at 100 °C and stripped with N2 (200 ml/min). FTIR acquisition was maintained during heating to monitor the extent of desorption. The solution was analyzed at each saturation point (either rich or lean state) by 13 C qNMR with an internal reference to obtain an accurate measure of N and to evaluate the extent of evaporation.

The absorption-desorption cycle was repeated five times, the N calculated by qNMR for EDA 0.5 M are plotted in Fig. 28. The solution maintained its capacity during five subsequent cycles, displaying an average CC=0.99 ± 0.05, or a normalized CCN=0.5 ± 0.03. The residual CO2 detected by NMR in the lean state corresponded to monocarbamate 1-EDA, confirming that this is the most stable carbamate of the library.

A chain length increase should, as described in paragraph 1.2., improve the stability of the dicarbamate and diammonium ions and therefore change the thermodynamic profile of the absorption and desorption process. The results obtained from the NMR speciation study of 1.3diam, 1.4-diam and 1.5-diam are presented in the following section. Even if dicarbamate was detected in all the solutions, all the -diamines tested anticooperative in water, as the experimental distribution of 2-diam was always inferior to the statistical distribution (1/3 of the CO2-bound species, black reference line in Fig. 29).

Exploiting NMR speciation to assess monocarbamate isomerization constants

2-EDA is the most unstable diammonium dicarbamate of the series, as it is absent in the library at full loading. The presence of an additional methylene on the backbone influences the stability of the doubly charged species, as 2-1.3-diam is still present at full loading. With four methylene (1.4-diam) spacers there is a 50% increase of dicarbamate concentration at Nmax compared to 1.3-diam. The presence of a fifth methylene bridge has a strong effect at Nmax, where a 33% increase of dicarbamate concentration was obtained for with respect to 1.4-diam.

K15-d can be calculated as in eq.24 with the molar fractions of the species involved in the disproportionation equilibrium (1-diam, 2-diam, damm, eq.15-d, Fig. 21, page 46) measured by qNMR. Diammonium (damm) cannot be distinguished from the amine by NMR, so we assumed that its molar fraction (xdamm) was equal to the one of dicarbamate (x2-diam).

K 15-𝑑 exp = [x 2-diam ] * [x damm ] [x 1-diam ] 2 ⁄ = [x 2-𝑑𝑖𝑎𝑚 ] 2 [x 1-𝑑𝑖𝑎𝑚 ] 2 ⁄ eq.24
The excellent agreement between the values of K15-d calculated with the virtual thermodynamic cycle (pKa) or with qNMR can be appreciated in Fig. 30. NMR-measured K15-d increase upon a chain elongation, in agreement with the previous calculations based on pKa.

(CH2)n+1- length (n=1 -4). G=-RTlnK15-d (R=8.314 J*K -1 mol -1 , T=273 K). 0.00E+00 5.00E-03 1.00E-02 1.50E-02 2.00E-02 2.50E-02 0.00E+00 1.00E+03 2.00E+03 3.00E+03 4.00E+03 5.00E+03 6.00E+03 7.00E+03 8.00E+03 9.00E+03 1 2 3 4 kJ/mol n [ -(CH 2 ) n+1 -] ΔG K15-d NMR K15-d calc
An exception within the series is observed for 1.4-diam, which displays a higher K15-d than the longer diamine 1.5-diam. This stabilization must originate for structural features that could not be appreciated by our semi-quantitative model. The experimental data prove the validity of the virtual thermodynamic cycle based on pKa which we described in section 1.1 of this chapter.

In conclusion, we were able to detect and quantify dicarbamates in all the -diamines + CO2 libraries and to identify a structure-stability relationship between carbamation states upon an increase of the chain length. We did not extend the scope of our study beyond 1.5-diam as 1.6-diam (0.5 M) precipitated upon CO2 loading.

The rest of our study will focus on aliphatic capture agents with at least three amine groups, to further explore the cooperativity of more complex scaffolds.

2. Study of the Diethylenetriamine-CO 2 library

Virtual combinatorial library of carbamate species

A total of 20 species, differing for their protonation and carbamation or carbonation state, can be generated upon CO2 capture with DETA. The species are represented in Fig. 31 in stacks relative to their occupancy state (carbamation/carbonation or protonation). A simplified snapshot of five carbamates, where the protonation state is neglected, and bicarbonate is presented below Fig.

31 with the related nomenclature and a color code which will remain constant throughout this document.

For the DETA-CO2 libraries, monocarbamates will be indicated as 1-DETA and dicarbamates as 2-DETA. The a/s letter indicates if the -(CH2)2groups of the carbamates are symmetric or asymmetric with respect to the position of CO2 on the amine backbone, and consequently if they are magnetically equivalent.

The carbamation pattern of DETA can be evaluated by 13 C NMR as the carbamate species (1a-DETA, 1s-DETA, 2s-DETA, 2a-DETA) differ in the position (regioisomery) and number of carbamate groups on the amine scaffold. As proton exchange is too fast to be observed spectroscopically, the signals in the NMR spectra correspond to an average of the differently protonated species in the library.

The statistical distribution of carbamate microstates for DETA is represented in Fig. 32: based on this scheme, DETA may display cooperative binding if the proportion of dicarbamates was higher than 14/25 (56%) of the carbamate subsystem.

Study of the Diethylenetriamine-CO 2 library in water a. qNMR Speciation

The following paragraph presents the results obtained by 1 H and 13 C qNMR speciation of DETA with CO2 in water (0.5 M, 5.15 % w/w). The DETA speciation published by Hartono et al. was used to confirm the attribution of the NMR signals to the library members, which was completed by bidimensional NMR. 78 The 1 H NMR spectra displayed a strong overlap between signals of different species (stack of 1 H qNMR spectra upon increasing  available in Annexes, Fig. A1 page 216), while 13 C NMR spectra displayed a higher resolution, which consequently enabled a more reliable quantification (stack of 13 C qNMR spectra in Annexes, Fig. A2). The speciation graph in Fig. 33 was obtained from 13 C qNMR integrations with respect to an internal reference. In agreement with literature, the first species formed in the library upon CO2 capture is the asymmetric monocarbamate 1a-DETA, whose concentration increases steadily up to N=0.25 where it represents 40% of the library (Fig. 33). At higher loading, 1a-DETA is gradually consumed, and it is almost absent at full loading ( max = 0.61).

Starting from =0.25, 1a-DETA undergoes a second carbamation either on the primary or secondary amine, which leads to the generation of 2s-DETA or 2a-DETA respectively (Scheme 20). The generation of dicarbamates implies a termolecular pathway, as two basic moieties are required to abstract the protons associated with each carbamate group.

Scheme 20: Scheme of dicarbamate (2s-DETA or 2a-DETA) generation via a second carbamation on 1a-DETA.

1s-DETA appears at  =0.1 and increases steadily up to Nmax, where it represents 66% of the library. Bicarbonate appears in solution around N=0.5 and rapidly increases up to xHCO3-=0.7 at full loading.

The comparison of the experimental distribution of dicarbamates with the statistical one revealed that the DETA capture system in water was anticooperative, as the sum of 2s-DETA and 2a-DETA molar fractions represented 22% of the total carbamate subsystem at full loading. Still, the system was closer to cooperativity than EDA, as both dicarbamates were still present within the library at full loading.

b. Rate of absorption

The stack of FTIR spectra obtained during CO2 absorption with a 0.5 M solution of DETA in water, with an attribution of the most intense carbamate and carbonate vibrations, is presented in Fig. 34. At the beginning of absorption, the first bands to appear in the IR spectrum were the N-CO2 - stretching at 1322 cm -1 and the primary carbamate's symmetric and asymmetric COO -stretching at 1486 cm -1 and 1560 cm -1 respectively, which partially overlap with water's H-OH stretching band. These observations agree with NMR speciation wherein 1a-DETA was the first CO2-bound species to be detected in the library at low loading. Both 2s-DETA and 2a-DETA bear a primary carbamate group so they cannot be distinguished from 1a-DETA within the FTIR spectrum and should contribute to the intensity of the vibration at 1322 cm -1 .

Bicarbonate's characteristic vibrations were observed at 1301 cm -1 (C-OH bending) and 1360 cm -1 (HO-CO2 -stretching).

The CO2 absorption experiment for DETA 0.5 M was conducted with the same operational procedure previously described for EDA (Exp). However, in this case, a FTIR-qNMR calibration was performed in order to correlate the intensities of R2N-CO2 -and HO-CO2 -vibrations with the corresponding xi of carbamates and carbonates (Exp).

To obtain the calibration curves, precise aliquots were withdrawn regularly from the solution during CO2 absorption, and they were analyzed by 1 H and 13 C qNMR with an internal reference. The xi of HCO3 -and of carbamates, calculated by NMR, were correlated with the IR intensities at 1360 cm -1 and 1320 cm -1 respectively. A cumulated molar fraction representing the total carbamate population was calculated with NMR data following eq.25, as the sum of the molar fraction of all carbamate species (xc), each multiplied with the respective number of CO2 bound on the molecular scaffold (nc).

𝑥 𝑁𝐶𝑂2-= ∑ 𝑛 𝑐 𝑥 𝑐 𝐶𝑂2 𝐶𝑂2 𝑐

eq.25 nc= number of CO2 bound on carbamate c xc CO2 = molar fraction of carbamate c

The absorption coefficients (A) were extracted from the slopes of the calibration curves (Exp, Fig. E1, page 200), and they were used to obtain the plot in Fig. 35, showing the increase of carbamate and bicarbonate xi and of  with the time of CO2 injection.

As expected, in the beginning of absorption the main contribution to the loading increase originates from carbamates, whose xi/t slope is two times higher than the one of bicarbonate in the first 15 minutes of absorption. The absorption rate (), expressed in mol CO2 captured/second, can be calculated as the loading difference between two measures separated by a 1 second interval (eq.26-2 for DETA).

= molCO2t1 -molCO2t0 t1= t0 + 1 s [mol CO2/s] eq.26-1 𝛂 𝐍 = mol CO 2 mol amine * nN DETA= (t1*3) -(t0*3)
eq.26-2

The "initial" absorption rate is calculated in the part of the plot where  increases linearly upon time of CO2 injection, the first 15 minutes for DETA (Fig. 35). In our operating conditions (DETA 0.5 M in water, T=25 °C, p=1 atm, pure CO2 200 ml/min) the initial absorption rate of DETA is =0.066 ± 0.004 mol CO2/s. After the initial phase, the slope of the  vs t plot decreased as the system approached thermodynamic equilibrium.

c. Cyclic Capacity

The CC of DETA 0.5 M in water was measured upon five consecutive absorption-desorption cycles (Fig. 36). Stripping at 100 °C for one hour induced the release of 80.5% of the previously absorbed CO2, resulting in an average CC=0.91 ± 0.02. The normalized Cyclic capacity, CCN=0.46 ± 0.01, resulted 9% inferior to EDA 0.5 M.

After desorption, the FTIR and NMR spectra displayed residual carbamate peaks corresponding to 1a-DETA, while bicarbonate was completely stripped.

The results obtained from the study of the DETA-CO2 system in water show that this library encompassed four carbamate species, including two dicarbamates, but it behaved in an anticooperative manner as monocarbamates were the most abundant capture products in the speciation.

The next section will present the study of the DETA-CO2 library in methanol, where cooperative binding was obtained thanks to the inferior dissociative character of this solvent compared to water. The speciation study of DETA in methanol had been previously performed in our group by 1 H and 13 C qNMR (Fig. 37a). 3 In methanol, bicarbonate is usually not generated due to the low water content. The asymmetric dicarbamate, 2a-DETA is also not formed in this solvent. The most abundant species at low loading in methanol is 1a-DETA, but starting from  =0.1 it captures a second CO2 molecule yielding 2s-DETA.

Modeling showed that 2s-DETA formation was assisted by 1s-DETA, which acted as a base. The concomitant increase of 1s-DETA and 2s-DETA within the speciation, starting from  =0.2 (Fig. 37a) suggests that 1s-DETA plays a template role for 2s-DETA. These two carbamate strands associate via supramolecular interaction generating a dimer 1s-2s-DETA (Fig. 37b). The supramolecular dimer is stabilized by three ammonium-carbamate salt bridges, and it is the most concentrated (xi=0.5) species in the library at full loading.

The dimer was characterized experimentally by CRYO ESI-MS, 1 H DOSY NMR and in-silico by density functional theory (DFT) calculations at the B3LYP-D3BJ/def2TZVP level (Optimized structure in Fig. 37c). 3 A control experiment by NMR speciation with 4-methyl-Diethylenetriamine (MDETA, Fig. 14,page 33) indicated that in the absence of the secondary nitrogen, and consequently of 1s-DETA, the xi of dicarbamate (2s-DETA) was reduced compared to the DETA library (x2s-DETA=0.15 at max), thereby confirming the template effect of the secondary monocarbamate (1s-DETA) towards 2s-DETA in methanol. 3 The switch from the low loading state, where 1a-DETA is the most abundant species, to the high loading state, where 1s-2s-DETA is the dominant species (eq.15-D2), represents a cooperative CO2 capture event favored by the stabilizing supramolecular interactions within the dimer. The experimental concentration of the dimer equals 50% of the library at full loading, so it is close to the statistical threshold defining a cooperative system for DETA (56% of the carbamated library, Fig. 32, page 59). We thereby decided to try obtaining the isothermal absorption plot for this capture system, whose shape could eventually confirm the presence of a cooperative CO2 capture mechanism.

b. Isothermal absorption plots

As mentioned in the bibliographic study, an isothermal absorption plot for a CO2 capture system is obtained by correlating the loading increase of a sorbent ( or ) with increasing CO2 partial pressure.

The experiment consists in injecting in solution a series of gas mixtures of CO2 with an inert gas (N2) containing increasing pCO2 and in measuring the corresponding loading once the system has attained thermodynamic equilibrium. This experiment is far from trivial from the operational perspective, due to the following issues:

-CO2 and N2 must be mixed effectively at the appropriate ratio. Our setup (represented in Exp) involved two mass-flow controllers connected to a cylinder of CO2 and N2 upstream, and to a T-valve mixer on the other end. The gases were mixed in different proportions and were injected at a fixed total flow rate of 40 ml/min. -Due to the volatility of methanol, solvent evaporation upon gas stripping can be substantial.

This issue is particularly critical for the lowest pCO2, as the system requires several hours to reach equilibrium. To limit the evaporation, we introduced, prior to the reactor, a prebubbler wherein the gas mixture was saturated with MeOH vapors before being injected into the amine solution. The saturator must be gas tight to avoid leakage or gas demixing. -An appropriate detection method should be applied to measure the loading corresponding to each pCO2 value. In our case, we measured  by gravimetry and by FTIR.

The gravimetric measure consisted in regularly weighing the samples until the weight stopped increasing, indicating the saturation of the system for the selected pCO2. The loading was calculated as in eq.3 (page 7), with the mol of CO2 calculated from the gravimetric weight increase.

For the FTIR measure, the  corresponding to each pCO2 value was extrapolated from the intensity of carbamate IR bands at saturation. We assumed for this experience that the IR intensity of carbamates increased linearly with their concentration (respecting Lambert Beer's law).

Consequently, the intensity at 1320 cm -1 at saturation for a given pCO2 was compared to the intensity obtained upon saturation with pure CO2 (Exp).

Both sets of experimental points (gravimetry, FTIR) could be successfully fitted with a model describing a symmetric sigmoidal plot (Fig. 38), which confirmed the presence of a cooperative CO2 capture process in the DETA-MeOH solution The step in the plots corresponds, from the molecular perspective, to the switch from the monocarbamate (1a-DETA) to the 1s-2s-DETA regime (eq.15-D2, Fig. 37). The step is centered before 0.1 atm of CO2 so it should adapt well to the industrial conditions (pCO2=0.15 atm if the gas is injected at atmospheric pressure).

A discrepancy can be observed between the two plots in Fig. 38 concerning the pCO2 corresponding to the step of the isothermal plot, which is centered around 0.1 mbar for the gravimetric plot and around 0.05 mbar for the FTIR plot. However, these measures should be deemed as qualitative tests, as the accurate detection of these absorption plots would require a finer tuning of the operational parameters listed above.

We did not push further our studies of the DETA-CO2 library in methanol, as we judged that it would have been highly time consuming for a system which is not well adapted to an industrial application.

In fact, methanol is a volatile and toxic organic solvent which would imply a harder liquid management in the PCC plants and would not be sustainable for a large-scale industrial deployment.

Even if the focus of our work is the study of supramolecular interactions between amines and CO2, we also want to maintain the capture systems as "green" as possible. Anyhow, we estimate that the results presented in this section are sufficient to confirm the cooperativity of CO2 capture by DETA in methanol.

This example shows the relevance of supramolecular interactions in CO2 capture systems, as the cooperativity is granted by the stabilizing salt bridges between separate carbamate strands.

Unfortunately, the cooperative effect was absent in water, which represents a better suited solvent for industrial PCC.

The results reported in the Chapter II of this thesis, presenting the development of template species for members of the polyamine-CO2 libraries, started as an attempt to increase the stability of polycarbamated states in order to develop a cooperative homogeneous capture system in water. The accurate measure of the absorption rates, expressed as mol CO2 captured/s, for EDA would require a previous FTIR-NMR calibration, equivalent to the one which was obtained for DETA. As this measure has not yet been completed, the graph in Fig. 39 enables a qualitative comparison of the different absorption rates. The y-axis presents the FTIR intensity of carbamates (1320 cm -1 ) for EDA and DETA and of bicarbonate (1360 cm -1 ) for PMDETA, normalized with respect to the intensity at saturation, plotted against the time since the beginning of absorption (x-axis, CO2 injection at 200 ml/min).

Comparison between EDA, DETA and PMDETA capture properties

DETA presents the fastest initial absorption rate of the three amines as it reaches full saturation upon approximately one hour of absorption. EDA is characterized by a fast absorption rate in the first ten minutes, then it slows down reaching full saturation after 1 hour and 35 minutes of absorption. For meaningful comparison, the absorption experiment with EDA should be repeated with a 1.5 times more concentrated solution than the DETA one, in order to have the same concentration of binding groups in solution. PMDETA absorbs CO2 exclusively through eq.12b (Scheme 3, page 18) by catalyzing bicarbonate formation, which is usually a slower capture pathway compared to carbamation. Expectedly, its absorption rate is inferior to EDA and DETA as it saturates after 2 hours of absorption. CC was measured for the three 0.5 M amine solutions in the same operational conditions. The  calculated by 13 C qNMR upon five consecutive absorption-desorption cycles are reported in Fig. 40.

PMDETA displays the highest capacity amongst the three amines, with a CC=1.89 ± 0.09 and a CCN= 0.63 ± 0.03. The extent of both absorption and desorption (min=0 in the first three cycles) is higher than EDA and DETA solutions. Bicarbonate was completely absent in the lean PMDETA solutions, since its formation enthalpy is 15-20 kJ/mol inferior to ammonium carbamates, and consequently it requires less energy to release CO2 compared to carbamates.

The CC of DETA and EDA are in a close range to one another (CCNEDA=0.50 ± 0.03, CCNDETA=0.46 ± 0.01).

The results reported in this paragraph highlight the positive thermodynamic features of bicarbonate-based CO2 capture systems, which display a higher CC compared to carbamate-based capture systems. However, primary and secondary amines (DETA, EDA) display favorable kinetic features, as they are more reactive towards CO2 than tertiary amines and they reach saturation faster.

The effort of combining fast kinetics and favorable thermodynamics inspired the results reported in Chapter IIIB, where we managed to amplify bicarbonate formation in primary and secondary polyamine solutions while maintaining the fast absorption rate typical of carbamation agents.

Study of the tris(2-aminoethyl)amine-CO 2 library in water by qNMR speciation

Figure 41: Individual component speciation within the TREN + CO2 system (H2O). xi and  calculated from q 13 C NMR integrations.

Our polyamine scope was extended to Tris(2-aminoethyl)amine (TREN, Fig. 14, page 33), a polyamine presenting three primary and one tertiary amine moiety. Some previous works had exploited TREN as CO2 capture agent in combination with superbases or grafted on solid supports. 97,98 We performed the qNMR speciation of TREN (0.5 M) in water. The species that can be generated in the library are presented above Fig. 41 as a simplified library where the protonation state is neglected. Due to the presence of three primary amine groups, TREN can theoretically generate a mono (1-TREN), a di (2-TREN) and a tricarbamate (3-TREN). All species are symmetric, so in the 13 C NMR spectra the signals of the carbamated branches present a different chemical shift than the uncarbamated ones. A high production of bicarbonate was expected in this system due to the presence of the central tertiary amine.

We succeeded in the attribution of the NMR signals in this untrivial system (stack of 13 C qNMR upon increasing  in Annexes, Fig. A3,A4) obtaining the speciation graph in Fig. 41.

In agreement with the previous observations for EDA and DETA, the first species to capture CO2 is 1-TREN. 2-TREN appears starting from =0.17 and increases steadily up to =0.37, where it represents 40 % of the library, but then it starts decreasing until disappearing at full loading. Tricarbamate 3-TREN was detected in solution: it is present all throughout speciation but remains low in concentration compared to the less charged adducts.

Bicarbonate (HCO3 -) appears in solution around =0.4 and increases rapidly until reaching the final value of xi=0.3 at full loading. From the shapes of the different curves, it may be hypothesized that, similarly to EDA, dicarbamate hydrolysis (eq.14-T, Scheme 21) is a contributive pathway to bicarbonate formation, as 100 % of the dicarbamate reservoir gets hydrolyzed at full loading. This mechanism is supported by the regeneration of TREN at high loading (Fig. 41, blue line), whose xi reaches a minimum (xTREN=0.15) at =0.4 and then increases up to xTREN=0.45 at =0.6.

Scheme 21: Hydrolysis of 2-TREN to regenerate TREN and HCO3 -at high loading.

Due to the absence of polycarbamated species at high loading, with the exception of 3-TREN (xi < 0.1), TREN results anticooperative in water.

Study of a tetrabenzenacyclohexadecaphane-CO 2 library in MeOH

Scheme 22: Ideal dicarbamate product 2-Cy obtained upon carbamation of Cy (3,7,11,15-tetraaza-1,5,9,13(1,3)tetrabenzenacyclohexadecaphane).

In addition to the industrial polyamines presented in the previous sections, we chose to explore the CO2 capture chemistry of 3,7,11,15-tetraaza-1,5,9,13(1,3)-tetrabenzenacyclohexadecaphane (Cy, Scheme 22), a tetraazacyclophane which was synthesized following a procedure reported in the literature (Exp). 93 We anticipated that the preorganization of this polyfunctional macrocycle could favor the generation of a dicarbamate, stabilized by intramolecular salt bridges with ammoniums (2-Cy, Scheme 22).

The CO2-absorption experiments were performed in deuterated methanol, as Cy is insoluble in water. The system was studied by mono and bidimensional 1 H and 13 C NMR. The experiments were performed by introducing CO2 at the slowest possible flow rate (10 ml/min) to avoid MeOD evaporation as to enable an accurate gravimetric quantification of loading.

Upon the capture of one equivalent of CO2 (0.944 eq., measured by gravimetry) a white precipitate separated from the solution. Gravimetry was not possible after this stage as the weight of the sample started decreasing due to solvent evaporation. The solid was isolated by centrifugation, washed, dried and analyzed by volumetry and 1 H NMR.

Volumetry is an analytical method which enables to measure the amount of CO2 absorbed in a solid phase (details in Exp). The sample is inserted in a gas-tight system, where it is hydrolyzed with a precise quantity of an acidic solution (H2SO4 or D2SO4 1M). A graduated burette is used to measure the volume of CO2 released from the sample, which can be related to the mol of CO2 via a calibration performed beforehand. The utilization of a D2SO4 solution enables the analysis the sample after hydrolysis.

The volumetric measure confirmed that the white solid was a monocarbamate, as predicted from the gravimetric detection of the  corresponding to the precipitation event.

The solid redissolved upon further injection of CO2, but the viscosity of the solution increased. The clear aspect of the solution at full loading brought us to predict further CO2 absorption after the generation of the insoluble monocarbamate. The fully loaded sample was analyzed by 1 H, 13 C and 2D NMR. The virtual carbamate library based on Cy presents four species: a monocarbamate (1-Cy), a dicarbamate (2-Cy), a tricarbamate (3-Cy) and a tetracarbamate (4-Cy) (Fig. 42). 1-Cy, presents a symmetry plane passing through N-CO2 -, thereby it presents four groups of geometrically equivalent protons (Ha, Hb, Hc, Hd), which should appear as four singlets at different chemical shift in the 1 H NMR spectra.

Since ten signals were observed in the 1 H aliphatic range (between 3.5 -4.6 ppm, Annexes, Fig. A6), we postulated the presence of a second carbamate species other than 1-Cy. This hypothesis was supported by the presence of two -CO2 -signals in the 13 C NMR spectrum in the sp 2 range, at 162.41 and 163 ppm ( 13 C NMR spectrum in the Annexes, Fig. A5).

The attribution of the signals was completed via bidimensional NMR (COSY, HMBC, HSQC, ROESY).

A magnification of the aliphatic 1 Hcarbamate 13 C correlations of the HMBC spectrum is presented in Fig. 43.

The most downfield 13 C carbamate peak (163.61 ppm) correlated with the most downfield aliphatic proton (4.44 ppm): the signals were assigned to the -CH2-closest to the carbamate in 1-Cy ( 1 Ha). The integration of the 1 H signal at 4.44 ppm was equivalent to the integrations at 4.2, 3.92, 3.79 ppm, which were assigned respectively to 1 Hc, 1 Hb and 1 Hd. The less intense 13 C carbamate peak at 163.35 ppm (N-CO2 -( 2), Fig. 43) showed a double correlation at 4.34 and 4.3 ppm, which brought to identify these two 1 H peaks as a doublet. Another doublet was present at 4.04 ppm. Both spanned over 0.1 ppm and presented a J coupling of 4.33 and 4.02 for the most downfield and upfield respectively.

The attribution was completed thanks to ROESY (Rotating frame Overhauser Effect Spectroscopy) NMR, an experiment which shows 1 H-1 H correlations based on spatial proximity. The spectrum (Fig. 45, next page) showed that all the signals in the aliphatic range correlated with one another:

1 Ha (4.44 ppm) correlated with the doublet at 4.36 -4.32 ppm, while 1 Hc (4.21 ppm) correlated with the doublet at 4.05-3.99 ppm.

These correlations proved that the species were, in fact, two identical monocarbamates (1-Cy and 1-Cy', Fig. 44), differing because of geometric reasons. The most abundant carbamate, 1-Cy, binds CO2 on a nitrogen and gets protonated on the opposed one. The ammonium behaves in a classical way, exchanging at a fast rate with the solvent so their protons are not detected.

The second monocarbamate (1-Cy') presents the ammonium on one of the two closest positions to CO2. Due to their proximity, the R-NH2 + and R-CO2 -groups interact by H-bond. The H-Bond imparts a higher rigidity to the monocarbamate structure, and the exchange between ammonium protons and the solvent is slower, so they can be detected on the 1 H NMR time scale: consequently, the two doublets at 4.0 and 4.3 ppm belong to R-NH2 + protons of 1-Cy.

Unfortunately, the stabilizing interactions between N-CO2 -and NH2 + prevent further carbamation, so polycarbamates were not generated in solution. Despite the structural preorganization of Cy, which could have favored dicarbamate formation on paper, the system resulted anti cooperative as monocarbamates were the only species generated upon CO2 capture.

However, the detection of the stabilizing H-Bond between adjacent carbamate and ammonium groups suggests that the preorganization could indeed increase the stability of carbamates species.

A more advanced molecular design is required to increase the probability of obtaining a dicarbamate. Ongoing synthetical studies are focusing on the development of a water-soluble homologue of Cy, to avoid 1-Cy precipitation and favor the binding of a second equivalent of CO2.

III. Conclusion to Chapter II

In Chapter II we presented the screening of a series of commercial and synthetic polyamines as CO2 capture agents. The members of the dynamic polyamine-CO2 libraries were characterized qualitatively and quantitatively by NMR speciation for each system.

The study of the isomerization equilibrium for -diamines enabled the identification of a direct relation between the length of the carbon chain and the extent of dicarbamate formed in solution.

It also highlighted that the main destabilizing contribution to the isomerization of mono into dicarbamates comes from the generation of the diammonium cation.

The capture properties of DETA were studied in water and in methanol. The speciation in water showed that the most abundant species at high loading were the secondary monocarbamate (1s-DETA) and bicarbonate. Despite the detection of two dicarbamate species in solution (2s-DETA and 2a-DETA), the system resulted anticooperative, as the experimental distribution was inferior to the statistical prediction. The absorption rate and Cyclic Capacity were measured by FTIR-qNMR combined procedures.

The cooperativity of CO2 capture displayed by DETA in methanol, which had been previously predicted in our group based on NMR speciation, was confirmed by obtaining two isothermal absorption plots, which presented the characteristic sigmoidal shape of a cooperative binding.

The speciation of TREN showed that the monocarbamate was the most stable member of the library. A tricarbamate was detected in solution, even if its concentration remained significantly lower than the other species.

The comparison between EDA, DETA and PMDETA in water highlighted the thermodynamic advantages coming from the adoption of carbonation agents (tertiary amines) and the kinetic advantages coming from the adoption of carbamation agents (primary and secondary amines).

A tetraazacyclophane was synthetized and characterized for its CO2 capture properties in MeOH. Despite the structural preorganization and proximity of the four binding sites, the 2D NMR experiments showed that monocarbamates were the only species generated in solution.

Overall, the results reported in this Chapter provide a deep understanding of the concentration of carbamate and bicarbonate species in polyamine solutions for CO2 capture. These data guided us towards the second part of this study, where we managed to influence the distribution of the members in the dynamic polyamine-CO2 libraries by the introduction of templates.

Chapter III: Tuning the covalent chemistry of CO 2 capture by supramolecular effects

Objectives

In Chapter III we will describe the design and application of template species for the modulation of the CO2 capture equilibria with polyamines. The objective of our study was the development of additives able to improve the CO2 capture properties of polyamine solutions through supramolecular interactions.

One of our interests was to develop a template species able to stabilize multiply charged states, in order to obtain a homogeneous cooperative capture system. We were also interested in amplifying bicarbonate formation in primary and secondary polyamine solutions, as this is the most thermodynamically efficient and naturally followed CO2 capture mode.

Achievement

Part A will present a bibliographic study of anion recognition by supramolecular binders, that inspired the design of our template species. A focus will be put on guanidines/guanidiniums, the most common H-bond donor groups applied to oxyanion recognition. The application of basic guanidines or amidines to CO2 capture, as CO2 BOLs or iminoguanidines, will be presented. A brief bibliographic section will list some examples of anion recognition with Cobalt(III) complexes, relevant to the third part of this chapter.

Part B will present the results that we obtained by exploiting bisguanidinium salts as bicarbonate templates. Polyguanidinium sulfate, nitrate and chloride salts were synthetized and characterized as additives to Ethylenediamine (EDA), Diethylenetriamine (DETA) and Tris(2-aminoethyl)amine (TREN) aqueous solutions for CO2 capture. The influence of the additives was assessed by NMR speciation and calculation of the Amplification Factors (AFi) for each component of the library. The Cyclic Capacity and absorption rate, measured by FTIR coupled with qNMR, showed that the templates induce an increase of the CC while maintaining the fast absorption kinetics characteristic of carbamation with primary/secondary amines.

Part C will present the study of [Co(DETA)2]X3 complexes as polycarbamate binders, as a follow-up of the previous data presented by our group about the template effect of this coordination compound for bicarbamate in the DETA-CO2 system. The supramolecular binding properties of the synthesized complexes were studied by UV-Vis and NMR titrations with dicarboxylic acids, exploited as static models for bicarbamates. The influence of carbamate association to the complex on the oxidation of Co(II) to Co(III) was studied by spectroelectrochemistry, which showed an acceleration of the oxidation rate thanks to the increased affinity of the "hard", oxidized complex for oxygenated anionic ligands.

I. Part A: Bibliographic study 1. Anion recognition by supramolecular chemistry

Anion receptor chemistry started to gain interest in the 1970s, following the work of the groups of Lehn and Schmidtchen. [99][100][101] The study of anion recognition processes by supramolecular interactions saw a slower development compared to cation ones. 102 This depended on the weaker nature of these supramolecular binding processes with respect to the cationic ones, and the consequently less trivial measurement of the binding constants.

Anion binders with an open or closed structure are defined "receptors" in the literature, even if this term, in supramolecular chemistry, is usually applied to the macrocyclic hosts. In natural and synthetic receptors, anion binding is typically mediated via electrostatic interactions such as Hbonds, neutral or charge-assisted (salt bridges).

A good anion receptor should present a positive charge, H-bond donor groups and a geometry adapted to the target anion. 103 Guanidines, amidines and (thio)ureas in their neutral or cationic state are the most common H-bond donor (HBD) moieties applied to anion recognition (Fig. 46). The strength of the binding between a host (receptor) and a guest (target, anion) is measured with the binding constant K, calculated as in eq.27 with the molar concentration of the host [H], the guest [G] and the formed complex [HaGb] at equilibrium, with the respective stoichiometric factors (a,b). The a/b ratio (N) indicates the binding stoichiometry. For a 1:1 host-guest binding, K will be expressed in l/mol, simplified to M -1 .

𝑎𝐻 + 𝑏𝐺 ⇄ 𝑎𝐻𝑏𝐺 𝐾 = [𝑎𝐻𝑏𝐺] [𝑎𝐻][𝑏𝐺]

𝒆𝒒. 𝟐𝟕 Some examples of supramolecular receptors based on electrostatic interactions and H-bonds will be presented in the following sections. The enthalpic and entropic contributions to anion recognition processes will be discussed in Section 1.3.

Anion binding by electrostatic interactions

Figure 47: Anion receptors synthetized by Lehn et al. 99 Counterion (SO4 2-) omitted for clarity.

Target

Receptor LogK H2PO7 2) MeOH/water 9/1 at T=25 °C by potentiometric titration. 99 Dietrich, Lehn et al. 99 were amongst the first to tackle the challenge of anion recognition in the 1970s, when they introduced the application of polyammonium and polyguanidinium cations as anion receptors. Their study was based on the potentiometric determination of binding constants (LogK) between polycationic hosts (Fig. 47) and polyoxygenated anions, such as phosphates and carboxylates, in water.

The screening of more than twenty receptors led the authors to draw some general conclusions regarding the relevant factors for anion recognition processes.

Receptors 1 -11 in Fig. 47 formed relatively stable complexes with phosphate in water (log Ks in the 2.0-4.0 M -1 range for PO4 3-in water) and with carboxylates in a moderately dissociative medium (methanol/water 9:1, log Ks= 1.0-5.0 M -1 ).

The strength and selectivity of recognition mainly depend on electrostatic factors: the anions and receptors with the highest charge density yielded the most stable complexes. For example, the association between ligand 4a 2+ and PO4 3-was two times stronger (K=2.4 M -1 ) than with HPO4 2- (K=1.2 M -1 ).

Polyammoniums generally displayed one order of magnitude higher K than polyguanidiniums (Table 6) due to the absence of charge delocalization (4H + vs 1H + in Fig. 46, page 80) and the consequent higher density of charge. However, the binding properties of ammoniums strongly depend on pH, as they can be easily deprotonated in water (R=H in 4H + , Fig. 46 These receptors present a higher degree of structural preorganization with respect to the open ones discussed previously. Spectroscopical data showed that anions could be hosted in the cavity of the receptors, with a selectivity dependent on their dimensions. Host 12 bound selectively to halides, while host 13, which presents a wider cavity radius, bound selectively to carboxylate anions. 105 This example enables the introduction of a fundamental issue for anion receptor design, i.e. the presence of competing species. In fact, a disadvantage of charged cationic receptors is that they must be accompanied by a counterion, which will compete with the target anion for the binding to the cationic sites.

To avoid this issue, Schmidtchen et al. developed the zwitterionic receptor 14: as a consequence of the absence of competing anions, 14 displayed an improved affinity for halides in water with respect to the cationic hosts 12 and 13 (one order of magnitude increase of LogK, Fig. 48b). 104 This study shows the importance of rational design for anion receptors, which should not only focus on the complementarity with the target anion, but also on the presence of competing species and on the influence of the solvent and of the pH.

Anion binding by Hydrogen bonds

Hydrogen bonds are highly directional interactions which are often exploited for the design of supramolecular receptors with a well-defined shape. Depending on the pKa of the H-bond donor group 106 and of the solvent, the receptors can be neutral or positively charged. In general, neutral H-bonds operate only in organic solvents (CHCl3, MeCN, DMSO) while in water most of the HBD groups will be protonated at pH < 13 (Table 7).

In the following paragraphs, the H-bond donor groups will be indicated as "HBD", while the H-bond acceptor groups (anions) will be indicated as "HBA" 106 .

a. Anion binding by neutral H-bond donors

An example of neutral HBD is the urea-based receptor 15 (Fig. 49). Urea and thiourea moieties present a Y-geometry adapted to carboxylate anion recognition.

15 was applied to carboxylate binding in DMSO: the binding constants (2.17 < LogK < 3.39) increased upon an increase of the anion's basicity (pKb), in agreement with the generation of increasingly stronger hydrogen bonds (Fig. 49). 107 The guanidinium cation (1H + , Fig. 46, page 80) is the most exploited functional group for anion receptor syntheses, due to the following properties:

• It is protonated over most of the pH range available in water (12 < pKa < 13.5).

• It is a H-bond donor and can interact with anions through salt bridges.

• It possesses a defined planar geometry due to the delocalization of the positive charge. Guanidiniums are highly selective for the recognition of carboxylate anions that present a complementary bidentate planar geometry (Fig. 50). 108 Nature employs guanidines and amidines as oxyanion receptors in a range of applications. Polycationic arginine rich polypeptides and proteins play a major role in chromatin and chromosome structure and in DNA nucleation. 108 Most natural anion recognition processes take place in the hydrophobic core of proteins, where electrostatic interactions are stronger than in water: taking inspiration from nature, supramolecular chemists have exploited guanidiniums to perform anion recognition in apolar solvents.

The incorporation of the guanidinium group in a cyclic structure can increase the solubility of the receptor in organic solvents and can also limit its rotational freedom, resulting in increased binding constants compared to more flexible receptors.

Cyclic guanidinium receptor 16 (Fig. 51) was applied to the recognition of carboxylate anions in chloroform (MeCl), where it displayed high binding constants, up to 7*10 6 M -1 for the 1:1 association with p-nitrobenzoate. 108 The structurally similar receptor 17 gave a 10 3 smaller K (K=1.6*10 3 M -1 ) due to the competitive character of the chloride counterion, which has a higher charge density and is more coordinating than tetraphenylborate (BPh4 -). 108 To bind to the receptor, the target anion must compete with the solvent. Water and alcohols are the strongest competing solvents to the binding due to their polar and coordinating character, which make them interact strongly with the cationic moieties of the receptors. To achieve anion complexation in water, one option is the multiplication of the binding sites on the receptor. This solution was adopted by Lehn and Dietrich with polyfunctional ammonium and guanidinium receptors, and by Schmidtchen with polyammonium macrocyclic cages. 99 Anslyn et al. also exploited this approach and developed the triguanidinium receptor 18 (Fig. 52), which bound citrate in water through H-bonds with a relatively high constant (K=6.9*10 3 M -1 ). 109 The receptor displayed a high selectivity for citrate, as it bound to this anion, with a slightly reduced binding constant (K= 4.3*10 -3 M -1 ) in crude orange juice, which contains other carboxylate anions (malate, ascorbate, succinate).

Many examples in the literature prove the high affinity of guanidiniums for oxyanions. The multiplication of HBD sites on the receptors increases the strength of the associations with oxyanions in strongly dissociative conditions, such as in water. These observations inspired our design of polyguanidiniums salts as templates for bicarbonate, which will be discussed in Part B of this chapter. The design of a selective receptor should take into account the properties of the target anion. The Hofmeister series (Fig. 53) was initially developed to list the effect of inorganic salts on the solubility of proteins, but it also represents an useful tool to evaluate the hydrophobicity, and therefore the degree of solvation, of anions. 110 The series can not only help with the design of a selective receptor (hydrophilic or hydrophobic) for the selected anions, but it can also indicate the most adapted (less competitive) counterions to couple with cationic receptors.

Thermodynamics of anion binding a. Anion influence

Hunter et al. provided another classification of anions based on their HBA properties. 106,111 The equilibrium constants of HBD-HBA associations can be obtained with eq.28, where G0 represents the free energy of formation of a 1:1 receptor-anion complex through a single hydrogen bond.

G0=-RT lnK = -(-s)(-s)+c [kJ/mol] eq.28

 : HBD parameter of donor species : HBA parameter of the acceptor species s : HBD parameter of the solvent s : HBA parameter of the solvent c: adverse free energy due to the formation of a host-guest complex in solution (6 kJ/mol) UV-Vis titrations were used to study the complexation of anions (halides, sulfonates, acetate, benzoate, phosphate diester, perrhenate, nitrate, and perchlorate) with neutral receptors in organic solvents (191, 192, 193, Scheme 23). The experimental data were exploited to calculate the HBA parameters () for the anions listed in Scheme 23.

Carboxylates showed the strongest HBA character of the series (BzO -and AcO -, =15), in agreement with the many examples in literature of strong H-bonded complexes with these anions. The weakest HBA of the series was PF6 -(=7), making it a well-suited counterion for cationic receptors.

Scheme 23: HBD Receptors (191, 192, 193) and HBA anions selected by Hunter et al. for the calculation of  and  parameters. 106 (TBA=tetrabutylammonium, TBMA= tributylmethylammonium). The binding constant K is of paramount importance to assess the strength of an anion-receptor interaction. However, the detailed analysis of the enthalpic and entropic contributions to a binding event is relevant to understand what the driving force of the complexation is.

b. Solvent influence

The thermodynamic analysis of a supramolecular binding event can be approached through a virtual thermodynamic cycle, by decomposing the complexation event (I, Fig. 54) in multiple steps which are analyzed individually. The complexation (host-guest, target-receptor) can be divided in the following steps:

II. Desolvation of the isolated host and guest. The cleavage of the solvation shell around the host and the guest requires energy, so this step in enthalpically unfavored (Hdesolv) but entropically favored due to the liberation of solvent molecules.

III.

Host-guest association in the gas phase. This step results in a complex with decreased rotational and translational degrees of freedom with respect to the isolated species. It is thus entropically unfavored (Sass), though enthalpically favored (Hass) due to the generation of stabilizing host-guest interactions.

IV. Solvation of the formed complex, unfavored entropically (Ssolv) due to the reorganization of some solvent molecules around the complex but favored enthalpically (Hsolv) thanks to the generation of solvent-complex stabilizing interactions.

The enthalpy and entropy of binding (I, Fig. 44) can be obtained as a sum of the individual contributions (eq.29a, eq.29b):

H= Hdesolv +Hass +Hsolv eq.29a

-TS = -TSdesolv -TSass -TSsolv eq.29b

Solvating effects are of upmost importance for anion binding, as the final complex is solvated by an inferior number of solvent molecules than the ones that were involved in the cumulated solvation shells of the separate species (Fig. 54).

The contribution of the solvent to the thermodynamics of the binding process strongly depends on its polarity and on the strength of its interactions with the charged species, as demonstrated by the examples presented in the following section.

c. Energetic contributions to anion binding

To date, there are few thermodynamic studies of anion recognition available. The energetic study of these processes is complicated by the weak nature of the interactions, especially in polar solvents such as alcohols or water. The investigation method must quantify at the same time the strength of the association and the contribution of solvation and desolvation.

Isothermal Titration Calorimetry (ITC) is a powerful tool for the thermodynamic analysis of hostguest associations. ITC delivers the reaction enthalpy (H) as a primary parameter to evaluate the endo or exothermicity of the process. The fitting of the titration curve enables to extrapolate G (and thus the binding constant K) and the stoichiometric ratio (N) of the association. S can then be calculated with the Gibbs-Helmholtz equation.

The first measurements of the enthalpic and entropic contributions to anion recognition processes were performed by Berger and Schmidtchen by ITC. 101 Bisguanidinium receptors (20, Fig. 55) were studied as receptors for sulfate (SO4 2-).

Figure 55: Bisguanidinium cationic host for sulfate (SO4 2-) studied by Schmidtchen et al. 101 In MeCl, the binding process resulted endothermic (H= +7.7 kcal/mol), which indicated that the energy required to break the solvent-receptor and solvent-anion interactions (Hdesolv, Fig. 54, page 88) was higher than the energy liberated from the association (Hass, Fig. 54, page 88). However, the treatment of the ITC data indicated high binding constants in the 10 5 -10 6 range for the 1:1 stoichiometric association of 20 with SO4 2-. In the selected experimental conditions (T=28 °C), the binding was in fact entropy driven, since the TS term outweighed H. 101

This study highlights the relevance of solvating effects for anion-receptor binding, as the entropic gain from solvent liberation can be sufficient to compensate unfavorable enthalpic contributions and to induce the complexation.

Hamilton et al. provided a more thorough analysis of the role of the solvent by the NMR and ITC study of dicarboxylate binding by synthetical receptors. 112 Receptors with increasingly acidic HBD groups (urea 21, thiourea 22, guanidinium 23, 24, Fig. 56) were exploited to bind dicarboxylate anions (25, 26 Fig.56) in DMSO, MeOH and water. displayed negative enthalpies and near-zero entropies, while the latter presented positive enthalpies and positive entropies (Table 8).

These observations, corroborated by subsequent studies 113 , proved that in organic solvents anion recognition processes are driven by the strength of the host-guest association, and thus by the reaction enthalpy (H, I, Fig. 54, page 88). In organic solvents, solvation and desolvation effects represent a small contribution to the binding, as the solvation spheres of receptors and anions are smaller than in water and the interactions of the substrates with the solvent molecules are weak.

On the contrary, in polar solvents (water, alcohols), the solvation spheres are extended and the interactions of the substrates with the solvent are strong, so the energy required to break these interactions (Hdesolv, Fig. 54, page 88) is higher than the energetic gain from the host-guest complexation. The entropic gain from solvent liberation (S, Fig. 54, page 88) drives the binding processes in polar solvents, as the solvent molecules switch from a highly ordered state (coordination spheres of anion, receptor), where they are strongly coordinated, to a more disordered state.

2. Strong bases for CO 2 capture by carbonation 2.1. Superbases Amidines and guanidines are labelled "superbases" (SB) due to their stronger basicity compared to alkylamines (pKa in Table 9 for SB compounds in Fig. 57).

The nature of the reaction of superbases with CO2 has been extensively discussed in the scientific community. Amidines and guanidines are generally non nucleophilic moieties and would thus be expected to react with CO2 only in the presence of water, similarly to tertiary amines (eq.12b, Scheme 3, page 18). The formation of zwitterionic carbamates (1-SB, Fig. 57) in anhydrous conditions has been postulated on the basis that these species could profit from the delocalization of the positive charge on the guanidinium/amidinium cations. 51 The first evidence accounting for the generation of 1-SB was obtained in 2011 with the NMR and XRD study of the solid material obtained upon CO2 capture with TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) in THF (Fig. 58, next page). 115 The product was obtained at low temperature, under CO2 atmosphere and in strictly anhydrous conditions, to avoid the competitive reaction of SB with water (Scheme 24), which would have led to bicarbonate generation.

Even if 1-SB were characterized in the solid state, they were never detected in the liquid or neat (solventless) state. The solid observed by Perez et al. was, indeed, a bicarbonate salt (HCO3-DBU) generated because of water traces in the system. The reaction of DBU with CO2 is in fact based on the Bronsted acidbase equilibrium eq.31b (Scheme 25), where DBU abstracts a proton from water and generates a OH -, that binds CO2 as a bicarbonate, and results in the solid amidinium-bicarbonate salt

Base

[DBUH + ][HCO3 -].

Superbase + Nucleophile blends

In anhydrous conditions, superbases can deprotonate weaker bases such as alcohols and amines, thereby increasing their nucleophilic character and their reactivity with CO2.

SB can activate the nucleophilic attack of amines on CO2, resulting in the generation of an amidinium/guanidinium carbamate salt (Scheme 26). 118

Scheme 26: Reaction of amine and SB (guanidine) blend with CO2.

Heterocyclic amines such as imidazoles or pyrrolidones can also be activated towards CO2 binding in the presence of a superbase (Scheme 27). The capture products are molten salts defined Protic Ionic Liquids (PIL), due to the presence of an acidic proton on the cationic group (guanidinium in Scheme 27). 119

Scheme 27: Reaction of imidazole and SB (MTBD) blend with CO2. Generation of a PIL. Apart for the application as SPS, the SB-alcohol blends are also efficient absorption agents for CO2. SPS specifically designed for CO2 capture, named CO2 BOLs (CO2-Binding Organic Liquids), were developed by Heldebrant et al. 65,[121][122][123] These sorbents are characterized by a higher capture capacity, a lower heat capacity and comparable enthalpies of absorption compared to alkanolamines in water. CO2 BOLs operate in anhydrous conditions, capturing CO2 through the reactions depicted in Fig. 59, by generating liquid alkylcarbonate -protonated base salts.

CO 2 -Binding Organic Liquids

CO2 BOLs display a higher gravimetric capacity than alkanolamines: a 1:1 mixture of DBU with 1hexanol can capture 1 mol of CO2 per DBU (15 wt%) plus a 4 % wt. by physical adsorption, resulting in a total capacity of 19% wt (147 g CO2/l), 2.7 times higher than 30% w/w aqueous MEA (108 g CO2/L, 7% by weight). 65 One of the strongest advantages of these sorbent is their inferior energetic requirement for CO2 desorption compared to aqueous alkanolamine sorbents. Some of these blends can also desorb CO2 at room temperature, even if heating is required to achieve a fast desorption rate 120 . The easier desorption depends on the inferior strength of the H-bonds in the 1C-SBH + salts compared to the Hbonds of ammonium carbamate and ammonium bicarbonate salts obtained in water.

In addition, the specific heat of CO2 BOLs is 50% lower than water. These favorable thermodynamic features imply that 60% less energy is required to release CO2 from CO2 BOLs at the same temperatures exploited with aqueous MEA (90-120 °C). 122 From the thermodynamic perspective, the absorption enthalpy strictly depends on the choice of the SB. 122 For example, the absorption enthalpy for the 1-hexanol/TMG blend (H=-160 kJ/mol) is more exothermic by 20 kJ/mol than for DBU/1-hexanol (H=-140 kJ/mol), because TMGH + can donate two H-bonds to the carbonate anion while DBUH + can only provide one (Fig. 59). However, the negative entropy counterbalances the reaction enthalpy of the TMG/1-hexanol mixture (S=-530 kJ/mol*K), due to the high organization of the H-bonded capture product (1-TMGH + , Fig. 59), resulting in a positive G (1.6 kJ/mol) at 25 °C. On the other hand, the reaction is spontaneous for the DBU/1-hexanol mixture (G=-9.4 kJ/mol at 25 °C) due to inferior value of S (S=-440 kJ/mol) compared to TMG.

One of the disadvantages of these CO2-capture agents is the volatility of the organic SB and alcohols, which limits their industrial applicability. 38 To avoid this issue, one option is the utilization of alkanolamidines or alkanolguanidines, which present both reactive sites (acid and base) on the same backbone (Scheme 28). 121

Scheme 28: Reaction of one-component CO2 BOLs with CO2.

These second-generation CO2 BOLs react with CO2 through nucleophilic attack and intramolecular proton abstraction, resulting in zwitterionic capture products. Alkanolamidine and alkanolguanidines display an inferior volatility but an increased viscosity compared to the twocomponent systems. The exponential increase in viscosity of some of these capture agents is detrimental to their capture capacity, as it reduces the mass transfer from the gas to the liquid phase. 38,121 In addition to their volatility, another limitation of CO2BOLs is that they must operate in strictly anhydrous conditions, as water traces could interfere with the binding mechanism by generating bicarbonate (Scheme 24, page 93), that could precipitate as a salt with the protonated superbase. Arguably, even if bicarbonates were generated, they could be cleaved by heating at lower temperatures compared to carbamate desorption (120 °C for MEA 30 w/w), and the process would still gain in efficiency compared to classical sorbents, due to the lower specific heat of CO2 BOLs with respect to water. 65 However, even if bicarbonate was cleaved, the CO2 BOL would undergo progressive dilution upon consecutive absorption-desorption cycles, which could hamper their recyclability.

Iminoguanidines

Iminoguanidines are basic compounds that were recently exploited as anion receptors and as CO2 capture agents. The extended conjugation of the guanidine groups with the imine moieties was exploited to impart rigidity to the structures and thereby to increase the strength of the H-bonds with oxyanions. 124 Due to their planar geometry, these receptors tend to generate highly ordered crystalline structures upon neutralization with oxyanions, and they often precipitate from aqueous solutions enabling the isolation of the anion-receptor complexes. 125 Glyoxal-bis(iminoguanidine) (GBIG, Fig. 60a) was exploited as CO2 capture agent. GBIG (pKa=12 in water) captured CO2 in water by generating OH -(eq.12b, Scheme 3, page 18), as previously seen with tertiary amines and SB. The resulting bisguanidinium bisbicarbonate salt was isolated as a hydrated crystalline species (GBIG(HCO3)2 , Fig. 60b), with 89% crystallization yield. 126 XRD analysis showed the formation of centrosymmetric bicarbonate dimers with 1.71 Å O-H …-O H-bonds (Fig. 60c) in the capture product. Each bicarbonate was stabilized by three additional H-bonds with guanidiniums and two with water, resulting in a highly ordered bidimensional crystalline layer.

The low solubility of the GBIG(HCO3)2 salt enabled the separation of the product from the aqueous solution and the application of the desorption process in the absence of water, resulting in the annulation of the sensible heat contribution to Qreg. The enthalpy of CO2 capture with GBIG is -61 kJ/mol, which falls in the lower part of the Habs range (-60/-80 kJ/mol) for alkanolamines in water. 124 However, additional energy is required to break the stable crystalline lattice in order to release CO2. Complete desorption and regeneration of GBIG required heating of the isolated solid at 120 °C, i.e. the same desorption temperature commonly applied to MEA 30% w/w, proving that the benefit from the annulation of sensible heat was compensated by the energy penalty for crystalline lattice disruption. Iminoguanidine-based sorbents were exploited to perform CO2 capture from air, an approach defined Direct Air Capture (DAC). As CO2 is diluted in the atmosphere (≈ 420 ppm in 2021), in order to perform DAC, the receptor should display a high affinity for CO2, i.e. a logKCO2 ≥ 3.7 at 25 °C, corresponding to a G ≤ 21 kJ/mol. 124 The conjugation of two iminoguanidine groups with a pyridine ring delivered PyBIG, a capture agent of increased acidity compared to GBIG, which performed DAC by precipitating from water as a salt with carbonate (CO3 2-, Fig. 61). 127 The low solubility of PyBIGH2-CO3 (Kps=1.0*10 -9 ) acted as a driving force towards the generation of the capture product, despite the low concentration of the target (CO2).

Unfortunately, the strength of the association implies a higher Habs and thus a higher energetic requirement for desorption: the enthalpy of CO2 release for PyBIGH2-CO3 is 288 kJ/mol, almost two times higher than GBIG (152 kJ/mol). 128

The DAC system based on PyBIG was hampered by a slow absorption rate. To overcome this issue, PyBIG was coupled with aminoacid promoters, such as glycine (Gly). 124 The combined process (Fig. 62) starts with CO2 capture by the aminoacid (1-32). This last example is relevant to the results that will be presented in the Results and discussion section of this chapter, as the combination of a carbamation and a carbonation agent delivered a capture agent of increased termodynamic and kinetic performances. 129 Still, a significant difference between these last examples and our work should be highlighted.

Iminoguanidines were exploited for their basic character, which led them to generate (bi)carbonate, which eventually precipitated with the protonated base. The innovative approach which we will discuss in Part B of this chapter is based on supramolecular interactions between preformed guanidinium salts and CO2-bound species. The results will show that guanidiniums interact with bicarbonate and stabilize it through salt bridging, inducing an amplification of this species with respect to carbamates in polyamine solutions. The mediation of bicarbonate amplification via supramolecular interactions does not involve a pH change in solution, and it affords a system with improved capacity but unaltered absorption rate and desorption properties. At the same time, the polyaminebisguanidinium salts combinations are soluble in water, so they present an inferior volatility and viscosity compared to CO2 BOLs.

Diethylenetriamine-based Ionic Liquids for CO 2 capture

Recent results showed that the partial protonation of Diethylenetriamine could improve its thermal resistance during CO2 desorption and its normalized capture capacity ().

DETAHNO3 is as a viscous/semi-solid material that was obtained by the reaction of DETA with HNO3, which was presented as an easily accessible Ionic Liquid for CO2 capture by Doyle et al. 130 The addition of PEG200 afforded a homogeneous system which was used as CO2 capture agent. DETAHNO3 could be exploited to perform DAC, as upon exposure to ambient air, it crystallized as a monocarbamate 1-DETAHNO3 (Scheme 29), whose structure was confirmed by XRD analysis by Ilioudis et al. 131 The capacity (max) of DETAHNO3 is inferior to DETA, as one binding site is sequestered by protonation. The main advantage of this capture agent is the two orders of magnitude decrease of the vapor pressure of DETAHNO3 (42.16 Pa at 120 °C) with respect to DETA (5006 Pa at 120 °C).

Scheme 29: DETAHNO3 reaction with CO2 in anhydrous conditions with the generation of the zwitterionic carbamate 1-

DETAHNO3.

A similar study was conducted by Chen et al. 132 with a IL obtained by the reaction of DETA with HBr (DETAHBr), with PEG200 as a cosolvent. Upon CO2 injection in anhydrous conditions, the IL captured CO2 as a zwitterionic monocarbamate (1-DETAHBr, Scheme 30) and it reached a max=0.523. In the presence of water traces (4.7% w/w) the system showed a 13% increased capacity, reaching max =0.592. The authors did not propose an explanation for this event, but it arguably depends on the generation of bicarbonate (eq.33-3, Scheme 30), catalyzed by the basic character of the amine moieties.

Scheme 30: DETAHBr reaction with CO2 with generation of the zwitterionic carbamate 1-DETAHBr in anhydrous conditions and of a bicarbonate salt DETAHBr-HCO3 in the presence of water.

These examples suggest that the stability and normalized capture capacity of DETA increase upon protonation of one or two amine groups. These studies are relevant to the results that will be presented in Part B of this chapter, where we obtained a capacity increase by the addition of bisguanidinium salts to polyamines in water.

with a 2:1 anion:receptor stoichiometry. 135 XRD showed the presence of an extended H-bond network for the disubstituted complex [Co(NH3)6]Cl(C9H11SO3)2 (Scheme 31).

This example represents a proof of concept of the possibility of performing anion binding with Werner complexes. However, the affinity and selectivity of the anion-receptor interaction cannot be assessed by this type of study, as the generation of the solid complexes could be driven by the Kps of the substituted products more than by their stability.

3.2.

[Co(DETA) 2 ]X 3 as anion binder a. Synthesis and stereochemistry Octahedral Bis(diethylenetriamine)Co(III)X3 ([Co(DETA)2]X3, CoDX3) complexes were extensively studied in the 1970s due to their rich stereochemistry. 136 In fact, depending on the coordination geometry of the two DETA ligands around the metal, the complex can be obtained as one of three diastereomers (Fig. 63):

• The mer (meridional) isomer, where the two DETA ligands are coordinated in trans position relative to the metal. The primary amines of each DETA ligand are opposed, generating two perpendicular planes with Cobalt. • The s-fac (symmetrical, facial) isomer, where the ligands are in facial disposition (each DETA occupies adjacent positions defining a face of the octahedron) and the secondary amine groups of each ligand are adjacent. This isomer presents a center and planes of symmetry. • The u-fac (unsymmetrical, facial), where both ligands are in facial disposition, but the secondary nitrogens are opposed. This isomer is optically active: the chirality depends on a configurational effect on the ethylenic fragments. 136 The chloride complex ([Co(DETA)2]Cl3, CoDCl3) is synthetized by reacting CoCl2 with two equivalents of DETA and one of HCl, in the presence of activated charcoal to catalyze the oxidation of the metal (Fig. 63).

The isomer ratio in the final product is influenced by the nature of the anion (X -) due to the preferential affinity of the latter for one of the three geometric isomers. This effect was highlighted by quantifying the isomer proportions obtained by the application the same synthetical procedure (Fig. 63) onto a series of CoX2 salts. 137 In the presence of oxyanions such as SO4 2-, SeO3 2-, PO4 3-, or HPO4 2-, the production of the s-fac isomer was amplified (from 7 for Cl -to 59%) compared to mer (from 65 for Cl -to 12%). The amplification depends on the complementarity between the R2NH/RNH2 groups (HBD) of the s-fac complex and the oxygen atoms of oxyanions (Fig. 64 for PO4 3- ), which results in the generation of three stabilizing H-bonds between the cation and the oxyanion. 137 This observation shows the relevance of the geometric arrangement of the functional groups for the interaction of the CoDX3 complexes with anions, and their potential as oxyanion binder.

b. Anion binding in the second coordination sphere

CoDX3 is inert to M-L exchange in the first coordination sphere. 138 However, it can bind to anions in its second coordination sphere as it presents 10 HBD groups (R2NH/RNH2), a positive charge and three geometric isomers with differential selectivity for anions. 137 Only few research groups managed to measure the binding constants of CoDCl3 with oxyanions. 137,[139][140][141] Keene and Searle obtained the values of the binding constants of phosphate with the three isomers of CoDCl3 (K=5460 for s-fac, 850 for u-fac and 84 for mer) in water by UV-Vis spectroscopy. 137 Tatehata et al. reported in 1998 the analysis of the association between the three geometric isomers of CoDCl3 with two anionic Cobalt complexes, [Co(gly)(ox)2] 2-and [Co(p-ala)(ox)2] 2-(gly= glycinate, p-ala = /3-alaninate, ox = oxalate). 140 These anions were chosen because they present an unhindered carboxylate face available for H-bonding (Fig. 65). Differently from mer, s-fac and u-fac isomers of CoDCl3 also present a triangular face adapted to coordinate to the carboxylate side of the selected anions through H-bonds. The binding constants were obtained through conductimetric titrations in water and were comprised between 350 -600 M -1 , so in a close range to the values obtained by Keene and Searle for phosphate. The order of strength, s-fac>u-fac>mer, also corresponded to the one found for PO4 3- , confirming the preferential affinity of polyoxygenated anions for the facial isomers of CoD 3+ . 137 Sharma, Bala et al. obtained the constants for the binding of CoDCl3 with benzoate anions (sodium salts) in water. 142 The measures were performed by UV-Vis titration in water, which resulted in K in the 10 2 -10 4 range (Log K= 2.11 for benzoate, LogK= 3.64 for para-chlorobenzoate and LogK = 3.66 for para-nitrobenzoate).

The higher values of K reported in this paper compared to the previous ones could depend on the absence of ionic strength control in the UV-Vis titration experiments. We encountered this same issue in our binding experiments between oxyanions and CoDX3 complexes, presented in the Results and discussion, where we highlighted that the ionic strength increase upon addition of the guest to the host solution could contribute to 50 % of the UV-Vis spectral variations.

Few other groups attempted the measure of the binding constants of CoDCl3 with anions, which all resulted in small values in the 10 1 -10 2 range, independently on the nature of the anion, indicating the weak nature of these supramolecular interactions. 139,141 c. Polycarbamate binding in the second coordination sphere

Previous work in our group studied the effect of the addition of sub-stoichiometric quantities of metal salts on the CO2 capture properties of DETA in MeOH, as previously discussed in the Chapter I for LaCl3. 39 The influence of the metal salts on the distribution of the carbamated species was evaluated from the comparison of the NMR speciations in the presence and in the absence of the metal.

Amplification Factors were calculated for each member of the library: this value, calculated as in eq.34, is obtained from the difference between the concentration of the i-species/population (library component) in the templated library (xi T ) with its concentration in the untemplated library (xi 0 ) at the same loading  (or ). The difference is normalized with the molar fraction of template in solution (xT). The plot of AFi against increasing  (or ) enables the assessment of the template's influence on the amine-CO2 library, as AFi are proportional to x0 T .

𝐀𝐅 𝐢 𝐓 (𝛂) = x i T (α) -x i 0 (α)

x T 0 𝐞𝐪. 𝟑𝟒

In the presence of CoCl2, an amplification of dicarbamate (2s-DETA) and of free DETA was observed in the solution at full loading (Fig. 66b). The amplification of 2s-DETA was compensated by a proportional decrease of monocarbamate (1a-DETA) with respect to the untemplated library. These data were interpreted as a Cobalt-induced disproportionation, where two equivalents of monocarbamate convert into one equivalent of amine and one of dicarbamate (eq.15-DCo, Fig. 66a). ESI-MS (in cryogenic conditions to limit the breaking of the supramolecular interactions) revealed that the metal was completely oxidized to Co(III) in the fully loaded solution, and that Bis(Diethylenetriamine)Co(III) complexes, with chlorides and carbamates as counterions, were the major Cobalt-containing species in solution. The complex was in fact generated spontaneously in solution, where it bound monoanionic dicarbamate (2s-DETAH + ) in its second coordination sphere through H-bonds, stabilizing this species against 1a-DETA.

The molecular structure of the CoDCl22s-DETA complex, obtained by DFT modeling at the B3LYP-D3BJ/def2TZVP level, shows that 2s-DETA is stabilized by up to five salt bridges with the HBD groups (R2NH/RNH2 of the two DETA ligands, Fig. 66c).

These observations agreed with the available literature on the anion binding properties of Cobalt Werner complexes and brought our interest to the study of these species as templates for polycarbamates. Eventually, the amplification of polycarbamates in polyamine solutions could lead to the generation of cooperative homogeneous CO2 capture mechanisms. The synthesis of some CoDX3 complexes and their characterization as oxyanion receptors will be presented in Part C of this chapter.

II. Part B. Bisguanidinium salts as supramolecular triggers for bicarbonate amplification. Results and discussion.

Context

Scheme 32: Non-covalent salt bridge exchange between guanidinium salts and ammoniumcarbamate (Nu= R1R2NH) or bicarbonate (Nu=OH -).

The reactions of CO2 capture with amines, alcohols or water were discussed in the previous Chapters. Most amine scrubbing agents are based on the carbamation reaction of primary amines such as MEA. Sorbents that capture CO2 by generating bicarbonate, such as tertiary amines, present optimal heat of absorption and desorption, [START_REF]was obtained by performing the same anion exchange procedure described in the previous paragraph (i) on a solution of diastereomerically pure mer-CoDCl2PF6. The product could be isolated in 99% yield by recrystallization in water and methanol[END_REF] but suffer from slow kinetics. Second generation sorbents were developed to combine the high capacity and low Qreg of hindered/tertiary amines with the fast kinetics of primary amines.

In alternative to amine design, we chose to explore the addition of supramolecular triggers to modulate the covalent CO2 capture reactions of aqueous solutions of amines. Our goal was to amplify bicarbonate production in aqueous solutions of primary and secondary polyamines by supramolecular interactions. This study is based on the previous work by our group which showed the possibility of modulating the properties of CO2 capture systems by a Dynamic Combinatorial approach (agonists, antagonists, Chapter I, Section II.6.).

Guanidines and guanidiniums have been widely applied for their oxyanion binding properties. We anticipated that these species, in their protonated state, could template the switch from the carbamate to the carbonate CO2 capture pathway in amine solutions. Saturated guanidiniums cannot directly capture CO2 but they can, upon dissociation of the X -counterion, engage in supramolecular interactions with anionic species, and thus carbamates or carbonates in CO2capture systems (eq.35, Scheme 32).

The behavior of polyamine-CO2 aqueous solutions in the presence of preformed bisguanidinium salts will be explored in the following discussion.

1. Polyguanidinium salts syntheses 1.1. Polyguanidinium Sulfates (G i SO 4 )

Diethylenetriamine (DETA), Ethylenediamine (EDA) and Tris(2-aminoethyl)amine (TREN) were chosen as model amines for this study. For geometric complementarity reasons, we synthetized three polyguanidiniums sulfate salts based on the same three polyamines.

The synthetic procedure from Bottcher et al. 143 was followed to obtain the bisguanidinium sulfate salts of EDA (1,1'-(ethane-1,2-diyl)diguanidine sulfate, G2SO4) and DETA (N,N-di(2guanidinylethyl)amine sulfate hydrate, G3SO4). The synthetical procedures are detailed in the Experimental section. The trisguanidinium sulfate salt of TREN (N,N′′′,N′′′′′′-(Nitrilotri-2,1-ethanediyl)trisguanidine, G4SO4) was synthetized following the procedure from Lehn et al. 99 in a pressure reactor (Exp). The bisguanidinium scope was extended by exchanging the sulfate anion of G2SO4 with Cl -(G2Cl2) and NO3 -(G2(NO3)2). The anion exchange was performed by two methods:

• For G2Cl2 and G2(NO3)2 both, by anion exchange chromatography with an Amberlyst® A26 OH-form stationary phase (procedure in Exp).

Study of the polyamine-bisguanidinium salt-CO 2 libraries in water

Once the scope of bisguanidinium salts obtained, we proceeded to evaluate their influence on the capture properties of polyamine solutions. The systems were analyzed by qNMR speciation, with the calculation of the amplification factors, and FTIR/qNMR to assess absorption rate and Cyclic Capacity.

The guanidinium groups were not expected to react with CO2 directly, as they should remain protonated (pKa≈13.5) in the pH range of the polyamine solutions (lean state pH≈12, rich state pH≈7). The sulfate salts (G2SO4, G3SO4, G4SO4) were soluble in water up to 0.1 M concentration. Due to the ditopic nature of the bisguanidinium salts, the sorbent solutions for CO2 capture were prepared with a 2:1 polyamine:bisguanidinium salt stoichiometry. The results obtained from the NMR speciation of a DETA solution containing a half equivalent of G2SO4 in water are discussed in this section. The solutions were prepared by solubilizing G2SO4 and DETA in deionized water in a graduated flask.

The speciation was performed with the operating procedure reported in the Experimental Section and previously exploited for the polyamines presented in Chapter II. The speciation graph obtained for DETA + 1/2 G2SO4 is reported in Fig. 67b, adjacent to the graph of DETA in the absence of the salt (Fig. 67a) to enable a comparison between the different systems.

As expected, the xi of G2SO4 remains constant upon increasing CO2 loading. In the absence of CO2, G2SO4 presents two peaks, one in the aliphatic range (sp3) at 40.04 ppm and one in the downfield range at 157.04 ppm (sp2). Both signals were static upon increasing , confirming that this species does not react directly with CO2 and it is not influenced by pH changes.

The carbamate subsystem evolved similarly to the DETA speciation in the absence of G2SO4. 1a-DETA was the most concentrated species at low loading. Around αN=0.2, dicarbamates 2s-DETA and 2a-DETA started to concomitantly increase in the system. 2s-DETA was still present at the end of speciation, while 2a-DETA disappeared around =0.5.

The most remarkable difference between the two graphs in Fig. 67 can be observed in the evolution of HCO3 -, whose xi exponentially increases starting from =0.2. In the absence of G2SO4, 1s-DETA is more concentrated than HCO3 -up until max, while in the presence of the salt, HCO3 - concentration is always higher than 1s-DETA, and at full loading bicarbonate is more concentrated than DETA (xHCO3-=1.16). At saturation, xHCO3-equals exactly two times the molar fraction of G2SO4, indicating that the amplification is proportional to the number of guanidinium groups on the cation.

The experimental points were fitted with polynomial equations of degree 3-5. Amplification Factors were obtained via eq.34 (page 105), for 0 <  < max ( =0.02), with xi calculated for each  with the polynomial fits from speciation. The AFi calculated with respect to G2SO4 concentration (xT=0.5) are plotted upon increasing  in Fig. 68.

AF were calculated for each species (plot of individual AF against  available in Annexes, Fig. A10) and, for clarity, for carbamate and carbonate subsystems. The xi of the carbamate subsystem (R-NHCO2 -) was calculated as the sum of each carbamate's molar fraction (xi) multiplied for the corresponding number of bound CO2.

The graph in Fig. 68 shows that bicarbonate is amplified in the presence of G2SO4 up to AFmax=0.63 at =0.5. The amplification is compensated by a reduction of carbamate concentration with respect to the G2SO4-free systems, in particular 2a-DETA (AF=-0.23 at =0.54, Annexes, Fig. A10). At any given , the sum of all AFi, each multiplied by the corresponding number of bound CO2, should be equal to zero: the amplification of a sub-population induced by a template must be compensated by the decrease of another. The sum of AFi(α) was calculated for each  and is presented in the graph as an indicator of the "cumulated deviation of the model". Only amplifications outside this interval of non-confidence were considered.

AF indicate that G2SO4 induces a switch between covalent CO2 capture chemistries, as bicarbonate is amplified to the same extent at which carbamates are consumed (Fig. 68). The selectivity of capture is shifted by the addition of G2SO4: a 60/40 ratio of HCO3 -/R2NCO2 -is obtained at Nmax in the guanidinium-containing library, compared to the 40/60 ratio obtained in the absence of the salt.

The capture capacity of DETA profits from bicarbonate amplification, as max=0.64 instead of 0.61 for the guanidinium-free library. The observations derived from NMR speciation brought us to construct the ion exchange equilibrium in Fig. 69, describing bicarbonate amplification from the molecular perspective. The hydrolysis of carbamates to generate bicarbonate (eq.14) is favored in the presence of G2SO4. This depends on the additional stability of bicarbonate, due to the association with the guanidinium cations by salt bridges. This supramolecular association is favored by the complementary geometry of the two planar bidentate ions (G2(HCO3)2, Fig. 69), as we discussed in the bibliographic study of this Chapter. As it induces the stabilization and consequently an amplification of HCO3 -, G2SO4 can be effectively assessed as a template for bicarbonate.

The generation of the guanidinium-bicarbonate product implies the dissociation of the initial ionic couple with sulfate, which is justified by the free amine amplification observed in the AF graph (Fig. 68). DETA gets protonated twice to balance the negative charges of bicarbonate and binds to the sulfate anion, after its replacement by bicarbonate, through ammonium-sulfate interactions (eq.35-1a, Fig. 69,DETAH2SO4). The anion exchange process is made explicit in Fig. 69 for 2a-DETA, the weakest dicarbamate of the DETA library due to the strong electrostatic repulsion between the two anionic groups. This species was subject to the most relevant decrease compared to the untemplated library, and it was thus the strongest contributor to bicarbonate generation.

The results of NMR speciation show that G2SO4 acts as supramolecular trigger, inducing a switch between covalent capture chemistries with DETA in water, and as a template for bicarbonate. The following sections will present the results obtained from NMR speciation for other bisguanidiniumpolyamine combinations.

b. DETA + ½ G3SO4

Scheme 39: G3SO4 carbamation reaction.

Differently from G2SO4, G3SO4 presents a secondary amine moiety which can react with CO2. We performed its speciation in the absence of amine, confirming the generation of a secondary monocarbamate (1-G3SO4, Scheme 39). The library at full loading (max=0.5) contains 20% carbamate and 30% HCO3 -(Speciation of G3SO4 available in Annexes, Fig. A9, page 224).

In combination with two equivalents of DETA, G3SO4 induces an even stronger bicarbonate amplification than G2SO4, with a AFmax=0.9 at =0.5 (Individual AF graphs in Annexes, Fig. A11). The molar fraction of bicarbonate in the fully loaded solution is also proportional to G3SO4 concentration (Fig. 70b), indicating that each guanidinium group binds to a bicarbonate through salt bridges, similarly to G2SO4.

The xi of 1-G3SO4 is 3 times higher in the presence than in the absence of DETA, however the carbamation on the secondary amine of does not affect the capacity of the guanidiniums to interact with HCO3 -. Despite the stronger amplification of HCO3 -at αN=0.5, at full loading the AF was equivalent to the one obtained with G2SO4. A 60/40 ratio between bicarbonate and carbamates was obtained at saturation with G3SO4, similarly to the solution containing G2SO4.

The speciations of DETA with 1/2 G2SO4 and G3SO4 showed that bisguanidinium salts resulted effective in amplifying bicarbonate instead of carbamates. We then turned to test the same bisguanidinium additives in combination with EDA, in order to extend their application range to other polyamines.

c. EDA + ½ GiSO4

The coupling of bisguanidinium sulfate salts with EDA gave less satisfactory results than with DETA. Bicarbonate amplification was observed in the EDA + ½ G3SO4 solution, but to a lesser extent then with DETA (AFmaxHCO3 -= 0.4, Fig. 71a).

The EDA + ½ G2SO4 system is the least effective among the sulfate series: the AF are small and they strongly deviate from the model, so their evolution upon N is not reliable (Fig. 71b).

These observations suggest that the structure of the amine plays a role on the efficiency of the ion exchange process. Following eq.35-1 (Fig. 69, page 111) the amine should interact with sulfate through diammonium-sulfate salt bridges. The inferior efficiency of the anion exchange process with EDA + ½ GiSO4 systems could depend on the fact that the second pKa of EDA (pKa2= 7.15) is close to the pH of a fully loaded amine solution (pH≈7 at Nmax), so EDAH2 + will be less abundant at full loading compared to DETAH2 + (pKa2=9.21).

d. TREN + ½ G4SO4

The scope was extended to the study of TREN with the correspondent trisguanidinium sulfate salt, G4SO4 (Scheme 35, page 107). The speciation (Annexes, Fig. A19) indicates a similar behavior to the amine-bisguanidinium systems presented before, with a bicarbonate amplification up to AFmax=0.49 at =0.44 (Fig. 72), compensated by a decrease of polycarbamate concentration (2-TREN, 3-TREN).

The AF of HCO3 -drops at higher loading (> 0.44), suggesting that the amplification of bicarbonate induced by G4SO4 is mostly effective at intermediate loading. In fact, bicarbonate was already generated with high xi (xHCO3-=1.39) in the template-free TREN speciation.

Interestingly, HCO3 -amplification is not accompanied, in this case, by a free amine amplification: instead, 1-TREN is amplified (red, Fig. 72), suggesting that this species gets doubly protonated and associates with sulfate upon ion exchange (eq.35-1a, Fig. 69, page 111).

Cyclic Capacity of DETA + ½ GiSO4 libraries

The influence of the bisguanidinium sulfate salts (GiSO4; i=2,3) on the CC of DETA in water was evaluated with the same operational procedure exploited for the systems presented previously in this text (Exp), by performing three consequent absorption/desorption cycles.

The FTIR spectra in the solutions containing GiSO4 presented the same vibrations of the guanidinium-free libraries. The CO2 loading, calculated by NMR at each saturation point (max and min) for DETA + 1/2 GiSO4 are plotted in Fig. 73. The 13 C NMR of the solutions after desorption showed that bicarbonate was completely released upon heating, while some residual monocarbamate (1a-DETA) was still present.

The comparison between the max and min in Fig. 73 shows that the extent of absorption and desorption is higher in the presence of G2SO4 (green, CCN=0.58 ± 0.01) and G3SO4 (blue, CCN=0.53 ± 0.03) than in their absence (red, DETA, CCN=0.46 ± 0.01). A 15% increase of the CC is obtained through the addition of G2SO4, and a 8.7 % increase with G3SO4.

The higher extent of desorption in the presence of bisguanidinium sulfate salts depends on two main factors:

• Bicarbonate, the most abundant capture products in the libraries containing GiSO4, requires less energy to release CO2 in comparison with carbamates. • Upon desorption, the lean amine and the guanidinium are stabilized by interaction with the sulfate (Fig. 74, next page).

The increase of CCN in the presence of GiSO4 shows that the stabilization of bicarbonate by interaction with the guanidinium groups does not confer to this species an excessive stability, which would have played a detrimental effect on the extent of CO2 release upon heating. The NMR analysis of the solutions after desorption showed that G3SO4 was subject to thermal degradation. The degradation involves a reaction between the secondary nitrogen and the guanidinium group of G3SO4, since the extent of carbonation decreased after the first desorption step. A 15% reduction of capture capacity for the DETA + ½ G3SO4 system was observed in the second absorption step compared to the first. Despite presenting the highest max upon the first absorption, the DETA + ½ G3SO4 system is thus unfit as CO2 capture agent as it cannot be recycled after desorption.

The DETA + ½ G2SO4 solution is then the most efficient of the capture systems based on sulfate salts, as it presents a 15% superior CCN to DETA and it is not degraded upon three consequent absorption/desorption cycles. CO2 hydration by tertiary amines is characterized by a slower absorption rate than carbamation, which represents the main issue limiting the adoption of carbonation agents for amine scrubbing. 41 To evaluate the influence of the bisguanidinium sulfate salts on the kinetics of bicarbonate formation in polyamine solutions, we performed an absorption rate study on DETA in the presence of ½ G2SO4 and G3SO4 by real-time FTIR (standardized condition of an absorption rate measure, procedure in Exp). The  corresponding to each FTIR measure (one spectrum per second) were obtained from IR intensities at 1320 cm -1 (N-CO2 -) and 1360 cm -1 (HO-CO2 -) through the absorption coefficients of DETA's FTIR-NMR calibration curves (Exp, Figure E1). Figure 75 presents the increase of  with the time of CO2 injection (fixed flow rate, 200 ml/min). PMDETA is also presented as a reference, as this tertiary amine exclusively captures CO2 by generating bicarbonate.

Absorption rate of DETA + ½ GiSO4 libraries

The graph shows that the bicarbonate amplification imparted by bisguanidinium sulfate salts does not play a detrimental role on the kinetics of capture, as the DETA solutions saturate at approximately the same rate in the presence and in the absence of ½ GiSO4.

The initial absorption rates (first 10 minutes), indicating the mol of CO2 absorbed per second, were calculated with eq.26-1 (page 63) and were respectively =0.056 ± 0.017 for DETA + ½ G2SO4, =0.052 ± 0.012 for DETA + ½ G3SO4 and =0.033 ± 0.004 for PMDETA (DETA =0.066 ± 0.003). The faster rate of bicarbonate formation with the polyamine + bisguanidinium salt systems compared to tertiary amines depends on the fact that bicarbonate is generated through the "fast" pathway of dicarbamate hydrolysis (eq.35-1, Scheme 40) instead of the "slow", direct pathway of CO2 hydration (eq.12b).

Scheme 40: Fast (carbamate hydrolysis in the presence of guanidinium sulfate) and slow (CO2 hydration) bicarbonate formation pathways.

Bisguanidinium sulfate salts proved to be efficient supramolecular additives to DETA, as their combination resulted in a capture system of increased Cyclic Capacity, maintaining the fast absorption rate characteristic of primary and secondary polyamines.

Diethylenetriamine with bisguanidinium dinitrate salt (G 2 (NO 3 ) 2 )

The bibliographic study highlighted the relevance of the counterion coupled with a cationic receptor for anion binding processes. We thereby chose to explore the influence of the substitution of sulfate (SO4 2-) with Nitrate (NO3 -) on the bicarbonate amplification properties of the G2 2+ bisguanidinium cation.

a. NMR speciation G2(NO3)2 resulted poorly soluble in water (Cmax= 0.06 M), so the 13 C qNMR spectra for the speciation with DETA were characterized by a low S/N ratio. All experimental points with a mass balance inferior to 80% with respect to the initial amine concentration were excluded from the speciation graph (reported in the Annexes, Fig. A16).

In combination with DETA, the nitrate salt apparently induces a stronger bicarbonate amplification than G2SO4 (AF in Fig. 76). The highest AF (AFmax=1) is obtained at =0.57, but a 25% AF imbalance between the model and the experimental points is observed at high loading, so the quantification could be biased.

b. Cyclic Capacity

The cyclic capacity of the DETA + ½ G2(NO3)2 system was evaluated upon two consecutive absorption-desorption cycles. The absorption and desorption efficiency decrease after the first cycle, by respectively 13% and 19% (Fig. 77). The system displays an average CCN=0.53 ± 0.04 (CC=1.07 ± 0.09).

Despite the stronger bicarbonate amplification observed by NMR compared to the sulfate salt, G2(NO3)2 can be deemed a less efficient bicarbonate template then G2SO4 due its lower solubility in water and inferior Cyclic Capacity.

Diethylenetriamine with bisguanidinium dichloride salt (G 2 Cl 2 )

The influence of sulfate substitution with chlorides on the G2 2+ cation will be presented in this section. The G2Cl2 salt resulted strongly soluble in water, up to 5 M concentration. To enable a meaningful comparison with the previous results, the speciation of the DETA + ½ G2Cl2 was performed in the same concentration of the solutions with the sulfate salts (DETA 0.2 M, G2Cl2 0.1 M).

The system remained homogeneous upon CO2 loading at 25 °C, but during refrigeration at 4 °C prior to NMR analysis some crystals formed in the loaded samples. The following paragraph will present the XRD analysis of the CO2-rich solid product. a. XRD analysis of G2(HCO3)2

The crystals were stored in refrigerated conditions to avoid CO2 desorption and they were analyzed by XRD. To our pleasure, the structure corresponded to G2(HCO3)2, i.e. a bisguanidinium bisbicarbonate salt (Fig. 78a), the species whose generation was postulated in Fig. 69 (page 111) to justify bicarbonate amplification.

G2(HCO3)2 spontaneously crystallized from the CO2-loaded solution in a dry lattice (no water molecules in the crystalline structure) despite its intrinsic hydrophilicity. A highly ordered structure is obtained by the association of one bisguanidinium unity with two bicarbonates, stabilized through multiple salt bridges (Fig. 78a).

The crystal contains a centrosymmetric bicarbonate dimer, similar to the one observed by Custelcean et al. 128 , with contact distances of 1.798 Å. Each charged oxygen atom of a bicarbonate unit accepts a H-bond from one of the guanidinium protons: the two O-H …-O bonds have approximately the same length (1.9 Å, 1.994 Å) and angles near to 180° (Fig. 78b).

As predicted, the bidentate nature of the guanidiniums is effective for the coordination to bicarbonate. In total, seven hydrogen bonds stabilize each bicarbonate anion (Fig. 78c) and result in a stable supramolecular aggregate, that develops in three dimensions in a highly ordered fashion.

Every HB donor and acceptor group is saturated in the product. The solid was also characterized by SSNMR and ESI-MS, that confirmed the stoichiometry of two bicarbonates for one guanidinium (SSNMR reported in the Annexes, Fig. A18). To complete the speciation set of the DETA + ½ G2Cl2 system, we quantified the mol of HCO3 - precipitated in the CO2-rich samples by volumetric titration (Exp) and added this quantity to the one detected in solution by 13 C qNMR.

b. NMR Speciation

The speciation (Annexes, Fig. A17) highlighted that this system displayed the strongest affinity for bicarbonate of the combinations analyzed up to now, as 68 % of CO2 was captured as bicarbonate, compared to the 40% with DETA and 61% with DETA + ½ G3SO4. The amplification factors (Fig. 79) seem to indicate an inferior amplification with respect the G3SO4 or G2(NO3)2 systems, but the 30 % deviation from the model at high loading could influence the quantification of AFs.

To fully assess the efficiency of this capture system, we measured the Cyclic Capacity for this system.

c. Cyclic Capacity

The CC for the DETA + ½ G2Cl2 system in water was measured with two different amine concentrations, 3 and 0.3 M. The strong solubility in water of G2Cl2 enables to perform capture in high concentration, closer to industrial amine scrubbing.

Unfortunately, G2(HCO3)2 precipitates at room temperature in the 3 M DETA solution upon CO2 absorption. The nature of the solid was confirmed by acid hydrolysis with volumetry and NMR. To quantify the CC, max was obtained by adding the  of the liquid fraction (q 13 C NMR) to the one of the solid, calculated from the guanidinium's mass imbalance in solution with respect to the initial concentration. A precipitation event can generate processing issues at the industrial scale if the separation of the phases is required. We thus chose to perform desorption directly on the heterogenous system to avoid the separation step. Unfortunately, despite heating at 100 °C and stripping with N2, the solid did not redissolve completely in solution. The NMR of the liquid phase after desorption showed that DETA had undergone degradation. The solid obtained at the end of desorption was analyzed by volumetry which indicated that it did not contain CO2, suggesting that CO2 was in fact released from G2(HCO3)2, while the degradation resulted in another solid product, whose nature has yet to be confirmed.

The degradation had a negative impact on the CC: a 39 % reduction in absorption capacity was observed between the first and the second absorption-desorption cycle in the most concentrated solution (DETA 3 M, Fig. 80, CCN= 0.46 ± 0.06). The desorption capacity was reduced with respect to the sulfate systems, as only 78% of absorbed CO2 was desorbed in the first heating step (with respect to 89% for DETA + ½ G3SO4). A ten-fold dilution of the solution (DETA 0.3 M, orange line in Fig. 80, CCN=0.42 ± 0.14) did not improve the CC of the system, which also underwent degradation after the first heating step.

The degradation of DETA could be limited by performing desorption on solid G2(HCO3)2 upon isolation from the amine solution, but this requirement would imply an additional separation step to the capture process.

Control tests of bicarbonate formation in polyamine solutions with organic and inorganic salts

Some control experiments were performed by quantifying AFHCO3, by qNMR, in DETA and EDA solutions containing different stoichiometric ratios of organic and inorganic salts. These tests were performed to rule out the influence of an ionic strength increase on bicarbonate amplification, and thus to fully assess the role of supramolecular effects on the properties of the aminebisguanidinium-CO2 libraries. The scores obtained for each system are listed in Table 10: we calculated the molar fraction of HCO3 -with respect to the amine, (xHCO3 vs amine), Nmax and the AFmax(HCO3 -). We also calculated the xHCO3 with respect to the total amount of CO2 captured, as in eq.36, to evaluate the percentage of CO2 captured as bicarbonate compared to carbamates.

𝑥 𝐻𝐶𝑂 The following considerations are based on the comparison between the AF listed in Table 10 for all the salts tested as additives to DETA and EDA:

-1:1 amine:bisguanidinium sulfate stoichiometry. An increase of bisguanidinium sulfate salt concentration resulted in a bicarbonate amplification to the same extent, or inferior to the 2:1 stoichiometry. This could depend on the fact that two amine moieties are protonated -Inorganic Na + salts (Na2SO4, NaNO3, NaCl). The addition non H-bond donor cations to DETA and EDA resulted in a smaller amplification of HCO3 -(0 < AF(HCO3 -) < 0.37) with respect to the untemplated library, proving that ionic strength plays a role in bicarbonate amplification. However, the amplifications were substantially inferior to the ones obtained with the HBD bisguanidinium salts. -The highest amplification was obtained with the DETA + ½ G2Cl2 combination, which presents the highest xHCO3-=1.26, the highest % of CO2 captured as HCO3 (68 %) and the highest maximal loading (Nmax= 0.63).

Counterintuitively, the AFHCO3-of the DETA + ½ G2Cl2 system is inferior to the one obtained with G3SO4. The explanation depends on the different evolution of xHCO3-with αN for the different capture systems. In fact, the x(HCO3 -) vs  plot for with G3SO4 presents an inflection point around =0.5, while the plot for DETA alone increases linearly starting from =0.2 (comparison between speciations, Fig. 70, page 113). The difference between the plots is thus more relevant at αN=0.5, where the highest bicarbonate amplification is detected, than it decreases at full loading. With G2Cl2 on the other hand, the xHCO3 vs  plot evolves linearly (Annexes, Fig. A17). In this case, the precipitation of solid G2(HCO3)2 favors bicarbonate generation at Nmax, where the strongest amplification is detected (Fig. 79, page 122).

Conclusion to ChapterIIIB

The results presented in Chapter IIIB demonstrate the possibility of switching between covalent capture chemistries in polyamine solutions by the addition of preformed, protonated bisguanidinium salts.

The amplification of bicarbonate against carbamates, proved experimentally by the calculation of the Amplification Factors from 13 C quantitative NMR speciation data, depends on the stabilization of this capture product by salt bridging with the guanidinium cations. At the microscopic level, bicarbonate amplification is obtained through an anion exchange process which implies the association of bicarbonate with guanidiniums and the binding of the counterion (sulfate, nitrate, chloride) by the free amine which gets doubly protonated. The proposed mechanism was confirmed by the isolation of crystalline G2(HCO3)2 from a CO2-loaded solution, which highlighted the extended H-bonding between the dication and two equivalents of bicarbonate.

The switch from the carbamate to the carbonate capture chemistry depends exclusively on supramolecular effects, thereby the thermodynamics and kinetics of the capture process are not drastically modified despite bicarbonate amplification. The most efficient amongst the polyamine/polyguanidinium combinations tested was DETA + ½ G2SO4, which showed a 15% increase of the Cyclic Capacity with respect to the untemplated library.

The templated libraries maintained the fast absorption rate characteristic of primary and secondary amines even if CO2 hydration, with bicarbonate formation, was the predominant capture pathway in solution. This remarkable effect depends on the fact that bicarbonate is generated through carbamate hydrolysis, which displays a faster rate compared to CO2 hydration by OH -.

At the present day, the polyamine-polyguanidinium salt sorbents are not fully adapted to the industrial amine scrubbing process because of the limited solubility of the salts, which would result in a reduced working capacity with respect to concentrated amine solutions. However, the results reported in this section prove that supramolecular chemistry has the potential to deliver effective improvement to Carbon Capture technologies.

These interesting and innovative results should be further exploited by finding a highly soluble polyguanidinium salt enabling bicarbonate amplification while maintaining the system homogeneous. The additive should also be resistant to thermal and oxidative degradation and ideally it should be applicable to different amine solutions, such as to MEA and PZ in water.

III. Part C. Bis(Diethylenetriamine)Co(III)X 3 complexes as supramolecular templates for dicarbamates. Results and discussion.

Context

The following section presents the study of Bis(Diethylenetriamine)Co(III)X3 (CoDX3, X -= anion) complexes as dicarbamate templates (T, Scheme 41).

This study was inspired by previous results from our group which assessed the template properties of the CoDCl3 complex for 2s-DETA (Bibliographic study, Chapter III.4., page 117). Experimental and theoretical results showed that the complex was generated in-situ in a methanolic DETA solution containing substoichiometric quantities of CoCl2, and that it stabilized the dicarbamate by donating H-bond with its R-NH2/R2NH moieties.

We were interested in synthesizing the complex ex-situ and in studying its interactions with oxyanions. The goal was to obtain a template for polycarbamates which could stabilize these highly energetic species and amplify their production in polyamine solutions for CO2 capture (eq.15T, Scheme 41). We envisioned that the stabilization of polyanionic states by supramolecular interactions could eventually lead to the development of a homogeneous, cooperative CO2 capture system in water.

Scheme 41: Scheme of ideal polycarbamate amplification in polyamine-CO2 libraries by Bis(Diethylenetriamine)Co(III) (CoDX3) complexes.

We were also interested in studying the oxidation rate of Co(II) to Co(III) in the presence of the carbamates of DETA. In fact, thanks to the previous observations from our group, we anticipated that the oxidation of the metal would be accelerated by the preferential binding of "hard" anionic ligands (carbamates) with "hard" cations, and thus with Co(III). Keene and Searle (Fig.63, page 102). 136 We attempted the chromatographic separation of the mer, s-fac and u-fac diastereomers, but as it was highly time consuming and resulted in an incomplete separation, we decided to conduct the binding studies with the mixture obtained from the synthesis.

As discussed in the Bibliographic study and in Part B of this chapter, the nature of the counterion coupled to a cationic receptor influences its solubility and the strength of its supramolecular interactions with the target anions. The bibliographic study concerning anion binding also indicated that the strongest anion receptors are those presenting a high positive charge density.

To be in the most favorable conditions to study polycarbamate binding by CoDX3 complexes, the chloride counterion was exchanged with three less coordinative anions. Figure 81 presents the electrostatic potential maps of the four CoDX3 complexes synthetized for this purpose, listed by order of decreasing charge density (a. X -= Cl -, b. X -= ClO4 -, c. X -= 2Cl -,PF6 -, d. X -= BPh4 -). As expected, the positive charge density on the cation increases from a. to d. (increasingly darker blue color) due to the weaker electrostatic interactions with the counterions.

Bis(Diethylenetriamine)Co(III) trisperchlorate ([Co(DETA)2](ClO4)3), CoD(ClO4)3) was synthetized from Co(ClO4)2 and DETA following the same synthetical procedure described for CoDCl3 (Exp). 137 An anion exchange procedure was exploited to substitute the chloride counterions of CoDCl3 with hexafluorophosphate. The addition of solid ammonium hexafluorophosphate (NH4PF6) to a saturated solution of CoDCl3 in hot water resulted in the precipitation of the monosubstituted, diastereomerically pure, mer-Bis(diethylenetriamine)Co(III) dichloride hexafluorophosphate ([Co(DETA)2]Cl2PF6, mer-CoDCl2PF6) complex (Fig. 82).

The selective precipitation of the mer isomer from the mixture with s-fac and u-fac could depend either on the preferential self-assembling of the PF6 -anion with the mer isomer or on the inferior water solubility of this complex compared to the others. mer-CoDCl2PF6 was isolated in crystalline form via recrystallization in water. XRD analysis showed the presence of a water molecule in the crystalline unit, which interacted via H-Bond with a R-NH2 unit. This interaction is sufficiently strong to mediate the electrostatic attraction between the cationic complex and two anions, PF6 -and Cl -. Another H-Bond connects a R2NH secondary unit to a Cl -anion.

The insertion of water in the crystalline structure of the complex suggests that mer-CoD 3+ interacts strongly with the solvent, which could consequently behave as a strong competitor against anion binding.

Anion exchange with a sodium tetraphenylborate (NaBPh4) solution was performed on CoDCl3 and on mer-CoDCl2PF6 in water to obtain Bis(Diethylenetriamine)Co(III) tristetraphenylborate ([Co(DETA)2](BPh4)3, CoD(BPh4)3 or diastereomerically pure mer-CoD(BPh4)3, Scheme 42), the only complex of the series which was insoluble in water but soluble in organic solvents. The complete substitution of Cl -and PF6 -by BPh4 -was assessed by quantitative 1 H NMR, which confirmed the 1:3 ratio between cation and anions.

This complex is a good candidate for the detection of weak anion binding events, as BPh4 -has a low coordinating character, and it will consequently represent a weak competitor against oxyanions. In addition BPh4 -grants the solubility of CoD(BPh4)3 in organic solvents, where supramolecular interactions are stronger than in water.

Scheme 42: Synthesis of (mer-)CoD(BPh4)3 from anion exchange with sodium tetraphenylborate (NaBPh4) on CoDCl3 and mer-CoDCl2PF6.

Study of oxyanion binding with Bis(Diethylenetriamine)Co(III)X 3 complexes

The following paragraphs present the most relevant results obtained from the spectroscopical study of the binding between the synthetized CoDX3 complexes and (poly)carboxylates, which were used as static models of (poly)carbamates. UV-Vis titrations and 1 H NMR analyses were exploited to detect the association of oxyanions in the second coordination sphere of the CoDX3 complexes. The UV-Vis maxima may shift in position and/or intensity upon the addition of a target anion to a CoDX3 solution. 142 However, the process of anion exchange in the second coordination sphere of the complexes, thereby far from the metal center responsible for d-d transitions, may have weak effects on the UV-Vis spectra. Consequently, the titrations experiments were conducted with the highest concentration of complex possible (15 mM) in order to favor the detection of binding while avoiding the saturation of the spectrophotometer detector.

UV Vis titrations

Ionic strength variations can significantly influence the spectral changes in a titration experiment based on charged species. The best suited option to avoid these secondary influences is to buffer ionic strength by preparing the solution with an appropriate salt in high concentration. This represents an issue for anion binding studies, as the salts soluble at more than 1 M concentration in water usually present chloride anions (KCl, NaCl), but Cl -is a strong competitor against organic anions. In methanol we encountered some solubility issues due to the precipitation of BPh4 -with tetramethylammonium and tetrabutylammonium cations, which were tested with Cl -or ClO4 - counterions as ionic strength buffers.

We consequently decided to conduct the anion vs CoDX3 titrations without ionic strength control, whilst performing a control titration beforehand with a salt presenting an innocuous anion (X -).

a. UV-Vis titration of CoD(ClO4)3 with citrate in water

Two independent UV-Vis titration were performed in water to characterize the binding of Citrate (C6H5O7 3-), exploited as static model of a tricarbamate, with CoD(ClO4)3. The perchlorate complex was chosen as anion binder among the three water-soluble complexes synthetized as it presents the least coordinative counterions.

Scheme 43: Anion exchange equilibrium CoD(ClO4)3 vs Citrate (TMA=tetramethylammonium) in water. Green: HBD groups, Brown: counterion (anion), RED: HBA groups (target anion), Blue: counterion (cation).

A 15 mM solution of the complex was prepared in deionized water, 600 l were transferred in a quartz cuvette (optical path l=1 cm). Tetramethylammonium (TMA) Citrate (0.135 M) was prepared in-situ by neutralization of citric acid with tetramethylammonium hydroxide pentahydrate (TMAOH*5H2O) in deionized water. The pH of anion and receptor solutions were set to 7 ± 0.2 prior to mixing. At neutral pH, citric acid (AH3) should present two negative charges (ratio of AH 2-/AH2 - =10 2.2 at pH=7, pKa1= 3.1; pKa2=4.8; pKa3=6.4).

The guest solution was added to the binder solution by sequential 12.2 l increments with a micropipette: each addition corresponded approximately to 0.2 equivalents of ligand with respect to CoD(ClO4)3. A UV-Vis spectrum was recorded in the 200 -800 nm spectral range after each addition. In total, five equivalents of citrate were added to the CoD(ClO4)3 solution. A multiplicative factor accounting for the dilution due to guest solution addition was applied to each spectrum.

The most relevant spectral changes involve the intensity of absorption at 465.5 nm. The average absorbance variation (A=A-A0, A0: absorbance for Cguest=0) at this wavelength is plotted against guest concentration (equivalents of Citrate with respect to CoD 3+ ) in Fig. 84. The experiment was duplicated to obtain mean value and rmsd, reported in Fig. 84 as vertical error bars for each point. The variations display a small amplitude (A=0.08 a.u.) with substantial deviations between the duplicate measures. Nonetheless, the experimental points can be fitted with a model describing a 1:1 host/guest association (fitted curve reported in Annexes, Fig. A21), which results in a small binding constant K35-2= 8.64 ± 2.89 M -1 with a 30% uncertainty. The consistent experimental errors obtained by this experiment and the inferior K compared to the values reported in the literature (K≈ 10 2 -10 3 M -1 ) could depend either on the weakness of binding between citrate and CoD 3+ or on the limited influence of second-sphere coordination on the UV-Vis spectra.

A control titration was performed by adding a TMAClO4 solution to a CoD(ClO4)3 solution in water. The addition of perchlorate, the same counterion of the Co(III) complex, should be innocuous with respect to anion exchange in the second coordination sphere, thereby enabling the evaluation of ionic strength effects. The spectral variations at 465.5 nm, plotted in Fig. 84 (grey dots) indicate that this parameter partially influences the absorbance of the complex (A=0.08 a.u.), even if at a smaller extent compared to the titration with Citrate.

A second UV-Vis titration was performed with CoD(BPh4)3 in methanol, a solvent where supramolecular interactions are stronger than in water and should consequently be easier to detect.

b. UV-Vis titration of CoD(BPh4)3 with ammonium carbamate in methanol

The binding of ammonium carbamate with CoD(BPh4)3 was studied by UV-Vis titration in MeOH. As ammonium carbamate is a monoanionic species, the association with the complex could result in a 1:1, 2:1 or 3:1 anion:receptor stoichiometry (eq.35-2b, Scheme 44 for a 3:1 binding).

Scheme 44: Anion exchange equilibrium CoD(BPh4)3 vs ammonium carbamate in MeOH. Green: HBD groups, Brown: counterion (anion), RED: HBA groups (target anion), Blue: counterion (cation).

The stability of ammonium carbamate, which at ambient temperature can degrade into NH3 and CO2, was assessed by 13 C NMR monitoring in D4-MeOD, which showed no variation of the 1 H and 13 C NMR spectra over the course of 4 hours at room temperature (average time required for a titration).

For the titration experiment, 20 l aliquots of carbamate solution (0.18 M solution, 0.4 equivalents of anion vs receptor) were added to the solution containing the complex (0.015 M solution, 600 l in a quartz cuvette). UV-Vis spectra were recorded in the 200 -800 nm spectral range after each guest solution increment. In total, twelve equivalents of ammonium carbamate were added to the complex (30 points). Each absorbance value was corrected with the corresponding dilution factor due to guest solution addition. Two independent titrations were performed to obtain mean values and rmsd. The average spectral variations at 465.5 nm are plotted in Fig. 85 (next page) against carbamate equivalents, with error bars indicating the rmsd between the two experiments.

UV-Vis spectral variations are stronger in MeOH than in water. Even if the measures present a consistent rmsd, there is a clear increase in absorbance at 465.5 nm upon carbamate addition, which stabilizes around six equivalents of anion versus receptor. Similarly to the titration presented in the previous paragraph, the fitting of the experimental points for a 1:1 association provides a weak binding constant with substantial uncertainties (K35-2b= 61.11 ± 16 M -1 , fitted curve available in Annexes, Fig. A22). The fit is unsuccessful for a 1:2 stoichiometry, while the 1:3 stoichiometry is not implemented in the fitting software which we exploit (http://app.supramolecular.org/bindfit/).

The control titration to evaluate ionic strength influence (grey dots in Fig. 85) was performed by titrating CoD(BPh4)3 with NaBPh4 in MeOH. The A increase at 465.5 nm, though inferior with respect to the actual binding experiment, clearly contributes to the spectral variations by almost 50%.

We attempted the subtraction of the control titration from the actual titration, but the resulting points could not be fitted nor for a 1:1 nor for a 1:2 binding stoichiometry. To avoid ionic strength influence, a suitable salt should be applied as a buffer, or the titration should be performed in more diluted conditions. Due to the difficulties encountered during UV-Vis titrations, depending on the weak spectral variations upon addition of the guest solution to the host, we turned to 1 H NMR as a more straightforward method to detect the second-sphere association of anions with CoD 3+ complexes.

NMR detection of CoDX 3 binding with oxyanions

The formation of H-bonds may be detected by 1 This trend reflects the decreasing strength of the interactions between the cationic complex and its counterions, which should lead to an inferior competitivity against oxyanions.

1 H NMR detection of mer-CoD(BPh4)3 binding with acetate in D6-DMSO Scheme 45: Anion exchange equilibrium mer-CoD(BPh4)3 vs ammonium acetate in D6-DMSO (25 °C). Green: HBD, Brown: counterion (anion), RED: HBA (target anion), Blue: counterion (cation).

The supramolecular binding of mer-CoD(BPh4)3 to acetate (Scheme 45) was explored by adding one equivalent of solid ammonium acetate to a D6-DMSO solution containing the complex.

The 1 H NMR spectrum of isolated mer-CoD(BPh4)3 in D6-DMSO presents one R2NH and four RNH2 signals (Fig. 87, blue, next page). At t0 after acetate addition (green, Fig. 87) the spectrum shows a significant downfield shift of the amine protons,  = 0.7 ppm for the R2NH signals,  =0.3 for the RNH2 protons at 4.7 ppm and  =1.4 ppm for the RNH2 protons at 4 and 4.2 ppm. No further chemical shift variation is detected with the passing of time (t1=24h, red, Fig. 87). The inferior intensity of the R2NH/RNH2 signals upon oxyanion addition depends on the presence of water traces in the ammonium acetate solution. The remarkable changes in the NMR spectra of the mer-CoD(BPh4)3 complex upon acetate addition could indicate the R2NH/RNH2 groups with the oxyanion. To fully characterize the binding event, a NMR titration should be performed in order to obtain the binding constant K. However, as discussed previously, the reliable detection of the anion-receptor association would require an accurate method to control ionic strength and pH.

As the binding experiments were highly time consuming and presented several issues connected with the solubility of either the complexes or the substitution products, and with the weakness of the spectroscopic variations, we decided to not go further with these experiments. Our main objective was to assess the potency of CoDX3 complexes as polycarbamate templates, but the results reported in this section suggest the thermodynamic weakness of the interactions between carbamates/carboxylates and the complexes.

However, we reasoned that the second-sphere binding of oxyanions to the Co(II) complexes could operate on another level, i.e. the kinetics of Co(II) oxidation into Co(III). We characterized this phenomenon by spectroelectrochemistry, which will be illustrated in the following section.

Spectroelectrochemical study of the CoCl 2 -DETA-CO 2 library in methanol

The oxidation of Co(II) into Co(III) plays a key role in the disproportionation equilibrium eq.15D-Co (Fig. 66, page 105), resulting into the in-situ formation of the CoD 3+ complex, with 2s-DETAH and a Cl -as counterions.

The discussion about agonism and antagonism in CO2 capture (Chapter I, Section II.6.) indicated that upon carbamation amines are converted from "soft", neutral ligands to "hard", oxygenated, anionic ligands. Previous studies from our group proved that ammonium carbamates display a preferential affinity for "hard" metal cations, and should consequently bind preferentially to Co(III) than to Co(II).

Scheme 46: Reaction studied by spectroelectrochemistry: in-situ formation of Bis(Diethylenetriamine)Co(III) complex starting from CoCl2 in the presence of an excess of DETA, under CO2 and O2 atmosphere.

We therefore decided to characterize the oxidation of Co(II) in the presence of an excess of DETA, with and without CO2, through spectroelectrochemistry, i.e. electrochemistry accompanied by a spectroscopic detection method (UV-Vis in this case). The results were obtained in LC of ENS-Lyon (Laboratoire de Chimie, École Normale Supérieure de Lyon) in collaboration with prof. Christophe Bucher.

The solutions containing CoCl2*6H2O and DETA were prepared in water (KCl 0.1 M as supporting electrolyte) or in methanol (TBACl 0.1 M). A 4:1 DETA:CoCl2 stoichiometric ratio was selected to enable the coordination of the metal by two triamine equivalents and the availability of two more DETA molecules for 2s-DETA generation (eq.37, Scheme 46).

The setup for the electrochemical experiments was made of a one-compartment electrolytic cell containing a Reference Electrode (RE: SCE -Saturated Calomel Electrode), a Counter Electrode (CE: Pt) and a Working Electrode (WE: RDE, Rotating Disk Electrode).

Voltammetries were performed to analyze the electrochemical response of the species in solution.

The utilization of a RDE is advisable when performing steady-state studies of a redox reaction. The rotating movement of the electrode continuously sweeps the reduced species from the electrode surface: the current will consequently reach a plateau at completion of a redox process, differently from the more usual Cyclic Voltammetry, which is performed with static electrodes.

The oxidation rate of the metal was measured by Open Circuit Voltammetry (OCV). This experiment consists in measuring the voltage difference between WE and RE when they are disconnected from a circuit. This means that no external potential is applied on the solution, and the only voltage variations detected will depend on the redox processes that are spontaneously taking place in solution. OCV were coupled with in-situ UV-Vis spectroscopy with an immersion probe, which enabled the correlation of the oxidation process to the spectra of the species in solution.

Study of Co(II)/Co(III) oxidation rate in the presence of DETA in water

The preliminary electrochemical tests were performed in water (KCl 0.1 M). The reactants, CoCl2*6H2O and DETA, were stripped with Ar prior to mixing to avoid O2 and CO2 traces.

UV-Vis acquisition (1 spectrum/s) was started before DETA addition to the solution containing CoCl2 to detect the complexation of the metal. Upon amine addition, the UV-Vis absorption intensity increased at 535 and 687 nm, indicating the generation of the reduced Bis(Diethylenetriamine)Co(II)Cl2 complex (CoDCl2), which was complete 4 seconds after amine addition (stack of UV-Vis spectra versus time under Ar in Annexes, Fig. A20).

This indicates that in all the CoCl2 + n DETA systems previously studied in our group, the metal was always doubly coordinated by two triamine molecules. The voltammetry of the solution under Ar (black line, Fig. 88a) shows an oxidation wave at -0.65 V vs SCE, relative to the monoelectronic Co(II)/Co(III) oxidation.

CoDCl3, synthetized ex-situ, was analyzed by voltammetry to enable a comparison with the complexes obtained in-situ. The voltammetry (grey line, Fig. 88a) shows an irreversible mono electronic reduction centered as E=-0.46 vs SCE, corresponding to the Co(III) /Co(II) reduction.

The oxidation rate of the CoCl2 + 4 DETA solution upon exposure to air was monitored by OCV and in-situ UV-Vis (200 -1000 nm). Upon air fluxing into solution, the voltage increased from the starting value E=-0.55 V towards more positive values (Fig. 88b). OCV reached a stationary state 30 minutes after air introduction, indicating the completion of the oxidation process.

Interestingly, the initial voltage of the OCV (E=-0.55 V, point A in Fig. 88b,) corresponds to the beginning of the Co(II)/Co(III) oxidation peak that had been observed by RDE before exposure to air (Fig. 88a, point A), and the final stationary state in the OCV (E=-0.3 V, point B in Fig. 88b) corresponds to the starting point of the reduction wave for CoDCl3 (Fig. 88,point B). This suggest that the most abundant species in solution are respectively CoDCl2 under Ar and CoDCl3 in the presence of O2.

A voltammetry experiment was performed upon stabilization of the OCV, and it confirmed that the metal had been completely oxidized (blue line in Fig. 88a, absence of oxidation peaks at E > 0 V). Two reduction waves were detected, starting at E1=-0.4 V and E2=-0.6 V.

The first one (E1) coincides exactly with the monoelectronic CoDCl3 reduction (grey line for the synthetized complex), assessing the in-situ generation of the complex.

The second one (E2), was attributed to oxo/peroxo Co(III) complexes, intermediates in the generation of CoDCl3 (Fig. 89). The synthetic procedure to obtain CoDCl3 requires activated charcoal and 36 hours of fluxing with air. In the absence of activated charcoal, as in the current case, the system stops on metastable, intermediate oxidized states and cannot fully convert into the final product. 147 The intermediate species are also responsible for the UV-Vis spectra differences between the CoCl2 + 4 DETA solutions and isolated CoDCl3.

Upon exposure of the solution to air, the absorbance increases at 465.5 nm and 400 nm (Fig. 90, next page). The first band corresponds to the d-d absorption of CoDCl3 (Fig. 83, page 130). The second d-d absorption of CoDCl3 at 322 nm is covered by the large and intense absorption of the intermediate peroxide species, which spans from 400 to 200 nm and saturates the spectrophotometer detector. The oxidation study was repeated in methanol, the solvent where the amplification of 2s-DETA by CoD 3+ was originally observed.

The voltammetry and OCV experiments in methanol are similar to the ones in water, despite an inferior resolution of the voltammetry plots (Fig. 91a). The OCV in MeOH (Fig. 91b) shows the spontaneous evolution of the voltage from 0 to -0.2 V, which is complete within 400 seconds (6.6 minutes). The apparent faster oxidation rate in MeOH could depend on the different operating conditions compared to the experiment in water, as a mass-flow controller was used in this case to regulate the flux of compressed air (fixed flow rate, 100 ml/min).

The initial (A) and final (B) voltage of the OCV correspond to the start of the Co(II)/Co(III) oxidation and to the Co(III)/Co(II) reduction in the voltammetry (Fig. 91b) respectively, as previously observed for the experiment in water.

b. With CO2

The measurement of the oxidation rate in methanol in the presence of carbamates was performed with an OCV procedure identical to the previous one, but the CoCl2 + 4 DETA solution was presaturated with pure CO2 (100 ml/min for 30 minutes) prior to air introduction (fixed flow rate, 100 ml/min), to ensure the full loading of the amine.

The UV-Vis spectral evolution in the loaded DETA solution coincided with the one in the absence of CO2, in agreement with the small spectral variations observed upon UV-Vis titrations with oxyanions (section 2.1. of this Chapter).

Yet, the OCV highlights a remarkable acceleration of the oxidation rate in the CO2-rich solution, as the oxidation of Co(II) into Co(III) was complete in 300 second, 1.5 times faster than in the CO2-lean state (Fig. 92), confirming the initial hypotheses.

A mechanistic interpretation to justify the acceleration is proposed in Figure 93 (next page) as a graph representing the energetics of the oxidation process in the presence or absence of DETA-CO2.

CoDCl2 is generated within few seconds from solution preparation (CoCl2 + 2 DETA, (1-37)), as demonstrated by UV-Vis. The oxidation of the complex starts as soon as the species is contacted with O2, generating oxo and peroxo intermediates (2-37). This step is characterized by a low energy barrier, as suggested by the sudden increase of the UV-Vis absorbance between 200-400 nm upon exposure to oxygen.

The requirement to add activated charcoal for the synthesis of CoDCl3 indicates that the full conversion of the intermediate species into the final complex is the rate limiting step of the reaction, thereby it should be characterized by a high activation energy (Fig. 93, (4-37)). The beneficial action of carbamates on the oxidation rate should consequently be focused on this step. Carbamates probably bind to the intermediate oxo and peroxo complexes in their second coordination sphere and lower the activation energy for the transition to the fully oxidized state (5-37). Although the thermodynamic features of binding could not be measured by our titration experiments, we managed to highlight a favorable kinetic effect exerted by the supramolecular binding of carbamates with CoDX3 complexes.

Conclusion to ChapterIIIC

Chapter IIIC presented the study of the supramolecular assembly between Bis(Diethylenetriamine)Co(III) complexes and carbamates/oxyanions.

Four CoDX3 complexes with different counterions (X -) were synthetized. CoDCl3, CoD(ClO4)3 were obtained following the synthetic procedure reported by Keene and Searle 137 , while CoD(BPh4)3, diastereomerically pure mer-CoDCl2PF6 and mer-CoD(BPh4)3, were newly synthetized with ion exchange procedures.

The binding constants of CoDX3 complexes with oxyanions were measured by fitting UV-Vis titration data, which led to K in the 10 1 -10 2 range. During these binding experiments, we encountered some experimental issues regarding the solubility of the complexes or of the byproducts of anion exchange, the weakness of the spectral variations associated with the binding and the control of the ionic strength.

Even if the thermodynamic effect of anion binding by CoDX3 complexes could not be fully assessed, the spectroelectrochemical study of Co(II) oxidation into Co(III) in the presence of loaded DETA highlighted a remarkable acceleration of the process imparted from carbamate binding to the complex. In the presence of DETA carbamates the oxidation of CoDCl2 was 1.5 times faster than in the CO2-free system. This behavior was attributed to a lowering of the energy barrier for the transition from intermediate oxo and peroxo complexes to the fully oxidized CoDCl3 product.

Our initial goal was to obtain further experimental proof assessing the activity of CoDX3 as a polycarbamate template, but as we continued our trials, we realized that we were moving further away from an industrially applicable process. Our interest in supramolecular chemistry must not be separated from the need to maintain the CO2 capture process as effective and cheap as possible.

We thus chose to stop the experiments on Cobalt and focused our energies on more promising templates, such as the polyguanidiniums discussed in Part B of this chapter.

III. Conclusion to Chapter III and future prospects

The results reported in Chapter III prove the effectiveness of the supramolecular approach for the modulation of the CO2 capture properties of polyamine solutions.

Polyguanidinium salts were characterized as efficient supramolecular templates for bicarbonate. These additives induced a Cyclic Capacity increase (up to 15%) in polyamine aqueous solutions while maintaining fast absorption kinetics. The prospects for the follow-up of this project aim at increasing the applicability of these additives by finding a highly water soluble and thermally resistant salt which could fulfil the industrial requirements for a large-scale amine scrubbing process. The ideal scenario would be the development of a universal polyguanidinium additive that could be added in high concentration to amine solutions to improve their Cyclic Capacity while maintaining the system homogeneous both in the lean and rich state.

Despite the efforts, we did not manage to find a template for polycarbamates able to induce a cooperative capture mechanism in water. The most relevant results for the association of CoDX3 complexes with carboxylates were obtained in organic solvents, while in water the binding constants were small and difficult to detect, which should translate into negligible effects in the amine solutions for CO2 capture. Our experimental results, in agreement with the literature available on the subject, suggest that polycarbamates can only be stabilized in organic solvents in anhydrous conditions, while in water the generation of zwitterionic monocarbamates or bicarbonate is the most favoured binding event. That is why the following efforts for the improvement of the CO2 capture properties of (poly)amine solutions should focus on the amplification of bicarbonate, as a more accessible target compared to polycarbamates.

Scheme 47: Displacement of the disproportionation equilibrium of 1-diam to damm and bicarbonate by A4, which acts as template for diammoniums (damm).

The future prospects for the implementation of supramolecular chemistry to CO2 capture are based on the development of template species for diammonium ions. This idea started from the observation that the most destabilizing contribution to eq.15-d (disproportionation of mono into dicarbamates, Scheme 15, page 34) came from the generation of diammonium ions.

Previous work in our group had assessed the high binding constants (10 6 -10 7 M -1 ) for the complexation of -diammonium ions by A4 (2,3,5,6,8,9,11,12-octathia-1,4,7,10(1,4)tetrabenzenacyclododecaphan-12,15,42,45,72,75,102,105-octacarboxylic acid), a water soluble tetrameric paracyclophane obtained by the thiol to disulfide oxidation of 1,4-dithiophenol-2,5dicarboxylate units (Scheme 47). 144 Ongoing experiments are studying the influence A4 addition to polyamine-CO2 libraries in water. The preliminary results, obtained by qNMR, suggest that A4 induces the complete cleavage of the carbamate bonds, which results in the insertion of the protonated polyamines in the cavity of the supramolecular host (Scheme 47). Released CO2 converts to bicarbonate to compensate the positive charges of the polyammoniums cations, and the system displays an increased capacity of one CO2 captured for each primary/secondary amine group.

Even though these experiments should be replicated and optimized, they represent an additional proof of the potential of supramolecular effects for the improvement of CO2 capture solutions.

after few hours in aerobic conditions, i.e. exposed to dioxygen from air, and after Iron removal, suggested the presence of an oxidization process involving on the amine substrate.

In sealed conditions, i.e. in the presence of a limited amount of air in the headspace of the reaction container, the solutions, which initially presented a bright red color characteristic of Fe(III) complexes, became transparent. Some crystals could be isolated, and their XRD analysis showed the generation of the Bis(Diethylenetriamine)Fe(II) dichloride complex, firstly characterized by us, where Iron was in the reduced Fe(II) state (Fig. 94). These preliminary observations highlighted the presence of a spontaneous aerobic reaction in solution, involving the redox couple Fe(III)/Fe(II) and the amine substrate. The DETA-FeCl3 system proved to be inappropriate to perform redox-triggered CO2 capture/release but attracted our interest in the study of this unexpected reactivity.

Objectives

Chapter IV will present the most relevant results obtained from the study of the aerobic reaction between DETA and FeCl3 in MeOH.

Despite being based on simple compounds (FeCl3, DETA, MeOH, O2), the reaction led to the generation of structurally complex products. The analysis of the system was far from trivial due to the hydrophilic nature of both reactants and products, and the subsequent difficulties with their isolation and purification.

Our main objective was to characterize the products issued from the reaction and to understand its mechanism. We also wanted to explore the specificity of the reaction toward different amine substrates, in order to apprehend its scope of applicability.

Finally, we aimed at optimizing the reaction conditions so as to propose a meaningful application of the reactive system to the synthetic or catalytic world.

Achievement

The results reported in this Chapter were obtained during this PhD and during the internship of Mr Nathan de Riggi (March -August 2021).

A standardized preparation and treatment procedure was developed to enable meaningful comparison between different aliquots and to eliminate Iron(III) from solution in order to enable NMR analyses. Two internal references were used to calculate the conversion and the yield of the reaction by 1 H NMR. The standardized protocol was applied to a scope of differently substituted polyamines to gain insight on the key moieties required for the reaction.

Mass Spectrometry (MS) and 1 H NMR were exploited to monitor the evolution of the species with the time of reaction in aerobic and anaerobic conditions. The data retrieved by HPLC-MS (Highperformance Liquid Chromatography -MS) and by bidimensional (2D) NMR analyses enabled to identify the most abundant products of the reaction and to propose a plausible mechanism, in agreement with the literature concerning Iron-catalyzed imidazole syntheses.

I. Bibliographic study 1. Imine syntheses

Imines, also known as Schiff bases, are versatile compounds often used as synthons in pharmaceutical syntheses. 150 Imines are traditionally prepared through condensation reactions between carbonyls and amines (Fig. 95a). Condensation is a thermodynamically controlled equilibrium: to increase yield and reaction rate, dehydrating agents or Lewis acids are often added to the reactive systems. 151,152 Several efforts were made in the last decades to develop one-pot procedures for imine synthesis from alcohol and amine oxidation. The synthetical approaches can be gathered into three groups (Fig. 95) 150 : b) Oxidative cross-coupling of alcohols with amines: the only by-product is water, and the reaction tolerates different starting substrates. The main challenge is the selectivity of the oxidation of alcohols to aldehydes. c) Oxidative self-coupling of primary amines. Homocoupled imines are easily obtained while heterocoupled ones are less accessible. Many by-products can be formed in this reaction (nitrile, amide, azo compounds).

d) Oxidative dehydrogenation of secondary amines, which usually displays a high selectivity as nitrile formation is prevented. Imine formation can be promoted by metal catalysts. Most metal-catalyzed syntheses employ O2 as oxidant, but some noble-metal catalysts (Ir, Rh, Ru) can promote amine and alcohol dehydrogenation under inert atmosphere.

This approach can lead to the re-hydrogenation of the condensation product to an amine, through a "borrowing-hydrogen" (BH) process. 153 Scheme 49: Borrowing Hydrogen (BH) and Acceptorless Dehydrogenative Coupling (ADC) strategies for C-N bond formation.

This strategy (Scheme 49) usually involves the dehydrogenation of an alcohol (1-38), operated by a catalyst which "borrows" H2 (2-38). The aldehyde undergoes condensation with an amine (3-38), which results in the generation of an imine. The imine can be reduced by the metal catalyst with the H2 borrowed from the alcohol (4-38).

Alternatively, if the dehydrogenation results in the generation of an aromatic system, the last hydrogenation step does not occur and H2 is liberated as a by-product (5-38). 153 The second pathway is defined Acceptorless Dehydrogenative Coupling (ADC) and it constitutes an efficient method for the generation of N-heteroaromatic compounds. 154

The dehydrogenation methods are highly atom economic, but usually require expensive metal complexes as catalysts. 153 The literature concerning imine synthesis is extremely rich. This bibliographic section will present some selected examples of amine oxidation for imine and imidazole syntheses with Iron-based catalysts, which are the most relevant references for the FeCl3polyamine reactive system which will be presented in the Results and discussion section.

Aerobic amine oxidations promoted by Iron catalysts

Aerobic imine syntheses are advantageous with respect to other synthetic pathways as they exploit O2 from air as non-polluting oxidant, they can be catalyzed by abundant metals, they take place in mild conditions and usually generate water as by-product. 150 Many studies presenting Iron catalyzed aerobic imine syntheses are reported in the literature.

A remarkable examples is the Fe(NO3)2 and TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl) catalyzed aerobic synthesis of imines developed by Zhang et al. (Scheme 50). 155 This system, based on sustainable reactants and catalysts, gave access to a panel of differently substituted imine products. Good yields were obtained for the homocoupling of primary amines, up to 97% for benzylamine (1-39) and for the cross-coupling of benzylamine with benzyl alcohol in the presence of catalytic amounts of KOH (2-39).

Even though the role of the Iron catalyst and of TEMPO should have been postulated, no mechanism was proposed to explain this reactivity.

Scheme 50: (1-39) Primary amine homocoupling and (2-39) primary alcohol and primary amine condensation with a Fe(NO3)3 /TEMPO catalytic system. 155 Another iron-based method for alcohol and amine oxidation (Scheme 51, next page) was proposed by Zhai et al. 156 In this case the Fe(III) catalyst, ((NH4)3[FeMo6O18(OH)6], I40), was supported by inorganic materials (POM= polyoxometalate).

A control experiment showed that the system did not react in anaerobic conditions, thereby ruling out the role of Fe(III) as an oxidant.

A mechanism, inspired from Iron enzymatic catalysis 157 , was proposed by the authors. I40 requires activation by molecular oxygen, which coordinates to Fe(III) delivering a Fe(IV) peroxide complex (II40, Scheme 51). This highly reactive species undergoes O-O heterolysis, generating a Fe(V) oxocomplex (III40). The high valence Fe(V) catalyst is the key active species which coordinates the amine substrate (IV40) and oxidizes it into an imine.

The catalyst then releases the oxidized substrate and a water molecule, regenerating the starting Fe(III) complex. After being released from the catalyst, the imine is hydrolyzed and then undergoes condensation with an amine molecule (3-40), resulting in the final homocoupled imine product.

Scheme 51: Reaction and mechanism proposed for the alcohol/amine oxidation operated by the Fe(III)POM catalyst. 156 To overcome the large energy barriers for the direct oxidation of amine substrates to imines with O2, nature employs multiple enzymes and coenzymes as Electron Transfer Mediators (ETM).

An impressive example of aerobic imine synthesis by ADC, catalyzed by Iron and inspired by the respiratory chain, was recently published by Guethmundsson et al. (Scheme 52, next page). 158 In this case, the catalytic system was based on a Fe(III) catalyst with a hydroquinone/Cobalt complex as ETM. Various primary and secondary amines could be oxidized in the optimized conditions with good to excellent yields (NMR yields, 70-95%).

The first step of the oxidative mechanism is the activation of the Fe(III) catalyst (I41) by CO abstraction, operated by trimethylamine N-oxide (TMANO), delivering the active catalyst I41 ' . I41 ' can catalyze the oxidation of the amine substrates to imines by dehydrogenation (1-41). The Co/hydroquinone complex (II41) acts as ETM by re-oxidizing the reduced Iron catalyst I41'H (2-41).

The ETM is then regenerated by reaction with molecular oxygen, with the formation of H2O as sole by-product of the reaction.

The examples reported in this paragraph show the possibility of exploiting Iron complexes or salts to catalyze the aerobic oxidation of amines, yet they usually require a cocatalyst (TEMPO, II41) or sophisticated Iron(III) complexes.

The following bibliographic study will present Iron-catalyzed syntheses of heterocyclic compounds, in particular imidazoles, as more pertinent examples to our experimental results.

The reaction then proceeded through condensation of the aldehyde with one of the aromatic amines (2-42), followed by the intramolecular ring closure (3-42) of a five-membered imidazoline cycle. A second ADC (4-42), catalyzed by I42 ' , afforded the aromaticity of the final benzimidazole product.

Iron trichloride (FeCl3) is an efficient metal catalyst applied to a growing number of chemical transformations. 161,162 Two studies exploiting this easily accessible catalyst to obtain imidazolecontaining products are presented hereafter.

Zhu et al. reported a procedure for the synthesis of tetrasubstituted imidazoles from amidines and chalcones, through FeCl3/I2 catalyzed oxidative coupling in aerobic conditions. 163 In aerobic conditions, the products were obtained with high selectivity and good yields (85% for the reaction in Scheme 55). The yield dropped to 14% in anaerobic conditions, indicating the fundamental role of O2 for the reaction. The addition of TEMPO as radical scavenger did not affect the yield, thereby ruling out a radical mechanism.

Scheme 55: FeCl3/I2 catalytic system for the synthesis of tetrasubstituted imidazoles from oxidative coupling of aldimines and chalcones in aerobic conditions. 163 Quenching of the reaction after 2 hours showed imidazolines (343, Scheme 55) as the most abundant products, while after 10 hours they were fully replaced by imidazoles (443). This suggested that the reaction underwent in the first hand the cyclization of the amidine and chalcone to form imidazolines, followed by oxidative dehydrogenation to obtain the imidazole.

The complex mechanism proposed for this system is presented in Scheme 55. Similarly to the previous examples, Fe(III) plays an indirect role in the oxidation of the amine, by activating the chalcone (543) towards the addition of the amidine. The addition leads to intermediate 643, which undergoes tautomerization generating enol 743. The latter should react with iodine, delivering 843. This species undergoes intramolecular cyclization, resulting in imidazoline 343 and liberating Iodine. The final imidazole product, 443, should be obtained by aerobic oxidation of 343, while I2 would be regenerated by redox reaction with Fe(III). Fe(II) can be reoxidized by O2 to complete the catalytic cycle.

This mechanism is hypothetical and should be confirmed by experimental evidence, especially regarding the last oxidation step from 343 to 443, which was not explained by the authors. Arguably, FeCl3 could play an active role in the activation of the substrate towards oxidation with O2, as demonstrated by the previous and following examples from literature.

Recently, Foss Jr. et al. reported a FeCl3-catalyzed aerobic cross-coupling of mono and disubstituted phenylenediamines (PDA) for the synthesis of 1.2-disubstituted benzimidazoles (Scheme 56). 164 The Iron salt, in its hexahydrate form, was the sole catalyst of the reaction, which was conducted in DMF at 90-100 °C and resulted in elevated yields for differently substituted PDAs (up to 90%, calculated by NMR).

Scheme 56: Benzimidazole synthesis by cross-coupling of ortho-disubstituted phenylenediamines in aerobic conditions, catalyzed by FeCl3*6H2O. 164 The catalytic mechanism presented in Scheme 56 was supported by UV-Vis and EPR analyses. The first step of the mechanism is the coordination of PDA to Fe(III) (1-44). A ligand-to-metal charge transfer (LMCT, (2-44)), results in the generation of a Fe(II) complex. The coordination of the Fe(II) complex by O2 (3-44) generates a high valence Fe(IV)-oxo complex.

A high-valence Iron intermediate was also present in the mechanism proposed by Zhai et al. (Scheme 51).This high valence active species reminds the Compound I intermediate observed in Horseradish (HRP) and lignine peroxidase, where a low-spin Fe(IV)-oxo complex is coordinated by a porphyrin radical cation (P . ). 165 A second LMCT (4-44) completes the oxidation of the organic substrate and reduces Iron back to the original state, i.e. Fe(III). The oxidized PDA (PDA 2+ ) is released from the complex with H2O2 as byproduct (7-44). PDA 2+ undergoes intramolecular cyclization (8-44) delivering a benzimidazoline intermediate, which then is subject to a second aerobic oxidation catalyzed by Fe(III), resulting in the final benzimidazole product (9-44).

All the aerobic, Iron catalyzed reactions presented in this bibliographic study involved Fe(III) as an initiator of the reaction or mediator of the electronic transfer to and from the organic substrates, but molecular oxygen was always required to achieve the activation of the catalyst and the oxidation of the substrates. Imidazoles were obtained from Fe(III) catalyzed reactions starting from preorganized organic substrates (PDA), thereby requiring previous synthetic steps, or in the presence of complex ligands (I42) or co-catalysts (I2).

The experimental data we retrieved concerning the aerobic reaction of DETA with FeCl3 in methanol, which will be discussed hereafter, showed the generation of imidazoles from simple aliphatic polyamines through a FeCl3 catalyzed aerobic process in the absence of cocatalysts or additional ligands. The optimization of this catalytic mechanism could provide a simpler and more sustainable alternative for imidazole synthesis compared to the examples reported in this bibliographic section.

II. Results and discussion

Originally, the spontaneous reactivity between DETA and FeCl3 was detected in methanolic solutions containing the amine in 0.5 M concentration and substoichiometric quantities (33% mol) of anhydrous FeCl3. Other solvents (EtOH, H2O, toluene, MeCN, DMF, DMSO) and Fe(III) salts (FeCl3*6H2O, Fe(NO3)3, Fe(ClO4)3, Fe(TfO)3) were tested, but most of the combinations with DETA resulted either insoluble or unreactive. Ongoing experiments are being performed to evaluate the influence of other solvents and reactants on the system, but they are not included in this dissertation.

The following discussion will present the results that were obtained from the study of the FeCl3 -DETA solutions in methanol, the system that was studied the most during this thesis.

Standardized preparation and work-up of FeCl 3 -DETA solutions in methanol

In solution, Iron should coordinate to DETA: the estimated constants for the binding of polyamines to Fe(III) are in the 10 25 -10 35 range (Kc, Scheme 57). 149 In order to perform NMR analyses and to quantify the organic species, the decoordination of Fe(III) and its removal from solution were required. The most straightforward method we identified for the work-up of the reaction was to precipitate Iron(III) as a hydroxide. Fe(OH)3 precipitates from acidic pH in water (Kps=1.6*10 -39 , for [Fe 3+ ]=0.1 M the precipitation starts at pH=1.3), but in methanol the addition of an excess (10 equivalents) of aqueous NaOH was required to break the coordinative bonds and to reach a basic pH, granting a complete precipitation of the metal (Keq, Scheme 57).

A standardized procedure was developed to reliably process the FeCl3-DETA solutions as to enable the comparison between different aliquots. The procedure, briefly presented in Scheme 57 and discussed hereafter, is detailed in the Experimental Section.

Scheme 57: Schematic standardized procedure for polyamine-FeCl3-MeOH solutions processing. R1 (TMPCl) and R2 (Pinacol) indicate the two internal references used for NMR quantification (Fig. 96).

The solutions were prepared in round bottom flasks by adding precise quantities of amines to a stirred FeCl3 methanolic solution. For the experiments in aerobic conditions, the flasks were filled to half their volume, to maximize the contact surface of the solution with air, they were left uncapped, and the solution was stirred at a fixed 500 rpm rate. At the end of the reaction, concentrated aqueous sodium hydroxide was added in solution to precipitate Iron as Fe(OH)3.

The resulting suspension was centrifuged to separate solid Fe(OH)3 aggregates from the solution containing the organic products. The liquid fraction was retrieved, then methanol was evaporated up to 1/5 of the initial volume. The sample was filtered on Celite to eliminate the remaining solid residues, then the solvent was evaporated to dryness. The dried sample was solubilized with distilled water in a graduated flask: the precise final volume was required to enable quantification by NMR.

For NMR quantification purposes, two reference compounds were exploited:

-R1: Tetramethylphosphonium Chloride (TMPCl, Fig. 96) was used as internal reference to calculate the yield by NMR. R1 was added to the solution at the beginning of the reaction in order to enable the absolute quantification of amine conversion and products formation. R1 is soluble in MeOH, unreactive towards the substrates and displays 1 H and 13 C NMR signals in a non-overlapping spectral range with respect to reactants and products.

[TMPCl 1 H NMR (D2O): δ 1.83 ppm (d). 13 C NMR (D2O): δ 8.65 ppm (d)].

-R2: Pinacol was used as a second internal reference. A precise volume of a R2 stock solution (D2O) was added to a precise volume of the solution obtained at the end of the standardized treatment (aliquot taken from the graduated flask, Scheme 57). The ratio between R2 and R1 integrations was exploited to determine the loss of organic species due to the precipitation procedure. The NMR signals of pinacol did not overlap with the ones of the substrates/products or R1.

[Pinacol 1 H NMR (D2O): δ 1.14 ppm (s). 13 C NMR (D2O): δ 23.89 ppm, 72.62 ppm] The preliminary analyses of the reaction, such as the kinetic monitoring by 1 H NMR and HRMS, were conducted on 0.5 M DETA solution containing 33% mol of FeCl3. In these conditions, the reaction was complete, i.e. no further evolution of the species observed by 1 H NMR or MS monitoring after 4 hours from preparation.

After testing different catalyst concentrations, we adopted a 10% mol FeCl3 with respect to DETA (0.5 M) as a more relevant catalytic charge. In these conditions, the same products as in the more concentrated catalytic system were obtained, but at a slower rate, as the reaction was complete in 16 hours.

The most significant results obtained from the study of the reaction are presented hereafter. A qualitative monitoring of the evolution of the organic species in solution with time was performed by 1 H NMR on a DETA 0.5 M solution containing FeCl3 (33% mol) in deuterated methanol (D4-MeOD).

For the monitoring in aerobic conditions (detailed procedure in Exp), the solution was stirred magnetically at 500 rpm in an open round bottom flask. 100 l aliquots were extracted regularly from the solution and they were treated with deuterated sodium hydroxide (NaOD 40% w/w) to eliminate Fe(III). 600 l of MeOD were added to solubilize the organic species and the resulting mixture was filtered on Celite to recover the liquid fraction, which was analyzed by 1 H NMR.

The stack of 1 H NMR spectra recorded upon increasing reaction time is presented in Fig. 97. The spectral areas showing the strongest variations are the "aliphatic" upfield R-(CH2)2-R range, between 2 and 4 ppm, and the "aromatic" downfield R-CH=R range, between 6.5 and 8 ppm.

The signal relative to DETA (at t0, symmetric multiplet, 2.6 -2.8 ppm) decreased in intensity with the passing of time, indicating the amine's consumption. A singlet at 2.7 ppm, corresponding to a byproduct, appeared in the spectrum after 30 minutes of reaction. After 4 hours this was the most intense signal, in an area presenting a high number of low-intensity peaks (2.6 -4.3 ppm). By combining 1 H and 13 C NMR, which showed uncoupled singlets in the appropriate spectral range ( 1 H (300 MHz):  2.53 (s) 13 C (300 MHz):  42.89) and ESI-MS analyses, the 2.7 ppm 1 H signal was assigned to the protons (HE) of Ethylenediamine (EDA).

After one hour of reaction three signals appeared in the "aromatic" (R-CH=R) range around 6.5 -8 ppm (Fig. 97). A singlet also appeared at 8.4 ppm, a spectral range characteristic of protons bound to electron withdrawing (EWD) groups.

A 13 C DEPT (Distortionless Enhancement by Polarization Transfer, available in Annexes, Fig. A24,A25) analysis performed at the end of the reaction highlighted the presence of eight saturated sp 3 aliphatic carbons in the 38.5 -51.7 ppm spectral range and of three saturated -CH= carbons between 115 -145 ppm.

A second 1 H NMR qualitative monitoring performed under N2 atmosphere (Annexes, Fig. A23) showed that the system was unreactive in anaerobic conditions, highlighting the vital role of molecular oxygen in the reaction mechanism.

These preliminary observations suggest that DETA undergoes oxidization in aerobic conditions, resulting in the generation of products presenting an aromatic structure. The oxidation of the amine is mediated by the Fe(III)/Fe(II) redox couple, whose participation in the reaction was proven by the isolation of the [Fe(DETA)2Cl2] complex, generated spontaneously in anaerobic conditions. The attribution of the 1 H and 13 C NMR signals to the corresponding products was completed by bidimensional (2D) NMR (following paragraph).

2D NMR analyses for products identification

Bidimensional NMR experiments (COSY, HMBC, HSQC) were performed after work-up at completion of the reaction to characterize the structure of the products. The 2D NMR spectra presented in Fig. 98 led to the attribution of the three 1 H and 13 C aromatic signals to N-substituted Imidazole (Im) derivatives. COSY NMR highlights the inter-correlations between the three aromatic protons (Fig. 98,1 Ha, 1 Hb, 1 Hc). HSQC shows that the same three protons correlate with the three unsaturated carbons observed at  120 ppm ( 13 Cc), 128 ppm ( 13 Cb) and 139 ppm ( 13 Ca). HMBC confirms the attribution by highlighting the J2-J3 correlation of Ha with Cc and Cb, of Hc with Cb and Ca, of Hb with Ca.

Some correlations between Im 13 C aromatic signals and aliphatic protons were highlighted by the COSY and HMBC spectra. The correlations, though weak in intensity, could be observed up to the second exocyclic carbon ( 1 He), suggesting that Im were N-substituted with aliphatic side arms. The spectra proving these correlations are reported in the Annexes (Fig. A26).

The presence of different aliphatic side arms on the Im products justifies the low resolution of the 1 H NMR spectra in the upfield, aliphatic range, as the protons of the ethylenic groups (R2N-(CH2)2-N R2) will display a similar chemical shift and each of them will be subject to J3 coupling.

The 1 H singlet at 8.4 ppm did not correlate with other protons in the COSY experiment but it correlated with a downfield 13 C signal at 171 ppm in the HSQC spectrum (Annexes, Fig. A27). These signals were attributed to sodium formate (NaHCO2) after verification by adding the commercial product to the NMR tube and observing the correspondence of the chemical shifts.

The oxidation of MeOH to formaldehyde ((1-45), Scheme 58), which could eventually undergo a second oxidation leading to formate production (2-45), agrees with the generation of imidazole (Im) structures, which require a C1-source for the closure of the five-membered cycle starting from DETA ((3-45), (4-45), (5-45), Scheme 58).

Scheme 58: Plausible condensation of DETA with formaldehyde and intramolecular transimination leading to the generation of a N-substituted imidazole (Im).

1 H NMR Quantification

The 1 H and 13 C NMR analyses indicate that a mixture of differently substituted products is generated from the reaction. The isolation and quantification of the products represents the main unresolved challenge of the reactive system at this day.

The separation is complicated by the hydrophilicity of both reactants and products, making liquid/liquid separation ineffective, and by their structural similarity, as all display multiple R2NH/R-NH2 or imine groups separated by ethylenic fragments, rendering the purification by chromatography untrivial.

The derivatization of the amine groups by reaction with ethyl chloroformate, delivering ethyl carbamates, followed by GC-MS analysis was attempted, but we did not succeed with the attribution of the great number of peaks to the corresponding species. 166 The standardized precipitation of Iron with an excess of NaOHaq, followed by centrifugation and filtration, and the analysis of the final sample by NMR with two internal references (R1 and R2, Fig. 96) is, at the present day, the most straightforward treatment that we could develop for the quantification of the products.

The NMR signals corresponding to DETA were absent both in the 1 H and 13 C spectra at the end of the reaction, which suggested that the amine was fully converted during the reaction.

R2 was exploited to evaluate the loss of organic species after the treatment to remove Iron (Detailed calculation in Exp). The comparison between R2 and R1 integrations highlighted a 39.2 % loss with respect to the initial R1 concentration. To verify that the organic species were retained to the same extent of R1, the 1 H NMR signals of Im and EDA were integrated, normalized with the corresponding number of protons and related to R2 to calculate the total concentration of organic molecules in the final sample. This calculation also indicated a 41.1 % loss with respect to initial DETA concentration, proving that the retention of the species is not chemoselective.

However, the NMR spectra might not be fully representative of the sample composition, especially if the mass loss involved selected species which could be preferentially retained during the separation procedure.

Scheme 59: Schematic reaction of DETA with FeCl3 10% mol in MeOH in aerobic conditions generating N-substituted imidazoles (Im).

The approximate reaction yield, calculated as the ratio between Im concentration at the end of the reaction ( 1 H qNMR) with respect to initial DETA concentration was 11.7 % (details of the calculation in Annexes, page 233). The mass loss quantified by R1/R2 comparison should be considered to justify the low yield. In addition, Im could be obtained by alternative mechanisms involving multiple equivalents of DETA, which could influence the calculation of the yield.

The following paragraphs will present some further analyses performed by MS, which provided additional data about the structure of the products and the reaction mechanism.

The most intense peaks were assigned to the corresponding molecular formula with high precision, which can be appreciated in the table reported under each spectrum, showing the experimental and calculated m/z with the related error.

At t0 (Fig. 99), most of the species detected were coordination compounds of DETA with Iron in its oxidized (Fe(III), orange in Fig. 99) or reduced (Fe(II), green in Fig. 99) state, indicating that the redox activity of the metal started as soon as the reactants were mixed. In the oxidized state, Fe(III) was coordinated by one DETA molecule and by some smaller ligands, such as Cl -, MeOH or H2O (m/z=189.0562, Fe(DETA)(MeO) + , Fig. 99). In the reduced state, probably because of its higher ionic radius, Fe(II) was coordinated by one or two DETA molecules (m/z=297.1256, Fe(DETA)2Cl + , Fig. 99).

Uncoordinated amines such as DETA, EDA and PZ were observed at low m/z. Amongst the organic species, the peak at m/z=145.1451 corresponded to o-TETAH + (C6H17N4 + oxidized triehylenetetramine), which can be obtained through the reactions listed in Scheme 60.

In the following Schemes, the species will be represented in their protonated state to enable the comparison with the experimental m/z of the MS spectra. At this stage, the oxidant species (Ox) is left unassigned and will be described further on in the discussion. ). An EDA fragment is eliminated, resulting in the generation of o-TETA (3-46). The generation of EDA as a byproduct of transimination agrees with its detection by 1 H and 13 C NMR at the end of the reaction and by HRMS at t0 (Fig. 99).

The mechanism in Scheme 60 is made explicit for the condensation of two DETA molecules, but the nucleophile agent of (1-46) could also be EDA, ammonia or any of the primary amine moieties in solution.

Among the low molecular weight species, the 128.1185 peak at t0 (Fig. 99) corresponds to the molecular formula C6H14N3 + . Multiple structures could be written for this same m/z, but they should present at least one cyclic moiety to justify the 2:1 ratio between carbon and nitrogen atoms (as C-C bonds can neither be formed nor broken in our experimental conditions). quaternary iminium is obtained. This highly reactive group is rapidly attacked by the primary amine attached on the aliphatic side arm of the piperazine, resulting in the generation of an imidazoline cycle condensed with the piperazine. The final polycyclic product (I1) corresponds to the experimental m/z with high precision.

The CRYO-HRMS spectrum at reaction completion (t= 4 hours) is presented in Fig. 100. At the end of the reaction, most peaks in the spectrum correspond to unsaturated or polycyclic structures.

With increasing reaction time, I1 is replaced by a species presenting a m/z inferior by exactly 2 u.m.a. units (m/z= 126.1023, I2, Scheme 62). This species should be related to the 251.1974 peak observed in all MS spectra starting from t=1 h, whose m/z equals two times 126.1026. Some species with an odd number of carbon atoms were detected at t=4 hours (114.1023 C5H12N3 + Im1, 155.1288 C7H14N4 + Im2, Fig. 100), an observation consistent with the generation of Im derivatives from DETA condensation with methanol, which had been previously evocated to justify their detection in the NMR spectra (Scheme 58, page 163).

The results of the CRYO-MS monitoring agreed with NMR observations. Most of the fragments observed by MS could be attributed to structures containing an imidazoline (I) or imidazole (Im) moiety. The presence of ethylenic fragments in the polycyclic species (I1, I2, 2I2) and as substituents on the Im structures (Im1, Im2) contributes to the signals observed in the aliphatic range of the 1 H and 13 C NMR. Fully oxidized imidazoles (Im1, Im2) were detected at the end of the reaction, justifying the presence of the aromatic 1 H and 13 C signals in the NMR spectra.

Mass Spectrometry is not a quantitative measurement: the height or even the presence of a peak in the MS spectrum strictly depends on the ionization properties of the corresponding species, but not on its concentration. To gain some further insight on what species were most abundant in solution, we performed an HPLC-MS experiment (HPLC= High Pressure Liquid Chromatography).

HPLC-MS separation for the identification of the most abundant products

A partial separation of the products issued from the reaction of DETA with FeCl3 in methanol in aerobic conditions was achieved by HPLC (elution conditions in Exp), which was coupled with HRMS to identify the products. Even though the separation of the products was not fully achieved, it enabled the evaluation of the most and least abundant species in the mixture. Even if the structure of these species requires further analyses to be attributed, I2 seemingly does not contain an aromatic moiety.

Scope of polyamines tested in standardized reaction conditions

The standardized reaction and workup protocol was applied to a series of commercial polyamines, presented in Fig. 103. The objective of this screening was to determine which fragments of the amine backbone were essential for the reaction, in order to increase its applicability. The benchmark reaction was performed with DETA. In the presence of tertiary amine groups, such as with MDETA and IDETA, no Im or imine formation was detected. This could depend on the higher steric hindrance of tertiary amines compared to primary and secondary ones, which could influence their coordinative character and consequently the interaction with the metal catalyst. In addition, the bibliographic study concerning the degradation of polyamines for CO2 capture (Chapter II, page 51) suggests that tertiary amines are subject to a lower oxidation rate than primary and secondary ones.

The addition of PMDETA and TEDETA to the FeCl3 methanolic solution resulted in an instantaneous precipitation phenomenon. The system could not be homogenized despite heating and acidification.

Di-(2-picolyl)amine also resulted unreactive, probably because of its stronger steric hindrance compared to DETA, which hampered its coordination to the metal and the subsequent oxidation and transimination reactions.

A unitary increase of the number of carbon atoms between amine groups (N-(EDA)-1.3-DPA) hampered the oxidation, suggesting that the reaction requires a substrate with two amines (primary and secondary), separated by an ethylene fragment.

Traces of Im were detected in the 1 H NMR spectra after 16 hours of reaction with EDA and TETA.

Despite their structural similarity to DETA and the proof of their involvement in the reaction mechanism (HRMS spectra), these amines undergo oxidation at a significantly slower rate.

At the present day, DETA is the amine displaying the most efficient reactivity with FeCl3 and MeOH.

The remarkable selectivity of the reactive system to the amine substrate suggests that the concomitant presence of primary and secondary amines is essential for the reaction. The former act as nucleophiles towards the latter, that are oxidized to imines.

In addition, the two carbon atoms spacing between amine moieties enables the obtention of the five membered Im cycle by condensation with formaldehyde, or the 6-membered cycles by intramolecular transimination. The requirement for an amine substrate with a C2 ethylenic spacer could also depend on the stronger binding constants that these ligands display for small 1 st row dmetals (Fe(III)), while metal cations with a higher ionic radius (Hg, Pb) are known to coordinate to 3C or 4C polyamines.

Further tests are being performed to extend the applicability of the reaction by identifying other active substrates or alternative reaction conditions.

Aerobic oxidation-transimination mechanism

The results obtained by NMR and MS analyses enabled the construction of mechanisms justifying the formation of imidazoles (Scheme 58) and heterocyclic products (Scheme 62, 63, 64).

The plausible role of Iron in the reaction is presented in Scheme The same mechanism, involving Iron switching from the (II) to the (III) and (IV) oxidation state, can be exploited to justify both methanol and amine oxidation. The plausible steps are detailed hereafter for methanol oxidation (O1, Scheme 63).

In the beginning, solvated Fe(III) coordinates to MeOH (1a-49): this assumption is confirmed by the observation of some Fe(III)-MeOH coordination compounds in the HRMS spectrum at t0 (Fig. 99). Upon a ligand-to-metal charge transfer (LMCT, (2a-49)), Iron is reduced to Fe(II) and a radical cation MeOH .+ is obtained. Molecular oxygen intervenes by coordinating to the metal (3a-49), which is oxidized to the high valence Fe(IV) state. A proton transfer from MeOH .+ to O2 generates an intermediate oxo-complex. The strong oxidative character of Fe(IV) induces a second LMCT (4a-49), which reduces the metal back to the original Fe(III) state and completes the oxidation of methanol.

Formaldehyde is eliminated with hydrogen peroxide as by-product of the oxidation, and the catalyst is restored (5a-49).

Formaldehyde could either react with amines through condensation or undergo a second oxidation pathway (O1) delivering formic acid, which was detected by NMR.

The same mechanism (O2, (1b-49)-(5b-49)) can be exploited to explain the oxidation of amines to imines, and thus the generation of aromatic Im structures.

In the mechanism, Iron acts as a mediator of the electron transfer from the ligands (MeOH, amine) to molecular oxygen. The coordination of the metal by O2 is required to oxidize the metal complex to the high-valence Fe(IV) state and consequently to induce the complete oxidation of the organic substrates.

At the present day this mechanism, even if self-consistent, remains theoretical. To confirm the hypotheses, data accounting for H2O2 or Fe(IV) generation, for example by EPR spectroscopy, should be obtained. The characterization of the Iron complexes involved in the oxidation catalysis could help to assess the specificity of the system, which at the present day is limited to DETA, and hopefully to increase its applicability.

The generation of Im structures from DETA reminds the degradation to cyclic urea discussed in the bibliographic section of Chapter II (Fig. 17, page 38). Arguably, the catalytic system presented in this Chapter could represent a scale up of the thermal degradation of DETA induced by the reaction at high temperature with the stainless-steel walls of the PCC tanks.

From another perspective, this reaction, which starts from simple synthons and generates a variety of differently oxidized and substituted compounds, catalyzed by Fe(III) with dioxygen, could remind some prebiotic mechanisms. 166 The generation of polycyclic imidazolines and imidazoles from the condensation of linear aliphatic amines with formaldehyde could resemble the primordial generation of nitrogen bases.

III. Conclusion to Chapter IV and future prospects

In Chapter IV we identified a novel catalytic system to access N-substituted imidazoles and polycyclic compounds starting from DETA and methanol, which react spontaneously through oxidation and transimination reactions catalyzed by FeCl3 in aerobic conditions. The monitoring experiments performed by 1 H NMR and CRYO HRMS, the 2D NMR experiments and the HPLC-MS analyses led us to the identification of the main products generated by the reaction and to the elaboration of a mechanistic proposal inspired by the available literature on similar systems.

From a synthetical perspective, the reaction, at the present day, presents unresolved issues mostly related to the low selectivity and yield. The main perspectives for the completion of this study are focused on the optimization of the reaction conditions and on the extension of its applicability. An efficient method to isolate the products from the mixture should also be developed in order to propose a meaningful application of this synthetic procedure. The reaction we analyzed in this Chapter enables the generation of molecular complexity starting from simple linear molecules resembling natural polyamines. The condensed polycyclic structures detected by MS are structurally close to nitrogen bases and to the PLP (Pyridoxal phosphate) coenzyme (Fig. 104). An interesting perspective would thus be the study of this system from a prebiotic approach, looking for similarities between the oxidative abiotic metabolism that we identified in this Chapter and the reactions involved with the origin of life.

Chapter V: Integrated CO 2 capture and electrochemical conversion by a supported Mn-bipyridil catalyst

Context

In an effort to contribute to the integrated CCUS field, we combined our knowledge of CO2 capture with amines with the expertise of the research group led by prof. Roberto Gobetto in Turin concerning the electrochemical transformation of CO2. [START_REF] Marocco Stuardi | Current Opinion in Green and Sustainable Chemistry[END_REF] The transformation of CO2 by catalytic electroreduction is a sustainable option to convert CO2 into high-added value products such as CO, formate (HCO2 -), MeOH and others. Usually, these processes are performed in electrolytic cells containing homogeneous or heterogeneous catalysts, wherein CO2 is introduced as a pure gas and it is reduced, at a negative potential, by electron transfer mediated by the catalyst.

The development of an integrated Carbon Capture and Recycling (CCR 167 ) process would drastically reduce the energy requirement of amine scrubbing by avoiding the desorption, compression and transportation of removed CO2 (Bibliographic section on Integrated CCUS, Chapter I).

As only few research groups had previously approached the electroreduction of CO2 on loaded amine solutions, we decided to tackle this untrivial challenge. I. Bibliographic study 1. CO 2 reduction CO2 conversion inevitably suffers from a high thermodynamic and kinetic barrier due to the intrinsic stability of the molecule. One of the available methods to transform pure CO2 is by the application of an electrochemical reduction process.

The one electron reduction of CO2 into radical anion CO2 .-occurs at a very negative potential in water (-1.9 V vs SCE). 169 Catalytic strategies have thus been developed to avoid the generation of CO2 ▪-and to obtain one of the many alternative products of CO2 reduction at an inferior energetic cost (Chart 1).

Chart 1: Standard Electrochemical Reduction potentials of CO2 vs NHE in water (Standard Hydrogen Electrode). 173

Electrochemical reduction can be exploited to convert CO2 to more reactive molecules such as CO, CH3OH, H2CO, H2CO2 or CH4, which all present a higher commercial value compared to the starting material. 174 For example, CO is used in combination with H2 in industrial processes such as MeOH production, while formic acid can be used for the production of fuel cells or for hydrogen storage materials.

Catalysis of CO 2 reduction by Mn bipyridyl complexes

The high energy barrier for CO2 reduction can be lowered with organometallic catalysts. 174 Coordination compounds of transition metals attracted particular interest as homogeneous catalysts, as they are easy to synthetize, their properties can be tuned by ligand modification and the reduction mechanisms can be easily investigated. 174 The reduction properties of metal complexes are usually studied under inert atmosphere by Cyclic Voltammetry (CV), which shows the evolution of the current at the working electrode (WE) upon a linear variation of the potential. The presence of negative current peaks during the negative potential sweep indicates the presence of a reduction process. Depending on the presence and position of a re-oxidation peak in the CV, the individual reductions of a complex can be "reversible", if a re-oxidation peak is observed in a close potential range to the reduction, or "irreversible", if the re-oxidation peak is either far from the reduction peak or absent.

These different behaviors depend on the chemical processes taking place in solution. A fast electron transfer will usually result in a reversible reduction process. A slow electron transfer, or an electron transfer followed by a chemical step which transforms the complex in solution (such as the loss of a CO2 + e--> CO2 The first monoelectronic reduction (r1) generates the anionic complex MnbpyBr -1 . This event is irreversible as the complex undergoes Br -loss (r2) generating a neutral species, Mnbpy 0 , which presents a coordination vacancy. Two Mnbpy 0 units dimerize spontaneously yielding Mnbpy2 0 (r3).

The dimer can be reduced through a reversible bielectronic process at E=-1.80 V in MeCN 5% H2O, leading to the pentacoordinate anionic complex Mnbpy -(r4).

The CO2 reduction activity of a catalyst usually manifests as a current increase in the CV under CO2 atmosphere, at a negative potential (E<0) corresponding to the generation of the active catalytic species. For Mnbpy, the current increase upon CO2 introduction was observed at E=-1.80 V, indicating that the anionic complex, Mnbpy -, generated from r4 was the active catalyst for CO2 reduction (Fig. 108).

A reliable evaluation of a catalyst's performance relies on Controlled Potential Electrolysis (CPE). This experiment consists in applying a constant reduction potential on the catalytic system and in monitoring the current (I) evolution and the generation of reduction products. Gaseous products such as CO, H2 or CH4 can be quantified in real-time by GC, while soluble products such as H2CO or H2CO2 are quantified a posteriori by chromatography or NMR analysis.

CPE enables the calculation of the TON (turnover number, mol of reduction product generated by a mol of catalyst) and TOF (turnover frequency, mol of reduction product generated by a mol of catalyst per time unit) for each reduction product, with the related Faradaic Efficiency (FE).

FE is an evaluation of the efficiency of electron transfer from the electrodes to the products. It can be calculated as the ratio between the moles of electrons that were used to obtain a reduction product (Qexp: the product between the moles of a product with the number of electrons required for the reduction and the Faraday constant) with the total moles of electrons that were delivered to the system (Qth, calculated as the product of the current intensity in the CPE experiment with time).

For example, in the case of the bielectronic reduction of CO2 to HCO2 -: In this case, the FE will be equal to 1 (or 100%) if HCO2 -is the only product and no e -is lost in the system. If multiple reduction products are obtained, the FE corresponding to each one will indicate the selectivity of the process.

Qth= I * t [C] Qexp= 2 * F * molHCO2- [C]
For Mnbpy in MeCN (5% H2O), a TONCO=14 was obtained after 4 hours of CPE at E=-1.7 V. The system displayed a FE=100% with Carbon monoxide (CO) as the only reduction product.

The reduction of CO2 to CO with organometallic catalysts usually follows pathway (4-49) in Scheme 64. The reduced metal catalyst binds CO2, activating the C-O bond cleavage in the presence of H + .

Depending on the Bronsted acidity of the proton source, a hydride intermediate could be generated (1-49). This species can eventually act as a nucleophile and bind CO2 (3-49), leading to formate production.

The presence of a Bronsted acid in solution also opens the way to the production of H2 via the HER (Hydrogen Evolution Reaction, (2-49)) competitive side-reaction. To avoid this reaction and favor formate production, the hydride intermediate should be "protected" from the interaction with a second H + . Alternatively, the presence of specific ligands on the catalyst can favor H + insertion in the metal-CO2 bond, thereby leading to formate production without the generation of the hydride (5-49). 177

Scheme 64: Competing CO2 reduction pathways from reduced transition metal catalysts (M) in the presence of protons. 178 Despite their well-understood reduction mechanism, homogeneous complexes present some disadvantages, mainly due to their limited stability and low TON.

One option to address these issues consists in grafting the homogeneous catalyst on a conductive support of high SSA (Specific Surface Area). This approach can improve the durability, efficiency and recyclability of catalysts.

Carbon cloth (CC) is a graphitic material with low electrical resistance and large SSA which was previously exploited in the Turin group as conductive support for the heterogenization of the Mnabpy catalyst (Fig. 109, next page). The presence of an aniline substituent on the bipyridyl ligand enabled its grafting through C-C bond formation via in-situ reduction of the corresponding diazonium salt (50-2).

The surface coverage of a CC of standardized dimension (3.4 cm 2 , corresponding to an effective electrochemical surface area of 19.95 cm 2 ), evaluated by electrochemical methods, corresponds to 1.4 *10 -9 mol/cm 2 . 168 Mn-CC displayed improved CO2-reducing efficiency and durability compared to the Mnabpy catalyst in homogeneous conditions. In fact, CO was produced with a remarkable TON=33200 in water during 10 hours of electrolysis at E=-1.35 V, while in homogeneous conditions (MeCN with 5% H2O) Mnabpy led to a TONCO=12 during 90 minutes of electrolysis at E=-1.73 V, successively undergoing deactivation. 168 The heterogenization of Mnabpy on Carbon Cloth was exploited for the CCR experiments which will be presented in the Results and discussion section of this Chapter. A good example of the tuneability of homogeneous catalysts is provided by the concept of "local proton source" developed by Gobetto et al. in Turin. 170,171 The insertion of a pendent diphenolic group on the bipyridyl ligand of the Mnbpy complex (MnbpyOH, Fig. 110) enabled to perform CO2 reduction in anhydrous MeCN, as the hydroxyl function acted as a local proton source and activated the catalyst.

Modulation of Mnbpy reduction selectivity by the introduction of a local proton source

The catalyst modification afforded a selectivity switch in terms of CO2 reduction products, as a mixture of CO, H2 and formate (HCO2 -) was obtained after CPE at E=-1.8 V. Carbon monoxide was produced at a similar extent (TONCO=19) than with Mnbpy but with a reduced FE (70%). A nonneglectable quantity of formate, which was not obtained with the Mnbpy catalyst in MeCN, was detected with MnbpyOH (TON=6, FE=22%). The 8% faradaic loss was due to the generation of H2 via the HER pathway.

The selectivity switch was attributed to a change in the reduction mechanism due to the presence of the local proton source. The detection of H2 and formate suggested the generation of a Mn-H hydride intermediate, which can be obtained via proton transfer from the hydroxyl group to the metal (Fig. 110). The formation of Mn-H was confirmed by the observation of characteristic CO stretching bands by Infrared-Spectroelectrochemistry (IR-SEC) and by the presence of a reduction peak at -1.7 V which was absent in the CV of Mnbpy in MeCN. 170 The dimerization reaction (r3) was not observed with this complex, probably due to the increased bulkiness of the phenolic substituent with respect to a non-substituted bipyridine.

This example shows how the fine-tuning of the electronic properties and steric hindrance of the ligands on Mnbpy catalysts can modify the selectivity of electrocatalytic processes. It also shows that formate can be produced by CO2 reduction with Mnbpy catalysts, providing that the reduction mechanism leads to the generation of the Mn-H hydride intermediate.

Integrated CO 2 capture and recycling (CCR) with Mnbipy complexes

Examples in the literature of utilization of captured CO2 as substrate for reductive transformations are scarce. 177 Some studies, such as the works by Rezayee 42 and Prakash 43 presented in the discussion on integrated CCUS (Chapter I, Bibliographic study, page 30), have shown the possibility of hydrogenating CO2 to methanol in the presence of amines. However, in the harsh reaction conditions required for hydrogenation CO2 was probably released from the amine by thermal stripping. Some remarkable examples of CCR obtained by performing CO2 reduction with Mnbpy catalysts in the presence of amines were published recently.

Bhattacharya et al. reported a CCR system combining CO2 capture with morpholine (0.1 M concentration) with its reduction operated by Mnbipy in MeCN (Scheme 65). 167 A significant selectivity shift of reduction products was obtained by the addition of morpholine, as formic acid (H2CO2) was produced with a 30.77 % FE while CO with a 9.5 % FE. However, dihydrogen was in fact the most abundant reduction product of the process, as in the optimized conditions it was produced with a 59.73 % FE. (CPE performed in MeCN at E= -2.2 V, 1 mM homogeneous catalyst, 0.1 M morpholine).

The selectivity shift with respect to Mnbpy in MeCN in the absence of morpholine was attributed by the authors to the acidic character of morpholinium carbamic acid (A-H, Scheme 65), which protonates the reduced Mnbpy -complex (3-50) generating Mn-H. In the presence of morpholine, the formation of Mn-H is kinetically competitive with respect to the direct coordination of CO2 to Mnbpy -(1-50), which yields CO as reduction product.

CPE control experiments were performed by substituting morpholine with a weak acid (PhOH). In this case, CO was obtained and HCOO -production was suppressed, confirming the fundamental role of the amine for the stabilization of the hydride intermediate. Our knowledge of CO2 capture chemistry suggests that carbamic acid should be excluded from mechanistic considerations, as this species is only transient in the amine-CO2 libraries. Arguably, the morpholinium cation (A-H', Scheme 65) could be the species transferring the proton to Mnbpy -1 and favoring hydride generation, as it is more acidic (pKa=8.49 in water) than PhOH (pKa=15.4 in water), and it is the most abundant source of protons in the CO2-loaded solution.

However, in that case, CO2 could not be stripped from the carbamate (ACO2 -, Scheme 65) due to the absence of available protons to restore the amine moiety. This means that the global reaction scheme would consume at least two equivalents of morpholine to produce one equivalent of formic acid (plausible chemical equation of the process in the lower part of Scheme 65), which would represent an issue for the industrialization of the process due to the consumption of this expensive chemical.

Even if the role of the amine (or its carbamate) in the mechanism was not fully elucidated, this study proved the possibility of combining CO2 capture and reduction to obtain a high added-value product such as formate.

In a similar study, Rønne et al. showed that the presence of tertiary amine groups attached to the bipyridyl ligands on a Mnbpy catalyst could shift the selectivity of CO2 reduction from CO to formate. 179 The reduction experiments were performed by CPE at -2.17 V in MeCN in the presence of alcohols (ROH) as proton donors. In the presence of 2.2.2-trifluoroethanol in 2M concentration in MeCN, the Mn catalyst depicted in Scheme 66 led to the preferential production of formate as CO2 reduction product, as it was obtained with FE= 60% (catalyst in 1 mM concentration, TFE 2 M, E=-2.17 V, MeCN, Bu4NBF4 0.2 M as electrolyte. FECO=FEH2=5%).

A detailed mechanistic study was performed by combining experimental results with DFT calculations, resulting in the mechanism presented in Scheme 66. The modelling studies confirmed the central role of the amine and the alcohol for formate generation. In fact, the amine ligand on the complex acts as Bronsted base and binds to the alcohol via H-bond. This coordination assists the reaction of TFE with CO2, resulting in a putative alkylcarbonate capture product (3-52). The protonated amine can then transfer a proton to the reduced Mn complex (4-52), thereby generating the hydride intermediate through a faster pathway compared to the protonation of Mnbpy -from the alcohol. Mn-H then reacts with CO2 and is reduced at E=-2.17 V, leading to formate generation. Similarly to the previous example, the reduction process consumes at least one TFE molecule per equivalent of formate generated, as depicted by the chemical equation represented in the lower part of Scheme 66. As in the previous case, CO2 could not be released from the carbonate product due to the sequestration of the proton for formate production.

Both studies presented in this paragraph show that amines can assists the generation of the Mn-H intermediate by either donating a proton or favoring the transfer of a proton to the reduced Mnbpy catalyst. However, in both cases, either the amine (morpholine) or the alcohol (TFE), were consumed in the reduction to donate a proton to CO2, in order to obtain formate (or formic acid).

The gradual loss of these expensive reactants would imply an additional cost to the CCR process and would thus not be adapted to an industrial scale-up. In addition, both processes are based on homogeneous catalysts which display an inferior lifetime and reduction efficiency compared to heterogeneous ones.

The CCR study which will be presented in the Results and discussion section resulted in a similar selectivity switch of CO2 reduction from CO to formate in the presence of amines, but with some improvements with respect to these examples, as it is based on a resistant and efficient heterogeneous Mnbpy catalyst and it exploits a less expensive "sacrificial" proton source.

II. Results and discussion

The following section presents the results obtained from the study of CO2 reduction by Mnbpy complexes in methanol, in the absence and in the presence of amines.

Methanol was chosen as a solvent as it enabled the obtention of homogeneous solutions upon carbamate formation (which is not the case in MeCN), while favoring the supramolecular interactions between carbamates (differently from water, Chapter II).

As the electrochemical behavior of Mnbpy complexes had never been explored in this solvent, we performed some preliminary explorative CVs in homogeneous conditions. We then turned to a heterogeneous approach, by grafting the catalyst on a conductive support, to increase the efficiency and lifetime of the catalyst.

1. Homogeneous catalysis of CO 2 reduction with Mnabpy in methanol a. Cyclic Voltammetry

From the beginning of the experiments, we were planning to adopt the heterogeneous approach for our integrated CCR system. We thereby exploited Mnabpy ([apbpy=4-(4-aminophenyl)-2,2'bipyridine)]), Fig. 111) as a catalyst for the explorative reduction tests in homogeneous conditions, as the presence of the aniline substituent on bpy enables the functionalization on Carbon Cloth by diazonium salt reduction (Fig. 109, page 181).

The reduction of the solvent (MeOH), leading to the unwanted production of H2 (HER), started at E=-1.9 V in our experimental conditions (blank CV under Ar in Annexes, Fig. A28, page 236). The reductive potential applied for CO2 reduction was therefore limited to higher values to avoid HER.

Mnabpy is poorly soluble in MeOH (Cmax=0.25 mM). The CV of the complex under Ar (Fig. 111, blue) displays a single irreversible reduction wave at E=-1.25 V. This first reduction should correspond to reactions r1-r3 listed above for Mnbpy. Anyhow, the dimerization with Mnabpy is probably inhibited by the bulkiness of the aniline substituent, in agreement with the results obtained previously Mnbpy-OH (page 197). This could lead to the generation of the Mn-H hydride intermediate as a kinetically favored species compared to the dimer.

Under CO2 atmosphere (Orange, Fig. 111) a 5-fold current increase was observed in correspondence of the reduction peak at -1.35 V, indicating CO2 reducing activity.

b. Cyclic Voltammetry with DETA

To first assess the influence of the amine on Mnabpy as a CO2 reduction catalyst, we performed a CV under CO2 atmosphere in the presence of DETA (25 mM).

The amine solution was loaded with pure CO2 for 20 minutes before adding the catalyst. This precaution was adopted following some preliminary experiments where we observed a rapid deactivation of the catalyst when exposed to the amine solution in the lean state. This detrimental effect was probably due to the strong coordinative character of DETA, which can coordinate to the activated catalyst upon Br -elimination (r1) thereby preventing the interaction with CO2. We observed that the loading of the amine solution with CO2 prior to the addition of the catalyst avoided this effect, so we adopted the pre-loading step for all subsequent electrochemical tests. comparing the TON of different reduction products obtained from a standardized CPE experiment of 22 hours at E=-1.35 V, which was performed for each different set of conditions.

CPE of Mn-CC

Mn-CC was tested by CPE at E=-1.35 V in methanol (TBAPF6 0.1 M) in the absence of amines under CO2 atmosphere. Figure 113 shows the increase of the TON of gaseous reduction products (CO and H2), quantified in-time by GC during the electrolysis.

The system displayed a remarkable efficiency, as it actively reduced CO2 to CO during 13 hours of electrolysis. CO production grew consistently during the first 10 hours, then it reached a stationary phase around a TONCO=30000 with a FECO=51%. This high TONCO value was close to the one previously obtained in MeCN (5% H2O). 168 Hydrogen production was constant during the electrolysis, reaching a TONH2=33000 after 20 hours, with a FE=20%. A control CPE experiment, performed with a pristine Carbon Cloth, showed a similar constant production of H2 with time, confirming that this product was originating from solvent reduction (CPE in Annexes, Fig. A29).

At the end of electrolysis with Mn-CC, an aliquot of the solution was analyzed by 1 H NMR with an internal reference, showing a remarkable production of formate with TONHCOO-=16400, and FE=26%.

In general, the Mn-CC catalyst in methanol showed a better reduction performance and extended lifetime with respect to the homogeneous catalyst. The TON of CO was close to the one obtained in MeCN, but a clear selectivity switch toward formate production was obtained in methanol. The production of formate requires the generation of the Mn-H intermediate, which, in this case, could be obtained by proton transfer from MeOH towards the Mnbpy -complex. The main structural difference between PMDETA, DETA and DEA is that the former presents exclusively tertiary amine groups. We consequently tested some other tertiary amines, NEt3, TMEDA and a guanidine (TBG), but the FE, TONs and overall catalytic performances were less satisfactory than the Mn-CC + PMDETA combination.

Mechanistic considerations

A mechanistic interpretation, accounting for the influence of methanol and of the different loaded amines tested on the CO2 reduction properties of Mn-CC, is currently still debated. Anyhow, some conclusions can be drawn from the experimental results.

During some parallel NMR experiments, we observed that in millimolar concentration primary and secondary amines do not yield carbamates as the major capture products in water. The experiments consisted in performing progressive dilutions on 0.5 M solutions of primary and secondary amines loaded with CO2. The diluted samples were re-loaded with CO2 to compensate eventual desorption due to the dilution, and they were analyzed by 1 H qNMR to quantify the loading (). These tests showed that the Nmax of amines gradually decreases upon dilution, until reaching complete desorption (N=0) in millimolar concentration (Annexes, Fig. A30, 1 H qNMR EDA-CO2 with progressive dilution). The explanation depends on the fact that at high dilution, bicarbonate formation is favored against carbamation due to the higher availability of water molecules instead of amines. However, we could not quantify bicarbonate in the diluted samples (5 mM) as the concentration was too low to perform 13 to the reaction of PMDETA with water traces and with CO2, could herein explain the strong selectivity switch towards formate production obtained with this amine. However, the inferior formate production observed with other strongly basic tertiary amines such as NEt3 and TMEDA, and with TBG, which should also generate bicarbonate, suggests that this explanation should be discarded. In addition, the Mn-CC catalyst had previously been studied by us in water in the presence of KHCO3 as supporting electrolyte, and no formate was detected in this case. 168 The interaction of amines with the Mn-CC catalyst should thus be independent on their CO2 capture products, as carbamates are not generated when the amine is in millimolar concentration, and the production of formate varied significantly in the presence of different tertiary amines.

Instead, the selectivity switch from CO to formate could be explained by a stabilizing effect played by the amine towards the Mn-H hydride intermediate. To relate the influence of the different amines to their structure, we surveyed some properties listed in Table 13.

Table 13: List of relevant thermodynamic parameters for the panel of amines tested with Mn-CC for CO2 reduction in methanol.

Emin and Emax were calculated by semi-empirical modelling or experimentally,  corresponds to the Hydrogen Bond Acceptor (HBA) parameter and  to the Hydrogen Bond Donor (HBD) parameter (which can be deduced from Emin and Emax), Log P is a measure of the lipophilicity of amines, and pKa measures the acidity of the amine groups.

Some remarks can be formulated based on these values:

-DEA has the highest  of the list. A strong H-bond donor character could translate into a strong coordination to the active center of the catalyst, consequently blocking the interaction with CO2. PMDETA has a lower HBD character, similarly to the other tertiary amines (TMEDA, TBG, NEt3).

-The pKa values are relatively close among the series of amines, apart from TBG (pKa=13.81 in water), which was the additive displaying the lowest production of formate. This could be due to the increased stability of the protonated guanidinium species compared to the ammoniums, and consequently on the lower Bronsted acidity of the acidic form which should transfer a proton to the catalyst to form Mn-H.

-The most lipophilic amines of the series are tertiary amines. Their lipophilic character, coupled with their weak HBD character (), make them the less polar amines, and thus the amines which will probably interact more weakly with the catalyst. This could partially explain the selectivity switch, as tertiary amines could create a protective environment/shell around the hydride intermediate, favoring CO2 insertion in the Mn-H bond. The bulkiness of the amine could also play a role on the stabilization of Mn-H, as TMEDA has a higher log P than PMDETA, but it displayed an inferior formate production.

These preliminary considerations agree with the available literature dedicated to similar CCR systems, which highlighted the selectivity shift imparted by amines towards formate production.

A sharp shift towards formate production was obtained in our CCR system (with PMDETA, FEH2CO=66%, FECO=5%), higher that the two CCR examples presented in the bibliographic section (Bhattacharya et al. 167 FEH2CO=59%, FECO=10%, Rønne et al. 179 FEH2CO=60%, FECO=5%).

In our case, the "sacrificial" proton donor is not an expensive amine/alcohol but methanol, as the high production of formate was observed even in the absence of amines. The amines still play a role in the amplification of formate, probably by creating a protective hydrophobic shell around the Mn-H hydride intermediate. The exploitation of methanol as proton source implies an improved industrial appeal of our CCR process compared to the previous examples in the literature, based on expensive sacrificial proton donors (TFE, morpholine).

CO2 + MeOH + 2 e -→ HCO2 -+ MeO - (E=-1.35 V)

The consumption of methanol as proton source would imply the generation of methanolate, which could eventually bind CO2. The generation of this species and its influence on the catalytic system have yet to be verified.

Our study provided fundamental evidence accounting for the influence of amines in CCR processes by electroreduction, proving that CO2 capture adducts, either carbamates or bicarbonate, should be excluded from consideration as reduction substrates, as the selectivity switch towards formate production depends on the presence of Bronsted acids (ammonium, TFE, MeOH), whose proton transfer is mediated by amines. DFT calculations and additional experiments are undergoing to complete the characterization of the reduction mechanism and to fully account for the influence of amines on the reduction mechanism of Mn-CC.

We identified a remarkably efficient CO2 reduction system by combining Mn-CC with PMDETA. The system displayed increased TONs and lifetime compared to the homogeneous case, proving the effectiveness of the heterogenous approach for CO2 reduction. In addition, the screening of different amine additives enabled the observation that tertiary amines are the best suited additives to increase formate production.

III. Conclusion to Chapter V and future prospects

Chapter V presented the results obtained in collaboration with the research group of prof. Gobetto in the University of Turin concerning the development of an integrated CO2 capture and electroreduction process.

The study of the reduction properties of Mnabpy in methanol in homogeneous conditions highlighted the fast deactivation of the catalyst both in the presence and in the absence of DETA.

The heterogenization of the complex by grafting on a conductive Carbon Cloth support resulted in catalyst of increased reduction efficiency, enabling a high production of carbon monoxide (CO) and of formate (HCO2 -) over the course of Controlled Potential Electrolysis at E=-1.35 V. Performing the reduction in methanol showed a remarkable selectivity shift in the reduction products compared to other solvents, as formate was obtained with a FE=26%, while it had not been detected in MeCN nor in water.

The screening of various polyamines as additives to Mn-CC in millimolar concentration showed an increase of the reduction efficiency and selectivity for formate production. The highest TON (28000) and FE (66%) of formate were obtained in the presence of PMDETA. The positive influence of this species is probably due to a combination between its lipophilicity and steric hyndrance, which could assist the protonation of Mn from methanol to form the hydride intermediate (Mn-H), and create a protective shell around this unstable intermediate, thereby facilitating CO2 binding.

Mechanistic studies by DFT are undergoing to fully understand the mechanism of this efficient system for CO2 reduction.

Unfortunately, in the presence of amines in molar concentration, thereby closer to the industrial amine scrubbing process, the catalyst's efficiency and lifetime were drastically decreased. The ideal scenario for the continuation of this project would be to identify a Mn-CC/amine combination enabling to perform CO2 reduction on concentrated amine solutions and to fully switch the selectivity of reduction towards the production of formate.

I. General Methods 1. Reagents and solvents

All reagents were obtained from Sigma-Aldrich, ThermoFischer Scientific, Fluorochem or TCI chemicals. Solvents were purchased from Sigma-Aldrich or VWR and were used without further purification.

CO2 (99,95%), N2 (>99.9%) and Ar (>99.9%) were obtained from Air Liquid. Deuterated solvents were purchased from Eurisotop.

TCL were performed with Macherey-Nagel silica gel 60 F254 and revealed with UV lamp at 254 nm or with a ninhydrin solution. Purification by chromatography was performed with a Macherey-Nagel Silica 60 M (0.04 -0.063 mm).

Anion exchange procedures were performed with an AmberlystA26 hydroxide form resin.

NMR

All liquid NMR experiments ( 1 H, 13 C, q 13 C, q 1 H, COSY, HSQC, HMBC, DOSY, ROESY) were performed in the CCRMN center at the University of Lyon on Bruker spectrometers AVL 300, AV 300, AV400 or AV500 MHz.

Chemical shifts are indicated in ppm with respect to TMS. Abbreviations used for the multiplicity of the NMR peaks are s (singlet), d (doublet), t (triplet), m (multiplet). Solid state NMR were recorded in IRCELYON on a Bruker AVANCE III 500 WB spectrometer equipped with a Bruker 4 mm MAS H/X/Y (magic angle spinning) probe.

In-situ FTIR

Infrared measurements were performed with Attenuated Total Reflectance Fourier transform infrared (ATR-FTIR) technology using ReactIR 15 workstation from Mettler Toledo. The instrument was equipped with a .3 mm AgX DiComp probe (Wetted Materials: Alloy C22; Diamond; Gold).

Mass Spectrometry

Mass Spectrometry (MS) experiments were performed at the CCSM of the University of Lyon. Low resolution mass spectroscopy (ESI-MS) was performed on a linear quadrupole ion trap LCQ Advantage (50-2000 m/z) apparatus from Thermofinnigan with an Electrospray ionization (ESI) method.

High Resolution Mass Spectroscopy (HRMS) was performed on a QTOf-MicroTof QII (50 -20000 m/z) apparatus form Bruker. CRYO-HRMS spectra were acquired using a hybrid Quadrupole -Time-of-Flight (QTOF) mass spectrometer (Impact II, Bruker, Bremen, Germany) equipped with a cold-spray (CSI) ionization source (CryoSpray, Bruker, Bremen, Germany).

UV-Vis

UV-Vis spectra were recorded on a Shimadzu UV-2401 PC spectrophotometer in the 200 -800 nm wavelength () range.

Volumetric titrations

Titration for the determination of the CO2 loading in solids (αS) was performed with a Chittick apparatus according to the method described by the Association of Official Analytical Chemists (AOAC), with a rmsd ≤ 5%. 180

Gravimetric Analyses

Gravimetric analyses were conducted on a Mettler Toledo XSR high precision (± 0.01 mg) analytical balance.

pH-measurements

pH in water were measured at 25 °C on a T5 titrator excellence by Mettler-Toledo.

Electrochemistry

UV-Vis Spectroelectrochemistry (Chapter IIIB) measures were performed in ENS Lyon. The setup involved a single compartment cell (40 ml) with a three-electrode setting. A UV-Vis immersion probe with an optical path of variable length (1mm -1 cm) was exploited for the real time detection of the UV-Vis spectra in the 200-1000 nm range.

Cyclic voltammetry (CV) and CPE experiments (Chapter V) were performed in the University of Turin. The experiments were performed with an Amel 7050 potentiostat. Tetrabutylammonium hexafluorophosphate (0.1 M TBAPF6) was used as supporting electrolyte. A single compartment cell (20 ml) was used for CV measurements. A double compartment cell (cathodic, anodic) was used for CPE measurements (40 ml).

Gaseous reduction products were detected and quantified by an Agilent 490 Micro GC. Soluble reduction products were quantified by 1 H NMR (with an internal reference) recorded on a JEOL ECP 400 FT-NMR spectrometer ( 1 H operating frequency 400 MHz) at 25 °C ± 0.1.

XRD

Single crystal X-ray diffraction spectra were recorded on an Xcalibur, Atlas, Gemini ultradiffractometer in Centre de Diffractométrie Henri Longchambon in Lyon. Powder Xray Diffraction patterns were measured with a Bruker D8 Advance diffractometer using CuKα radiation with λ = 1.54060 Å, with a BraggBrentano geometry and equipped with a Bruker Lynx Eye detector.

II. NMR Methods 1. Quantitative NMR spectra acquisition

Liquid state quantitative 1 H and 13 C NMR spectra were recorded on an Avance III 500MHz spectrometer, with a standard BBFO probe from Bruker. Accuracy of the quantitative analyses was ensured by running the experiments using 90 degrees pulses and relaxation delay > 5*T1. T1 relaxation time measurement of 1 H and 13 C were run with the standard inversion recovery experiment from Bruker pulse program library. Spectra were recorded at 25 °C ± 0.1, and samples were kept at 4 °C before measurement.

Solid-state NMR spectra were recorded at room temperature on a Bruker AVANCE III 500 WB spectrometer (11.7 T, 13 C at 125.73 MHz) equipped with a Bruker 4 mm MAS H/X/Y (magic angle spinning) probe. HPDEC (High power decoupling) spectrum was recorded at a spinning rate of 10 kHz, a π/2 pulse of 3.7μs was applied on 13 C and 69 kHz 1 H Spinal-64 decoupling was applied during acquisition. The spectral width was set to 400ppm (50.5KHz), the recycle delay was set to 60s and 1400 scans were accumulated. For data processing, the spectrum was referenced to adamantane (38.5ppm) and line broadening of 20 Hz was applied.

NMR speciation procedure

A stock solution containing the selected capture agent was prepared by dissolution with deionized H2O in a volumetric flask (10 ml). The solution was divided into 1 ml aliquots which were individually loaded with increasing amounts of CO2. Pure CO2 was delivered at a fixed flow rate (5 or 10 ml/min) through a mass-flow controller.

For amine concentrations higher or equal to 1 N, the extent of loading was evaluated by gravimetry, by weighing the samples before and during CO2 absorption.

After absorption, a 550 µL aliquot of each sample was transferred into an NMR tube containing 50 µL of a TMPCl stock solution (0.5 M) in D2O as internal reference. The samples were stored at 4 °C before analysis to avoid CO2 desorption.

Each sample was analysed by q 1 H and q 13 C NMR. The concentration of each library component was obtained by integrating the corresponding 13 C peaks with respect to the internal reference. For species displaying multiple 13 C peaks, the corresponding concentration was calculated by averaging the individual integrations with respect to the internal standard, delivering mean value and rmsd.

The accuracy of quantification was assessed by comparing the total concentration of amine calculated by NMR with the concentration of the mother solution. The experimental points were neglected if the mass balance was inferior to 85%.

The speciation graphs present the plot of the molar fraction of each i-species xi, calculated with respect to the total concentration in amine, against normalized loading (N). N was calculated following eq.3 (main text, page 7) with molar fractions (xi) issued from NMR quantification.

Calculation of the Amplification Factors

Amplification factors (AFi) were calculated to quantify the extent of the covalent equilibrium displacement induced by non-covalent template effects in CO2 capture systems. This score quantifies at each  the variation of the molar fraction xi of a given species or population i with respect to the corresponding value in the untemplated system xi 0 . This difference is normalized with the amount of template T introduced xT 0 (eq.E1).

At any given , the sum of all AFi, each multiplied with the corresponding number of bound CO2, should be equal to zero: the amplification of a component induced by a template must be compensated by the decrease of another component (eq.E1-7). The sum of AFi(α) was calculated for all systems containing a template and it was plotted on the AFi(α) graph as an indicator of the "cumulated deviation from the model".

III. FTIR -NMR methods

Real-time, in-situ IR spectra were recorded with a 60 s interval. 128 scans were accumulated perspectrum, in the 4000 -650 cm -1 optical range, with a =2 cm -1 resolution. All solutions were thermostated at 25 °C during absorption.

FTIR-NMR calibration for DETA in water

A FTIR-NMR calibration was performed on a DETA 0.5 M solution to obtain the absorption coefficients for carbamate and carbonate vibrations at 1320 cm -1 (N-CO2 -stretching) and 1360 cm -1 (HO-CO2 -stretching) respectively. 100 ml of sorbent solution were prepared and transferred in a round bottom flask containing a magnetic stirrer. The FTIR probe was inserted in solution and the acquisition was started prior to CO2 introduction. Pure CO2 was injected at a fixed flow rate of 200 ml/min using a mass flow controller equipped with a gas inlet. The intensity of selected IR frequencies was continuously monitored during CO2 injection (1 spectrum per minute, ∆ν= 2 cm -1 ) until saturation was reached.

550 L aliquots were withdrawn regularly from the mother solution (every 15 min) and they were transferred into an NMR tube containing 50 µL of a TMPCl 0.5 M solution in D2O (as internal reference) and analysed by q 1 H and q 13 C NMR.

The absorption coefficients for carbamates (A NCO2-=0.2199) and for bicarbonate (A HCO3-=0.4466) were extracted from the calibration curves in Fig. E1. 201

Absorption rate

The same operational setup described in the previous paragraph, with the exception of the regular sampling and qNMR analysis, was exploited to evaluate the rate of absorption of amine solutions (other than DETA).

100 ml of sorbent solution were prepared and transferred in a round bottom flask containing a magnetic stirrer. The FTIR probe was inserted in solution and the acquisition was started prior to CO2 introduction. Pure CO2 was injected at a fixed flow rate of 200 ml/min using a mass flow controller equipped with a gas inlet. The intensity of selected IR frequencies was continuously monitored during CO2 injection (1 spectrum per minute, ∆ν= 2 cm -1 ) until saturation was reached.

For DETA, DETA + 1/2 GiSO4 and PMDETA solutions the absorption coefficients A obtained from the calibration (previous paragraph) were used to calculate the xHCO3-(molar fraction of bicarbonate) and xNCO2-(molar fraction of carbamate) corresponding to FTIR intensities. The molar fractions of capture products were exploited to calculate the  corresponding to each IR spectrum and to obtain the  vs time plots. For these systems, the initial absorption rate, expressed in mol CO2 captured / second, was calculated from the  difference between two measurements separated by a 1 second interval (eq.25, main text, page 62).

Cyclic Capacity measurements

100 ml of absorbent solution were prepared in deionized H2O and transferred into round bottom flask containing a magnetic stirrer. The temperature and FTIR probe were inserted in solution, the stirring rate was set to 700 rpm. The FTIR acquisition was started prior to CO2 introduction (1 spectrum per minute, 128 scans, ∆ν = 2 cm -1 ). Pure CO2 was injected at a fixed flow rate of 200 ml/min through a mass-flow controller. The gas was saturated with water vapor in a pre-bubbler prior to introduction in the sorbent solution in order to limit evaporation in the sample.

The FTIR intensities at 1320 cm -1 and 1360 cm -1 were monitored during absorption. Upon saturation of the solution (i.e. FTIR intensities reaching a plateau), 550 µl of solution were transferred in an NMR tube containing 50 µl of a TMPCl solution (0.5 M in D2O) as internal reference for qNMR analyses ( 1 H, 13 C).

Desorption was performed by heating the loaded solution at 100 °C and stripping with N2 (fixed flow rate 200 ml/min) while performing in-situ FTIR monitoring and NMR sampling when the system reached equilibrium, as for the absorption step.

Quantitative NMR enabled the accurately quantification of the loading in the rich (max Nmax) and lean (min,Nmin) state. After each desorption step, the concentration was re-adjusted to initial values with deionized water and the consequent absorption/desorption cycle was performed. The CC and CCN were calculated with eq.6 (main text, page 8) by averaging the max and min values calculated by qNMR to obtain mean value and rmsd. The isothermal absorption plot for the DETA 0.5 M solution in methanol was obtained by gravimetry and FTIR spectroscopy.

Isothermal absorption plot for DETA in methanol

For the FTIR measure, the DETA 0.5 M solution was inserted into a 100 ml three neck reactor containing the FTIR probe, a temperature probe and the gas inlet. A series of gaseous mixtures made by different proportions of CO2 and N2, containing increasing percentages of CO2, were obtained by branching two separate mass flow controllers via a T-valve (Figure E2). The streams exiting each mass-flow controller were combined to obtain a total 44 ml/min flow rate. The solution was fluxed with each gas mixture until it attained equilibrium, i.e. the IR intensity stopped increasing at 1320 cm -1 (N-CO2 -stretching). The loading corresponding to each pCO2 was calculated by comparing the corresponding IR intensity at equilibrium with the IR intensity of the solution loaded with pure CO2 (eq.E1-8).

𝛼 =

I 1320 CO2 (p CO2 ) I 1320 CO2 (CO 2 100%) eq.E1-8

IV. Organic Syntheses 1. Syntheses from Chapter II a. 3,7,11,15-tetraaza-1,5,9,13(1,3)-tetrabenzenacyclohexadecaphane C0 was synthetized following the protocol from Chang et al. 93 Isophtalaldehyde (798.7 mg, 5.95 mmol) was dissolved in 250 ml CHCl3 in a 500 ml round bottom flask. 1.3-phenylenedimethanamine (809.01 mg, 5.94 mmol) was added. The solution was heated at 50 °C and stirred for 48 hours under Ar. After cooling down, the solution was added to an excess of NaBH4 (9.07 g, 239 mmol) dissolved in in 600 ml anhydrous MeOH. The solution was left to react under stirring at 25 °C for 1 hour. The solvent was evaporated to dryness. The remaining solid was transferred in a separating funnel and it was extracted with CH2Cl2 and water. The combined organic layers were dried with Na2SO4, filtered and evaporated to dryness.

The mixture was purified by chromatography on SiOH with a strongly polar and basic eluent (CH2Cl2/MeOH 19%/NH3 1%). The pure cyclophane was isolated as a light-yellow solid (1.53 g, 3.2 mmol, 53.8% yield). Ethylenediamine was added (95 ml, 85.5 g, 1.42 mol) in the flask. The solution was stirred at RT for 24 hours. A white viscous precipitate formed, it was separated by filtration on sintered glass and washed with few ml of iced cold water and EtOH.

Synthesis from

The dried white solid was recrystallized in water. To favor the crystallization of G2SO4 upon reflux, the solution was cooled down with a water bath first at room temperature, then at 0 °C. The white platelet crystals were filtered and washed with ice water and Et2O (26.056 g, 30.6 % yield).

1 H NMR (300 MHz, D2O): δ 3.42 (s, 4H, CH2-CH2). Diethylenetriamine (526 mg, 5.09 mmol) was added to a 50 ml S-methylisothiourea hemisulfate (1.417 g, 10.2 mmol) solution (H2O/EtOH 1:1). The solution was stirred at 25 °C for 24 hours. A white precipitate formed, it was filtered on sintered glass and washed with 10 ml of iced water and EtOH.

Once dried, the solid was recrystallized in water to obtain white acicular crystals (423 mg, 1.48 mmol, 29.2 % yield) which were filtered off and analyzed by NMR.

Procedure 2. Ion methatesis

A BaCl2 aqueous solution (1.006 g, 4.12 mmol) was added to a solution of G2SO4 (998 mg, 4.12 mmol). A white precipitate formed instantaneously.

The solution was filtered on hydrophilic membrane to eliminate solid BaSO4. Water was lyophilized to obtain G2Cl2 as a white solid (821 mg, 3.78 mmol, 91.7 % yield). The complete sulfate substitution was verified by ESI-MS in negative mode. The NMR spectra corresponded to the product obtained through ion exchange.

LR ESI-MS (-) 215. 10 (G2Cl2-H, 215.06 predicted) in water to obtain white acicular crystals, which were washed with iced water and Et2O and dried (644 mg, 1.54 mmol, yield 19.3 %).

1 H NMR (300 MHz, D2O): δ 3.29 (t, J = 3.3 Hz, 6H, 2x G-CH2-), 2.69 (t, J = 2.69 Hz, 6H, -CH2-NH-CH2-) 13 C NMR (75 MHz, D2O): δ 156.89 (3x C=NH), 52.28 (3x CH2-NH-CH2), 39.01 (3x G-CH2-)

Data matched with literature.

f. Bis(Diethylenetriamine)Co(III) trichloride (CoDCl 3 )

The procedure described by Keene and Searle was exploited to synthetize Bis(Diethylenetriamine)Co(III) trichloride (CoDCl3). 137 CoCl2*6H2O (22.95 mg, 0.096 mmol) was dissolved in 50 ml deionized water. Diethylenetriamine (19.9 mg, 0.19 mmol) and HCl 1 M (100 l) were added in the flask. The solution was stirred and fluxed with compressed air in the presence of active charcoal. After 36 hours charcoal was filtered off, leaving a bright red solution. Water was evaporated to a small volume (10 ml) and the product was precipitated with EtOH. The orange solid was filtered and washed with EtOH and Et2O (31.95 mg, 0.086 mmol, yield 86%).

1 H NMR (300 MHz, D2O) δ 2.65 -3.35 (m, 16H, 8x-CH2-) 13 C NMR (75 MHz, D2O): δ 42. 22, 45.89, 47.18, 50.35, 53.14, 54.81 (-CH2-of the three geometric isomers)

g. Bis(Diethylenetriamine)Co III triperchlorate (CoD(ClO 4 ) 3 )

The procedure described by Keene and Searle was followed. 137 CoClO4*6H2O (36.72 mg, 0.1003 mmol) was dissolved in 50 ml of deionized water. Diethylenetriamine (20.8 mg, 0.20 mmol) and 100 l of HCl 1 M were added to the solution. The solution was stirred and fluxed with compressed air in the presence of active charcoal. After 36 hours the solution was filtered to remove charcoal, leaving a bright red clear solution. The filtrate was evaporated to a small volume and the product was precipitated with EtOH. The orange solid was filtered and washed with EtOH and Et2O (43 mg, 0.077 mmol, yield 77%).

208 1 H NMR (300 MHz, D2O) δ 2.73 -3.41 (m, 16H, 8x-CH2-) 13 C NMR (75 MHz, D2O): δ 42. 27, 43.32, 43.41, 45.87, 47.17, 50.37, 53.16, 54.79, 54.97 (-CH2-of the three geometric isomers)

h. mer-Bis(Diethylenetriamine)Co III bischloride monohexafluorophosphate (CoDCl 2 PF 6 )

Diasteromerically pure mer-CoDCl2PF6 was firstly synthetized via an anion exchange procedure. 1.5 ml of deionized H2O were saturated with CoDCl3 (1 g, 2.69 mmol). The mixture was stirred at 80 °C until complete dissolution of the solid. Three equivalents of NH4PF6 (solid, 1.31 g, 8 mmol) were added. A solid appeared in the hot solution as soon as the salt was added. The solid was collected from the cooled down solution by filtration and it was recrystallized in water. The product was retrieved as red-colored platelet shaped crystals (776 mg, 1.61 mmol, yield 60 %).

1 H NMR (500 MHz, D6-DMSO) δ 2.30 -2.73 (m, 16H, 8x-CH2-), 3.91 (s, 2H, -NH2), 4.22 (s, 2H, -NH2), 4.39 (s, 2H, -NH2), 4.73 (s, 2H, -NH2), 7.35 (s, 1H, -NH-) 13 C NMR (130 MHz, D6-DMSO): δ 45. 85, 47.16, 50.33 (-CH2-) i. Bis(Diethylenetriamine)Co III tristetraphenylborate (CoD(BPh 4 ) 3 )

This complex was firstly synthetized via an anion exchange procedure. A NaBPh4 (1 g, 2.9 mmol) aqueous solution was added to a stirred aqueous solution of CoDCl3 (370 mg, 1 mmol). A precipitate formed instantaneously. The solid was filtered, washed with water and crystallized in a water/methanol mixture. Acicular orange crystals were obtained by leaving methanol to evaporate slowly (1.137 g, 0.93 mmol, yield 93%).

V. Chapter IV: Methods 1. Standardized preparation and work-up procedure for FeCl 3 + DETA (MeOH) solutions

Tetramethylphosphonium chloride (TMPCl, R1, 80.4 mg, 0.625 mmol) was solubilized in 12 ml of MeOH in a 50 ml round bottom flask. FeCl3 (202.8 mg, 1.25 mmol, Cf=0.05 mol/l) was introduced in the solution and solubilized via magnetic stirring. Diethylenetriamine (1.289 g, 12.5 mmol, Cf=0.5 mol/l) was solubilized in 12 ml of MeOH and added in the flask.

The solution was stirred at a fixed 500 rpm rate. For reactions performed in aerobic conditions, the flask was left open in order to maintain the contact between the solution and dioxygen from air.

After 16 hours, 1.25 ml of NaOH 10 M (10 equivalents with respect to FeCl3) were added to the solution, which was stirred for 5 minutes after the addition. The suspension was transferred in 10 ml falcon tubes and it was centrifuged. The liquid fraction was separated from the solid, which was washed with 1 ml of clean methanol to maximize the recuperation of the organic products. The liquid fractions were resembled, and methanol was evaporated to 1/5 of the initial volume under reduced pressure. The remaining solution was filtered on Celite (2 -3 g) to eliminate the remaining solid Fe(OH)3 aggregates. The filtrate was evaporated to dryness.

The remaining substances were transferred in a 25 ml graduated flask whose volume was completed with distilled water. The samples for ESI-MS analyses were extracted at this stage.

For NMR analyses, a 5 ml aliquot was taken from the graduated flask and evaporated to dryness in a previously weighed round bottom flask. The sample was redissolved in 1 ml D2O containing the second internal reference (Pinacol, R2, 0.1 M), a 500 l aliquot of this sample was analyzed by 1 H q NMR.

1 H NMR qualitative monitoring in aerobic and anaerobic conditions

For the monitoring in aerobic conditions, FeCl3 (135 mg, 0.83 mmol) was solubilized in 5 ml D4-MeOD in a 10 ml round bottom flask. Diethylenetriamine (258 mg, 2.5 mmol) was added to the stirred FeCl3 solution. The solution was exposed to air and stirred at a constant 500 rpm rate. 100 l aliquots were extracted from the solution at increasing reaction times with a micropipette. Fe(III) was precipitated as Fe(OD)3 by adding 16.6 l of deuterated NaOD (40 % w/w, 10 M). The mixture was sonicated, then 600 l of MeOD were added to solubilize the organic products. The suspension was filtered on Celite to separate the solid Fe(OD)3 from the liquid. The eluted liquid was analyzed by 1 H NMR.

The same procedure was followed for the kinetic monitoring in anaerobic conditions, but the reactants (MeOH, FeCl3, DETA) were stripped with N2 prior to mixing, and the solution was kept under N2 to avoid O2 leaking.

HPLC-MS conditions

HPLC-MS was performed with a Kinetex 1,7 µm EVO C18 100 A 100x2.1 mm column, heated at 35 °C. The products were separated with a mixed water/acetonitrile eluent. The pH of the eluent was set to 10 with NH4OH. The sample was injected with a 0.4 mL/min fixed flow-rate. The minimal length of an experience was 15 minutes.

VI. Chapter V: methods 1. Carbon Cloth functionalization with Mnabpy A 3.4 cm 2 Carbon Cloth was pre-treated with the following three consecutive steps: sonication in a iPrOH/H2O 1:1 for 15 minutes, sonication in distilled water for 20 minutes and drying in oven at 60 °C.

In a single-compartment electrochemical cell, Mnapbpy (0.0233 g, 0.05 mmol) was dissolved in 10 ml of anhydrous MeCN under Ar atmosphere. The cell was immersed in an iced water-acetone-NaCl bath (temperature inferior to 0 °C) to avoid diazonium salt decomposition. Trichloroacetic acid (30 µL) and isoamyl nitrite (20 µL) were added to obtain the diazonium salt of Mnabpy in-situ (reaction in Fig. 109, page 181, main text). The solution was stirred for 20 minutes, then TBAPF6 (0.387 g, 0.1 M) was added as supporting electrolyte and the CE and RE were immersed in the solution. Carbon Cloth was branched to a conductive wire and immersed in solution.

The functionalization was performed by applying five CVs scans at reduction potential (E<0), which induced the reduction of diazonium salt on the carbonaceous surface of the carbon cloth, and the subsequent formation of C-C. After functionalization, Mn-CC was sonicated in MeCN for 10 minutes and then dried under inert gas (Ar).

Controlled Potential Electrolysis Procedure

The performances of functionalized Mn-CC electrodes were tested in methanol (TBAPF6 0.1 M as supporting electrolyte) by Controlled Potential Electrolysis (CPE) in a three-electrode cell with two gastight compartments. A pre-bubbler filled with methanol was exploited to saturate the gas with solvent vapors in order to avoid evaporation in the cell.

Procedure with no addition of amines: after saturation of the solution with CO2 (30 mL/min, 20 minutes), electrolysis was started with a set potential of -1.35 V vs Ag/AgCl.

Procedure with addition of amines: specific aliquots of the amines were added in the cathodic compartment. The solution was saturated with a constant flow of CO2 (30 mL/min). The electrolysis was started by setting the potential at -1.35 V vs Ag/AgCl. The negative potential was maintained during 22 hours.

CPE experiments were performed for 22 hours, during which the GC performed automatic sampling of the gas phase in the upper part of the cell every 5 minutes. The electrochemical cell was protected from light during CPE.

c. Crystallographic data

c. Tetraazacyclophane

General Conclusion

The objective of this doctoral thesis was to apply the principles of supramolecular chemistry to the development of efficient absorption agents for CO2 capture by amine scrubbing.

Chapter I presented the study by quantitative NMR speciation of a series of commercial and synthetic polyamine solutions (EDA, DETA, TREN, -diamines, 3,7,11,15-tetraaza-1,5,9,13(1,3)tetrabenzenacyclohexadecaphane). The characterization and quantification of each component of these dynamic polyamine-CO2 libraries provided a deep understanding of the structural parameters and of the electrostatic forces which drive polyfunctional capture agents towards the generation of selected library members. A thorough thermodynamic study was performed for -diamines to analyze the disproportionation of monocarbamates into dicarbamates. Thanks to a virtual thermodynamic cycle and to NMR speciation, we highlighted that the generation of diammonium ions represents the strongest unfavorable contribution weighing on the equilibrium constant of the reaction. We characterized the CO2 cooperative capture mechanism of DETA in methanol, which is favored by the generation of a double stranded polycarbamated dimer stabilized by multiple salt bridge interactions. Chapter II indicated that the generation of polyanionic states, and particularly of polycarbamates, is an unfavored event in water, which is the best suited solvent for industrial amine scrubbing.

Dynamic combinatorial chemistry principles were applied to develop template species able to stabilize selected components of the polyamine-CO2 libraries. Chapter IIIB discusses our effort to amplify polycarbamate formation, by synthetizing a series of Bis(Diethylenetriamine)Co(III)X3 complexes (X=Cl -, ClO4 -, PF6 -, BPh4 -). The oxyanion-binding properties of these species were characterized by UV-Vis titrations and by NMR. The thermodynamic effect of anion binding to the complexes could not be fully assessed, but the affinity of carbamates for the oxidized complex was confirmed by the spectroelectrochemical study of Co(II) into Co(III) oxidation rate, which was accelerated in the presence of the carbamates of DETA.

Chapter IIIA described the study of polyguanidinium salts as templates for bicarbonate in polyamine aqueous solutions. The presence of these additives induced a 2-fold amplification of bicarbonate with respect to the untemplated libraries, due to a stabilization of the anion by salt bridges with the guanidiniums. Bisguanidinium salts were characterized as efficient templates for bicarbonate and as supramolecular triggers, able to induce a switch between covalent CO2 capture chemistries. As the templated libraries showed an increased Cyclic Capacity and a fast absorption rate, these results represent the clearest prove of the advantages of supramolecular chemistry implementation to CO2 capture solutions.

In Chapter IV, we presented an innovative reaction based on the oxidative abiotic metabolism of DETA in the presence of FeCl3 in methanol. The reaction was studied by spectroscopy, which highlighted the spontaneous generation of N-substituted imidazoles and polycyclic structures, obtained through oxidation and transimination reactions initiated by the Iron(III) catalyst, activated by O2. The optimization of this reaction could represent a remarkably more sustainable alternative to the current synthesis of imidazole compounds. From another perspective, the reaction could be identified as a prebiotic mechanism, starting from simple compounds and evolving spontaneously towards to the generation of structural complexity.

The collaboration with the University of Turin, aimed at designing an integrated CO2 capture and electroreduction process, brought to the identification of an efficient catalytic system based on a heterogeneous Mn bipyridyl catalyst in methanol in the presence of amines. In agreement with literature, amines increased the selectivity of reduction of the Mn-bipyridyl catalyst towards formate production. By screening a scope of amines with a different structure, we could not only identify the most efficient additive (PMDETA) to increase formate production, but also propose some relevant mechanistic considerations assessing the selectivity shift observed in CCR processes.

In conclusion, in this thesis we demonstrated how supramolecular chemistry has the potential to become a game-changer in carbon capture chemistry. More broadly, the results reported in this dissertation show the great diversity that can be originated from the molecular study of dynamic systems and from the application of supramolecular chemistry to fields where it is not commonly exploited.

Optimisation de la chimie covalente des polyamines avec le CO2 par méthodes supramoléculaires

Le procédé de lavage aux amines est la méthode plus mature à ce jour pour limiter les émissions de CO2 dans l'atmosphère.

Tuning the covalent chemistry of CO2 and Polyamines by Supramolecular Means

The Amine Scrubbing process is the most advanced Post-Combustion CO2 capture method at this day. This thesis presents the development of innovative and efficient CO2 capture agents by the application of supramolecular chemistry. Amines react with CO2 by generating libraries of carbamates and carbonates that evolve in a dynamic fashion, thanks to the reversibility of the N-C and O-C bonds. The molecular study of multiple polyamine-CO2 libraries, by NMR, IR spectroscopy and mass spectrometry, highlighted some relevant structure-reactivity relations. Some "template" species were developed to influence the capture equilibria by selectively stabilizing some members of the libraries. With the purpose of designing a cooperative capture mechanism, some Cobalt coordination complexes were applied polycarbamate templates. In water, polyguanidinium salts were exploited as additives to polyamines to amplify bicarbonate against carbamates, leading to capture systems of increased thermodynamic and kinetic efficiency. An integrated CO2 capture and conversion process, by electrochemical reduction with an organometallic catalyst, was developed in collaboration with the University of Turin. Finally, a part of this thesis focused on the study of the spontaneous reactivity between Diethylenetriamine and methanol in the presence of Iron(III) and dioxygen from air, leading to the formation of imidazole products.

Key Words : CO2 capture, Supramolecular Chemistry, Amine Scrubbing, Dynamic Combinatorial Chemistry, Template, Imidazole

Figure 1a :

 1a Figure 1a : Réactions de captage du CO2 étudiées. (1) Carbamatation des polyamines avec la formation de carbamate d'ammonium zwitterionique. Pont salin intramoléculaire indiqué par le trait hachuré. (2) Formation de bicarbamate de diammonium par isomérisation des monocarbamates. Formation de ponts salins intermoléculaires. (3) Hydratation du CO2 catalysée par les amines. Formation de bicarbonate d'ammonium.

Figure 2a :

 2a Figure 2a : Déplacement des équilibres de captage du CO2 par l'introduction de molécules cibles (Templates). (4) Complexes de coordination du Cobalt utilisés comme récepteurs pour les polycarbamates. (5) Sels de polyguanidinium utilisés comme récepteurs pour le bicarbonate.

Figure 3a :

 3a Figure 3a: Réaction de la Diéthylènetriamine avec le Fer(III) et le dioxygène dans le méthanol. La réaction génère un mélange de produits cycliques oxydées, où les produits majoritaires contiennent des groupes imidazole.

  Abbreviations used for amines 1.3-diam: 1.3-diaminopropane 1.4-diam: 1.4-diaminobutane 1.5-diam: 1.5-diaminopentane AMP: 2-amino-2-methyl-1-propanol BUMEA: 2-(butylamino)ethanol DBU: 1,8-Diazabicyclo(5.4.0)undec-7-ene DMA: N,N-Dimethylamine DMAPA: Dimethylaminopropylamine DEA: Diethanolamine DETA: Diethylenetriamine DIPA: Diisopropylamine DMEDA: N,N-Dimethylethylenediamine EDA (1.2-diam): Ethylenediamine EMEA: Ethyl Monoethanolamine Gly: Glycine IDETA: N 1 -(2-(dimethylamino)ethyl)ethane-1.2-diamine MAPA: diamine 3-(methylamino)propylamine MEA: Monoethanolamine MEDA: N-Methylethylenediamine MDEA: N-Methyldiethanolamine MDETA: N-Methyl Diethylenetriamine MMEA: N-Methylethanolamine MMEN: N,N'-Dimethylethylenediamine MTBD:7-Methyl-1,5,7-Triazabicyclo(4.4.0)dec-5-ene NEt3: Triethylamine PMDETA: N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine PZ: Piperazine

Figure 1 :

 1 Figure 1: Visual representation of the Global carbon budget. Reproduced from Le Quéré et al. 5

Figure 2 :

 2 Figure 2 : Scheme of the Petra Nova PCC plant. Reproduced from https://www.nrg.com/case-studies/petra-nova.html.

Figure 3 :

 3 Figure 3: CO2 capture reactions with related thermodynamic parameters. eq.1: carbamation, eq2: carbonation.

Figure 4 :

 4 Figure 4: Piperazine (PZ) carbamation reaction and related calculation of theoretical loading (max th ) and normalized theoretical loading (Nmax th ).

  Qreb=(fCO2HCO2) + (m̄solvCp(Tin-Tout))+(nvap,H2OHvap,H2O) eq.7 where fCO2= CO2 stripping molar flow rate [kg/s] HCO2= reaction enthalpy [kJ/kg] m̄solv= average mass flow rate of solvent through the stripper [kg/s] Cp= specific heat capacity of the solvent [kJ/kg] Tin-Tout = Temperature difference of solvent entering and exiting the stripper [K] nvap,H2O = molar flow rate of water vapor [kg/sec] Hvap,H2O = latent heat of water vaporization [kJ/kg]

  Coal-fired power plants have a Levelized Cost of Electricity (LCOE, average net cost of electricity generation for a plant over its lifetime) of €48/MWh, which rises up to €65-70/ MWh upon PCC implementation.21

Figure 5 :

 5 Figure 5: a) Schematic representation of a PCC unit adapted from http://www.frames-group.com/Products/Galloxol b) Simplified technical-economic analysis of PCC adapted from the study by Li et al. 17 c) Reaction of CO2 capture by Monoethanolamine (MEA) with the generation of an ammonium carbamate capture product (1-MEA).

Figure 6 :

 6 Figure 6: a) Worldwide distribution of large-scale CCUS plants. Reproduced from: https://co2re.co/FacilityData b) Number of operational large-scale CCUS plants in stacks relative to storage (Geological storage) or combined utilisation and storage (EOR=Enhanced Oil Recovery) method.

Figure 7 :

 7 Figure 7: Representation of the most mature CCS (Geological Storage) and CCUS (EOR, Enhanced Oil Recovery) methods.

Figure 8 :

 8 Figure 8: a) Worldwide distribution of CCU plants. Reproduced from: https://co2re.co/FacilityData b) Number of operational and under development CCU plants in stacks relative to the product issues from CO2 transformation/use.

Figure 9 :

 9 Figure 9: Panel of reactions participating in the IAM process with CaO. 41

Scheme 3 :

 3 Scheme 3: Reactions regulating CO2 capture by amines in water (B= Base, Nu=Nucleophile).

Scheme 5 :

 5 Scheme 5: Equilibria involved in CO2 hydration. K= acid dissociation constant, k= reaction rate constant.

Scheme 8 :

 8 Scheme 8: Isomerization of 1-PZ to obtain a dicarbamate (2-PZ) for piperazine.

Scheme 9 :

 9 Scheme 9: Carbonation reaction of TBAE in solventless conditions.

Figure 11 :

 11 Figure 11: Schematic representation of a Dynamic Combinatorial library. 1

Figure 12 :

 12 Figure 12: a) DETA's carbamate species detected in MeOH (protonation neglected for clarity). Green arrows indicate isoenergetic transcarbamation equilibria. b) Transcarbamation equilibrium of 1a-DETA to 2s-DETA + DETA templated by La 3+ , which precipitates as a neutral complex with 2s-DETA.

Figure 13 :

 13 Figure 13: Visual representation of the main objectives and results of this thesis.

Figure 14 :

 14 Figure 14: Molecular structure and abbreviated names of the polyamines cited in this chapter.

Figure 15 :

 15 Figure 15: Intramolecular H-bond in zwitterionic monocarbamate (1-diam) of -diamines (diam). Structures of 1-1.2diam and 1-1.3-diam from modelization.82 Intramolecular HB indicated by a broken line.

Figure 16 :

 16 Figure 16: PZ + CO2 individual component speciation upon increasing normalized loading in CO2(5 M, H2O, 40 °C,

89

 89

Figure 18 :

 18 Figure 18: Shape of a positive (blue line) and negative (black line) cooperative binding plots. The concentration of bound/unbound states increases abruptly upon a small change in conditions, while the concentration of intermediates is low. Reproduced from Hunter et al.[START_REF] Hunter | [END_REF]

Figure 19 :

 19 Figure 19: Ideal isothermal absorption plots for a) a classic absorption system (MEA 30 % w/w in water) b) a cooperative absorption system. Adapted from Long et al.90

Figure 20 :

 20 Figure 20: Statistical distribution of carbamate microstates in -diamine (diam) -CO2 library (1-diam= monocarbamate, 2-diam = dicarbamate) at full loading ( th =0.5).

Figure 21 :

 21 Figure 21: eq.15-d regulating carbamate isomerization for -diamines (diam). 1-diam= zwitterionic monocarbamate, 2-diam= dicarbamate, damm=diammonium.

  can virtually be transformed into an aminoacid (aa), by substituting the two C-N bonds of the carbamate group with two C-C bonds ((1-22), Fig.22, next page). The unitary increase of the carbon chain length accounts for the substitution of a C-N-C by a C-C-C unit.Applied on 2-diam, the exact same bond substitution process yields an -dicarboxylate (dac), through the replacement of two C-N bonds on each carbamate moiety by two C-C bonds((2-22), Fig.22).

Figure 22 :

 22 Figure 22: Modelization of carbamates with static species. 1-diam as aminoacid (aa), 2-diam as a dicarboxylate (dac).

Figure 23 :

 23 Figure 23: Deconstruction of aa in individual ammonium (am) and acid (ac) compounds for the evaluation of the stabilization imparted by intramolecular H-Bond.

Figure 24 :

 24 Figure 24: Deconstruction of dac and dam in individual acid (ac) and ammonium (am) compounds for the evaluation of the destabilization imparted by electrostatic repulsion.

Figure 25 :

 25 Figure 25: EDA + CO2 (H2O). Stack of 1 H and 13 C quantitative NMR spectra upon increasing normalized loading with CO2 (N). a) 1 H qNMR b) 13 C qNMR. Zoom on the sp 3 (on the right side) and sp 2 (on the left side) spectral range.

Figure 26 :

 26 Figure 26: Individual component speciation within the EDA + CO2 library (H2O). xi and  calculated from q 13 C NMR integrations.

Figure 27 :

 27 Figure 27: Plot of the increase of carbamate (N-CO2-1320 cm -1 ) and bicarbonate (HO-CO2-1360 cm -1 ) stretching intensities (I) with the time of CO2 injection for EDA 0.5 M in H2O. (CO2 fixed flow rate: 200ml/min for 100 ml of amine solution, T=25 °C).

Figure 28 :

 28 Figure 28: Cyclic capacity of EDA 0.5 M (H2O). Normalized loading αN calculated by13 C qNMR upon five consecutive absorption-desorption cycles.The Cyclic Capacity (CC) of EDA in 0.5 M concentration was measured with a standardized FTIR-NMR procedure (detailed in Exp). The setup and experimental conditions for the CC experiment were

Figure 29 :

 29 Figure 29: Experimental ( 13 C qNMR) x2-diam (dicarbamate molar fraction) plotted upon increasing normalized loading (N) for diam of increasing chain length (n=-(CH2)n-, 2<n<5).The 13 C qNMR speciations of 1.2-diam (EDA), 1.3-diam, 1.4-diam and 1.5-diam were performed in water in 0.5 M concentration. The superposition of the corresponding dicarbamate molar fractions upon increasing  is presented in Fig.29.

Figure 30 :

 30 Figure 30: Correlation between experimental (NMR) and calculated (pKa) K15-d values for -diamines of -(CH2)n+1-length (n=1 -4). G=-RTlnK15-d (R=8.314 J*K -1 mol -1 , T=273 K).

Figure 31 :

 31 Figure 31: Virtual carbamation combinatorial library based on CO2 and DETA. Numbers refer to occupancy rate (protonation or carbamation).

Figure 32 :

 32 Figure 32: Statistical distribution of carbamate microstates in the DETA-CO2 library at full loading. ( th =0.5)

Figure 33 :

 33 Figure 33: Individual component speciation within the DETA + CO2 library (H2O). xi and  calculated from q 13 C NMR integrations.

Figure 34 :

 34 Figure 34: DETA (0.5 M) + CO2 (H2O). Stack of FTIR spectra during CO2 absorption. (CO2 fixed flow rate 200 ml/min, 1 spectra/sec, 128 scan, =2 cm -1 ).

Figure 35 :

 35 Figure 35: Plot of  , xHCO3-, xNCO2-vs time (CO2 injected at a fixed flow rate, 200 ml/min) for a DETA 0.5 M solution (100 ml).

Figure 36 : 2 .

 362 Figure36: Cyclic capacity of DETA 0.5 M (H2O). Normalized loading αN calculated by13 C qNMR upon five consecutive absorption-desorption cycles. Error bars representing the standard between two independent measures.

Figure 38 :

 38 Figure 38: Isothermal absorption plots for DETA 0.5 M in methanol . () vs pCO2 measured by a) gravimetry or b) FTIR. Non-linear fit for a symmetric sigmoidal plot applied to each set of points. (https://mycurvefit.com/)

Figure 39 :

 39 Figure 39: Absorption rate comparison between EDA, DETA and PMDETA (0.5 M) in water. Normalized IR intensities calculated at 1320 cm -1 (N-CO2 -) for EDA and DETA and at 1360 cm -1 (HO-CO2 -) for PMDETA against time of CO2 injection.The graph in Fig.39enables a comparison of the absorption rates of three equimolar (0.5 M) solutions of EDA, DETA and PMDETA, the permethylated equivalent of DETA (Fig.14, page 33), in water during three absorption experiments performed in the same operating conditions (Exp).

Figure 40 :

 40 Figure 40: Cyclic capacity study for EDA, DETA and PMDETA in 0.5 M concentration. Normalized loading αN calculated by 13 C qNMR upon five consecutive absorption-desorption cycles.

Figure 42 :

 42 Figure 42: Virtual carbamate library of Cy. The -CH2-functions with the same predicted chemical shift are highlighted with the same colour code. For 1-Cy, the -CH2-groups are labelled 1Ha, b, c, d depending on their proximity to the carbamate group.

Figure 43 :

 43 Figure 43: Cy at full loading with CO2 (MeOD): HMBC spectrum optimized for J=5 Hz. Zoom on the correlations of aliphatic -CH2-protons ( 1 Hhorizontal) with downfield carbamate ( 13 Cvertical).

Figure 44 :

 44 Figure 44: Equilibrium of Cy carbamation with generation of 1-Cy and 1-Cy' monocarbamates.

Figure 45

 45 Figure 45: 1 H ROESY NMR of Cy (MeOD) at full loading with CO2.

Figure 46 :

 46 Figure 46: Most common anion recognition moieties. 1) guanidine 1H + ) guanidinium 2) amidine 2H + ) amidinium 3) X=O urea, X=S thiourea 3H + ) X=O protonated urea, X=S protonated thiourea 4) amine 4H + ) ammonium.

  , pKa=9.25) so their anion receptor properties are limited to acidic solutions, while guanidiniums can be exploited in a wider range of conditions (R=H in 1H + , Fig.46, pKa=13.5). Schmidtchen et al. developed macrotricyclic quaternary ammonium receptors for anion recognition in water (Fig.48) 104 .

Figure 48

 48 Figure 48: a) Macrotricyclic polyammonium receptors (12, 13, 14) b) LogK for the association with halides in water.

Figure 49 :

 49 Figure 49: Urea-based receptor for carboxylate recognition. K measured in DMSO at 25 °C. 107

Figure 50 :

 50 Figure 50: Complementary guanidinium/carboxylate geometry.

Figure 51 :

 51 Figure 51: Bicyclic guanidinium receptors for carboxylates. Syn and anti conformation of the guanidinium group (TBA + = tetrabutylammonium, BPh 4-= tetraphenylborate). 108

Figure 52 :

 52 Figure 52: Triguanidinium receptor for citrate in water synthetized by Anslyn et al. 109

Figure 53 :

 53 Figure 53: The Hofmeister series, listing anions for their hydrophobic/hydrophilic character.

Figure 54 :

 54 Figure 54: Virtual thermodynamic cycle of receptor-anion binding in solution.

Figure 56 :

 56 Figure56: HBD Receptors and HBA anions studied byHamilton et al. 112

Figure 57 :

 57 Figure57: Superbases (SB) commonly exploited as CO2 capture agents and proposed reaction (eq.30) with CO2 in anhydrous conditions affording a zwitterionic carbamate(1-SB).

Figure 58 :

 58 Figure 58: Synthesis and XRD structure of 1-TBD.115

Figure 59 :

 59 Figure 59: SPS/CO2 BOLs based on equimolar mixtures of SB (DBU, TMG) and alcohols (ROH). Generation of amidinium/guanidinium alkylcarbonate salts upon CO2 capture. Representation of the hydrogen bonds in the capture adducts by broken lines (black).

Figure 60

 60 Figure 60: a) GBIG reaction with CO2 and water with the generation of the H-bonded GBIG-HCO3 product b) XRD structure of GBIG-HCO3 c) HB of bicarbonate dimers. Bond distances are indicated in Å. Adapted fromWilliams et al. 126

Figure 61 :

 61 Figure 61: Reaction of PyBIG with CO2 and water generating crystalline PyBIGH2-CO3.

Figure 62 :

 62 Figure 62: CO2 capture reaction of the combined Glycine + PyBIG system. 129

Figure 63 :

 63 Figure 63 : Synthesis of the Bis(diethylenetriamine)Co(III) trichloride (CoDCl3) complex 136 , represented with its three geometric isomers.

Figure 64 :

 64 Figure 64: Proposed structure for s-fac-[Co(DETA)2]PO4 complex. Reproduced fromKeene, Searle. 137

Figure 65 :

 65 Figure 65: Simplified structures of the three geometric isomers of CoDX3, showing the triangular unhindered HBD face (on s-fac and u-fac isomers) and simplified structure of [Co(gly or p-ala) (ox)] 2-anions, showing the triangular unhindered carboxylate face (HBA). Readapted from Tatehata et al. 140

Figure 66 :

 66 Figure 66: a) Disproportionation equilibrium of monocarbamate (1a-DETA) into bicarbamate (2s-DETA) templated by CoCl2 in MeOH. b) Amplification factors for the CoCl2 + DETA system in MeOH. c) DFT optimized structure of the substituted complex [Co(DETA)]Cl22sDETAH , drawn in d) for clarity.

  Scheme 33: 1,1'-(ethane-1,2-diyl)diguanidine sulfate (G2SO4) synthesis.

Scheme 35 :

 35 Scheme 35: N,N′′′,N′′′′′′-(Nitrilotri-2,1-ethanediyl)trisguanidine sulfate (G4SO4) synthesis

Figure 67 :

 67 Figure 67: Comparison between individual component speciation upon increasing normalized loading ( ) in water for a) DETA (experimental points hidden for the comfort of the eye) b) DETA + ½ G2SO4 (bisguanidinium sulphate salt of EDA, page 107).

Figure 68 :

 68 Figure 68: DETA + ½ G2SO4 + CO2 Amplification Factors calculated for carbamate/carbonate subsystems upon increasing normalized loading N.

Figure

  Figure69: Ion exchange process in polyamine + bisguanidinium sulphate + CO2 systems in water. eq.14: Carbamate hydrolysis with bicarbonate generation in the absence of guanidiniums. eq.35-1: Carbamate hydrolysis driven by ion exchange with guanidinium sulphate. eq.35-1a: 2a-DETA hydrolysis driven by ion exchange with G2SO4, with the generation of H-bonded G2(HCO3)2 and diammonium sulphate (DETAH2SO4) as exchange products.

Figure 70 :

 70 Figure 70: Comparison between individual component speciation upon increasing normalized loading ( ) in water for a) DETA (experimental points hidden for the comfort of the eye) b) DETA + ½ G3SO4 (bisguanidinium sulphate salt of DETA, page 107). (H2O) libraries upon increasing normalized loading .

Figure 71 :

 71 Figure 71: Amplification Factors upon increasing normalized loading N, calculated for carbamate/carbonate subsystems for a) EDA + ½ G3SO4 (bisguanidinium sulphate salt of DETA, page 107) b) EDA + ½ G2SO4 (bisguanidinium sulphate salt of EDA, page 107).

Figure 72 :

 72 Figure 72: TREN + ½ G4SO4 (trisguanidinium sulphate salt of TREN, page 107) speciation. Amplification Factors calculated for individual species upon increasing normalized loading N.

Figure 73 :

 73 Figure 73: Cyclic capacity of DETA in the presence and in the absence of ½ GiSO4 (i=2,3. Polyguanidinium sulphate salts, structures in page 107) salts in water. Normalized loading αN calculated by 13 C qNMR.

Figure 74 :

 74 Figure 74: Stabilization of lean amine and guanidinium by H-bonding with sulfate.

Figure 75 :

 75 Figure 75: Rate of CO2 absorption (normalized loading  vs time of CO2 injection) monitored by in-situ FT-IR for DETA, DETA + ½ G2SO4, DETA + ½ G3SO4 and PMDETA solutions (H2O). (Structures of polyguanidinium sulphate salts in page 107).

Figure 76 :

 76 Figure 76: DETA + ½ G2(NO3)2 + CO2 Amplification Factors calculated for carbamate/carbonate subsystems upon increasing normalized loading N. (G2(NO3)2 bisguanidinium nitrate salt of EDA. Page 108).

Figure 77 :

 77 Figure 77: Cyclic capacity of DETA ½ G2(NO3)2. Normalized loading αN Calculated by 13 C qNMR. (G2(NO3)2 bisguanidinium nitrate salt of EDA. Page 108).

Figure 78 :

 78 Figure 78: a) XRD structure of G2(HCO3)2. H-Bonds indicated by red broken lines b) interatomic HB distances in Å and bond angles in ° c) Crystalline lattice.

Figure 79 :

 79 Figure 79: DETA + ½ G2Cl2 + CO2 Amplification Factors calculated for carbamate/carbonate subsystems upon increasing normalized loading N. (G2Cl2 bisguanidinium dichloride salt of EDA. Structure in page 108).

Figure 80 :

 80 Figure 80: Cyclic capacity of DETA + ½ G2Cl2. Normalized loading αN Calculated by 13 C qNMR. (G2Cl2 bisguanidinium dichloride salt of EDA. Structure in page 108).

Figure 81 :

 81 Figure 81 : Electrostatic potential maps (Emax = 200 kJ/mol; Emin = -200 kJ/mol) of CoDX3 complexes from semi-empirical calculations at the PM6 level in the gas phase, performed with the Spartan'14 package. X = Cl -(a); X = 2 Cl -, PF6 -(b); X = ClO4 -(c) and X= BPh4 -(d).

Figure 82 :

 82 Figure 82: Anion exchange procedure on CoDCl3 with NH4PF6 and XRD structure of diastereomerically pure mer-CoDCl2PF6.

  All the synthetized complexes (CoDCl3, CoD(ClO4)3, CoDCl2PF6, CoD(BPh4)3) absorb visible light and have a bright orange color. The UV-Vis spectrum of CoDCl3 and CoD(ClO4)3 presents two absorption bands at 465.5 nm and 343 nm relative to d-d transition (blue, grey lines in Fig.83), in agreement with literature.142,145 The 343 nm d-d transition of CoD(BPh4)3 (orange in Fig.83) is covered by the absorption of BPh4 -, whose aromatic moieties absorb UV radiation starting from =300 nm.

Figure 83 :

 83 Figure 83 : UV-Vis spectra of 15 mM 1) CoDCl3 (grey line, H2O) 2) CoD(ClO4)3 (blue line, H2O) 3) CoD(BPh4)3 (orange line, MeOH).

Figure 84 :

 84 Figure 84: Blue points: average absorbance variation (A) at 465 nm for two UV-Vis titrations of CoD(ClO4)3 with CitrateTMA3 (H2O, 25 °C). Error bars represent rmsd. Grey points: Ionic strength control titration CoD(ClO4)3 vs TMAClO4.

Figure 85 :

 85 Figure 85: Blue points: average absorbance variation (A) at 465 nm for two UV-Vis titration of CoD(BPh4)3 with NH2CO2NH4 in methanol. (25 °C) Error bars equal to rmsd. Grey points: Ionic strength control titration CoD(BPh4)3 vs NaBPh4.

  H NMR, provided that the HBD groups, in this case the R2NH/RNH2 protons on the DETA ligands of the CoDX3 complexes, are detected in the spectra. The R2NH/RNH2 protons of CoDX3 complexes can be detected by 1 H NMR in aprotic solvents, such as D6-DMSO. Due to the concomitant presence of three diastereomers, the 1 H NMR spectrum of CoDCl3 in D6-DMSO displays nine R2NH/RNH2 signals, spread over a wide range of chemical shifts (3.0 -8.5 ppm, Fig.86, next page). A preliminary attribution of the R2NH peaks to the corresponding isomer was done by comparison with literature values, 146 but the separation of the isomers would be required to complete the task for the R-NH2 protons. The chemical shifts of the R2NH/RNH2 signals of the four isolated complexes (CoDCl3, CoD(ClO4)3, mer-CoDPF6Cl2, mer-CoD(BPh4)3) vary significantly in the 1 H NMR spectra in D6-DMSO. A progressive upfield shift of the HBD signals emerges from the stack of 1 H NMR spectra in Fig.86 (next page).

Figure 86 :

 86 Figure 86 : Stack of 1 H NMR spectra of CoDCl3, mer-CoD(Cl2,PF6), CoD(ClO4)3, mer-CoD(BPh4)3 in D6-DMSO.

Figure 87 : 1 H

 871 Figure 87: 1 H NMR (D6-DMSO) issued from the mer-CoD(BPh4)3 + ammonium acetate sample. a) mer-CoD(BPh4)3 b) t0, mer-CoD(BPh4)3 + ammonium acetate c) t1= 24 h.

Figure 88 a

 88 Figure 88 a) Voltammetries in water (KCl 0.1 M, WE: RDE carb3mm, 500 tr/min RE: SCE). Grey line: CoDCl3 5 mM. Black line: CoCl2 (5mM) + 4 DETA under Ar. Blue line: CoCl2 (5 mM)+ 4 DETA after exposure to air and completion of oxidation process (OCV). b) OCV: spontaneous evolution of the potential (V) in the CoCl2 (5 mM) + 4 DETA solution (KCl 0.1 M) upon exposure to air.

Figure 89 :

 89 Figure 89: Plausible oxo/peroxo complexes intermediates in CoDCl2 oxidation to CoDCl3. Counterions omitted for clarity.

Figure 90 :

 90 Figure 90: UV-Vis spectral evolution CoCl2 + 4 DETA (H2O) upon air fluxing. (1 spectrum/s).

Figure 91 :

 91 Figure 91: a) Voltammetries in methanol (TBACl 0.1 M, WE: RDE carb3mm, 500 tr/min, RE: Ag/Ag+). Black line: CoCl2 (5 mM) + 4 DETA under Ar. Blue line: CoCl2 (5 mM) + 4 DETA after exposure to air and completion of oxidation process (OCV). b) OCV: spontaneous evolution of the potential (V) in the CoCl2 + 4 DETA solution (MeOH, TBACl 0.1 M) upon exposure to air.

Figure 92 :

 92 Figure 92: OCV of CoCl2 (5 mM) + 4 DETA in MeOH (TBACl 0.1 M). Three electrodes setting (WE: RDE, CE: Pt, RE: Ag/Ag + ). Black line: CoCl2 + 4 DETA + Air; Red line: CoCl2 + 4 DETA-CO2 + Air. Air fluxed at a fixed flow rate 100 ml/min.

Figure 93 :

 93 Figure 93: Representation of the activation energy for the oxidation reaction in CoCl2 + n DETA solutions with the in-situ formation of the Bis(Diethylenetriamine)Co(III) complex. Grey background: under Ar, spontaneous formation of Bis(Diethylenetriamine)Co(II) complex. Blue background: Upon contact with Air (O2), generation of the intermediate peroxo dinuclear Co(III) complexes. Slow transition towards the final mononuclear product (Co(III)(DETA)2). Red background: In the presence of CO2 and oxygen, formation of intermediate dinuclear complexes, and fast transition towards the final complex thanks to the supramolecular binding with the dicarbamate.

Figure 94 :

 94 Figure 94: XRD structure of [Fe(DETA)2]Cl2.

Figure 95 :

 95 Figure 95: Synthetic methods for imine formation. a) Traditional method, condensation of carbonyls with amines b) Oxidative cross-coupling of alcohols with amines c) Oxidative self-coupling of primary amines d) Oxidative dehydrogenation of secondary amines. 150

Figure 96 :

 96 Figure 96: NMR internal references. R1: TMPCl, R2: Pinacol.

2 .

 2 NMR study of FeCl 3 -DETA solutions in methanol 2.1. 1 H NMR qualitative monitoring

Figure 97 :

 97 Figure 97: Stack of 1 H NMR spectra of DETA (0.5 M)-FeCl3 (33% mol) in MeOD upon increasing reaction time in aerobic conditions. Zoom on the aliphatic (2.5 -3.8 ppm) and aromatic (7 -8 ppm) spectral range.

Figure 98 :

 98 Figure 98: 2D NMR spectra (COSY, HSQC, HMBC) confirming the generation of imidazoles(Im) from the reaction of DETA with FeCl3 in MeOH. Spectra are zoomed on the "aromatic" 1 H (6.5 -8.5 ppm) and13 C (120 -140 ppm) spectral range.

Scheme 60 :

 60 Scheme 60: Mechanism of o-TETA generation from oxidization and transimination of two DETA molecules.The first step of o-TETA formation is the oxidation of the secondary amine of DETA (1-46). The electrophilic imine can react through intramolecular transimination with the nucleophilic primary amine of a second DETA molecule (2-46). An EDA fragment is eliminated, resulting in the generation of o-TETA(3-46). The generation of EDA as a byproduct of transimination agrees with its detection by 1 H and 13 C NMR at the end of the reaction and by HRMS at t0 (Fig.99).

Scheme 61 :

 61 Scheme 61: Plausible intramolecular condensation of o-TETA to obtain the fragment at m/z (exp) = 128.1185 (I1). A plausible mechanism of intramolecular transimination starting from o-TETA and leading to the generation of a fragment corresponding to this experimental m/z is proposed in Scheme 61. The mechanism starts with a o-TETA oxidized on one of the terminal moieties. Intramolecular nucleophile attack (1-47) generates a six-membered cycle corresponding to a N-substituted piperazine structure. After elimination of the pendent amine group as ammonia (2-47), an unstable

Figure 100 :

 100 Figure 100: CRYO-HRMS spectrum of DETA + FeCl3 (MeOH) at t=4 hours. The most intense peaks are attributed to the corresponding products.Table showing the agreement between experimental and calculated m/z.

Scheme 62 :

 62 Scheme 62: Plausible reactions leading to I2 (m/z= 126) and 2I2 (m/z=251) generation.I2 could be obtained from the oxidation of I1 ((1-48), Scheme 62), that could involve either the imidazoline or the piperazine heterocycles. The same m/z could also correspond to Pi1, a doubly oxidized N-substituted piperazine which could be originated from o-TETA intramolecular transimination ((1-47), (2-47)) followed by oxidation. Alternatively, I2 could also be plausibly obtained by the intramolecular transamination of o-TETA ((2-48)-(5-48)) depicted in the lower part of Scheme 62.

Figure 101 :

 101 Figure 101: HPLC-HRMS identification of the three most abundant products issued from the aerobic reaction of DETA with FeCl3 in methanol.

Figure 102 :

 102 Figure 102: HPLC-HRMS identification of three minority products issued from the aerobic reaction of DETA with FeCl3 in methanol.

Figure 103 :

 103 Figure 103: Scope of polyamines tested with the standardized reaction conditions with FeCl3 (10% mol) in MeOH.

Figure 104 :

 104 Figure 104: Schematic representation of the main products issued from the oxidative abiotic metabolism of DETA in the presence of FeCl3 and methanol, leading to products structurally close to nitrogen bases or PLP.

Figure 105 :

 105 Figure 105: Scheme of Integrated Carbon Capture and Recycling by catalytic Electroreduction on amine solutions.

F=

Figure 109 :

 109 Figure 109: Schematic preparation of Mn-CC (Carbon Cloth functionalized with Mnabpy) via in-situ diazonium salt reduction. (TCA= trichloroacetic acid).

Figure 110 :

 110 Figure 110: MnbpyOH as CO2 reduction catalyst presenting a local proton source. Mn-H, hydride intermediate responsible for formate production from CO2 reduction.

Figure 111 :

 111 Figure 111: CV of Mnabipy (0.25 mM) in MeOH (TBAPF6 0.1 M). Blue: under Ar. Orange: under CO2. (WE: glassy carbon, CE: Pt, RE: SCE. Current range 50 mA, scan rate 100 mV/s).

Figure 112 :

 112 Figure 112: CV of Mnabipy (0.25 mM) in MeOH (TBAPF6 0.1 M) under CO2. Orange: without DETA, Blue: with DETA 25 mM, Grey: with DETA 50 mM. (WE: glassy carbon, CE: Pt, RE: SCE. Current range 50 mA, scan rate 100 mV/s).

Figure 113 :

 113 Figure 113: TON H2 (blue) and CO (red) vs t (min) obtained from the CPE under CO2 atmosphere with Mn-CC in MeOH (TBAPF6 0.1 M). (E=-1.35 V, WE Mn-CC RE : SCE, CE : Pt)

  𝑁 ) = molar fraction of species i, at a given αN, in the templated library 𝑥 𝑖 0 (𝛼 𝑁 ) = molar fraction of species i, at a given αN, in the templated library 𝑥 𝑇 = molar fraction of the template 𝑛 (𝐶𝑂 2 )𝑖 = number of CO2 molecules bound on species i 𝑛 𝑁 = number of CO2-binding sites on the sorbent molecule 200

Figure E 1 :

 1 Figure E 1: FTIR-NMR Calibration curves obtained from DETA 0.5 M, leading to the FTIR absorption coefficients for bicarbonate stretching at 1360 cm -1 and for carbamate stretching at 1320 cm -1 .

Figure E 2 :

 2 Figure E 2: Operational setup for the isothermal absorption plot measure for DETA in MeOH by FTIR.

13 C

 13 NMR (75 MHz, D2O): δ 157.15 (2x C=NH), 40.06 (2x -CH2-). LR ESI-MS, m/z: 145.12 [M-HSO4 -] + (calc.: 145.12) Data matched with literature.b. N,N-di(2-guanidinylethyl)amine sulfate hydrate (G 3 SO 4 ) G3SO4 was synthetized following the protocol by Bottcher et al.143

205 13 C

 13 NMR(75 MHz, D2O): δ 156.99 (2x C=NH), 46.5 (CH2-NH-CH2), 40.55 (2x G-CH2-) LR m/z: 286.13 (M+H, predicted 286.13), 188.16 predicted) Data matched with literature.c. 1,1'-(ethane-1,2-diyl)diguanidine dichloride (G 2 Cl 2 )Procedure 1: Anion exchange G2Cl2 was synthetized by anion exchange starting from G2SO4 on an ion exchange resin. Amberlyst® A26 hydroxide form (50 g, 50 mmol NaOH) was sonicated for 30 min in 200 ml of 1M HCl. The resin was filtered and washed with deionized water until neutral pH to eliminate the excess of HCl. The resin was loaded into a 320 mm long column of 3 mm diameter, and it was packed with compressed air. 500 mg of G2SO4 (1.75 mmol) were dissolved by heating at 60 °C in 10 ml of deionized H2O. The solution was loaded onto the column. Elution was performed with 3x volumes of deionized H2O. The eluted solution was lyophilized to collect the exchanged salt as a white solid. The products were analyzed by ESI-MS in positive and negative mode to confirm complete SO4 2-substitution by Cl -. (363.35 mg, 1.69 mmol, 96.6% yield)1 H NMR (300 MHz, D2O): δ 3.81 (s, 4H, -CH2-CH2-)13 C NMR (75 MHz, D2O): δ 156.96 (2x C=NH), 40.01 (-CH2-CH2-) LR ESI-MS (-) 215.10 (G2Cl2-H, 215.06 predicted)

Figure A 2 :

 2 Figure A 2: Stack of quantitative 13 C NMR spectra of DETA 0.5 M upon increasing normalized loading N. (H2O) a) sp 3 carbons b) sp 2 carbons.

  219

Figure A 4 :

 4 Figure A 4: Stack of q 13 C NMR spectra of TREN 0.5 M upon increasing normalized loading N. (H2O) sp 2 carbons.

Figure A 5 :Figure A 6 :Figure A 9 :

 569 Figure A 5: Cy stack of 13 C NMR in the lean and rich state. (MeOD)

Figure A 12 :Figure A 11 :Figure A 13 :Figure A 15 :Figure A 16 :Figure A 17 :

 121113151617 Figure A 12: Individual component speciation within the EDA + ½ G2SO4 + CO2 system.

Figure A 18 :Figure A 28 :Figure A 29 :

 182829 Figure A 18: SSRMN 13 C CPMAS 10KHz on G2(HCO3)2.

Table 1 :

 1 Principal products obtained from CO2 utilization, with related production rate and CO2 consumption rate expressed in ton / year.

  Scheme 1: Integrated CO2 capture and catalytic Hydrogenation by Rezayee et al.

42 DMA= Dimethylamine. DMF= Dimethylformamide.

Table 2 :

 2 Thermodynamic and kinetic parameters of CO2 absorption for polyamines and MEA.

	Values reproduced from a)

EDA and PZ stand out from the series as they present an inferior Habs to MEA by 18 kJ/mol, which should translate into an inferior Qreg.

suggested that Amine max b,c max CC c Initial absorption rate a, c Initial desorption rate a Habs (kJ/mol) b,c

	the Nmax of -diamines (diam) increased with the number of -CH2-spacers between the amine
	groups. 82
	MEA	0.55 c	0.55	0.83	3.78 c	1.23	-88.91
					0.67 a
	PZ	0.81 c	0.40	1.36	9.12 c		-70.00 c
	P1	1.44 c	0.48	0.50	5.00 c
	P2	1.43 c	0.72	0.65	4.02 c
	P3	1.30 c	0.65	0.84	4.89 c
	EDA	0.95 b	0.48		0.57 a	1.42	-70.44 b
	DETA	1.33 b	0.44		0.59 a	2.35	-89.48 b
	TETA	1.66 b	0.42		0.67 a	2.63	-84.91 b
	TEPA	2.12 b	0.42		0.74 a	4.14	-85.87 b

Table 3

 3 lists the % of degradation of some linear polyamines, PZ and MEA for comparison. The values were obtained by two separate studies as the % increase of the Gas Chromatography peaks

of Amine Amine loss % (1) Amine loss % (2)

	degradation products upon heating of loaded amine solutions for multiple days ((1) T=140 °C for 15
	days 86 , (2) T=120 °C for 3 days 53 ).
	Kim et al. showed that, expectedly, PZ was the most thermally resistant amine amongst the ones
	tested, presenting a 0.03 % of degradation after three days at 120 °C, 140 times less than MEA in
	the same conditions (4.2 %). 53
	MEA	42 (a)	4.2
	PZ		0.03
	TMEDA	15 (a)
	TMPDA	21 (a)
	TMBDA	80 (a)
	PMDETA	89 (b)
	P1		39.9
	P2		61.3
	P3		89.5

Table 4 :

 4

Calculated LogK15-d for the isomerization of 1-diam to 2-diam and damm for -diamines of increasing chain length (-(CH2)n+1-).

Table 6 :

 6 LogK

	for the binding of H2PO7 3-and fumarate anions by ammonium (b) and guanidinium (a) receptors. LogK
	calculated in (1) water or (

Table 7 :

 7 Tabulated pKa values of HBD precursors for anion receptors.

	HBD	Structure	pKa (water, 25 °C)
	Guanidine (CH5N3)		13.27 ± 0.70
	Acetamidine (C2H6N2)		12.49 ± 0.40
	Urea (CH4N2O)		14.43 ± 0.50
	Ammonia (NH3)		9.25

Table 8 :

 8 Binding constants (logK) and thermodynamic contributions (H and S) obtained by ITC or NMR titration for

	HBD	HBA	logK	H [kJ/mol]	S [kJ/mol*K]
	21 a	26	2,89	-4,0	+0,3
	22 a	26	4,04	-4,8	+2,0
	23 b	26	2,86	+3,8	+26
	24 b	26	2,92	+2,5	+26

the association of receptors 21 and 22 (DMSO) and 23 and 24 (MeOH/H2O 3/1) with 26 at T=20 °C.

The thermodynamic contributions to the association differed remarkably for neutral

(21, 22)

receptors, studied in DMSO, and charged receptors

(23, 24)

, studied in MeOH/H2O (3/1). The former

Table 9 :

 9

pKa values of superbases in water and acetonitrile

(MeCN)

.

114

  The blends are liquid both in the lean and in the rich state and CO2 can be released by stripping with an inert gas (N2) at room temperature. 120 CO2 capture induces a marked polarity switch in the solvent: a DBU/1-hexanol mixture presents a polarity close to MeCl in the lean state, while it is close to DMF in the loaded state. Consequently, SPS can be exploited to simplify separation procedures, switching from one state to the other by stripping with either CO2 or an inert gas.38

Jessop et al. introduced switchable polarity solvents (SPS) in 2005. 120 SPS are solvents based on equimolar blends of amidines/guanidines with alcohols/amines, that react with CO2 in anhydrous conditions yielding liquid amidinium/guanidinium-alkylcarbonate/carbamate salts. The reaction with CO2 involves a nucleophilic attack by the alcohol/amine, assisted by the SB which abstracts a proton, resulting in an alkylcarbonateprotonated SB salt product (1C-SBH + , Fig.

59

, 1C-TMGH + and 1C-DBUH + ).

  The crystals can be heated to release CO2 and then they can be recycled for another capture cycle.129

The carbamate is hydrolyzed (3-32) by the OH -generated from water dissociation, catalyzed by PyBIG (2-32), which regenerates the aminoacid. Carbonate is obtained from bicarbonate deprotonation (4-32), and it precipitates with PyBIGH2 + (5-32).

Table 10 :

 10 CO2 Capture indicators of polyamine sorbents in the absence or presence of various salts.

xHCO3 vs amine: molar

	fraction of bicarbonate with respect to the amine; xHCO3 vs CO2tot: % of CO2 captured as bicarbonate; max: maximal
	normalized loading; AFmax(HCO3 -) maximal HCO3 -amplification factor

  Figure 69, page 111), thereby EDA and DETA saturate for the 2:1 amine to salt ratio and can no longer contribute to the amplification.-Monovalent guanidinium sulfate G1SO4. Guanidinium sulfate (C2H12N6SO4) can be considered as the precursor of G2SO4 lacking structural preorganization. The smaller bicarbonate amplification detected with this salt (AFHCO3-=0.25 for DETA + ½ G1SO4 vs AFHCO3-=0.56 for DETA + ½ G2SO4) proves the relevance of the supramolecular approach, as the preformed bisguanidinium salts induces a 2 times stronger bicarbonate amplification than G1SO4.

	upon anion exchange (
	Amine	T	xHCO3 vs amine xHCO3 vs CO2tot (%) max	AFmax(HCO3 -)
		0	0.73	40	0.61	-
		0.5 G1SO4	0.43	28	0.51	0.15
		1 G1SO4	0.54	29	0.49	0.21
		2 G1SO4	0.84	59	0.48	0.24
		0.5 G2SO4	0.64	56	0.64	0.56
		G2SO4	0.74	50	0.51	0.34
	DETA	0.5 G2Cl2 0.5 G2(NO3)2	1.26 1.25	68 67	0.63 0.62	0.87 1.01
		0.5 G3SO4	0.81	61	0.63	1.07
		G3SO4	0.93	46	0.52	0.59
		0.5 Na2SO4	0.47	31	0.51	0.24
		1 Na2SO4	0.76	23	0.47	0.37
		2 Na2SO4	0.89	59	0.55	0.27
		NaCl	0.32	24	0.43	-0.04
		NaNO3	0.48	31	0.52	0.13
		0	0.31	26	0.56	-
		0.5 G1SO4	0.44	37	0.60	0.25
	EDA	0.5 G2SO4	0.21	34	0.63	0.24
		G2SO4	0.67	57	0.60	0.35
		0.5 G3SO4	0.33	36	0.55	0.38
		G3SO4	0.59	40	0.51	0.51

  63, which is inspired from the mechanism recently proposed by Foss et al. for the aerobic synthesis of 1.2-substituted imidazoles catalyzed by FeCl3. 164

	Scheme 63: Mechanism of FeCl3-catalyzed oxidation of DETA and methanol. O1) Catalytic mechanism of MeOH oxidation
	to formaldehyde. O2) Catalytic mechanism of amine oxidation to imine. Ligands (Ln, solvent molecules) omitted for
	clarity.

Table 12 :

 12 TON (Turnover Numbers) and FE (Faradic Efficiencies) obtained by CPE with Mn-CC electrode in MeOH (TBAPF6 0.1 M as supporting electrolyte) with and without amines (1 mM concentration). Time refers to the lifetime of the catalyst before deactivation, but all solutions were subject to electrolysis for 22 hours at E=-1.35 V.

  Chapter III a. 1,1'-(ethane-1,2-diyl)diguanidine sulfate (G 2 SO 4 ) G2SO4 was synthetized following the protocol by Bottcher et al.143 S-methylisothiourea hemisulfate (196 g, 0.704 mol) was solubilized in 100 ml H2O/EtOH 1:1 in a round-bottom flask. The solid was dissolved by heating at 60 °C and stirring at 1400 rpm.

  Cette thèse présente le développement d'agents de captage du CO2 innovants en faisant appel aux principes de la chimie supramoléculaire. Les amines réagissent avec le CO2 en générant des librairies de carbamates et carbonates qui évoluent de façon dynamique grâce à la réversibilité des liaisons N-C et O-C. L'étude du point de vue moléculaire, par spectroscopies RMN, IR et spectrométrie de masse de plusieurs librairies de polyamines avec le CO2 a permis d'identifier des relations structure-réactivité. Des espèces cible (« template ») ont été développées pour influencer les équilibres de captage en stabilisant sélectivement certains adduits. Dans le but d'obtenir un mécanisme de captage coopératif, une série de complexes de coordination du Cobalt ont été étudiés en tant que « templates » pour les polycarbamates. Dans l'eau, des sels de polyguanidinium ont été exploités comme additifs aux polyamines pour amplifier la formation du bicarbonate, menant à des systèmes de captage plus efficaces thermodynamiquement et cinétiquement. Un système intégré de captage et conversion du CO2 par voie électrochimique a été développé en collaboration avec l'Université de Turin. Finalement, une partie de cette thèse est consacrée à l'étude de la réactivité spontanée entre la Diéthylènetriamine et le méthanol en présence de Fer(III) et de dioxygène, menant à des structures macrocycliques type imidazole.

Mots Clés : Captage du CO2, Chimie Supramoléculaire, Lavage aux amines, Chimie Dynamique Combinatoire, Template, Imidazole

H NMR (400 MHz, MeOD): δ 3.77(s, 16H, -CH2-), δ 7. 16H, Data matched with literature.

H NMR (300 MHz, D2O): δ 3.31 (t, J =

2.80 Hz, 4H, 2x G-CH2-), 2.80 (t, J = 3.31 Hz, 4H, -CH2-NH-CH2-).

H NMR (300 MHz, MeOD): δ

 16H,, δ 6.79 (d, 2H, Ar-H), δ 6.99 (t, 2H, Ar-H), δ7.31 (m, 1H,

Annexes

C qNMR.

Remerciements

Je tiens tout d'abord à remercier les membres du jury, Mme Micheline Draye, Mme Elsje Alessandra Quadrelli, Mr Bruno Andrioletti pour avoir accepté d'évaluer mon travail. Merci à Mr David J. Heldebrant et à Mr Joseph Moran pour avoir examiné mon manuscrit.

Anion recognition by Cobalt Werner complexes

The anion receptors presented in the previous sections were based on organic HBD groups such as guanidiniums and ammoniums. This section presents a less common type of anion receptor, receptors based on Co(III) Werner complexes, i.e. coordination compounds where the ligands coordinate to the metal cation via their lone electronic pairs. The association of anions with Werner complexes implies the displacement of the counterions that are initially associated with the cation. Thereby, the binding in this case is based on an anion exchange process taking place in the second coordination sphere of the complexes.

[Co(NH 3 ) 6 ]Cl 3 as anion binder

Sharma, Bala and coworkers exploited Co(III) complexes as receptors for oxyanions, showing that they could associate in their second coordination sphere with a variety of anions by acting as H-bon donors. [133][134][135] Their studies consisted in performing anion exchange reactions between chloride or bromide amine-Co(III) complexes with a salt of the target anion. The complexes were analyzed by XRD as they generated crystalline insoluble aggregates. Hexammine Cobalt(III) chloride ([Co(NH3)6Cl3], Scheme 31) presents six ammonia ligands in octahedral coordination: each proton can donate a H-Bond. This complex was studied as a receptor for aromatic sulfonate (Ar-SO3 -) anions. The chloride complex was reacted with a stoichiometric quantity of the sodium salt of mesitylene sulfonate (Scheme 31), resulting in a crystalline product Scheme 36 : 1,1'-(ethane-1,2-diyl)diguanidine dinitrate (G2(NO3)2) synthesis by anion exchange.

Scheme 37 : 1,1'-(ethane-1,2-diyl)diguanidine dichloride (G2Cl2) synthesis by anion exchange.

• Only for G2Cl2 by anion metathesis with BaCl2 (Exp).

Scheme 38 : 1,1'-(ethane-1,2-diyl)diguanidine dichloride (G2Cl2) synthesis by ion metathesis.

The exchange products were analyzed by NMR and ESI-MS in positive and negative mode to confirm full substitution of sulfate.

Chapter IV: Aerobic, Iron catalyzed process to access substituted imidazoles

Context

The template properties of CoCl2 for bicarbamate in MeOH were highlighted in our group 3 and were described in Chapter III, Part C of this document.

Cobalt is a strategic metal used in crucial technologies and subject to an increasing demand due to the growing production of consumer electronics and electric vehicles, which contain batteries based on La2Ni9Co alloys. 148 We thereby chose to explore the bicarbamate/carbonate template properties of FeCl3 as a more sustainable alternative to Cobalt.

Scheme 48: Scheme of ideal polycarbamate amplification by FeCl3 with electrochemically induced CO2 desorption.

We were also interested in exploiting the redox properties of the metal templates to trigger CO2 desorption. We reasoned that the electrochemical reduction of the metal could favor CO2 release, due to the increased affinity of the lean amine for the reduced cation (Scheme 48). This idea was inspired by the Electrochemically Mediated Amine Regeneration (EMAR) process developed by Stern et al., which exploits the differential affinity of polyamines for Cu and Cu(II) to induce CO2 desorption. 149 The reduction of Fe(III) to Fe(II) is a well-known reaction participating to many biological processes and which can be reversed by reaction with molecular oxygen. Consequently, the Fe(III)/Fe(II) redox couple seemed a good candidate for the combined CO2 capture and EMAR process in Scheme 48. Surprisingly, during the preparation of the FeCl3-DETA solutions in MeOH we detected a spontaneous reactivity in solution prior to CO2 introduction. The 1 H NMR analysis of the solution

Iron-catalyzed Imidazole syntheses

Scheme 53: Van Leusen reaction for imidazole synthesis.

Imidazoles are relevant compounds for pharmaceutical industry and agrochemistry. 159 The traditional synthesis is based on the Van Leusen reaction, wherein an aldimine reacts with TosMIC (p-toluenesulfonylmethyl isocyanide) in the presence of a base (Scheme 53). 160 Many efforts were made to develop new imidazole syntheses avoiding the utilization of hazardous isocyanide. Most of the alternative reaction pathways rely on metal catalysis.

A recent example of imidazole synthesis with an Iron catalyst was reported by Putta and coworkers. 154 The authors presented a catalytic method to access 1.2-disubstituted benzimidazoles starting from primary alcohols and phenylenediamines (PDA) through ADC with an Iron catalyst. In this case, the reaction was conducted in anaerobic conditions.

The Fe(III) catalyst (I42, Scheme 54), similar to the one exploited by Guethmundsson et al., was activated by CO abstraction by TMANO. I42' acted as H2 borrower in the oxidation of the primary alcohol (1-42), delivering an aldehyde.

Scheme 54: ADC catalysed by Iron for 1,2-substituted benzimidazole synthesis. 154

Mass spectrometry study for product and mechanism identification

High resolution mass spectrometry (HRMS) enabled the real-time analysis of the species in solution with high precision, as it showed the ratio of ion mass with charge number (m/z) with deviations below 5 ppm with respect to the calculated m/z. The study of the individual fragmentations of selected ions by MS/MS (tandem mass spectrometry) was exploited to identify the molecular structures corresponding to some m/z. Multiple structures could be drawn for a same m/z ratio. The most probable ones, considering the starting materials and the signals detected by 1 H and 13 C NMR, are proposed in the following discussion. Some mechanisms are also presented to justify the generation of the most abundant species. However, to complete the characterization of the molecular structures and of the mechanism, the isolation of the products from the mixture would be required.

CRYO-MS monitoring in aerobic conditions

A real-time monitoring of the reaction was performed by HRMS in cryogenic (CRYO) conditions, i.e. with a cold-spray ionization source (CSI), on aliquots extracted from a DETA + FeCl3 (33% mol) solution in MeOH. The refrigeration and the mild ionization method (Electrospray Ionization, ESI) were exploited to avoid the cleavage of the coordination bonds. This type of measure had already been exploited in our group for the MS detection of carbamates. 3 The samples could be injected directly in the spectrometer without the pre-treatment to eliminate Iron, which should increase the accuracy of detection with respect to NMR.

In total, seven HRMS spectra were measured over the course of the 4 hour-long reaction. The first (t0, Fig. 99) and last (t=4 h, Fig. 100) spectra are displayed and commented in this section.

Objectives

The goal of the collaboration with the research group of prof. Gobetto in the University of Turin was the development of an integrated CCR process enabling to transform CO2 into high-added value products by the direct application of an electroreduction process on loaded amine solutions.

We aimed at integrating CO2 reduction by heterogeneous Manganese (Mn) bipyridyl (Mnbpy) catalysts onto the CO2 capture system based on DETA in methanol. This absorbent was selected because it presents a cooperative capture mechanism, favored by the supramolecular interactions between carbamate and ammoniums in methanol. Consequently, we reasoned that this solvent would also favor the interactions between the catalyst and carbamates.

The ideal scenario for this CCR process was that carbamates would act as a reservoir of activated CO2 in the electrolytic cell, increasing the concentration of the target in proximity of the reduction catalyst. CO2 would have eventually been released from the amine in a "pre-activated", bent state (red 2, Fig. 106) and it would have reacted with the catalyst with a reduced thermodynamic and kinetic barrier compared to when the gas is in the pure, dissolved state. Alternatively, the reduction could have involved carbamates, resulting in organic reduction products (red 1, Fig. 106).

Achievement

The preliminary experiences were performed in November 2019 in the University of Turin. The results were continuously consolidated up to now thanks to the collaboration between the research groups in Lyon and in Turin.

The reduction systems were studied by Cyclic Voltammetry (CV) and Controlled Potential Electrolysis (CPE) coupled to GC and NMR analysis to identify and quantify the reduction products.

To reduce CO2 we exploited Mn bipyridyl complexes, well-known and efficient reduction catalysts which have been widely exploited and improved over the last years in the research group from Turin. [168][169][170][171][172] At first, the catalyst was applied in homogeneous conditions to characterize its reduction mechanism in methanol, as it had never been studied before in this solvent. To increase the durability and efficiency of the catalyst, we proceeded with its functionalization on a conductive support, an approach which brought to remarkably high Turnover Numbers (TON) of CO2 reduction products, with a remarkable selectivity for formate (HCO2 -). ligand), will translate into an irreversible reduction. Examples of reversible and irreversible reductions will be presented hereafter. Mn and Re bipyridyl catalysts, such as fac-Re(bpy)(CO)3Cl (Rebpy) 175 and fac-Mn(bpy)(CO)3Br (Mnbpy) 176 (Fig. 107), are considered the most promising family of transition metal catalysts for CO2 reduction. The following discussion will focus on Mn-based catalysts, a more sustainable alternative to Re-based ones due to the higher availability and inferior costs of the former metal. The reduction mechanism of Mnbpy in MeCN, with 5% H2O as proton source, was characterized for the first time by CV by Bourrez et al., and it follows reactions r1-r4 (next page). 176 The CV of Mnbpy under Ar presents two reduction peaks at E=-1.65 and -1.80 V (Fig. 108) corresponding respectively to reductions r1-r3 and r4.

The CV showed a 3-fold increase of the catalytic current at E=-1.35 V (Fig. 112, blue) compared to the amine-free system (Fig. 112, orange). No further increase in reduction current was obtained by increasing DETA concentration (50 mM, Fig. 112,grey).

The CV suggests a positive influence of DETA on the catalyst's CO2 reducing activity. However, this experiment only shows effects operating on a time scale of few seconds, after which the catalyst could be deactivated. The true evaluation of the catalytic efficiency was obtained by CPE, an experiment taking place during multiple hours.

c. Controlled Potential Electrolysis

We performed some CPE experiments with Mnabpy in homogeneous conditions in methanol. In MeCN, Mnabpy gave CO as the main product of CO2 reduction, with a TON=12 (t=90 min, E=-1.73 vs Ag/AgCl, FE=93%). 168 Carbon monoxide production in methanol (E=-1.35 V) is inferior than in MeCN, as a TONCO=2.6 was obtained at the end of electrolysis (Table 11). Interestingly, formate (HCO2 -), which was not produces nor in MeCN nor in water, 168 was detected in methanol with a non-negligible TONHCOO-=6.

The activity of the Mnabpy catalyst was completely suppressed in CO2-loaded solutions of DETA in 50 mM and 25 mM concentration. The efficiency did not improve by lowering the amine concentration to millimolar values, as in the presence of DETA 1 mM no reduction products were detected (Table 11). The unsatisfactory results obtained by CPE in homogeneous conditions proved that, despite the current increase observed by CV, Mnabpy was rapidly deactivated in methanol, and even more rapidly in presence of DETA despite the pre-loading with CO2. We consequently turned to the heterogeneous approach to improve the stability and efficiency of the catalyst.

Catalyst

Heterogeneous catalysis of CO 2 reduction with Mn-CC in methanol

Mn-CC was obtained by the procedure schematized in Fig. 109 (page 181) and detailed in the Experimental section. The functionalized material was used as working electrode (WE) in the cathodic compartment of the electrolytic cell for CPE.

Due to the high surface area of Mn-CC, explorative CVs could not be performed with this type of electrode, as the potentiostat would saturate upon the application of a potential. Consequently, the reduction efficiency of Mn-CC, in the absence and in the presence of amines, was evaluated by

CPE of Mn-CC in the presence of amines

Different commercial amines (and a guanidine), presented in Fig. 114, were tested as additives in the cathodic compartment of the electrolytic cell during a CPE experiment with the Mn-CC catalyst in MeOH.

Our original objective was to apply electroreduction on concentrated amine solutions loaded with CO2. Unfortunately, we observed that loaded amine solutions in concentrations higher than millimolar values had a detrimental effect on the lifetime and activity of the Mn-CC catalyst. Consequently, the CPE experiments were conducted in the presence of amines in 1 mM concentration.

The amines were added in precise quantities in the cathodic compartment. The electrolytic cell was stripped with CO2 for 30 minutes before inserting the Mn-CC catalyst. CPE were performed in standardized conditions by applying a constant reduction potential E=-1.35 V for 22 hours, during which the gaseous reduction products were analyzed by automated GC sampling. Formate was quantified by NMR analysis at the end of electrolysis. The results of CPE experiments in the presence of different amines are listed in Table 12 (next page) with the corresponding TON of CO, H2, HCOO - and FE (%). In general, in the presence of amines formate production was amplified against CO. The highest levels of formate production were obtained with PMDETA, the permethylated equivalent of DETA, with a TONHCOO-=28000 and a FEHCOO-=66%. The high selectivity for formate was compensated by a reduction of CO generation compared to the amine-free system (FECO=5% with PMDETA vs FECO=51% without the amine). The 11% FE loss for the system containing PMDETA was due to H2 production by HER.

The addition of loaded DETA to Mn-CC similarly proved detrimental for CO production but induced a 3-fold increase in formate production (TON=16160) with respect to the amine-free system. DETA was tested at various concentrations (1-70 mM), temperatures (25 °C, 30 °C) and reduction potentials (-1.35 V, -1.3 V), but the yields did not improve with respect to the standardized conditions.

The addition of DEA gave less satisfactory results compared with PMDETA and DETA, as the TON of formate was close to the amine-free system but more H2 was produced.

Experimental Section

The same anion exchange protocol described for G2Cl2 synthesis was exploited to obtain the nitrate salt G2(NO3)2.

Amberlyst® A26 hydroxide form (50 g, 50 mmol NaOH) was sonicated for 30 min in 200 ml of 1M HNO3. The resin was filtered and washed with deionized water until neutral pH to eliminate the excess of HNO3. The resin was loaded into a 320 mm long column of 3 mm diameter, and it was packed with compressed air. 500 mg of G2SO4 (1.75 mmol) were dissolved by heating at 60 °C in 10 ml of deionized H2O. The solution was loaded onto the column. Elution was performed with 3x volumes of deionized H2O. The eluted solution was lyophilized to collect the exchanged salt as a white solid. The products were analyzed by ESI-MS in positive and negative mode to confirm complete SO4 2-substitution by NO3 -.

(326.0 mg, 1.21 mmol, yield 69%) G4SO4 was synthetized following the protocol from Dietrich et al. 99 Tris(2-aminoethyl)amine (1.17 g, 8 mmol) and S-methylisothiourea hemisulfate (3.5 g, 24.8 mmol) were solubilised in 10 ml concentrated NH4OH in a 100 ml pressure reactor containing a magnetic stirrer. The solution was stirred at 60 °C for 12 hours, during which a precipitate formed. The solvent was evaporated under reduced pressure, leaving a viscous orange solid. The solid was crystallised

VII. XRD methods

All structures have been deposited in the Cambridge Crystallographic Data Centre (CCDC).

a. mer-Bis(Diethylenetriamine)Co III dichloride monohexafluorophosphate

Crystallization method

Single yellow block-shaped crystals of CoDCl2PF6 were obtained by recrystallisation in water of the solid issued from ion exchange from CoDCl3 with NH4PF6 (Fig. 82, page 129).

XRD measure

A suitable crystal 0.29×0.19×0.13 mm 3 was selected and mounted on a nylon loop in perfluoroether oil on an Xcalibur, Eos, Nova diffractometer. The crystal was kept at a steady T = 150 K during data collection. The structure was solved with the ShelXT (Sheldrick, 2015) structure solution program using the Intrinsic Phasing solution method and by using Olex2 (Dolomanov et al., 2009) as the graphical interface. The model was refined with version 2018/3 of ShelXL (Sheldrick, 2015) using Least Squares minimisation. A 0.15 M solution of G2Cl2 was prepared in water. Two equivalents of Diethylenetriamine were added (final amine concentration 0.3 M). The solution was fluxed with pure CO2 until full loading (verified by gravimetry). The solution was refrigerated at +4 °C. Colourless block crystals of G2(HCO3)2 formed spontaneously over few hours in refrigerated conditions. They were collected by filtration on sintered glass and washed with few ml of ice water and Et2O.

Crystal

XRD measure

A suitable crystal with dimensions 0.62 × 0.32 × 0.17 mm 3 was selected and mounted on a nylon loop in perfluoroether oil on a Xcalibur, Atlas, Gemini ultra-diffractometer. The crystal was kept at a steady T = 150.01 (10) K during data collection. The structure was solved with the ShelXS (Sheldrick, 2008) solution program using direct methods and by using Olex2 181 as the graphical interface. The model was refined with ShelXL 2018/3 (Sheldrick, 2015) using full matrix least squares minimisation on F2. Chapter II Additional NMR spectra a. DETA qNMR speciation in water Assessing monocarbamate disproportionation for a -diamines from pKa of amines and acids n=1 case: pK a for K (309, 465.5, 337.5) for two independent titrations. (http://app.supramolecular.org/bindfit/)

Crystal

Chapter IV Kinetic monitoring under N 2 Yield / DETA: the mmol of Im, EDA, HCO2 -in the NMR tube (mmol tube) were calculated by comparing their 1 H NMR integrations (I) with respect to R2 mmol. At the end of the standardized treatment, one tenth of the substances issued from the reaction should be contained in the NMR tube. Consequently, mmol tube was multiplied for a factor 10 to obtain the total mmol of products obtained from the synthesis (mmol tot(1)).

Yield calculation

%rec.1: percentage of recuperation of organic species after work-up, calculated from the R1/R2 ratio. The mmol of R1 in the final sample (mmol tot) are calculated by comparison with R2. The mmol tot value is compared with the mmol of R1 added at the beginning of the reaction (mmol i=0.625), obtaining the %rec.1=60.8. The loss (39.2 %) is obtained as the difference from 100%.

%rec.2: percentage of recuperation of organic species after work-up calculated from the sum of their mmol tot(1) (Im, EDA), calculated with respect to R2. This value is compared with the initial mmol of DETA (mmol i=12.5), obtaining %rec.2=58.9 and the related loss of 41.1%.

The correspondence between %rec.1 and %rec.2 proves the reliability of the double referencing method, indicating that the separation procedure is not chemoselective, as R1 and the organic products were retained to the same extent. Additional NMR spectra (