Julien Bonin

Thomas Devic

Sébastien Blanchard

Laurence Rozes

Norbert Stock D'

Adèle, José, Paolo, Jingjing, Raphaël, Iulia Ellie Tam

Huan Yun Bruno

Nathalie Leclerc Pour L'atg

Flavien Bourdeux

Muriel Bouttemy

Solène

J'aimerais dans un premier temps remercier mes directeurs, et encadrants, de thèse Caroline Mellot-Draznieks, Pierre Mialane et Anne Dolbecq. Je les remercie de m'avoir permis de mener ce projet durant près de 3 ans et demi. Merci aussi à Maria Gomez-Mingot pour son implication. Merci à tous les quatre pour la qualité de nos discussions scientifiques, pour votre patience (je suis conscient du problème que je rencontre parfois avec les deadlines), pour votre soutien tout au long de cette thèse. Je remercie également Marc Fontecave pour m'avoir accueilli au laboratoire, et pour ses conseils scientifiques.

Merci à Mathis pour ces 3 années, je suis persuadé qu'avoir un autre doctorant dans le même groupe avec qui échanger et me plaindre m'a beaucoup aidé. Merci à Alex pour le côté scientifique et humain, merci pour les super échanges et le travail qu'on a pu faire sur l'UiO, pour les sorties et soirées, ma seule déception étant de ne pas t'avoir vu sauter, 15 m ce n'est pas rien. Thanks to Albert (switching to English so you won't have to google translate it, even though seeing how you progress in French maybe it's not needed), thank you for your hard work on all the different projects we had in common, it hasn't always been easy to understand everything but it has been very enriching each time. Thank you for your support, in particular in this last step, which is by far the worse, the manuscript writing up, and thanks for the gossips it helped a lot! Un grand merci à Grégoire, pour m'avoir fait découvrir les MOFs lors d'un super stage, et aidé lors de ma première année de thèse, et pour le reste. J'aimerais par ailleurs remercier Lauren Dalecky et Khadija Talbi pour leur aide, respectivement sur les projets P4Mo6 et POM@MOF-545, sans qui je n'aurais probablement pas eu le temps de faire tout ça. Merci aussi pour votre bonne humeur en toutes circonstances.

Merci à mes amis en dehors du laboratoire et du monde scientifique, je pense notamment à mes ministres. Merci Adam et Val de nous accueillir aussi souvent, Victor « Otcho » Miremont pour ces vacances à Vienne, mes frères Reda et Yassin et tous ces appels fréquents qui m'auront toujours fait beaucoup rire, le copaing Thibault et Féfé pour ces WE à trainer, Victor G, Virgile et Romain toujours un plaisir de vous voir. Merci aussi à Somia pour les repas mensuels, et à Axel. Une pensée enfin aux amis que je n'ai pas eu la chance de voir aussi souvent, mais qui n'en sont pas moins importants : Quentin, Julien et Baptiste.

Le plus important pour la fin, j'aimerais remercier mes parents qui m'ont soutenu jusqu'ici et toujours poussé à aller plus loin. Mon frère, ma soeur, ma tante, mon oncle et ma grand-mère qui ont été présents quotidiennement, ainsi que le reste de ma très grande famille.

General introduction

Facing the increasing demand for energy, growing from 59 000 to 171 000 TWh in the last fifty years, and the limitation of fossil fuels, developing new energy sources is of immediate need in order to assure the energetic transition (Figure 1.1). 1 Developing renewable energies (solar, wind turbines, hydrokinetic turbines etc.) is considered a very promising solution. However, the intermittent nature of most of these energies, including those derived from solar energy, makes it a challenging alternative for fossil fuels as our energy consumption is not intermittent.

Indeed, storing the excess of energy produced over favorable weather periods is a key issue. Storing this energy in the form of chemical bonds, which can be transformed later on to deliver this energy back, has been considered widely for the past few decades driving the ever-growing research effort in the chemical reduction of CO2 (Figure 1.2, right). This process can be observed in nature where plants, algae and bacteria can harvest light and use water during the light phase of photosynthesis to stock energy and release O2, then use the stored energy to transform CO2 into sugars, cellulose or starch during the dark phase of photosynthesis. Using CO2 as a C1 carbon feedstock to store energy in chemical bonds is thus an attractive strategy when working with intermittent renewable energies. In fact, carbon dioxide is largely produced from the combustion of fossil fuel. As shown in Figure 1.2 (left) the annual CO2 emissions coming from fossil sources exceeds 30 Gt per year, and is in most parts of the world still increasing.

The chemical transformation of CO2 is thus beneficial in two points: i) climate change theories explain the global warming mainly through the increase carbon dioxide concentrations in the atmosphere and its greenhouse effect, ii) the gigantic amount of CO2 available to be used as a substrate for chemical reaction, as only 150 Mt is used in the industry per year. Chemical reduction of CO2 on an industrial scale still faces two challenges, i) the difficulty to capture CO2 due to its dilution in ambient air that makes it an expensive process and ii) the chemical reduction of carbon dioxide in energy-dense molecules (CH4, ethanol, methanol, ethylene etc.) is a fascinating fundamental challenge because of the high thermodynamic stability of CO2 and thus the endergonic nature of its reduction (Table 1.1).

Table 1.1: Equation, free enthalpy, Gibbs energy and potentials of CO2 and proton reduction.

Equation Reaction

H 0 (kJ•mol -1 ) G 0 (kJ•mol -1 ) Scheme 1.1: Equations and potentials of CO2 reduction and water splitting (vs. NHE, at pH 7).

Focusing on CO2 photocatalytic reduction, many parameters are to be taken into account when developing new catalysts: their efficiency, selectivity, stability, recyclability associated with the necessity to use a photosensitizer able to harvest visible light (often based on noble metals) and, of course, their cost when considering an economic point of view. As detailed below, the immobilization of molecular catalysts in porous materials is a valuable approach to increase both their stability and recyclability, while maintaining their catalytic activities when compared to those observed in homogeneous conditions. In this context, the main objective of my PhD thesis was to develop new heterogeneous photocatalytic systems for CO2 reduction based on Metal-Organic Frameworks (MOFs) and polyoxometalates (POMs).

In this regard, the first chapter of this PhD thesis will describe in a non-exhaustive fashion current strategies of the literature making use of molecular catalysts and co-catalysts for CO2 reduction and MOFs as immobilizing platforms. We will focus mainly on molecular complexes with polypyridyl groups, porphyrins and polyoxometalates. Concerning MOFs, we will shortly describe the mostly used ones for immobilizing catalysts before focusing on the subfamily of zirconium-based MOFs, which were specific MOF we used during this PhD. Finally, we will describe the various existing catalyst immobilization procedures and the main physicochemical methods used to characterize the resulting complex composites.

The three following chapters will be dedicated to the presentation of my results over the PhD.

Three distinct families of photocatalytic systems for CO2RR involving molecular catalysts, MOFs and POMs will be presented over the three following chapters.

The second chapter will be dedicated to the co-immobilization of a Keggin polyoxometalate (PW12) and a rhodium-based catalyst, Cp*Rh(bpydc)Cl, in the zirconium-based MOF UiO-67, leading to the (PW12,Cp*Rh)@UiO-67 composite. We will describe its synthesis, characterization and photocatalytic properties for CO2 reduction. A mechanism for carbon dioxide and proton reduction to formate and H2 is proposed with a tentative interpretation of the role of the POM in that process.

In the third chapter, we will focus on an all-in-one platform for CO2 reduction, making use of the porphyrinic zirconium-based MOF-545. This MOF can by itself reduce CO2, i.e. without the addition of any external photosensitizer or immobilized molecular catalyst. Similarly, we will describe the synthesis, characterization, photocatalytic properties and a proposed mechanism for carbon dioxide reduction to formate. In a second part, we will detail our attempts made in immobilizing three different molecular catalysts and various Keggin polyoxometalates in MOF-545.

Lastly, in the fourth chapter, we report three new polyoxometalate-based crystalline non-porous solids able to reduce carbon dioxide to methane and carbon monoxide in heterogeneous conditions. We will describe the hydrothermal synthesis, the structure of these novel POMbased materials, their photocatalytic properties and propose a plausible mechanism supported by DFT calculations.

Chapter 1: Molecular catalysts and polyoxometalates, immobilization in Metal-Organic Frameworks

We mentioned previously the need of a catalyst in order to reduce CO2 photochemically. The nature of these catalysts can vary a lot from oxides, metal nanoparticles to organometallic complexes. Organometallic complexes have been widely studied as photocatalysts, or photocatalytic complexes, for CO2 reduction reactions (CO2RR) due to their tunable structures. [3][4][5] The possibility to tune these catalysts at a molecular level can indeed allow a better control on the different CO2RR key points such as the efficiency, the selectivity or stability of the catalyst. Polyoxometalates represent another family of molecular catalysts, recently identified as potentially interesting for CO2RR, amongst others (see below). They can be functionalized with a wide variety of metals proven to be active for CO2RR, including Mn, Fe, Cu, Co. Moreover, organometallic complexes can be linked covalently to polyoxometalates and bring additional properties to the hybrid species, such as electron relay. 6 Nonetheless, these homogeneous catalysts suffer from two main drawbacks, i) their stability is often limited, with the catalyst deactivating after few hours and ii) the complexes being dissolved in solution cannot be easily recovered. Immobilizing these molecular catalysts in porous materials, namely metal-organic frameworks, is a commonly used approach in order to circumvent the above drawbacks. Indeed, MOF porosity and versatility regarding their organic building units enable the incorporation of molecular catalysts via covalent grafting or supramolecular interactions, thus increasing the stability of the catalyst as a result of its heterogenization. In this chapter we will first describe, in a non-exhaustive way, the commonly used organometallic complexes for CO2RR and POMs. We will then present the main metal-organic frameworks used to heterogenize these molecular species. Finally, we will discuss the different synthetic methods used to immobilize catalysts in MOFs and go through the usual characterization methods for such composite materials.

Homogeneous molecular catalysts

Before introducing molecular catalysts, we will briefly present the different components needed for photocatalytic reduction: a solvent, a sacrificial electron donor for reduction reactions, a photosensitizer, light and lastly, a catalyst.

-Choosing the solvent is the first important step as gas concentration differs depending on the solvent and pH. For example, CO2 concentration in acetonitrile is almost ten times higher than in water at pH 7 (0.28 M vs. 0.033 M respectively). 7 Moreover, the molecular catalyst as well as the sacrificial agent have to be soluble and stable in the chosen solvent.

-For light driven CO2 reduction, the photosensitizer should be able to be excited by photons preferably in the visible region. It must possess properties such as light-harvesting ability, long excited-state lifetime and strong oxidizing/reducing power in its excited state in order to receive an electron from the sacrificial electron donor or reduce the substrate. Two possible quenching routes of the excited state of the photosensitizer are possible: oxidative quenching or reductive quenching as illustrated in Figure 1.3. 8 In the case of oxidative quenching, the excited state of the photosensitizer reduces the substrate before being reduced by the sacrificial electron donor.

In the reductive quenching mechanism, the excited state of the photosensitizer is first reduced by the electron donor before being oxidized by the substrate.

-The electron-donor will react with the photosensitizer (PS* or PS + form). A list of usual electron donors (triethylamine, triethanolamine, benzyl-dihydronicotinamide, dimethylphenylbenzimidazoline) as well as their reaction mechanism has been reported

recently by Odobel and Pellegrin. 8 -The last key component is the catalyst. The catalyst performances can be evaluated by four main factors: the catalyst's selectivity, turnover number, turnover frequency and quantum yield. The highest the catalyst's selectivity, the better as it avoids having a mixture of products.

Turnover numbers (TONs) can be calculated as follows and correspond to the number of reductions occurring per catalytic site:

𝑇𝑂𝑁 = ∑[𝑝𝑟𝑜𝑑𝑢𝑐𝑡] [𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡]

Turnover frequencies (TOFs) are generally measured at the initial stage of the reaction when the product formation is still linear. It can be calculated with the following formula and is usually expressed in s -1 , min -1 or h -1 :

𝑇𝑂𝐹 = 𝑇𝑂𝑁 𝑡𝑖𝑚𝑒

Apparent quantum yield (Φ) is the amount of product in respect to the number of induced photons used for the reaction. The number of induced photons can be calculated with the Plank-Einstein relation where Etot is measured beforehand with a photodiode power sensor. The nature of the catalyst, including both its metal center and organic ligands, will impact greatly its performances. In the following subsection, we will present briefly several families of catalysts for CO2 photoreduction. The catalyst performances will not be detailed as they can be found in recent reviews. 4,9 1.1 Molecular catalysts for CO2RR

In this subsection we separate the molecular complexes in two categories, where the first one gathers noble metal polypyridyl molecular catalysts for carbon dioxide reduction, and the second one, noble-metal-free polypyridyl catalysts. Noble metal catalysts described in this paragraph contain ruthenium and iridium. We will also consider rhenium complexes, even if rhenium is not usually reported as noble metal. Rhodium cyclopentadiene based photocatalytic systems will be treated in chapter 2.

Noble metal-based CO2 reduction catalysts Figure 1.4: Examples of noble metal-based photocatalysts for CO2RR. 4 Rhenium polypyridyl catalysts were first reported in 1993 by Hawecker et al. 10 working on Re(CO)3(bpy)X (X = Cl) (Figure 1.4). The catalyst could reduce CO2 in CO as the sole product, in DMF with TEOA as an electron donor and could reach a TON of 48 after 4h. The solution was irradiated with visible light ( > 400 nm) without the use of external photosensitizer. The same catalyst with other X groups (X = CN, SCN, Br) was studied by different groups. [11][12][13][14] It was demonstrated that with X = SCN, the rhenium catalyst exhibits the best CO2RR properties, due to the easy dissociation of the SCN -group after the first one-electron reduction of the complex. 13 Indeed, to reduce CO2 the rhenium is first excited, then quenched by TEOA leading to the reduced state of the complex. At this step, the X linker is lost and CO2 can bind to the rhenium atom (Figure 1.5). Besides, Kurz et al. 15 showed that the modification on the aromatic group (bipyridine-4,4'-dicarboxylic acid, phenanthroline, azopyridine, etc.) induced a decrease of their fluorescence lifetime, thus of their catalytic activity.

Figure 1.5: Proposed mechanism for CO2 photoreduction with Re(bpy)(CO)3NCS. 13 The efficiency and catalytic conditions of the different catalysts presented in this subsection are summarized in Table 1.2. The first report of CO2 photoreduction with a ruthenium polypyridyl catalyst was published in 1985 by Hawecker et al. 16 where they studied [Ru(bpy)3] 2+ . The ruthenium catalyst, now widely used as a photosensitizer, is able to reduce CO2 to CO and formate in a dimethylacetamide/water mixture with benzyl-dihydronicotinamide (BNAH) as an electron donor. During the process,

[Ru(bpy)3] 2+ loses a bipyridine ligand which is replaced by two solvent molecules leading to [Ru(bpy)2(solvent)2] 2+ which is the actual catalyst for CO2 reduction. To facilitate the formation of the active form of ruthenium polypyridyl complexes, [Ru(bpy)2R1R2] complexes (Figure 1.4) were investigated (where R = H, Cl, CO, MeCN, DMF). No clear trend on which R substituent was the most efficient for CO2 reduction, but the difference between the catalysts is outstanding. When R1 = R2 = Cl a TON of 51 could be achieved, when substituting a Cl for a CO, the TON was increased to 326 after 2h. 17 Besides, the selectivity of Ru(bpy)2 complexes towards CO and formate production is highly dependent on the pH. For example, in presence of TEOA (pH ~10) the only product observed was HCOOH. In contrast, when using BNAH, the formation of both HCOOH and CO could be obtained at a neutral or acidic pH. The main issue with Ru(bpy)2-type catalysts is their poor stability. Indeed, after a few hours a black precipitate could be observed inducing a slowdown in the catalyst performances. 18,19 Finally, ruthenium catalysts bearing only one bipyridine and two CO ligands were studied with different bipyridine functionalizations (Figure 1.4). 20,21 The various catalysts have shown similar properties as the previously described ones, with a mixture of CO and HCOOH as reduction products.

Iridium complexes have been barely studied as photocatalysts for CO2 reduction. Two complexes are presented in Figure 1.4: [Ir(R2-tpy)(R1-ppy)Cl] and [Ir(tpy)(bpy)] (tpy = terpyridine and ppy = phenylpyridine). These complexes can reduce CO2 selectively in CO in acetonitrile with TEOA as an electron donor. When R1 = R2 = H, the catalyst could exhibit TONs up to 38, 22 functionalization on the aromatic rings with methyl groups could slightly increase the catalyst's efficiency with turnover numbers up to 50. Genoni et al. 23 have shown that functionalizing the tpy linker with an electron donating group, an anthryl, could drastically increase the TONs that can reach 310.

Another emerging strategy is the covalent grafting of photosensitizer on molecular catalyst to form a diade. This strategy allows an easier electron transfer from the photosensitizer to the catalyst thus increasing impressively the system efficiency, often reaching TONs of several thousands. 4 As we have seen, noble metal-based molecular catalysts present many advantages: their high selectivity, efficiency and light adsorbing properties. They generally do not need the addition of external photosensitizer. The main critical issue is their rare nature and thus the high price of the metal needed to design these catalysts. When considering a large-scale production, the metal prices would be a huge drawback with prices ranging from ~20 k€/kg for ruthenium to almost 500 k€/kg for rhodium. Developing noble-metal-free catalysts is thus a necessity for considering sustainable photocatalytic systems. For comparison, the price of nickel is ~20 €/kg and that of cobalt ~50 €/kg which makes these more abundant metals a thousand times less expensive.

Metalloporphyrins.

Metalloporphyrins are well known for their electron transfer roles in a myriad of redox systems in nature (chlorophyll, heme, vitamin B12, cytochrome P450, etc.) as well as highly effective photocatalysts thanks to their strong absorption in the 400-450 nm region (Soret band) and weak absorption in the 500-700 nm region (Q-bands). Metalloporphyrins and related derivatives are strong light absorbers with extremely large extinction coefficients in the visible spectral region.

However, yields of active catalysts formed after light absorption are very low, mainly due to the short (picosecond) lifetimes of their excited states. Metalloporphyrins and related derivatives studied for CO2 reduction include metallocorrins, metallophthalocyanines and metallocorroles. These different systems have been largely studied by Savéant and his group in the 1990s. Among the metalloporphyrins, they have shown that iron porphyrins are highly selective for CO in protic solvents in presence of a weak Brønsted acid (phenol, trifluoroethanol) or a Lewis acid. In early stages of the development of the field, the porphyrin efficiency was low and photodecomposition of the catalyst could be observed. 24 Addition of external photosensitizer can, sometimes, be beneficial for the reduction reaction. Dhanasekaran et al 25 . have shown that the addition of p-terphenyl in solution with iron tetrakis(phenyl)porphyrin (FeTPP) could promote the reduction of Fe II to Fe I and Fe I to Fe 0 .

The presence of p-terphenyl could thus multiply by ten the CO production. The same study was performed with CoTPP and also showed a higher CO production, both photo and electrochemically. 26 Conditions and TONs are detailed in Table 1.3.

Some metalloporphyrins bear functional groups with negative or positive charges at the meso substituents, designed to enhance efficiency of their catalytic properties. Bonin et al. 27 reported in 2014 the functionalization of FeTPP phenyl groups with alcohol (CAT) or fluorine (FCAT) (Figure 1.6). The introduction of OH groups on the phenyl rings allows stabilizing the CO2 adduct owing to the internal hydrogen bonding between -OH groups and the CO2 molecule bound to the metal center. FeTPP could reach TONs of 5.5 for H2 and less than 0.5 for CO after 1h irradiation but was poorly selective, with only 8% selectivity for CO. In contrast, CAT and FCAT were highly selective for CO, with a product selectivity reaching 93% for CAT and 85%

for FCAT. All the catalytic tests were conducted in acetonitrile with TEA as an electron donor, with or without trifluoroethanol and without external photosensitizer. The production rate of CO is linear for a few hours before decaying, showing the limited stability of the molecular catalyst.

Figure 1.6: Iron porphyrins used as molecular catalysts. 27 The same group reported a few years later the catalytic activity of CAT with the addition of [Ir(ppy)3] + or 9-cyanoanthracene as photosensitizer. 28 The addition of a photosensitizer not only increases the TONs of the system but also its stability, with no decaying production even after 50h.

A last example of porphyrin design was reported by the same group in 2017. Rao et al. 29 reported the catalytic activity for CO2 reduction of a trimethylammonium substituted FeTPP on the p-position in water with purpurin as a photosensitizer. The catalyst is 95% selective for CO and reaches TONs of 120 after 47h. Interestingly, when working with [Ir(ppy)3] + as a photosensitizer the catalyst is highly selective for CH4 (82%). They propose that the high reduction property of [Ir(ppy)3] + might be necessary to reduce the carbonyl from the Fe II -CO adduct.

Manganese pyridyl complexes

The CO2 reduction catalytic activity of manganese analogues to [Re(bpy)(CO)3] + complexes was first reported in 2011 by Chardon-Noblat, Deronzier and co-workers. 31 They performed the electrocatalytic reduction of CO2 to CO using the catalysts Mn(bpy)(CO)3Br and Mn(4,4'-Mebpy)(CO)3Br at the potential of -1.70 V vs. Ag/Ag + . When used in photocatalytic conditions with [Ru(4,4'-Me-bpy)3] 2+ as a photosensitizer, Mn(bpy)(CO)3Br would be expected to yield CO as the only product as observed electrocatalytically. 32 It was shown instead to lead to the formation of a mixture of CO and formate in DMF/TEOA solution (4:1, v/v) with BNAH as an electron donor. This observation was independently confirmed in two reports of Kubiak and co-workers, 33,34 in which Mn(bpy)(CO)3Br and Mn(bpy)(CO)3CN (Figure 1.7) were assessed for CO2RR in the same photocatalytic conditions. The selectivity towards CO or formate in the photosensitized system with Mn(bpy)(CO)3CN was shown to be dependent on the nature of the solvent, with formate production favored in DMF and CO production in MeCN. In DMF it could reach TON of 130 for formate after 15h, whereas the TONs for CO where only of 8.

Moreover, it must be noted that Mn(bpy) catalysts tend to dimerize irreversibly under catalytic condition leading to an inactive form of the catalyst. HCOOH in aqueous solution. 36 The photocatalytic activities of various macrocyclic Ni-based dimeric complexes for CO2RR were also studied. 37 When [Ni2(6,6'-Bi(Me2-cyclam))] 4+ (Figure 1.8 c) was used as a catalyst, the production of CO was 6.8 times higher than that of the corresponding mono-macrocyclic Ni complex. Later on, Chang et al. reported a series of Ni II complexes based on distorted squareplanar tetradentate ligands (Figure 1.8 d). 38 The catalyst could selectively reduce CO2 to CO when irradiated in the presence of [Ir(ppy)3] + and afforded a TON as high as 98 000 within 7 h.

Nonetheless, this TON was reached with a concentration of catalyst equal to 2 nM, which might be too low to have a proper quantification. With a more conventional concentration (0.2 µM), the TON at 7h for CO was equal to 1500. We presented a variety of molecular catalysts. As shown, the presence of organic linker allows an easy modification of the catalyst to improve its properties. Nonetheless, these complexes, in homogeneous conditions, tend to be rather fragile. Another kind of molecular catalyst that are worth of interest is polyoxometalates. Even though only few POMs have been reported for CO2

reduction they display properties that can support molecular catalysts during CO2 reduction processes, such as electron and proton relay properties. 39 1.2 Polyoxometalates, fully inorganic molecular catalysts

Polyoxometalates (POMs) are anionic molecular metal oxides of the early transition metals, which have been recognized for their remarkable redox activity and their ability to undergo reversible multi-electronic reduction processes. They have accordingly found numerous applications in electrocatalysis, energy storage, and photochemical processes, assorted with a wide variety of structures, as described in more detail below. POMs have been discovered in 1826 by Berzelius, 40 but structurally characterized only a century later by Keggin, in 1933. 41

Polyoxometalate structures

POMs are constituted of MOx polyhedra, mainly octahedra (x = 6), but polyhedra with x = 4 or 5 are also encountered, with M being mainly W(VI/V), Mo(VI/V) and V(V/IV). The polyhedra are linked by their vertices, edges or faces with bridging oxygen atoms noted (µ-O) for vertices, (µ-O)2 for edges and (µ-O)3 for faces (Figure 1.9). Polyoxometalates are exclusively anionic species, their charge is compensated either by protons for their acidic form, or by alkaline cations. These counterions can be exchanged by organic counter cations, for example tetrabutylammonium (TBA) in order to increase the POM's solubility in organic solvents, 42 or to form ionic liquids. 43 [H2PW11O39(cyclopentadiene)Rh(H2O)] 3-complex which is active for CO2RR, nonetheless favoring H2 production when compared to the parent rhodium bipyridine complex. 47

Polyoxometalates in catalysis

Polyoxometalates have shown applications in various fields including biology, 48,49 magnetism, energy 50,51 and mostly catalysis. 50,[52][53][54] POMs are constituted of a wide range of metals in high oxidation states. They can be reduced several times and used as an electron and proton reservoirs. 39,[START_REF] Liu | Ti-Substituted Keggin-Type Polyoxotungstate as Proton and Electron Reservoir Encaged into Metal-Organic Framework for Carbon Dioxide Photoreduction[END_REF] This reduction process is reversible for most POMs, and can be beneficial for redox reactions. This is the case for "sandwich" type POMs which have been used for several catalytic reactions such as water oxidation or reduction. [START_REF] Lv | Polyoxometalate water oxidation catalysts and the production of green fuel[END_REF][START_REF] Geletii | Polyoxometalates in the Design of Effective and Tunable Water Oxidation Catalysts[END_REF] In 2008, Sartorel et al. [START_REF] Sartorel | Polyoxometalate Embedding of a Tetraruthenium(IV)-oxo-core by Template-Directed Metalation of[END_REF] reported the synthesis of a ruthenium sandwich POM [Ru4(μ-O)4(μ-OH)2(H2O)4(γ-SiW10O36)2] 10-(Ru4). Ru4, in water, with (NH4)2Ce(NO3)6 as sacrificial agent, is able to oxidize H2O in O2 with an impressive TON of 488 and TOF of 450 h -1 . At the same moment, the same POM with different counterions was reported by Hill and co-workers [START_REF] Geletii | An All-Inorganic, Stable, and Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation[END_REF] for the same reaction. In their conditions, without sacrificial cerium acceptor the TONs were of 18.

However, as mentioned previously, in regard of noble metal prices, building noble-metal-free catalysts is a necessity. In 2011, Hill and co-workers [START_REF] Huang | Efficient Light-Driven Carbon-Free Cobalt-Based Molecular Catalyst for Water Oxidation[END_REF] To our knowledge, the number of reports about POMs for CO2 reduction is very limited. 52,54 This can probably be explained by the unfavorable electrostatic interaction between CO2 and POMs [START_REF] Streb | New trends in polyoxometalate photoredox chemistry: From photosensitisation to water oxidation catalysis[END_REF] and because POM redox potentials are too low to reduce CO2. [START_REF] Hiskia | Comparison of the photoredox properties of polyoxometallates and semiconducting particles[END_REF] showing that even with unfavorable interactions between POM and CO2, the reduction might still be possible.

As most of the POM are not active for CO2 reduction, the grafting of catalytic organometallic complexes able to reduce CO2 has been considered. This has been reported previously by Ettedgui et al. [START_REF] Ettedgui | Photoreduction of Carbon Dioxide to Carbon Monoxide with Hydrogen Catalyzed by a Rhenium(I) Phenanthroline-Polyoxometalate Hybrid Complex[END_REF] who grafted a rhenium-based catalyst [Re(L)CO3(MeCN)] + with L being a sodium 15-crown-5 crown ether covalently linked to a Keggin POM (Figure 1.13 a). The hybrid POM is able to reduce CO2 to CO under UV light. DFT investigations of the role of each component of this system was reported a few years later in a computational chemistry paper.

Cie et al. 6 proposed that the rhenium metal center is the catalytic site, whereas the Keggin POM is the photosensitizer (with UV light) and acts as an electron and proton donor (Figure 1.13 b).

More recently, Proust and co-workers 47 We have seen in this subsection a wide variety of catalysts, going from organometallic complexes to functionalized polyanions. These catalysts often show a good selectivity and efficiency for CO2 reduction but also often lack stability. Supporting these catalysts on a solid could improve their performances, especially with respect to stability. Metal-organic frameworks represent an immobilization platform of choice. Also, with their chemical versatility, they can provide what molecular catalysts are lacking. For example, by considering a photosensitive MOF, one can avoid using an external photosensitizer which is often noble metal-based. Besides, their high porosity allows different encapsulation procedures. We will in the next subsection describe the most used MOFs for catalysts and POM encapsulation before focusing on Zr-based MOF more specially used over the period of my PhD research work

Metal-Organic Frameworks

Metal-organic frameworks, also called porous coordination networks (PCN), are crystalline and porous solids. MOFs are hybrid materials composed of organic linkers coordinated to metal ions or metal oxoclusters. The diversity of possible metal precursors and organic linkers leads to a huge variety of MOFs in terms of structure and chemical composition. Moreover, the same {metal ion, organic linker} combination can give several different metal-organic frameworks, making their phase diagram particularly complex and sensitive to synthetic conditions. [START_REF] Mckinstry | Effect of Synthesis Conditions on Formation Pathways of Metal Organic Framework (MOF-5) Crystals[END_REF] In order to differentiate these unique structures, they are labeled with a prefix (usually MOF, PCN or the university where the MOF has been discovered) and a number. Nonetheless, the same structure can sometimes be discovered by two different teams at the same moment resulting in two different names for the same MOF. To date, more than a hundred thousand MOFs have been synthesized and more than half a million predicted. [START_REF] Moosavi | Understanding the diversity of the metal-organic framework ecosystem[END_REF] MOFs present several advantages such as thermal and chemical stability, high surface area and the possibility to accommodate the molecular catalytic guest into their cavities, with, however, some variations from one MOF to the other. Looking at a recent review we published, [START_REF] Mialane | Heterogenisation of polyoxometalates and other metal-based complexes in metal-organic frameworks: from synthesis to characterisation and applications in catalysis[END_REF] it can be concluded that the Cr-based MIL-101 MOF has been by far the most studied one for molecular catalyst's immobilization. We can propose several reasons: MOF-101 can be synthesized in water with high yields, and its Cr-based version is very stable while possessing large cavities and windows capable of accommodating a large range of bulky species (see below). However, its main drawback is that it contains chromium ions which are considered as one of the most toxic heavy metal ions. MIL-100(Fe) and NH2-MIL-101(Al) have thus also been largely investigated as alternative platforms. Zr-based MOFs also appear more ecofriendly and among them, UiO-66 and UiO-67 are the most commonly used but the porphyrinic Zr-based MOF-545 has also been encountered. Finally, among the other MOFs, HKUST-1 and ZIF-8, copper-and zinc-based MOF respectively, seem to be the most popular. A representation of the main MOF hosts with their formulas and the dimensions of their cavities is given in Figure 1.14 and a short description of their structure as well as their stability is given below.

Both the structure and stability of the MOF are obviously important parameters to take into account when choosing a MOF as a platform to immobilize a catalytic species.

MOF structural description

In this subsection, we will introduce the main metal-organic frameworks used as immobilization platforms for catalysts. Then we will emphasize on zirconium-based MOFs, which are the main focus of this PhD thesis.

HKUST-1, also called Cu-BTC, is one of the oldest MOF, first synthesized in 1999 by Chui et al.. [START_REF] Chui | A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n[END_REF] HKUST-1 is composed of dimeric cupric tetra-carboxylate units. Each copper dimer is linked to four different benzenetricarboxylate through 8 oxygen, Cu ions complete its coordination with aqua ligands. Each BTC linker is connected to three Cu dimers leading to a paddle-wheeled framework. Cu-BTC has 18.6 Å diameter hexagonal cages and 13 Å diameter square pore windows. [START_REF] Chui | A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n[END_REF] This MOF exhibits a relatively low surface area of 690 m²•g -1 . Cu-BTC is known to be unstable to water and steam leading to a partial transformation after a short period of time but is stable to air and oxidative conditions (5% H2O2) for a few days. [START_REF] Leus | Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks[END_REF] ZIF-8 [START_REF] Park | Exceptional chemical and thermal stability of zeolitic imidazolate frameworks[END_REF] is a zeolitic imidazolate framework where two zinc tetrahedra are bridged with an imidazole forming a sodalite-type network. ZIF-8 has a unique pore type of 12 Å diameter and a window made of six zinc clusters with an aperture diameter of 3.4 Å. The empty ZIF-8 BET surface area is equal to 1630 m²•g -1 . ZIF-8 has been claimed to be highly stable to air, water, steam showing no degradation over a few days in those conditions. It can withstand acid and basic condition (from ~ pH 4 to pH 12) for several days and oxidative conditions for the same duration. [START_REF] Leus | Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks[END_REF] MIL-101(M) (M = Cr III , Fe III , Al III ) [START_REF] Ferey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] is built on M III octahedra trimers connected with benzenedicarboxylate (BDC) linkers giving the general formula M3O(BDC)3(H2O)2X (X = F, Cl, NO3, CH3COO -, C6F5COO -). [START_REF] Rallapalli | HF-free synthesis of MIL-101(Cr) and its hydrogen adsorption studies[END_REF] The structure displays two types of mesoporous cages, the first one with a free diameter of 29 Å and pentagonal windows of 12 Å, and the second and larger cage with a free diameter of 34 Å and both pentagonal windows of 14x14 Å and hexagonal windows of 16x16 Ų. [START_REF] Ferey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] MIL-101(M) displays a gigantic cell volume of 702 000 Å 3 and a huge BET surface area of around 4500 m²•g -1 allowing the hosting of large molecules.

Moreover, MIL-101(M) is easily functionalized via post synthetic modification or via direct pre-synthetic modification of the BDC linker (adding NH2, NO2, SO3 or other functions) [START_REF] Lammert | Single-and Mixed-Linker Cr-MIL-101 Derivatives: A High-Throughput Investigation[END_REF] which allows great control and uniformity of the MOF. MIL-101(M) stability is highly linked to the nature of the metal and the BDC functionalization. MIL-101(Cr) is highly stable to water (as solvent, moisture or even steam) and can last several days in boiling water without any structural degradation. The MIL-101 Fe and Al homologues show much lower stability toward hydrolysis, with for example NH2-MIL-101(Al) transforming into the more thermodynamically stable NH2-MIL-53(Al) after only 5 min exposure to water. [START_REF] Wittmann | Enhancing the Water Stability of Al-MIL-101-NH2 via Postsynthetic Modification[END_REF] MIL-101(Cr) is stable to both acidic and basic conditions for several weeks, and stable to oxidative conditions (5% H2O2) for a few days. Again, Al and Fe homologues show poor stability toward acidic or basic condition with partial transformation into MIL-53 and high loss of porosity. [START_REF] Bhattacharjee | Chromium terephthalate metal-organic framework MIL-101: synthesis, functionalization, and applications for adsorption and catalysis[END_REF] MIL-100 [START_REF] Férey | A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction[END_REF] is built on metal octahedra trimers connected by their summit and interconnected by BTC linkers giving the general formula M3O(BTC)2(H2O)2X (X = F, Cl, SO4). [START_REF] Akiyama | Highly Porous and Stable Coordination Polymers as Water Sorption Materials[END_REF] [START_REF] Wu | Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal-Organic Framework UiO-66 and Their Important Effects on Gas Adsorption[END_REF] UiO-66 is stable to water and organic solvents for several weeks and can withstand long exposure to acidic or basic pH ranging from 4 to 12. It also does not show any XRD change after several weeks in oxidative conditions (5% H2O2) but lacks stability toward air and moisture, leading to its partial destruction after only 3 days. [START_REF] Leus | Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks[END_REF] Stability tests on UiO-66-NH2 have shown that using functionalized linkers not only doesn't reduce its stability to water but increase its stability to air.

UiO-67 [START_REF] Cavka | A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability[END_REF] with the formula Zr6O4(OH)4(bpdc)2 is an analogue to UiO-66, replacing the BDC linker with 4,4'-biphenyldicarboxylic acid (bpdc), and thus increasing the distances between each Zr6 node, the pore sizes and the pore windows (Figure 1.16). UiO-67 is composed of octahedral cages of 18 Å of diameter and tetrahedral cages of 11.5 Å 88 diameter and displays a 3000 m²•g -1 BET surface area. [START_REF] Cavka | A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability[END_REF] UiO-67 is less stable than UiO-66: it exhibits a total loss of porosity after 3 days in air/moisture, partial loss of porosity at pH 4 and 12 after 3 days, loss of crystallinity at pH 4 after several weeks, and a total destruction after 3 days in 5% H2O2. [START_REF] Leus | Systematic study of the chemical and hydrothermal stability of selected "stable" Metal Organic Frameworks[END_REF] The lack of stability to water has been attributed to the linker hydrolysis. UiO-67 allows the grafting of several catalysts seen in the first subsection. Indeed, UiO-67 can be built with biphenyldicarboxylate linkers partially substituted with bipyridinedicarboxylate ones, thus allowing the covalent binding of pyridyl and polypyridyl-type catalysts. The main drawbacks for this MOF are its relative water sensitivity but also its non-photosensitive linkers, thus requiring the addition of an external PS, often noble-metal based, for photocatalytic purposes. Immobilizing catalysts inside a photosensitive MOF could thus avoid the use of external PS and allow the design of three-in-one noble-metal-free platforms: catalytic, porous and photosensitive. This strategy was been adopted by a former PhD student of our group, Grégoire Paille, [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF] whereby he immobilized Co4 sandwiches POMs in MOF-545, a Zr6 porphyrin MOF for the purpose of water oxidation. We will now describe the Zr6 porphyrin MOF family (Figure 1.17).

Figure 1.17: Structures of Zr6-TCPP MOFs.

The rotational freedom of the porphyrin carboxylate groups allows the formation of six different Zr6-TCPP MOFs: MOF-525, [START_REF] Morris | Synthesis, structure, and metalation of two new highly porous zirconium metal-organic frameworks[END_REF][START_REF] Liu | Topology-Guided Design and Syntheses of Highly Stable Mesoporous Porphyrinic Zirconium Metal-Organic Frameworks with High Surface Area[END_REF] PCN-224, [START_REF] Feng | Construction of Ultrastable Porphyrin Zr Metal-Organic Frameworks through Linker Elimination[END_REF] PCN-223, [START_REF] Feng | A Highly Stable Porphyrinic Zirconium Metal-Organic Framework with shp-a Topology[END_REF] MOF-545, [START_REF] Morris | Synthesis, structure, and metalation of two new highly porous zirconium metal-organic frameworks[END_REF][START_REF] Feng | Zirconium-Metalloporphyrin PCN-222: Mesoporous Metal-Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts[END_REF][START_REF] Chen | Biomimetic Catalysis of a Porous Iron-Based Metal-Metalloporphyrin Framework[END_REF] PCN-225 [START_REF] Jiang | An Exceptionally Stable, Porphyrinic Zr Metal-Organic Framework Exhibiting pH-Dependent Fluorescence[END_REF] and NU-902. [START_REF] Deria | Topology-dependent emissive properties of zirconium-based porphyrin MOFs[END_REF] Controlling accurately the synthesis media is thus extremely important, as any of these six

MOFs can be synthesized with the same reactants. In this thesis, we will focus on MOF-545, of formula Zr6O8(H2O)8(TCPP)2. [START_REF] Morris | Structure, and Metalation of Two New Highly Porous Zirconium Metal-Organic Frameworks[END_REF] Each TCPP linker is bound to four distinct Zr6 subunits on two different layers, creating two long porous tunnels, one triangular of 12 Å diameter and a bigger hexagonal one of 36 Å diameter with a total BET surface area of 2200 m².g -1 . The huge tunnel size allows easy post-synthetic impregnation of various bulky molecules or species.

Moreover, the photosensitivity of TCPP and its easy metalation render the composite promising for catalytic reactions such as CO2 photoreduction or water splitting. [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF],100 MOF-545 is stable in organic solvents as well as in water even in strong acidic conditions (concentrated HCl solution), but decomposes at pH above 10. 101 MOF-545 with its gigantic pore of 3.6 nm diameter, and UiO-67 with its biphenyldicarboxylate linkers that can be substituted for bipyridinedicarboxylate ones, are both ideal candidates for catalyst immobilization. Moreover, as formerly detailed, the photosensitive and catalytic nature of porphyrins is ideal for building noble metal-free catalysts. In the following subsection, we will discuss the immobilization protocols, their advantages and limitations.

Encapsulation strategies

The synthesis of the Cat@MOF composites (Cat being a molecular catalyst immobilized in the MOF) can be performed using various strategies according to i) the chemical nature of the catalyst, ii) its size compared to that of the pores or windows and iii) its stability (Figure 1.18).

It can be deduced from a recent review we published on catalyst encapsulation in MOFs 71 that PW12 represents around 35 % of the POMs immobilized in MOFs, for the possible following reasons: i) it is a well-known catalyst for several catalytic reactions, 102 ii) it is stable in the conditions used for the most common MOFs synthesis and also in impregnation conditions, iii) its charge and size are among the smallest for a POM which allows to obtain a high POM loading, allowing minimal linkers defects for charge compensation, iv) it is commercially available at low cost and v) it possesses characteristic IR and 31 P NMR signatures useful to check the stability of the POM after immobilization, before and after catalysis.

Synthetic encapsulation

Molecular catalysts can be encapsulated within the cavities of MOFs by direct synthesis also called "bottle-around-the-ship" or "synthetic encapsulation" method. The catalysts are added as reactants during the MOF synthesis. This method thus requires that the catalysts are soluble and stable in the conditions of the MOF's synthesis. For example, hydrothermal conditions, with temperatures of 180°C (during 72 h) or 220°C (during 8h) are usually described for the synthesis of MIL-101(Cr) composites. Note that microwave conditions have also been reported

for the synthesis of PW12@MIL-101(Cr), 103 PW11Zn@NH2-MIL-101(Al) 104 and PW12@NH2-MIL-53(Al) 105 composites. Zr-based MOFs composites are usually synthesized under solvothermal conditions in DMF at 120°C for 6 to 24 h while HKUST derivatives are isolated by heating the reactants in water at 180°C for 6 to 72 h or in mixed H2O/EtOH solvents, at lower temperatures (even at RT). An original method which attracts a growing interest, the mechanochemical synthesis, 106,107 has been described for PMo10V2@HKUST-1, 108 PW12@HKUST-1, 109 XM12@ZIF-8 (X = P, M = W, Mo; X = Si, M = W) 110 and PMo12-xVx@rho-ZIF (x = 0-3). 111 This method which consists in grinding the solid precursors in an agate mortar or using a ball-milling grinder, is very fast and easy, does not consume any solvent and does not require any heating.

The synthetic encapsulation method is mostly applied to Cats having dimensions greater than the sizes of the MOF windows. In this case, the advantage is that catalyst leaching is then impossible. The major drawback is that the pores can be blocked and that the diffusion of reactants through the framework can thus be hindered.

Impregnation

Some catalysts are not stable in the conditions used for the MOF synthesis (acidic medium, high temperature, etc.) and thus can only be immobilized by impregnation, provided they are small enough to diffuse through the MOF windows. In this milder synthetic method, the catalyst is dissolved in a solvent in which it is stable, usually water or CH3CN for POM catalysts with respectively protons and alkaline or TBA counter-cations and various organic solvents (acetone, CH2Cl2, CHCl3, MeOH, toluene, DMF) for coordination complexes. The crystallites of MOFs are then added and the suspension is stirred for a determined amount of time which can go from 2 hours to 4 days, usually at room temperature. Note that the condition that the catalyst is stable in the impregnation solution is not sufficient to ensure its integrity after its immobilization inside the MOF. Indeed, for example, 31 P NMR as well as IR spectroscopy have shown that the hybrid sandwich-type POM [(PW9O34)2Co7(OH)2(H2O)4(O3PC(O)(C3H6NH3)PO3)2] 14-(noted P2W18Co7(Ale)2) evolves into P2W18Co4 inside the cavities of the mesoporous MIL-101(Cr) material. 112 Two explanations have been proposed to account for the instability of the hybrid Co-containing POM under the impregnation conditions. The first one is the presence of Brønsted acid sites inside the MOF. The second one relies on the observation that the stability of the POM depends on the nature of the counter-cation. These observations might explain why there are so far no examples of hybrid organic-inorganic POMs immobilized in MOFs, these species being less stable than the fully inorganic POMs. The drawbacks of the impregnation method are i) a limited amount of catalyst loading for MOFs in which the immobilization of POMs is based on an ionic exchange, ii) possible leaching and iii) lower homogeneity than for the synthetic encapsulation.

A few strategies have been reported to prevent leaching in composites synthesized by impregnation. For example, carboxylated functionalized Rh and Ru complexes were coimmobilized in NH2-MIL-101(Cr). 113 The authors show that the interactions between the carboxylate groups of the complexes and the MOF framework prevent their leaching. In the same way, Chen et al. described the post-synthetic modification (PSM) of the linker after the encapsulation in order to block the apertures of the MOF cavities and avoid leaching. 114 A chiral Co(III)-salen catalyst was first immobilized in the cavities of the Zn-MOF IRMOF-3. In a second step, the free-amino groups of the BDC-NH2 linker were acylated by reaction with acetic anhydride, the acylamide groups blocking the windows as a fence. ICP analyses indicate that catalyst leaching was important in the sample before PSM treatment after soaking in CHCl3, which was not the case after PSM treatment.

Finally, it can also be noticed that dissociative linker exchange reactions can allow to encapsulate molecular guests larger than the aperture size of a MOF host. For example, Li et . 115 The authors thus show that this aperture-opening process exists even in a robust MOF and is highly dependent on the identity of the solvent used. Protic solvents such as MeOH are perfectly adapted to this process. On the contrary aprotic solvents do not favor dissociative linker exchange reactions and are thus required for the catalysis reaction, preventing linker dissociation and leaching.

Charge compensation

In most of the cases, the molecular catalyst is charged, with a negative charge for POMs, and most often a positive charge for coordination complexes, and the question thus arises of the charge compensation of the catalysts after their immobilization. Discussions on this matter are quite rare in articles. For composites isolated by impregnation or in situ, the catalyst can be immobilized with the counterions present in the catalyst precursor, with thus no modification of the composition of the MOF. This is for example the case of POM@HKHUST-1 composites, where the Cu-based MOF framework is neutral and the negative charge of the encapsulated Keggin ions is compensated by TMA cations and/or protons. [121][122][123][124][125][126] Interestingly, TMA cations can be exchanged by K + ions without loss of crystallinity 126 or be eliminated by thermal treatment (180°C under vacuum). 127 The cationic Zn(II) porphyrin complex [Zn II NMeTPyP] 4+

was also immobilized in the Zr porphyrinic MOF PCN-224 with its iodide counterions as shown by EDS analysis. 128 In the CoW12@ZIF-8 composite, isolated by synthetic encapsulation, the absence of K + counter-ions (present in the POM precursor) was evidenced by elemental analysis and the authors propose that charge compensation of the [CoW12O40] 6-POM comes from solution protons trapped by the composite. 129 The localization of the protons is however not discussed.

The immobilization of anionic catalysts by impregnation in MIL-101(Cr) has been described as based on ion exchange reactions. It must be noted that, if the solvent is water, the value of the pH determines the charge of the MOF. Zeta potential can be measured to determine surface charge characteristics of MOFs in aqueous solution as a function of pH. For example, zeta potential measurements showed that particles of MIL-101(Cr) are positively charged for pH below 7, 130 which favors the encapsulation of anionic species such as POMs. MIL-101(Cr)

possesses negatively charged counterions (mostly F -or NO3 -counterions) which can be exchanged by negatively charged Cats. This implies a limitation in the catalyst loading in impregnation reactions, the total charge brought by the catalysts being equal, at the maximum, to the initial negative charge of the counter-ions. 112 The immobilization of the anionic In a similar way, the immobilization of the P2W18Co4 sandwich-type POM by impregnation of MOF-545 in an aqueous solution of the POM is also based on an ionic exchange reaction.

Indeed, the MOF-545 framework is positively charged due to the protonation of the Zr6 clusters and possesses Cl -counter-anions. These Cl -counterions can be exchanged by POMs, leading to a composite with the following formula: [Zr6O16H18][TCPPH2]2[P2W18Co4]0.2. [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF] Likewise, in PW12@NU-1000, the charge compensation of the POM results from protons which go on the hydroxyl groups on the Zr6 nodes and the synthesis of the composite is thus performed in acidic pH (pH 2.5) in order to favor the protonation of the MOF. 132 In POM@UiO-67 composites, which are synthesized by in situ methods, it has been proposed that the negative charge of the POM guests is also compensated by the positively charged framework but this positive charge does not result from the protonation of the MOF but is rather due to linker defects. [START_REF] Salomon | Immobilization of polyoxometalates in the Zr-based metal organic framework UiO-67[END_REF] For example, three POM@UiO- The catalyst immobilization is the first step when developing new cat@MOF for catalytic purposes. A throughout characterization is needed, as one needs first to be sure of the stability of the MOF host as well as the integrity of the catalyst immobilized. Depending on the catalyst encapsulated on the MOF a wide range of characterization techniques, such as NMR, XRD, N2 adsorption, TGA, etc., can be performed. Last but not least, the quantification of the amount of catalyst immobilized in the MOF guest is primordial. Indeed, to probe the catalytic performance of the cat@MOF, as most of the time only the catalyst immobilized is active, a miscalculated loading could lead to wrong conclusions. In the following part we will discuss the different characterization methods for host/guest materials.

Characterizations of Cat@MOF composites

4.1 Determination of the composition (ICP, solution NMR, UV-vis, TGA, EDX)

A combination of several techniques can be used to determine the composition of the Cat@MOF materials and give access to the Cat loading. The control of the loading is important in view of catalytic applications. A material with only a small amount of immobilized catalyst will have low performances. On the other hand, overloading can block the cages and windows and thus the accessibility of the reactants to the catalytic species. This has for example been described for PW12@MOF-808X (X = F, A, and P where the coordinated monocarboxylate group is formic, acetic or propionic, respectively). 138 The precise knowing of the composition of the Cat@MOF is also essential for the determination of the turn over numbers (TONs) in catalytic studies. The most common and also the most precise technique of determination of the composition is the ICP analysis of digested solutions of Cat@MOFs. For example the Co/In

ratio of a composite [Cp2Co]@[R4N]3[In3(BTC)4] (R = Et, nPr, nBu) digested in diluted HNO3
(1/10 v/v) was measured by ICP-OES and allowed to quantify the amount of immobilized complex. 137 Note that ICP is also a powerful technique to monitor uptake of catalysts during impregnation reactions by an analysis of the impregnation solutions and has for example been used for PW12@NU-1000. 132 This analytic method also allowed to follow POM leaching when the same material was dispersed in acidic aqueous solutions.

Several other techniques can also be used to apprehend the composite composition. . 113 This study allowed to determine the catalyst loading and to find the best solvent for their immobilization. The analysis of digested solutions of the composite instead of the supernatant solutions should allow a more accurate determination of the catalyst loading. Indeed, due to Cat species adsorbed at the surface of the MOF crystallites that go out after careful washing, the amount of immobilized Cats is overestimated by an analysis of the supernatant solutions. [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF] Thermogravimetric analysis (TGA) can also be used to confirm the composition of a Cat@MOF. A typical example is represented in Figure 1.21 for a PW11Zn@MIL-101(Cr) composite. 143 The first weight loss is attributed to the departure of solvent molecules, the second weight loss to the decomposition of the framework. The total weight loss for the POM@MOF is lower than for the parent MOF due to the presence of POM guests which decompose into oxides. Thus, the comparison of the weight loss calculated from the results of elemental analysis with the experimental weight loss allow confirming the formula of the composite and in particular the guest loading. TGA also allows studying the thermal stability of the composites.

It can be noted that the temperature of decomposition of POM@MOF composites is usually very close to that of the MOF, which indicates that the insertion of POMs only slightly affects their thermal stability. This is also the case for the rare examples of TGA curves reported for Complex@MOFs. A few exceptions can be noticed. For example, the SiW12@HKUST-1 (also named NENU-1) composite, after thermal treatment (180°C under vacuum) to remove TMA + cations and H2O molecules (NENU-1a material), is stable up to 300°C while the POM-free MOF is stable only up to 240°C. The authors attributed this difference to physicochemical interactions between the POM and the MOF framework. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are largely used to characterize the morphology and composition of the composite materials, after synthesis and sometimes also after the catalytic reaction. The SEM images allow showing that the composite is still crystalline and EDX analyses indicate the distribution of the elements of the catalyst within the MOF. It can be noted that in all the reported cases, SEM-EDX analyses suggest a uniform distribution of the elements. However, one has to be cautious for composites isolated by impregnation reactions because SEM-EDX mapping is not sufficient to prove the homogeneity of the distribution. [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF]144 For example, SEM experiments on P2W18Co4@MOF-545 seem to indicate a homogeneous repartition of the Co, W and P elements of the POMs while HRTEM showed higher concentrations of these elements at both extremities of the crystal rods of the MOF (Figure 1.22). [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF] This distribution is consistent with the alignment of the channels of the MOF along the c axis, that is, the longest dimension of the rods, and an accumulation of the POMs at the extremities of the channels during the impregnation reaction.

EDX analysis can also be used to follow anion exchange reactions during impregnation Regarding the morphology of the composite crystallites, while in almost all cases, the crystals of the Cat@MOF composites have morphologies similar to that of the parent MOF, there are a few reports where the crystals have altered morphologies. For example, PW12@UiO-66 crystallites obtained by synthetic encapsulation do not exhibit regular shapes but look like agglomerated small particles, with irregular inter-grown microcrystalline poly-octahedra morphologies. 145 SEM images of (Fe-salen,SiW12)@ZIF-8 also show the uneven surface of the nanocrystals of the composite, when compared with the smooth surface of the nanocrystals obtained in the absence of POM. 146 This provides strong evidence in support of the growth of POMs on ZIF-8, which was also suggested by the very large amount of POM determined by ICP-OES analysis.

IR and Raman spectroscopy

In all the reported Cat@MOF studies, IR spectra are recorded to check whether the catalyst and MOF structures are preserved. This technique is also routinely performed to study the stability of the composites after the catalytic experiments. For example, for the PW12 catalyst, characteristic P-O, W=O and W-O-W bands are observed around 1000, 980 and between 800 and 900 cm -1 respectively. Red shifts have sometimes been described between the POM precursor and the encapsulated POM. A red shift of about 30 cm -1 was thus observed for the P-O and W=O bands of PW12 immobilized in MOF-808X. 138 The P-O vibration was also shifted from 1030 to 1048 cm -1 from P2W18Co4 to P2W18Co4@MIL-101(Cr) (Figure 1.23 a). 112 These shifts were attributed to strong interactions between the POM and the MOF. IR spectroscopy can also evidence the catalyst transformation after its immobilization in the MOF. Semi-quantitative analysis can also be performed on the IR spectra of Cat@MOFs. For example, the relative intensities of the bands at 1077 cm -1 attributed to the P-O vibration of the Keggin ion and at 1180 cm -1 attributed to C-C and C-N vibrations of the linker were followed in the IR spectra of PW12@Zr6-TCPP-Fe (where Zr6-TCPP-Fe is a MOF built from the connection of Zr6 clusters by Fe-metallated porphyrintetracarboxylate linkers). The evolution of the ratio between both intensities reveals the maximum amount of POM that can be immobilized in the MOF by synthetic encapsulation experiments. In some cases, the IR bands of the guest cannot be observed, for example in P2W18Co4@MOF-545, [START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF] PMo12@UiO-67, 148 and CoW12@ZIF-8, 129 probably because of low loading.

In addition to IR, or when the catalyst bands cannot be clearly identified, Raman spectroscopy is sometimes used to characterize the materials. However, in the case of POM@MOFs, the intensities of these bands are also often quite weak. For example, the Raman spectrum of SiW11@MIL-101(Cr) exhibits a small band at approximately 978 cm -1 assigned to a W=O stretching mode of SiW11 (Figure 1.23 b). 147 In P2W18Co4@MIL-101(Cr) the W=O band of the POM at 982 cm -1 is clearly visible in the Raman spectrum of the composite but also with a rather weak intensity. 149 In rare examples, the intensity of the characteristic bands of the catalyst is stronger. 150,151 Raman spectroscopy was also used to characterize Complex@MOFs. For example, the bands attributed to the iron tetrasulfophthalocyanine FePcS are clearly visible in the Raman spectrum of FePcS@MIL-101(Cr) at the same position as in the bulk FePcS precursor. [START_REF] Meyer | Hydrothermal synthesis, structure, and sorption properties of the new microporous ferric molybdenum phosphates [(CH3)4N]2(NH4)2Fe2Mo12O30[END_REF] Furthermore, no new bands are observed. This indicates the integrity of the complex after its immobilization.

BET measurements

N2 porosimetry is also a technique largely used to characterize MOF derivatives. The significant reduction of the N2 amount adsorbed in Cat@MOF compared with the MOF is considered as a clear indication of the successful incorporation of the catalyst. The Brunauer-Emmett-Teller (BET) specific surface area and total pore volume of Cat@MOF composites are found to be lower compared to the guest-free MOF, as a result of both the mass increases due to the introduction of the catalyst and of the pore space occupied by the guests. For example, it was shown that the surface area of PW12@MIL-100(Fe) depends on the amount of POM incorporated. While even for high loadings (35 wt %) PW12@MIL-100(Fe) exhibits a high BET surface area of 1046 m 2 •g -1 , this one is indeed significantly lower than that of the POM-free MOF (2055 m 2 •g -1 ) (Figure 1.24). 153 It is also noticeable that the mesopores and microporous windows are also retained, which ensures that substrates enter into the pores and come into contact with the POM.

Figure 1.24: a) N2 adsorption-desorption isotherms at 77 K and b) pore size distribution of MIL-100(Fe) and PW12@MIL-100(Fe) with 7 wt % (PW12@MIL-100(Fe)-7%), 16% (PW12@MIL-100(Fe)-16%), and 35% (PW12@MIL-100(Fe)-35%) loading in POM. 153 Besides, Juan-Alcañiz et al. deduced from the comparison of N2 isotherms of various samples of PW12@MIL-101(Cr) that in the sample isolated by direct synthesis the dispersion of the POMs in the MOF matrix was better than in the sample obtained by impregnation. 144 The encapsulated samples with high POM loadings show indeed an adsorption behavior similar to that of the POM-free MOF, with two steps corresponding to the filling of the two types of cages.

This strongly suggests that the filling of the pores by the POM molecules is quite homogeneous, with no indication of blocking of the cavities. In contrast, for the impregnated samples, only the large cavities are occupied and most of these cavities are blocked. Besides, N2 adsorption/desorption isotherms allowed to show that the bulky metal phthalocyanines (FePctBu4)2N are not incorporated into the mesoporous structure of MIL-101(Cr) but adsorbed at the surface of the crystallites. Indeed, the total pore volume of the composite decreases by only 5 %, which corresponds to the weight gain of 5.2 wt % after loading. 154

Solid-State NMR

For the POMs which contain P atoms, and in particular for the most studied POM, PW12, 31 P cross-polarization magic angle spinning (CP MAS) NMR has proved a valuable tool to study their stability after immobilization, before and after catalysis. In most cases, a single peak is observed after immobilization of the POM into the MOF. For example, a single peak at -12.2 ppm is observed for PW11@MIL-101(Cr), confirming the integrity of the POM. [START_REF] Ferey | A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area[END_REF] However, a few studies have reported the presence of several 31 P NMR peaks after the immobilization of PW12. For example, Maksimchuk et al. showed the existence of three main peaks at -16.4, -14.8 and -11.8 ppm in PW12@MIL-101(Cr). 155 They proposed that these peaks could correspond to different modes of interaction between the POM and the MOF surface, as IR spectra did not indicate any noticeable change and thus any degradation of the POM. In another composite with PW12, namely PW12@MOF-808A with 42% POM loading, the 31 Solution NMR is also useful to study the formation of composites. For example 31 P solution NMR was also used to study the interactions between Cu 2+ and PMo12 ions during the synthetic process of PMo12@HKHUST-1, with a shift from 3.72 ppm for the 31 P signal of a solution of PMo12 to 3.00 ppm after addition of Cu 2+ and 3.09 ppm after subsequent addition of BTC. 157 Shifts in the 183 W NMR spectra of a solution of PW12 also support the existence of strong interactions between the copper ions and the POM during the synthesis of PMo12@HKHUST-1 and confirmed the integrity of the POM.

X-ray diffraction

Powder X-ray diffraction is generally used to check the crystallinity and confirms the phase purity of the MOF host after the immobilization of a catalyst guest. In almost all cases of reported Cat@MOF, some redistribution of the intensities of the XRD peaks was observed for POM@MOF composites compared with the pristine MOF, but both the positions and width of the X-ray reflections usually remain unchanged, indicating that the MOF does not lose its crystallinity after the immobilization of the catalyst. A detailed study on the influence of the catalyst loading on the relative intensity of the peaks was made on CuPhen@MIL-100(Al). 156 The authors showed that the 2 = 4.0° Bragg intensity peak gradually decreased with increasing CuPhen loadings. Such variation was not observed for a sample prepared by impregnation of the MOF with a solution of the CuPhen complex. In some rare examples of Cat@MOF obtained by synthetic encapsulation, it has been observed that the crystallinity of the MOF depends on the amount of encapsulated guests. For example, in PW12@MOF-808X, the crystallinity was improved compared to the pristine MOF for POM loadings lower than 32%. The authors attributed this result to the modulator role of the POMs for low loading, while higher loadings induce the presence of defects (by inhibiting the coordination between the zirconium ions and ligands) and thus leads to a decrease in crystallinity. 138 Some authors have reported an increase of the intensity of some peaks; this is the case for the peaks around 5° in a P2W18Co4@MIL-101(Cr)composite. 149 Also, the apparition of new diffraction peaks in the powder X-ray diffraction patterns of such composites has been observed. This phenomenon has been observed in PW12@MIL-101(Cr) 158 and PW12@NH2-MIL-101(Cr). 159 The positions of the new peaks are consistent with the unit cell of MIL-101 but had a null intensity in the POM-free MOFs. The authors proposed that the electron density of the crystal structure of the MOF increases after the incorporation of the POM, thus creating the observed diffracted intensity at these peak positions. These various studies highlight the fact that the localization of the POMs in the cavities of the MOF can have a great influence on the PXRD patterns of the composites and that their analysis can thus allow making hypotheses on the structure of the POM@MOF. It is possible to go further in the structural characterization by using Pair Distribution Function (PDF) and Difference envelope density analysis (DED).

Once fully characterized, the MOF and cat@MOF materials can be probed for their catalytic performances. It is important to keep in mind that some MOFs can be active for CO2 reduction without any need of immobilized catalyst. It is for example the case of various MIL-125(Ti)-NHX (with X = H, Me, Et, iPr, Bu, etc.) 160,161 as well as UiO-66-NH2 162 which has been found to be selective for HCOOH production, BDC-NH2 acting as a photosensitive linker and also helping CO2 adsorption. Porphyrin-based MOFs have also been able to reduce by themselves CO2 to formate or CO/CH4. Their activity toward CO2 reduction will be discussed in the introduction of the third chapter of this manuscript. Other examples of catalytic MOFs for CO2 reduction can be found in a recent review from Luo et al. 4 where they discuss molecular catalysts, MOFs, and immobilization of catalysts in MOFs. The review we recently published also discuss the rich catalytic properties of cat@MOF composites, highlighting that these materials are able to perform a wide range of reactions going from small molecule activation for energy (OER, HER, CO2RR) to desulfurization, alcohol oxidation etc. [START_REF] Mialane | Heterogenisation of polyoxometalates and other metal-based complexes in metal-organic frameworks: from synthesis to characterisation and applications in catalysis[END_REF] With all this developed knowledge, we propose in the present work the design of new heterogeneous and stable photocatalytic systems for CO2 reduction. In the following chapter, we describe a first strategy whereby we co-immobilize a rhodium-based organometallic molecular catalyst and a POM in the Zr-UiO-67 for CO2 and proton reduction to formate and H2, assisted by the [Ru(bpy)3]Cl2 photosensitizer. In the third chapter, we managed to avoid the use of noble metal by using the active porphyrinic MOF-545 and optimized its efficiency towards CO2 reduction by metalating the porphyrin. We also attempted to immobilize different molecular catalysts in its wide pores, which we will briefly describe. In the last fourth chapter, we present the CO2 reduction properties of three new polyoxometalates in heterogeneous conditions.

Chapter 2: Co-immobilization of a Keggin POM and a Rhcatalyst in UiO-67 MOF: in-depth characterization and CO 2 photosensitized reduction

As mentioned in the first chapter, catalyst incorporation in porous materials increases in most cases its stability, eventually its efficiency while allowing the final composite to be collected back after the catalytic run, thus enhancing its recyclability. This chapter highlights the coimmobilization of a rhodium catalyst, namely Cp*Rh analysis (PDF) of the POM@MOF and (POM,cat)@MOF materials, Mohamed Haouas performed the ssNMR. My involvement to this project is in two points: first, the synthesis of all POM@MOF, cat@MOF and (POM,cat)@MOF composites as well as most of their physicochemical characterization; second, the photochemical CO2 reduction tests. In this chapter, I will emphasize on my contribution. Nevertheless, a subpart is dedicated to the work of my coworkers since without their efforts, this study would have been incomplete. Lastly, Dr.

Albert Solé-Dora succeeded in computing a plausible mechanism for the CO2 reduction with our system which I will briefly describe. Later, Deronzier 5 and his team worked with the same molecular complex reporting for the first time the use of rhodium bipyridine systems for CO2 electrocatalytic reduction. The main reaction products observed were formate and H2 with a varying selectivity depending on reaction conditions. In acetonitrile, with tetrabutylammonium perchlorate as an electrolyte and at Eapp = -1.60 V vs. Ag/Ag + , no catalytic effect can be observed, requiring the addition of 3% of water to start the catalytic reaction. Further addition of water increases the faradaic yield for both formate and H2 where at 5% water content the current efficiency of formate reaches 25% while that of H2 reaches about 10%. The best faradaic yield is reached at 20% water content where the formate current efficiency is at 36% and H2 at 30%. The suggested mechanism involves a two electrons -one proton reduction of the rhodium center going from Rh(III) to Rh(I), followed by a single electron transfer on the bipyridine ligand leading to the active species. The presence of water increases the proton concentration leading to higher faradaic yields.

State of the art for Rhodium bipyridine complexes for CO2 reduction

The Cp*Rh complex has been first studied for its ability to photoreduce CO2 in 2015 by a previous postdoc of our laboratory at LCPB, Matthew Chambers, 1 and his co-workers.

Surprisingly, no study of the Cp*Rh for CO2 photoreduction was indeed conducted so far, although this complex was first reported in 1988 for H2 photoreduction and in 1997 for CO2 6 and to an iridium complex immobilized in a nonporous coordination polymer based on ytterbium. 7 Research on catalyst immobilization in porous materials has been since then intensively developed, whereby a range of different works can be found in a recent review. 9 The aim of the previous work 1 of our group at LCPB was two-fold: first, to successively immobilize this catalytic complex in a porous host, the UiO-67 MOF, as part of the framework (Figure 2 The investigation of heterogeneous complexes thus showed that the catalytic activity was retained and comparable to that found for the homogeneous Cp*Rh(bpydc) catalyst (Figure 2 In 2019, Todorova et al. 13 10O40] is necessary for the reduction of CO2 to CO catalyzed by a [Re(bpy)(CO)3] + complex. The mechanism of this reaction has been later on studied by Poblet et al. 16 In 2017, the same team has again described a similar catalyst for the reduction of CO2 to CO. 17 A dirhenium catalyst was prepared with a modified bipyridine (6,6′′-(hydrazine-1,1-diyl)di-2,2′-bipyridine) (Figure 2 Few years later, the use of a Keggin POM immobilized in a MOF porous solid as an electron and proton relay for CO2 photoreduction was reported by Liu et al.. 18 The disubstituted K7PW10Ti2O40 Keggin, already known for its ability to photoreduce CO2, 19 HKUST-1 is the equivalent MOF without the POM inside the pores. 18 The catalysts have been used as synthesized, the conversion yield of CO2 into CO, H2 and CH4 being shown in Figure 2.7 a). The authors report 12.8 µmol•g -1 •h -1 for CO, 2.6 for H2 and 2.1

for CH4 with Au@NENU-10, whereas the yields for Au@NENU-3 are of 0.15 µmol•g -1 •h -1 for CO and 0.5 for H2 with no CH4 production, leading to an 85-fold increase in CO production and 5-fold in H2 production. The catalytic properties in homogeneous media have also been investigated where the same effect can be observed, although the yields are lower. Indeed, having both AuNP and PW12 in solution produces only H2 (0.42 µmol•g -1 •h -1 ) whereas with AuNP and PW10Ti2 the whole range of products can be detected (CO 2.1, H2 0.29 and CH4 0.35 Overall, the catalytic properties of any (POM,cat)@MOF solid rely on the integrity of its (POM, cat, MOF) components and on subtle interactions between the immobilized (POM, cat) species and the MOF host. Rietveld refinement methods from powder X-ray diffraction (PXRD) patterns, based on the Bragg peaks analysis arising from symmetry in polycrystalline solids, may be used to locate guest species while retrieving the host's crystal structure. However, such techniques -even using high-resolution synchrotron data -are not suitable when the immobilized guests exhibit a strong positional disorder in the porous host. Although many POM@MOF composites have been reported for catalysis purposes, 20,21 there has been so far very few report of structural single-crystal data determination 22 and no report on POM@MOF crystal structure determination from powders, besides the proposed positioning of PW12 in NU-1000 by Farha et al. from difference electron density maps. 23,24 Instead, pair distribution function (PDF) data collection, which includes both Bragg and diffuse scattering, is considered as a valuable technique to overcome the above limitations as detailed below and retrieve reliable structural information on hosted molecular species.

µmol•g -1 •h -1 ).
The PDF signal reflects the probability of finding a pair of atoms at a distance r in the structure and may be described as a one-dimensional function that exhibits peaks at r-values corresponding to characteristic interatomic distances within the sample. The strength of this method lies in the quantitative structural information provided about short, middle and longrange orders, including all the deviations from the average structure. 25 In the present POM@MOF and (POM,cat)@MOF materials, the contributions arising from both the host and the guests are expected to induce a strong peak overlap making the interpretation of the PDF data challenging. To overcome this limitation, an elegant approach was proposed by Chapman et al., namely the differential PDF (d-PDF) method, 26 whereby the local structure of the guest, the host-guest interactions and the structural modifications of the host are retrieved through the subtraction of the PDF of the guest-free host (as a reference) from that of the guest-loaded PDF.

In this context, Chapman et al. studied N2 adsorbed in Prussian blue whereby the d-PDF refinement showed a slight expansion of the host network and a confinement of the loaded N2 molecules close to the pore centers. 25 In another challenging case, Platero-Prats et al.

demonstrated the formation of Cu-oxo clusters within the small pores of NU-1000 during the atomic layer deposition of Cu. 27 The further refinement of the d-PDF highlighted the formation under reducing atmosphere of metallic Cu 0 nanoparticles of 4 nm and < 1 nm sizes within the pores, followed by their transformation into cuprite Cu2O nanoparticles under oxidative atmosphere. However, the use of PDF and d-PDF methods is still limited in the field of guest@MOF materials, with only a few reports, most of which concerning the adsorption of small molecules. 25,[28][29][30] To the best of our knowledge, the investigation of POM@MOF and (POM,cat)@MOF composites by differential PDF analysis is the first of the kind. Considering this information, we have decided to co-immobilize Cp*Rh and PW12 in the same material in order to investigates its catalytic properties and the eventual effect that PW12 could have on the catalysis. Indeed, introducing an electron/proton relay near the Cp*Rh catalytic center might increase the system efficiency for CO2 reduction, as shown with NENU-10. 18 Besides, the stability and structural complexity of PW12@UiO-67 make it a perfect proof of concept for d-PDF studies of POM@MOF.

Synthesis and Characterizations of the (PW12,Cp*Rh)@UiO-67 composite

The (PW12,Cp*Rh)@UiO-67 compound was synthesized in two steps combining the two reported procedures for the synthesis of PW12@UiO-67 14 and Cp*Rh@UiO-67 1 (Figure 2.8). The presence of PW12 inside the MOF cavities leads to a more difficult linker exchange during the PSE. We made the hypothesis that it might be due to some steric hindrance effects. In order to achieve similar amounts of exchanged linkers in both the POM-containing and POM-free UiO-67 materials, a higher initial amount of Cp*Rh linker in the reactive medium was thus necessary when working with the POM@MOF than with the MOF alone (see details in experimental section). The percentage of bpdc linkers exchanged in the MOF as well as the amounts of POMs immobilized in the cavities were evaluated by EDS and elemental analyses.

Firstly, EDS analysis allowed us to estimate the amount of both POM and Cp*Rh by monitoring the W% and Rh% with respect to the Zr%. The proposed formula and metal% for (PW12,Cp*Rh)@UiO-67 were confirmed by ICP-AES and elemental analysis. Once all the samples were synthesized with the same amount of guests inside the MOF pores, systematic characterization steps have been made, ranging from usual characterization methods (PXRD, IR, UV-vis, etc.) to more specialized ones (ssNMR, PDF). The powder X-ray diffraction measurements (Figure 2.10) of the (PW12,Cp*Rh)@UiO-67 composite, compared to those of UiO-67 and PW12@UiO-67, confirm that the crystallinity of the MOF host is maintained upon the in situ incorporation of the POM and upon the PSE of the Cp*Rh complex. For the PW12@UiO-67 and (PW12,Cp*Rh)@UiO-67 composites, we could unambiguously assign the additional broad signal visible at low (2) angles (below 4°) under the Bragg peaks to the diffuse scattering signal of the encapsulated [PW12O40] 3-in the MOF cavities, while excluding the possible presence of a separated amorphous phase, thanks to the PDF analysis (see below).

Figure 2.10: Powder X-ray diffraction patterns (PXRD) of POM@MOF and (POM,cat)@MOF composite solids. PXRD are shown for UiO-67, PW12@UiO-67, Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 before and after catalysis.

All the diffraction peaks are unambiguously attributed to the UiO-67 crystalline structure. 8 Apart from the diffuse scattering observed in PW12@UiO-67 (common to all POM-containing composites), the PXRD patterns of UiO-67 and PW12@UiO-67 are strictly comparable in terms of 2θ peak positions and intensities. However, the quality of the diagrams prohibits any further structural investigations such as Rietveld refinement. It shows here the limit of conventional PXRD analysis to retrieve structural information in the case of the immobilization of a guest inside an MOF porous network (low POM content, disorder). The 1 H and 13 C MAS NMR spectra (Figure 2.15) of Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67, when compared to those of Cp*Rh, UiO-67, and PW12@UiO-67, confirm the incorporation of the Cp*Rh complex into the UiO-67's framework. The 1 H NMR spectra of UiO-67 and PW12@UiO-67 exhibit the expected resonances of aromatic linker's protons (7.1 and 7.9 ppm) and of hydroxyl groups of the inorganic node (0.2-3.7 ppm) including those of defect sites. 14,32 New resonances at 1.1-1.2 ppm observed in Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 solids correspond to methyl protons of the Cp* moiety, however, significantly shielded with respect to the resonance at 1.9 ppm observed in the spectrum of the Cp*Rh(bpydc)Cl2 precursor. Furthermore, the absence of the signal at 12.5 ppm characteristic of the acid protons of the carboxylic groups of the precursor indicates the full complexation of the bpydc linker by the inorganic node of the MOF upon the ligand exchange process, confirming the absence of unreacted Cp*Rh(bpydc) inside the MOF pores. 13 C NMR also confirms the inclusion of both the Cp* complex and bpydc ligand with the appearance of their characteristic resonances at ca. 9 and 98 ppm, and ca. 154 ppm, respectively. It should be mentioned that the excessive line broadening observed in both 1 H and 13 C spectra of (PW12,Cp*Rh)@UiO-67 when compared to those of PW12@UiO-67 or Cp*Rh@UiO-67 should indicate major disorder within the structure when the POM and the Cp*Rh complex are introduced together. PW12@UiO-67 reflecting a homogeneous environment of the POM in UiO-67, 14,33 while two main resonances at -15.2 and -14.4 ppm can be distinguished in the case of (PW12,Cp*Rh)@UiO-67. This latter result suggests that the POMs occupy two distinct structural positions within the pores of Cp*Rh@UiO-67.

To attribute the two different environments of PW12 when in presence of the Cp* moiety, finer characterization methods are needed. The results of those methods will be presented in the next part.

3. In-depth Characterizations of the (PW12,Cp*Rh)@UiO-67 composite

In view of the level of structural complexity reached in the (PW12,Cp*Rh)@UiO-67 solid, we use here, besides standard characterizations, an in-depth combination of Density Functional Theory (DFT) calculations and Pair Distribution Function (PDF) analysis using laboratory Xrays. Also, the mutual interactions between the MOF host and the POM and catalyst guests were probed using 1 H, 13 C, and 13 P solid-state NMR, in line with the computational chemistry findings.

2D Solid-state NMR

As seen earlier, two different 31 The calculations also show that in both "out" and "in" positions short-range interactions of the PW12 with the linkers of the MOF are maintained through a series of hydrogen-bond type interactions within the 2.3-3 Å range of distances.

Pair Distribution Functions analysis

In a first step, PDF analysis of the PW12@UiO-67 composite was performed by Dr. Alex

Lemarchand using UiO-67 as a reference, which will act as a proof-of-concept for the applicability of the d-PDF method to our composites. The experimental PDF profile of the UiO-67 (Figure A2.3) is characterized by a series of narrow peaks on the selected r-range (1-40 Å)

which typically attest the high crystallinity of the sample, i.e. with short, middle and long-range orders. All intense peaks were assigned in a similar fashion than previously done on the related UiO-66 material, 34 reflecting the reported crystal structure of UiO-67. 35 More details may be found in the appendix. Turning to the PW12@UiO-67 composite, its experimental PDF profile contains all the expected features of the UiO-67 host while presenting particularly intense additional peaks below 10.0 Å (Figure 2 36 and residual profile (green). A-H labels of peaks correspond to the indicated refined distances in the POM components as illustrated in (c) for the PO4 tetrahedron, the WO6 octahedron, the trimer of WO6 octahedra, and within the full PW12 polyoxometalate structure. WO6, blue octahedra, PO4, pink tetrahedron, O, red spheres, W, gray spheres, P pink sphere. (d) Experimental d-PDFs (numbered 1 to 3) for PW12 in PW12@UiO-67 (blue), in (PW12,Cp*Rh)@UiO-67 before catalysis (magenta) and in (PW12,Cp*Rh)@UiO-67 after catalysis (navy blue) determined by subtracting the PDF of UiO-67 from the PDF of PW12@UiO-67, (PW12,Cp*Rh)@UiO-67 before and after catalysis, respectively.

Alex Lemarchand compared the d-PDF with the calculated PDF of an isolated PW12 extracted from reported crystallographic data (JCPDS 00-50-0304 CIF file) 36

Photocatalytic reduction of CO2

Photocatalytic activity

Considering that Cp*Rh@UiO-67 was active for the CO2 reduction under visible-light irradiation, 1 the photocatalytic activity of the (PW12,Cp*Rh)@UiO-67 composite was investigated in the same conditions. Briefly, 0.8 mg of Cp*Rh@UiO-67 or 1.4 mg of (PW12,Cp*Rh)@UiO-67 is dispersed in 0.8 mL of a 0.1 mM Ru(bpy)3Cl2 CH3CN/TEOA (5:1) solution. The sample was degassed for 20 min using CO2 before being irradiated for 3h with a 280 Xe lamp equipped with a UV and infrared cutoff (415 <  < 800 nm).

The production of H2 and formate were thus compared for the Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 materials (Figure 2.20), no other reaction products were detected neither with Cp*Rh@UiO-67 nor (PW12,Cp*Rh)@UiO-67 (oxalate, methane, carbon monoxide, methanol, ethylene and ethanol were tested). The H2 production performed by the (Cp*Rh,PW12)@UiO-67 catalyst increases progressively before reaching a plateau after 3 h, in a similar fashion than observed with Cp*Rh@UiO-67 (Figure 2.20 a). Remarkably, we estimate at 3 h that the H2 production is enhanced by a factor of ~2.5 in the (PW12,Cp*Rh)@UiO-67 composite with respect to that of the POM-free Cp*Rh@UiO-67 composite (Figure 2.21 b). The production of formate follows the same trend, being similarly enhanced but by a factor of ~2 (Figure 2.21 b). Thus, the presence of the POM in the Cp*Rh@UiO-67 photosystem seems to enhance both the hydrogen and formate production.

The same trend is also observed on the initial TOF values: for H2, without POM the TOF is 6.8

h -1 with the POM it increases to 17.8 h -1 ; as for formate without the TOFs are 4.2 and 8.4 h -1

for Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 respectively. The TOFs were calculated at 30min.

The amount of (PW12,Cp*Rh)@MOF catalysts used during the photochemical tests has been adapted to have the same catalyst molarity as Cp*Rh@UiO-67 system. The presence of the POM contributing to the compound mass, a higher amount of powder is needed with (PW12,Cp*Rh)@UiO-67 (1.4 mg vs. 0.8 mg for the POM free composite). Control experiments without irradiation did not show any formate production nor hydrogen production. A very minor H2 production can be observed with the Rh-catalyst-free PW12@UiO-67 (Figure 2.20 a). By comparison with the experiment in absence of composites, this production can be fully assigned to some catalytic activity of the Ru(bpy)3Cl2 complex (Figure 2.20 a)). Furthermore, blank experiments with argon instead of carbon dioxide led to no formate production, the catalyst was still active for proton reduction. Photoreduction experiments with 13 C could be planned to confirm the formic acid production is not coming from the catalyst or the MOF degradation. As MOFs are obtained mostly as polycrystalline powders, depositing them on transparent conducting supports allows developing easy-to-use and recyclable setups. Investigating the stability of the catalyst on the powder often causes catalyst's mass loss after recollection by centrifugation. Moreover, the catalyst being highly porous, weighing the powder after each catalytic test would lead to a bias since the solvent uptake would contribute non-innocently to the composite mass. Having this in mind, the stability and recyclability of the (PW12,Cp*Rh)@UiO-67 composite upon photocatalysis were assessed utilizing thin films rather than suspensions, by drop casting (DC) (PW12,Cp*Rh)@UiO-67 on an indium tin oxide (ITO) plate. Only a slight decrease of the formate production was detected after the first run (8%) and the second run of irradiation (9%) (Figure 2.22). Furthermore, it can be noticed that much higher formate TONs are achieved with DC-thin films than with the suspension (175 compared to 14.6, at 3h). This can be assigned to the better illumination of the crystallites deposited on the ITO plate in contrast with usual photocatalytic experiments performed on a suspension, as we recently observed for the P2W18Co4@MOF-545 photosystem (Figure 2.23). 37 Indeed, while all particles deposited on ITO are exposed to light, the less efficient exposure of crystallites to light when in suspension (random orientation of crystallites, light diffusion by the suspension…) may be responsible for the lower TONs observed. optimal (random orientation of crystallites, light diffusion); (b) as a thick layer on ITO obtained using electrophoresis (EP); the absorption of light is limited to the fraction of the crystallites in contact with ITO; (c) deposited on ITO by drop casting; DC allows depositing small amounts of POM@MOF dispersed on ITO so that all crystallites may be illuminated. 37

Post-catalysis characterization

Importantly, no significant differences are detected in the XRD (Figure 2.10), IR (Figure 2.11)

and EDS-SEM analyses (Figure 2.24) between the composite before and after the photocatalytic reaction, confirming its stability upon both illumination and the catalytic event.

Mechanistic investigation

This subpart describes DFT calculations performed by Albert Solé-Daura in view of our results in photocatalysis, aiming at understanding the possible role of each component of the complex (PW12,Cp*Rh)@UiO-67 system. They are briefly summarized here in order to provide key comparative features between the POM-free and the POM-containing catalyst for CO2 reduction.

[Ru(bpy)3]Cl2 is used as an external photosensitizer. When irradiated by visible light, an electron from the ruthenium complex is promoted to the bipyridine linker as a result of a metal ligand charge transfer, forming an MLCT excited singlet state, which rapidly evolves to a longlived triplet MLCT via intersystem crossing. 38 The reduced bipyridine in the 3 MLCT state can act as an electron donor to reduce the POM with up to 2 electrons in a step-wise process that involves two different molecules of photosensitizer. This 3 MLCT is also reducing enough to transfer electrons to the Cp*Rh catalyst, yielding a Rh(I) complex and releasing the Cl -ligand, while consuming two molecules of photoexcited Ru(bpy)3. The oxidized form of the photosensitizer can be then reduced by TEOA in solution, regenerating the Ru(II)bpy3 species while generating TEOA + radical cation. The latter TEOA + is then quenched by a second TEOA molecule, leading to the formation of HTEOA + which was found to act as the source of protons, and a neutral TEOA  radical, a reducing agent able to reduce both PW12 and Cp*Rh.

These preliminary steps are the same no matter the presence of PW12.

Starting from the Cp*Rh(I) shown in This is due to the low concentration of electron donors in solution (namely TEOA  and photoactivated Ru(bpy)3). Thus, it is expected that an equilibrium between Cp*Rh(III)H (at -2.4 kcal•mol -1 ) and (Cp*H)Rh(I) (at -7.2 kcal•mol -1 ) is reached, the latter being more stable.

Interaction between (Cp*H)Rh(I) and an electron donor, followed by another intramolecular hydrogen transfer from the Cp* moiety to the Rh leads to the formation of the catalyst in its active form (-3.6 kcal•mol -1 ). This last intramolecular hydrogen transfer is the reaction limiting step with a relatively high G ‡ of 26.3 kcal•mol -1 (shown in red arrow). This limiting step could be avoided with a higher concentration of electron donors since the reduction would occur before the first intramolecular hydrogen exchange. ii) The presence of the doubly-reduced PW12(2e -) (blue pathway in Figure 2.26) plays a huge role in the catalytic process, by being confined next to the Cp*Rh and able to act as an electron donor. The free-energy barrier for the single-electron transfer (SET) from the doubly-reduced POM to the Rh-catalyst (+13.9 kcal•mol -1 ) is lower than that for the POM-free process whereby an H-transfer allows forming the Rh(I) species (+16.9 kcal•mol -1 ). Thus the presence of the POM allows bypassing the formation of the highly stable intermediate highlighted in It is important to keep in mind that this mechanism occurs once the POM is reduced twice, meaning that the nature of the POM still has an impact on the overall properties. The POM should be easily reducible in order to be reduced twice, but also able to transfer electron easily in order to act as an electron relay. Building a similar system with other Keggin POMs would help us better understand the role of each moiety.

Conclusions and perspectives

We successfully co-immobilized a POM entity and a Cp*Rh catalyst in the (PW12,Cp*Rh)@UiO-67 material and showed that the reduction of CO2 into formate and the production of H2 are significantly enhanced when compared to those observed with the POMfree Cp*Rh@UiO-67 material. Catalytic studies were performed on crystallites in suspensions.

The (PW12,Cp*Rh)@UiO-67 composite was also deposited as films on ITO plates, providing evidence for the stability and the recyclability of the system as well as improved performances compared to those of suspensions. Importantly, differential Pair Distribution Function analysis was used for the first time to characterize a POM@MOF composite and allowed to prove the integrity of the POM before and after catalysis. Also, solid-state NMR nicely confirmed the two environments of the POMs within the MOF's cavities as determined from Monte Carlo and DFT calculations, with the Cp*Rh moiety pointing either towards a POM-free cavity or towards a POM. Such POM-Cp*Rh interactions might explain the beneficial role of the POMs in the photocatalytic reduction of CO2. DFT calculations also helped to provide a plausible mechanism. However, the precise role of the POM in the catalytic reaction remains to be confirmed. With this goal in mind, we started different projects detailed in the next part.

As mentioned in the conclusion above, and at the time of the publication of this study, the investigations on the mechanism and on the role of each component of our complex system were still at an early stage. Different hypotheses on how the PW12 could play a role in this catalytic system, the most plausible one being a role of either electron or proton relay, were envisioned.

PMo12@UiO-67

In order to confirm the role of the POM in the system, we tried to change the nature of the Keggin anion immobilized in the Rh-MOF. Changing the red/ox properties of the POM, where PMo12 is easier to reduce than PW12, 39,40 would influence its capacity to transfer electrons to the Cp*Rh moiety. Considering the proposed mechanism, the limiting step is not the electron transfer from the POM to the catalyst but from the active form of the catalyst to the CO2 or H + .

The presence of reduced POM allows to bypass the previous limiting step. Thus, having an increased amount of reduced POM near the catalyst, either by having a higher loading, or by having a POM that is more easily reduced, should increase furthermore the system efficiency for CO2 reduction. If no change is to be observed in the catalytic performances between the two different (Keggin,Cp*Rh)@UiO-67 composites, here (PMo12,Cp*Rh)@UiO-67 and (PW12,Cp*Rh)@UiO-67, that would lead to two hypotheses. The first one is that even though PMo12 can be reduced more easily, it might not transfer the electron as well as PW12, and the second that the effect on the catalysis does not depend on the nature of the POM immobilized inside the pore.

The first step to test this hypothesis was to synthesize PMo12@UiO-67. The PMo12@UiO-67 system has already been studied by a former PhD student at Institut Lavoisier de Versailles, William Salomon. He has shown that a direct synthesis, such as the one used for the incorporation of PW12 inside the UiO-67 frameworks, was unsuccessful. The "POM@MOF" PXRD pattern indeed did not match with that of the pristine UiO-67 (Figure 2.28). In addition, the peaks were compared to those of the simulated XRD pattern obtained with the CIF of Strandberg et al. and do not match with those of H3PMo12O40 neither. 41 Moreover, a color change corresponding to the reduced PMo12 could be observed. The in-situ reduction of the POM can be the reason why the final composite is amorphous. In order to avoid this process, we tried to add a strong oxidizing agent in the reaction media, Br2. However, the final compound was still amorphous. We thus turn to another POM, nonetheless trying other synthetic conditions, such as ambient temperature synthesis, or microwave synthesis could maybe solve the crystallinity issues.

Figure 2.28: PXRD or UiO-67 and PMo12@UiO-67.

PW10Ti2@UiO-67

We have thus started to study the incorporation of the PW10Ti2 POM inside the UiO-67. The idea was also to work with a POM showing better electron/proton relay properties 18 than PW12 and able to reduce CO2 by itself. 19 To synthesize the PW10Ti2@UiO-67, we have used the same method as for PW12@UiO-67. The proportion of PW10Ti2 was adapted to have a similar stoichiometry with respect to ZrCl4. The PXRD data show that the MOF is still crystalline after the synthesis. Infrared spectra show no additional peak that could be attributed to PW10Ti2 in the POM@MOF composite, probably due to the low POM loading. Nevertheless, the presence of PW10Ti2 inside the MOF and the loading has been confirmed by EDS analysis, with a W/Zr

2

PMo 12 @UiO-67 + Br 2 PMo 12 @UiO-67 ratio of 0.18. EDS analysis also revealed the presence of KCl (with ~30% K and 30% Cl compared to 32% Zr, 6% W and 1.5% Ti). Several washing with EtOH (96%) were necessary to get rid of the KCl without harming the POM@MOF integrity. The first problem we encountered with this sub-project was the low content in PW10Ti2 in the MOF. As seen previously in this chapter, the POM is meant to be immobilized in the MOF without its counterions, meaning that a certain amount of the MOF's linkers is removed to guarantee an electroneutral composite. The necessity of those defects to immobilize the POM could thus lead to a lower loading when the POM has a higher charge. PW10Ti2 bears seven negative charges, more than twice that of PW12. It results that the POM@MOF loading is only equal to 0.1 POM per Zr6 cluster (about one POM every ten cavities). We tried to increase the loading by adding a higher amount of PW10Ti2 (from one to three times the initial amount) but the resulting powder became amorphous. However, the next step would still be to exchange the biphenyl linkers for the catalytic Cp*Rh on the low-content POM@MOF composite and to test the composite activity for CO2 reduction. Although the loading is low, as the POM could show better CO2 boosting effect, a significant improved catalytic effect could not be excluded even at low loading. for one hour. Interestingly, we can see an increase in the H2 production when the POM is added to the complex when compared to the POM-free polymer. Also, the catalyst is still active after three catalytic cycles. However, surprisingly, the formate production is not enhanced by the presence of the POM (Figure A2.5) while both the formate and H2 productions were increased when PW12 and Cp*Rh were co-immobilized in the MOF. Co-immobilization of the POM and Cp*Rh catalyst in the POP did once again increase the system efficiency for CO2 reduction, confirming that the use of POM to enhance the catalysis can be used with other supports.

Interestingly the H2 production increase is similar in both UiO-67 and POP system; however, we are still working to understand the reasons of this increase in H2 only by DFT calculations.

Experimental Section

If no contrary indication, all chemicals have been bought either from Sigma, TCI Chemicals or Alfa Aesar and used without any further purification.

Synthesis of Keggin POMs.

H3PW12O40 was synthesized as previously described in Ions minéraux condensés. 42 H3PMo12O40 and K7PW10Ti2O40 were synthesized following reported procedures. 19,43 Synthesis of Cp*Rh(bpydc).

Cp*Rh(bpydc) was synthesized according to the protocol described by Chambers et al.. Synthesis of Cp*Rh@UiO-67.

Cp*Rh@UiO-67 with a Cp*Rh loading of 7% was synthesized by a slight variation of the reaction conditions used for the synthesis of the Cp*Rh@UiO-67 with a Cp*Rh loading of 10%.

suspended in 10 mL of degassed deionized water. The solution was stirred at room temperature for 24 h. The solid was isolated by centrifugation and thus suspended in 5 mL of DMF. After standing at room temperature for 1 h, the suspension was centrifuged and the solvent was decanted. The obtained yellow solid (~ 40 mg) was then washed with ethanol and finally dried under reduced pressure at room temperature.

Synthesis of (PW12,Cp*Rh)@UiO-67.

In a 10 mL glass flask, PW12@UiO-67 (50 mg, 1.87•10 -5 mol) and Cp*Rh(bpydc)Cl2 (17 mg, 3•10 -5 mol) were suspended in 5 mL of degassed deionized water. This solution was stirred at room temperature for 24 h. The solid was isolated by centrifugation and suspended in 5 mL of DMF. After standing at room temperature for 1 h, the suspension was centrifuged and the solvent was decanted. The obtained yellow solid (40 mg) was then washed with ethanol and finally dried under reduced pressure at room temperature.

Synthesis of PMo12@UiO-67.

The synthesis of PMo12@UiO-67 follows the same protocol as PW12@UiO-67 but using H3PMo12O40 instead of H3PW12O40.

Synthesis of PW10Ti2@UiO-67.

The synthesis of PW10Ti2@UiO-67 follows the same protocol as PW12@UiO-67 but using K7PW10Ti2O40 instead of H3PW12O40.

Thin film preparation.

Indium thin oxide 4 cm² was bought from SOLEMS and cut in 0.6 cm x 2 cm plates. A plate was sonicated in ethanol for 20 min and then dried under airflow. A solution of 1 mg•mL -1 of catalyst (Cp*Rh@UiO-67 or (PW12,Cp*Rh)@UiO-67) was sonicated for 20 min in parallel.

After the sonication, 10 µL of the suspension was drop-cast on the ITO plates, dried under air and then in an oven at 100°C.

General photocatalytic procedure.

Photochemical Reactions were performed using a 280 W, high-pressure Xe arc lamp (Newport Instruments). The beam was passed through a water infrared filter, a collimating lens and a filter holder equipped with a 415 nm band pass filter (Asahi Spectra). Samples were prepared in a 1 cm path length quartz sealed cuvette (Hellma) which was placed in a temperature-controlled cuvette holder (Quantum Northwest) maintained at 20 °C with a circulated water bath. A CH3CN/TEOA mixture 5:1 was used as a solvent (CH3CN and TEOA purchased from Sigma Aldrich and used without further purification). The photosensitizer utilized was a 1.0 mM solution of Ru(bpy)3Cl2 (purchased from Sigma Aldrich and used without further purification). 1.4 mg of (PW12,Cp*Rh)@UiO-67 and 0.9 mg of Cp*Rh@UiO-67, which corresponds to 0.17 µmol of Cp*Rh catalyst, were used for the photocatalytic studies. Samples were saturated with CO2 via directly bubbling CO2 through the solution mixture for 20 min.

During irradiation, the samples were vigorously stirred and aliquots of gas and liquid were analyzed. H2 measurements (aliquots of 50 µL of the headspace) were performed by gas chromatography on a Shimadzu GC-2014 equipped with a Quadrex column, a Thermal Conductivity Detector and using N2 as a carrier gas. The formate concentration was determined using a Metrohm 883 Basic IC plus ionic exchange chromatography instruments, using a The peak absence at middle-and long-range order on the experimental d-PDF can then be explained on one hand by the random immobilization of the POM inside the UiO-67 network and, on the other hand, by the POM rotational disorder from one octahedron to another one.

Structural data

Refined values Reference (JCPDS 00-50-0304) 6 Structural parameters a (Å) As mentioned in the previous chapter, the (PW12,Cp*Rh)@UiO-67 catalytic system had two major limitations: i) the use of noble metal (Rh, Ru), which potentially limits its large scale utilization and ii) the necessity of an external photosensitizer (PS), Ru(bpy)3.Cl2, that deactivates itself after few hours due to its relative unstability. The use of noble metal-free Zr6-TCPP (TCPP = tetrakis(4-carboxyphenyl)porphyrin) porphyrinic MOFs could solve these two problems at once as MOFs incorporating Zr6 oxoclusters (Figure 3.1 left) have been already reported to be able to reduce CO2 1-3 while Zr6-TCPP MOFs, (Figure 3 As presented in the first chapter, depending on the Zr6 oxocluster coordination (6, 8 or 12 TCPP linker per Zr6 node) a large variety of Zr6-TCPP MOFs have been synthesized so far (Figure 3.2). [5][6][7][8][9][10][11][12] Moreover, the TCPP linker can obviously be metalated with a wide range of transition metal cations (Mn 3+ , Co 2+ , Cu 2+ , Zn 2+ , Fe 3+ , Pd 2+ , Rh Despite these reports, 13 the mechanism for CO2 reduction with these composites remains unclear. So far, three different mechanisms have been proposed:

-TCPP acts as a PS, transferring an electron to the Zr6 oxocluster which plays the role of a catalyst.

-TCPP acts both as a PS and a catalytic center.

-The MOF is a dual catalyst, whereby TCPP acts as PS and both Zr6 and TCPP act as catalysts.

The different mechanisms will be developed in the next part, as well as our approach to clarify the mechanism we believe is at work in our system.

State of the art of Zr6-TCPP MOFs for CO2 reduction

The first report on the photocatalytic reduction of CO2 by Zr6-TCPP MOFs has been published in 2015. Xu et al. 1 have shown that the MOF-545 can photoreduce CO2 into formate in acetonitrile with TEOA as both electron and proton donor. MOF-545 produces a significantly greater amount of formate than the homogeneous TCPP counterpart, going from 2 to 30 µmol (or 120 µmol.g -1 .h -1 considering 10h reaction time and 25 mg of catalyst) of formate respectively. This is also much higher than the previously reported values for the UiO-66-NH2 These two publications sum up the work that has been published claiming that the inorganic node is the actual catalyst for CO2 photoreduction with Zr6-TCPP MOFs. But knowing the catalytic properties of metalated porphyrins in homogeneous conditions for CO2 reduction (mainly giving CO as a product), both via electro and photochemical ways, it cannot be excluded that in our systems, when metalated, the porphyrin center acts as the catalyst.

This has been proposed in 2016 by Zhang et al. ions. 2 The same trends as in the previous work can be observed, MOF-545(Zn) produces 120

µmol•g -1 •h -1 .
As for the mechanism, they propose a dual catalytic route for all three MOFs where both the Zr6 node and the metalated porphyrin are able to reduce CO2 to formate.

Importantly, the effect of a co-catalyst immobilization in porphyrin MOFs has been studied by a few teams. [18][19][20] For example, Choi et al. 20 grafted covalently the Re(bpy)(CO)3Cl complex, known to yield CO in homogeneous conditions. It was observed that the grafting of the catalyst changed the selectivity toward CO2 reduction. While the catalyst free MOF-545 was reported to produce only formate, once the Re(I) catalyst immobilized the cat@MOF is indeed selective for CO production, reaching TONs of 1900 and a production rate of 1070 µmol•g -1 •h -1 . In our attempts to rationalize the CO2 photoreduction behavior of porphyrin MOFs, we will not consider the particular case of cat@MOF systems as the presence of the cat can influence the MOF selectivity.

We can summarize the different work with Zr6-porphyrin MOFs for CO2 reduction, their obtained products and claimed catalytic centers in the following table (Table 3.1). Noticeably, Zr6-porphyrin MOFs can reduce CO2 to both liquid (formate) and gas products (CO/CH4).

Moreover, the literature is still unclear on which part of the MOF plays the role of a catalyst. Taking advantage of the modularity of these systems, our objective was to synthesize a series of metalated MOF-545(TM) (TM = Mn, Fe, Co, Cu, Zn) and investigate their visible-light photocatalytic activity towards CO2RR in acetonitrile using triethanolamine (TEOA) as sacrificial donor in order to go deeper in the understanding of the CO2RR mechanism in these systems. Overall, we can consider two distinct cases: i) the metalated porphyrin acts as a cocatalyst, and a mixture of CO/CH4 and HCOOH is expected or ii) the porphyrin acts only as a photosensitizer, and HCOOH is thus the only product. Still, the MOF-545(TM) efficiency as PS may vary according to the metal center at the porphyrin linker. We have also considered a nanosized MOF-545-based system with the aim of boosting the catalytic activity as the result of i) a higher density of catalytic sites available due to a significant increase in specific surface area and ii) a faster and more facile substrate-catalyst interaction due to lower diffusion barriers and shorter diffusion distances. 21 Moreover, thanks to Albert Solé-Daura, we present a comprehensive mechanistic study by means of DFT calculations which reveals a new and unexpected role for TEOA in such catalytic systems as CO2 reducing agent once photo-oxidized by the MOF linkers. Very recently, Fujita and co-workers described several supportive roles played by TEOA in the photo-reduction of CO2 by Ru-based molecular catalysts. 22 However, this is to our knowledge the first time that TEOA is proposed to play the role of CO2 reducing agent within a mechanistic scenario that redefines it as an up to three-electron donor.

In a second part, we will describe our preliminary results concerning the immobilization of potential co-catalysts and of POMs for CO2 reduction. Indeed, metalation and size effect being optimized, adding a co-catalyst able to assist the MOF during the CO2 reduction would be a way to increase to a higher step the MOF efficiency for CO2RR. Furthermore, as described in Chapter 2, POMs can act as an electron/proton relays and boost the catalytic activity. Three catalysts were selected for this purpose, as well as a series of POMs. The selected catalysts for encapsulation will be described in the beginning of the second subpart of this chapter.

Synthesis and characterizations of metalated and nanosized MOF-545

MOF-545 was synthesized following a reported synthetic procedure using 1,2-dichloroacetic acid as modulator. 23 Regarding the synthesis of nanosized crystals of MOF-545, following a high-throughput screening study, 24 it has been reported in DMF at 130°C and later applied for the preparation of nanocomposite reverse osmosis membranes. 25 Also, the use of various amounts of acetic acid as a modulator allowing the control of the crystals size in DMF at 65°C was reported. 26 Still, the reported yields are quite low in both syntheses. Furthermore, there has been no report, to our knowledge, on the synthesis of nanosized MOF-545(Fe). Thus, we developed a synthetic

Photocatalytic reduction of CO2

The photocatalytic activities of the MOF-545(TM) (TM = Mn, Fe, Co, Cu, Zn) series of solids for CO2RR were investigated in acetonitrile with TEOA as sacrificial electron/proton donor under visible light illumination. In all cases, it has been observed that no dihydrogen was produced. Blank experiments in the absence of light, of CO2 or of catalyst have been conducted, with close to no formate production (Table 3.2).

Figure 3.12: Formate production for a) MOF-545 (light red), MOF-545(Cu) (brown), MOF-545(Co) (violet), MOF-545(Mn) (khaki), MOF-545(Zn) (blue), MOF-545(Fe50%) (light green) and MOF-545(Fe) (green). Reaction conditions: catalyst 2 mg, CH3CN/TEOA (10:1) 2 mL (λ > 415 nm, 280 W).

NanoMOF-545(Fe) exhibits indeed a remarkable formate production of 1160 µmol•g -1 •h -1 , well above that of MOF-545(Fe) (Figure 3.13). This value also exceeds by far those previously reported for most MOF-based photocatalytic systems (Table A3.1). Analogously, the nonmetalated NanoMOF-545 possesses a higher activity than that of MOF-545 (400 vs 159 µmol•g - 1 •h -1 , respectively) (Figure 3.13). These results highlight how the photocatalytic activity of MOF-545(TM) for CO2RR can be optimized by playing both on the nature of the porphyrin metal center and the size of the catalyst particles. Kinetics of visible-light-driven formate production was investigated for NanoMOF-545(Fe) (Figure 3.14). Importantly, it appeared that a significant amount of the formate produced during the catalytic process remained adsorbed into the MOF. Indeed, on the first attempts of making the kinetic study, we noticed that the amount of formate produced was not only non-linear, but also sometimes decreasing (Figure A3.1). Considering the porous nature of metal-organic framework, and their affinity toward HCOOH, we concluded that part of the formate produced was stuck in the pores of the MOF. The yields presented in Figure 3.14 (left) were thus obtained by considering the amount of formate measured both in the supernatant and retrieved upon washing the MOF with water at 60°C at the end of each photocatalytic test. It is observed that the formation of HCOO -is slowed down after ca. 2 h. This phenomenon may be assigned to the poisoning of active sites in the pores by the formed carboxylate, in agreement with its experimentally observed strong affinity for the MOF, as mentioned above and discussed in the following sections. Finally, recyclability experiments evidence that the catalyst remains stable after 3 runs (Figure 3.14 right). The small loss of activity observed is probably due to powder loss after the several washing steps and not to catalyst degradation.

4.

Reaction mechanism for CO2 reduction to formate in MOF-545

DFT Calculation

After the gained knowledge from the above experimental study, Albert Solé-Daura next conducted DFT calculations to investigate the reaction mechanism governing the reduction of CO2 to formate at an atomistic level. Previous computational studies have shown that the reduction of CO2 to formate typically involves the initial protonation of an electron-rich catalyst to form a metal hydride that can hydrogenate an incoming CO2 molecule. [28][29][30] However, this mechanism cannot apply in unmetalated MOF-545 or MOF-545(TM) (supposing that the same mechanism would be at play in unmetalated and metalated MOFs), since, as discussed above, d(Zr) orbitals are too high in energy to accommodate electrons for the subsequent reduction of a proton, in line with the absence of H2 as a byproduct. It is also unlikely that a porphyrinic metal hydride participates in the reaction mechanism, as CO2 is also reduced to formate when using the free-base MOF-545 as the catalyst 1 or even with other MOFs such as NNU-28, 31 PCN-136, 32 and the aminated UiO-66, 33 which are built on the same Zr-oxo cluster but lack metal centers in their linkers. Altogether, the joint analysis of the preceding works strongly suggests that the Zr-oxo clusters are involved in the reaction mechanism, although their role is not that of reducing CO2. Instead, we propose a new mechanism for CO2 reduction that benefits from the Lewis acidity of Zr IV sites and the unprecedented role of the photo-generated TEOA • radical species as a hydride-donor. This mechanism was characterized and comprehensively studied by means of DFT calculations as detailed below. Its feasibility was then further evaluated with additional experimental assays presented in the following subsection. Relative energies of species that contain a TEOA • fragment account for the energy penalty of 2.7 kcal•mol -1 associated to displacing the equilibrium depicted in Scheme 3.1.c towards the free reducing agent in solution. The free-energy cost for the product release considers the product as a non-covalent HCOOH•••TEOA adduct in solution. Red-dashed lines represent an alternative, less favorable mechanism that is higher in energy.

The geometry of TS2-4 is represented in Figure 3.17. Although the organic hydroxyl proton is still attached to TEOA in the transient species structure, we were unable to locate any intermediate in the reaction coordinate between TS2-4 and the product. This indicates that this proton is spontaneously transferred to the hydroxyl group of the Zr6 cluster after the hydride transfer from TEOA • to CO2, forming an aqua ligand.

Thanks to the Lewis acidity of Zr IV , the free-energy barrier for the hydride-transfer is decreased by 14.1 kcal•mol -1 with respect to the non-catalyzed process, which entails an overall barrier of 25.0 kcal•mol -1 . The non-catalyzed process corresponds to the hydride transfer from TEOA • radical to a non-activated CO2. It is worth noting, though, that despite being relatively high, the latter is still affordable at room temperature and might explain the small yield of formate produced by the homogeneous porphyrin (vide infra). Importantly, the initial oxidation of TEOA is strictly required to create reactive allylic protons near a heteroatom. Such a pattern facilitates the hydride-transfer by delocalizing the positive charge generated in the process, as recently observed for structurally similar allylic alcohols capable of transferring a hydride to an electrophilic peroxo group. 34 This pinpoints that every component in the reaction mixture plays an essential role in this complex reaction mechanism, which altogether allows the efficient reduction CO2 when synergistically combined within the three-dimensional structure of the MOF.

Figure 3.17: Optimized geometry for TS2-4, in which a TEOA • molecule transfers a formal hydride to CO2 to generate a formate anion coordinated to the Zr6 cluster. The relative Gibbs free energy with respect to the reactants is given in parentheses in kcal•mol -1 . Main distances and angles are given in Å and degrees.

Notably, the formation of coordinated formate in 4 is rather exergonic and confers an irreversible character to the reaction, with a reverse free-energy barrier of ~30 kcal•mol -1 . Upon releasing an aqua ligand, species 4 may evolve to the slightly more stable species 4ʹ where the formate anion coordinates two neighboring Zr ions in a bridged fashion. After reducing CO2, TEOA • turns into a depleted, aldehyde-like radical species that can be reductively quenched by the reduced linker or by a second TEOA • molecule and protonated by HTEOA + (Scheme A3.1)

to form an aldehyde by-product, previously reported as a photo-degradation product of TEOA. 35 Post catalytic 1H NMR of the solution confirms the presence of the aldehyde (Figure A3.2).

The final de-coordination of formic acid from species 4 to regenerate 1ʹ is endergonic by 9 kcal•mol -1 , in agreement with the experimental observations reporting the strong adsorption of the product in MOF-545. Owing to the large excess of TEOA in solution, this energy cost was computed assuming the formation of a H-bonded adduct between formic acid and TEOA that stabilizes the product by 4.4 kcal•mol -1 , facilitating its release as pointed out by Fujita and coworkers in a recent contribution. 22 This energy cost is then compensated by the reduction of another CO2 molecule, which provides the thermodynamic driving force for completing the reaction.

Despite TS2-4 governing formate production, it is not necessarily the fastest interaction that 2 and TEOA • may undergo. We characterized another transient species lower in energy, TS2-3, that leads to species 3 through the formation of a C-C bond between the Zr-O=C=O group and the vinylic C atom of TEOA • overcoming a very smooth barrier of 1.6 kcal•mol -1 from 2-TEOA • (vs. 10.9 kcal•mol -1 to reach TS2-4). Species 3 is a zwitterionic complex where both positive and negative charges are stabilized by the electron-donating and electron-withdrawing character of the alkyl chains and of the Zr IV center, respectively. Notably, this makes 3 lie slightly below the reactants in terms of free energy, and therefore, it is expected to act as the resting state of the catalytic cycle. Thus, the overall free-energy barrier (3⟶TS2-4, highlighted with a red arrow in Figure 3.16) accounts for a moderate value of 22.0 kcal mol -1 that can be overcome at the experimental conditions.

Alternatively, we have evaluated whether TEOA • acting as a hydride donor could serve to generate a Zr-hydride from species 1ʹ. As shown in Figure 3.15, the Zr-hydride intermediate is already 3.5 kcal•mol -1 higher in energy than TS2-4, suggesting that this pathway might not be operative. A sequential electron transfer and proton transfer mechanism from 2 + TEOA • was also predicted to be unlikely, involving a one-electron reduced Zr-(OCO •-) intermediate that lies 30.9 kcal•mol -1 above the reactants.

Finally, we sought for an explanation for experimental catalytic trends. Thanks to the gained knowledge on the reaction mechanism, the outperformance of metalated MOFs over MOF-545 might be rationalized by the ability of metalated porphyrins to coordinate TEOA through the N atom, 36 allowing a more efficient inner-sphere photo-oxidation and leading in turn, to a higher concentration of TEOA • . Among them, the high catalytic activity of MOF-545(Fe) and MOF-545(Mn) might be attributed to the fast and highly efficient metal-centered photo-reduction (from TM III to TM II ) of the porphyrin linkers to produce TEOA • . On the other hand, the higher activity of MOF-545(Zn) compared to its TM II -containing partners may be also explained by a boost in the concentration of TEOA • , caused either by a more favorable association with TEOA or by a superior light-harvesting ability. 37-39

Complementary photocatalytic tests

To validate the role of TEOA in our mechanistic proposal, we performed a new set of catalytic tests with the free-base MOF-545 in the presence of an additional amount of molecular Fe III Cl-TCPPOMe (TCPPOMe = tetramethoxyphenylporphyrin) in solution, the iron derivative being the material providing the highest formate yield in the present work. The ester form of the porphyrin was used for solubility reasons. According to our mechanism, the porphyrin added in solution is expected to act as an auxiliary electron acceptor boosting the concentration of the TEOA • and in turn, accelerating the rate of the CO2RR. In fact, according to a ΔΔG ‡ of 3 kcal•mol -1 (22.0 vs 25.0 kcal•mol -1 for the Zr6-catalyzed and non-catalyzed paths, respectively), the formate produced on the Zr sites would account for the 99.4 % of the overall yield at 25 °C. Further supporting our mechanistic proposal, we noticed that most of the experimental works that reported the formation of formate with Zr-based porphyrinic MOFs also employed TEOA as "sacrificial donor" (Table A3.1). The remaining ones performed the catalytic experiments in the presence of other aliphatic alcohols structurally similar to TEOA that might display analogous reactivity, such as ethylene glycol 2 or triisopropanolamine. 32 To conclude, a computational study supported by experimental data allowed proposing for the first time a full description of the reaction mechanism at play in the photocatalytic reduction of CO2 into formate by Zr-oxo-clusters in a porphyrinic MOF in presence of TEOA, which is depicted in Figure A3.3. The proposed mechanism consists of three main processes: i) the photo-generation of the active reducing species at the porphyrinic linkers; ii) the thermallyactivated reduction of CO2 on the Zr6 nodes and iii) the termination via radical quenching. It appears that the photoreduction of the linkers by TEOA yields a TEOA • radical, which, beyond being a mere depleted sacrificial donor, was identified as the real reducing species, being able to transfer a hydride to a CO2 molecule activated on a Zr IV Lewis acid center of the Zr6 node.

The relative catalytic activity of the different materials is rationalized on the basis of their ability to oxidize TEOA under visible-light irradiation: Fe III -and Mn III -containing MOFs, with an oxidation state of +3, permit a highly efficient metal-centered photoreduction, whilst the coordination of TEOA to metalated linkers allows more efficient inner-sphere charge-transfer process compared to free base TCPP linkers.

Overall, these findings represent a step forward towards understanding the catalytic processes that make use of photo-active materials in combination with TEOA (or structurally similar)

"sacrificial donors" and is expected to inspire the design of new multi-component catalytic platforms for photo-reduction reactions with improved activity and selectivity.

Co-catalyst immobilization in MOF-545

Another way to increase the activity of MOF-545 would be to use its porous character in order to immobilize a co-catalytic guest able to reduce CO2. The large 36 Å long pore allows the incorporation of a wide variety of catalysts, from small molecules to larger complexes. 40 The combination of the photosensitive porphyrin and a catalytic guest could allow the reduction of CO2 without addition of external photosensitizer, in contrast with (PW12,Cp*Rh)@UiO-67 seen in the previous chapter. The choice of the catalyst is a crucial step to build the photoactive composite. A molecular catalyst already reported to reduce CO2 in presence of porphyrins as PS would be the best choice. Nonetheless, as shown by Paille et al., 40 the heterogenization of the TCPP porphyrin in metal-organic frameworks can change its photoactivity. Indeed, the molecular [(PW9O34)2(H2O)2Co4] 10-(Co4) polyoxometalate catalyst reported for H2O oxidation could not, in homogeneous conditions with TCPP, oxidize water in O2. However, once Co4 was immobilized in MOF-545, the system could act as a heterogeneous catalyst for water splitting,

showing that the energy levels of the TCPP can be modified between the molecular species in homogeneous conditions and the PS immobilized in the MOF as linker. Considering these facts, we have turned to three catalysts previously reported for CO2 reduction: Mn(bpy)(CO)3Br 41,42 (noted Mn(bpy)), Ni(cyclam)Cl2 [43][44][45] (noted Ni(cyclam)) and 5,10,15,20tetrakis(pentafluorophenyl)porphyrin (noted F20) (Figure 3.19). We have immobilized Ni(cyclam) and Mn(bpy) in MOF-545(Zn). This MOF was chosen because Zn(TPP) is known to be the best PS 46,47 among the metalated porphyrins. The immobilization of F20 was first performed in the unmetalated MOF-545 to test the possibility of encapsulating this porphyrin catalyst in a porphyrinic MOF. In 2016, Zhang et al. 41 reported the photoreduction of CO2 by a Mn(bpy)(CO)3Br complex in homogeneous conditions with the zinc tetrakis(phenyl)porphyrin as a photosensitizer. The reaction was carried out in CH3CN/H2O (20:1) with 0.1 M of triethylamine as an electron donor and 0.5 mM of catalyst. The catalyst over ZnTPP ratio was modulated ranging from 1:2 to 4:1.

Surprisingly, the lower the amount of ZnTPP, the higher the catalyst efficiency. Indeed, the 4:1 ratio reached remarkable TONs of 119 for CO and 19 for HCOOH compared to 8 and 6 for CO and HCOOH, respectively, when the Mn(bpy):ZnTPP ratio was 1:2 (Table 3.3). The authors did not propose any explanation for this ratio effect.

Table 3.3: Photocatalytic CO2 reduction with Mn(bpy)(CO)3Br and ZnTPP. 3h irradiation, 500W Xe Lamp, CH3CN/H2O (20:1), TEA 0.1 M. 41 Fei et al. 42 reported in 2015 the immobilization of Mn(bpy) in UiO-67 MOF. They used a mixed linker UiO-67 including a bipyridine moiety to act as an anchoring site for the Mn(bpy) catalyst.

The Mn(bpy)@UiO-67 activity was compared to that of the Mn(bpy) and Mn(bpydc) (bpydc = bipyridine dicarboxylate) complexes dissolved in a DMF/TEOA (4:1) solution and photosensitized by a ruthenium complex, with additional BNAH (BNAH = 1-benzyl-1,4dihydronicotinamide) as an electron donor. After 18h, the cat@MOF was close to twice more active than the complexes in homogeneous conditions. However, the homogeneous catalyst only shows a slight deactivation after a few catalytic runs 41 whereas the Mn(bpy)@UiO-67 composite loses half its efficiency after only one catalytic run. The authors indicate that the loss of activity is due to the loss of CO ligands from the Mn catalytic sites and to the degradation of the MOF framework.

Regarding Ni(cyclam), it was first described for CO2 reduction in 1987 by Grant et al.. 43 Ni(cyclam) was used in water with ascorbic acid as an electron donor and [Ru(bpy)3]Cl2 as the photosensitizer. The catalyst was highly stable; however, the catalyst was poorly efficient reaching TONs of 4.8 after 22h for CO and was poorly selective as 90% of the product was H2.

Moreover, the sample was irradiated at 1000W, which is a far more powerful output than any of the studies presented so far. Few reports on Ni(cyclam) immobilization, on ZrO2 44 or in a Gd MOF, 45 show a significant increase in the catalyst efficiency.

F20 was selected to serve as a proof of concept of porphyrin molecular catalyst immobilization in a porphyrin MOF because, although F20 has not been reported for CO2 photoreduction, closely related porphyrins have shown interesting catalytic performances. 48 Furthermore, F20 is commercial and relatively cheap and easy to quantify thanks to the 20 fluor born by the phenyl rings. We can note also that several publications dedicated to catalytic reactions involving the cobalt metalated F20 have been published, including oxygen evolution reactions, 49 cyclohexane oxidation 50 or hydroalkoxylation of alkynes. 51 This study was performed in collaboration with Julien Bonin and Marc Robert (Laboratoire d'Electrochimie Moléculaire, Université de Paris)

Synthesis, characterization and photocatalytic activity of the cat@MOF composites

To synthesize the cat@MOF-545 composites, we adapted the protocol described by Paille et al.. 40 Briefly, 10 mg of MOF-545(TM) are dispersed and stirred for 24h in a 5 mM solution of the molecular catalyst. For Ni(cyclam)Cl2 and Mn(bpy)(CO)3Br, the catalysts were dissolved in acetonitrile and heated at 25 or 60°C. The cat@MOF composites were collected by centrifugation and washed several times with the same solvent than the one used for the encapsulation. A last washing step with acetone was necessary before drying the powder in the oven at 100°C. The quantity of catalyst immobilized in the MOF was estimated by EDS, through the evaluation of the Mn/Zr and Ni/Zr ratios for Mn(bpy) and Ni(cyclam), respectively (Table 3.4). We thus propose the following formulas:

Zr6O16H18(C48H24N4O8Zn)2Cl2(Mn(bpy))0.8 Zr6O16H17.2(C48H24N4O8Zn)2Cl2(Ni(cyclam))0.4 at room temperature and Zr6O14H10(C48H24N4O8Zn)2(Ni(cyclam))4.0 at 60°C. The previous formula considers the charge compensation by a proton loss on the zirconium cluster as no additional chlorine has been detected by EDS, meaning Ni(cyclam) is encapsulated as a cation.

Table 3.4: EDS analysis of Mn(bpy)@MOF-545(Zn) and Ni(cyclam)@MOF-545(Zn).

Mn/Zr Ni/Zr Mn(bpy)@MOF-545(Zn) 0.13 -Ni(cyclam)@MOF-545(Zn) RT -0.07 Ni(cyclam)@MOF-545(Zn) 60°C -0.66 the amount of impregnated F20, the temperature was increased to 120°C, leading only to a clearly insufficient F20/Zr6 ratio of 0.04. We thus turned to a "successive encapsulation" method. After each 24h encapsulation, the solid was collected by centrifugation and washed several times. The same solid was then dispersed in a fresh solution of 5 mM F20 in DMF and stirred for 24h at 120°C. This way, we obtained larger F20/Zr6 ratio up to 0.2 after the third cycle. (Table 3.5).

Table 3.5: EDS analysis of F20@MOF-545 composites obtained after one (F20@MOF-545-1), two (F20@MOF-545-2) and three (F20@MOF-545-3) impregnation cycles in DMF (24h, 120°C, 5mM F20).

F/Zr F20/MOF F20@MOF-545-1 0.14 0.04 F20@MOF-545-2 0.47 0.14 F20@MOF-545-3 0.69 0.20

The composites have been characterized at each step by infrared spectroscopy and PXRD (Figure 3.21). As for Mn(bpy)@MOF-545(Zn) and Ni(cyclam)@MOF-545(Zn), the different PXRD patterns are similar to that of MOF-545, assessing the structural integrity of the composite even after 72h at 120°C and several washing steps. Like for the two previous cat@MOF composites, it was not possible to identify the presence of the catalyts on the IR spectra. Next, the activity for CO2 reduction of the three cat@MOF composites were tested. We have considered different combinations between water and acetonitrile as solvent and TEA, TEOA and ascorbic acid as sacrificial electron donors. For both F20@MOF-545 and Ni(cyclam)@MOF-545(Zn), no matter the combination, no CO2 reduction product were detected after several hours of irradiation. Nonetheless, for Mn(bpy)@MOF-545(Zn) a large amount of CO was detected by gas chromatography using acetonitrile as solvent and TEOA as an electron donor (Figure 3. 22). In order to confirm the catalytic nature of the reaction, we performed blank experiments. Without light, no CO was detected. However, we could still detect CO without bubbling CO2 in the solution, meaning that the CO detected by GC is not issued from CO2 reduction but from the Mn(bpy)(CO)3Br decomposition. Indeed, after 8h

Synthesis and characterization and photocatalytic activity of POM@MOF composites

As seen in the previous chapter, the presence of an electron-rich POM near the catalytic center can enhance greatly the catalytic performances of the composite. Moreover, the pores of MOF-545 being large enough to proceed to post synthetic encapsulations, we worked on immobilizing various POM anions in the MOF-545.

In a preliminary study conducted before the MOF-545 CO2RR catalytic activity optimization seen in subpart two, we investigated the effect on the catalysis of the immobilization of four The synthesis of the POM@MOF-545 composites was performed using 5 mM of POM DMF/H2O (1:1 v/v) mixtures. We quantified the formate production rates in two ways, first by considering the whole sample as the catalyst and thus presenting the rate in µmol•g -1 •h -1 , second by considering that the POM does not act as catalyst and thus presenting the rates in µmol•g -

1 MOF•h -1 (Table A3.2).
Two POM@MOFs, Ni4@MOF-545 and SiW12@MOF-545, present a significant boosting effect on the catalysis compared to the POM-free MOF, whereas the two others (Co4@MOF-545 and PW10Ti2@MOF-545) have proven a limited increase. The value of the HCOOH production in µmol•g -1 •h -1 of Ni4@MOF-545 is greater than that of SiW12@MOF-545 (268 vs 218). In contrary, when considering the MOF only, SiW12@MOF-545 shows the greatest increase (378 vs 423) (Figure 3 Moreover, two new peaks at 4.1 and 9.0° appeared. To date, these peaks could not be attributed to known porphyrinic MOF phases nor to the initial POMs. The reason of the drastic changes in the PXRD pattern could be attributed to the highly concentrated solutions of POMs used during the impregnation steps probably resulting in high loading of POMs. Indeed, for POM@MOF composites, the negative POM charge is often compensated by linker defects and thus high loadings can lead to a partial destruction of the MOF.

Next, we tried to play around the impregnation protocol to avoid the changes in PXRD patterns.

We thus decided to decrease the amount of POM in solution during the encapsulation processes, even if this could lead to significantly lower loadings in POM. We performed the encapsulations The POM@NanoMOF-545 composites activity for CO2 reduction has been tested.

Unfortunately, no significant CO2 reduction has been found. This is highly surprising as the MOF alone has shown to be active (see above). Thus, this could be due to the presence of the POM but also to the POM impregnation conditions. To check this last hypothesis, the POMfree NanoMOF-545 has thus been stirred in DMF/HCl (0.1M) solution (i.e. the impregnation conditions for the obtaining of the POM@MOF materials). Despite being highly crystalline, the resulting MOF has shown only a small activity (Table 3.6), strongly suggesting that the loss of activity is indeed due to its treatment in DMF/HCl (0.1M). The NanoMOF-545 has also been stirred in a DMF/H2O mixture, a medium where alkaline Keggin salts are soluble. Interestingly, a slighter activity loss can be observed, going from 1050 µM of formate for the starting NanoMOF-545 to 723 µM after the DMF/H2O treatment. This supports that the loss of activity is due to the acidity of the medium used for POM encapsulation.

Moreover, it has been found that supercritical CO2 treatment can help the NanoMOF-545 to recover partially its activity after the DMF/H2O treatment as the formate produced increases from 723 to 867 µM (with and without supercritical CO2 activation respectively).

To conclude this study, we have seen that the impregnation conditions could lead to a drastic loss of activity of the MOF. Conditions where the MOF is both crystalline and active after POM encapsulation still have to be clearly established. Further investigations on the encapsulation conditions are thus necessary. We could think of working with monosubstituted Keggin.

Monosubstituted PW11M Keggin anions are indeed stable at pH higher than non-substituted ones. This pH can, depending on the nature of the heterometal M, reach pH 7; 52 thus, encapsulation in DMF/H2O can be considered for these species.

Conclusions

The photocatalytic activity of non-metalated and metalated MOF-545(TM) towards CO2RR in 10:1 CH3CN/TEOA solutions has been evaluated by systematically varying the transition metal coordinated to the porphyrinic linkers. All the prepared catalysts showed 100% selectivity towards formate production and the yield increases from free-base to Cu II < Co II < Mn III < Zn II < Fe III , reaching 566 µmol•g -1 •h -1 for MOF-545(Fe). The activity of MOF-545(Fe) could be boosted thanks to the preparation of nanocrystals (150 nm length) allowing an increased density of accessible catalytic sites. This strategy enabled reaching a formate production of 1160 µmol•g -1 •h -1 , being one of the highest productions of formate amongst MOF catalysts reported to date. Recyclability tests show that the MOFs maintain their catalytic activity over several runs.

Concerning the encapsulation of Mn(bpy) and Ni(cyclam), a high loading in porphyrinic MOFs can be reached in rather mild conditions (60°C in acetonitrile). The resulting cat@MOF-545(Zn) have been characterized and tested for their catalytic CO2 reduction properties but, unfortunately, did not give promising results. Indeed, no products were detected when working with Ni(cyclam). Besides, with Mn(bpy) a high amount of CO was detected but corresponded to the catalyst degradation, releasing its former covalently bound CO ligands. Considering the high loading of both catalysts, it could however be interesting to test the cat@MOF-545(Zn) composites for CO2 electroreduction as it has been reported that Ni(cyclam) 53 is more active and Mn(bpy) 54 more stable in such conditions than under photocatalytic conditions. As for F20, we successfully encapsulated the porphyrin in MOF-545. The encapsulation is more challenging than Mn(bpy) and Ni(cyclam) as it involves several successive encapsulations to reach a loading of 0.20 F20/MOF, but again, no significant CO2RR activity has been observed.

Lastly, we attempted to encapsulate Keggin anions in NanoMOF-545. Due to the low stability of the polyoxometalates used in non-acidic conditions and the stability issues of MOF-545 toward water, simple encapsulation in pure aqueous media was excluded. Reactions have thus been performed in DMF/HCl (0.1M) (1:1 v/v). A very high POM loading has been obtained, greatly affecting the crystallinity of the MOF as evidenced by the PXRD patterns which are altered, with in particular the appearance of two new intense peaks. Moreover, the resulting POM@MOF composites are inactive for CO2 reduction. We assume that treatments in acidic media deactivate the MOF. A non-acidic DMF/H2O mixture has thus been considered as potential solvent for further impregnation experiments. Interestingly, it has been found that this time, the NanoMOF-545 conserves its activity, which makes it possible to obtain in future studies active POM@MOF composites.

Experimental section

NanoMOF-545 synthesis. 137 mg of ZrOCl2.8H2O, 30 mg of TCPP and 300 µL of 1,2dichloroaceticacid were dissolved in 10 mL DMF in a 100 mL Teflon vial. The mixture was heated for 20 min under microwave (130°C, 800 W) with vigorous stirring. After cooling to room temperature, a powder was collected by centrifugation. The MOF was then heated at 130°C for two hours in a 10 mL DMF / 1 mL 1M HCl mixture. After centrifugation, the obtained material was rinsed twice with DMF and twice with acetone. Lastly, the powder was dispersed in 20 mL of acetone and stirred overnight. The final compound was collected by centrifugation and rinsed twice with acetone then dried at 100°C (m = 28 mg).

NanoMOF-545(Fe) Synthesis. 137 mg of ZrOCl2.8H2O, 32 mg of (Fe)TCPP and 900 µL of 1,2-dichloroaceticacid were dissolved in 10 mL DMF in a 100 mL Teflon vial. The mixture was heated for 20 min under microwave (130°C, 800 W) with vigorous stirring. After cooling to room temperature, a powder was collected by centrifugation. The MOF was then heated at 130°C for two hours in a 30 mL DMF / 3 mL 1M HCl mixture. After centrifugation, the obtained material was rinsed twice with DMF and twice with acetone. Lastly, the powder was dispersed in 20 mL of acetone and stirred overnight. The final compound was collected by centrifugation and rinsed twice with acetone then dried at 100°C (m = 30 mg). EDX calcd.

(exp.): Fe/Zr 0.33 (0.32).

Photocatalysis experiments. 2 mg of catalyst and 2 mL of a CH3CN/TEOA (10:1) mixture were added to a 4 mL sealed quartz cuvette and sonicated for 10 min. The cuvette was then submitted to a CO2 flux for 20 min before being irradiated for 4 h with a 280 W Xenon lamp equipped with a 415 nm UV cutoff. Gas products were analyzed via gas chromatography while liquid products were assessed by ionic chromatography. After the catalytic process, the catalyst was recollected by centrifugation, dispersed in 2 mL of milli-Q water and stirred at 60°C for 1

h. An aliquot of the liquid phase was collected and analyzed by ionic chromatography to quantify the remaining formate leached by the MOF.

Scheme A3.1: Proposed pathway and transient intermediates involved in the formal hydridetransfer process from TEOA • to CO2. In this chapter, we will focus on POMs for the CO2 reduction in heterogeneous conditions.

Noticeably, although numerous studies have revealed their potentialities as catalysts for water splitting, [1][2][3] there have been only few reports on POMs for CO2RR. 4 However, besides rare examples of transition metal (M)-substituted polyoxotungstates [5][6][7] as seen in Chapter 2, reduced polyoxomolybdates (i.e. containing Mo V ions) emerged in 2019 as promising candidates.

Illustratively, {-Mo V 12Mo VI 4O52} reduces CO2 into formaldehyde under UV-light in homogeneous conditions in water 8 and the related {-PMo V 8Mo VI 4Zn4O52}, as the building unit of a POM-based Metal-Organic Framework (POMOF) NNU-29, selectively reduces CO2 into formate in heterogeneous conditions under visible light in water, with [Ru(bpy)3] 2+ as a photosensitizer. 9 Another family of solids made of reduced POM building units have been recently evidenced as heterogeneous photocatalysts for CO2RR; these solids made of {P4Mo V 6} POM units connected by Mn II or Co II ions can reduce CO2 into CO and CH4 in the presence of [Ru(bpy)3] 2+ . [10][11][12] 1. Polyoxometalates for CO2 photoreduction P4Mo6 oxoclusters were first described in 1989 by Haushalter et al. 13 who report a structure containing [Mo6P4O24(OH)7] 5-anionic clusters. In this anion, three {Mo V 2O4} dimeric units are connected by edges around a central phosphate group (Figure 4.1). Three peripheral phosphate groups complete the molecular structure. The Mo(V) ions form short Mo-Mo bonds (2.6 Å) within the dimeric unit. The central and the three peripheral phosphate groups lie in the same side of the plane defined by the six Mo ions. The P4Mo6 subunits are linked together by bridging Na + cations forming 1D strings. In the following decade several groups have reported a variety of related structures with different transition metals (Zn II , Fe II/III , Co II , Mn II ) as bridging cations for the P4Mo6 subunits. 14-1 The hydrothermal synthesis of insoluble solids with P4Mo6 polyoxometalates units connected to transition metal ions has been previously studied in our group by a former PhD student, Charlotte du Peloux. In 2004 our group thus reported the partial substitution of the bridging metal cations in order to give bi-metallic P4Mo6 derivatives. 18 Six out of ten manganese atoms in the Na15Mn10[(Mo2O4)3(HPO4)(PO4)3(OH)3]4(PO4)•48H2O solid were replaced by Fe ions.

To date, this composite had only been studied for its magnetic properties. In fact, reducing CO2

was not a concerning topic at that time. The structure of Ru(bpy)-Mn has similarities with that of Ru(bpy)-Co, with Mn(II) ions replacing Co(II) ions. However, the two compounds are not isostructural and present some differences. In Ru(bpy)-Mn, there are two independent P4Mo6 units with two independent Mn(II) ions (Mn1 and Mn4) connecting them (Figure 4 Finally, Fe-Mn does not contain additional transition metal complexes as counterions as it is the case for the three other structures reported herein. It is thus a fully inorganic structure with P4Mo6 units linked by Fe and Mn metal ions. Its structure is reminiscent of that of the previously reported molybdenum(V) phosphates Na15Mn10[(Mo2O4)3(HPO4)(PO4)3(OH)3]4(PO4), 28 Na7Mn4Fe6[(Mo2O4)3(HPO4)2(PO4)2(OH)3]4(PO4), 27 NENU-605 11 and NENU-606, 11 which all crystallize in the tetragonal I41/acd space group and contain Mn(P4Mo6)2 sandwich-type units.

One additional common feature of these structures is the presence of an extra phosphate ion which does not belong to a P4Mo6 unit. The difference between them lies in the nature of the transition metal ions connecting these units. In Fe-Mn, the Mn(P4Mo6)2 sandwich-type units are connected to each other and to the extra phosphate group via Fe(II) and Fe(III) ions (Figure 4.9 a). BVS calculations unambiguously allowed to determine that Fe1 is a Fe(III) ion while The experimental X-ray powder patterns and of the powder pattern calculated from the structure solved from single-crystal X-ray diffraction data (Figure 4.13) are in accordance, showing no crystalline impurities mixed with the crystals after the several washing steps.

Photocatalytic properties

The photocatalytic properties of the four compounds were studied in aqueous suspensions of ground crystals, with triethanolamine (TEOA) as an electron donor (H2O/TEOA = 14:1).

Despite the presence of [Ru(bpy)3] 2+ species in the framework of Ru(bpy)-Co and Ru(bpy)-Mn, neither the formation of gaseous products nor that of formate in the liquid phase was observed under visible light irradiation in absence of additional photosensitizer (PS). However, in presence of [Ru(bpy)3]Cl2 dissolved in the catalytic medium, the formation of CO and CH4 could be observed in the gas phase for Ru(bpy)-Mn and Fe-Mn, but not for Ru(bpy)-Co and Ni(cyclam)-Mn. For the two former compounds, CH4 is the major product and its production increases almost linearly with the irradiation time (Figure 4.14). In contrast, the production of CO is much lower and stops after a few hours. The CH4 production amounts to 48.0 nmol (i.e. 600 nmol•g -1 •h -1 ) and 115.2 nmol (i.e. 1440 nmol•g -1 •h -1 ) after 8 h, with selectivity of 85.2% and 92.6% for Ru(bpy)-Mn and Fe-Mn, respectively. Fe-Mn is thus the most efficient catalyst, and its activity is comparable to that observed for the related fully inorganic P4Mo6-based heterometallic molybdenum(V) compounds NENU-605 (894.7 nmol•g -1 •h -1 ) and NENU-606 (1747.8 nmol•g -1 •h -1 ) studied in similar conditions. 11 However, the selectivity of Fe-Mn for CH4 (92.6%) is better than the selectivity of these two compounds (76.6% and 85.5% for NENU-605 and NENU-606, respectively).

Mechanistic study

Previous computational studies have shown that the reduction of CO2 to CO requires the coordination of CO2 to an active metal center through the carbon atom to induce the charge transfer from electron-rich ligands or from the metal itself to form a M-COO •-intermediate. 30- and in turn, more likely to participate in the reaction mechanism compared to those sandwiched between two P4Mo6 units, which are hexacoordinated to bridging oxygen of the POM.

Owing to the presence of d electrons in fully reduced polyoxomolybdates such as {P4Mo V 6}, it was initially postulated that upon light absorption, POMs may be able to transfer electrons to interstitial metal centers to generate a M(I) ion where CO2 molecule could be then reduced upon coordination. 12,39 In such a scenario, the role of the [Ru(bpy)3] 2+ would be that of regenerating the fully reduced POM complex with the help of a sacrificial electron donor. More recently, photophysical studies suggested that the reduction of CO2 is attained by the [Ru(bpy)3] 2+ complex rather than by {P4Mo V 6}-containing materials, as the transient photovoltage of [Ru(bpy)3] 2+ is quenched in the presence of CO2, whereas that of {P4Mo V 6}-based crystals was not significantly altered. 15 Similar findings were also reported for larger Mo(V)-phosphatecontaining POMs. 21 To investigate the processes that may occur when irradiating such a complex multi-component system, we evaluated the feasibility of possible interactions between the POM catalyst, the Ruphotosensitizer and the TEOA sacrificial donor. On the one hand, [Ru(bpy)3] 2+ is well-known to undergo a metal-to-ligand charge transfer (MLCT) upon visible-light absorption, leading to a singlet MLCT state that rapidly evolves towards a long-lived triplet MLCT state, 3 [Ru III (bpy)2(bpy •-)] 2+,* . 40 On the other hand, the {P4Mo6} POMs can also interact with visible light, leading to internal transitions of metal-centered (MC) and metal-to-metal charge transfer (MMCT) character that involve the Mo(V) ions of the cluster. Our DFT calculations predicted that both the POM and the [Ru(bpy)3] 2+ systems are susceptible of generating a hole at a suitable energy level to be quenched by TEOA after absorbing a photon, in agreement with previous photophysical measurements. 10,21 It is also important to note that the 3 [Ru III (bpy)2(bpy •-)] 2+,* excited state is not reducing enough to spontaneously transfer an electron to the ground state of the POM, although an excited state of {P4Mo6} may reduce [Ru(bpy)3] 2+ . Overall, this analysis indicates that the Ru complex is the species that acts as electron acceptor, while the {P4Mo6} clusters in the crystal could act as electron shuttles, facilitating a formal electron transfer from TEOA to [Ru(bpy)3] 2+ .

The gained knowledge about the charge-transfer processes underlying to the {P4Mo6}/Ru(bpy)3 photosystem prompted us to propose a novel mechanism for the reduction of CO2 to CO in which [Ru(bpy)3] 2+ acts as the catalyst and the POM-based material as a co-catalyst. This mechanistic proposal is schematically represented in Figure 4.17 and evaluated by means of theoretical and experimental approaches. First, the formation of the one-electron reduced forms of the POM and [Ru(bpy)3] 2+ (highlighted in yellow and green, respectively) occurs photochemically as described above. The absorption of a second photon by [Ru II (bpy)2(bpy •- )] + could promote its photoionization via a ligand-to-solvent charge transfer (LSCT), generically known as charge transfer to solvent (CTTS), to regenerate the initial [Ru II (bpy)3] 2+ species yielding a solvated electron (e - aq). In the presence of CO2, solvated electrons tend to localize in an empty π* MO of CO2, resulting in a CO2 •-radical anion of bent structure, 41,42 and accomplishing the first electron injection. Aside from evolving to CO on the crystal surface, we can envisage an alternative pathway that CO2 •-may undergo, which is the radical coupling with another CO2 •-to form an oxalate anion (C2O4 2-), which may prevail in the absence of coordination sites. In view of this possible route, we re-performed blank experiments in the absence of POM-based crystals, not only verifying the absence of CO and CH4, but more importantly revealing the formation of oxalate anions as demonstrated by 13 C NMR (Figure 4.18) and also confirmed by IC measurements, thus supporting the generation of CO2 •-anions in solution by the Ru complex. Furthermore, this also evidences that [Ru II (bpy)3] 2+ alone is not sufficient for yielding CO and thus, in absence of metal centers where CO2 •-can bind to carry on with the reaction, the generated CO2 •-ions quench among them to form oxalate, which is fully consistent with the proposed reaction mechanism and the roles of the Ru complex and the POM crystal as catalyst and co-catalyst, respectively. The peak at 160 ppm in the 13 C NMR spectrum of the overnight irradiated ruthenium solution can indeed be attributed to oxalate. However, formic acid shift being close to 160 ppm in the same conditions, we performed a DEPT 135 NMR experiment in order to confirm the nature of the product. The disappearance of the peak in DEPT 135 assesses the absence of H bond to the carbon, excluding the presence of formate.

Finally, we attempted to propose a plausible mechanism to explain the subsequent reduction of CO to CH4. Very recently, we have characterized the TEOA • species that origins from the oxidation of TEOA as a hydride-donor that is capable of hydrogenating a Lewis-acid activated CO2 to formic acid (see Chapter 3). Although we have not studied this process in detail, a preliminary analysis of the reaction profile suggests that CO could be reduced to formaldehyde, then to methanol and finally to methane on Mn(II) sites via sequential hydride-transfer steps from the allylic carbon of TEOA • to the carbon atom of the substrate. According to this, homogeneous [Ru(bpy)3] 2+ would not be strictly required to reduce any of these intermediates to methane and indeed, additional catalytic experiments showed that in aqueous solutions with TEOA, Ru(bpy)-Mn is capable of reducing methanol to methane by itself ( Although these facts are consistent with TEOA • being related to the CO to CH4 reduction, further experimental and theoretical analyses are required to fully understand this process.

However, this is beyond the scope of this work aimed at reporting and understanding how these new photo-systems activate and reduce CO2.

Finally, concerning our two new structures not reducing CO2, the absence of activity for Ru(bpy)-Co despite previous reports 10,11 showing that Co bridging cation helps the CO2 photoreduction, supports the importance of the crystalline assembly. This importance can be confirmed with Ni(cyclam)-Mn: looking at the structure carefully we can notice that Mn(II) ions of Ni(cyclam)-Mn are scarcely accessible to an incoming CO2 molecule and thus a mechanism similar to that observed for Ru(bpy)-Mn is not possible.

As for [Ni(cyclam)] 2+ , it does not play the role of co-catalyst as the POM-based composite cannot reduce CO2. However, it must be recalled that the activity of this complex towards CO2 reduction depends strongly on the nature of the electron donor as well as of the solvent. 43

Conclusion

In conclusion, four novel structures containing the P4Mo6 anionic building unit connected by first-row transition metal ions were successfully isolated by hydrothermal synthesis. Their Overall, this study further evinces that heterometallic molybdenum(V) phosphates can act as stable heterogeneous catalysts for the photocatalytic reduction of CO2 into the high-valued CH4 hydrocarbon with high selectivity. Moreover, it provides insights into the complex mechanism governing the photo-reduction of CO2 at an atomic level, which might serve for designing a next generation of more efficient catalysts.

Experimental section

[Ru(bpy)3]Cl2•6H2O 44 was synthesized according to reported procedures. All other reagents were purchased from commercial sources and used as received. Molybdenum powder 1-5 µm with purity >99.9% was used for the synthesis. Hydrothermal syntheses were carried out in 23 mL polytetrafluoroethylene lined stainless steel containers under autogenous pressure. All reactants were briefly stirred before heating. The mixture was heated to 180 °C over a period of 1 h, kept at 180 °C for 70 h and cooled down to room temperature over a period of 20 h. The product was isolated by filtration after sonication and washed with ethanol. Other attempts in P4Mo6 based POM synthesis. Powder X-ray diffraction data were obtained on a Brüker D5000 diffractometer using Cu radiation (1.54059 Å). EDX measurements were performed on a JEOL JSM 5800LV apparatus.

Thermogravimetry analyses (TGA) were performed on a Mettler Toledo TGA/DSC 1, STARe System apparatus under oxygen flow (50 mL•min -1 ) at a heating rate of 5°C min -1 up to 700°C. We sought to understand why despite being structurally similar, the Ru(bpy)-Mn (active) and Ru(bpy)-Co (inactive) lead to remarkably distinct catalytic activities. Experimental UV-Vis spectra (Figure 4.10) already anticipated that the photochemical properties of both materials are rather alike. Thus, the nature of the POM-material should not alter the ability of the Ru complex to reduce CO2 to CO2 •-. This strongly support that the peripheral Mn(II) centers in the Ru(bpy)-Mn crystal participate explicitly in the reaction mechanism, in line with the mechanism proposed in Figure 4.17 In a second part, we worked on the photocatalytic properties of the (PW12, Rh@MOF-UiO-67)

Appendix

composite. The immobilization of the Cp*Rh complex, rhodium-based molecular catalyst, in the UiO-67 MOF was reported by a former postdoctoral researcher of our group. It was shown that upon immobilization in UiO-67, the Cp*Rh complex could not only keep its activity toward CO2 reduction but also increase the stability of the heterogenized catalyst, being recyclable up to five times without significant activity loss. Capitalizing on these results and knowing the properties of PW12 Keggin polyoxometalate as "electron reservoir", one sub-project of my PhD consisted in studying the impact of the co-immobilization of the PW12 Keggin POM and the Cp*Rh complex in the UiO-67 host. The first step of this project was to synthesize both Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 with a similar rhodium catalyst loading, in order to have the proper POM-free reference composite for further catalytic studies. Due to the POM presence in the MOF, we had to investigate different Cp*Rh concentrations to find a matched loading with the POM-free composite. The second step was to compare the photocatalytic activity toward CO2 reduction of Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 composites in the same conditions. This study showed interesting results: when co-immobilized with the Keggin POM, the Cp*Rh catalyst was two to three times more effective, achieving higher TONs and TOFs. A mechanistic pathway based on DFT calculations was proposed by Dr. Albert Solé-Daura, pointing that the reduced form of the Keggin, POM(2e -) can act as an electron relay and is the reason for the catalytic increase in the (PW12,Cp*Rh)@UiO-67 system. Concerning this project, we can point out two limits: first, the use of an external photosensitizer [Ru(bpy)3]Cl2, and second, the presence of noble metal in the catalyst. We can thus propose few perspectives to enhance the attractiveness of this system:

-As seen in the state-of-the-art chapter, a wide range of transition metal pyridyl complexes can be immobilized in UiO-67 via post-synthetic exchange. We could then avoid the use of noble metal in the catalyst by using catalysts such as Mn(bpy)(CO)3Br

or Cu(quaterpyridine) both reported to actively reduce CO2 to CO.

-The mechanism suggests that the presence of a reduced PW12, acting as an electron relay, is responsible for the enhancement of the catalytic properties of the (PW12,Cp*Rh)@UiO-67 when compared to the POM-free one. Under photocatalytic conditions, the POM can be reduced with up to two electrons. However, it is still unclear which of the one or the two-electron reduced POM assists the catalysis via photoinduced or thermal/photoinduced electron transfer to the catalyst, respectively. In this regard, photoelectrochemical experiments could be planned to clarify the nature of the mechanism.

-Finally, other types of POM could be investigated. As described in Chapter two perspectives section, varying the redox and acidic properties of the POM could indeed help us to experimentally support the proposed mechanism, and possibly enhance, furthermore, the catalytic activity of such composites. First POM to investigate would be PW10Ti2 as it has been described having better electron relay properties than PW12, we could thus expect a greater increase in the catalytic CO2 photoreduction.

The last sub-project I carried during my PhD thesis was the investigation of the effect of porphyrin metalation in MOF-545 on its CO2RR photocatalytic activity. MOF-545 was selected as an interesting platform for performing CO2 photocatalytic reduction in heterogeneous conditions: porphyrins are known to act as photosensitizers as well as catalysts for CO2 reduction, and MOF-545 is highly porous and may thus be easily loaded with guest molecular (co)catalysts. MOF-545 can thus be considered as an interesting three-in-one platform combining its porosity, its ability to harvest light and to catalytically reduce CO2. We first investigated how the porphyrin metalated with a transition metal might modify the MOF catalytic activity towards CO2RR. We found that MOF-545(Fe) was the one leading to the highest formate production with a 4-fold increase in activity when compared to that of the freebase MOF. A second strategy was investigated to further enhance MOF-545 catalytic efficiency, whereby the synthesis of nanoparticles was attempted. Indeed, as MOF are used in suspension, the smaller the particles, the higher the active surface area. We developed a microwave synthesis of NanoMOF-545(Fe), which to our knowledge had never been described so far. Working with those nanoparticles could increase by another two-fold the formate production, making an overall increase of 8-fold. Importantly, thanks to DFT calculations, the non-innocent role of TEOA in the CO2 photoreduction mechanism was revealed: that the photooxidized form of TEOA is indeed responsible for the formation of the formate product with the assistance of a Zr(IV) site acting as a Lewis acid to release formate. This mechanistic proposal was further validated with additional photocatalytic experiments and allowed rationalizing the experimentally observed reactivity trends on the basis of the ability of the material to promote the initial photo-oxidation of TEOA.

In a second part, we immobilized different molecular catalysts in MOF-545 (Mn(bpy), Ni(cyclam), tetrakis(pentafluorophenyl)porphyrin) (noted F20). For Mn(bpy)@MOF-545(Zn)

and Ni(cyclam)@MOF-545(Zn), after only a few hours a high loading of 0.8 Mn(bpy)/Zr6 and

Figure 1 . 1 :

 11 Figure 1.1: Evolution of the global annual energy consumption. 1

Figure 1 . 2 :

 12 Figure 1.2: Global annual CO2 emission, RoW = rest of the world (left) and CO2 chemical valorization research reports (right).2

Figure 1 . 3 :

 13 Figure 1.3: Oxidative and reductive quenching mechanisms. SD: Sacrificial Donor, PS:Photosensitizer, SUB: Substrate.8

Figure 1 . 7 :

 17 Figure 1.7: Manganese pyridyl complexes used for catalytic CO2 reduction.

Figure 1 . 8 :

 18 Figure 1.8: Nickel macrocyclic complexes for CO2RR.

Figure 1 . 9 :Figure 1 . 10 :

 19110 Figure 1.9: Polyoxometalates building bricks and connectivity by their vertices (μ-O), edges (μ-O)2 or faces (μ-O)3.

Figure 1 .

 1 Figure 1.11: a) Detailed representation of [α -PW12O40] 3-; b) Keggin POM 5 isomers, red trimetallic subunits have been rotated by 60°.

  reported the synthesis of a cobalt sandwich POM [Co4(H2O)2(PW9O34)2] 10-(Co4) able to oxidize H2O into O2 with [Ru(bpy)3]Cl2 as a photosensitizer under visible light irradiation. At this time, the catalyst's stability was not studied and the longest reported time for catalysis was only 15 min. Nonetheless, the catalyst reaches impressive TOF > 1000 h -1 .Concerning reduction reactions, the same group reported proton reduction with two related POMs [Mn4(H2O)2(VW9O34)2] 10-(Mn4) and [Ni4(H2O)2(PW9O34)2] 10-(Ni4). The two system have shown an impressive stability with photoreduction of protons for a week without substantial activity loss. Concerning their activity, Mn4 has shown poor efficiency with TONs of 41 after 7 days, in contrary Ni4 could efficiently reduce H + with TONs of 6500 after the same period.[START_REF] Lv | Visible-light-driven hydrogen evolution from water using a noble-metalfree polyoxometalate catalyst[END_REF][START_REF] Lv | A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution[END_REF] Other nickel-based POMs have also been reported for the same reaction, the dimeric nickel POM [Ni3(OH)3(H2O)3P2W16O59] 9-described by Guo et al.[START_REF] Guo | Structural Characterization, and Catalytic Properties of Diand Trinickel Polyoxometalates[END_REF] and the [{Ni4(OH)3AsO4}4(B-α-PW9O34)4] 28-tetrameric one (Ni16) described by Lv et al..[START_REF] Lv | A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution[END_REF] The latter POM is able to reduce protons in a CH3CN:DMF mixture, with TEA as an electron donor and with an iridium photosensitizer, with a TON of 580 after 5h (Figure1.12).

Figure 1 .

 1 Figure 1.12: a) Ni16 POM, WO6 light blue octahedra, AsO4 orange tetrahedra, PO4 yellow tetrahedra, Ni green balls, O red balls and b) Photochemical H2 evolution at different catalyst concentrations under otherwise identical conditions: LED light (20 mW, 455 nm), [Ir(ppy)2(dtbbpy)] + (0.2 mM), TEOA (0.25 M), H2O (1.4 mM), catalyst (20 µM or 320 µM as noted), CH3CN/DMF (1:3 v/v, 2 mL) degassed with Ar. 64

  studied the grafting of a rhodium-based catalyst [Rh(Cp*)] 2+ on a [-H2PW11O39] 5-POM. They reported the electroreduction of CO2 to formate and of protons to H2. Compared to [Rh(Cp*)(bpy)Cl]Cl, the hybrid POM favored the reduction of protons.

Figure 1 .

 1 Figure 1.13: a) Hybrid POM [Re I (L)-(CO)3(MeCN)][MHPW12] 68 and b) Proposed mechanism for CO2 photoreduction with the hybrid POM. 6

Figure 1 . 14 :

 114 Figure 1.14: Schematic representation of the structures of the main MOF guests with the dimensions of their cavities, the formula of the linker and of the MOFs.

Figure 1 . 16 :

 116 Figure 1.16: Structure of the UiO-67.

Figure 1 . 18 :

 118 Figure 1.18: Schematic representation of the various synthetic pathways used for the synthesis of pore-functionalized Cat@MOFs.

  al. have described the encapsulation of the Ru catalyst ( tBu PNP)Ru(CO)HCl ( tBu PNP = 2,6bis((di-tert-butyl-phosphino)methyl)pyridine) in the pores of UiO-66 by exposing MOF crystallites to a solution of the catalyst in MeOH for 5 days (Figure 1.19)

Figure 1 . 19 :

 119 Figure 1.19: Schematic representation of the aperture-opening process used for the encapsulation of a Ru complex in the cavities of UiO-66. 115

Figure 1 . 20 :

 120 Figure 1.20: Schematic illustration for the "Ship-in-a-bottle" synthetic strategy to prepare [Ni(bpet)(H2O)2]@Ru-UiO-67. 116

[

  Co(CO)4] -complex in MIL-101(Cr) is also based on an ion-exchanged process and is performed in two steps: first the charge-balancing F -anions, which are directly coordinated to the Cr(III) sites in the as-synthesized MIL-101(Cr), are exchanged by Cl -anions (by reaction with AlCl3) then in a second step the mobile Cl -ions are exchanged by the negatively charged complex by reaction with Na[Co(CO)4].131

  UV-vis spectroscopy can thus allow monitoring the incorporation of the catalyst in the MOF during the impregnation and to estimate the amount of Cat immobilized in the MOF. For example, the decrease of the bands around 250 nm (assigned to the charge transfer from the bridging Oatoms to W-atoms) allowed to follow qualitatively the immobilization of PW9, 139 PW11, 140 SiW11, 140 PW11Ti 141 and PW11Co 141 in MIL-101(Cr). A detailed UV-vis study was also performed to measure the catalyst concentration in the supernatant after the impregnation experiment of various Ru catalysts in NH2-MIL-101(Al) and MIL-101(Cr) MOFs 142 and of a Rh catalyst and a Ru photosensitizer in NH2-MIL-101(Al)

Figure 1 . 21 :

 121 Figure 1.21: Typical TGA curve of a Cat@MOF composite: TGA curves under air of a PW11Zn@MIL-101(Cr) composite (red curve) and of the parent MIL-101(Cr) (dashed blue line). 143

  reactions. For example, EDX showed the exchange of F -anions by Cl -anions (absence of the F Kα peak, presence of the Cl Kα peak) followed by the exchange of F -by [Co(CO)4] -anions (absence of the Cl Kα peak and presence of the Co Kα and Lα peaks) in the two-step procedure leading to [Co(CO)4]@MIL-101(Cr).131

Figure 1 . 22 :

 122 Figure 1.22: Comparison of a) STEM-HAADF images of P2W18Co4@MOF-545 and EDS mapping of the various elements contained in the POM and the MOF and b)-e) SEM imagesand EDS elemental mapping.[START_REF] Paille | A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation[END_REF]

Figure 1 .

 1 Figure 1.23: a) IR spectra of composites synthesized by immobilizing Co-substituted POMs in MIL-101(Cr); the composite named PWCo@MIL results from the immobilization of the hybrid P2W18Co7(Ale)2 POM. b) Raman spectra of SiW11Fe, MIL-101(Cr) and SiW11Fe@MIL-101(Cr). 112,147

  P MAS NMR spectrum exhibited three resonances at -15.1, -16.2, and -17.6 ppm, much broader in comparison with those for the pure POM. The existence of these three signals was attributed as either different protonation states of PW12 or POM distortion resulting from strong interactions with the host framework. 138Solid-state NMR has also been used to characterize Complex@MOFs. For example,13 C CP-MAS NMR of CuPhen@MIL-100(Al) presents signals assigned to the aromatic carbons and of the carboxylate groups of the MOF linkers as well as of the Phen ligand. Only one broad peak was observed at around 15 ppm in the 31 P{ 1 H} MAS NMR spectrum of [Ir(cod)(PCy3)(py)]@NaSO3-MIL-101(Cr) and was assigned to PCy3 ligands. The absence of peaks characteristic of PF6 -anions suggests a complete cation exchange.

  1 and the Keggin type POM PW12 in the UiO-67 MOF used as an heterogenization scaffold. Both the immobilization of Cp*Rh and PW12, in two separated studies, have been previously achieved by previous fellows from the laboratories. Cp*Rh once in the UiO-67 is still able to reduce CO2 in formate and H2, while PW12 can act as an electron and/or proton relay. The co-immobilization of the two moieties in the UiO-67 framework could show synergetic effects and enhance the overall composite efficiency for CO2 photoreduction. This work has been conducted in close collaboration with Mathis Duguet, Alex Lemarchand, and Mohamed Haouas, student, postdoctoral fellow and researcher respectively, at the time of the study. Mathis Duguet conducted DFT calculations to find where the POM could be positioned inside the MOF pores while Alex Lemarchand focused on the pair distribution

  Rhodium bipyridine complexes were first used as catalysts for CO2 electroreduction byBolinger et al. in 1985 2 and 19883 . They have based their research on the cis-[Rh(bpy)2X2] + complexes (X = Cl -or trifluoromethanesulphonate OTf -). The catalysts were investigated by cyclic voltammetry which revealed an enhanced cathodic current density in presence of CO2, attributed to catalytic CO2 reduction. These complexes were further studied by bulk electrolysis at -1.93 V vs. Fc + /Fc in MeCN, and afforded the production of mixtures of H2 and formate. The faradaic yields varied depending on the total charge passed, starting close to 100% and decreasing as more coulombs were passed. For example, the faradaic yield for formate dropped from 83% to 52% and 36% as the charge passed increased from 77 to 111 and about 150 C. By contrast, the faradaic yield for H2 rose slightly, from 10% to 20% as the current increased. This change of selectivity is assigned by the authors to the irreversible reduction of Rh and bpy ligand.Meanwhile, Kölle and Grützel published in 19874 the synthesis of a new Rh-bipyridine based catalyst, namely [Cp*Rh(ppy)L] n+ (with n = 1 or 2 depending on L charge, L = H2O, OH -, Cl - and I -and ppy = 2,2'-bipyridine, 2,2'-bipyridine 4,4'-dicarboxylic acid, 4,4'diacetyl-2,2'bipyridine or 1,10-phenanthroline) (Figure 2.1). The catalysts were built by the reaction of a solution of {Cp*RhCl(µ-Cl)}2 dimers and an oligopyridine ligand in methanol at ambient temperature for 30 min. The different catalysts were investigated for proton photoreduction and represent the first report of these rhodium bipyridine systems for photochemical catalysis. At pH < 7, irradiation of 5 µmol of [Cp*Rh(bpy)L] n+ in 20 mL of a solution containing 25 mg of TiO2 leads after 3h to the formation of 0.5 mL of H2 which corresponds to a TON of 4.5.

Figure 2 . 1 :

 21 Figure 2.1: Scheme of [Cp*Rh(2,2'-bipyridine)Cl]Cl

  Brief reminder, UiO-67 elemental formula is Zr6O4(OH)4(C14O4H8)6 and is constituted of Zr oxo-clusters linked by 4,4'biphenyldicarboxylate (BPDC) linkers. UiO-67 is composed of octahedral and tetrahedral cages of 18 Å and 11.5 Å internal diameters, respectively (Figure 2.2 a). Its zirconium cluster is fully saturated with a coordination of 12. The pores are big enough to host a large variety of molecules. 8

Figure 2 . 2 :

 22 Figure 2.2: a) Representation of the UiO-67 framework and b) the Cp*Rh@UiO-67. Zr6O4(OH)4 clusters are represented in orange, C in grey, Rh in cyan, N in deep blue and Cl in green.

  photocatalytic reduction of CO2 was conducted in ACN:TEOA (5:1) with 1 mM of [Ru(bpy)3]Cl2 as an external photosensitizer. The authors reported for the homogeneous catalyst a good activity during the CO2 photocatalytic reduction process with a selective production of formate (TON = 125 for Cp*Rh(bpy) and 42 for Cp*Rh(bpydc)). The significant decrease with Cp*Rh(bpydc) is attributed by the authors to the electron-withdrawing nature of COOH which decreases the efficiency for CO2 reduction. No other carbon products were detected and H2 was the only other product with TON = 55 and 38 for Cp*Rh(bpy) and Cp*Rh(bpydc), giving a similar selectivity than in electrochemical conditions.

Figure 2 . 3 :

 23 Figure 2.3: a) Scheme of the Cp*Rh@UiO-67 composite preparation : Schematic PSE of biphenyl dicarboxylic acid by the Cp*Rh(bpydc) catalyst and b) formate production over time by the homogeneous Cp*Rh(bpydc) (blue) and heterogenous Cp*Rh@UiO-67 (red) catalysts. 1

. 3 b

 3 ), with TON = 47 vs 42 for formate and TON = 36 vs 38 for H2. The recyclability experiments were taken with 4 mg of 10%-Cp*Rh@UiO-67 irradiated for six cycles of 16h. Between each cycle, the cat@MOF was collected by centrifugation and the formate was assessed by ionic chromatography, showing a retained activity even after six cycles. Lastly, different materials %Cp*Rh@UiO-67, differing by the amount of catalyst introduced in the MOF, have been tested (Table2.1), the authors showing that the lesser Cp*Rh in the MOF, the better the activity, whereby the 5%-Cp*Rh@UiO-67 was found as the most active compound. Other examples of immobilized Cp*Rh in MOF have been later reported where, for example Cp*Rh has been coimmobilized with [Ru(bpy)2(bpydc)] 2+ in the MIL-101-NH2(Al) MOF giving a photosensitive and photocatalytic composite.11,12

  highlighted the mechanism for CO2 reduction with [Cp*Rh(bpy-R)Cl]Cl derivatives. Combining both experimental and theoretical results, they investigated the effect of the substituent of bipyridine (Figure 2.4) on the selectivity toward HCOOH and the overall efficiency. They focus the DFT calculations on the C3 and C8 derivatives (see Figure 2.4) corresponding to an electron-donating and -withdrawing group, respectively. In both cases, the first step is a two-electron reduction of the Rh(III) to Rh(I) accompanied by the loss of the Cl -counterion. This step is followed by the protonation of the Rh then a first proton transfer from the Rh to the Cp* ring, which is strongly facilitated by the presence of electronwithdrawing groups, leading to an equilibrium between [RhH] and [Cp*H], with [Cp*H] being the more stable by 2 to 3 kcal•mol -1 . CO2 reduction happens by a direct hydride transfer from [RhH] to CO2 to form the [Rh•••HCO2 -] adduct, a protonation then leads to the formation of HCOOH. A hydride transfer from the [Cp*H] to CO2 is less likely to occur as the transition state is much higher in energy. The nature of the substituents plays a huge role on the selectivity as electron-donating ligands can facilitate the hydride transfer to formate by lowering the energy barrier thus increasing the HCOOH:H2 ratio (Figure 2.4).

Figure 2 . 4 :

 24 Figure 2.4: Structures of [Cp*Rh(bpy-R)Cl]Cl complexes (left) and correlation between theHammett constant and the TOF(HCOOH)/TOF(H2) of the complexes (right).13

  .5 a). The addition of the rhenium complex ([Re-cat]) and the POM in acetonitrile leads to the formation of the hybrid catalyst through salt formation between the basic nitrogen of the pyridine ring and the acidic protons of the POM. This catalyst has then been reduced twice electrochemically in dimethyl acetamide (DMA) at -1.302 V vs. Fc/Fc + leading to the formation of [Re-cat]-H5PW V 2W VI 10O40. The feasibility of the electron transfer from the reduced PW V 2W VI 10 POM to the rhenium complex was demonstrated by UV-Vis experiment (Figure 2.5 b). The reduced POM (leading to a blue solution) was reacted with the rhenium catalyst showing a decrease of the large band between 600 and 900 nm characteristic of POM-reduced species. After 3h irradiation no more absorbance in this range was observed, suggesting an electron transfer from the reduced POM to the rhenium complex. Lastly, the authors have conducted photo-electro catalytic experiments where the POM was reduced at Eapp = -1.30 V vs. Ag/Ag + , the reduced PW V 2W VI 10 POM thus transferring two electrons to the rhenium catalyst, allowing the further reduction of CO2 in carbon monoxide. The presence of the POM as an electron relay thus led to a higher activity of [Re-cat].

Figure 2 . 5 :

 25 Figure 2.5: a) Strategy for the synthesis of the dirhenium complex; b) UV-Vis spectra following the reduction of H3PW12O40 and its re-oxidation induced by visible light in the presence of [Re-cat]. 17

  has been used to synthesize the POM-based MOF NENU-10. NENU-10 being non-catalytic, gold nanoparticles (AuNP) were co-immobilized in the POM@MOF. The study compares different systems, HKUST-1 being the POM-free MOF, NENU-3 containing the PW12 Keggin, NENU-10 containing PW10Ti2, and the gold nanoparticles loaded MOF Au@NENU-3 and Au@NENU-10 (Figure 2.6).

Figure 2 . 6 :

 26 Figure 2.6: Reaction between POM, Cu 2+ and BTC to form NENU-3 and NENU-10 MOF.HKUST-1 is the equivalent MOF without the POM inside the pores.18

Figure 2 . 7 :

 27 Figure 2.7: a) Production yields of CO, H2 and CH4 from CO2 b) recycling tests ofAu@NENU-10.18

  Cp*Rh has shown great catalytic properties for CO2 reduction to both formate and H2, by electroreduction at first, and more recently by photochemical ways where the catalyst has been immobilized in the UiO-67 MOF and kept its activity. Moreover, reports show that Keggin POM can be used as an electron and proton relay to increase the CO2 reduction activity of different kinds of catalysts including a rhenium bipyridine based catalyst, which is close to rhodium bipyridine catalysts in terms of activity. Lastly, Salomon et al. succeeded in incorporating PW12 inside the same MOF where the Cp*Rh catalyst has been immobilized.

First, the encapsulation

  of the POMs in the octahedral cavities of UiO-67 was achieved by in situ synthesis in DMF at 120°C. The resulting PW12@UiO-67 composite has been washed several times with heated DMF and acetone to dissolve all unreacted compounds (c.a. benzoic acid; biphenyl dicarboxylic acid) before being reacted with the molecular complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2'-bipyridine-5,5′-dicarboxylic acid). UiO-67 being unstable to water in oxidative conditions, in order to avoid the MOF degradation, two sealed vials were degassed with argon for 20 min, one containing the MOF and the Cp*Rh linker, the other one filled with water. Degassed water was then added to the powder containing flask. The reaction media was stirred at RT for 24h.

Figure 2 . 8 :

 28 Figure 2.8: (a) Schematic representation of the two-step in situ synthesis and linker's postsynthetic exchange (PSE) procedure to obtain the composite material (PW12,Cp*Rh)@UiO-67.

Figure 2 . 9 :

 29 Figure 2.9: (PW12,Cp*Rh)@UiO-67 components: the PW12 polyoxometalate, the Zr-based inorganic subunit of UiO-67, the [Cp*Rh(bpydc)Cl] + molecular catalyst and the bpdc linkers. WO6, blue polyhedra; ZrO8, orange polyhedra; PO4, pink polyhedron; Zr, orange spheres; O, red spheres; C, gray or black spheres; N, blue spheres; Rh, dark green spheres; Cl, green spheres.

Figure 2 . 11 :

 211 Figure 2.11: Infrared spectra of the different precursors and MOFs composites.

Figure 2 . 12 :

 212 Figure 2.12: Raman spectra of POM, (PW12,Cp*Rh)@MOF, Cp*Rh@MOF and PW12 and Cp*Rh solids measured at 532 nm excitation wavelength.

Figure 2 . 13 :

 213 Figure 2.13: SEM image of (PW12,Cp*Rh)@UiO-67 (a) and EDS mapping of Rh L1 (b), W M1 (c) and Zr L1 (d).

Figure 2 .

 2 Figure 2.14: a) N2 adsorption/desorption isotherms of UiO-67 and the PW12@UiO-67, and (PW12,Cp*Rh)@UiO-67 composites (77K, P/P0 = 1 atm.). Inset shows the pore distributions.b) Plot of the (Ah) 2 versus photon energy (h) of (PW12,Cp*Rh)@UiO-67 (red), Cp*Rh@UiO-67 (blue), PW12@UiO-67 (green) and UiO-67 (black).

Figure 2 .Figure 2 .

 22 Figure 2.15: a) Solid state1 H MAS NMR spectra of PW12@UiO-67, Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 composites compared to that of the Cp*Rh complex precursor and UiO-67. b) Solid state13 C{ 1 H} CPMAS NMR spectra of PW12@UiO-67, Cp*Rh@UiO-67 and (PW12,Cp*Rh)@UiO-67 composites compared to that of the Cp*Rh complex precursor and UiO-67.

  P peaks are observed in solid-state NMR suggesting two environments for the PW12 moiety. Further evidence of two environments was provided by 2D NMR 31 P-1 H HETCOR experiments (Figure 2.16). The spectrum shows that the 31 P signal at -15.2 ppm correlates preferably with the 1 H signal of aromatic protons at 8.3 ppm, while the second 31 P signal at -14.4 ppm correlates with the Cp* moiety peak at 1.6 ppm. This clearly indicates that two POM environments co-exist within the (PW12,Cp*Rh)@UiO-67 material, i.e. one near the Cp* moieties (-14.4 ppm) and another closer to the bipyridine organic linker (-15.2 ppm). Quantitatively, they are distributed almost equally (47:53 for -14.4 and -15.2 ppm respectively).

Figure 2 . 16 :

 216 Figure 2.16: 31 P{1H} CPMAS NMR, 1 H MAS NMR and 31 P{ 1 H} HETCOR spectra of (PW12,Cp*Rh)@UiO-67 (left), proposed relative position of PW12 and Cp*Rh considering the different 31 P{ 1 H} HECTOR signals (right).

  In a first step, simulated annealing (SA) and dispersion-corrected DFT-D3 geometry optimizations were performed by Mathis Duguet to identify the most likely position of the POM in UiO-67 and qualify its host-guest interactions with the hybrid framework. SA simulations provided recurrent successful insertion of PW12 in the octahedral cage of UiO-67. Figures 2.17

a) and A2. 2

 2 illustrate its position and interactions in the solid. The POM is located at the center of the cavity and interacts equally with the 12 bpdc linkers of the octahedral cage. It is stabilized through a network of (PW12)O … H(bpdc) van der Waals interactions between its terminal oxygen atoms and two aromatic hydrogen atoms of each bpdc linker within 2.4-2.7 Å distances.Overall, the host-guest interaction energy, which emanates from electrostatic interactions including multiple van der Waals interactions, is estimated from DFT calculations around 200 kcal•mol -1 . This high affinity of the MOF for the POM results from the confinement of the POM in the rather small size pores of UiO-67.Mathis Duguet then investigated the positioning of the Rh catalytic complex in the PW12@UiO-67 solid considering the replacement of 1 bpdc linker per octahedral cage by one Cp*Rh(bpydc) catalytic linker (Figures 2.17

  and A2.2). Two situations were distinguished whereby the Cp*Rh catalytic moiety is oriented either towards an empty, i.e. POM-free, neighboring cage (Figure2.17 b), position "out") or towards the same cage as the PW12 guest (Figure2.17 c), position "in"). The DFT-D3 level geometry optimization of the "out" position shows a similar {POM, MOF} interaction energy than in the catalyst-free MOF. By contrast the comparison of the "out" and "in" models shows that interactions between the POM and the Cp*Rh component of the catalytic linker are favored by ~46 kJ•mol -1 when they are located within the same cage.

Figure 2 . 17 :

 217 Figure 2.17: Computed PW12@UiO-67 and (PW12,Cp*Rh)@UiO-67 models. (a) PW12 in PW12@UiO-67 and (b)-(c) in (PW12,Cp*Rh)@UiO-67 as obtained from SA and DFT-D3 level geometry optimizations. The POM and the Rh-catalytic center are modeled within different cages ("out" position, b) or within the same cage ("in" position, c). Interaction energies between PW12 and the MOF host are given for each model. WO6, blue polyhedra; ZrO8, orange polyhedra; Rh, dark green sphere; O, red spheres; C, gray spheres; N, dark blue spheres; Cl, green spheres. H atoms are omitted.

  .18 a). Knowing the respective scattering factors of the POM and MOF atoms (fW >> fZr >> fP > fO > fC), these narrow and intense peaks clearly emanate from the heavy W scatterers and indicate their short-range structural organization in line with the presence of the encapsulated POM. Importantly, the two experimental PDF profiles perfectly overlap at long-range order (15.0 to 40.0 Å), confirming the similar high crystallinity of the UiO-67's framework upon the encapsulation of PW12 by in situ synthesis, as already observed from their PXRD patterns (Figure 2.10). This long-range feature allowed the calculation of the differential PDF in optimal conditions. The resulting d-PDF (Figure 2.18 a) is dominated by intense and narrow peaks in the low r region with a strong dampening above 10.0 Å.

Figure 2 . 18 :

 218 Figure 2.18: (a) Experimental PDF data for PW12@UiO-67 (black) and the guest-free UiO-67 (green) and the corresponding d-PDF for PW12 encapsulated in UiO-67 (blue) determined by subtracting the PDF of UiO-67 from the PDF of PW12@UiO-67. (b) Comparison of the calculated PDF of an isolated PW12 (black) and the experimental d-PDF of PW12 in UiO-67 (blue dotted lines), superimposed with the refined d-PDF (red) fitted using the [PW12O40] 3- structural model from CIF files (JCPDS 00-050-0304)36 and residual profile (green). A-H labels of peaks correspond to the indicated refined distances in the POM components as illustrated in (c) for the PO4 tetrahedron, the WO6 octahedron, the trimer of WO6 octahedra, and within the full PW12 polyoxometalate structure. WO6, blue octahedra, PO4, pink tetrahedron, O, red spheres, W, gray spheres, P pink sphere. (d) Experimental d-PDFs (numbered 1 to 3) for PW12 in PW12@UiO-67 (blue), in (PW12,Cp*Rh)@UiO-67 before catalysis (magenta) and in (PW12,Cp*Rh)@UiO-67 after catalysis (navy blue) determined by subtracting the PDF of UiO-67 from the PDF of PW12@UiO-67, (PW12,Cp*Rh)@UiO-67 before and after catalysis, respectively.

  over the 1-10 Å r-range (Figure2.18 b). The two profiles match remarkably well whereby all peaks of the experimental d-PDF are equally well described (shape and position) in the calculated PDF of PW12, allowing an exhaustive assignment of its characteristic interatomic distances. The first short distance peak at 1.5 Å (A) corresponds to the P-O bond length of the central PO4 tetrahedron of the POM. The second and third peaks (B, C) at 2.0 and 2.4 Å, respectively, correspond to the length of various W-O bonds constituting the WO6 octahedra.The intense peaks (D-G) at 3.5, 3.7, 5.0, 6.2 and 7.1 Å, are assigned to various W-W distances inside the PW12 structure. In addition, a very good agreement is found over the 1-8 Å r-range between the observed d-PDF and the theoretical one calculated from the DFT models of UiO-67 and PW12@UiO-67 (FigureA2.4 a). The disagreement between the experimental and theoretical d-PDF profiles at long range (> 8 Å) emanates from the absence of POM's ordering in the PW12@UiO-67 composite, by contrast with the virtual ordering induced by the DFT periodic models.Overall, the structural integrity of the PW12 upon encapsulation in UiO-67 is thus supported by PDF analysis. Nonetheless, it can be pointed out that the lacunary polyoxotungstate PW11 POM would display the same W-O lengths as in PW12. Solid-state NMR is thus necessary to unambiguously confirm the stability of the POM. Alex Lemarchand further attempted a refinement of the POM's atomic positions whereby it is constrained to the space group Pn-3m (Figure2.18 b), TableA2.1). The relatively good quality of the refinement (Rw~28.2 %) and the corresponding structural model (Figure2.18 c) indicate that the original structure of the POM is indeed fully preserved upon its immobilization in UiO-67. The residual difference between experimental and calculated d-PDFs might be assigned to errors induced by the PDF subtraction step at low r-values, and/or potential positional atomic disorder and symmetry loss within the POM structure induced by the proximity of UiO-67's ligands and nodes.In a second step, the d-PDF related to the (PW12,Cp*Rh)@UiO-67 catalyst was investigated using UiO-67 as a reference. The PDF of the (PW12,Cp*Rh)@UiO-67 exhibits all expected features of UiO-67 showing that the linker exchange did not induce major changes in the structure or important loss of crystallinity. The perfect overlap of (PW12,Cp*Rh)@UiO-67's and UiO-67's PDF profiles at large r-values (15-40 Å) allowed us to calculate the related d-PDF (Figure2.19) reflecting the POM's structure. Consistently, this d-PDF is very similar to the d-PDF obtained for the POM in PW12@UiO-67, i.e., prior to the linker exchange (Figure 2.18 d), confirming that the post-synthetic linker exchange has no impact on the structural integrity of PW12. It is worth mentioning here that further attempts to identify the contribution of the catalytic Cp*Rh ligand in the d-PDFs were unsuccessful due to the low amount of Rh in the sample and to the coincidence between Rh-Cl distances ( 2.4 Å) and W-O distances characteristic of the POM.

Figure 2 . 19 :

 219 Figure2.19: Experimental PDF data for (PW12,Cp*Rh)@UiO-67 before catalysis (black) and the POM-free UiO-67 (red). Bottom: the resulting d-PDF for PW12 in (PW12,Cp*Rh)@UiO-67 before catalysis (magenta) determined by subtracting the PDF of UiO-67 from the PDF of (PW12,Cp*Rh)@UiO-67 before catalysis.

Figure 2 . 20 :

 220 Figure 2.20: Kinetics of (a) H2 and (b) HCOO -production during CO2 photoreduction reaction by (PW12,Cp*Rh)@UiO-67 compared to Cp*Rh@UiO-67. Reactions conditions: 0.17 µmol of catalyst, 0.8 mL of CH3CN/TEOA 5:1, 1 mM Ru(bpy)3Cl2, 280 W,  > 415 nm.

Figure 2 .

 2 Figure 2.21: a) Photoreduction of CO2 overtime by a (PW12,Cp*Rh)@UiO-67 composite. A fresh solution of Ru(bpy)3Cl2 photosensitizer was added to the reaction after 120 min b) Formate (green) and H2 (orange) TONs during a CO2 photoreduction reaction by (PW12,Cp*Rh)@UiO-67 and Cp*Rh@UiO-67 composites after 180 min illumination.

Figure 2 . 22 :

 222 Figure 2.22: Recyclability of (PW12,Cp*Rh)@UiO-67 thin films for photocatalytic CO2 reduction. Normalized formate TONs (a) after 30 min illumination, (b) after 180 min (CH3CN/TEOA 5:1, Ru(bpy)3Cl2 1 mM, 280 W, > 415 nm). The TON after 180 min is equal to 175 for the first run for (PW12,Cp*Rh)@UiO-67 thin films and 126 for (Cp*Rh)@UiO-67 thin films.

Figure 2 . 23 :

 223 Figure 2.23: Schematic representation of the P2W18Co4@MOF-545 photosystem under various setups upon light illumination: (a) in suspension, light capture by the crystallites is notoptimal (random orientation of crystallites, light diffusion); (b) as a thick layer on ITO obtained using electrophoresis (EP); the absorption of light is limited to the fraction of the crystallites in contact with ITO; (c) deposited on ITO by drop casting; DC allows depositing small amounts of POM@MOF dispersed on ITO so that all crystallites may be illuminated.37

Figure 2 .

 2 Figure 2.24: a) SEM image of (PW12,Cp*Rh)@UiO-67, b), c), d) EDS mapping of Rh, W and Zr, respectively, before catalysis and e) SEM image, f), g), h) EDS mapping of Rh, W and Zr, respectively, after catalysis.

Figure 2 . 25 :

 225 Figure 2.25: Experimental PDF data for (PW12,Cp*Rh)@UiO-67 after catalysis (black) and the POM-free UiO-67 (red). Bottom: the resulting d-PDF for PW12 encapsulated in (PW12,Cp*Rh)@UiO-67) after catalysis.

Figure 2 . 26 ,

 226 in presence of HTEOA + a proton exchange can occur and form the Cp*Rh(III)H intermediate. From this species, we can envisage two possible processes to occur: the first one (in black) where no POM is near the Cp*Rh, and the second one (in blue) with a doubly-reduced PW12(2e -) immobilized near the catalyst. i) In the absence of POM (black pathway in Figure2.26), an intramolecular hydrogen transfer from the Rh center to the Cp* ligand is expected to be faster than the direct bpy ligand reduction by the electron donor (leading to Cp*Rh(III)H(bpy -)) to form the active species of the catalyst.

Figure 2 . 26 :

 226 Figure 2.26: Proposed mechanism for CO2 photoreduction by Cp*Rh@UiO-67 (in black and red) and (PW12,Cp*Rh)@UiO-67 (in blue). Relative Gibbs free energies are given in kcal.mol -1 .

Figure 2 .

 2 26 and going directly from the Cp*Rh(III)H(bpy) to the active Cp*Rh(III)H(bpy -) at a lower activation energy cost. The new limiting step (arrows in blue) is then either the CO2 transformation into HCOO -or the H2 formation with G = 13.6 kcal•mol -1 for the CO2 reduction and G = 13.9 kcal•mol -1 for proton reduction. This change in the nature of the limiting step and the significant reduction of the overall free-energy barrier by ca. 12 kcal•mol -1 can qualitatively explain the experimentally observed enhancement of the catalytic activity induced by the presence of the POM. Moreover, the fact that the POM acts as an electron relay suggests that any POM with suitable energy levels may be approachable to increase the activity of the catalyst without significantly altering its reactivity or selectivity (Figure2.27).

Figure 2 . 27 :

 227 Figure 2.27: HOMO and LUMO levels of reduced PW12(1e), two time reduced PW12(2e) and Rh(III)(H)(Cp*).

Figure 2 . 29 :

 229 Figure 2.29: PXRD patterns of UiO-67 and the PW10Ti2@UiO-67 composites, in blue after the EtOH washing, and in green with increase amount (HL stands for High Loading) of PW10Ti2 during the synthesis (left), and IR spectra of UiO-67, PW10Ti2 and PW10Ti2@UiO-67 (right).

  Cp*Rh)@POP Lastly, we have worked in collaboration with the group of Dr. Jérôme Canivet in IRCELyon to co-immobilize the Cp*Rh and PW12 in another kind of porous material. The objective was to see if the impact of the PW12 POM observed in the UiO-67 on CO2 photocatalytic reduction could be reproduced on another similar porous however non-crystalline polymer. Dr. Ashta Chandra Ghosh thus worked on the immobilization of the POM in a bipyridine/viologen based porous organic polymer (POP) as a first step before adding the Cp*Rh catalyst by post synthetic modification methods (Figure 2.30).

Figure 2 . 30 :

 230 Figure 2.30: Synthesis steps of (PW12,Cp*Rh)@66%VioBpyMP-1

Figure 2 . 31 :

 231 Figure 2.31: Production yield of H2 with RhCp*@66%VioBpyMP-1 and(PW12,RhCp*)@66%VioBpyMP-1 (left) and recyclability tests with (PW12,RhCp*)@66%VioBpyMP-1 as a catalyst (right).

1 A 50 mL

 150 reaction flask was charged with 0.506 g (0.820 mmol, 1 equivalent) of [Cp*RhCl2]2 and 0.400g (1.64 mmol, 2 equivalent) of 2,2'-bipyridine-5,5′-dicarboxylic acid. 25 mL of MeOH was added to the reaction flask and the mixture was allowed to stir at room temperature. After one hour, the homogeneous deep orange solution was concentrated under vacuum. Excess diethyl ether was added to the reaction flask to yield a yellow precipitate and a colorless solution. The yellow precipitate was collected on a Buchner funnel and dried under vacuum for 2 hours at which point the dried powder was transferred to a vial.Synthesis of UiO-67.UiO-67 was synthesized as described by Salomon et al.. 14 ZrCl4 (116 mg, 0.5 mmol), biphenyldicarboxylic acid (121 mg, 0.5 mmol) and benzoic acid (1.83 g, 15 mmol, 30 equivalents) were dissolved in 10 mL of dimethylformamide (DMF) in a 23 mL polytetrafluoroethylene-lined stainless steel container. Hydrochloric acid 37% (83 μL) was then added to the solution. All reactants were stirred briefly before heating. The mixture was heated to 120 °C over a period of 1 h, kept at 120 °C for 24 h, and allowed to cool down to room temperature. The solid was filtrated, washed with dry DMF and dry acetone and dried in an oven at 90°C overnight. The presence of chlorine has not been detected by EDX analysis indicating that the linker deficiency has been compensated by hydroxyl ligands in the formula. Final mass: 102 mg (yield 60% based on Zr).

  Metrosep A. Supp 5 column and a conductivity detector. A typical measurement requires the sampling of 200 μL of solution, followed by 100-time dilution in deionized 18 MΩ water and injection of 20 μL into the IC chromatograph. All TONs and TOFs were assessed by normalizing the production of H2 and HCOO -with the number of nmol of Cp*Rh catalyst.

Figure A2. 1 :

 1 Figure A2.1: Raman spectra of A, B) PW12, (PW12,Cp*Rh)@UiO-67 and Cp*Rh@UiO-67 and C) PW12@UiO-67, (PW12,Cp*Rh)@UiO-67 and Cp*Rh solids measured at 532 nm excitation wavelength.

Figure A2. 2 : 3 Å

 23 Figure A2.2:Computed PW12@UiO-67 and (PW12,Cp*Rh)@UiO-67 models. Computed position of PW12 in PW12@UiO-67 and (PW12,Cp*Rh)@UiO-67 as obtained from SA and DFT-D3 level geometry optimizations. The POM and the Rh-catalytic center are modeled within the same cage ("in" position) or different cages ("out" position). For describing the "out" position, one inorganic Zr-cluster was disregarded to illustrate the interactions of the POM with the Cp*Rh moiety. Interatomic distances and interaction energies between PW12 and the MOF host are given for each model. WO6, blue polyhedra; ZrO8, yellow polyhedra; Rh, dark green sphere; O, red spheres; C, H, gray; N, dark blue; Cl, green spheres.

Figure A2. 3 : 5 Figure A2. 4 :

 354 Figure A2.3: PDF analysis of UiO-67. (a) PDF data of UiO-67. A-F labels of peaks below 6.5 Å correspond to the indicated correlations within the Zr6O4(OH)4 clusters. (b) Two Zr6O4(OH)4 units connected by a bpdc linker and relevant Zr-Zr distances corresponding to the longer r features (G-K). Zr, gray spheres; O, red spheres; C, light gray spheres; H, white spheres. Presentation inspired from Bennett et al.5

  3- polyoxometalate obtained from d-PDF refinement from a PW12@UiO-67. For a comparison purpose, the structural parameters of the Keggin [PW12O40] 3-reference are reminded.

Figure A2. 5 :Chapter 3 :

 53 Figure A2.5: Production yield of formate with RhCp*@66%VioBpyMP-1 and (PW12,RhCp*)@66%VioBpyMP-

  .1 right) can act as PSs as they bear TCPP linkers.4

Figure 3 . 1 :

 31 Figure 3.1: Structure of the Zr6 oxocluster (left) and the TCPP linker (right)

  3+ ,…) leading to an even greater chemical variety of Zr6-TCPP MOFs. Importantly, previous reports have shown that MOF-525, MOF-545, PCN-223 and PCN-224 are able to reduce CO2 into different products (CO, CH4 and/or formate) without any additional co-catalyst or PS. 13 {Zr 6 O 4 (OH) 4 } Tetrakis (4-carboxyphenyl) porphyrin

Figure 3 . 2 :

 32 Figure 3.2: Topology of the different Zr6 porphyrinic MOFs.

( 26 .

 26 4 µmol•g -1 •h -1 ) and MIL-125-NH2 (16.3 µmol•g -1 •h -1 ) MOFs under similar conditions. The authors propose that the presence of deep trap states on the porphyrin, when immobilized in the MOF, increases the lifetime of the photoinduced electrons in MOF-545 and thus the formate production. Transient absorption spectroscopy combined to steady-state photoluminescence measurements supports this hypothesis, but no additional details about the trap states are given.Nonetheless, they propose a mechanism where the porphyrin acts only as a PS, the catalyst being the Zr6 oxocluster. Besides, electron paramagnetic resonance (EPR) measurements suggest the formation of Zr(III) centers, implying that an electron would be transferred from TCPP to the Zr6 cluster (Figure3.3), in a similar fashion to the Ti(IV)Ti(III) reduction observed in the titanium-based MOF, MIL-125-NH2, in photocatalytic conditions.14

Figure 3 . 3 :

 33 Figure 3.3: CO2 photoreduction with MOF-545 (red), the homogeneous TCPP (blue) and blank experiments without TEOA, CO2 or catalyst (black) (left) and proposed mechanism for the reaction (right). 1

Figure 3 . 4 :

 34 Figure 3.4: Proposed mechanism for coupled water oxidation and CO2 reduction withZr/TiMOF-525.15

3

 3 They report the CO2 photoreduction of metalated MOF-525, namely MOF-525(Co) and MOF-525(Zn), compared to its free-base homologue. They have shown that under visible light, in acetonitrile with TEOA as an electron/proton donor, MOF-525(M) is able to reduce CO2 to a mixture of CO and CH4. The nature of the metal in the porphyrin plays a key role in the catalyst efficiency, with the range of performances as follows: MOF-525(Co) > MOF-525(Zn) > MOF-525 >> TCPP in homogeneous conditions (Figure 3.5). As in the previous reports, 1,15 their EPR experiments show the presence of a signal at g = 2.0048 which has been attributed to the formation of Zr(III) and at g = 1.9984 which has been attributed to the formation of Co(I), coinciding to an electron transfer from the excited porphyrin to the Zr nodes in absence of CO2. Nonetheless, under irradiation and in presence of CO2 the signal corresponding to Zr(III) disappears, suggesting that the electron transfer from Co(II) to Zr(IV) is quenched in presence of the substrate. The authors thus suggest, with the support of DFT calculations, a mechanism where the porphyrin acts as both photosensitizer and catalyst in the MOF-525 system.

Figure 3 . 5 :

 35 Figure 3.5: CO2 photoreduction with metalated or free-base MOF-525 composites (left) andproposed mechanism for the reaction (right).3

Figure 3 . 6 :

 36 Figure 3.6: Proposed mechanism for the CO2 photoreduction with PCN-224(Rh).16

  Metalated MOF-545(TM) (TM = Mn, Fe, Co, Cu, Zn) were obtained via an analogous protocol but using the required (TM)TCPP ligand instead of the non-metalated porphyrin derivative as starting reactants. Powder X-ray diffraction (PXRD) patterns and infrared (IR) spectra (Figure 3.7) support the formation of the targeted materials. It can be noticed that the additional peak at ~1000 cm -1 present in most of the IR spectra of MOF-545(TM) corresponds to N-TM vibrations.

Figure 3 . 7 :

 37 Figure 3.7: Left: Comparison of the experimental X-ray powder patterns of MOF-545(Co) (red), MOF-545(Cu) (blue), MOF-545(Mn) (green), MOF-545(Zn) (black) and of the powder pattern calculated from the structure solved from single-crystal X-ray diffraction data 5 for MOF-545 (purple). Right: FT-IR spectra of MOF-545 (black), NanoMOF-545 (red), MOF-545(Fe) (green), NanoMOF-545(Fe) (blue), MOF-545(Co) (cyan), MOF-545(Cu) (pink), MOF-545(Zn) (purple) and MOF-545(Mn) (khaki).

  Solid-state UV-Vis spectra evidence the porphyrin ligands Soret band at ca. 420 nm and the Q bands in the 500-800 nm range (Figure3.8).

Figure 3 . 8 :

 38 Figure 3.8: Electronic absorption spectra of MOF-545 (black), MOF-545(Fe) (blue), MOF-545(Zn) (purple), NanoMOF-545 (brown), NanoMOF-545(Fe) (red), MOF-545(Cu) (orange), MOF-545(Mn) (green) and MOF-545(Co) (cyan).

  protocol to obtain NanoMOF-545 and NanoMOF-545(Fe) under microwave conditions (130°C, 800 W, 20 min) allowing getting ~30 mg of NanoMOF for each MOF analog (see Experimental section). The PXRD patterns are in full agreement with those of MOF-545, with an expected widening of the Bragg peaks (Figure3.9 a). The IR spectrum of NanoMOF-545(Fe) exhibits the characteristic Fe-N vibration at 997 cm -1 , close to that of MOF-545(Fe) (1001 cm -1 ) (Figure3.9 b).27 SEM images indicate sizes of ca. 3000, 200 and 150 nm for MOF-545(Fe), NanoMOF-545 and NanoMOF-545(Fe), respectively (Figure3.9 c).

Figure 3 . 9 :

 39 Figure 3.9: a) Comparison of the experimental X-ray powder patterns of NanoMOF-545(Fe) (red), NanoMOF-545 (blue), MOF-545 (black) and of the simulated powder pattern of MOF-545 (purple). b) IR spectrum of NanoMOF-545(Fe). c) SEM images of MOF-545, NanoMOF-545 and NanoMOF-545(Fe).

Figure 3 . 10 :Figure 3 . 11 :

 310311 Figure 3.10: TGA curves of NanoMOF-545 (black) and NanoMOF-545(Fe) (red).

1 )Figure 3 . 13 :

 1313 Figure 3.13: Formate production for MOF-545 (light red), MOF-545(Fe) (dark red), NanoMOF-545 (dark green) and NanoMOF-545(Fe) (dark green). Reaction conditions: catalyst 2 mg, CH3CN/TEOA (10:1) 2 mL (λ > 415 nm, 280 W).

Figure 3 . 14 :

 314 Figure 3.14: HCOO -production over time with 2 mg of NanoMOF-545(Fe) (left) and recyclability test of the NanoMOF-545(Fe) (right).

Figure 3 . 15 :

 315 Figure 3.15: Experimental PXRD patterns of NanoMOF-545(Fe) before catalysis (red) and after 4h catalytical run (black) and simulated PXRD pattern of MOF-545 (purple).

Figure 3 .Scheme 3 . 1 :

 331 Figure 3.16 represents the calculated Gibbs free-energy profile for the reduction of CO2 to formic acid catalyzed by the Zr6 nodes of MOF-545 and MOF-545(TM). After the photoreduction of the linker to generate a TEOA • radical (Scheme 3.1), the thermally-activated reduction of CO2 starts with the displacement of a labile aqua ligand from an open Zr-site of the Zr6 cluster 1.Ligand displacement generates a coordination vacancy in 1ʹ where CO2 can bind through one of its oxygen atoms to the Zr center to give species 2. The stepwise process is endergonic by 12.6 kcal•mol -1 . In species 2, the polarization of the C=O bond of CO2 by the effect of the Zr IV ion acting as a Lewis acid increases the electrophilicity of the carbon atom, i.e. activating it towards receiving a nucleophilic attack from an external hydride.

Figure 3 . 16 :

 316 Figure 3.16: Gibbs free-energy profile (kcal•mol -1 ) for the reduction of CO2 to HCOOH catalyzed by MOF-545. The red arrow highlights the overall free-energy barrier (3→TS2-4).Relative energies of species that contain a TEOA • fragment account for the energy penalty of 2.7 kcal•mol -1 associated to displacing the equilibrium depicted in Scheme 3.1.c towards the free reducing agent in solution. The free-energy cost for the product release considers the product as a non-covalent HCOOH•••TEOA adduct in solution. Red-dashed lines represent an alternative, less favorable mechanism that is higher in energy.

Figure 3 .

 3 18 shows that indeed, the addition of the iron-porphyrin increases the yield of formate produced by the free-base MOF-545 by a factor of 1.75. This enhancement cannot be ascribed to the iron-porphyrin acting as an additional catalytic site by itself, because i) the formate production of the {MOF-545, Fe III Cl-TCPPOMe} system exceeds by far the sum of the individual formate production of MOF-545 and Fe III Cl-TCPPOMe and, ii) experiments with the Fe-porphyrin alone give much lower amounts of formate than with the MOF-545. This indicates much slower kinetics of CO2 reduction when using Fe III Cl-TCPPOMe rather than MOF-545, whereby the catalytic pathway involving the MOF should prevail over that involving the molecular porphyrin in the mixed {MOF-545, Fe III Cl-TCPPOMe} system.In line with experiments, our calculations indicate that the consumption of reactants (CO2 and TEOA • ) should be much faster on the Zr6 nodes of the MOF than with the molecular Feporphyrin in solution. Therefore, the yield obtained experimentally by combining both MOF-545 and Fe III Cl-TCPPOMe species should be attributed to the catalytic activity of the MOF exclusively, while the Fe-porphyrin added in solution allows increasing the concentration of the reducing agent by accelerating the one-electron oxidation of TEOA.

Figure 3 . 18 :

 318 Figure 3.18: Formate production upon CO2 reduction with Fe-TCPPOMe (1.2 mg) and/or free-base MOF-545 (2 mg) allowing ~1:1 ratio between porphyrins in solution and in the MOF. Experiments were conducted in 2 mL CH3CN/TEOA (10:1) for 4h under 280 W Xenon Lamp. The amount of formate detected was normalized regarding MOF-545.

  Mn(bpy)(CO)3Br 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin iron chloride Ni(cyclam)2+

Figure 3 . 19 :

 319 Figure 3.19: Representation of Mn(bpy), Ni(cyclam) and F20 considered for immobilization in MOF-545

Figure 3 . 21 :

 321 Figure 3.21: PXRD (left) and infrared spectra (right) of the successively encapsulated F20@MOF-545.

1 F 20 @

 120 MOF-545-2 irradiation, the CO production measured without CO2 and the CO production found when bubbling CO2 were similar (640 nmol).

Figure 3 . 22 :

 322 Figure 3.22: CO release in presence of CO2 of a 2 mL of CH3CN/TEOA (5:1) mixture with 0.5 mg of Mn(bpy)@MOF-545(Zn) (24 h irradiation with a 280W Xe lamp equipped with a 415nm UV cut-off).

  POMs which differ by their composition, charge and shape: [(PW9O36)2(H2O)2Ni4] 10-(Ni4), [(PW9O36)2(H2O)2Co4] 10-(Co4), [PW10Ti2O40] 7-(PW10Ti2) and [SiW12O40] 4-(SiW12) (Figure 3.23).

Figure 3 . 23 :

 323 Figure 3.23: Structure of Ni4/Co4, PW12 and PW10Ti2

  .24).We thus decided to investigate the incorporation of different Keggin anions in MOF-545, the main purpose of this study being to rationalize the effect of the POM charge, redox potential and acidity on the catalysis. The compounds K5[BW12O40], Na4[SiW12O40], Na2H[PW12O40],Na2H[PMo12O40] and K4[SiMo12O40], were considered. Also, based on the results described above, the nano version of the MOF-545 was considered.

Figure 3 . 24 : 1 MOF•h - 1 . 1 )Figure 3 . 25 :

 324111325 Figure 3.24: CO2 photoreduction rates for the studied POM@MOF composites. The plain bars correspond to the rates in µmol•g -1 •h -1 and the hatched bars in µmol•g -1 MOF•h -1 .

  with three different concentrations (1, 0.5 or 0.25 equivalents of Na2HPMo12O40 per Zr6 units). This corresponds to concentrations of 2, 1 and 0.5 mM of POM. The new PXRD patterns are shown in Figure 3.26. It is indeed observed that the lower the POM concentration, the closer to the NanoMOF-545 PXRD pattern. To compromise between the crystallinity and the POM loading we have decided to work with the 0.5 eq encapsulation methods.

Figure 3 . 26 :

 326 Figure 3.26: PXRD patterns of PMo12@nanoMOF-545 composites prepared from solution incorporating 2.5 (black), 1 (red), 0.5 (blue), 0.25 (green) equivalent of POM per Zr6 units.

Figure A3. 1 :

 1 Figure A3.1: Formate production over time with NanoMOF-545(Fe), 280W Xe, CH3CN/TEOA (10:1), 2 mg of catalyst. Results are before formate desorption protocol.

Figure A3. 2 :

 2 Figure A3.2: 1 H NMR spectrum of the reaction mixture after the catalysis. The triplets at 2.62 and 3.54 ppm are assigned to the alkyl protons in TEOA; whereas smaller peaks at 8.46 and 9.67 ppm are assigned to the C-bound proton of formic acid and the proton of the aldehyde function in the oxidized form of TEOA respectively.

Figure A3. 3 .

 3 Figure A3.3. Schematic summary of the proposed reaction mechanism for the photocatalytic reduction of CO2 to formic acid with TEOA promoted by Zr-based porphyrinic MOFs.

Figure 4 . 1 :

 41 Figure 4.1: Ball and stick representation of the [(Mo2O4)3(HPO4)x(PO4)4-x(OH)3] (9-x)-(P4Mo6) anion (left) Polyhedral representation of the sandwich-type structure formed by the connection of two P4Mo6 anions (right)

  The first report of P4Mo6-based material active for CO2 reduction was published fifteen years later. Xie et al.11 reported the reduction of CO2 into CH4 in water by two new compounds: NENU-605 and NENU-606. NENU-605 is a purely inorganic material with four P4Mo6 polyoxometalate subunits connected to 10 manganese metal centers, while NENU-606 is a bimetallic POM-based material containing 7.5 Mn and 2.5 Co centers every four P4Mo6 units.

Figure 4 . 2 :

 42 Figure 4.2: a) Simplified cluster constitutive of NENU-605, with Mn in green dots and Mo6 rings in blue; b) surrounding environment of the simplified cluster, with outer Mn in purple; c) 3D Structure of NENU-605. 11

Figure 4 . 3 :

 43 Figure 4.3: Amounts of CH4 and CO produced as a function of the time of visible-light irradiation using (a) NENU-605 and (b) NENU-606; (c) total product yield and the selectivity of gaseous products in the photocatalytic CO2RR. 11

Figure 4 . 5 :

 45 Figure 4.5: Polyhedral representation of the sandwich-type structure formed by the connection of two P4Mo6 anions by a Co(II) or Mn(II) ion in octahedral coordination.

Figure 4 . 6 :

 46 Figure 4.6: a) View of the chain formed by the connection of Co(P4Mo6)2 sandwich-type POMs by Co(II) ions in the structure of Ru(bpy)-Co; b) side view and c) top view of the chain surrounded by [Ru(bpy)3] 2+ cations.

  .7 a). These Mn(P4Mo6) sandwich-type ions stack into chains like Co(P4Mo6)2 analogues in Ru(bpy)-Co but while these entities are parallel in Ru(bpy)-Co, the chain adopts a zig-zag shape in Ru(bpy)-Mn with tilted anionic units. Furthermore, the POMs are connected by four Co(II) ions in Ru(bpy)-Co but there are only two Mn(II) ions linking the anionic entities in Ru(bpy)-Mn. Mn2 and Mn3 are in a pseudooctahedral coordination environment, being connected to three terminal water ligands and to three oxygen atoms of phosphate ions (Figure A4.1 b). Contrarily to what is observed in the structure of the Co analogue, the two Mn ions are not directly connected via an oxygen atom of a bridging water molecule but are separated by a sodium ion. The chains are surrounded by [Ru(bpy)3] 2+ counterions (Figures 4.7b and c).

Figure 4 . 7 :

 47 Figure 4.7: a) View of the chain formed by the connection of Mn(P4Mo6) sandwich-type POMs by Mn(II) ions in the structure of Ru(bpy)-Mn, b) side view and c) top view of the chain surrounded by [Ru(bpy)3] 2+ cations.

Figure 4 . 8 :

 48 Figure 4.8: a) View of the chain formed by the connection of Mn(P4Mo6)2 sandwich-type POMs by Mn(II) ions in the Ni(cyclam)-Mn crystal structure ; b) view of one sandwich-type POM surrounded by four Ni(II) complexes and c) view along the c axis of the supramolecular structure formed by the connection of the POMs and the [Ni(cyclam)] 2+ cations.

Fe2

  is in the +II oxidation state, Fe1 and Fe2 being in octahedral environments. Fe1 is connected to six oxygen atoms of phosphate groups and Fe2 is bound to four oxygen atoms of phosphate groups and to two water molecules (Figure A4.1 d). The four oxygen atoms of the extra PO4 group are bound to Fe1 ions. The Fe2 ions connect directly phosphate groups of P4Mo6 units. The connection of the POMs via these Fe-O(PO3) bonds generates a compact 3D structure (Figure 4.9 b and c).

Figure 4 . 9 :

 49 Figure 4.9: a) View of the structural unit in Fe-Mn, which consists in a Mn(P4Mo6)2 sandwich-type anion, one Fe(III) ion (Fe1), one Fe(II) ion (Fe2) and one extra PO4 group; b) view highlighting the extra PO4 group bound to four Mn(P4Mo6)2 sandwich-type anions via Fe-O(PO3) bounds; c) view along the b axis of the 3D structure.

Figure 4 . 10 :

 410 Figure 4.10: Solid-state UV-vis spectra of Ru(bpy)-Co (black), Fe-Mn (green), Ru(bpy)-Mn (red) and Ni(cyclam)-Mn (blue).

Figure 4 . 11 :

 411 Figure 4.11: TGA curves of Fe-Mn, Ni(cyclam)-Mn, Ru(bpy)-Co and Ru(bpy)-Mn (air, heating rate 5°C/min).

Figure 4 . 12 :

 412 Figure 4.12: Infrared spectra of Ru(bpy)-Co (black), Fe-Mn (blue), Ru(bpy)-Mn (red), Ni(cyclam)-Mn (green), Ni(cyclam)-Cl2 (purple) and [Ru(bpy)3]Cl2 used for the synthesis (pink).



  C-C and  C-N cyclam and bpy

Figure 4 . 13 :

 413 Figure 4.13: PXRD for Ru(bpy)-Co, Ru(bpy)-Mn, Ni(cyclam)-Mn and Fe-Mn, in red experimental data, in black simulated data.

Figure 4 . 14 :

 414 Figure 4.14: CO and CH4 production from CO2 over time with Fe-Mn (a), Ru(bpy)-Mn (b), H2O:TEOA and (c) compared yield at 8h. (14:1 v/v), [Ru(bpy)3]Cl2 0.01 mmol, 280 W,  > 415 nm.

Figure 4 . 15 :

 415 Figure 4.15: 48h catalytic CO2 photoreduction (left) and recyclability tests (right) and with Ru(bpy)-Mn as a catalyst.

Figure 4 . 16 :

 416 Figure 4.16: Infrared spectra and PXRD of Ru(bpy)-Mn and Fe-Mn before (black) and after (red) catalysis.

Figure 4 . 17 :

 417 Figure 4.17: Proposed mechanism for the photoreduction of CO2 to CO catalyzed by {P4Mo6}/Ru(bpy)3 systems using TEOA as sacrificial donor. Reaction Gibbs free energies are given in kcal•mol -1 . Values in parentheses indicate relative Gibbs free energies. MLCT, ISC, MMCT, MC and LSCT stand for metal-to-ligand charge transfer, intersystem crossing, metalto-metal charge transfer, metal-centered and ligand-to-solvent charge transfer, respectively.

Figure 4 .

 4 Figure 4.18: a) 13 C NMR of 0.01 mmol Ru(bpy)3.Cl2 in 30 mL H2O:TEOA (14:1) after overnight irradiation; b) DEPT 135 NMR of the same solution.

Figure 4 .

 4 19). Nonetheless, when [Ru(bpy)3] 2+ is added to the reaction mixture the yield of CH4 increases significantly due to an enhanced photooxidation of TEOA caused by the electron flow from reduced POMs to [Ru(bpy)3] 2+ and by the role of the latter as an additional TEOA photooxidant.

Figure 4 . 19 :

 419 Figure 4.19: Methanol photocatalytic reduction to CH4 in presence (right) or in absence (left) of Ru(bpy)-Mn. Experiments were conducted for 50 h under 280W Xenon Lamp equipped with a 415 nm cutoff, with or without 10 mg of catalyst, in 30 mL of H2O/TEOA/MeOH (13/1/1), in the absence of additional [Ru(bpy)3]Cl2.

  dimensionality depends on the nature of the counterions. Ru(bpy)-Co, Ru(bpy)-Mn and Ni(cyclam)-Mn exhibit a 1D structure with M(P4Mo6)2 (M = Co or Mn) sandwich-type POMs connected by additional Co(II) or Mn(II) ions. The [Ru(bpy)3] 2+ cations introduced in the synthesis of Ru(bpy)-Mn to play the role of counterions do no act as photosensitizer, and additional Ru complexes must be added in the catalytic solution. Since it does not play any catalytic role either, comparing the Ru(bpy)-Mn to its homologue without Ru(bpy) would provide useful information concerning its necessity, as addition of Ru(bpy)3.Cl2 might be mandatory to obtain the POM-based crystals.. DFT calculations allowed giving insights into the photocatalytic CO2RR mechanism. Analysis of the electronic structure and photochemical properties of the different species revealed that both the POM units of the crystals and [Ru(bpy)3] 2+ can photooxidize the TEOA sacrificial donor and in turn, reduced POMs can reductively quench a photoexcited [Ru(bpy)3] 2+ . As Rubpy complexes with delocalized electrons in the bpy ligands are known for being photo-ionized, we propose that the absorption of a photon by the one-electron reduced Ru complex, i.e.[Ru II (bpy)2(bpy •-)] + , generates a solvated electron, which can then reduce a CO2 molecule to form a CO2 •-anion in solution. In the presence of metal sites with labile ligands and suitable energy levels such as Mn(II) ions in Ru(bpy)-Mn, CO2 •-can coordinate to the metal center, being further reduced with one electron by the material and protonated to generate a Mn-COOH intermediate. The protonation of the latter induces the C-O(H) bond cleavage to yield a Mncoordinated CO product releasing a water molecule. This step determines the overall cost of the thermally activated process, which needs to overcome a moderate free-energy barrier of ca. 18 kcal•mol -1 . Remarkably, this mechanism is consistent with three key experimental facts: i) the reduction of CO2 to CO does not take place in the absence of any of the components; ii) oxalate is the sole product in the absence of POM crystal, since [Ru(bpy)3] 2+ can promote the formation of solvated electrons and CO2 •-ions (which quench each other to give oxalate) but it cannot accomplish the formation of CO; iii) despite having very similar structures, Ru(bpy)-Co does not yield CO because the coordination of CO2 •-to Co(II) centers results in a spontaneous electron transfer to form a Co-Co bond, regenerating an oxidized CO2 molecule (more details can be found in the appendix). Understanding the following steps for the reduction of CO to CH4 requires additional experimental and computational work that is under way in our groups, although preliminary studies point towards the critical role of a photogenerated TEOA • species as hydride donors.

  Preparation of [Ru(bpy)3]2CoII 5[P4Mo6O26(OH)5]2•22H2O (Ru(bpy)-Co): A mixture of Na2MoO4·2H2O (0.235 g, 0.98 mmol), metallic Mo (0.030 g, 0.31 mmol), concentrated (85 wt%) H3PO4 (73 µL, 1.11 mmol), CoCl2·6H2O (0.420 g, 1.76 mmol), [Ru(bpy)3]Cl2.6H2O (0.200 g, 0.27 mmol) and H2O (4 mL) was stirred and the pH was adjusted to 2.0 with 4 M HCl (pHf = 2.0). Red crystals were isolated after filtration and sonication in ethanol (0.067 g, 14.7 % based on Mo). Metal composition was checked by EDX analysis: P/Mo = 0.55 (calc 0.67); Ru/P = 0.29 (calc 0.25); Co/P = 0.70 (calc 0.62). IR (ν/cm -1 ): 1624 (w), 1601 (w), 1460 (w), 1441 (w), 1421 (w), 1310 (m), 1166 (w), 1122 (w), 1060 (m), 1001 (s), 959 (s), 926 (s), 882 (s), 769 (m), 729 (m), 677 (m), 608 (w), 530 (m). Preparation of Na2[Ru(bpy)3]2Mn II 3[P4Mo6O25(OH)6]2•20H2O (Ru(bpy)-Mn): A mixture of Na2MoO4·2H2O (0.470 g, 1.95 mmol), metallic Mo (0.030 g, 0.31 mmol), concentrated (85 wt%) H3PO4 (290 µL, 4.42 mmol), MnCl2·4H2O (0.119 g, 0.59 mmol), iminodacetic acid (0.100 g, 0.75 mmol), [Ru(bpy)3]Cl2.6H2O (0.100 g, 0.13 mmol) and H2O (4 mL) was stirred and the pH was adjusted to 2.5 with 4 M HCl (pHf = 2.5). Red crystals were isolated after filtration and sonication in ethanol (0.065 g, 8.8 % based on Mo). Metal composition was checked by EDX analysis: P/Mo = 0.61 (calc 0.67); Ru/P = 0.30 (calc 0.25); Mn/P = 0.30 (calc 0.37); Na/P = 0.14 (calc. 0.25). IR (ν/cm -1 ): 1634 (w), 1600 (w), 1464 (w), 1441 (w), 1421 (w), 1118 (m), 1001 (s), 958 (s), 923 (s), 768 (m), 726 (s), 687 (m), 603 (m), 528 (m). Preparation of Na7Mn II 2Fe II 4Fe III 4[P4Mo6O26(OH)5]4(PO4)•52H2O (Fe-Mn): A mixture of Na2MoO4·2H2O (0.470 g, 1.95 mmol), metallic Mo (0.030 g, 0.31 mmol), 8 M H3PO4 (530 µL, 4.22 mmol), Fe(SO4)2(NH4)2·2H2O (0.280 g, 0.70 mmol) and H2O (4 mL) was stirred and the pH was adjusted to 2.5 with 4 M HCl (pHf = 2.6). Dark red hexagonal crystals were isolated after filtration and sonication in ethanol (0.270 g, 44.0 % based on Mo). Metal composition was checked by EDX analysis: P/Mo = 0.71 (calc.: 0.67); Mn/Fe = 0.25 (calc 0.25); Fe/Mo = 0.33 (calc :0.48); Fe/Na 1.10 (1.14). IR (ν/cm -1 ): 1624 (m), 1427 (m), 1190 (m), 1112 (sh), 1073 (s), 1043 (s), 1001 (s), 955 (s), 918 (s), 785 (m), 701 (m), 595 (m), 537 (m). Preparation of Na2[Ni(cyclam)]3Mn II 2[P4Mo6O25(OH)6]2•12H2O (Ni(cyclam)-Mn): A mixture of Na2MoO4·2H2O (0.470 g, 1.95 mmol), metallic Mo (0.030 g, 0.31 mmol), concentrated (85 wt%) H3PO4 (290 L, 4.42 mmol), MnCl2·4H2O (0.119 g, 0.59 mmol), iminodiacetic acid (0.100 g, 0.75 mmol), [Ni(cyclam)]Cl2 (0.050 g, 0.25 mmol) and H2O (4 mL) was stirred and the pH was adjusted to 4.0 with 8 M NaOH (pHf = 3.9). Bright red hexagonal crystals were isolated after filtration and sonication in ethanol (0.070 g, 10.4 % based on Mo). Metal composition was checked by EDX analysis: P/Mo = 0.60 (calc 0 .67); Mn/P = 0.25 (calc 0.25); Mn/Ni = 0.49 (calc. 0.67); Na/Mn = 1.03 (calc 1.00). IR (ν/cm -1 ): 1620 (w), 1468 (w), 1449 (w), 1425 (w), 1234 (w), 1164 (w), 1127 (m), 1007 (s), 947 (s), 915 (s), 733 (m), 691 (m), 668 (m), 608 (m), 527 (m).

Figure A4. 1 :

 1 Figure A4.1: Co, Mn and Fe environments in the structures of a) Ru(bpy)-Co, b) Ru(bpy)-Mn, c) Ni(cyclam)-Mn and d) Fe-Mn

  , playing a critical role that the Co(II) centers in Ru(bpy)-Co cannot. Indeed, analysis of the key coordination step of CO2 •-to Co(II) centers Ru(bpy)-Co revealed that, unlike Mn(II) centers in Ru(bpy)-Mn and Fe(II) centers of the also catalytically active Fe-Mn, Co(II) ions cannot stabilize an M-COO •-intermediate. Instead, {Co(II)}2 dimers act as an electron sink taking the extra electron from the CO2 •-fragment to form a Co-Co bond and regenerate CO2. This can be explained by the fact that virtual Cocentered orbitals are deeper in energy than those of Mn (FigureA4.2), being more easily filled by a reducing agent, here the CO2 •-anion. Importantly, the lack of CO/CH4 products reported for Ru(bpy)-Co can be explained through its inability to coordinate and stabilize a reduced CO2 molecule, further supporting our mechanistic proposal.

Figure A4. 2 :

 2 Figure A4.2: Comparison of the Density of States for Ru(bpy)-Mn (a) and Ru(bpy)-Co (b) models.

Figure A4. 3 :

 3 Figure A4.3: Ball and stick representation with partial atomic labeling scheme and selected bond distances (Å) of the P4Mo6 unit in Ru(bpy)-Co. Bond Valence Sum (BVS) are given for Mo ions and for selected bridging atoms. Short Mo-Mo bonds around 2.6 Å also indicate the +V oxidation state of the Mo ions.

Figure A4. 4 :

 4 Figure A4.4: Ball and stick representation with partial atomic labeling scheme and selected bond distances (Å) of the P4Mo6 unit in Ru(bpy)-Mn. Bond Valence Sum (BVS) are given for Mo ions and for selected bridging O atoms. Short Mo-Mo bonds around 2.6 Å also indicate the +V oxidation state of the Mo ions.

Figure A4. 5 :

 5 Figure A4.5: Ball and stick representation with partial atomic labeling scheme and selected bond distances (Å) of the P4Mo6 unit in Ni(cyclam)-Mn. Bond Valence Sum (BVS) are given for Mo ions and for selected bridging O atoms. Short Mo-Mo bonds around 2.6 Å also indicate the +V oxidation state of the Mo ions.

Figure A4. 6 :

 6 Figure A4.6: Ball and stick representation with partial atomic labeling scheme and selected bond distances (Å) of the P4Mo6 unit in Fe-Mn. Bond Valence Sum (BVS) are given for Mo ions and for selected bridging atoms. Short Mo-Mo bonds around 2.6 Å also indicate the +V oxidation state of the Mo ions.

Table 1 .

 1

	Catalyst	Photosensitizer	Conditions	TON (Time) / Product
	Re(CO)3(bpy)Cl 10	-	DMF:TEOA (5:1)	16 (2h) CO
	87 µM
	Re(CO)3(bpy)Br 10	-	DMF:TEOA (5:1)	14 (2h) CO
	79 µM
	Re(CO)3(bpy)CNS 13	-	DMF:TEOA (5:1)	30 (25h) CO
	0.5 mM
	Ru(bpy)3 16	-	DMF:TEOA (4:1)	27 (24h) HCOOH
	11 mM
	Ru(bpy)2Cl2 17	[Ru(bpy)3] 2+	DMF:TEOA (4:1), 15% water	51 (2h) HCOOH
	3.3 µM	3.3 µM
	Ru(bpy)2(CO)Cl 17	[Ru(bpy)3] 2+	DMF:TEOA (4:1), 15% water	326 (2h) HCOOH
	3.3 µM	3.3 µM
	Ir(tpy)(ppy)Cl 22	-	CH3CN:TEOA (5:1)	38 (5h) CO
	0.5 mM
	Ir(R-tpy)(ppy) 23	-	CH3CN:TEOA (5:1)	310 (5h) CO
	0.5 mM

2: Comparison of CO2 photoreduction experiments with noble-metal based polypyridyl catalysts.

  30

Table 1 . 3 :

 13 Comparison of catalytic CO2 reduction with different porphyrin-based catalysts.

	Catalyst	Photosensitizer	Conditions	TON (Time) / Product
	FeTPP 25	p-terphenyl (TP)	CH3CN, 5% TEA	20 (6h) CO
	100 µM	3 mM		25 (6h) HCOOH
	CoTPP 26	-	CH3CN, 5% TEA	80 (200h) CO
	10 µM			320 (200h) HCOOH
	FeTPP 27	-	CH3CN, 0.36 M TEA, 50 mM	7 (10h) CO
	10-50 µM		trifluoroethanol	23 (10h) H2
	CAT 27	-	CH3CN, 0.36 M TEA, 50 mM	30 (10h) CO
	10-50 µM		trifluoroethanol	10 (10h) H2
	FCAT 27	-	CH3CN, 0.36 M TEA, 50 mM	23 (10h) CO
	10-50 µM		trifluoroethanol	12 (10h) H2
	CAT 28	[Ir(ppy)3] +	CH3CN, 0.36 M TEA	140 (60h) CO
	2 µM	0.2 mM		10 (60h) H2
	CAT 28	9-cyanoanthracene	CH3CN, 0.36 M TEA	40 (45h) CO
	2 µM	0.2 mM
	FeTPP(TMA) 29	Purpurin	CH3CH:H2O (1:9), 50 mM TEA	60 (47h) CO
	2 µM	0.2 mM		3 (47h) H2
	FeTPP(TMA) 30	[Ir(ppy)3] +	CH3CN, 50 mM TEA	140 (102h) CH4
	2 µM	0.2 mM		28 (102h) H2

  MIL-100 MOFs are mainly stable in acidic pH and start degrading at pH 7 as shown for MIL-100(Fe) by Bezverkhyy et al..[START_REF] Bezverkhyy | Degradation of fluoride-free MIL-100(Fe) and MIL-53(Fe) in water: Effect of temperature and pH[END_REF]

	linker defect.
	2.2	Zirconium-based MOF
	Zirconium-based MOFs are made of zirconium oxoclusters. The zirconium building bricks can
	either be consisted of 6 or 8 Zr(IV) ions. 85 The MOFs studied in this PhD thesis are solely
	composed of Zr6 oxoclusters with formula Zr6O4(OH)4. The Zr6 cluster can be coordinated to
	either 6, 8 or 12 ligands, being fully coordinated with 12 ligands (Figure 1.15). When the Zr6
	cluster is unsaturated (coordination less than 12), the free sites are occupied by OH or H2O
	groups. In this thesis, we will focus on two MOFs, namely UiO-67 (12 coordinated Zr6) and
	MOF-545 (8 coordinated Zr6). We will thus describe the UiO MOF family and the Zr6-
	porphyrin MOF family in more detail.
	Figure 1.15: a) Zr6 building brick; b) 12 coordinated Zr6 oxocluster and c) 8 coordinated Zr6
		oxocluster.
	Four metal UiO isoreticular series of MOF includes UiO-67, UiO-68 and UiO-69, these MOFs are
	trimers are linked together by BTC forming a supertetrahedron (ST), the STs being linked to differentiated by the number of benzene ring of their linkers going from one for UiO-66 to four
	each other by BTC forming a pentagonal mesoporous cage (20 STs) or a hexagonal mesoporous for UiO-69. UiO-66 with the formula Zr6O4(OH)4(BDC)6 is one of the most studied Zr-based
	cage (28 STs). The smallest cage has a diameter of 25 Å and pentagonal windows of 4.8x5.8 MOF. First synthesized in 2008, 86 it consists of Zr6 clusters linked together with BDC linkers
	Å. The largest cage displays a diameter of 29 Å and has hexagonal windows with an aperture forming octahedral mesoporous cages with a diameter of 11 Å and tetrahedral cages of 8 Å
	of 8.6x8.6 Ų. Those two cages lead to a huge surface area of 3100 m²•g -1 . MIL-100 MOFs diameter. The accessible space inside the framework amounts to 1200 m²•g -1 . As with MIL-
	101, the functionalization of the BDC ligand leads to a variety of UiO-66 derivatives. The high
	connectivity (12 linkers per Zr6 unit) allows maintaining high crystallinity even with up to 25%

display high water stability and no degradation after 24h at 323K

[START_REF] Küsgens | Characterization of metal-organic frameworks by water adsorption[END_REF]

and are stable in most solvents. However, again, the thermal stability is highly linked to the nature of the metal node, being stable up to 270, 325 and 370°C for MIL-100(Fe), MIL-100(Cr) and MIL-100(Al) respectively.

[START_REF] Haouas | Monitoring the Activation Process of the Giant Pore MIL-100(Al) by Solid State NMR[END_REF]

Table 2 .

 2

1: Catalytic CO2 photoreduction with varying amount of substituted Cp*Rh linker in %Cp*Rh@UiO-67.

1

Table 2 .

 2 Zr in the UiO-67, we can consider about 0.22 POM per Zr6 cluster. A former PhD student in our group previously demonstrated by CHN elemental analysis and EDS that the POM gets inside the pores without its counterions meaning that its charge of -3 causes a linker defect in order to ensure the electroneutrality of the compound.14 We can thus propose, for the

	2: EDS Analysis
	PW12@UiO-67	Cp*Rh@UiO-67	(PW12,Cp*Rh)@UiO-67
	W/Zr	0.45	-	0.38
	Rh/Zr	-	0.073	0.071
	In the case of PW12@UiO-67 the W/Zr ratio is 0.45, knowing that there is twice as much W in
	PW12 than

PW12@UiO composite I have synthesized, the formula: Zr6O4(OH)4(C14O4H8)5.67(PW12O40)0.22, which represents approximately the incorporation of one POM in the pores of UiO-67 every 5 Zr-clusters. As for the Cp*Rh@UiO-67, both the catalytic rhodium linker and the pristine bpdc linker are twice negatively charged. We propose a formula without linker defects, which is the following: Zr6O4(OH)4(C14O4H8)5.56(RhC22H21N2O4Cl2)0.44. We compared the theoretical composition in C, H, N, W, Zr and Rh calculated from the proposed formula, to the experimental values collected by elemental analysis, ICP and EDS (Table

2

.3). Considering the good correlation between calculated and experimental values, we can thus conclude that the linker exchange does not induce any linker defect. Lastly, for the (PW12,Cp*Rh)@UiO-67 composite, a chlorine defect has been identified by EDS suggesting a loss of chloride ions in the Cp*Rh complex. The electroneutrality is ensured by the Zr6 cluster contributing for 12 positive charges, by the bpdc (-2), Cp*Rh without a chloride anion (-1) and PW12 (-3) for the

Table 2 .

 2

	3: Calculated and experimental values of elemental analysis of (PW12,Cp*Rh)@UiO-
	67.
		C	H	N	W	Zr	Rh
	Calc.	38.22	3.17	3.28	14.48	16.33	1.29
	Exp.	38.27	4.79	3.55	14.80	16.70	1.40

Table 3 .

 3

	1: Zr6-TCPP MOF selectivity toward CO2 photoreduction
	MOF	Product	Solvent (Sacrificial agent)	Claimed catalytic center
	MOF-545 1	HCOOH	Acetonitrile (TEOA)	Zr6 node
	MOF-525 15	CO / CH4 / O2	Gas phase (Water)	Zr/Ti node
	MOF-525(M) (M= Zn,	CO / CH4	Acetonitrile (TEAO)	Porphyrin
	Co) 3
	PCN-224(Rh) 16	HCOOH	Acetonitrile (TEOA)	Zr6 node + porphyrin
	MOF-545(Zn) 17	HCOOH	H2O (Ethyleneglycol)	Zr6 node + porphyrin
	MOF-545(Zn), PCN-	HCOOH	H2O (Ethyleneglycol)	Zr6 node + porphyrin
	223(Zn), PCN-224 (Zn) 2

Table 3 . 6 :

 36 Photocatalytic CO2 reduction of NanoMOF-545 after different treatments in 10:1 CH3CN/TEOA solutions.

	Impregnation solvent	Activation	[Formate] after 2h.
	No impregnation	No activation	1052 µM
	DMF/HCl (0.1 M)	No activation	339 µM
	(1:1)
	Borate buffer pH 8.0	No activation	238 µM
	Borate buffer pH 8.0	Supercritical CO2	460 µM
	DMF/H2O (1:1)	No activation	723 µM
	DMF/H2O (1:1)	Supercritical CO2	867 µM

Table A3 . 2 :

 A32 Photocatalytic reduction of CO2 with POM@MOF composites. 4h irradiation, 280W Xe,  > 415 nm, 2 mg of catalyst in 2 mL of CH3CN/TEOA (10:1), 20°C.

	NR 2								NR 2				NHR 2
	O H		H	O C	O	OH		H O	O C O H	OH	SET	H O	O C O H	OH
	HO	Zr		O O H		Zr	OH 2	HO	Zr	O O H		Zr	OH	HO	Zr	O H O	Zr	OH 2
				PCET
										NR 2					NR 2
	O	C	H						O	O	C	H			O	H	C O
	H 2 O	O Zr	O H O	OH Zr	OH 2	HO H	Zr O	O O	Zr OH	OH 2	HO H	Zr O	O O	Zr OH	OH 2
				4				R 2 N	O						H-transfer
									H
	Catalyst					Loading	µmol•g -1 •h -1	µmol•g -1	MOF•h -1	TON
						(POM/Zr6)
	MOF-545								-			159		159	1.53
	MOF-545(Fe)							-			566		566	5.84
	Co4@MOF-545					0.2			200		282	2.73
	Ni4@MOF-545					0.2			268		378	3.66
	PW10Ti2@545						0.3			209		275	2.67
	SiW12@545							0.8			218		423	4.09

Table 4 . 1 :

 41 Synthesis protocol for 1-A to 4-J. All mass values are in mg. Crystallographic data are given in Table A4. Selected bond distances and valence bond calculations are given in Figures A4.1-A4.4. Valence bond calculations confirm the oxidation state of the Mo, Mn, Co and Fe ions and indicate the presence of protons on bridging oxygen atoms of the POMs.

		1-A	1-B	1-C	1-D	1-E
	Mo	30	30	30	30	30
	Na2MoO4	235	175	117	117	470
	H3PO4 (85%)	73 µL	37 µL	37 µL	37 µL	290 µL
	Fe(SO4)2(NH4)2.6H20					240
	CoCl2.6H2O	420	105		52
	MnCl2.4H2O			176	132	60
	Iminodiacetic acid					100
	Ru(bpy)3	200	100	100	100	100
	H2O (mL)	4	4	4	4	4
	pHi	2	2	2	2	2.5
		1-F	1-G	1-H	1-I	1-J
	Mo	30	30	30	30	30
	Na2MoO4	470	470	470	470	470
	H3PO4 (85%)	290 µL	290 µL	290 µL	290 µL	290 µL
	Fe(SO4)2(NH4)2.6H20	118		118	118

Table A4 . 2

 A42

	Empirical formula C60H102Co5Mo12N12O8	C60H80Mn3Mo12N12Na	C30H112Mn2Mo12N12N	Fe8H82Mn2Mo24Na7O1
		4P8Ru2	2O72P8Ru2	a2Ni3O76P8	59P17
	Formula weight / g	4231.36	3933.34	3588.34	6173.31
	g Crystal system	Monoclinic	Triclinic	Triclinic	Tetragonal
	Space group	C2/c	P-1	P-1	I41/acd
	a / Å	25.734(3)	13.4831(10)	14.204(3)	27.4482(10)
	b / Å	13.656(2)	19.2059(14)	14.319(2)	27.4482(10)
	c /Å	35.406(4)	21.6348(15)	14.888(3)	37.9136(19)
	a / °	90	95.036(3)	112.527(8)	90
	 / °	106.329(5)	91.690(3)	108.688(9)	90
	 / °	90	98.550(3)	101.510(8)	90
	V / Å 3	11941(2)	5513.6(7)	2463.4(8)	28564(3)
		4	2	1	8
	calc / g cm -3	2.354	2.369	2.419	2.871
	 / mm -1	2.356	2.146	2.532	3.332
	Data / Parameters	17433 / 879	32042 / 1570	14439 / 548	10403 / 531
	Rint	0.0276	0.0314	0.0410	0.0357
	GOF	1.175	1.057	1.063	1.271
	R (>2(I))	R1 a = 0.0516	0.0332	0.0424	0.0431
		wR2 b = 0.1368	0.0894	0.1111	0.1164

: Crystallographic data for Ru(bpy)-Co, Ru(bpy)-Mn, Ni(cyclam)-Mn and Fe-Mn. Ru(bpy)-Co Ru(bpy)-Mn Ni(cyclam)-Mn Fe-Mn a R1 = Fo-Fc/Fo. b wR2 = [w(Fo 2 -Fc 2 ) 2 /w(Fo 2 ) 2 ] 1/2

In a 10 mL glass flask, UiO-67 (50 mg) and Cp*Rh(bpydc)Cl2 (17 mg, 3•10 -5 mol) were

Remerciements

Moreover, in all cases, the selective formation of formate was systematically observed, with no CO nor methane production. Interestingly, we observe that the production of HCOO -increases with TM = Cu < Co < Mn < Zn < Fe50% < Fe, reaching a maximum of 566 µmol•g -1 •h -1 with the MOF-545(Fe) photocatalyst (Figure 3.12). The MOF-545(Fe50%) activity is slightly higher than the expected value of ~ 380 µmol•g -1 •h -1 . However the determination of the iron content was estimated by EDX which is not a very precise technique, meaning that the metalation might be slightly underestimated.

The metallation of the porphyrin with Fe was thus selected as the best candidate for investigating further the effect of nanosized particles on the photocatalytic performances of metalated MOF for CO2RR.

The different compounds were characterized by usual means, PXRD and infrared spectroscopy. Considering F20, the catalyst was purchased from Sigma and used as received. The encapsulations were conducted in DMF rather than acetonitrile to heat the solution at a higher temperature. A first encapsulation attempt was carried at 80°C for 24h. However, the EDS analysis showed a negligible amount of F20 incorporated into MOF-545. In order to increase the absence of cobalt the Ru(bpy)3 complex is the photosensitizer while Na{P4Mo6}2 is unable to reduce CO2 making the Ru(bpy)/Na{P4Mo6} system inactive. Furthermore, the use of organic molecules as counter cations and shaping agents has been previously described for polyoxometalates-related composites (i.e. crown ether complexes, organometallic salts). 19,20 Considering the photocatalytic CO2 reduction reaction, it is potentially interesting to use [Ru(bpy)3] 2+ as a counterion whereby it could play the role not only of a photosensitizer but also of a catalyst, as pointed by Du et al. with their Co{P4Mo6} system and in other studies. 21,10,22 This strategy has been in particular explored for POMOFs exhibiting catalytic properties for proton reduction. 23 Another example is the transformation of the cesium salt of the [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] 16-POM into the corresponding [Ru(bpy)3] 2+ salt. This compound was studied as a heterogeneous photocatalyst for water oxidation and showed improved performances when compared to those of the cesium salt, the authors proposing that the formation of the photosensitizer/catalyst pair in the solid state is beneficial for the electron transfer event and for the long-term stability of the photosensitizer. 24 Our objective was thus to synthesize new P4Mo6-based materials with a varying range of bridging cations in order to try to rationalize the effect of the metal center for the CO2 reduction.

Moreover, we used [Ru(bpy)3] 2+ and [Ni(cyclam)] 2+ , small active cations for CO2 photoreduction, 25,26 as counterions with the aim to increase the overall activity. The synthesis protocols and the characterization of the new crystalline solids will be presented in the following section, then we will focus on the photocatalytic activity of the synthesized systems before finally focusing on the mechanism for CO2 reduction with P4Mo6 based POMs.

Synthesis and characterization

Synthesis

Hydrothermal synthesis is the method of choice for the preparation of heterometallic molybdenum(V) phosphates. Indeed, these conditions are necessary to promote the reduction of Mo(VI) by metallic Mo to form the {Mo V 2O4} dinuclear fragments. Phosphate ions are added in excess. An acidic pH, between 2 and 4, is also crucial to promote the formation of the molybdenum(V) phosphates. A mixture of Na2MoO4, Mo, concentrated H3PO4 and metallic salts (Fe(II), Co(II), Ni(II) or Mn(II)) in aqueous solution is thus heated at 180°C for 70 h. In addition, [Ru(bpy)3] 2+ (noted Ru(bpy)) and [Ni(cyclam)] 2+ (noted Ni(cyclam)) have been introduced in the reaction medium to play the role of counterions. Finally, iminodiacetic acid [HN(CH2COO)2] 2-has been added in some syntheses to improve the crystallization process, following previous observations. 27,28 We modulated the metal salt and the amount of salt used as described in Table 4.1. We tried over 60 different combinations of metals/counterions, many of them did not lead to any crystal formation. We mainly focused on Mn, Co, Fe and Ni as bridging metals for the POMs. Out of the 60 conditions screened less than 20 yielded crystals (Table 4.2), but only four of them gave reproducible synthesis with homogeneous crystals suitable for single crystal diffraction.

Those four crystals are labeled Ru(bpy)-Mn, Fe-Mn, Ru(bpy)-Co and Ni(cyclam)-Mn (highlighted in light orange in Table 4.2) will be the main focus of this chapter, their synthesis is described in -Ni(cyclam)-Mn:

In parallel, despite several attempts, we never managed to synthesize NENU-605 and NENU-606 in order to compare their photocatalytic activity to those of our systems in the same conditions. Detailed synthesis conditions can be found in the experimental section.

Structure description

The four structures contain the [(Mo2O4)3(HPO4)x(PO4)4-x(OH)3] (9-x)-(x = 2-3) (P4Mo6) anion.

In all four structures, as almost exclusively observed with heterometallic molybdenum(V) If no contrary indication, all photocatalytic experiments were run in H2O/TEOA 14:1 (v/v), with 30 mL of solution, 10 mg of catalyst finely ground in a mortar and 0.01 mmol (7.48 mg) of [Ru(bpy)3]Cl2. Photochemical reactions were performed using a 280 W, high-pressure Xe arc lamp (Newport Instruments). The beam was passed through a water infrared filter, a collimating lens, and a filter holder equipped with a 415 nm band-pass filter (Asahi Spectra).

Samples were saturated with CO2 via directly bubbling CO2 through the solution mixture for 30 min. During irradiation, the samples were vigorously stirred and aliquots of gas were analyzed..

Single crystal X-ray diffraction

Data collections were carried out by using a Bruker AXS BV diffractometer equipped with a CCD bidimensional detector using the monochromatised wavelength (Mo K) = 0.71073 Å.

Absorption corrections were based on multiple and symmetry-equivalent reflections in the data set using the SADABS program based on the method of Blessing. The structures were solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package. In the structure of Ru(bpy)-Co, one of the Co ions is disordered over two positions (Co3A and Co3B, with space occupation factors equal to 0.7 and 0.3 respectively) as well as one of the P atoms (P4A and P4B with space occupation factors equal to 0.5 and 0.5 respectively). A disorder on one of the phosphate groups is also observed in the structure of Ru(bpy)-Mn.

General Conclusion

The process of the photocatalytic reduction of CO2 is known to be kinetically and thermodynamically challenging. Developing new catalysts is a necessary step to overcome this challenge. Many parameters have to be taken into consideration when developing new catalysts, precisely their efficiency, selectivity and stability. In the past decade, many efforts have been dedicated to the synthesis of new molecular catalysts for the photocatalytic reduction of CO2.

Nonetheless, due to their molecular nature they often lack stability and cannot be easily recycled. The objective of this PhD thesis was thus, thanks to the unique properties of Metal-Organic Frameworks and polyoxometalates, to develop new heterogeneous catalysts for CO2 reduction.

In a first study, we synthesized three novel compounds containing P4Mo6 anionic building units connected by transition metals and studied a fourth structure already reported by our group, respectively Ru(bpy)-Mn, Ru(bpy)-Co, Ni(cyclam)-Mn and Fe-Mn. The P4Mo6 negative charges are compensated with coordination complexes, such as [Ru(bpy)3] 2+ and [Ni(cyclam)] 2+ . Both these complexes have been reported to be catalytically active in the reduction of CO2, and were selected hoping to provide co-catalytic properties to the final POMbased composite. Under photocatalytic conditions, two of the four POM-based composites could reduce CO2 to CO and CH4 with methane as the major product (selectivity reaching 93%

for Fe-Mn). DFT studies allowed shedding light on the mechanism involved, showing that CO2 is reduced to CO2 4 Ni(cyclam)/Zr6 can be obtained, although three successive encapsulations were needed before reaching a 0.2 F20/Zr6 ratio, probably because of the bulkier nature of F20. We then tested the three systems for CO2 reduction. Unfortunately, we could not assess any activity. Nonetheless, we have shown that small molecular catalysts can be immobilized with high loadings. Lastly, in a preliminary study, we investigated the catalytic activity for CO2 reduction of different POM@MOF-545 materials using four different POMs, Co4, Ni4, PW12 and PW10Ti2 showing that SiW12@MOF-545 was the most promising candidate among the POM@MOF series. We thus investigated five Keggin POMs, PW12, BW12, SiW12, SiMo12 and PMo12. Due to their highly negative charge (from -3 to -5), aiming at moderate loading is mandatory to avoid damaging the resulting POM@MOF composite. Finally, photocatalytic tests on these materials have shown that the acidic encapsulation media is degrading the MOF activity for CO2 reduction. To avoid this problem, we could work with PW11M Keggin POM as they are stable in neutral pH.

Regarding the MOF-545 sub-project, the work reported here opens up numerous perspectives.

Iron porphyrins and porphyrin-based MOFs have been widely reported for electrocatalytic CO2

reduction. As we have shown here how efficient NanoMOF-545(Fe) is for CO2 photocatalytic reduction, we may thus investigate its performance for CO2 (photo)electroreduction by considering the deposition of NanoMOF on porous electrodes (gas diffusion electrodes).

Another potentially interesting area of exploration is the use of MOF-545 for performing dual catalysis, i.e. both water oxidation and CO2 reduction, using the ability of porphyrins to also oxidize water or the ability of the MOF to host a molecular water oxidation catalyst. For instance, the synthesis of Ti-doped homologues (Zr/Ti)MOF-545 could allow promoting charge separation upon illumination, while being able to oxidize water at the porphyrin linker and reduce CO2 at the Zr/Ti centers. Another route for designing a MOF-545-based system working both for water oxidation and CO2 reduction would be the immobilization of the sandwich cobalt-based POM, Co4, in the NanoMOF-545(Zn) CO2RR catalyst, taking advantage of the photocatalytic activity of Co4@MOF-545 we previously reported for water oxidation, with the challenging issue of finding the appropriate photocatalytic conditions.

Overall, the experimental work reported herein, supported by theoretical calculations, has allowed highlighting mechanisms at work in the photocatalytic reduction of carbon dioxide by MOFs, extended POMs and related POM@MOF composites. I hope that in a near future this work will help to create new systems with largely improved performances in CO2RR.