+ H2O -0.48 CO2 + 6e -+ 6H + → H3COH + H2O -0.38 CO2 + 8e -+ 8H + → CH4 -0.24

Table 1-1. Reduction potentials of various reduction reactions of CO2. Reaction E° / V vs SHE CO2 + e -→ CO2 •- -1.90 CO2 + 2e -+ 2H + → CO + H2O -0.53 CO2 + 2e -+ 2H + → HCOOH -0.61 CO2 + 4e -+ 4H + → H2CO

iii Acknowledgments I am forever grateful for this journey. I still could not believe that someone who initially did not plan to do PhD studies due to fear and anxiety of the challenges that science can give is now a full-pledged doctor. PhD life is tough, but I learned a lot. It sparked a great interest in answering fundamental questions and even more in asking trivial things. I am truly humbled that I have achieved this feat with great support from a lot of people in so many ways.

First and foremost, I would like to thank my supervisors Winfried Leibl and Ally Aukauloo for picking up a lost soul, for motivating me to stand up on my feet, and for guiding me all the way. Since I started my M2 internship with them, their continuous support and encouragement is remarkable. I want to also extend my gratitude to my co-supervisors Zakaria Halime and Annamaria Quaranta, who supplemented a much-needed guidance. I consider these people as my second family here in France. They are always there answering my random curiosities and taking care of me from all the (mis)adventures during my stay: from breaking my arm in a ski strip, to having a worsened slip disk needing an epidural injection, to having a retinal detachment just after coming from a conference abroad, to losing my passport in Barcelona during a vacation, and to all the other surprises that I bring. These are stories that I will always remember and tell. Regis Guillot (XRD), Christian Herrero (EPR), Tanya Inceoglou (MS), Jean Pierre Baltaze and Amandine Dos Santos (NMR), Emilie Kolodziej (GC-MS), and Eric Riviere (SQUID). I want to also thank Essyllt Louarn from Laboratoire de Chimie Physique for the attempts we had for characterizing the "mysterious gas" produced by one of my catalysts using a membrane inlet mass spectrometry. I thank all my colleagues in Service de Bioénergétique, Biologie Structurale et Mécanismes (CEA Saclay): Sandra Andrianambinintsoa, Anja Krieger-Liszkay, Klaus Brettel, Pavel Muller, Sun Un, Diana Kirilovsky, Pierre Setif and Alberto Mezzetti for the informal discussions during coffee sessions or randomly in the hallway and cafeteria. I would like to extend my appreciation to my friends Simona, Fernando, Pablo, Alex and the whole gang in the CEA groups' "Sushi Fridays" and "Beauty and The Beer" for keeping me sane all these years. I thank the interns Edouard, Max, Lei, Mewenn, and Yen who I supervised in the laboratory and the SERP Chem Batch of 2018 who I taught a three-hour inorganic chem course, for giving me the opportunity to share my knowledge and experiences. I thank the postdocs Maria, Jully, Khaled, Tania and Rajaa, the alumni Stephanie, Clem, Linh, Antoine and Yiting, and the current PhD students Ju, Asma, Adel, Yiting, Amanda, Laura, Adama, Axel, Nico, Sophie, Riya, Arthur, Gabriel, and Jeremy, to whom I have shared some random questions, jokes, and stories. I especially thank Dr. Tam, my running mate in this PhD journey -we finally did it! To some extent, I want to thank again my SERP-Chem family headed by our program director Sandrine Lacombe, for giving me the chance to be in Europe in the first place during my master studies and for opening a lot of opportunities from then on. I thank our ever-supportive Eva and Kali who is always there to answer to my needs. I thank my professors Hynd Remita, Thomas Gustavsson, Benedikt Lasalle, Rachel Meallet-Renault, Remi Metivier, and Andrew Mayne for the chance encounters during conferences and some catch-up discussions. I want to extend my gratitude to Laure Catala for agreeing to be on my comité de suivi de thèse, for being there to listen and for giving me advices on how to clear up all the confusions and worries I had over the years.

Lastly, I offer all my achievements to my family: Mi, Di, Kuya Van, Kuya Mark, and El, for all your sacrifices and efforts to provide everything in my life.

Taos puso akong nagagalak at nagpapasalamat! v

Abstract

Heavy reliance on fossil fuels as a dominant energy resource and a key industrial carbon feedstock has caused alarming concentrations of carbon dioxide (CO2) in the atmosphere. The anthropogenic accumulation of this greenhouse gas has instigated critical effects on climate change and global warming. This necessitated a global consensus not just in decarbonizing the current energy mix by venturing on renewable energy resources but also in capturing and reducing current CO2 emissions. A look on the natural carbon cycle reveals an important role that plants play in sequestering CO2 just by using the renewable solar energy resource.

This inspired the strategies employed by the field of artificial photosynthesis in harnessing solar energy while addressing the current greenhouse problem. At the heart of this challenge, is the development of sustainable catalysts that can help capture, activate, reduce, and eventually valorize CO2.

This PhD project was focused on the design and characterization of molecular catalysts, inspired by natural systems, to understand how to efficiently harness solar energy and use it to drive CO2 reduction and transformative

reactions. An initial investigation was done on rhenium triscarbonyl bipyridyl complexes, photo-activating it with ruthenium trisbipyridine photosensitizers to perform a visible-light driven reduction of CO2 to CO. The photocatalytic activity of the system was optimized, and key processes were studied by spectroscopy, electrochemistry, and single flash photolysis to track the electron transfer reactions.

Because of the need to accumulate photo-induced electrons on the catalyst to perform CO2 reduction, double flash photolysis and photo-accumulation studies were employed. Such investigations allowed to disentangle productive and deleterious reaction pathways upon multiple excitations, and these fundamental insights are crucial to improve the efficiency of charge accumulation in order to optimize the photocatalytic systems.

In search for more efficient and stable catalysts, synthesis of novel complexes employing earth-abundant metals was pursued. New mimics of the Ni-centered active site of the CO dehydrogenase (CODH), the only enzyme known to reversibly reduce CO2 to CO, have been synthesized and investigated towards CO2 reduction activity. Our catalyst design strategy included consideration of an unsaturated coordination sphere of the metal complexes by employing penta-coordinated ligands, multi-point hydrogen bonding pillars, and a substrate-activating vi environment on the second coordination sphere. We have successfully engineered a series of modified iron porphyrins employing urea and imidazolium functions in a meticulous 3D pre-arrangement which significantly lowered the overpotential for CO2 reduction while maintaining good turnover frequency and selectivity.

Further inspiration from functional features of the CODH enzyme led us to establish a proof-of-concept for direct valorization of the CO produced by our photocatalytic system. The photocatalytic system was utilized to transform isotopically labeled 13 CO2 into isotopically labeled 13 CO and immediately use this product in aminocarbonylation reaction to produce isotopically labeled amide groups, a common motif in pharmaceutical drugs. This application shows the capability of our system for light-driven transformation of CO2 into valorized products, further opening opportunities on renewable energy utilization not only for energy but also for transformative chemistry.

vii

Résumé

La forte dépendance vis-à-vis des combustibles fossiles en tant que source d'énergie dominante et en tant que matière première industrielle de carbone a entraîné des concentrations alarmantes de dioxyde de carbone (CO2) dans l'atmosphère. L'accumulation anthropique de ce gaz à effet de serre a provoqué des effets critiques sur le changement climatique et le réchauffement de la planète. Cela a nécessité un consensus mondial sur non seulement la décarbonisation du bouquet énergétique actuel en exploitant des sources d'énergie renouvelables, mais également sur la captation et la réduction des émissions actuelles de CO2. Un regard sur le cycle naturel du carbone révèle le rôle important que jouent les plantes dans la séquestration du CO2 simplement en utilisant de l'énergie solaire.

Cela a inspiré les stratégies employées dans le domaine de la photosynthèse artificielle pour l'exploitation de l'énergie solaire tout en s'attaquant au problème actuel de l'effet de serre. Le développement de catalyseurs pouvant aider à capturer, activer, réduire et valoriser le CO2 est au coeur de ce défi. Une autre inspiration des caractéristiques fonctionnelles de l'enzyme CODH nous a amené à établir une preuve de concept pour la valorisation directe du CO produit par notre système photocatalytique. Le système photocatalytique a été utilisé pour transformer du 13 CO2 marqué isotopiquement en 13 CO et d'utiliser directement ce produit dans une réaction d'aminocarbonylation pour produire des fonctions amides marquées isotopiquement, un motif courant dans les molécules a intérêt thérapeutique. Cette application montre la capacité de notre système à transformer le CO2 en produits valorisés, ouvrant aussi de nouvelles possibilités d'utilisation de l'énergie renouvelable, non seulement pour l'énergie, mais également pour la chimie de transformation.

ix

The Great Concern

Everything comes with a price. The desire to search for a better life through advancement of economic and industrial sectors has inevitably caused a great dependence on energy sources. Indeed, energy is critical for providing life sustainability needs, basic social services, and income-generating opportunities.

Through the years, energy consumption has increased substantially, as shown in Figure 1-1, due to industrial revolution and the large increase in population. With the continuing increase in population and the goal of increasing global standard of living, it is expected that energy consumption will continue to increase. [1] Figure 1-1. Global energy consumption by fuel source type from 1965 to 201, with the inset showing percent contribution of each fuel type. Black solid line shows the related CO2 emission from the combustion of oil, coal, and gas. Data from BP Statistical Review of 2019 World Energy. [1] The global energy demands are still primarily met by fossil fuels, such as coal, natural gas, and oil/petroleum, at a total of 85% contribution to global energy consumption, as shown in the inset of Figure 1-1. Fossil fuels are products of natural photosynthesis and took millions of years to build up. They contain reduced carbon material with low (coal), average (oil), or high (natural gas) hydrogen content.

Burning the C-C and C-H bonds from these hydrocarbons in an exothermic reaction with oxygen releases the stored energy, and as such these products serve as energy carriers [2] Heavy reliance on such energy sources however presents a critical global concern as fossil fuels are finite and humans are using them at an alarming rate.

Further exhaustion of these resources not only concerns the growing energy crisis, but also the shortage of carbon sources to produce various synthetic organic compounds, polymers, and carbon materials, which are critical inputs for the industrial sector.

A much bigger concern is the emission of carbon dioxide, a dominant greenhouse gas, accompanied when using such energy carriers. As shown in the solid black line in Figure 1-1, for every billion ton of oil equivalent of fossil fuel consumed, around 2-3 billion tons of CO2 are emitted. What is worrying is that the rate at which human activities emit CO2 exceeds the rate at which natural photosynthetic systems are able to sequester CO2 leading to increasing mean concentrations in the atmosphere, as shown in Figure 1-2. [3] As CO2 is acting as a greenhouse gas, there is a high correlation between cumulative emissions of CO2 with global mean surface warming. The current atmospheric concentration of 411.57 ppm (as of June 2019) has already caused around 1 °C temperature increase compared to pre-industrial base temperatures (blue curve in Figure 12).

The 2014 Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) indicates that the unprecedented increase of such anthropogenic greenhouse gas emissions have critically disrupted the climatic system causing impacts on both natural and human systems in all continents and across the oceans. Some of these impacts include a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in several regions. [4] Continued emission of such greenhouse gas will cause further warming by the end of the 21 st century leading to a very high risk of severe, pervasive and irreversible global impacts. The report suggests that mitigation measures would be more cost-effective if used in an integrated approach that combines actions to reduce energy use and the greenhouse gas intensity of end-use sectors, decarbonize energy supply, reduce net emissions, and enhance carbon sinks in land-based sectors. [4] As such, the Paris Agreement was then adopted by 195 nations to the United Nations Framework Convention on Climate Change in December 2015 that aimed at strengthening the global response to the threat of climate change by "holding the increase in the global average temperature to well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels." [6] This in part accelerated the global research and implementation of carbon capture, storage, and utilization strategies.

Transforming Carbon Dioxide

Carbon dioxide is an extremely stable molecule generally produced by fossil fuel combustion and respiration. Returning CO2 to a useful state by activation is a scientifically challenging problem, requiring appropriate catalyst and energy input.

The initial step to activate CO2 involves an electron transfer (reduction process) that occupies the σ* anti-bonding lowest unoccupied molecular orbital (LUMO). This causes the increase in the C-O distance, manifested in the bending of the molecule, as shown in Figure 1-3. [7] This bent molecule can then interact with electrophiles and nucleophiles through its frontier orbitals. shows strong charge localization associated with the frontier orbitals. [7] This one-electron reduction of CO2 to CO2 •-however, is a highly endergonic reaction occurring at a reduction potential of E° = -1.90 V vs SHE, due to a large reorganizational barrier between the linear molecule and bent radical anion. Multiproton and multi-electron steps are more favorable than single electron reductions because they can lead to thermodynamically more stable molecules like carbon monoxide (CO), formic acid (HCOOH), formaldehyde (H2CO), methanol (H3COH), and methane (CH4) as products (Table 1-1). This thermodynamic ease comes however with a kinetic cost of bringing together all the protons and electrons in appropriate pathways to result in the desired reaction. As such, to transform CO2 by overcoming the thermodynamic and kinetic barriers of its reduction, catalysts are needed to stabilize intermediate transition states. In addition, energy is still needed to be introduced into the reaction mixture to drive the reduction reactions because of the intrinsic overpotential of catalytic systems. Ideally, the energy for this process will be supplied by a renewable source such as solar energy. The energy-rich reduction products then may serve as energy source in a closed carbon-neutral cycle.

Though products such as methanol and methane are more readily useful as fuels, direct reduction of CO2 to such products necessitates multiple electron and proton transfer and the kinetic barriers associated with these are very high. In comparison, the "considerably easier" strategy of 2-electron, 2-proton coupled reduction of CO2 to CO is equally valuable as CO can then be used in the wellproven d-metal catalyzed Fischer-Tropsch technologies to convert synthesis gas (CO and H2) to liquid hydrocarbons such as diesel fuel and gasoline. [8] Carbon monoxide also has a significant fuel value (∆Hc 0 = -283 kJ/mol) and can readily be converted into methanol (e.g., by the CuO/ZnO/Al2O3-catalyzed process) for use as a liquid fuel. Furthermore, CO serves as an important feedstock in industrial processes such as the production of acetic acid by Cativa-Monsanto processes, [9] purification of nickel in the Mond process, [10] and the production of phosgene, [11] useful for preparing isocyanates, polycarbonates, and polyurethanes. Indeed, in a technoeconomic perspective, the transformation of CO2 molecules to short-chain simple building blocks is currently the most economically compelling target because of the current efficiencies and selectivities of catalytic systems. [12] 1.

Artificial Photosynthesis as an Innovative Solution

A deeper look in the CO2 concentration trends in Figure 1-2 already reveals the potential of natural systems for sequestering CO2. The high concentrations during winter are significantly lowered when the first signs of spring arrive due to the process of photosynthesis. Plants, algae, and most autotroph bacteria are known to use free and sustainable resources such as solar energy and water to capture and store CO2 in the form of carbohydrates thereby providing essentially all energy input into the biosphere. This biological process provides an interesting strategy by employing sustainable solar energy to transform CO2 into an energy vector, closing the carbon cycle and alleviating global warming. [13] This process is also the basis of the field of artificial photosynthesis. By learning and mimicking the natural photosynthetic systems, artificial technologies can be designed to produce renewable solar fuels and synthetic building blocks from CO2 as a feedstock and as such, address at the same time the intermittency and storage concerns of solar energy harnessing, as envisioned in Figure 1234. arranged in an antenna-array configuration to efficiently capture solar energy and relaying them to the reaction centers. [14] The reaction centers use this excitation energy to transfer electrons from donors to acceptors, creating energetic chargeseparated states that resist recombination long enough to allow migration of the oxidizing and reducing equivalents to the catalytic sites. Water oxidation to molecular oxygen occurs in the manganese-calcium complex of the photosystem II (PS II) reactions while the electrons are delivered through a series of uphill and downhill steps to generate energy-rich intermediates in the photosystem I (PS I)

reactions. The process results in the synthesis of the biological two-electron reducing agent nicotinamide adenine dinucleotide phosphate (NADPH) and generates a transmembrane proton gradient that drives the production of adenosine triphosphate (ATP). [15] Both NADPH and ATP are then used in a series of lightindependent reactions to form C-C covalent bonds of carbohydrates. In the artificial system, the goal is to design a photochemical cell that similarly contains a photosensitizer that captures solar energy, creating charged separated states. The holes are used by an oxidation catalyst to oxidize water to molecular oxygen producing electrons and protons which are then utilized by a reduction catalyst to reduce protons to dihydrogen gas or reduce carbon dioxide to produce high-energy chemicals called solar fuels, as shown in Figure 1-6. At the core of this strategy is the fundamental understanding on how the energy of a photon of light is transformed and leveraged to drive redox reactions that are otherwise endergonic in the absence of light. As such, this presents an interesting way of harnessing and storing the intermittent renewable solar energy as chemical energy which can be exploited at the later part of the process chain.

Enzymatic Conversions of CO2

In nature, there are six pathways known to fix carbon dioxide as organic material for biomass. The involved enzymes perform carboxylation of various sugars, coenzyme-A (CoA) esters and carboxylic acid, and reduction of CO2 (to CO or formate). [7] The dominant process is the reductive pentose phosphate (Calvin- precursor for a series of interconversions to form the six-carbon sugar fructose-1,6bisphosphate). [16] The other five pathways all involve net reactions of producing acetyl-CoA from CO2 for anabolic (biosynthesis) and catabolic (energy) purposes:

reductive acetyl-CoA pathway, reductive citric acid cycle, dicarboxylate/4hydroxybutyrate cycle, 3-hydroxypropionate/4-hydroxybutyrate cycle, and 3hydroxypropionate bi-cycle. [17] Though the thermodynamically favorable multi-electron reductions of CO2 are very important reactions, they are surprisingly not observed in natural systems, which instead use discrete two-electron reduction steps. For example, the Wood-Ljungdahl pathway for autotrophic growth and acetate synthesis involves initial two-electron reduction steps: the CO dehydrogenase (CODH)-catalyzed reduction of CO2 to CO and the formate dehydrogenase (FDH)-catalyzed reduction of CO2 to formate. Similar discrete two-electron steps are involved in the eight-electron reduction reactions observed for methanogenic archaea catalyzing CO2 to methane and for acetogenic bacteria catalyzing reduction of CO2 to acetic acid. Reasons for this strategy include (i) the versatility of these two-electron reduced intermediates to branch off into various pathways for the synthesis of cellular metabolites, and (ii) the final products (e. g. methane, acetic acid) are usually just by-products in the overall energy conservation scheme. [18] As such, fundamental learnings from the biological two-electron reduction of CO2 to CO and formate would be particularly interesting in designing synthetic CO2 reduction catalysts.

CO-dehydrogenase (CODH) catalyzes the two-electron, two-proton interconversion between CO2 and CO. active site. [7] The [NiFe]-centered active site catalyzes the reversible reduction of CO2

to CO with turnover frequency (TOF) of 45 s -1 for CO2 reduction and TOF of 40,000 s -1 for CO oxidation. [19] Crystallographic studies of the oxidized form of this [NiFe] C

cluster site shows a Ni center bridged by an Fe3S4 electronic buffer that rigidly positions the Ni metal in close proximity to an Fe metal (structure 1 in Figure 1234567).

The Ni species is coordinatively unsaturated binding three sulfur ligands in a distorted T-shaped configuration. In its reduced form (reaction with Ti III citrate) and in the presence of CO2 substrate (through bicarbonate ion source), CO2 is observed to bridge the Ni and Fe metals, as shown by structure 3 in Figure 1-7. CO2 is bound to the reduced, formally Ni 0 metal by the C atom completing the square planar coordination of Ni while the carboxylate oxygen atom is bound to the pendant Fe atom, as well as forming hydrogen bond with a lysine (Lys) residue. In addition, the second carboxylate oxygen atom seems to be within a hydrogen bonding distance from a protonated histidine (His) residue. This bifunctional activation, reminiscent of a frustrated Lewis acid-base pair, works in synergy with the second coordination sphere activation to cause only minor changes from the T-shaped coordination geometry to square planar. [20] This structure promotes the heterolytic cleavage of the C-O bond, resulting in a CO coordinated to the Ni metal and a hydroxide coordinated to the Fe metal. The CO is then released from this Ni center to form back the oxidized state, completing the proposed mechanism in Figure 1-7. [7] The low reorganizational energy observed from these experimentally-determined intermediate structures explains the high turnover rates of the enzyme. [MoCu] CODH has similar structure to that of the [NiFe]-centered CODH with the presence of heterobimetallic metal centers in the active site though it is observed to only catalyze the oxidation of CO to CO2 at a much lower TOF of 100 s - 1 . [21,22] The oxidized form of the active site shows a Mo metal with an apical oxo group linked to two sulfur atoms of the molybdopterin cytosine dinucleotide cofactor, as shown in structure 5 in Figure 1-8. The second coordination sphere similarly contains amino acid residues such as glutamine and glutamate that are within hydrogen bonding distances from the oxo and hydroxyl ligands, respectively.

Based on n-butylisonitrile-inhibited enzyme studies, [22] the reaction of a Mo VI oxo/hydroxy species with CO leads to the formation of the reduced Mo IV state with a thiocarbonate insertion product, structure 6. This is a distinct difference with the

[NiFe] CODH as the carbon atom of the carboxylate-like intermediate is bound to the ligated sulfido group acting as the Lewis base, instead of the reduced Mo metal center. [7] Figure 1-8. Proposed structure-based mechanism for [MoCu] CODH towards oxidation of CO to CO2. [22] A similar sulfur-rich Mo-centered active site is found in the air-sensitive formate dehydrogenase (FDH). These enzymes catalyze both the reduction of CO2 to formate with a TOF of 280 s -1 and the oxidation of formate with TOF of 3400 s -1 . [23] The oxidized form of the active site contains a Mo VI metal center in a distorted trigonal prismatic geometry with four S atoms from two pyranopterin ligands, a selenium atom from a selenocysteine residue, and a sulfur atom in the sixth coordination site, as shown in structure 8 in Figure 1-9. In its reduced form, a square pyramidal coordination is produced with an apical sulfur atom and Chapter 1

13 decoordinated selenocysteine ligand. Based from nitrite-inhibited enzyme studies, one oxygen atom of the CO2 substrate is bound to the Mo metal while the other oxygen atom is in close contact with an arginine (Arg) residue assisting to orient the intermediate for proton delivery by the protonated histidine (His) residue, as shown in structure 11. [24] After this protonation step, the formate product is released, producing back the oxidized form and completing the proposed catalytic cycle in These enzymes that catalyze the two-electron and two-proton reduction to either CO or formate seemingly contain similar features. First, they consist of sulfur-rich ligand structures that can store and supply electrons (Fe3S4 cluster, and pterin-like cofactors). Second, they employ a substrate-activating environment either by bifunctional Lewis acid-base pair or second coordination sphere activation by nearby amino acid residues. Third, they promote good CO2 binding in meticulously pre-arranged active sites with minimal changes in the geometry, resulting in low reorganizational energies. Lastly, they take leverage of the flexibility of their aqueous environment to manage the proton supply, and promote protoncoupled electron transfer steps, significantly helping in the thermodynamic requirement of the reduction reactions.

Bio-inspired Strategy

Inspired by the ingenious scheme of natural photosynthesis in harnessing solar energy and the efficiency of enzymes in selectively performing discrete twoelectron CO2 reduction steps, we envisioned a bio-inspired strategy taking into account key insights from these natural systems. A modular molecular approach strategy was chosen to systematically design and investigate photo-and electrocatalysts for CO2 reduction. As shown by the general scheme in Figure 1

State-of-the-Art

Following the bio-inspired strategy, a look on the current state-of-the art on both photo-and electro-catalytic systems would provide a background on where our approach will commence. Extensive reviews [25][26][27] have been published in the literature but here it will be limited to the following: i) summary of photosensitizers and electron donors used in photo-catalytic systems, ii) summary of common structural motifs of CO2 reduction catalysts, and iii) summary on catalysts employing second coordination effects.

Evaluating Catalyst Performance

The difference in the photo-and electro-catalytic approach requires a look on the different criteria used to compare catalysts performance. Generally, a better performing catalyst should have high activity, high energetic efficiency, high stability, and high selectivity,

The activity of the catalyst is often reported by calculating the rate of the catalytic reaction in the form of turnover frequency (TOF) with a unit of s -1 . In a photo-catalytic system, this is measured by the initial linear slope of the plot between amount of products normalized by the amount of catalyst in the solution and the irradiation time. In an electro-catalytic system, a reaction layer is considered because not all of the bulk concentration of the catalyst participates in the reaction. The catalytic rate is measured based on the amount of active catalysts in the vicinity of the electrode that participates in the catalytic cycle at given point in time. This is determined by foot-of-the wave analysis of the cyclic voltammogram of the catalyst in a catalytic condition (presence of CO2 substrate and proton source). This is described in detail in the Supplementary Information in the Annex.

The higher the TOF, the more products are formed per unit of time.

The stability of the catalyst strongly depends on a variety of deactivation pathways such as decomposition, deleterious side reactions, and product inhibition, but the overall performance is usually evaluated in terms of turnover number (TON). In a photo-catalytic system, this is simply determined by dividing the maximum amount of products formed by the amount of catalyst in the solution.

In an electro-catalytic system, the TON is determined from the calculated TOF but more accurately described by the trend in the current as a function of bulk electrolysis time.

The energetic efficiency refers to how a source of energy (either by a photon for photo-catalytic systems or an electrical energy for electro-catalytic system) is efficiently utilized to perform the reduction of CO2. In a photocatalytic system, it is reported in the form of quantum efficiency calculated by dividing the equivalent amount of electrons from the amount of product formed by the amount of photons absorbed by the system. The higher the quantum efficiency, the more efficiently a photon of light is utilized to form the product. In an electro-catalytic system, it is reported in terms of Faradaic efficiency and overpotential. Faradaic efficiency is calculated by dividing the equivalent amount of electrons from the amount of product formed by the total amount of charge passed during bulk electrolysis. The higher the Faradaic efficiency, the more efficient is the charge utilized to form the product. Overpotential is similarly calculated as it refers to the additional potential needed to be applied to the system past the thermodynamic potential of the reaction. A lower overpotential indicates a less energetic requirement to perform the reaction.

Lastly, selectivity is as important as the other criteria because it refers to how a preferred reaction is being followed and it would be indicative of the additional steps necessary to perform the separation of other undesired products.

In the photo-catalytic system, it is calculated by dividing the amount of the desired product by the total amount of products formed. It is similarly determined for the electro-catalytic system, but the calculated Faradaic efficiency would already give a hint on the product selectivity of the system.

These criteria are often used for catalyst performance comparison but when considering scalability and industrial applications, the cost of the catalyst is as well important. The choice of the metal for metal-centered catalysts is usually related to its abundance, with first row transition elements appearing to be cheaper choices.

There should be a balance of choice however, as the complexity of the ligand framework (indicating expensive synthetic routes) could counterbalance the cost of the metal. In the end, as long as the catalyst is stable and efficiently performing, it almost translates to upscaling with some cost modifications, which is beyond the scope of this comparison.

Photosensitizers and Electron Donors

The first step in the photo-catalytic system is the harvesting of solar energy creating charge-separated states. As such, photosensitizers should i) absorb in the solar irradiation spectrum (preferentially with high absorption coefficient in the visible region), ii) have a long-lived excited state for efficient quenching process, and iii) provide the necessary redox power (either in the excited state or one-electron reduced species) matched for an electron transfer to the catalyst. [25] K. [28,29] Some common and notable photosensitizers utilized for the photocatalytic reduction of CO2 are listed in Table 1-2 with the structures shown in Figure 123456789101112. Because of the forbidden nature of transition from 3 MLCT to the singlet ground state, this results in a long-lived emission with typical lifetimes from 600 to 1000 ns. This excited state lifetime is long enough to allow chemical interactions with other components in the solution. In photocatalytic CO2 reduction, this excited state is usually reductively quenched by an electron donor to form the bipyridinecentered one-electron-reduced (OER) species, formally denoted as Ru I . From Table 1-2, the typical [Ru(bpy)3] 2+ has a reducing potential of -1.71 V vs Fc +/0 , [30] thermodynamically sufficient to transfer an electron to some CO2 reduction catalysts. Modifying this photosensitizer by introducing methyl donors in the 4,4'position (structure 13) increases the reducing power to -1.83 V vs Fc +/0 [31] or replacing the bipyridine ligands with phenanthroline ligands (structure 14)

increases the potential to -1.79 V vs Fc +/0 , [32] showing the versatility of this class of complexes in driving the photo-induced electron transfer to CO2 reduction catalysts.

In addition, this class of photosensitizers has spectrally defined characteristics that permit tracking of electron transfer steps in the system by time-resolved absorption measurements. [a] acetonitrile, [b] dimethylformamide, [c] dimethylacetamide, [d] from the triplet state

Catalysts that operate at more negative potentials (i.e. more negative than - [36] It can be noticed however from as it may play some non-innocent roles in the system. [25,37] Some of the commonly used EDs for photocatalytic CO2 reduction are shown in Figure 1-13.

Ascorbate, or monodeprotonated ascorbic acid, denoted as Asc in Figure 1-13, possesses a sufficient potential to reductively quench the excited state of [Ru(bpy)3] 2+ and is often used as a reversible electron donor [38] in time-resolved photophysical measurements. It is not very often used in the longer durations of photocatalytic CO2 conditions as its use as ED does not efficiently translate to good photocatalytic activity. It is reported that the oxidized species of Asc can disproportionate to generate dehydroascorbic acid, which is a mild oxidant that can thermodynamically oxidize the reduced PS -or the reduced Cat -, i.e. a short-circuit reaction. [38,39] These aliphatic amines are sometimes considered two-electron and one-proton donors, as the radical formed after deprotonation of the one-electron oxidized species has a more reducing power than the starting amine themselves. On the other hand, BNAH and BIH possess higher reducing power able to effectively quench the excited PS* (as such involved in most reductive quenching pathways).

BNAH is not as effective as BIH however because of the rapid dimerization of the deprotonated oxidized radical, and as such, it only acts as a one-electron oneproton donor. [40] BIH has been recently widely utilized in most photocatalytic systems because of the highly reducing nature of the deprotonated one-electron oxidized radical BI • (-2.06 V vs Fc +/0 ), making it an effective two-electron one-proton donor. [41] This is adventitiously used to drive catalysts whose thermodynamic potentials are not matched for a photo-induced electron transfer (and accumulation) from a photosensitizer. cyclams & other aza-macrocyclic complexes, and phosphines. [26,42,43] In this work, we will review only some notable catalysts relevant to our employed experimental strategies, deeply investigating the rhenium-based bipyridine catalysts and iron porphyrin catalysts.

Rhenium triscarbonyl bipyridine complexes

Pioneering work by the group Lehn in 1980s have shown both electro-and photo-catalytic activity of Re(bpy)(CO)3Cl for the reduction of CO2 to CO. [44,45] The catalysts possess two reduction potentials: a reversible first reduction at -1.68 V vs Fc +/0 attributed to a bpy-centered reduction and a second irreversible reduction at -1.98 V vs Fc +/0 attributed to a Re I/0 metal centered-reduction. Initial reports have shown the possibility of a one-electron and two-electrons pathway mechanisms for the reduction of CO2 to CO by this type of catalyst, depending on the applied potential [46] and solvent conditions [47][48][49] . In the one-electron reduction pathway (left cycle of Figure 1-15), after the first reduction of the catalyst and halide loss, it was proposed that a [Re(bpy)(CO)3] • radical reacts with two CO2 substrates to form CO and CO3 2-. Later studies confirmed the formation of a CO2-bridged dimer intermediate, [50] in the absence of proton source, and proposed a bimetallic disproportionation with another CO2 molecule leading to CO and CO3 2-. [48,51] Figure 1-15. Proposed one-electron and two-electron pathway mechanisms for the reduction of CO2 by Re(bpy)(CO)3Cl. [46][47][48][49] On the other hand, the two-electron reduction pathway (right cycle of Figure to CO. [46] This was later confirmed by IR spectroelectrochemistry and it was reported that such intermediate was necessary for the interaction with CO2 in acetonitrile. [47] Indeed the initial electrolysis experiments were done on the second reduction peak to produce higher current densities and high Faradaic efficiency for CO. [45] It was further confirmed by the group of Kubiak that such a doubly-reduced intermediate showed higher affinity for a CO2 substrate even in the presence of excess protons in the solution, resulting in a highly selective reduction towards CO, preventing the competing proton reduction. [49,52] Since then, many studies have utilized such rhenium-based catalysts and implemented a variety of modifications in the bipyridine ligand, the halide ligand and the metal center. The group of Alberto in 2006 [53] have systematically changed the diimine ligand from bipyridine (bpy) to phenanthroline (phen), dipyridoquinoxaline (dpq), and dipyridophenazine (dppz), and showed that increasing the aromaticity of the diimine significantly lowered the first reduction potential, as shown in Figure 1-16. This stabilization is due to the delocalization of the electron in the extended aromatic rings, helping to stabilize electron accumulation. This did not translate however to better photocatalytic activity towards CO2 to CO, as shown by the lowered TON, indicating that indeed the catalytic activity is not significantly dependent on the bpy-centered reduction. The group of Kubiak recently published another systematic study investigating the effect of 4,4'-substitutions in the bipyridine ligand of the Re catalyst. [54] The electronic effects of the nature of the para substituents, such as methoxy, methyl, tert-butyl, trifluoromethyl, and cyano groups, on the bipyridine ligand is clearly reflected in the linear relationship between the first reduction potential and the Hammett parameter as shown in Figure 1-17a. This indicates that the electronic character of the bpy ligand indeed affects the bpy-centered reduction potential. However, this electronic character does not necessarily translate to improved electrocatalytic activity which occurs at more negative potentials. It can be observed from the linearity of the plot of the catalytic overpotential vs log TOF in Figure 1-17b, that the improvements observed in the TOF are mainly dependent on the linear free energy relationship with the overpotential. The substituent effects do not correspond to the same trend observed for the electronic effects on the first reduction potential, indicating again that catalytic activity goes beyond the bpybased reduction. Though the tert-butyl substituted Re catalysts was initially reported as the most active [55] because of the steric bulkiness preventing deactivation by catalyst dimerization, it seems from the plot that this effect is mainly due to the electron donating ability of the group. The observed increase of the TOF comes with the cost of the higher overpotential. It was interesting to note that though CF3 and CN substituents follow this linearity, they are not stable and quickly deactivate during bulk electrolysis. This was reasoned out to be due to the highly destabilizing state of the extra electron density in the bipyridine ligand, leading to catalyst degradation. [54] In addition, their Faradic efficiencies for CO production are very low (3%) compared to the high Faradaic efficiencies of the unsubstituted Re catalyst (100%), indicating mechanistic deviations for the typical product formation, e.g. cyano groups react with CO2 to form carboxylamides. [56] Data from [54] .

The axial ligand was also systematically studied using Cl, Br, H2O, SCN, CN, P(OEt)3, CH3CN, and pyridine ligands as shown in Table 1-3. [53,54,57,58] Replacing the halide ligand with a H2O ligand results in a cationic complex, and as such, this complex shows the most anodically shifted first reduction potential (-1.55 V vs Fc +/0 ). This does not translate however to an improved photocatalytic activity as shown by the lower photocatalytic TON (1.8). A much better improvement was noted with a SCN ligand due to the long-term stability of the resulting complex. This enhanced stability was reasoned to be coming from the suppression of the formation of a catalytically inefficient formato complex [Re(HCOO)(bpy)(CO)3] as an undesired side product in the halide-coordinated Re catalyst. [53] Recently, the group of Kubiak showed that the cationic complexes with CH3CN or pyridine as labile ligands made it also easier to reduce the modified bpy ligand compared to neutral complexes. This didn't significantly improve the catalytic overpotential nor the electrocatalytic TOF, concluding that the ligand loss does not determine such described parameters. This is in contrast to the photocatalytic reactions where the loss of the halide ligand is key to the reactivity. [59] CH3CN -1.73 -3.92 [54] pyridine -1.68 -3.90 [54] The effect of the metal center was also studied by substituting the Re metal with the isoelectronic Mn [61][62][63] , as shown in Figure 1-18. The Mn alternatives was first reported by the group of Deronzier [62] with Mn(bpy)(CO)3Br and Mn(dmbpy)(CO)3Br and showed reasonable efficiencies and selectivities, with lower overpotential compared to the Re analogue. [64] Various modifications were similarly performed in the bipyridine ligands [61,63,65,66] to improve the catalytic activity of the Mn catalyst. The best performing catalyst so far (in terms of TOF and overpotential)

is the Mn complex possessing bulky mesityl groups in the 6,6' position of the bipyridine ligand that prevents dimer formation. [61] A systematic study was also done investigating W-and Mo-centered complexes with (tBu-bpy)(CO)4 ligand structures, [67] as shown in Figure 1-18. These group 6 transition metals are often observed in natural systems and the dianionic forms of these complexes are formally isoelectronic to the pentacoordinated active form of the group 7 (Mn and Re) catalysts. These complexes were shown to be active towards reduction of CO2 to CO but suffered from low TOF mainly attributed to the stronger back donation to CO compared to the group 7 counterparts.

Re(tBu-bpy)(CO)3Br Mn(tBu-bpy)(CO)3Br Mn(mes-bpy)(CO)3Br W(tBu-bpy)(CO)4 Mo(tBu-bpy)(CO)4

Log The photocatalytic activity of the Re catalyst has been greatly improved when photosensitized in the visible region through the construction of supramolecular catalysts with a Ru-based photosensitizer unit. [25,31,43,[68][69][70][71][72] This has been extensively investigated by the group of Ishitani who has shown that these supramolecular photocatalysts are performing much better compared to separate bimolecular units. [31] From TON of 5-20 when the photocatalytic activity was first published, current improvements show TONs of ca. 3000, as shown in Table 1-4. [73] These systematic studies have arrived at some important conclusions concerning the molecular architecture for better catalytic activity. First is the establishment of directionality of electron flow by having the π* orbital energy of the peripheral ligand of the PS to be equal or higher than that of the bridging ligand. [68] Second is the establishment of a sufficiently high π* orbital energy of the bridging ligand to maintain the high reducing power of the catalyst towards CO2 reduction. As such, establishment of conjugated ligands, which anodically shifts and stabilizes the potential of the catalyst, are not recommended as it consequently lowers the reducing power of the catalyst. [72] Most improved systems involve the use of short aliphatic bridges [31,68,70,73] and the highly efficient two-electron one-proton donor BIH. [73] Table 1-4. Photocatalytic performances of supramolecular catalysts. [25,43] Chapter 1

Structure R Z L L' ED TON Ref 1 X = (4dmbpy)2Ru, Y = Re(CO)3Cl BNAH 14 [68] 1 X = Re(CO)3Cl, Y =(4dmbpy)

29

The synthetic tunability and mechanistic flexibility (either through electro-or photo-catalytic pathways) of the Re polypyridyl-type complexes has indeed resulted in vast investigations and optimizations. Some mechanistic studies have been provided both experimentally and theoretically but the sensitivity of mechanistic pathways on various modification strategies has made it difficult to generalize certain criteria for the improvement of catalytic activities. In addition, there is a lack of bridge between the trends observed and optimized in electro-catalytic measurements with that of the photo-catalytic measurements, possibly because of the intrinsic differences of catalyst activation. This might indicate an interesting point for further understanding and investigation.

Iron porphyrin complexes

Another well-studied and efficient CO2-to-CO reduction catalyst is the class of iron porphyrins, pioneered and extensively investigated by the group of Savéant. [74] Addition of weak Brönsted acids such as water, trifluoroethanol, and phenol [75] or addition of any Lewis acids like magnesium cations, [76] have shown to greatly improve the catalytic activity. In a general proposed mechanism, the iron (III) tetraphenylporphyrin chloride, shown in Figure 1-19, undergoes three reversible reductions as evidenced by three reversible peaks in its cyclic voltammetry. This corresponds to the formal reductions from Fe III to Fe II to Fe I and finally to the active form of Fe 0 . Recent studies have suggested that the last two additional electrons reside in the π* orbitals of the porphyrin ligand. [77] As such, the last two reductions can be formulated as calculations have shown that reaction goes through the Fe I asymmetrical adduct rather than the Fe II symmetrical adduct because of the asymmetrical roles of the carbon-oxygen bond. [75] This adduct is stabilized by a hydrogen bond donor such as weak acids and further protonation from a second acid molecule leads to C-O bond cleavage. CO is then released after homogeneous one-electron reduction of the Fe II -CO adduct by a Fe 0 species. Recent studies have confirmed such mechanism by employing resonance Raman spectroscopy at low temperature. [78] The group has also developed benchmarking strategies to critically compare electrocatalytic CO2 reduction performance by a simple technique of cyclic voltammetry (CV) and the corresponding analysis of catalytic Tafel plots from the foot-of-the-wave of the CV. [75,[79][80][81] This provided a quick estimation of catalyst performance independent of the side phenomena that interfere at high current densities. [79] Catalytic Tafel plots correlate the turnover frequency to the overpotential as shown in Figure 1-20a with a corresponding linear correlation in Figure 1-20b. [82] For example, the modification of the porphyrin ligand by throughstructure substituents affects the electronic structure of the catalyst, resulting in correlated changes with respect to both the overpotential and TOF. The addition of electron withdrawing groups such as fluorine atoms is expected to lower the overpotential because it lowers the electron density near the metal active site. Thus, it is easier to inject an electron into the catalyst, shifting the reduction potential anodically, resulting in a lower overpotential. This can be observed for the addition of increasing number of fluorine atoms from 5 to 10 to 20, resulting in a lower overpotential compared to the nonfunctionalized FeTPP. [82,83] This comes however, with a decrease in the catalytic activity due to the decrease in the affinity between the active catalyst (now with decreased Lewis basicity) and the substrate in the framework of an electronic-push-pull mechanism. On the other hand, putting electron donating group, increases the Lewis basicity of the active site able to efficiently transfer electrons to the CO2 substrate. This comes however with cost of more negative catalytic potentials. This implicates that substituents that alter the electronic structure of the catalyst favor one parameter but disfavors another one. Similar linear scaling between the overpotential and TOF was observed for modifications in the Re-based catalyst, as shown in Figure 1-17. This suggests that in order to achieve a better catalytic performance, with higher catalytic rates at lower overpotential, strategies should go beyond the electronic through-structure effect. Substantial improvements were indeed observed when employing throughspace substituent effects stabilizing or activating key reaction intermediates. This strategy has already been well-documented to be at work in the catalytic sites of enzymes known to perform at much higher efficiencies and stabilities. Employing such strategy in catalyst design would then require careful optimization of the second coordination sphere of the catalyst to incite such through-space interactions.

[

Catalysts Employing Second Coordination Sphere Effects

Second coordination sphere relates to the distal structure of the catalyst beyond the primary coordination sphere of the ligand structure with the metal center that may influence metal-bound intermediates and may or may not affect the electronic structure of the catalyst. Strategies relying on such effect have initially been extensively implemented in various synthetic catalysts for the reduction of oxygen and reduction of protons, mimicking features observed in natural systems.

Recently, this has been applied to the development of CO2 reduction catalysts and specific approaches can be generally categorized in the following: establishing local proton sources, tethering hydrogen bond relays, leveraging on cationic groups, and employing bimetallic structure.

Local proton source

The observation that addition of weak Brönsted acids [75] enhanced the catalytic reduction of CO2 already hinted the possibility of placing local acid proton sources in the periphery of the catalyst. This has been initially reported by the group of Savéant by placing pendant hydroxyl groups in FeTPP [84] as shown in structure 20 in Figure 1-21. These pendant proton donors resulted in enhanced catalytic activity (log TOFmax of 3.8) and a decrease in overpotential (η of 0.66 V) in DMF with 2 M H2O. Control experiments with methoxy (-OCH3) groups instead of the -OH groups showed a poorer activity (log TOFmax of 2.5) at a much larger overpotential (η of 1.04 V). This highlights the crucial role of high local concentrations of phenolic protons in a push-pull (electron-proton) mechanism towards metal carboxylate intermediates. [75] Nocera extended the investigation by establishing pendant acid sources (phenolic and sulfonic groups) in an iron hangman porphyrin configuration, [85] shown in structures 21 and 22 in Figure 1-21. The pendant -OH groups caused a thermodynamically favored -5.0 kcal/mol stabilization of the CO2 adduct. However, the introduction of sulfonic groups didn't improve the catalytic activity of the system because once deprotonated, they cannot be reprotonated by the weaker external PhOH donor (pKa of SA is 3 vs pKa of PhOH is 18 in DMF [86] ). The steric congestion and electrostatic repulsion from the negatively charged sulfonate resulted in a mismatch of orientation where the sulfonic groups are no longer pointed towards the bound CO2 substrate. This highlights the importance of external acid sources in regenerating the pendant local proton sources to maintain the secondary sphere coordination effect. The -OH bonds in the bipyridine ligand are cleaved by stepwise reduction, which was proposed to dearomatize the doubly-reduced bpy ligand. The slow CO release and decomposition of the intermediates at applied potentials limit the activity of the 6,6' substituted complex, highlighting the importance of proper positioning of these local proton sources. Asymmetric attachment of phenol moieties has been employed

in Mn-based catalysts (26 and 27 in Figure 1 -22). This led to enhanced activity in comparison to the nonfunctionalized Mn catalyst, [87,88] including the activity in the absence of proton source. [87] Control experiments with methoxy groups showed insignificant catalytic activity, again highlighting the enhancement specifically made by the local phenolic protons. The competitive dimerization was, however, still apparent in both modified catalysts. Similar phenolic ligand structure was employed in a Re-based catalyst which prevented the dimerization usually observed for Mn-based catalysts, but upon photocatalysis, it showed higher selectivity towards formation of formate (TONHCCO-of 86, selectivity of 75 %) rather than CO, indicating the mechanistic pathway deviation caused by such bpy modification.

Carboxylic acids have been tethered in Re-based catalyst, shown in structure 25 [53] ,

but didn't improve the photocatalytic activity (TONCO of 7.7) compared to the nonfunctionalized Re catalyst (TONCO of 13). Such groups have also been asymmetrically positioned in a Ni cyclam catalyst (adsorbed in Hg-Au amalgam electrode) as shown in structure 28 [89] which showed improved electro-catalytic Chapter 1 34 activity towards reduction of CO2 to CO in water compared to the nonfunctionalized catalyst. The enhanced activity was attributed to its stability and selectivity for CO2 reduction to CO even at pH 2, overcoming the competitive proton reduction. [89] Figure A bio-inspired approach was employed in Re-based catalysts by the group of Kubiak by incorporating peptide linkages of varying lengths containing tyrosine residues, [91] as shown in structures 29 and 30 in Figure 1

Hydrogen bond

Another approach for second coordination effects does not necessarily place local proton sources in the periphery of the catalyst, but places functional groups able to establish hydrogen bonding interactions either with the metal carboxylate adduct or with the external proton sources to synergistically stabilize CO2 reduction intermediates. This has been observed in Ni-cyclam catalysts, one of the first known catalysts to electrochemically reduce CO2 to CO in water. [92][93][94] DFT studies by the group of Kubiak have shown that a trans I isomer of the catalyst as shown in structure 31 in Figure 1-24, one of the six possible isomers based on the orientation of the H atoms on the four metal-coordinated secondary amines in the macrocycle, is more favorable in stabilizing a CO2 substrate by hydrogen bonding interactions. [95] Further DFT studies by the group of Ye have shown that this isomer (four cofacial H-atoms) is greatly stabilizing the CO2 adduct compared to the trans III isomer (two cofacial H-atoms) by approximately 3 kcal/mol. [96] N-alkylated A systematic study was performed by the group of Marinescu on the effect of pendant secondary amines on a cobalt pyridine macrocycle (structure 32 in Figure 1-24) that showed a linear dependence of catalytic rate on the number of pendant hydrogen bond donors. [97,98] Unlike the cyclams, the pendant amines lie completely outside the primary coordination sphere of the metal allowing isolated roles of the amines as hydrogen bond donors facilitating non-cooperative hydrogen bonds with the proton source in the solution. However, this improved catalytic activity comes with the cost of high overpotential (1080 mV). Kubiak reported a simple acetamidomethyl group in the 4 position of the bpy ligand (structure 33) which shifted the potentials anodically (220 mV shift for the first reduction and 440 mV shift for the second reduction) compared to the nonfunctionalized catalyst. [91] This positive shift in the potential was similarly observed if two acetamidomethyl groups were incorporated on the 4,4' positions of the bpy ligand. [51] The enhancement was mainly attributed to the formation of hydrogen-bonded dimers which catalyze the reductive disproportionation of CO2 to CO and CO3 2-in a bimolecular mechanistic pathway. Thus, the H-bond donors in the amide moieties do not directly interact with the metal carboxylate intermediate. In a recent report, the group of Neumann has asymmetrically tethered thiourea functions in the periphery of the Re catalyst, [99] as shown in structure 34 in Figure 1-24. 1 H NMR measurements showed that the proximal hydrogen atom of the thiourea moiety directly binds to the oxygen atom of the CO2 substrate. In this way, the thiourea group can effectively bind CO2 and stabilize the carboxylate intermediate and at the same time, act as a local proton donor. [99] However, addition of any external proton source (i.e. water) inhibited the catalytic activity of the system, likely interfering with the hydrogen bonding interactions pre-established by the thiourea group on the CO2 adduct. V) is better than a proximal one (ortho-1-amide, structure 37, log TOFmax of 4.35, η of 0.78 V). [100] This shows that proper positioning of the amide functions is critical in establishing suitable hydrogen bonding interactions with the metal carboxylate intermediate. They all improved the catalytic activity compared to nonfunctionalized FeTPP but the overpotentials were still not significantly improved. Recently, the group of Dey has synthesized a picket fence iron porphyrin that contains proximal amides, as shown in structure 38. This resulted in an improvement of the catalytic activity in the presence of 3 M phenol (log TOFmax of 5.71, η of 0.62 V) compared to the single-point amides in structure 37. This shows that aside from proper positioning, the number of such pendant amido groups can also affect the catalytic activity of the system.

Cationic Moieties

Substantial improvements were observed with the incorporation of cationic functionalities like trimethylammonium groups in iron porphyrin catalysts reported by the group of Savéant. [83] The ortho-positioning (structure 39, Figure 1-26) of the through-space electrostatic interactions between the positive charges of substituents and the negative charge developed in the metal carboxylate intermediate resulted in a significant decrease in overpotential (η of 0.25 V) while simultaneously increasing the TOF (log TOFmax of 6). [102] This αβαβ atropoisomer configuration of the substituents in the iron porphyrin was even better than the diffused electrostatic effects in the para-substituted alternative (structure 40), Recently, the group of Nippe even combined the synergistic effects of H-bond donors and electrostatic interactions by establishing imidazolium moieties in the periphery of the Re and Mn triscarbonyl bipyridine catalysts (structure 42 and 43

highlighting
in Figure 1-27). [103,104] Since this class of catalysts involves the loss of halide during the catalytic cycle, it was proposed that the C2-H carbon of the imidazolium moiety interacts with the halide promoting its release upon first reduction. Addition of another electron forms the doubly reduced pentacoordinated active species, which in the presence of CO2 forms a metal carboxylic acid intermediate. Subsequent addition of electrons and protons (from a water network) results to a tetra-carbonyl species, which upon reduction and CO dissociation regenerates the initial reactive species. [104] This resulted in an improved catalytic activity (log TOF of catalyst 42 is 2.44 at a scan rate of 4 V/s in ACN with 2.8 M H2O and η is 0.46 V; no data retrieved for catalyst 43) compared to the nonfunctionalized catalyst. In a control experiment, when the imidazolium C2-H was replaced with C2-CH3, the catalytic activity was lower indicating the critical role that C2-H plays in the catalytic cycle. [103] Though the overpotentials are lowered for these modified catalysts, the TOF and Faradaic efficiency for CO (77 % for catalyst 42 and 70 % for catalyst 43)

are still relatively low. Furthermore, addition of higher concentrations of water (beyond 2.8 M) significantly decreased the activity of the catalyst 42 which is usually not observed for the nonfunctionalized Re catalyst. This is indicative that such strategy inadvertently resulted in major alterations in the mechanistic pathways.

Bimetallic strategies

Another approach for secondary coordination effects is through the bimetallic approach, which has been similarly observed in the enzymes performing CO2 reduction, as discussed previously. The bimetallic system works cooperatively to store charges and to activate the CO2 substrate in a classical push-pull donoracceptor configuration. An initial bio-inspired mimic was reported by the group of DuBois by synthesizing a bimetallic palladium phosphine complex (structure 44 in Figure 1-28. This was strategically done to overcome the low CO2 binding of the monomeric counterpart, which was identified to be the rate determining step of the system. The complex showed low overpotential but suffered as well from low turnover number because of the formation of a Pd-Pd bond, which is considered an inactive form. Similar bimetallic strategy was employed by Naruta in modifying the well-known CO2-reducing iron porphyrin catalysts in a cofacial configuration (structure 45). [105] Controlling the positioning of the porphyrin platforms in the phenyl linker made it possible to control the distance between the Fe metals. A higher catalytic activity (log TOFmax of 3.63, Faradaic efficiency of 95%, η of 0.7 V)

was achieved compared to the mononomeric catalyst with an ortho configuration of the dimer where the Fe-Fe distance is expected to be 3.2 -4.0 Ǻ, suitable for binding of the CO2 substrate. The overpotential of the system was further optimized using electron-withdrawing substituents on the phenyl rings of the porphyrin achieving an overpotential of 0.4 V with a log TOFmax of 4.2. [106] Figure 1-28. Bimetallic (homonuclear and heteronuclear) strategy for improving CO2 reduction activity.

A rigid homobimetallic Re-based catalyst dimer was also reported by the group of Jurss using anthracene as a bridge to maintain proper distances between the metal centers. [107] Electrochemical studies show distinct mechanisms for two isolated isomers of the dimers: the cis conformer (structure 46) works via a cooperative bimetallic CO2 activation and conversion while the trans conformer reduces CO2 through well-established single-site and bimolecular pathways. Higher catalytic rates are observed for the cis conformer (log TOF of 1.55) relative to the trans conformer (log TOF 1.36), outperforming the monomeric Re catalyst (log TOF of 1.04).

It was previously discussed that a similar Re-based hydrogen-bonded supramolecular dimer was formed in situ when mixing acetoamidomethyl-modified bpy ligands. [51] This dimer operated at 250 mV lower overpotential compared to the monomeric catalyst but still suffers from a low TOF and Faradaic efficiency as well.

The group of Kubiak has further optimized the system by synthesizing a heterobimetallic Re-Mn dimer (structure 47) just by mixing 1:1 equivalents of the Re-and Mn-centered acetoamidomethyl-modified ligands. Enhanced catalytic activity was observed and it was proposed that the Mn metal acts to activate the CO2 substrate while the Re metal acts to transfer the electrons to the substrate.

This similarly mimics the heterobimetallic activity observed in the [NiFe] CODH (Ni metal as redox active center while Fe metal as substrate-activating). Recently the group of Lu has synthesized a variety of cryptates holding Ni-Ni, [108] Co-Co, [109] and even a heterometallic Co-Zn [110] catalyst (Complex 48) which show improved photocatalytic activity compared to mononuclear counterparts. This catalyst resulted in a TON of 65000, TOF of 1.8 s -1 , and a 98% selectivity for CO when photosensitized by a Ru phenanthroline photosensitizer in the presence of triethanolamine as ED. Control experiments with Co-Co and Zn-Zn dimers showed the need for the synergistic effect of both metal centers where the Co metal acts as the redox active site while the Zn metal acts to activate the CO2 substrate. [110] These studies show great opportunities and promise for further optimizations and translations to other existing monomeric forms. Substantial mechanistic analysis is however needed to fully validate the proposed pathways by which these bimetallic catalysts operate.

Thesis Objectives and Scope

Considering the developments of catalyst design for CO2 reduction presented above, there still seems a lot to be understood and optimized. The current performances of both photo-and electro-catalysts are still not at par to be operationally scaled, requiring optimum efficiency and stability. In response to this challenge, this thesis will focus on developing catalysts for CO2 reduction by employing key insights obtained from natural systems known to sustainably utilize solar energy in transforming CO2.

Chapter 2 will focus on the fundamental understanding of how the energy of a photon of light can be efficiently transformed to a reducing power able to reduce CO2. This will entail validation and further investigation on a photocatalytic system composed of a ruthenium (II) trisbipyridine [Ru(bpy)3] 2+ complex as photosensitizer and a rhenium (I) triscarbonyl bipyridyl [Re(CO)3(bpy)Br] complex as catalyst.

Chapter 3 will focus on catalyst design that employs first coordination sphere modifications on a nickel (II) catalyst bearing a tetradentate N2S2 ligand towards photo-catalytic CO2 reduction. Investigations will employ bio-inspired sulfur-rich pentadentate coordinating ligands on nickel catalysts that will be systematically studied using time-resolved spectroscopic techniques.

Chapter 4 will focus on employing second coordination sphere effects on CO2 reduction catalysts similarly observed in the active site of CO dehydrogenase enzymes. This will involve the establishment of functional groups such as amino, urea, amido, and cationic imidazolium moieties in the periphery of rhenium triscarbonyl bipyridine and iron porphyrin catalysts. Systematic investigation will also deal with the specific pre-arrangement of these functional groups in the vicinity of the catalyst, critical for the activity.

Chapter 5 will focus on leveraging the carbon monoxide produced from these studied systems by mimicking the CO dehydrogenase/acetyl-CoA synthase partnership. This will involve a two-chamber system that immediately uses the photo-produced CO from one chamber for an amino-carbonylation reaction in the other chamber. This will indicate further opportunities and value for the catalytic reduction of CO2.

1

Chapter 3

Designing Catalysts:

First Coordination Sphere Modifications for the oxidation of CO and 45 s -1 for CO2 reduction. [2] These enzymes operate near and Rhodospirillum rubrum CODH (Figure 3-1c) [3] with varying distances between the Ni atom and the axially-positioned S* atom from the cubanoid [Fe3S4] cluster. [1] Though several of these structures display partial atom occupancies and disordered positions, the described core structural features are still maintained.

The most accurate definition of the biologically-active structure was reported in 2007 by the group of Dobbek as shown in Figure 3-2a where the coordinatively unsaturated Ni II species binds three S ligands in an apparent T-shaped environment. [4] Upon addition of CO2 to this reduced state, CO2 binds to the redox active Ni II acting as a η 1 OCO ligand and completes the square planar coordination geometry found for Ni ions, as shown in Figure 3 Fe1 acts as Lewis acid that stabilizes the negative partial charge on O1 of CO2. [5] The [Fe3S4] cubanoid cluster provides a metal-sulfur frame in which Ni and Fe1 are held in place and serves as an electronic buffer to compensate for the electronic changes during the catalytic cycle. The low reorganization energy arising from the small structural changes of the active site during the catalytic cycle ensures the high efficiency and stability of the enzyme. Closest synthetic models of the CODH C-cluster have been extensively investigated by the group of Holm even before the advent of the protein structures.

They successfully synthesized the first [NiFe3S4] cubane model, [6] complex 1 in planar Ni II species as shown by the Ni-S2P2 type [1,7] in complex 2 and Ni-S4 type [8] in Complex 3. However, no activity of these complexes for CO2 reduction was reported. Functional models mimicking the sulfur-rich environment around the metal centers are rare for CO2 reduction but were extensively investigated for hydrogenase mimics for proton reduction. [9] Functional models of metal-sulfur enzymes have been pioneered by the group of Sellmann [10] leveraging the unique electronic flexibility of sulfur ligands (such as sulfides, thiolates, and thioethers) which can accommodate both hard and soft ancillary ligands. Synthetic analogues of the [NiFe] hydrogenase have been well-reviewed by Pickett [11] and Bouwman [12] showing various macrocyclic and acyclic nickel complexes with sulfur ligands, as well as mixed-donor ligand structures containing oxygen, phosphorus, and nitrogen donor atoms. In 2011, the group of Eisenberg and Holland has reported the photocatalytic activity of a Ni-N2P2 type Dubois' electrocatalyst [13] when coupled with a Ru or eosin Y photosensitizer (PS) and using ascorbate as electron donor (ED). [14] A Ni-N3S3 type nickel pyridinethiolate catalyst, shown as complex 4 in Figure 3-4, was also reported by the same group for the photocatalytic H2 evolution in the presence of fluorescein as the PS and triethylamine as the ED. One of the coordinating N atoms was proposed to be hemilabile and to play the role of proton relay during the formation of the H-H bond. [15,16] This role was attributed to an S atom in the photocatalytically active nickel complexes with bidentate dithiolenes (Ni-S4 type shown in complex 5) reported by the group of Fan [17] Only recently, the group of Kojima published a Ni complex bearing thioethers in the form of tetradentate N2S2 ligands (complex 6) that showed photo-catalytic activity towards CO2 reduction to CO with a Ru PS and 1,3-dimethyl-benzoimidazole (BIH) as ED. [18] A later study then showed that the same catalyst is active towards photocatalytic proton reduction when done in an ascorbate-containing acetonitrile solution. [19] The group of Fontecave has similarly investigated sulfur-rich ligands by synthesizing Ni(bisdithiolene) catalysts with the Ni-S4 type of coordination (complex 7) which are capable of electrocatalytically reducing CO2 to formate with an Hg/Au working electrode. [20,21] These dithiolene ligands mimic the natural molybdopterin cofactor present in the active site of formate dehydrogenase. The same group has also reported a molybdenum complex consisting of similar dithiolene ligands which showed a photocatalytic activity for the reduction of CO2 to formate when coupled to a Ru PS and using BIH as ED. [22] These few reports on Ni-centered CO2 reduction catalysts containing a sulfur-rich environment seem promising for investigation of bio-inspired catalyst design.

Objectives

Inspired by the sulfur-rich environment found in the Ni-centered active site of CO dehydrogenase, we employed a comparative study on the catalytic activities of

Syntheses and Structural Characterization

The pentadentate ligands N2S3 and P2S3, as well as the tetradentate N2S2 ligand were synthesized following some modifications of reported methods. [23,24] As shown in Figure 3

Electrochemical Characterization

The redox behavior of the modified catalysts was characterized by cyclic voltammetry (CV) as shown in Figure 3 ). [23] Thioethers are considered as soft ligands, indicating high polarizability of the electron density of the ligand. Its availability in a mixed coordination environment (with hard ligands like pyridine) stabilizes the Ni I state. [25] A much greater stabilization was achieved when the pyridines are replaced with the softer phosphine ligands, achieving an anodic shift of 240 mV when comparing Ni-N2S3

and Ni-P2S3. This even resulted in the reversibility of the reduction peaks in Ni-

P2S3.

A second irreversible reduction is observed at -1.47 V (vs Fc +/0 ) for Ni-N2S2 The observed shifts for the second reduction peak are not as significant however, as the shifts in the first reduction peak.

E / V vs Fc +/0 Ni-N2S2 Ni-N2S3 Ni-P2S3
The CVs of the modified catalysts under a CO2-saturated solution in the presence of water as a proton source was recorded to determine their electrocatalytic activity towards CO2 reduction. As shown in Figure 345678, there are no significant current enhancements observed for all catalysts. For the reported Ni-N2S2, a small current enhancement is observed just before the second reduction peak, while it is observed only after the second reduction in Ni-N2S3 and Ni-P2S3.

Kojima has proposed that such Ni catalysts need to be reduced to their Ni 0 state to be active for CO2 reduction. [18] Comparing the CVs of these catalysts in Figure 3-8d, it shows that an earlier onset in current enhancement is observed for Ni-N2S2, though a greater intensity is observed at more negative potentials for the Ni-N2S3.

Even though much greater stabilization of lower oxidation states was achieved by

Ni-P2S3, it doesn't seem to show any improved CO2 reduction activity.

I / µA E / V vs Fc +/0 Ni-N2S2 Ni-N2S3 Ni-P2S3 (a) (b) (c) (d) Ni-N2S2 Ni-N2S3

Ni-P2S3

It was observed though that the modified catalysts, in the presence of acidic proton sources but absence of any CO2 substrate, showed some proton reduction activity. This was similarly reported by the group of Kojima when they later published the proton reduction activity of the Ni-N2S2 catalyst. [19] As shown in nickel dithiolate complexes. [26] The intensity of the current enhancement for Ni-P2S3

though is not at par with that of Ni-N2S2 and Ni-N2S3, indicating again that the extra stabilization gained by such complex does not improve its catalytic activity. The case of Ni-N2S3 is interesting however as both electronic and structural changes induced an anodic shift of the first reduction potential but a cathodic shift of the second reduction potential, which resulted to higher current enhancements for both proton and CO2 catalytic reduction activities.

Though the differences observed in these complexes are interesting to be further probed by electrochemistry-based techniques (e.g. spectroelectrochemistry), a weakly-bound film is always adsorbed at the glassy carbon working electrode after each CV measurement, preventing any accurate succeeding measurements. As such, this required careful rinsing and polishing of the electrode after each measurement. Roberts [27] and Dempsey [28,29] have attributed similar observation to the vulnerability of Ni-S bonds which leads to the electrochemical degradation of the catalyst in acidic media. The homogenous catalysts then become a precursor for a heterogeneous catalyst that is more active towards proton reduction. This observation precluded any accurate comparative studies using electrolysis, as it would be difficult to distinguish the homogenous nature of the catalysis from the heterogeneous activity of the adsorbed film. The photocatalytic activity of the catalysts was instead investigated as described in the next section.

Photocatalytic Activity

The photocatalytic activity of the catalysts was investigated because it allows tracking spectral changes of the catalyst in the solution. Though this method of catalyst activation may result in a different mechanistic pathway as it involves other players (photosensitizer and electron donor) than electrocatalysis, the reduction potentials determined from electrochemistry are still relevant and useful for determining the thermodynamic driving forces of the photocatalytic system.

The photo-induced CO2 reduction activity of the modified catalysts was investigated in a typical catalyst/photosensitizer bimolecular system using ruthenium trisbipyridine [Ru(bpy)3] 2+ as the photosensitizer and 1,3-dimethylbenzoimidazole (BIH) as electron donor in a CO2-saturated dimethylacetamide solution containing 5.5 M water as the proton source. These conditions were chosen to match those reported in literature [18] for easier comparison. As shown in Figure 3

Ni-N2S2 Ni-N2S3 Ni-P2S3

proton reduction activity was investigated with the optimized condition of using Ru(bpy)3] 2+ as the photosensitizer but with an aqueous ascorbate buffer of pH 4

(1:1 v/v buffer:ACN) as both proton source and electron donor. Though both Ni-N2S3 and Ni-N2S2 have sluggish catalytic activity compared to other Ni catalysts reported in the literature, [14,15,30,31] a better photocatalytic proton reduction activity is observed for the pentadentate-coordinated Ni-N2S3 with a TOF of 3.

Tracking the First Photo-Induced Electron Transfer

To rationalize the observed differences in performance between the catalysts Ni-N2S3 and Ni-N2S2, time-resolved UV-visible spectroscopy was employed to determine eventual differences in the photo-induced electron transfer dynamics.

The investigations were focused on these two catalysts as they show differing photocatalytic activities which could be related to changes in the primary coordination sphere. The initial photo-induced electron transfer was investigated using similar conditions as for preparative photocatalytic reactions: [Ru(bpy)3] 2+ This is still being pursued in our laboratory. The main difficulty is the need to optimize the yield of the first photo-induced event as a prerequisite to identify the spectral features of the doubly reduced Ni catalyst in a second photo-induced event.

This is critical since the Ni catalysts have intrinsically low absorption coefficients.

X-ray Absorption Spectroscopy Measurements

The photocatalytic investigations were then coupled with static and time- species with a square bipyramidal geometry, where the solvent is no longer coordinated. After this intermediate, proceeding steps are proposed based on initial observations from cyclic voltammetry measurements that a second electron transfer is needed to form the active form of the catalyst. This second electron transfer is expected to be coupled to a proton transfer for proton reduction activity, but this might not be the case for CO2 reduction activity [15,18,19] . The reaction sequence in this proposed mechanism need to be confirmed by further experimental and theoretical investigations. Though a strategy of modifying the primary coordination sphere of the nickel catalyst can in most cases shed light on the structural features of the active site of the CODH enzyme, it seems that systematic changes in such primary coordination positions does not necessarily translate to expected systematic improvements in the catalytic activity. This is in part due to the interplay of electronic and structural changes that might critically affect the reaction pathways of the catalyst.

Furthermore, the similarity of the sulfur-rich environment with the active site of hydrogenase enzymes makes such strategy limited in controlling and preventing the competitive proton reduction activity.

Bibliography

Introduction

The criticality of global warming effects due to increasing emission of carbon dioxide (CO2) is challenging scientists to advance research in the development of catalysts that can help capture, activate, and reduce CO2. An emphasis is currently being put on the discovery of new cost-efficient molecular catalysts to drive the electrocatalysis of CO2 reduction and ultimately to use solar energy to drive these energetically uphill chemical transformations. [1][2][3][4][5][6][7][8][9][10] Still, the route towards industrial upscale applications requires much improvement in the efficiency of these catalysts while maintaining their stability and selectivity.

Rational strategies for catalyst design and improvement hinge on what we continually learn from natural systems, as they exhibit exceptional stabilities and performance. Carbon monoxide dehydrogenase (CODH) stands as the typical inspiration for scientists working on the field as it is known to catalyze the reversible reduction of CO2 to CO. Lessons from the structure and function of this enzyme point out to some important features that can be artificially mimicked by catalyst design: (1) a bifunctional activation of CO2 substrate by two metal centers,

(2) an iron-sulfur cluster acting as an electronic relay and buffer to the catalytic unit and (3) precisely positioned amino acid residues to form a hydrogen-bond stabilization network with the CO2 substrate. [11][12][13] Inorganic structural mimics of CODH active site were pioneered by the group of Holm synthesizing a closely-resembling [NiFe3S4] cubane model [14] (Complex 1 in Figure 4-1) and even maintaining a square planar configuration for the redox-active Ni atom (Complex 2). [15] The biomimicry was even extended by synthesizing a heterobimetallic analogue in the form of Complex 3 [16] resembling the [NiFe] bimetallic center in the CODH active site. Though these models achieve a substantial structural similarity with the active site of the enzyme, they were not reported to catalyze the reversible reduction of CO2 to CO. A similar barrier was faced when scientists tried to structurally mimic the active sites of hydrogenases to develop synthetic catalysts towards reversible reduction of protons to hydrogen. [17] As such, there is a growing consensus in the community of bio-inspired stynthetic chemistry that for the development of efficient catalysts, key principles observed in biologically active sites including the nearby biomolecular environment should be applied to systems possessing different ligands and metal centers, as shown in pioneered by the group of DuBois, have been shown to be highly active catalysts for CO2 reduction to CO at low overpotentials [18] but suffer from low CO2 binding, which is considered as the rate determining step in the system. A bimetallic Pd catalyst was synthesized (Complex 4) [19] to increase the affinity for CO2, mimicking the same activation in the metal centers in CODH. This strategy helped improve the electrocatalytic rate of the system but suffered from low turnover because of catalyst deactivation due to the formation of Pd-Pd bond. A similar bimetallic strategy was employed by Naruta in modifying the well-known CO2-reducing iron porphyrin catalysts in a cofacial configuration shown in Complex 5. [20] Controlling the relative position of the porphyrin platforms by using a phenyl linker made it possible to control the distance between the Fe metals and an optimized catalytic rate was attained with an ortho configuration of the dimer. In this case also, no improvement of the overpotential was observed going from the monomer to the diner. Recently the group of Lu has synthesized a variety of cryptates holding Ni-Ni, [21] Co-Co, [9] and even a heterometallic Co-Zn [10] catalyst (Complex 6) which all show improved photocatalytic activity compared to mononuclear counterparts. The Co-Zn system is one of the few dinuclear heterometallic catalysts closely reproducing the roles proposed for the bifunctional activation in CODH (Ni metal as redox active center while Fe metal as substrate-activating). Though recent progress in bimetallic strategy for CO2 reduction catalysts is promising, the synthesis of bimetallic catalysts remains complicated and the formation of the M-M inactive species seems to be unavoidable in some cases.

The latter strategy consisting of introducing a second coordination sphere on the structure of the catalyst offers more versatility as evidenced by the rich literature on catalysts for the activation of small molecules like H + /H2 [17,22,23] and O2/H2O [24][25][26] . Such strategy is just gaining momentum for improving CO2 reducing catalysts but has already shown some promising results, as discussed in Chapter 1.

DFT studies on the Ni cyclam catalyst [27] show that a trans I isomer of the catalyst (shown in Complex 7) is more favorable in stabilizing a CO2 substrate by hydrogen bonding to the NH protons of the ligand. A systematic study was performed by the Chapter 4

108 group of Marinescu on the effect of pendant secondary amines as proton relays on a cobalt pyridine macrocycle and showed a linear dependence on the number of pendant amines in the second coordination sphere. [28,29] Unlike the metal cyclams, the pendant amines lie completely outside the primary coordination sphere of the metal allowing different roles of the amines as proton relays facilitating noncooperative hydrogen bonds with the acids in the solution. However, this improved catalytic activity comes at the cost of high overpotential (1080 mV). Other H-bond donors are reported in the form of guanidines in an iron hangman porphyrin by the group of Nocera, [30] positional amides in similar iron hangman porphyrins by the group of Chang, [31] and recently, thioureas in a Re catalyst by the group of Neumann. [32] The relative position with respect to the metal center of such H-bond donors is critical for the resulting catalytic activity as observed in a systematic study by Chang where ortho amides are better than the corresponding para configuration and a distal positioning (ortho-2-amide, Complex 10) is better than a proximal one (ortho-1-amide). [31] Careful design must be put in place as the position can affect the mechanistic CO2 reduction pathway as shown by the loss of performance of the thiourea-tethered Re catalyst (Complex 11) in the presence of external proton source (i.e. water). [32] Though these pendant H-bond donors improve the catalytic rates compared to the catalyst, they still don't significantly improve the overpotential of the system.

Local proton sources on the periphery of the catalysts were also implemented in the form of phenols in iron porphyrins (Complex 9) by the group of Costentin, Robert and Savéant, [33][34][35] phenols in Mn catalyst by the group of Nervi [36] and Bocarsly, [37] tyrosine residues in Re catalyst by the group of Kubiak, [38] and carboxylic acids in water-soluble Ni cyclam [39] and Mn bipyridine [40] catalysts by the group of Cowan. Though such functionalities have been shown to stabilize the CO2 adduct, the modified catalysts didn't display significant improvements in overpotential and turnover frequency (TOF). Substantial improvements of both catalytic rates and overpotential were observed when cationic groups such as trimethylammonium were introduced in iron porphyrins (Complex 12) reported by Costentin, Robert, and Savéant. [41] The specific positioning in the ortho position of the aryls promotes through-space electrostatic interactions between the positive charges of substituents and the negative charge of an Fe-CO2 adduct resulted in a significant decrease in overpotential while simultaneously increasing the TOF. [5] The same class of catalyst was also reported to photo-catalytically reduce CO2 to CH4

Chapter 4

109 when coupled to an Ir-based photosensitizer. [42][43][44] Recently, the group of Nippe combined the synergistic effects of H-bond donors and electrostatic interactions by introducing imidazolium moieties in the periphery of the Re (Complex 13) and Mn bipyridine catalysts. [45,46] Though the overpotentials are lowered for these modified catalysts, the TOFs are still relatively low.

Objectives

Inspired by the exceptional efficiency and stability of CODH enzymes in reversibly reducing CO2 to CO, we envisioned a strategy of introducing substrate-

Implications of Proximal Amines

From the key observation of the presence of amino acid residues in the vicinity of the Ni active site in CODH, we initially investigated the effect of similar amino groups in the vicinity of the metal of CO2 reduction catalysts. Bio-inspired modifications of the well-known Re catalyst include the introduction of a functional group, such as phenol [38,47] , amide, [38] thiourea [32] , and imidazolium, [45] all designed to provide single-point hydrogen bonding interactions with the CO2 substrate. Disubstituted bipyridyl ligands on the 4,4' positions have been reported for biomimetic groups such as carboxylic acids, [48] hydroxymethyl groups, [49] peptidelength tyrosine residues [38] , as well as other non-biomimetic functions. [50] Substitutions on the 6,6' positions are rare [51] and recent bio-inspired reports [32,45,46] only show mono-substitutions. With the goal of further improving the efficiency of the Re catalyst by bio-inspired modifications, the synthesis of 6-amino-substituted bipyridyl ligand for ReAm1 and a 6,6'-diamino substituted bipyridyl ligand for ReAm2 was performed, as shown in Figure 4-3, to mimic the multi-point stabilization roles of the lysine and histidine residues found within the active site of CODH. The choice of the substitutions was motivated by the ease of synthesis starting from commercially available ligands. Systematic study of these catalysts would provide an understanding of the effects of proximal modifications on 6,6' positions of the bipyridyl ligand, as well as the effect of number of available H-bond donors, on the performance of the catalyst. [38,45,47] or di-substituted bipyridyl ligands. [48] These deviations don't affect the Re-Br bond length in the initial state of the catalyst, though they may play a role once the Br is lost in the doubly-reduced active form, and as such they might affect the resulting activity of the system.

The electrochemical features of the modified ReAm1 and ReAm2 catalysts were compared with those of nonfunctionalized Re catalyst, as shown in the Cyclic Voltammograms (CVs) in Figure 4-4. As the number of amines is increased, the first reduction potential is shifted towards more negative potential. The bpy-based first reduction potential decreases by about 100 mV for every amino group introduced on the 6,6' position. This shift can be mainly attributed to the strong electron donating nature of the amines via resonance effects with the bipyridine ligand. A loss of reversibility in the first reduction peak was also observed when the number of amine ligands increase, indicating a more labile halide once a first electron is transferred to the bipyridine ligand. The second reduction, which is reported to be a metal-centered reduction, is similarly shifted towards more negative potentials with greater cathodic shift for ReAm2 (330 mV) compared to ReAm1 (60 mV).

To rationalize the observed electrochemical differences between the two modified Re catalysts, infrared spectroelectrochemistry was employed to further understand the transformation induced by reduction process. In the unreduced forms, both ReAm1 and ReAm2 show the typical three sharp C=O stretching peaks at 2022 cm -1 , 1915 cm -1 and 1899 cm -1 for ReAm1, and 2023 cm -1 , 1916 cm -1 and 1896 cm -1 for ReAm2 (Figure 4-5 and Table 4-2). These IR peaks compare well to those of the nonfunctionalized Re catalyst, as shown in Figure 4-7a. Though structural differences are observed in the crystal structures of these catalysts, it seems that the deviation of the bipyridyl ligand from the horizontal plane does not have a significant effect on the stretching modes of the surrounding carbonyl ligands in the unreduced form of the catalysts.

Cat

E / V vs Fc +/0 1 st red 2 nd red Re -1.72 -2.03 [a] ReAm1 -1.82 [a] -2.09 [a] ReAm2 -1.93 [a] -2.36 [a] -40 [a] For irreversible peaks, potentials at half-wave point are reported. 4-6: black -unreduced species, red -singlyreduced species, blue -solvated doubly-reduced species, and green -unsolvated doubly-reduced species). Condition consisted of 100 s electrolysis per 50 mV stepwise reduction and as such, contributed to persistence of some peaks in the proceeding potential steps; intermediates attained at the following V vs Fc +/0 , ReAm1: -1. The one-electron reduced species show IR peaks at 2009 cm -1 and 1892 cm -1 for ReAm1 and similar peaks for ReAm2 (2010 cm -1 and 1896 cm -1 ). The absence for both catalysts of a peak at 1998 cm -1 , [52] attributed to the presence of the halide in The electrocatalytic activity of the modified catalysts was then investigated by performing CVs in the presence of CO2 substrate and proton source (Figure 45678). In the presence of CO2, some enhancement of the cathodic current is observed on the second reduction wave. More significant enhancements can be observed in the presence of weak Brönsted acids like water or trifluoroethanol (TFE). Similar behavior was observed when the nonfunctionalized Re catalyst was first reported by the group of Lehn [53] and in a systematic study by the group of Wong on the effect of Brönsted acids on such catalysts. The catalytic enhancement is also strongly dependent on the pKa of the acid added as evidenced by the difference in the Chapter 4

116 intensity of catalytic current when water (pKa in ACN = 38-41) [54] was replaced by the stronger acid TFE (pKa in ACN = 35.4). [55] For more thorough comparison of the performance of the modified catalysts with that of the nonfunctionalized Re catalyst, we used foot-of-the-wave (FOW) analysis method developed by the group of Costentin, Robert, and Savéant. [56] In this analysis, the kinetics of the catalytic reaction represented by turnover frequency (TOF) is associated with the overpotential (η) at which the catalyst operate (see Annex for calculation details). A catalytic Tafel plot can be extracted from this analysis with a plot of log of TOF versus overpotential. Good catalysts can easily be identified as having low overpotentials while having high TOF, which would translate to an upper left deviation of the plot. From the catalytic Tafel plots in Figure 4-8d, it can be observed that the plots of ReAm1 and ReAm2 are shifted to the right because of the higher overpotentials needed for the modified catalysts as initially hinted by electron donating effect of the amines. The higher the number of amines introduced in the 6,6' positions of the bipyridine, the greater is the catalytic overpotential. The plot shown in Figure 4-9 also displays that values for TOF are affected by the nature of the proton source.

When water (pKa in ACN = 38-41) [54] is used as a proton source, the TOFmax of ReAm1 is not too different from the nonfunctionalized Re catalyst but is significantly enhanced in the case of ReAm2. When a more acidic source of proton is used like TFE (pKa in ACN = 35.4), [55] there is some improvement in the TOFmax of ReAm1 but the TOFmax of ReAm2 is significantly lower than that of the nonfunctionalized Re catalyst. This trend of activities obviously goes beyond the electronic effects of increasing number of proximal amines in the bipyridyl ligand, as the typical electronic scaling would suggest a higher TOFmax for higher overpotentials, as observed in iron porphyrin catalysts. [5] This could be due to (i) the interplay of the effects of the amines on both the first and second coordination sphere, (ii) the unsuitable positioning of the amines relative to the CO2 adduct, and/or (iii) some unexpected mechanistic deviations during catalytic conditions.

From these investigations, key learnings for catalyst design would then include (i)

the need for isolation of the second coordination sphere from the first coordination sphere and (ii) the need for carefully designed ligand geometries to provide the necessary activation of the CO2 adduct.

Urea and Amide Moieties as Hydrogen-Bond Relays

Reports have shown promise for amide and urea as potential bio-inspired functions for activating the CO2 substrate. As previously discussed, a single amido group had been placed on the bipyridyl ligand of a rhenium catalyst by the group of Kubiak [38] and positional effects of second-sphere amide functions have been systematically investigated by the group of Chang in an iron hangman porphyrin. [31] Recently, the group of Das has reported a crystallographic structure of a neutral urea scaffold that fixes atmospheric CO2 as carbonate clusters held together by an extended H-bonding network. [57] The group of Chang have also leveraged this multipoint hydrogen bond donation of urea as an external additive to improve the electrocatalytic activity of nickel cyclam at the same overpotential. [58] Neumann and coworkers have even tethered a thiourea function on the second coordination sphere of a rhenium catalyst that improved the electrocatalytic activity in the absence of external proton source. [32] However, the presence of weak acids, such as water, inhibits the activity of the latter system which will be problematic when aligning with the sustainable goal of using water as a free source of electrons and protons.

In the quest of improving catalytic activities by a bio-inspired strategy mimicking the multipoint hydrogen bonding scheme of histidine and lysine residues in stabilizing the CO2 adduct in CODH, two super-structured iron porphyrins holding in one case four urea functions (FeTPPUr) and in another four amido groups (FeTPPAm) were synthesized as shown in Figure 4-10. Both functions carried a phenyl ring bearing two -CF3 motifs as electron withdrawing groups to enhance hydrogen bonding aptitude of the NH fragments. Unlike studies utilizing hangman, [31,59] and picket fence [33,60] structures of porphyrins for CO2 reduction, an (TAPPαβαβ) [61,62] leads respectively to porphyrin ligands bearing four urea (TPPUr) or four amido (TPPAm) groups. Iron insertion was then performed using ferrous bromide and 2,6-lutidine to attain the catalysts FeTPPUr and FeTPPAm. The CV of FeTPPUr showed three reversible redox waves at -0.468 V, -1.326

αβαβ
V, and -1.812 V vs Fc +/0 , corresponding respectively to the formal Fe III/II , Fe II/I , Fe I/0 couples (Figure 4-13). All three waves show an anodic shift when compared to those of FeTPP under similar experimental conditions. However, the third redox wave is markedly shifted by over 300 mV to more positive potential, a shift similar to that of FeTPPF20. This substantial shift cannot be attributed only to the withdrawing inductive effect of the two -CF3 groups on the aryl ring at the other end of each urea groups. In the case of FeTPPAm holding the same fluorinated substituted aryl groups, only a small anodic shift was noted. Instead, the observation can be attributed also to the strong electron delocalization on the urea groups helping to contain the addition of electron on the catalytic unit. of electron withdrawing groups, leads to a decrease in the TOF of the catalysts. [5] This can be observed by the downward left shift of the catalytic Tafel plot in Figure 123 with catalysts having a local proton source or favorable second sphere interactions. [33,41] In the case of FeTPPAm, where hydrogen bond interaction with amide groups can be expected, only a small upward left deviation is observed. This positive deviation is much more pronounced in the case of FeTPPUr catalyst which has comparable TOF to that of FeTPP but with ~300 mV less overpotential. These comparative studies indicate that the significant enhancement in FeTPPUr can be assigned to the multi-point hydrogen bonding induced by the urea groups compared to the single-point hydrogen bonding in FeTPPAm.

Cat

In a control experiment, the addition of an excess of urea as an external additive during the catalytic reduction of CO2 by FeTPP did not bring any improvement to either the catalytic current or the catalytic potential (Figure 45678910111213141516).

This result underlines the important aspect of the 3D pre-organization of the FeTPPUr catalyst which places the two urea groups on one side of the porphyrin platform in a position where all four hydrogen atoms of the urea groups can establish hydrogen bonding interactions with a CO2 substrate on the metal-center from two opposite meso-positions of the porphyrin macrocycle. This result is a clear demonstration on the importance of positioning the hydrogen bonding patterns for optimizing the electrocatalytic properties of the bio-inspired catalyst FeTPPUr. It is not the sum of the additional NH interactions as compared to FeTPPAm derivative but should rather be considered as a close mimic of the lysine and histidine residues of CODH active site. Indeed, based on the structure-function proposal for the enzyme, this pair of amino acids conspires to stabilize a carboxylate intermediate (M-CO2) through multi-point hydrogen bonding. [11] The choice of introducing urea groups in the second coordination sphere of an iron porphyrin catalyst was motivated by their ability to bind oxygen atoms of carbonyl groups as well as carbonates. [63][64][65] In some cases, urea groups can even capture atmospheric CO2 as carbonates/bicarbonate. [57] X-ray structures resolved In order to assess the contribution of the urea arms to the binding affinity of CO2, the CV of FeTPPUr was performed under CO2 but in the absence of a proton source. In these conditions, where the catalytic 2-electron and 2-proton reduction of CO2 cannot proceed, both a loss of reversibility and an anodic shift of the third reduction wave was observed (Figure 4-18c). These characteristics are unprecedented for FeTPP catalyst (Figure 4-18a) but were reported as indicative of a good CO2 binding for other macrocyclic catalysts. [66,67] For a thorough comparison, the CO2 binding rate constants was estimated using the reported method by Savéant based on irreversible kinetics upon CO2 binding (see Annex for calculation details). It was found that the CO2 binding rate constant of FeTPPUr to be an order of magnitude higher than that of reference compounds such as FeTPP or even The table displays the estimated CO2 binding rate constant based on reported method [68] considering irreversible kinetics upon CO2 binding (see Annex for calculation details).

FeTPPAm (Table inset in

However, this increased CO2 binding affinity may not be the only parameter to account for the high catalytic performance of FeTPPUr. As discussed, a stabilization of reactive intermediates such as Fe-CO2 species through hydrogen bonding interactions with the NH of urea groups may also contribute. To back up this hypothesis, density functional theory (DFT) calculations were performed on model compounds to investigate on the possible geometry of the CO2 adducts of the triply reduced iron porphyrins. In the case of FeTPPAm, only two weak hydrogen bonding interactions between the amide arms and the coordinated CO2 substrate were found, as shown in

I / μA E / V vs Fc +/0 FeTPPUr FeTPPAm FeTPP (a) (b) (c) Chapter 4 126 N•••(H)•••O distance of 2. 20 -15 -10 -5 0 5 -2.8 -2.3 -1.8 -1.3
802 Å comparable to those evidenced in the active site of the CODH (2.637 and 2.884 Å with Lys and His residues, respectively). This multipoint anchoring of CO2 is responsible of a free enthalpy of stabilization of 29 kcal mol -1 .

The proton source, a pre-requisite for the reduction of CO2, was further interrogated. A 5.77 normal kinetic isotope effect (KIE) was measured for the electrocatalytic reduction of CO2 by FeTPPUr with H2O/D2O as a proton source indicating that proton transfer is involved in the rate-determining step (Figure 4-20a). The investigation was extended to other sources of proton currently used in this field, i.e. trifluoroethanol (TFE) and phenol (PhOH), by comparing the catalytic rates estimated from the FOW analysis. Although H2O is less acidic (pKa in DMF = 31.5) than TFE (pKa in DMF = 24.0) or PhOH (pKa in DMF = 18.8), [69] surprisingly, it has by far the most marked effect on improving the catalytic rate compared to PhOH or TFE (Figure 4-20b). This result is in sharp contrast with a recent study by the group of Neumann for a thiourea-tethered rhenium catalyst, where the authors observed a severe drop in the catalytic rate upon addition of H2O or PhOH. [32] They suggested, based on DFT calculations, that external protons may disrupt the preestablished hydrogen-bond stabilization of the thiourea moiety with the CO2 adduct. In the case of FeTPPUr, water seems to work in synergy with the urea arms to manage the proton supply. Hints for this proposal come from the network of water molecules observed in the X-ray structure of FeTPPUr, as shown in Figure 4-21a, suggesting that such a network may also be in play during the catalytic reduction of CO2. A similar water network in the vicinity of the active site of CODH revealed by high resolution X-ray structure (Figure 4-21b) was proposed to have a major role in the enzyme reactivity. [11,13] The geometry of a model structure where a water molecule was inserted in between a urea arm of FeTPPUr and CO2 could be optimized by DFT calculation as a local minimum, as shown in Figure 4-22. This configuration is similar to one of the water molecules in the X-ray structure of FeTPPUr (O4 in Figure 4-21a). In contrast with the previously mentioned thioureatethered Re catalyst, the CO2 is in this case maintained in a quasi-unchanged topology while being put in contact with a water molecule held by the second urea arm. The less important effect of PhOH and TFE can be due to i) their higher acidity which can prevent the formation of pre-established intramolecular hydrogen bonds between urea groups and CO2 adduct, and/or ii) their steric hindrance which can limit their access to the catalytic center in between the two urea arms. Further calculations are needed to probe the reaction pathway involving such catalyst to rationalize the interesting effect of proton source. the rate constant (kcat) was calculated to be 2.76 x 10 3 s -1 , the TOF to be 455 s -1 , and the TON to be 3.28 x 10 6 . As shown in Figure 4

Cationic Imidazolium Moieties as Intermediate Stabilizers

A closer look on the active site of CODH shows the stabilization of the metal carboxylate intermediate by the amino acid residues, histidine 93 and lysine 563.

These amino acids are in hydrogen bonding distances to the oxygen atoms of the carboxylate intermediate and it has been proposed they may be in their protonated cationic forms. [11] As such, through-space electrostatic interactions between the carboxylate intermediate and these amino acid residues might play a crucial role in the activity of the enzyme. The role of electrostatic interactions in catalysis has initially been reported by the group of Masel when the use imidazolium-based ionic liquids as electrolytes have lowered the overpotential for the electro-reduction of CO2 in a heterogeneous catalyst consisting of a silver cathode. [70] It was proposed that the ionic liquid stabilizes the one-electron reduced CO2 •-intermediate, thereby lowering the initial reduction barrier. Implementation of imidazolium-based ionic liquid electrolyte as cofactor in catalysis has been shown by the group of Wallace and Officer to anodically shift the reduction potential of an iron porphyrin catalyst and enhance the kinetics of CO2 reduction. [71] Similar substantial improvement in the overpotential and TOF were observed by the group of Costentin, Robert, and Savéant when cationic trimethylammonium groups were tethered at the periphery of an iron porphyrin catalyst. [41] Recently, cationic methylimidazoliumfunctionalized Re [45] and Mn [46] catalysts were reported by the group of Nippe with improved electrocatalytic properties. It was proposed that the imidazolium units act to provide both hydrogen bonding and through-space electrostatic stabilization of the metal-carboxylate intermediate. Intriguingly, addition of water as a proton source resulted in a significant increase of the catalytic current at lower overpotential, but at higher concentrations of water, a drastic drop in the catalytic current together with positive potential shifts was observed. [45] With these recent reports, we extended the investigation of second coordination sphere approach by synthesizing an iron porphyrin derivative with methylimidazolium units attached on the periphery of the porphyrin macrocycle, denoted as FeTPPIm, as shown Figure 4-24. Inspiration for the targeted catalyst stems from the pioneering work of Collman and Boitrel where the semi-rigid ''Ushaped'' aryl-amide-aryl motif of the starting ligand was used to introduce coordinating or functional groups in a pre-organized fashion close to the coordination sphere of the metal ion in the center of the porphyrin ring. [61,[72][73][74] Iron (II) ion was inserted in the N4 coordinating cavity of this ligand using ferrous chloride in the presence of a base. The substitution of the chloro groups at the benzylic positions was then realized upon treatment with stoichiometric amounts of N-methylimidazole at 70˚C to prevent the formation of rotational isomers.

Interesting features of this catalyst include the picket fence α-4-tetraimidazolium tetraphenylporphyrin ligand structure that establishes a cationic sphere on top of the active site of the catalyst, and the presence of distal NH fragments from the cationic arms, resembling the weak hydrogen bonding interactions by the previously described FeTPPAm with the metal carboxylate intermediate (Section 4.4). As such, this system may differentiate the extent of contribution of hydrogen ). [75,76] By comparing these reduction waves to the CVs of the reference iron porphyrin catalysts (Figure 4-13), we observe that the third reduction wave of FeTPPIm is drastically shifted towards more positive potential in comparison with the nonfunctionalized FeTPP (ΔE of 415 mV), or the fluorinated analogues FeTPPF8

(ΔE of 290 mV), FeTPPF20 (ΔE of 100 mV), and even the FeTPPAm (ΔE of 310 mV).

We attribute this significant shift to the stabilization of the dianionic reduced form of the iron porphyrin through a space-charge interaction with the positively charged methylimidazolium groups. This result therefore provides convincing support that the topologically pre-organized aryl-amide-aryl arms bring the methylimidazolium groups close to the porphyrin platform which helps to support charge accumulation.

Under CO2 atmosphere and in presence of 5.5M water as a proton source, FeTPPIm shows the typical catalytic current on the third reduction wave but at a much lower overpotential compared to the other modified iron porphyrin catalysts.

Based on the catalytic Tafel plot and linear scaling relationship in Figure 4-26,

FeTPPIm is a much better catalyst as indicated by the upward left deviation (that is, lower overpotential and higher TOF) in comparison with the fluorinated reference catalysts FeTPPF8 and FeTPPF20. Even when compared to the FeTPPAm having similar NH fragments that provide weak hydrogen bonding interactions in the second coordination sphere, FeTPPIm is a much better catalyst. The catalyst improvement observed for FeTPPIm is even more evident when compared to FeTPPUr, which possesses strong multi-point hydrogen bonding stabilization, indicating the great extent through-space electrostatic interactions contributed to the enhancement of the catalysis. -

I / μA E / V vs Fc +/0 Ar CO2 CO2+H2O 80 -60 -40 -20 0 20 -2.3 -1.8 -1.3 -0.8 -0.3 0.2
Chapter 4

133

The tetracationic nature of FeTPPIm renders it water-soluble hence allowing to evaluate its catalytic performance for CO2 reduction in water as a convenient and clean solvent and proton source. The CV of FeTPPIm in Ar-degassed water containing 0.1 M potassium chloride (KCl) shows three redox processes similar to those observed in DMF. However, even though CO2 is less soluble in water (0.033 M) than in DMF (0.23 M), the CV displays a much higher catalytic current for CO2 reduction at a potential as low as E˚cat = -1.712 V vs Fc +/0 (Figure 4-25b). To explain this higher catalytic activity in water, we have to take into consideration the primary intermediate proposed for CO2 reduction by iron porphyrins which is

[Fe(porph)CO2] 2-described formally as a resonance of [Fe I (CO2 • )] 2-and [Fe II (CO2 •• )] 2-
. [5,69,78] As described earlier, the positive deviation from the expected electrochemical reactivity as already observed in DMF may be assigned to the stabilization of this intermediate through electrostatic interactions with the positive charges of the methylimidazolium groups. In water, which has a higher relative permittivity (εr,H2O = 78.4) compared to DMF (εr,DMF = 36.7), [79] the degree of dissociation of the embarked ionic liquid (methylimidazolium + Cl -) units is greater than that in DMF, [80] which leads to better stabilization of the [Fe(porph)CO2] 2-adduct through stronger space-charge interactions with more "free" methylimidazolium groups. hours of electrocatalysis at -1.642 V vs Fc +/0 , which corresponds to an overpotential of 418 mV (E ˚CO2/CO = -1.224 V vs Fc +/0 in H2O), and a current density of 1.06 mA cm -2 , led to the exclusive formation of CO with 91% Faradaic efficiency. Considering a two-electron process, the rate constant (kcat) of 2.44 x 10 5 s -1 , TOF of 14986 s -1

(log TOF = 4.18), and TON of 1.08 x 10 8 were calculated. These results show that the pre-organized ionic liquid environment confined within the active site of the catalyst promote not only significant lowering of the overpotential for CO2 reduction but also enhance the TOF of the reaction. This gives hints that the synergy of through-space electrostatic interactions as well as hydrogen bonding stabilization of the metal carboxylate intermediate promotes improved catalytic CO2 reduction, as observed in the comparison of the catalytic Tafel plots of the studied catalyst with notable CO2 reduction catalysts reported in literature (Figure 4-28 and Table 4-3). Ni cyclam -1.74 1.9 ACN H2O [27] Pd2(triphos)2 -1.45 1.5 DMF HBF4 [19] Re(bpy)(CO)3(py) -1.99 2.9 ACN TFE [82] Mn(tbu-bpy) (CO)3Br -2.09 2.8 ACN TFE [83] Mn(mesbpy) (CO)3Br -1.97 3.7 ACN MeOH

Conclusions

In conclusion, we have described a bio-inspired strategy for improving the We also extended our second coordination sphere approach to the introduction of cationic methylimidazolium units as local ionic liquids. This idea was inspired from a proposed role of protonated amino acids in the stabilization of [Fe I (CO2 • )] 2-adduct in the CODH active site. Significant decrease of the overpotential and an important increase in the TOF was achieved with this system which is attributed to some extent to the weak hydrogen bonding of the proximal amide functions similar to FeTPPAm, but also to a larger extent to the electrostatic Chapter 4

137 interactions between the cationic arms and the metal carboxylate intermediate, leading to superior performance compared to FeTPPUr. Because of the cationic nature of the catalyst, it was even possible to perform CO2 reduction in water as a convenient and clean solvent and proton source. Indeed, the careful design and systematic manipulation of the secondary coordination sphere of catalysts by applying key learnings from the structure-function studies of CODH was able to improve the catalytic activities. This serves as an important synthetic tool for scientists in developing and discovering catalysts with enhanced activities.

Bibliography

The way Nature uses sustainable resources to produce forms of energy which support all forms of life on earth inspires a particularly interesting learning point on how we can solve our global warming and energy crisis problems. Indeed, in these work we have shown that we can apply key insights learned from natural systems to improve artificially-designed photo-and electro-catalytic CO2 reduction systems.

In Chapter 2, we learned how visible light is used to reduce CO2 to CO using a photocatalyst system comprising of a rhenium triscarbonyl bipyridyl catalyst 162 intermediates leading to the formation of a photo-induced Ni I complex. This was not sufficient though to differentiate the observed experimental differences as there is a need to probe beyond the first electron transfer. This can be addressed by employing photo-accumulation studies but requires further optimization of experimental set-up and conditions to fully track the second electron accumulation.

We learned from this initial endeavor that though a strategy of modifying the primary coordination sphere of the nickel catalyst can, in most cases, shed light on the structural features of the active site of the CODH enzyme, it seems that systematic changes in such primary coordination does not necessarily translate to expected systematic improvements in the catalytic activity. This is in part due to the interplay of electronic and structural changes that might critically affect the reaction pathways of the catalyst. Furthermore, the similarity of the sulfur-rich environment to hydrogenase enzymes makes such strategy limited in controlling and preventing the competitive proton reduction activity.

In put in contact with a water molecule, explaining the synergistic effect of water as a sufficient proton source for the catalytic reduction. This similarly mimics the way CODH manages its proton supply through a nearby water network, which was reported to be critical for the enzymatic activity.

In addition, we learned the advantages of establishing cationic methylimidazolium units in a picket fence iron porphyrin structure in FeTPPIm.

Significant lowering of the overpotential and increase in the TOF was achieved which is attributed to some extent to the weak hydrogen bonding of the proximal amide functions similar to FeTPPAm, but to a larger extent to the electrostatic interactions between the cationic arms towards the metal carboxylate intermediate, exceeding the performance of FeTPPUr. Because of the cationic nature of the catalyst, it was even possible to perform CO2 reduction in water as a convenient and clean solvent and proton source. Indeed, the careful design and systematic manipulation of the secondary coordination sphere of catalysts by applying key learnings from the structure-function studies of CODH was able to improve the catalytic CO2 reduction activities.

In Chapter 5, we further learned on how Nature utilizes carbon monoxide as a chemical feedstock and applied it to valorize the catalytic reductions of CO2 to CO studied in this work. The direct valorization of CO produced from the visible-lightdriven catalytic reduction of CO2 at the scale of molecular photocatalysis was established towards an aminocarbonylation reaction to access amide groups, which are common motifs for bio-active pharmaceutically-relevant compounds. As such, CO2 as a starting material was transformed to simple C1 building blocks which open up further opportunities not only for valorizing the works done in the electroand photo-catalytic reduction of CO2, but also providing a cheap access to raw material for industries that uses carbonylation as a core chemical process.

This research has shown efforts to develop step-by-step strategies in improving CO2 reduction catalysts. Some of the approaches are still crude and need further mechanistic investigations. In perspective, there are still a lot of things to improve. We observe that Nature uses almost the same elemental players (e.g. metal centers, coordinating ligands, amino acid residues, etc.) in performing efficient processes, and the selectivity towards a certain reaction depends on how they are specifically and carefully arranged (ranging from the active sites of the enzymes to the supramolecular quaternary structures of proteins). This gives a hint that further improvement of the current molecular systems would entail a similar hyperstructuration. This may involve the synthesis of more complex ligand structures forcing an entatic state on the active site of the catalyst, or the heterogenization of the molecular catalysts in metal-organic frameworks, zeolites, and polymers, or even encapsulation on nanostructured reaction sites. A balanced decision should be made however between the complexity of the synthetic procedure, the cost of the material, and more importantly, the efficiency and stability of the system.

Harnessing solar energy and reducing carbon dioxide would, in the current state, entail the use of developments in photovoltaics to drive electrocatalytic CO2 reduction processes. The promise and potential for a direct photoelectrocatalytic processes still rely on a fundamental need of understanding and optimizing the system. In this end, photocatalytic CO2 reduction systems which when studied on their own rely on sacrificial electron donors should be incorporated into overall 165 economic assessment on a practical targeted product to valorize the envisioned tandem technologies. In the end, for these innovations to be globally accepted requires political, economic, and social influences to eliminate the NIMBY ("not in my backyard") mentality.

Electrochemical characterization

Cyclic voltammetry (CV) measurements were performed in an electrochemical cell composed of a glassy carbon (3 mm diameter) working electrode, Ag/AgNO3 (10 - 2 M) reference electrode, and a platinum wire counter electrode. Dimethylformamide (DMF) and acetonitrile (ACN) were mainly used as solvents and solutions of samples were prepared at a concentration of 1 mM. Tertbutylammonium hexafluorophosphate (TBAP) was used as supporting electrolyte and its concentration was maintained at hundred-fold excess compared to the sample.

Different proton sources (water, trifluoroethanol, phenol) were added, the solutions were purged with inert argon gas, and the cyclic voltammograms were measured at a scan rate of 100 mV/s (unless stated otherwise). Ferrocene was added to the solutions at the end of each measurement as a reference for standard comparison, and as such all potentials were reported versus the Fc +/0 couple. Standard comparisons with state-of-the-art catalysts were made by converting literature values to the Fc +/0 couple using reported conversion values. [4][5][6][7] Catalytic rates of the electrocatalytic systems were determined using the reported benchmarking strategy of foot-of-the wave (FOW) analysis of the CVs. [8] This quick estimation technique gives the catalytic rate constant, kcat and turnover frequency (TOF) preventing any contributions from side phenomena such as substrate consumption, catalyst deactivation, and/or product inhibition. The analysis is based on the linear correlation between i/ip 0 and 1/{1+exp[F/RT(E -E 0 cat)]} where i is the catalytic current in the presence of CO2, ip 0 is the peak current in the absence of CO2, F is the faraday constant, R is the gas constant, and T is the absolute temperature, E is the applied potential, and E 0 cat is the standard potential of the reversible reduction peak of the active form of the catalyst. Plotting i/ip 0 vs. This provides a precise characterization of the performance of the catalyst as a function of the applied overpotential. Taking the TOF at zero overpotential (TOF0) then gives the intrinsic activity of the catalyst:

𝑙𝑜𝑔𝑇𝑂𝐹 0 = 𝑙𝑜𝑔𝑘 𝑐𝑎𝑡 - 𝐹 𝑅𝑇𝑙𝑛10 (𝐸 𝐶𝑂 2 𝐶𝑂 ⁄ 0 -𝐸 𝑐𝑎𝑡 0 )
The maximal catalytic rate constants, TOFmax, asymptotically reached for large overpotentials, are also often reported which is just equal to the determined kcat. [9] The FOW analysis has been successfully applied to the iron porphyrin catalysts with estimations close to that obtained from bulk electrolysis experiments. [10] It has been addressed recently however, that for some catalysts, like the Re-based bipyridine catalysts, estimations using the FOW analysis yielded irreproducible kcat values. [11] The irreproducibility was attributed to the estimation of 𝐸 𝑐𝑎𝑡 0 which is sometimes beyond the reversible couple of the active form of the catalyst. There have been reports consolidating measurements of overpotential, and it was suggested that use of the half-wave potential (Ecat/2), defined as the point at which the homogeneous catalytic wave reaches half of its maximum current, is often recommended, when Ecat is difficult to determine. [12,13] Because of these difficulties, most authors turn to the method of scan-rate independent catalytic plateau currents to determine the catalytic rate, where n is the number of unique electron transfer processes that occur at the electrode per catalyst and n′ is the catalyst equivalents required per turnover:

𝑖 𝑖 𝑝 0 = 2.24 √ ( 𝑅𝑇 𝑛𝐹𝑣 ) 𝑛′𝑘 𝑐𝑎𝑡

Infrared spectroscopy and spectroelectrochemistry

The IR spectra of the catalysts were determined by dropping dichloromethane solution of the samples onto the diamond prism of an attenuated total reflectanceinfrared (ATR-IR) Thermo-Nicolet 6700 FTIR spectrometer equipped with a mercurycadmium-telluride (MCT) detector. The solvent was allowed to evaporate in a stream of nitrogen gas until a dry film is deposited. 200 sample scans were recorded with a resolution of 4 cm -1 , in the range of 4000 cm -1 to 500 cm -1 with a speed of 1.

Time-resolved optical measurements

Laser Flash Photolysis

Excited state absorption characteristics were measured using an Edinburgh

Instruments LP 920 laser flash photolysis spectrometer. Laser excitation pulses were generated from a Continuum Surelite Q-switched Nd:YAG laser coupled with a Continuum optical parametric oscillator (OPO) to produce an excitation pulse with 5 ns pulse duration at a wavelength of 460 nm (or 430 nm) and a typical laser energy of about 7-10 mJ per flash. The LP920 system is equipped with a 450 W Xenon arc lamp as the probe for the transient absorption measurements. Detection in the LP920 system is performed either via a Czerny-Turner blazed 500 nm monochromator (bandwidth: 1-5 nm) coupled with a Hamamatsu R928 photomultiplier tube (for kinetics mode), or via a 500 nm blazed spectrograph (bandwidth: 5 nm) coupled with a water-cooled ICCD camera Andor DH720 (for spectral mode). LP920 software was used for data acquisition, to calculate changes in optical density, for mathematical data treatment.

Chapter 2

Synthesis of Re(CO)3(2,2'-bpy)Br

The complex was synthesized with slight modifications from literature methods. [15] An equimolar amount of Re(CO)5Cl (0.3451 g, 0.85 mmol) and 2,2'bipyridine (0.136 g, 0.87 mmol) was dissolved in toluene (50 mL) ) and heated to 115 °C. The reaction mixture was stirred under reflux for 2 h. Afterwards the solution was removed from the heat source and cooled down by placing in an ice bath. The product precipitated from solution as a yellow powder and was filtered off.

Pure product was isolated from the reaction with an overall yield of 92% (0.3256g).

X-ray quality crystals were grown from the vapor diffusion of pentane into a chloroform solution of the complex.

Synthesis of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole (BIH)

Synthesis of NiN2S2

An equimolar solution of Ni(ClO4)2•6H2O (392 mg, 1.0 mmol, 1.0 eq) in MeOH (10 mL) was added to a solution of N2S2 ligand (292 mg, 1.0 mmol, 1.0 eq) in MeOH (10 mL). The mixture was stirred for 1 day at room temperature. The solvent was evaporated using rotary evaporator forming purplish oil. The oil was dissolved in acetonitrile and ethyl acetate was added to promote precipitation. X-ray quality crystals were grown by slow diffusion of ethyl acetate in the acetonitrile solution of the complex (376 mg, 0.61 mmol, 61% yield).

ESI-HRMS: calculated m/z = 432.9588 [M-(ClO4) -] + for C14H16N2S2NiClO4, found 432.9593.

Synthesis of N2S3

Na metal (2.48 g, 104 mmol, 4.0 eq) was dissolved in absolute ethanol (150 mL) under Ar atmosphere. 2,2′-Thiodiethanethiol (3.4 mL, 26.1 mmol, 1.0 eq) was added to the solution and the mixture was heated to reflux at 85 ˚C for 30 min.

Then, a solution of 2-picolyl chloride hydrochloride (8.50 g, 51.8 mmol, 2.0 eq) in absolute ethanol (100 mL) was added slowly to the mixture. The mixture was stirred and refluxed at 85 ˚C for 15 h under Ar. The solvent was removed using rotary evaporator and the residue was extracted with chloroform and H2O. The organic layer was washed with brine solution, dried over anhydrous CaCl2, and filtered. The volatiles of the filtrate were evaporated using rotary evaporator and further dried under vacuum to produce brown oil of bis(2-pyridylmetyl)-

Synthesis of NiN2S3

An equimolar solution of Ni(ClO4)2•6H2O (336 mg, 1.0 mmol, 1.0 eq) in MeOH (20 mL) was added to a solution of N2S3 ligand (366 mg, 1.0 mmol, 1.0 eq) in MeOH (20 mL). The mixture was stirred for 1 hour at room temperature. The solvent was evaporated using rotary evaporator forming purplish oil. The oil was dissolved in acetonitrile and ethyl acetate was added to promote precipitation. X-ray quality crystals were grown by slow diffusion of ethyl acetate in the acetonitrile solution of the complex (438 mg, 0.69 mmol, 69% yield).

ESI-HRMS: calculated m/z = 492.9622 [M-(ClO4) -] + for C16H20N2S3NiClO4, found 492.9621.

Synthesis of P2S3

Synthesis of 1-chloro-2-diphenylphosphinoethane: Under an Ar atmosphere, potassium diphosphide solution (75 mL, 0.5 M in THF) was added dropwise to dichloroethane (250 mL) for 1 hour at room temperature. The mixture was stirred further for 2 h and the solvent was evaporated using rotary evaporator. The residue was extracted with dichloromethane and water. The organic layer was washed with brine solution, dried over anhydrous MgSO4, and filtered. The volatiles of the filtrate were evaporated using rotary evaporator to produce yellow oil. The crude product was purified using column chromatography (silica gel) with chloroform as eluent, finally obtaining pure yellow oil (1.532 g, 16% yield).

Synthesis of NiP2S3

An equimolar solution of Ni(ClO4)2•6H2O (220 mg, 0.60 mmol, 1.1 eq) in MeOH (10 mL) was added to a solution of P2S3 ligand (320 mg, 0.55 mmol, 1.0 eq) in MeOH (10 mL). The mixture was stirred for 1 hour at room temperature. The solvent was evaporated using rotary evaporator forming dark green oil. The oil was dissolved in acetonitrile and ethyl acetate was added to promote precipitation. X-ray quality crystals were grown by slow diffusion of ethyl acetate in the acetonitrile solution of the complex (202 mg, 0.24 mmol, 44% yield).

ESI-HRMS

EXAFS Analysis

Athena software [17] was used for data processing. The energy scale for each scan was normalized using copper metal standard. Data in energy space were preedge corrected, normalized, deglitched (if necessary), and background corrected.

The processed data were next converted to the photoelectron wave vector (k) space and weighted by k. The electron wave number is defined as , E0 is the energy origin or the threshold energy. K-space data were truncated near the zero crossings k = 2.486 to 14 Å -1 for the nickel-based solution complexes, in Ni EXAFS before Fourier transformation. The k-space data were transferred into the Artemis Software for curve fitting. In order to fit the data, the Fourier peaks were isolated separately, grouped together, or the entire (unfiltered) spectrum was used.

The individual Fourier peaks were isolated by applying a Hanning window to the first and last 15% of the chosen range, leaving the middle 70% untouched. Curve fitting was performed using ab initio-calculated phases and amplitudes from the FEFF8 [18] program from the University of Washington. Ab initio-calculated phases and amplitudes were used in the EXAFS equation:

where Nj is the number of atoms in the j th shell; Rj the mean distance between the absorbing atom and the atoms in the j th shell; (,k, Rj ) is the ab initio amplitude function for shell j, and the Debye-Waller term accounts for damping due to static and thermal disorder in absorber-backscatterer distances. The mean free path term reflects losses due to inelastic scattering, where λj(k), is the electron mean free path. The oscillations in the EXAFS spectrum are reflected in the sinusoidal term , where is the ab initio phase function for shell j. This sinusoidal term shows the direct relation between the frequency of the EXAFS oscillations in k-space and the absorber-backscatterer distance. S0 2 is an amplitude reduction factor.

The EXAFS equation [19] was used to fit the experimental Fourier isolated data (q-space) as well as unfiltered data (k-space) and Fourier transformed data (Rspace) using N, S0 2 , E0, R, and  2 as variable parameters (Table 0-1). N refers to the number of coordination atoms surrounding Ni for each shell. The quality of fit was evaluated by R-factor and the reduced Chi 2 value. The deviation in E0 ought to be less than or equal to 10 eV. R-factor less than 2% denotes that the fit is good enough. R-factor between 2 and 5% denotes that the fit is correct within a consistently broad model. The reduced Chi 2 value is used to compare fits as more absorber-backscatter shells are included to fit the data. A smaller reduced Chi 2 value implies a better fit. Similar results were obtained from fits done in k, q, and R-spaces.

2 1 2 0 ] / ) ( 2 [  E E m k - = )) ( 2 sin( ) , , ( ) ( ) ( 2 2 2 2 0 2 2 k kR e e R k f kR N S k ij j k R k j eff j j j j j j j      + = - -  j eff f 2 2 2 k j e  - ) ( 2 k R j j e  - )) ( 2 sin( k kR ij j  + ) (k ij 
overlapped with an X-ray spot size of 100 µm(V) x 450 µm(H) and laser spot size of 170 µm(V) x 550 µm. With a liquid flow speed of around 3 m/s, the pumped laser volume was calculated to move out of the FWHM region in around 24 µs. This temporal range ensured that the excited state volume was probed more at the center and less at the edges where the excitation fraction would be less, due to movement of the sample. Beamline 11 ID-D has an automated data digitization system which allows for all X-ray pulses after laser excitation to be collected. Such a system, together with the larger X-ray beam spot size, was very useful for our experiments, as multiple X-ray pulses after laser excitation were averaged to monitor the dynamics for the formation and decay of Ni I transient species in the nsµs time regime. In addition, by averaging multiple pulses, we obtained a better resolution of the main features in the pre-edge and edge regions of the transient signals.

The delay between the laser and X-ray pulses was adjusted by a programmable delay line (PDL-100A-20NS, Colby Instruments) and the X-ray fluorescence signals were collected with two APDs positioned at 90˚ on both sides of the liquid jet. Moreover, a combination of Z-1 filters and soller slits with conical geometry were used to reduce the background from elastically scattered X-rays. A Ni metal foil was placed between two ionization chambers downstream to the X-ray beam, and its transmission recorded with each scan for energy calibration.

Chapter 4

Synthesis of ReAm1

An equimolar amount of Re(CO)5Cl (64.2 mg, 0.37 mmol) and 6-amino-2,2'bipyridine (150 mg, 0.37 mmol) was dissolved in toluene (30 mL) ) and heated to 115 °C. The reaction mixture was stirred under reflux for 1 h. Afterwards the solution was removed from the heat source and cooled down by placing in an ice bath. The product precipitated from solution as a yellow powder and was filtered off.

Pure product was isolated from the reaction with an overall yield of 68% (131 mg).

X-ray quality crystals were grown from the vapor diffusion of pentane into a chloroform solution of the complex.

Synthesis of ReAm2

An equimolar amount of Re(CO)5Cl (70.3 mg, 0.38 mmol) and 6,6'-diamino-2,2' -bipyridine (150 mg, 0.37 mmol) was dissolved in toluene (30 mL) ) and heated to 115 °C. The reaction mixture was stirred under reflux for 1 h. Afterwards the solution was removed from the heat source and cooled down by placing in an ice bath. The product precipitated from solution as a yellow powder and was filtered off.

Pure product was isolated from the reaction with an overall yield of 53% (106 mg).

X-ray quality crystals were grown from the vapor diffusion of pentane into a chloroform solution of the complex. (200 mg, 0.296 mmol) in dry dichloromethane under argon. After an overnight stirring at room temperature, the solvent was removed under vacuum. Finally, the residue was chromatographed on a silica gel and eluted with chloroform. The expected compound was precipitated by dissolving it in a minimum amount of dichloromethane and adding a large amount of pentane. After a filtration, washing with pentane and drying under vacuum, compound TPPUr was obtained as purple solid with 89 % yield (447 mg). Single crystals suitable for X-ray diffraction analysis were obtained by a slow evaporation of a TPPUr saturated solution in a 1:5 mixture of acetone:H2O. Due to the D2d symmetry of TPPUr porphyrin, the four urea arms are magnetically equivalent. 2D NMR (COSY, HSQC and HMBC) experiments were also performed in order to confirm the attribution of 1D 1 H NMR signals. UV-Vis (CHCl3): λmax / nm (ε, in M -1 cm -1 ): 423 (159900), 517 (8800), 553 (4500), 590 (6100), 647 (4400)

Synthesis of FeTPPUr

A solution of porphyrin TPPUr (200 mg, 0.188 mmol) and lutidine (273 µL, 2.360 mmol) in dry THF was degassed by argon bubbling for 30 min. Then, FeBr2 (178 mg, 0.826 mmol) was added and the reaction mixture was stirred overnight at room temperature under argon. The reaction mixture was then filtered to remove the excess of iron salts and the solvent was evaporated under vacuum. The reaction crude was dissolved in 50 mL of CHCl3 and 50 mL aqueous solution of 0.5 M HCl was added. After, 3 hours stirring the two phases were separated and the organic phase containing the iron complex was evaporated under vacuum.

Chromatographic purification on a silica gel using a mixture of methanol:CHCl3 (from 0:100 to 3:97) gives FeTPPUr (with a Cl -as axial coordinating anion) in 80 % yield (165 mg). Single crystals suitable for X-ray diffraction analysis were obtained by a slow evaporation of a FeTPPUr saturated solution in a 1:6 mixture of methanol:H2O, resulting in a crystal structure having a methoxide axial ligand. UV-Vis (CHCl3): λmax / nm (ε, in M -1 cm -1 ): 357 (32700), 422 (72100), 508 (9100), 575 (3100), 646 (2800), 673 (2400) IR wavenumber in cm -1 : NH stretching at 3411 and 3298, C=O stretching at 1689 and 1678

Synthesis of FeTPPBzm

The starting ligand, meso-tetra (o-(2-chloromethyl)benzamidephenyl) porphyrin with an  configuration (500 mg, 0.389 mol), was dissolved in 10 mL of Ar degassed THF then 2,6-lutidine (0.9 mL, 7.781 mol) and FeCl2 (345 mg, 2.723 mol) were added to the reaction mixture. After an overnight stirring at room temperature, the solvent was evaporated, and the crude materiel was dissolved in 100 mL of CHCl3 then extracted with 100 mL of 0.25 M solution of HCl. The organic phase was filtered through a small plug of silica gel then the solvent was evaporated to FeTPPBzm as a dark purple solid (470 mg, yield = 88 %). UV-vis (MeOH): λmax /nm (ε, in M -1 cm -1 ): 341 (29800), 419 (95900), 593 (6400), 656 (3500).

Synthesis of FeTPPIm

FeTPPBzm complex (100 mg, 73 mmol) was dissolved in 5 mL of dry DMF under Ar atmosphere, then 1-methylimidazol was added (23 μL, 291 mmol) to the reaction mixture at room temperature. After 24 hours stirring at 70° C, the reaction mixture was brought to room temperature then approximately 200 mL of diethylether were added. The obtained purple precipitate was filtered and washed with 2 x 50 mL of diethylether to give FeTPPIm (114 mg, yield = 92 %).

The  geometry (the four arms in the same side with regard to the porphyrin plane) of the porphyrin ligand is maintained at room temperature due to the blocked rotation of the aryl groups on the meso positions of the macrocycle. At higher temperature, this rotation becomes possible and can induce the formation of a mixture of four possible atropoisomers (, ,  and ). To confirm the retention of the  geometry during the synthesis of FeTPPIm, we performed a control experiment using the starting porphyrin ligand. In this experiment, a solution of the  porphyrin in DMF was heated at 70° C for more than 24 hours then TLC and 1 H NMR monitoring of this solution showed no modification of the porphyrin geometry. UV-vis (MeOH): λmax /nm (ε, in M -1 cm -1 ): 344 (29400), 421 (102800), 594 (5400), 640 (3100).

FOW analysis of modified iron porphyrin complexes

CO2 binding rate constants of modified iron porphyrins

The CO2 binding constant (KCO2) or binding rate constant (kCO2) was calculated based on procedures reported in literature by analysis of the CVs under Ar and CO2 in dry DMF (to avoid any catalytic turnover in the presence of proton source). For a reversible peak after CO2 binding, the method proposed by Ingle [27][28][29] is widely used. This has been applied for FeTPP, FeTPPF8 and FeTPPF20 to determine CO2 binding constant (KCO2 in M -1 ), with increased scan rate (1 V/s) helping to achieve reversible kinetics. However, for FeTPPUr and FeTPPAm, reversibility was not achieved even by increasing the scan rate indicating that the electron transfer and CO2 binding reaction are rapid, but the reverse reaction is slow. For such a condition, the method proposed by the group of Savéant [30,31] is used, assuming all the complexes follow an EC mechanism. A plot Ep as a function of ln v would give an intercept, where k (in M -1 s -1 ) is calculated. The calculated slope from Ep vs. ln v is 0.0275 which is different from the supposed RT/2F (0.0128), possibly indicating a different mechanism. The equation was then modified accounting for the experimental slope and from here the CO2 binding rate constant was estimated for the catalysts. The solubility of CO2 in DMF at 25 °C is 0.23 M. [32] Reversible kinetics upon CO2 binding:

Irreversible kinetics upon CO2 binding: set-up, CO2 content was estimated to be 1987 μmol based on the solubility of CO2 in the solvents and the amount in the headspace determined by gas chromatography. Chamber 1 was photo-irradiated with a blue LED lamp while Chamber 2 was covered with aluminium foil to avoid any illumination. The twochamber system was then held at room temperature for 21 hours under constant stirring. 50 μL gas aliquots were then withdrawn from the headspace of chamber 1 and subjected to gas chromatography and showed an amount of 440 μmol 12 CO in the headspace. Estimated amount of 12 CO dissolved in the solvents were minimal (39 μmol) because of its poor solubility.

𝐹𝑒 𝐼 +
For 13 CO2 manipulations using the Tritec® system, the modified twochamber system in Figure 0-20 was utilized. Chamber 1 was loaded according to "General protocol for chamber 1" and Chamber 2 was loaded with 4 mL DMF. The chambers were sealed with a screwcap fitted with a Teflon® seal. Chamber 1 was sealed by tightening the red knob while Chamber 2 was sealed by placing a rubber septum. The adaptor was then connected to the Tritec® system. The chambers were degassed three times using freeze-thaw method. 500 μmol of 13 CO2 was then injected to the chambers using the Tritec® system keeping the stopcock open between the two chambers. The loaded chambers were then removed from the Tritec® system and the solutions were left to thaw to room temperature. Once thawed, Chamber 1 was photo-irradiated with a blue LED lamp while Chamber 2 was covered with aluminium foil to avoid any illumination. The two-chamber system was then held at room temperature for 21 hours under constant stirring. 50 μL gas aliquots were then withdrawn from the headspace of chamber 1 and subjected to gas chromatography and showed an amount of 192 μmol 13 CO in the headspace. Estimated amount of 13 CO dissolved in the solvents were minimal because of its poor solubility.

Synthesis of n-hexyl-4-methoxybenzamide using 12 CO2 purging

COware® two-chamber system shown in Figure 0-19 was used. Chamber 1 was loaded according to "General protocol for Chamber 1" and Chamber 2 was loaded with Pd(dba)2 (14.5 mg, 0.025 mmol), P(tBu)3HBPF4 (14.5 mg, 0.05 mmol), 4iodoanisole (293 mg, 1.25 mmol), tetrahydrofuran (3.7 mL), n-hexylamine (0.33 ml, 2.5 mmol), and DABCO (280 mg, 2.5 mmol). The chambers were sealed with a screwcap fitted with a Teflon® seal. Chamber 2 solution was purged with argon gas Annex 201 for 10 minutes (keeping 1 atm pressure by putting an outlet syringe), after which, Chamber 1 was purged with carbon dioxide gas for 15 minutes (keeping 1 atm pressure by putting an outlet syringe). Chamber 1 was photo-irradiated with a blue LED lamp while Chamber 2 was covered with aluminium foil to avoid any illumination. The two-chamber system was then held at room temperature for 21 hours under constant stirring. After the reaction, the crude mixture in Chamber 2 was filtered and the solvent evaporated using rotary evaporator. It was then subjected to column chromatography (30% ethyl acetate in cyclohexane) and isolated as a yellowish solid (85.5 mg, 364 μmol, 83% based on CO, 18% based on CO2).

Synthesis of n-hexyl-4-methoxybenzamide using 13 CO2 Tritec®

Modified two-chamber system shown in Figure 0-20 was used. Chamber 1 was loaded according to "General protocol for Chamber 1" and Chamber 2 was loaded with Pd(dba)2 (14.5 mg, 0.025 mmol), P(tBu)3HBPF4 (14.5 mg, 0.05 mmol), 4iodoanisole (293 mg, 1.25 mmol), tetrahydrofuran (3.7 mL), n-hexylamine (0.33 ml, 2.5 mmol), and DABCO (280 mg, 2.5 mmol). Chamber 1 was sealed by tightening the red knob while Chamber 2 was sealed by placing a rubber septum. The adaptor was then connected to the Tritec® system. The chambers were degassed three times using freeze-thaw method. 501 μmol of 13 CO2 was then injected to the chambers using the Tritec® system keeping the stopcock open between the two chambers. The loaded chambers were then removed from the Tritec® system and the solutions were left to thaw to room temperature. Once thawed, Chamber 1 was photoirradiated with a blue LED lamp while Chamber 2 was covered with aluminium foil to avoid any illumination. The two-chamber system was then held at room temperature for 21 hours under constant stirring. After the reaction, the crude mixture in Chamber 2 was filtered and the solvent evaporated using rotary evaporator. It was then subjected to column chromatography (30% ethyl acetate in cyclohexane) and isolated as a yellowish solid (37 mg, 159 μmol, 82% based on 13 CO, 32% based on 13 CO2).

Synthesis of Moclobemide using 12 CO2 purging

COware® two-chamber system shown in Figure 0-19 was used. Chamber 1 was loaded according to "General protocol for Chamber 1" and Chamber 2 was loaded with Pd(dba)2 (36 mg, 0.0625 mmol), PPh3 (33 mg, 0.125 mmol), 1-chloro-4iodobenzene (298 mg, 1.25 mmol), dioxane (3.4 mL), 4-(2-aminomethyl)morpholine (0.33 ml, 2.5 mmol), and triethylamine (0.34 ml, 2.5 mmol). The chambers were sealed with a screwcap fitted with a Teflon® seal. Chamber 2 solution was purged with argon gas for 10 minutes (keeping 1 atm pressure by putting an outlet syringe), after which, Chamber 1 was purged with carbon dioxide gas for 15 minutes (keeping 1 atm pressure by putting an outlet syringe). Chamber 1 was photo-irradiated with a blue LED lamp and was held at room temperature while Chamber 2 was covered with aluminium foil to avoid any illumination and was held at 80°C (immersed in an oil bath) for 21 hours under constant stirring. After the reaction, the crude mixture in Chamber 2 was filtered and the solvent evaporated using rotary evaporator. It was then subjected to column chromatography (2% methanol in dichloromethane) and isolated as a brownish solid (122.4 mg, 455

μmol, 91% based on CO, 23% based on CO2).

Synthesis of Moclobemide using 13 CO2 Tritec®

Modified two-chamber system shown in Figure 0-20 was used. Chamber 1 was loaded according to "General protocol for Chamber 1" and Chamber 2 was loaded with Pd(dba)2 (36 mg, 0.0625 mmol), PPh3 (33 mg, 0.125 mmol), 1-chloro-4iodobenzene (298 mg, 1.25 mmol), dioxane (3.4 mL), 4-(2-aminomethyl)morpholine (0.33 ml, 2.5 mmol), and triethylamine (0.34 ml, 2.5 mmol). Chamber 1 was sealed Annex 203 by tightening the red knob while Chamber 2 was sealed by placing a rubber septum. The adaptor was then connected to the Tritec® system. The chambers were degassed three times using freeze-thaw method. 530 μmol of 13 CO2 was then injected to the chambers using the Tritec® system keeping the stopcock open between the two chambers. The loaded chambers were then removed from the Tritec® system and the solutions were left to thaw to room temperature. Once thawed, Chamber 1 was photo-irradiated with a blue LED lamp and was held at room temperature while Chamber 2 was covered with aluminium foil to avoid any illumination and was held at 80°C (immersed in an oil bath) for 21 hours under constant stirring. After the reaction, the crude mixture in Chamber 2 was filtered and the solvent evaporated using rotary evaporator. It was then subjected to column chromatography (2% methanol in dichloromethane) and isolated as a brownish solid (43.3 mg, 161 μmol, 85% based on 13 CO, 30% based on 13 CO2). Lastly, further understanding of CODH also led to a proof-of-concept that directly valorizes the photo-produced CO for the synthesis of isotopically-labelled amide bonds, a common motif in pharmaceutically-relevant drugs.

I

  am honored and grateful for all the distinguished scientists in my defense jury: Marc Fontecave, Marc Robert, Fabrice Odobel, Clifford Kubiak, and Sylvie Chardon-Noblat, for taking their time to evaluate my work and for a very insightful three-hour discussion. I want to thank all the people who I collaborated with during my PhD and without whom I could not have published the papers that I needed to graduate: Davide Audisio, Antonio del Vecchio, and Serena Monticelli from Laboratoire de Marquage au Carbone 14 (CEA Saclay) for the two-chamber experiments; Bernard Boitrel from Université Rennes 1 for providing the porphyrin precursors, which made the synthesis of the catalysts less complicated; Marie Sircoglou and Vincent Gandon from Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO) for the DFT calculations; Dooshaye Moonshiram from IMDEA Nanocienca in Spain for the time-resolved X-ray absorption spectroscopy measurements; and Trần Thu Trang, Minh Huong Ha Thi and Thomas Pino from Institut des Sciences Moléculaires d'Orsay for the double flash photolysis experiments. iv I thank all the people in ICMMO who I constantly interact with to characterize the complexes I synthesized and to set-up some needed experiments:

  Cette thèse portait sur la conception et la caractérisation de catalyseurs moléculaires, inspirés de la Nature, pour comprendre comment exploiter efficacement l'énergie solaire et l'utiliser pour des réactions de réduction et de transformation du CO2. Une première étude a été réalisée sur les complexes de rhénium triscarbonyl bipyridyle, photo-activés par des photosensibilisateurs de type ruthénium trisbipyridine afin d'effectuer une réduction du CO2 en CO induite par la lumière visible. L'activité photocatalytique du système a été optimisée et les processus clés étudiés par spectroscopie, électrochimie, et photolyse éclair pour suivre les réactions de transfert d'électrons. En raison de la nécessité d'accumuler des électrons photo-induits sur le catalyseur pour réaliser la réduction du CO2, des études de photolyse à double flash et de photo-accumulation ont été entreprises. De telles investigations ont permis de démêler des voies de réaction productives et délétères lors des excitations multiples, et ces informations fondamentales sont essentielles pour améliorer l'efficacité de l'accumulation de charges afin d'optimiser les systèmes photocatalytiques. Dans cette quête de catalyseurs plus performants pour la réduction du CO2, les recherches se sont également orientées vers le développement de nouveaux complexes à base de métaux abondants comme catalyseurs. De nouveaux mimiques du site actif centré sur le Ni de la CO déshydrogénase (CODH), la seule viii enzyme connue pour réduire le CO2 en CO, ont été synthétisées et étudiées en vue d'une activité de réduction du CO2. Notre stratégie de conception de catalyseur comprenait la prise en compte d'une sphère de coordination non saturée des complexes métalliques en utilisant des ligands penta-coordonnés, ou encore des piliers de liaisons hydrogènes multiples et un environnement de type liquide ionique dans la seconde sphère de coordination. Nous avons mis au point avec succès une série de porphyrines à fer modifiées utilisant les fonctions urée et imidazolium dans un arrangement en 3D méticuleux, ce qui a considérablement réduit le potentiel de réduction du CO2 tout en maintenant une bonne fréquence de cycle catalytique et une bonne sélectivité.

Figure 1 - 2 .

 12 Figure 1-2. Monthly mean carbon dioxide measured at Mauna Loa Observatory, Hawaii from 1958 to 2019, with the red curve showing mole fraction in dry air and the black curve showing mean values after correction for seasonal cycle magnified in the inset. Blue dashed lines correspond to the global surface temperature change from the base period of 1951-1980. Data from NOAA Earth System ResearchLaboratory.[3,5]

Figure 1 - 3 .

 13 Figure 1-3. Initial activation of the linear molecule requires an electron transfer that makes the linear molecule bent. The bent CO2 shows strong charge localization associated with the frontier orbitals.[7]

Figure 1 - 4 .

 14 Figure 1-4. Strategy of artificial photosynthesis in harnessing renewable solar energy while reducing carbon dioxide.

Figure 1 - 5 .

 15 Figure 1-5. General schematic process involved in natural photosynthesis in harnessing solar energy and transforming CO2 to carbohydrates.

Figure 1 - 6 .

 16 Figure 1-6. General schematic process involved in artificial photosynthesis in harnessing solar energy and transforming CO2 into high-energy chemicals.

Figure 1 - 7 .

 17 Figure 1-7. Proposed structure-based mechanism for the catalytic cycle of [NiFe]CODH towards reversible reduction of CO2 to CO.[20]

Figure 1

 1 Figure 1-9.

Figure 1 - 9 .

 19 Figure 1-9.Proposed structure-based mechanism of formate dehydrogenase towards reduction of CO2 to formate.[7,24]

  photo-catalytic system consists of a photosensitizer (PS) capable of absorbing light and creating charge separated states. The holes are replenished by an electron donor (ED), which ideally performs the oxidation of water in the final photoelectrochemical cell, and the electrons are used to activate a reduction catalyst (Cat) for CO2 reduction. On the other hand, an electro-catalytic system takes leverage of photovoltaics to deliver the necessary electrons to the catalyst. Central to both these approaches is the design of homogenous molecular catalysts taking into account a ligand framework able to accumulate charges, a metal-centered catalyst able to capture and bind CO2, and a second coordination environment able to activate the substrate, all working in synergy to efficiently reduce CO2. Such strategy would not only provide fundamental understanding of transforming solar energy into chemical energy but also important information for the development of efficient bio-inspired catalysts.

Figure 1 - 10 .

 110 Figure 1-10. Bio-inspired strategy employing a modular molecular approach in developing photo-and electro-catalytic systems for CO2 reduction. (ED = electron donor, PS = photosensitizer, Catred = reduction catalyst, Catox = oxidation catalyst PV = photovoltaics).

Figure 1 - 11 .

 111 Figure 1-11. Possible electron transfer quenching mechanisms of an excited photosensitizer (PS = photosensitizer, Cat = catalyst, ED = electron donor).

Figure 1 - 12 .

 112 Figure 1-12. Some common and notable photosensitizers used for the photocatalytic CO2 reduction.

Figure 1 - 13 . 37 ]

 11337 Figure 1-13.Electron donors (EDs) used in the photocatalytic CO2 reduction.[25,37]

Figure 1 - 14 .

 114 Figure 1-14. Common structural motifs of CO2 reduction catalysts containing a central metal atom and a ligand framework.

1- 15 )

 15 is accompanied by a fast loss of the halide ligand, producing a doublyreduced [Re 0 (bpy •-)(CO)3] -intermediate which is active towards the reduction of CO2

Figure 1 - 16 .

 116 Figure 1-16.Effect of modifying the diimine ligand on the first reduction potential (V vs Fc +/0 ) of the Re catalyst and the photocatalytic turnover number for CO2 to CO in DMF with TEOA as ED.[53]

Figure 1 - 17 .

 117 Figure 1-17. (a) Trend between electron donating character of 4,4'-bpy substituent and 1 st reduction potential of the Re catalyst in ACN. (b) Trend between electrocatalytic overpotential and log TOF in CO2-saturated ACN with 1 M PhOH.Data from[54] .

Figure 1 - 18 .

 118 Figure 1-18. Effect of changing the metal center in bipyridine carbonyl-based complexes on the electrocatalytic TOF and overpotential towards CO2 reduction to CO in ACN.

  Fe II TPP]/[Fe II (TPP • )] -and [Fe II (TPP • )] - /[Fe II (TPP •• )] 2-. Since these electrons are delocalized in the conjugated macrocycle, the corresponding electron transfer will involve marginal geometric distortions of the ligand, and hence a low reorganizational energy. The formal Fe 0 reacts with the CO2 substrate producing two mesomeric forms: [Fe I (CO2 • )] 2-and [Fe II (CO2 •• )] 2-. DFT

Figure 1 - 19 .

 119 Figure 1-19. Proposed mechanism for the CO2 to CO reduction by iron porphyrins in the presence of a proton source.

Figure 1 - 20 .

 120 Figure 1-20. (a) Catalytic Tafel plots of modified analogues of iron porphyrin with the structures shown at the bottom. (b) Correlation of TOFmax with the catalytic potential showing through-structure substituent effects. Data from Ref. [83] in DMF with 3 M PhOH.

Figure 1 - 21 .

 121 Figure 1-21. Introduction of local proton sources in iron porphyrin catalysts.[84,85]

1 - 22 .

 122 Figure 1-22. Establishment of local proton sources in Re-and Mn-based triscarbonyl bipyridyl complexes and Ni cyclams. [53,87-90]

  -23. Molecular dynamics simulation, 2D NMR experiments, and IR spectroelectrochemical experimentsshowed a stable configuration where the catalyst arm adopts a folded configuration that places the phenol moiety close to the bpy ligand, allowing interactions between the pendant proton and the metal-bound substrate. Catalyst 29 showed catalytic CO2 reduction activity but catalyst 30 precipitated on the electrode under electrocatalytic conditions. This peptide-linkage approach is nevertheless interesting as the inorganic Re catalyst can be inserted at any desired point in the linkage during peptide synthesis, allowing more avenues of exploration and optimization.

Figure 1 - 23 .

 123 Figure 1-23.Incorporation of peptide linkages containing tyrosine residues in a Rebased triscarbonyl bipyridyl catalyst.[91]

  derivatives of [Ni(di-methylcyclam)] 2+ and [Ni(tetra-methylcyclam)] 2+ showed diminished electrocatalytic CO2 reduction activities due to the absence of hydrogenbond induced stabilization effects and increased steric congestion. This further highlight the importance of amine protons in promoting hydrogen bonding interactions.

Figure 1 - 24 .

 124 Figure 1-24. Establishment of hydrogen bond in the periphery of CO2 reduction catalysts.

Figure 1 - 25 .Figure 1 -

 1251 Figure 1-25. Incorporation of amino and amido donors in the periphery of iron porphyrin catalysts. [85,100,101]

  the importance of suitable orientation of the electrostatic effect. On the other hand, the incorporation of negatively charged sulfonate substituents (structure 41) resulted in a decrease in the TOF as a result of electrostatic repulsions with the negatively charged metal carboxylate intermediate. These effects are clearly observed in the deviations in the catalytic Tafel plots and linear scaling shown in Figure 1-26, with better catalysts appearing in the upward left part of the plot while the poor catalysts deviating towards the downward right part.

Figure 1 -

 1 Figure 1-26. (a) Catalytic Tafel plots of modified analogues of iron porphyrin with the structures shown at the bottom. (b) Correlation of TOFmax with the catalytic potential showing through-space substituent effects. Data from Ref. [83] in DMF with 3 M PhOH.

FeTPPFigure 1 - 27 .

 127 Figure 1-27.Incorporation of a cationic imidazolium moiety in the periphery of Reand Mn-based catalysts with the proposed mechanism.[103,104]

3. 1 .

 1 IntroductionThe urgent need to decarbonize the current energy mix because of global warming has accelerated research in the development of catalysts that can both utilize renewable solar energy and CO2 and transform them into energy vectors that can be stored, transported, and used upon demand. Common forms would include the development of electrocatalysts and coupling them with photovoltaics harnessing solar energy, or the development of a direct photocatalytic system. Both forms still require the development of efficient and stable catalysts. A variety of catalyst design strategies have been reported for the homogenous reduction of CO2 as discussed in Chapter 1 and an interesting approach is the development of bioinspired catalysts.

Figure 3 - 1 .

 31 Figure 3-1. Crystallographic structures of the C-clusters in enzymes from three organisms: (a) dithionite-reduced C. hydrogenoformans CODH II at 1.1 Ǻ, (b) COtreated M. thermoacetica CODH at 1.9 Ǻ, and (c) R. rubrum CODH at 2.8 Ǻ. Figures taken from ref[1] .

  Figure 3-1. Crystallographic structures of the C-clusters in enzymes from three organisms: (a) dithionite-reduced C. hydrogenoformans CODH II at 1.1 Ǻ, (b) COtreated M. thermoacetica CODH at 1.9 Ǻ, and (c) R. rubrum CODH at 2.8 Ǻ. Figures taken from ref[1] .

  the thermodynamic potential of the CO2/CO couple, indicating a significantly low overpotential compared to synthetic CO2 reduction catalysts. Crystallographic studies show that the C-cluster active site of the enzyme generally consists of Ni and Fe metal centers bridged by a [Fe3S4] cluster that fix the positions of these two metal centers in close proximity, as shown in Figure 3-1. Various organisms possess enzymes which display different coordination around the nickel atom with a planar NiS4 coordination in Carboxydothermus hydrogenoformans CODH II (Figure 3-1a) or a distorted tetrahedral one in Moorella thermoacetica CODH (Figure 3-1b) (a) (b) (c)

  -2b. The change from the distorted T-shaped to the square planar coordination at the Ni ion only induces minor changes in the geometry of the cluster as observed in the superposition of the structures in Figure 3-2c. In the CO2 adduct, Ni acts as Lewis base while the nearby

Figure 3 - 2 .

 32 Figure 3-2. Crystallographic data on the reduced state (-600 mV vs SHE) of the active site of C. hydrogenoformans CODH II in the (a) absence and (b) presence of CO2, and (c) the corresponding superposition of the structures. Figures taken from ref[4].

  Figure 3-2. Crystallographic data on the reduced state (-600 mV vs SHE) of the active site of C. hydrogenoformans CODH II in the (a) absence and (b) presence of CO2, and (c) the corresponding superposition of the structures. Figures taken from ref[4].

Figure 3 - 3 .

 33 Figure 3-3. Other [NiFe3S4] complexes consist of the closely-resembling square

Figure 3 - 3 .

 33 Figure 3-3. Structural mimics of the Ni-centered C cluster site of CODH.

Figure 3 - 4 .

 34 Figure 3-4. Some notable functional models of sulfur-rich Ni-centered catalysts able to perform proton reduction to dihydrogen (complexes 4-5), CO2 reduction to CO (complex 6), and CO2 reduction to HCOOH (complex 7).

  Ni (II) complexes by modifying the primary coordination sphere of the reported Ni-N2S2 catalyst, with the structures shown in Figure3-5. Specifically, this chapter aims at: • investigating how (i) the extra thioether donor in a pentadentate Ni-N2S3 catalyst compared to the tetradentate Ni-N2S2, and (ii) replacement of the pyridine donors in Ni-N2S3 with phosphine donors in Ni-P2S3, affect the electrocatalytic reduction activity • investigating the differences in the photocatalytic CO2 and proton reduction activities of Ni-N2S2 and Ni-N2S3 and mechanistically follow the intermediate species by time-resolved UV-Vis and X-ray absorption spectroscopy.

Figure 3 - 5 .

 35 Figure 3-5. Synthesis of the catalysts Ni-N2S2, Ni-N2S3, and Ni-P2S3.

  -5, reaction of 2-picolyl chloride with 1,2-ethanedithiol yields the N2S2 ligand, while reaction of 2-picolyl chloride or 2-chloroethyldiphenylphosphine with 2,2′-thiodiethanethiol yields the N2S3 and P2S3 ligands, respectively. These ligands were characterized by proton NMR and mass spectrometry measurements as shown in the supplementary information in Annex. Nickel (II) complexes were then obtained by complexation of the ligands with a Ni perchlorate salt in methanol/acetonitrile solution resulting respectively in Ni-N2S2, Ni-N2S3, and Ni-P2S3, as confirmed by mass spectrometry measurements. Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of ethyl acetate in an acetonitrile (ACN) solution of the complexes. As shown in Figure 3-6, X-ray structures of the Ni-N2S2 and Ni-N2S3 complexes show a hexacoordinated Ni in a slightly distorted octahedral geometry. In both cases, the central metal is coordinated by two aromatic nitrogens from the pyridines and three or two sulfur atoms from the thioethers. The first coordination sphere is completed by one (for Ni-N2S3), or two (for Ni-N2S2) nitrogens from acetonitrile solvent. As shown in Table 3-1, the bond angles in Ni-N2S3 are more deviated from the ideal 90° compared to Ni-N2S2, indicating a greater distortion of the coordination sphere in Ni-N2S3. The bond with coordinated axial S1 atom from thioether is more elongated in Ni-N2S3 than in Ni-N2S2 and is even longer than the bond length of coordinated ACN solvent, indicating a weak coordination bond for the S1 atom. This is an indication that S1 may be relatively labile in solution for Ni-N2S3.

Figure 3 - 6 .

 36 Figure 3-6. Capped sticks representation of the X-ray crystal structures of (a) Ni-N2S2, (b) Ni-N2S3, and (c) Ni-P2S3. Counter anions and hydrogen atoms are omitted for clarity.

- 7 .

 7 It can be observed that the first reduction peak, attributed to the reduction of Ni II to Ni I , shifts anodically going from Ni-N2S2 to Ni-N2S3 and to Ni-P2S3, as seen in Table 3-2. The addition of the extra thioether in Ni-N2S3 resulted in a potential shift of 180 mV compared to Ni-N2S2. This was similarly observed in an earlier systematic study by the group of Adhikary on Ni complexes bearing N2Sx ligands (where x represents increasing number of sulfur donors

and - 1 .

 1 56 V for Ni-N2S3. Contrary to the anodic shift observed for the first reduction of Ni-N2S3 compared to Ni-N2S2, a 90 mV cathodic shift is observed for this second wave. This irreversible reduction wave was attributed by the group of Kojima to the Ni I/0 couple which involves a large structural change from an octahedral Ni I complex having a d 9 electronic configuration to a tetrahedral Ni 0 complex with a d 10 configuration. As such, the change from a tetradentate ligand structure in Ni-N2S2 to the sterically hindered pentadentate ligand structure in Ni-N2S3 can explain the cathodic shift in the second reduction process. However, the reversibility is gained, and an anodic shift of 70 mV is observed when the pyridines are replaced by phosphines in Ni-P2S3. This is indicative of the fact that even though Ni-N2S3 and Ni-P2S3 share the same pentadentate configuration, the high polarizability of the phosphine ligands in tandem with the thioether donors can stabilize the Ni 0 state.

Figure 3 - 7 .

 37 Figure 3-7. Cyclic voltammograms of 1 mM of catalyst Ni-N2S2 (red), Ni-N2S3 (blue), and Ni-P2S3 (green) in Ar-degassed ACN containing 0.1 M TBAP.

Figure 3 - 8 .

 38 Figure 3-8. Cyclic voltammograms of 1 mM of catalyst (a) Ni-N2S2, (b) Ni-N2S3, and (c) Ni-P2S3 in the absence (black) and presence of CO2 with 5.5 M H2O (red) in ACN containing 0.1 M TBAP. (d) Comparison of the CVs of the catalysts under catalytic conditions (CO2 + 5.5 M H2O).

Figure 3 - 9 ,

 39 Figure 3-9, the CVs of the modified catalysts in the presence of acetic acid showed catalytic current enhancement after the second reduction peak.

Figure 3 - 9 .

 39 Figure 3-9. Cyclic voltammograms of 1 mM of catalyst (a) Ni-N2S2, (b) Ni-N2S3, and (c) Ni-P2S3 in the absence (black) and presence of increasing concentrations of acetic acid in ACN containing 0.1 M TBAP. (d) Comparison of the CVs of the catalysts under catalytic conditions (50 mM acetic acid).

- 10 ,

 10 Ni-N2S2 largely outperforms Ni-N2S3 and Ni-P2S3 for the photocatalytic reduction of CO2 to CO with a TOF of 445 h -1 and TON of 1130 after 19 h. As shown in the inset of Figure 3-10, the pyridine-based Ni-N2S3 shows a better photocatalytic activity with a TOF of 48.6 h -1 and TON of 70 compared to the phosphine-based catalyst Ni-P2S3 (TOF 24.6 h -1 , TON 40).

Figure 3 - 10 .

 310 Figure 3-10. Photocatalytic production of CO using 5 µM of catalyst (Ni-N2S2 in red, Ni-N2S3 in blue, and Ni-P2S3 in green), 50 µM [Ru(bpy)3] 2+ , 100 mM BIH and 5.5 M H2O in CO2-saturated dimethylacetamide. Inset shows a magnified view comparing the activities of Ni-N2S3 and Ni-P2S3. Irradiation source: blue LED lamp centered at 463 nm.

8 h - 1 compared

 1 photocatalytic activities are too different compared to the electrocatalytic activities for both proton and CO2 reduction, indicating differing mechanisms at play. On the other hand, the Ni-P2S3 catalyst didn't produce any detectable amount of H2 gas possibly owing to the large overpotential observed for the catalytic proton reduction to occur.

Figure 3 - 11 .

 311 Figure 3-11. Photocatalytic production of H2 using 100 µM of catalyst (Ni-N2S2 in red and Ni-N2S3 in blue) and 250 µM [Ru(bpy)3] 2+ in Ar-saturated 1:1 dimethylacetamide : ascorbate buffer (pH 4, 100 mM).

  Ru II ) as photosensitizer, and ascorbate buffer (pH 4) as the reversible electron donor (denoted as ED) in aqueous acetonitrile solution (1:1 v/v). The thermodynamic potentials of each of these components were determined electrochemically and the energy diagram is mapped in Figure 3-12.

Figure 3 - 12 .

 312 Figure 3-12. Thermodynamic diagram associated with the photocatalytic cycle involving the catalysts Ni-N2S3 (blue) or Ni-N2S2 (red), Ru II as photosensitizer and ascorbate as electron donor (ED). Colored text indicates the spectral signatures followed for the time-resolved absorption measurements.

Figure 3 - 13 .

 313 Figure 3-13. Stern-Volmer plot of the emission quenching of Ru II* monitored at 613 nm in Ar-purged ACN:ascorbate buffer (pH 4, 1:1 v/v) at various concentrations of either ascorbate buffer (black) or Ni-N2S3 (blue).

Figure 3 - 14 .

 314 Figure 3-14. Transient absorption spectra of 24 μM [Ru(bpy)3] 2+ , 660 μM Ni-N2S3, in Ar-purged ACN:asc buffer (100 mM buffer at pH 4; 1:1 v/v). Laser energy = 8 mJ, λexc at 460 nm. Green solid arrow shows initial growth of Ru I , blue arrows show decay of Ru I with concomitant growth of Ni I , and black arrows show decay of Ni I .

Figure 3 - 15 .

 315 Figure 3-15. Transient absorption kinetics at 510 nm (left) due to the formation and decay of Ru I and 440 nm (right) due to the growth and decay of Ni I (Ni-N2S3) as a function of catalyst concentration in Ar-purged ACN/asc buffer (pH 4, 1:1 v/v).

Figure 3 - 16 .

 316 Figure 3-16. Transient absorption kinetics at 510 nm (left) due to the formation and decay of Ru I and at 440 nm (right) due to the growth and decay of Ni I (Ni-N2S2) as a function of catalyst concentration in Ar-purged ACN/asc buffer (pH 4, 1:1 v/v).

Figure 3 - 17 .

 317 Figure 3-17. Comparison of the plots of globally fitted k against concentration of Ni-N2S3 (red) and Ni-N2S2 (black) giving electron transfer rate constants of 1.11 x 10 8 M -1 s -1 and 2.16 x 10 7 M -1 s -1 , respectively.

Figure 3 - 18 .

 318 Figure 3-18. Decay associated spectra (DAS) of the transient absorption kinetics of (a) Ni-N2S3 and (b) Ni-N2S2 catalysts taken at 1 ms delay at various wavelengths with similar conditions as described in Figure 3-14. Black lines correspond to [(Ru I + Asc + ) -(Ru II + Asc)] while the red lines correspond to [(Ni I + Asc + ) -(Ni II + Asc)], with their respective lifetimes.

Figure 3 - 19 .

 319 Figure 3-19. DFT-optimized UV-visible absorption spectra for the proposed Ni II and Ni I geometries of the Ni-N2S3 catalyst. Inset shows the difference spectra between Ni II and Ni I . See Appendix for calculation details.

  resolved X-ray absorption spectroscopy (tr-XAS) techniques to determine the changes in the coordination environment and oxidation states of the Ni metal of the Ni-N2S3 and Ni-N2S2 catalyst during photocatalysis. This was done in collaboration with Dr. Dooshaye Moonshiram from IMDEA Nanociencia in Madrid. The X-ray absorption near structure (XANES) of Ni-N2S3 and Ni-N2S2 was first investigated in a mixture of acetonitrile and water, as shown in Figure 3-20. Although the metal centers in both catalysts have the same oxidation states, the local coordination spheres and geometries of both complexes are clearly different explaining the shift in energy and changes in the 1 s → 4 p main transitions along the rising edge. The energy shift is well reproduced by spectra simulated by time-dependent Density Functional Theory (td-DFT) methods (Figure 3-20 inset) showing that geometry optimizations and ab-initio XANES simulations used for both catalysts reproduce the experimental data well.

Figure 3 - 20 . 3 96

 3203 Figure 3-20. Normalized experimental Ni K-edge XANES of Ni-N2S3 (black) and Ni-N2S2 (red) in 1:1 (v/v) ACN:H2O. Inset shows the details of the pre-edge and rising edge regions in comparisons to the calculated XANES spectra of Ni-N2S3 and Ni-N2S2 using DFT-optimized coordinates (see Appendix).

Figure 3 - 21 .

 321 Figure 3-21. (a) Fourier transforms of k 2 -weighted Ni EXAFS of Ni-N2S3 (black) and Ni-N2S2 (red) in 1:1 (v/v) ACN:H2O. Inset shows the back Fourier transforms of the experimental results (solid lines) and fitting of k 2 χ(k) (dashed lines). (b) Simulated EXAFS spectra (solid lines); atomic coordinates were obtained from single crystal Xray diffraction structures of the catalysts and from DFT simulations (dashed lines).

Figure 3 - 22 .

 322 Figure 3-22. (a) Experimental difference spectra corresponding to the Ni I transient signal for Ni-N2S3 at ~12 µs. (b) Theoretical XANES simulations corresponding to the difference between Ni II with a bound acetonitrile molecule and Ni I with unbound solvent molecule. (c) Stacked spectra corresponding to a series of time-delays between laser and X-ray pulses. These measurements were carried out for a range of averaged time delays from 0-23 µs for a complete photocatalytic system when bubbled in N2 gas.

Figure 3 - 23 .

 323 Figure 3-23. (a) Calculated Ni I geometries with different coordinated ligands. (b) Calculated optical absorption spectra corresponding to the optical signatures of Ni I with a bound acetonitrile molecule (black), Ni I with a bound water molecule (red) and Ni I with no solvent molecule (cyan). (c) Theoretical XANES simulations corresponding to the difference between Ni II with a bound acetonitrile molecule and Ni I with a bound acetonitrile molecule (black), Ni I with a bound water molecule (red) and Ni I with no solvent molecule (cyan).

  the electronic and structural configuration of the reduced Ni I species through time-resolved XAS and DFT calculations, complemented the kinetics and spectral information attained by time-resolved visible absorption measurements. This information point to some interesting initial events, as proposed in Figure3-24. An acetonitrile-coordinated Ni II -N2S3, is initially observed when dissolved in the aqueous acetonitrile solution. In the presence of light, an excited Ru II * is formed which is efficiently quenched by the ascorbate electron donor producing the reduced Ru I . This species then transfers an electron to the catalyst with a close-to-diffusion-limited bimolecular rate constant of 1.11 x 10 8 M -1 s -1 . The reduced catalyst is described by having a metal-centered reduction giving a NiI

Figure 3 - 24 .

 324 Figure 3-24. Proposed catalytic cycle for the photocatalytic reduction of either protons (blue) or CO2 (red) by Ni-N2S3 with the colored boxes indicating the spectroscopic tracking of the intermediates performed in this work.

Figure 4 - 1 .

 41 Figure 4-1.

Figure 4 - 1 . 4 107

 414 Figure 4-1. Active site of CO dehydrogenase (CODH) and some notable synthetic analogues showing bio-mimetic (structural) and bio-inspired (bimetallic and substrate-activating) strategies.

••Figure 4 - 2 .

 42 Figure 4-2. General bio-inspired strategy focusing on second coordination substrate activation in improving CO2 reduction catalysts.

Figure 4 - 3 .

 43 Figure 4-3. Synthesis of rhenium triscarbonyl mono-amino (ReAm1) and di-amino (ReAm2) substituted bipyridyl catalysts.

Figure 4 - 4 .

 44 Figure 4-4. Cyclic voltammograms of 1 mM ReAm1 (red) and ReAm2 (blue) in comparison with the nonfunctionalized Re catalyst (black) in Ar-purged ACN with 0.1 M TBAP at a scan rate of 100 mV/s. Inset shows the effect on the first reduction potential peak of each addition of amine (slope = mV / amine).[a] For irreversible peaks, potentials at half-wave point are reported.

Figure 4 - 5 .

 45 Figure 4-5. Spectra of observed species during IR spectroelectrochemistry of (a) ReAm1 and (b) ReAm2 in ACN with 0.1 M TBAP under N2 atmosphere. (Colors correspond to cycle proposed in Figure4-6: black -unreduced species, red -singlyreduced species, blue -solvated doubly-reduced species, and green -unsolvated doubly-reduced species). Condition consisted of 100 s electrolysis per 50 mV stepwise reduction and as such, contributed to persistence of some peaks in the proceeding potential steps; intermediates attained at the following V vs Fc +/0 , ReAm1: -1.79, -1.89, -2.29; ReAm2: -1.84, -2.24, -2.54;

Figure 4 - 6 .Figure 4 - 7 .

 4647 Figure 4-6. Proposed catalytic cycle for the modified catalysts ReAm1 and ReAm2.

  the nonfunctionalized Re catalyst, indicate that the first electron reduction induces the loss of the halide and it is possibly replaced by ACN solvent, [Re(CO)3(bpy-R,R'))(ACN)]• . This indicates that the halide is more labile in these modified catalysts. The loss of reversibility of the first reduction peak observed in the cyclic voltammetry measurements is in accordance with this halide loss. It should be noted that no significant changes in the carbonyl stretching values were observed between the nonfunctionalized and the substituted catalysts.When a potential corresponding to the second reduction is gradually applied to the catalysts, the C=O stretching observed at 2009 cm -1 , attributed to the axial carbonyl ligand, is shifted towards lower wavenumbers in ReAm1 (1999 cm -1 ) and -cm -1 ) with a broadening of the stretching of the other carbonyls (1870 cm -1 for ReAm1 and 1876 cm -1 for ReAm2) is observed. The decrease of the wavenumber corresponds to better stabilization of the carbonyl ligands due to the increased π back-bonding from the reduced metal to the CO ligand. This reduced species can be formulated as [Re(CO)3(bpy-R,R'))(ACN)]-, with an ACN molecule in the coordination sphere, based on comparison of the IR peaks with literature reports on nonfunctionalized Re catalyst. Further electrolysis eventually removes this coordinated solvent, giving the penta-coordinated doubly reduced form,[Re(CO)3(bpy-R,R'))]-. This form is characterized by IR absorption bands at 1989cm -1 and 1868 cm -1 for ReAm1 and very similarly observed in ReAm2 (1988 cm-1 and 1866 cm -1 ). The IR peaks of this final active form are shifted to higher wavenumbers (higher vibrational energy) compared to the IR peaks observed in the nonfunctionalized Re catalyst shown in Figure4-7d. This is indicative of the destabilization of the C=O ligands either due to the presence of the amino groups in the proximal position of the bipyridyl ligand (6,6' position) or the relocation of electron density away from the carbonyl ligands once a pentadentate doubly reduced form is attained. This destabilization does not depend however on the number of amines (no difference in wavenumbers between ReAm1 and ReAm2), indicating that already one amine placed in the 6/6' position of the bipyridyl ligand causes such higher energy stretching of the carbonyl ligands of the doubly-reduced form. This effect of amino substitution observed by IR spectroscopy could indicate differences in the activity of the modified catalysts compared to the nonfunctionalized one. How it translates to catalytic activity is another aspect that needs to be further investigated because IR spectral changes from this active form are not easily tracked under catalytic conditions.

Figure 4 - 8 .

 48 Figure 4-8. Cyclic voltammograms of 1 mM of catalysts: (a) ReAm1 (b) ReAm2, and (c) Re, in the presence of Ar (black), CO2 (red), CO2 + 5.5 M H2O (blue), and CO2 + 700 mM TFE (green) in ACN containing 0.1 M TBAP at a scan rate of 100 mV/s. (d) Comparison of catalytic Tafel plots (Re -black, ReAm1 -red, ReAm2 -blue; with water (dotted line) or TFE (solid line) as proton source water; see Annex for calculation details).

Figure 4 - 9 .

 49 Figure 4-9. Comparison of the TOFmax and catalytic overpotentials of ReAm1 (red) and ReAm2 (blue) with the nonfunctionalized Re catalyst (black). Effect of proton source is shown with water (green line, circular markers) and TFE (pink line, square markers).

  atropoisomer was chosen with the target to provide two sets of hydrogen bonding network in a trans fashion towards a metal bound CO2 molecule and at the same time to confer two identical chemical faces on each side of the porphyrin molecular platform. As shown in Figure 4-10, the addition of four equivalents of 3,5-bis(trifluoromethyl)phenyl isocyanate or 3,5-bis(trifluoromethyl)benzoyl chloride to 5,10,15,20-tetrakis(2-aminophenyl)porphyrin with an αβαβ configuration

Figure 4 - 10 . 4 120X

 4104 Figure 4-10. Synthetic scheme for the preparation of FeTPPUr and FeTPPAm.

Figure 4 - 11 .

 411 Figure 4-11. Wireframe and capped sticks representation of the X-ray structures of FeTPPUr (left) and FeTPPAm (right) having methoxide anion as an axial ligand. Only urea and amide protons are explicitly shown, and solvent molecules are omitted for clarity.

Figure 4 - 12 .

 412 Figure 4-12. Reference complexes, including the nonfunctionalized FeTPP, and its fluorinated analogues FeTPPF8 and FeTPPF20 used for comparison.

Figure 4 - 13 .

 413 Figure 4-13. CVs of 1 mM FeTPP (black) and its modified analogues (green = FeTPPAm, blue = FeTPPF8, pink = FeTPPF20, red = FeTPPUr) in Ar-saturated DMF containing 0.1 M TBAP at a scan rate of 100 mV/s.

Figure 4 - 14 .

 414 Figure 4-14. (a) CVs of 1 mM FeTPP (black) and its modified analogues (green = FeTPPAm, blue = FeTPPF8, pink = FeTPPF20, red = FeTPPUr) in CO2-saturated DMF containing 0.1 M TBAP and 5.5 M H2O at a scan rate of 100 mV/s. (b) Catalytic Tafel plot comparisons of the catalysts with the red dot corresponding to bulk electrolysis result of FeTPPUr (see Annex for calculation details).

4 -Figure 4 - 15 .

 4415 Figure 4-15. Comparison of the TOFmax and catalytic overpotentials of FeTPPUr and FeTPPAm with the expected electronic scaling effect observed in FeTPP, FeTPPF8, and FeTPPF20.

Figure 4 - 16 .

 416 Figure 4-16. CVs of 1 mM FeTPP in the absence (dotted line) and presence of increasing amounts of external urea additive (solid line: 7 mM, 86 mM, 798 mM) in CO2-saturated DMF containing 5.5 M H2O.

  from single crystals obtained by slow evaporation of H2O/acetone-saturated solution of TPPUr porphyrin, revealed the presence of four acetone molecules bound to the four urea groups through N-H•••O=C hydrogen bonding interactions (Figure 4-17), confirming the stabilization of the carbonyls by urea moieties.

Figure 4 -

 4 18).

Figure 4 - 17 . 1 FeTPP 7 . 6 FeTPPAm 6 . 8 FeTPPUr 58 . 0 Figure 4 - 18 .

 41717668580418 Figure 4-17. Wireframe and capped sticks representation of the X-ray structure of TPPUr showing H-bonding interactions between the hydrogen atoms of urea and the carbonyl of acetone molecules.

Figure 4 -

 4 19, with H•••O donor-acceptor distances of 2.007 Å and 2.345 Å, with average N•••(H)•••O distance of 3.187 Å. In marked contrast, two strong hydrogen bonds were engaged by each urea arms of FeTPPUr yielding four short H•••O distances of 1.756 Å, 1.759 Å, 1.880 Å, and 1.888 Å, with an average

Figure 4 - 19 .

 419 Figure 4-19. Optimized geometries of model compound [FeTPPUr-CO2] 2-(left) and [FeTPPAm-CO2] 2-(right) showing main structural parameters.

Figure 4 - 20 .

 420 Figure 4-20. (a) Plot of calculated kcat (calculated from FOW analysis) of 1 mM FeTPPUr in CO2-saturated DMF as a function of concentration of added H2O (blue) or D2O (red). (b) plot of log kcat as a function of pKa of the acid (PhOH -black, TFEgreen) at 5 M concentration.

  was performed on FeTPPUr at -1.77 V vs Fc +/0 to measure the efficiency at which CO is produced and to evaluate the stability of the catalyst. Gas chromatography analysis of the reaction headspace during bulk electrolysis showed no hydrogen formation and the catalyst FeTPPUr operates with the exclusive formation of CO with a Faradaic efficiency as high as 91% (Figure4-23a). A stable current density of 0.77 mA cm -2 was achieved during the two-hour electrolysis run which confirms the good stability of the catalyst for the duration of

Figure 4 - 22 .Figure 4 - 21 .

 422421 Figure 4-22. Optimized geometries of model compound of [FeTPP-Ur-CO2] 2-where a water molecule was inserted showing main structural parameters.

  -14b, this calculated TOF value fit well in the catalytic Tafel plot of FeTPPUr estimated by FOW analysis, indicating reliability of such quick estimation on catalytic performances. These results show that bio-inspired catalyst design for improved CO2 reduction activity requires (i) careful choice of functional groups acting as substrate or intermediate stabilizers, (ii) specific 3D engineering of these functions within proper distances towards the active site, and (iii) careful management of proton supply within the vicinity of the active site.

Figure 4 - 23 .

 423 Figure 4-23. (a) Progress of CO evolution and the corresponding Faradaic efficiency and (b) current density changes during bulk electrolysis at -1.77 V vs Fc +/0 of a CO2-saturated DMF containing 0.5 mM FeTPPUr, 5.5 M H2O, and 0.1 M TBAP.

  interactions to the catalytic activity of iron porphyrin catalyst.

Figure 4 - 24 .

 424 Figure 4-24. Synthesis of imidazolium-tethered iron porphyrin catalyst, FeTPPIm.

Figure 4 - 25 .

 425 Figure 4-25. (a) CVs of 1 mM FeTPPIm in DMF solution containing 0.1 M TBAP in the absence (black) and presence of CO2 (red), and presence of CO2 + 5.5 M H2O (blue). (b) CVs of 0.5 mM FeTPPIm in H2O solution containing 0.1 M KCl in the absence (black) and presence of CO2 (red).[Reference electrode for aqueous solution was SCE but potentials are converted to Fc according to Ref.[77] for better comparison].

Figure 4 - 26 .

 426 Figure 4-26. (a) Catalytic Tafel plot of FeTPPIm (orange) in comparison with the modified catalysts described in Section 4.4 (black -FeTPP, blue FeTPPF8, pink -FeTPPF20, green -FeTPPAm, red -FeTPPUr) and (b) how FeTPPIm deviates from the linear scaling relationship.

Figure 4 - 27 .

 427 Figure 4-27. (a) Progress of CO evolution and the corresponding Faradaic efficiency and (b) current density changes during bulk electrolysis at -1.64 V vs Fc +/0 of a CO2-saturated H2O containing 0.5 mM FeTPPIm, and 0.1 M KCl.

[ 3 ]Figure 4 - 28 .

 3428 Figure 4-28. Catalytic Tafel plots comparing the electrocatalytic performances of the studied catalysts with notable reported catalysts.

  catalytic activity of well-known CO2 reduction catalysts by applying key learnings from the structure-function studies in CO dehydrogenase enzymes, known to reversibly reduce CO2 to CO at an exceptional efficiency and stability. Learning from the hydrogen bond stabilization of nearby lysine and histidine residues towards the metal carboxylate intermediate in the enzyme, we have systematically investigated the effect of hydrogen bond donors in the periphery of different catalysts.Introducing one or two proximal amines in the 6,6' positions of the bipyridyl ligand of the Re triscarbonyl catalyst increased the overpotential for CO2 reduction without significant increase in the TOF. This result is attributed to the improper positioning of the hydrogen bond donors affecting both the first and second coordination spheres. Installing functions in the periphery of such Re catalysts seems complicated because of the intrinsic mechanistic pathway involving the loss of a halide ligand resulting in a pentadentate active form, requiring more sophisticated considerations in catalysts design. Improved catalytic activity by hydrogen bonding stabilization was successfully achieved when urea functions were installed in an αβαβ iron porphyrin atropoisomer with significant decrease of the overpotential while maintaining a high TOF. Such a catalytic enhancement was achieved through a pre-organized positioning of the multi-point NH fragments on two urea arms within hydrogen bonding distances from the center. Our systematic study also highlighted the advantage of the multi-point hydrogen bonding from urea groups in FeTPPUr for CO2 electrocatalytic reduction when compared to the single-point Hbonding from amide groups in FeTPPAm. More importantly, DFT calculations showed that the metal carboxylate intermediate remains in a quasi-unchanged topology while a water molecule is inserted in the second coordination sphere, explaining the synergistic effect of water as a sufficient proton source for the catalytic reduction of CO2 in the case of the FeTPPUr catalyst.

[Re(CO) 3 ( 3

 33 photo-catalytic reduction of protons to hydrogen gas compared to the reference Ni-N2S2 catalyst. Time-resolved UV-vis and X-ray absorption spectroscopic techniques were able to track down and identify the nature and environment of the

Chapter 4 ,

 4 another bio-inspired strategy was employed learning from the hydrogen bonding stabilization of nearby lysine and histidine residues towards the metal carboxylate intermediate in the CODH enzyme. Systematic investigation was done on the effect of hydrogen bond donors in the periphery of CO2 reduction catalysts. Establishing one or two proximal amines in the 6,6' positions of the bipyridyl ligand of the Re triscarbonyl catalyst increased the overpotential for CO2 reduction without significant increase in the TOF. The effect is attributed to the improper positioning of the hydrogen bond donors affecting both the properties of first and second coordination spheres. We learned that installing proximal functionalities in the periphery of such Re catalysts seems tricky because of the intrinsic mechanistic pathway involving the loss of a halide ligand resulting to a pentadentate active form. As such, further modifications for this class of catalyst require more elegant considerations in catalysts design.We learned that improved catalytic activity by hydrogen bonding stabilization was successfully achieved when urea functions were installed in a αβαβ iron porphyrin atropoisomer with significant lowering of the overpotential while maintaining a good TOF. Such enhancement was achieved because of the preorganized positioning of the multi-point NH fragments on both urea arms in hydrogen bonding distances towards the metal carboxylate intermediate. The systematic study showed the need for the stronger multi-point hydrogen bonding in Chapter 6 163 urea functions in FeTPPUr compared to the weaker single-point H-bond donors in an amide analogue in FeTPPAm. More importantly, DFT calculations show that the metal carboxylate intermediate remains in a quasi-unchanged topology while being

  photoelectrocatalytic system where the electrons are sustainably provided by the coupled oxidation reaction, ideally using water as the substrate. Approaches may include grafting of the photosensitizers and catalyst on electrodes of a photoelectrochemical cell or grafting of supramolecular assemblies linking the photosensitizer unit and catalytic unit in a suitable directed fashion for efficient electron flow. The Z-scheme approach observed in the reaction centers in natural photosynthesis can be similarly applied by employing photoanodes and photocathodes delivering the necessary oxidizing and reducing equivalents to drive the redox reactions, eventually in tandem configuration of electrodes with complementary light absorption properties.

  89 kHz and aperture of 10 mm. IR-spectroelectrochemistry was performed by mounting a home-made electrochemical cell on top of the ATR prism. The three-electrode cell is employed with a glassy carbon (3 mm diameter) working electrode, Ag/AgNO3 (10 -2 M) reference electrode, and a platinum wire counter electrode. Extra care is needed to adjust the distance between the ATR prism and the active surface of the working electrode ensuring that a thin reactive layer is immediately within the detection limit of the ATR prism. A linear sweep voltammetry (LSV) technique is used in the spectroelectrochemical measurements where the potential is swept from the start to the end potential of the reactive species being studied with a scan rate of 50 mV/s, giving an interval time of 100 s. During this interval, 80 scans of IR spectra are recorded with a resolution of 2 cm -1 for the spectrum range of 4000 cm -1 to 500 cm -1 with a speed of 3.6 kHz and aperture of 10 mm. The IR scans take into account a background spectrum of the electrolyte solution (0.1 M TBAP in ACN), so that the resulting scans will provide the clean IR spectra related to the catalyst. As such, each potential step from the LSV corresponded to one IR spectrum. We found that this optimized set-up gives much sensitive changes in the IR signal compared to a chronoamperometric technique. It has been observed however that for some experiments, some preceding species persists on the next potential scan, resulting in observation of IR peaks from two intermediate species. The evolution of the IR spectra however is a good indication of the evolution of the intermediates during the potential sweep, with tested reference complexes agreeing with literature reports.

:

  calculated m/z = 735.0287 [M-(ClO4) -] + for C32H36P2S3NiClO4, found 735.0303.Steady-State XANES and EXAFS MeasurementsX-ray absorption measurements were performed in collaboration with Dr. Dooshaye Moonshiram from IMDEA Nanociencia in Madrid. The spectra were collected at the Advanced Photon Source (APS) at Argonne National Laboratory on bending magnet beamline 9 at electron energy 8.43 keV and average current 100 mA. The radiation was monochromatized by a Si(110) crystal monochromator. The intensity of the X-ray was monitored by three ion chambers (I0, I1 and I2 ) filled with 70% nitrogen and 30% helium and placed before the sample (I0) and after the sample (I1 and I2 ). A Ni metal foil was placed between the I1 and I2 and its absorption recorded with each scan for energy calibration. Plastic (PEEK) EXAFS sample holders (inner dimensions of 12 mm x 3 mm x 3 mm) filled with frozen solution samples were inserted into a pre-cooled (20 K) cryostat. The samples were kept at 20 K in a He atmosphere at ambient pressure. Data were recorded as fluorescence excitation spectra using a 4-element Si drift energy-resolving detector.Solid samples were diluted with BN powder, pressed between polypropylene and mylar tape, and measured in the cryostat in transmission mode. In order to reduce the risk of sample damage by X-ray radiation, 80% flux was used in the defocused mode (beam size 1 x 10 mm) and no damage was observed to any samples scan after scan. The samples were also protected from the X-ray beam during spectrometer movements by a shutter synchronized with the scan program. No more than 5 scans were taken at each sample position in any condition. The Ni XAS energy was calibrated by the first maximum of the second derivative of the cobalt metal XANES spectrum.

Figure 0 - 1 .

 01 Figure 0-1. A. Normalized experimental Ni K-edge XANES of NiN2S3 in pure ACN (black) and in 1:1 ACN:H2O (green) B. Fourier transforms of k 2 -weighted Ni EXAFS of NiN2S3 in pure ACN (black) and in 1:1 ACN:H2O (green).

1 H

 1 NMR (CD3CN, 298 K, 250 MHz): δH 5.99 (4H, br s), 6.87 (2H, d), 7.50 (2H, d), 7.70 (2H, t) ESI-HRMS: calculated m/z = 558.9386 [M+Na + ] + for C13H10BrN4NaO3Re, found 558.9355 Synthesis of TPPUr 3,5-Bis(trifluoromethyl)phenyl isocyanate (215 µL, 1.245 mmol) were added to a solution of -5,10,15,20-tetrakis (2-aminophenyl)porphyrin (TAPP )

1 H

 1 NMR ((CD3)2CO, 298 K, 400 MHz): δH 8.92 (8H, s), 8.48 (4H, d, J = 8.0 Hz), 7.91 (12H, m), 7.59 (8H, s), 7.56 (4H, td, J1 = 7.6 Hz, J2 = 0.8 Hz), 7.40 (4H, s), 7.31 (4H, s), -2.72 (2H,s) 13 C NMR ((CD3)2CO, 298 K, 100 MHz): δc 152.2, 141.3, 139.1, 134.8, 132.4, 131.2 (q, J = 32 Hz), 129.6, 123.2 (q, J = 269.7 Hz), 123.1, 122.9, 117.53, 115.5, 114.5. ESI-HRMS: calculated m/z = 1695.3455 [M+H + ] + for C80H47F24N12O4, found 1695.3455; calculated m/z = 1717.3274 [M+Na + ] + for C80H46F24N12O4Na, found 1717.3236.

  ESI-HRMS: calculated m/z = 1802.2654 [M-Cl -+MeO -+Na + ] + for C81H47F24FeN12NaO5, found 1082.2644. UV-Vis (CHCl3): λmax / nm (ε, in M -1 cm -1 ): 417 (52700), 517 (7500), 591 (2100), 652 (1700), 684 (1800) IR wavenumber in cm -1 : NH stretching at 3352, C=O stretching at 1713 and 1683. ESI-HRMS: calculated m/z = 1742.2218 [M-Cl -+MeO -+Na + ] + for C81H43F24FeN8NaO5, found 1742.2201; calculated m/z = 1756.2010 [M-Cl -+HCO2 -+Na + ] + for C81H41F24FeN8NaO6, found 1756.2005.

  ESI-HRMS: calculated m/z = 1336.2270 [M] + for C76H52Cl4FeN8O4, found 1336.2213.

  ESI-HRMS: calculated m/z = 381.1370 [M] 4+ for C92H76FeN16O4, found 381.1392. ESI-HRMS: calculated m/z = 305.1101 [M] 5+ for C92H76FeN16O4, found 305.1118.

Figure 0 - 2 .

 02 Figure 0-2. FOW analysis of 1 mM FeTPP in CO2-saturated DMF as a function of H2O concentration.

Figure 0 - 3 .

 03 Figure 0-3. Catalytic Tafel plots of FeTPP in CO2-saturated DMF at various concentrations of H2O (1.11 M to 5.55 M).

Figure 0 - 4 .

 04 Figure 0-4. FOW analysis of 1 mM FeTPPAm in CO2-saturated DMF as a function of H2O concentration.

Figure 0 - 5 .

 05 Figure 0-5. Catalytic Tafel plots of FeTPPAm in DMF at various concentrations of H2O (1.11 M to 5.55 M).

Figure 0 - 6 .

 06 Figure 0-6. FOW analysis of 1 mM FeTPPF8 in CO2-saturated DMF as a function of H2O concentration.

Figure 0 - 7 .

 07 Figure 0-7. Catalytic Tafel plots of FeTPPF8 in CO2-saturated DMF at various concentrations of H2O (1.11 M to 5.55 M).

Figure 0 - 8 .

 08 Figure 0-8. FOW analysis of 1 mM FeTPPF20 in CO2-saturated DMF as a function of H2O concentration.

Figure 0 - 9 .

 09 Figure 0-9. Catalytic Tafel plots of FeTPPF20 in CO2-saturated DMF at various concentrations of H2O (1.11 M to 5.55 M).

Figure 0 - 10 .

 010 Figure 0-10. FOW analysis of 1 mM FeTPPUr in CO2-saturated DMF as a function of H2O concentration.

Figure 0 - 11 .

 011 Figure 0-11. Catalytic Tafel plots of FeTPPUr in CO2-saturated DMF at various concentrations of H2O (1.11 M to 5.55 M).

Figure 0 - 12 .

 012 Figure 0-12. FOW analysis of 1 mM FeTPPUr in CO2-saturated DMF as a function of TFE concentration.

Figure 0 - 13 .

 013 Figure 0-13. Catalytic Tafel plots of FeTPPUr in CO2-saturated DMF at various concentrations of TFE (1 M to 5 M).

Figure 0 - 14 .

 014 Figure 0-14. FOW analysis of 1 mM FeTPPUr in CO2-saturated DMF as a function of PhOH concentration.

Figure 0 - 15 .

 015 Figure 0-15. Catalytic Tafel plots of FeTPPUr in CO2-saturated DMF at various concentrations of PhOH (1 M to 5 M).

Figure 0 - 16 .

 016 Figure 0-16. FOW analysis of 1 mM FeTPPUr in CO2-saturated DMF as a function of D2O concentration.

Figure 0 - 17 .

 017 Figure 0-17. Catalytic Tafel plots of FeTPPUr in DMF at various concentrations of D2O as proton source (1.11 M to 5.55 M).

Table of Abbreviations

 of

	ACN	acetonitrile
	ACS	acetyl CoA synthase
	ATR	attenuated total reflectance
	BNAH	1-benzyl-1,4-dihydronicotinamide
	BIH	1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole
	bpy	2,2'-bipyridine
	Cat	catalyst
	CoA	coenzyme A
	CODH	CO dehydrogenase
	CV	cyclic voltammetry
	DABCO	1,4-diazabicyclo[2.2.2]octane
	DMA	dimethylacetamide
	DMF	dimethylformamide
	DFT	density functional theory
	ED	electron donor
	EXAFS	Extended X-Ray Absorption Fine Structure
	Fc	ferrocene
	FDH	formate dehydrogenase
	FE	Faradaic efficiency
	GC	gas chromatography
	GHG	greenhouse gas
	IR	infrared
	LED	light emitting diode
	LFP	laser flash photolysis
	Me	methyl
	MLCT	metal-to-ligand charge transfer
	MS	mass spectrometry
	NaAsc	sodium ascorbate
	NHE	normal hydrogen electrode
	nm	nanometer
	NMR	nuclear magnetic resonance
	OER	one-electron reduced species
	Ph	phenyl
	PhOH	phenol
	PS	photosensitizer
	SHE	standard hydrogen electrode
	TBAP	tertbutylammonium hexafluorophosphate
	TEA	triethylamine
	TEOA	triethanolamine
	TFE	trifluoroethanol
	TOF	turnover frequency
	TON	turnover number
	TPP	tetraphenylporphyrin
	UV-Vis	ultraviolet-visible
	XAS	X-ray absorption spectroscopy
	XANES	X-ray absorption near edge structure
	XRD	X-ray diffraction
		x xi

Table of Contents

 of

	Acknowledgments	iii
	Abstract	v
	Résumé	vii
	Table of Abbreviations	ix
	Table of Contents	xi
	Chapter 1. Introduction
	1.1. The Great Concern
	1.2. Transforming Carbon Dioxide
	1.3. Artificial Photosynthesis as an Innovative Solution
	1.4. Enzymatic Conversion of CO2
	1.5. Bio-inspired Strategy
	1.6. State-of-the-Art
	1.6.0. Evaluating Catalyst Performance
	1.6.1. Photosensitizers and Electron Donors
	1.6.2. Structural Motifs of Functional CO2 Reduction Catalysts
	1.6.3. Catalysts Employing Second Coordination Sphere Effects
	1.7. Thesis Objectives and Scope

Table 1 -2. Photophysical

 1

	and electrochemical characteristics of some
	photosensitizers used for photocatalytic CO2 reduction, with labels and structures
	shown in Figure 1-12. Shaded region corresponds to the reducing power sought
	from these PS. Potentials are all reported versus the Fc +/0 reference couple.

PS λmax abs λmax em τ [d] / µs Ered / V Eox / V E*red / V E*ox / V Ref.

	[a] 452 nm	615 nm	1.10	-1.71	0.91	0.39	-1.19	[30]
	[b] 462 nm	639 nm	0.88	-1.83	0.73	0.21	-1.31	[31]
	[a] 477 nm	608 nm	0.60	-1.79	1.02	0.43	-1.20	[32]
	[a] 375 nm	494 nm	1.90	-2.57	0.39	-0.07	-2.11	[33]
	[a] 310 nm	606 nm	0.34	-1.78	0.88	0.55	-1.45	[34]
	[a] 375 nm	624 nm	3.19	-2.04	0.80	-0.02	-1.19	[35]
	[c] 343 nm	654 nm	4.30	-	-0.17	-	-2.07	[36]
	[c] 388 nm	506 nm	480	-	0.27	-	-2.18	[36]

Ir(ppy)2(bpy)] +

	Chapter 1
	possesses a sufficiently long-lived excited state (-1.90 µs) with a very reducing
	potential of -2.11 vs Fc +/0 which can reduce most CO2 reduction catalysts. [33] It can
	also be reductively quenched to form a more reducing OER with a potential of -2.57
	V vs Fc +/0 . Equally important is the reducing power of the modified [(structure 16) which possesses a spectrally well-defined bpy-centered OER once the
	excited state is reductively quenched.
	There is a current exploration of cheaper alternatives for photosensitizers for
	the photocatalytic CO2 reduction and some have emerged as promising alternatives
	for the Ru and Ir-based ones. Notable examples include the heteroleptic copper (I)
	complex possessing diimine and phosphine ligands (structure 17) which possesses
	a long-lived excited state (3.19 µs) and a sufficiently reducing power of -2.04 V vs
	Fc +/0 for the OER once reductively quenched by an electron donor. Organic dyes
	were also considered as potential PSs because of their long-lived excited state but
	most do not possess sufficiently reducing potentials. Recent advancements show
	the promise of N,N-5,10-di(2-naphthalene)-5,10-dihydrophenazine (structure 18)
	and 3,7-(4-biphenyl)-1-naphthalene-10-phenoxazine (structure 19) which access a
	long-lived triplet excited state (4.3 µs and 480 µs for 18 and 19, respectively) and at
	the same time possessing very reducing excited state potentials comparable with
	the Ir-based PSs (-2.07 V and -2.18 V for 18 and 19, respectively).
	1.9 V) require the use of cyclometalated iridium (III) tris-2-phenylpyridine, denoted
	as [Ir(ppy)3] shown in structure 15. Once excited in the near UV region (375 nm), it
	20

Table 1

 1

	-2, that PSs possessing more reducing power have
	absorptions only at the shorter wavelengths of the visible region. As such, a choice
	of PS should balance the need to maximize the solar irradiation spectrum and the
	thermodynamic matching with the CO2 reduction catalyst.
	The modular molecular approach for CO2 reduction requires the use of
	appropriate electron donors to either reductively quench the excited PS to produce
	the OER species of the PS or to reduce the oxidized PS + (formed from the oxidative
	quenching with the catalyst) to generate back PS (Figure 1-11). As such, it should
	possess the necessary thermodynamic potential (Eox of ED < E*red of PS* or Eox of
	ED > Eox of PS + ) and efficient quenching rate. Equally important in the long
	durations of photocatalytic conditions are the reaction pathways of the oxidized ED

1.6.2. Structural Motifs of Functional CO2 Reduction Catalysts

	Both photo-and electro-catalytic systems rely on the design and the
	optimized performance of the catalyst. As such, it is central to review the common
	structural motifs found in functional CO2 reduction catalysts. Most of the catalysts
	contain a central transition metal atom that is supported by a coordinated ligand
	framework, as shown in Figure 1-14. The metal center usually acts as the point of
	binding for the CO2 substrate and it is typically responsible for transferring the
	electrons to the substrate to perform reduction reactions. The metal can range from
	the initially reported better performances with the use of second (4d) d-block series
	metals such as Ru, Rh, & Pd, and third (5d) d-block series metals such as Re, Os,
	Ir, & Pt. There is a recent trend in exploring first (3d) d-block series metals such as
	Fe, Mn, Co, Ni, and Cu because of their relative abundance and the observation
	that Nature uses such metals in performing efficient catalytic reactions. Equally
	important as the redox active metal is the ligand framework which supports and
	stabilizes the reduced metal and/or accumulate charges in its framework allowing
	the storage of multiple reducing equivalents across the molecule. There is a vast
	literature on the ligand frameworks and they are generally categorized into the
	following family: polypyridyl complexes (e.g. bipyridines, phenanthrolines,
	terpyridines, quaterpyridines, etc.), porphyrins & similar macrocyclic complexes,

Table 1 -3. Effect

 1 of changing the nature of the axial ligand on the first reduction potential of the catalyst and the corresponding changes in the photocatalytic TON or the electrocatalytic TOF.

	Re Complex	X	E1/

2 / V vs Fc +/0 TON Log TOF Ref

	Cl	-1.68	8.2	-	[53]
	Br	-1.74	13.0	-	[53]
	H2O	-1.55	1.8	-	[53]
	SCN	-1.57	26.4	-	[53]
	CN	-1.58	4.5	-	[53]
	P(OEt)3	-1.59	7.5	-	[57,60]
	Cl	-1.86	-	3.79	[54]
	Br	-1.86	-	3.78	[54]

Table 3 - 1 .

 31 Selected bond lengths and angles of the crystal structures of the complexes shown in Figure 3-6.

	Parameter	Ni-N2S2	Ni-N2S3	Ni-P2S3
	Ni-S1, Å	2.4073(3)	2.4419(4)	2.4220(7)
	Ni-S2, Å	2.4108(5)	2.4157(3)	2.2643(7)
	Ni-S3, Å	-	2.3860(4)	2.2854(6)
	Ni-N1 / Ni-P1, Å	2.080(1)	2.099(1)	2.2146(7)
	Ni-N2 / Ni-P2, Å	2.102(2)	2.0659(9)	2.2001(7)
	Ni-N(ACN)	2.061(1)	2.065(1)	-
		2.052(1)
	∠S1-Ni-S2, °	89.40(1)	85.58(1)	90.14(2)
	∠N1/P1-Ni-S2, °	89.59(4)	94.03(3)	86.92(2)

Table 3 - 2 .

 32 Reduction potentials of the catalysts Ni-N2S2, Ni-N2S3, and Ni-P2S3 in ACN determined from the CVs in Figure 3-7.

	-2.7	-2.4	-2.1	-1.8	-1.5	-1.2	-0.9	-0.6	-0.3
						E / V vs Fc +/0
	Complex
				1 st reduction		2 nd reduction
	Ni-N2S2				-1.133				-1.465*
	Ni-N2S3				-0.952				-1.556*
	Ni-P2S3				-0.710				-1.484

*For irreversible peaks, potentials are reported from the half wave point.

Table 4 -

 4

	ReAm1	ReAm2

1. X-ray crystal structures (ORTEP thermal ellipsoids set at 50% probability) and selected bond lengths and angles of ReAm1 and ReAm2. Superposition of the crystal structures (red -ReAm1, blue -ReAm2) is also shown on the top left. Hydrogen atoms are omitted for clarity except the ones from amines. X-ray structures obtained from single crystals of ReAm1 and ReAm2 show the typical octahedral ligand arrangement with facial triscarbonyl moiety, even maintaining similar bond lengths, as reported in Table

4

-1. A distinguishing feature is observed however on the deviation of the bipyridyl ligand from the horizontal plane of the octahedron, clearly seen on the superposed structures. The presence of one amino group in the 6 position of the bipyridine in ReAm1 deviates the ∠C13-

Table 4 -

 4

79, -1.89, -2.29; ReAm2: -1.84, -2.24, -2.54; 2. Values of IR peaks observed for reduced species of ReAm1 and ReAm2 identified during IR spectroelectrochemistry in ACN containing 0.1 M TBAP under N2 atmosphere.

E / V vs Fc +/0 1st red 2nd red 3rd red

	FeTPP	-0.602	-1.480	-2.114
	FeTPPAm	-0.700	-1.414	-2.009
	FeTPPF8	-0.497	-1.380	-1.990
	FeTPPF20	-0.442	-1.280	-1.800
	FeTPPUr	-0.468	-1.326	-1.812

Table 4 - 3 .

 43 Electrocatalytic performances of some notable reported catalysts for CO2 reduction to CO in comparison with some of the modified catalysts in this work.

	Catalyst	Ecat / V vs Fc +/0 log TOFmax Solvent	H +	Ref
	FeTPP	-2.12	4.5	DMF	PhOH	[81]
	FeTPP(OH)8	-2.04	3.8	DMF	PhOH	[33]
	FeTPP-o-2-amide	-2.18	6.7	DMF	PhOH	[31]
	FeTPPAm	-2.01	3.9	DMF	H2O	work
	FeTPPUr	-1.81	3.8	DMF	H2O	work
	FeTPPIm	-1.78	6.0	DMF	H2O	work
	FeTPP-o-TMA	-1.64	6.0	DMF	PhOH	[41]

  and stirred at room temperature, closed, for 45 min. The solution turned to yellow. The reaction mixture was then poured into a stirring solution of water (700 mL) containing potassium hydroxide (3.5 g). 200 mL of diethyl ether was then added and stirred until both layers were transparent. The organic layer was decanted, and the same process was repeated with additional diethyl ether. The

	Synthesis of N2S2
		Na metal (300 mg, 13 mmol, 4.4 eq) was dissolved in absolute methanol (15
	mL) under Ar. 1,2-Ethanedithiol (250 μL, 2.97 mmol, 1.0 eq) was added to the
	solution and stirred at room temperature. Then, the solution of 2-picolyl chloride
	combined organic layers were washed with water, brine, dried over MgSO4, filtered, hydrochloride (1.06 g, 6.46 mmol, 2.2 eq) in absolute methanol (7 mL) was added
	and evaporated to get yellowish oil which turned into yellow precipitate upon slowly to the mixture. The mixture was stirred for 16 h at room temperature under
	vacuum drying. Yield: 4.019 g (91%). Ar. The solvent was removed using rotary evaporator and the residue was extracted
	1 H NMR (DMSO-d6, 300 MHz,) δH 7.86 (2H, d), 7.69 (1H, d), 7.63-7.56 (4H, m), 7.32-7.23 with chloroform and H2O. The organic layer was washed with brine solution, dried
	(2H, m), 3.88 (3H, s) over anhydrous MgSO4, and filtered. The volatiles of the filtrate were evaporated
	using rotary evaporator and further dried under vacuum to produce brown oil of
	bis(2-pyridylmetyl)-1,2-ethanedithiol, denoted as N2S2 (811 mg, 2.95 mmol, 99%
	yield).	Synthesis of 1,3-dimethyl-2-phenylbenzo[d]imidazolium iodide: 3.999 g of N-
	methyl-2-phenylbenzo[d]imidazole (19.2 mmol, 1.0 eq) was dissolved in 30 mL
	dichloromethane under stirring. 3.8 mL of methyl iodide (60.5 mmol, 3.2 eq) was 1 H NMR (CDCl3, 298 K, 360 MHz): δH 2.70 (4H, s), 3.85 (4H, s), 7.17 (2H, dd), 7.37 (2H, d)
	7.66 (2H, dd), 8.52 (2H, d)
	1 H NMR (DMSO-d6, 300 MHz): δH 8.15 (2H, br s), 7.98 (2H, br s), 7.91-7.78 (5H, m), 3.90
	(6H, s)
		Synthesis of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole (BIH): 4.247
	g of 1,3-dimethyl-2-phenylbenzo[d]imidazolium iodide (12.1 mmol, 1.0 eq) was
	dissolved in 160 mL methanol under inert atmosphere. 1.147 g of NaBH4 (30.3
	mmol, 2.5 eq) was slowly added to the reaction mixture in an ice-water bath. The
	resulting mixture was stirred under inert atmosphere at room temperature for 1
	hour followed by evaporation of solvent by rotary evaporator. Water was added and
	the product was extracted with diethyl ether. The organic layer was treated with
	Synthesis of N-methyl-2-phenylbenzo[d]imidazole: 2.25 g of potassium saturated NaCl and anhydrous MgSO4. The solvent was evaporated, and the white
	hydroxide (40 mmol, 2.0 eq) was dissolved in 70 mL DMSO and was stirred at room solid was recrystallized with minimum amount of 5:1 (v/v) ethanol:water mixture to
	temperature for 30 min. 4.135 g of 2-phenylbenzo[d]imidazole (21.3 mmol, 1.0 eq) form white needle-like crystals. Yield: 2.185 g (81 %).

was dissolved in 70 mL DMSO and was added to the previous mixture and was stirred at room temperature, closed, for 45 min. The solution turned from clear yellow to clear red. 1.4 mL of methyl iodide (22.3 mmol, 1.05 eq) was added to the reaction mixture added to the reaction mixture and was stirred at 30°C, closed, for 17 h. The solvent was evaporated using rotary evaporator and diethyl ether was added. The solid was filtered, washed with diethyl ether, and dried under vacuum to produce off-white powder. Yield: 6.681 g (99%). 1 H NMR (CDCl3, 300 MHz): δH 7.58 (2H, m), 7.41 (3H, m), 6.72 (2H, dd), 6.43 (2H, dd), 4.87 (1H, s), 2.56 (6H, s) ESI-HRMS: calculated m/z = 223.1230 [M-H -] + for C15H15N2, found 223.1230 Chapter 3 ESI-HRMS: calculated m/z = 299.0647 [M+Na + ] + for C14H16N2NaS2, found 299.0636.

  Ar atmosphere. 2,2′-Thiodiethanethiol (0.4 mL, 3.1 mmol, 1.0 eq) was added to the solution and the mixture was heated to reflux at 85 ˚C for 30 min. Then, a solution of 1-chloro-2-diphenylphosphinoethane (1.532 g, 6.2 mmol, 2.0 eq) in absolute ethanol (15 mL) was added slowly to the mixture. The mixture was stirred and

	refluxed at 85 ˚C for 17 h under Ar. The solvent was removed using rotary
	evaporator and the residue was extracted with chloroform and H2O. The organic
	layer was washed with brine solution, dried over anhydrous CaCl2, and filtered. The
	volatiles of the filtrate were evaporated using rotary evaporator. The residue was
	further purified with column chromatography (silica gel) with pentane containing
	30-80% (v/v) chloroform as the eluent, finally obtaining pure yellow oil of 1,11-
	bis(diphenylphosphino)-3,6,9-trithiaundecane, denoted as P2S3 (996 mg, 1.72
	mmol, 55% yield).
	1 H NMR (CDCl3, 298 K, 250 MHz): δH 2.30-3.00 (20H, m), 7.35 (4H, d), 7.40-7.58 (8H, m),
	7.74 (4H, dd)
	1 H NMR (CDCl3, 298 K, 250 MHz): δH 2.61 (2H, t), 3.63 (2H, t), 7.39-7.49 (10H, m)

Synthesis of 1,11-bis(diphenylphosphino)-3,6,9-trithiaundecane (P2S3): Na metal (296 mg, 12.9 mmol, 4.0 eq) was dissolved in absolute ethanol (20 mL) under ESI-HRMS: calculated m/z = 579.1527 [M+H + ] + for C32H37P2S3, found 579.1519.

  NMR (CDCl3, 298 K, 300 MHz): δH 7.77 (2H, d), 6.91 (2H, d), 6.06 (1H, br s), 3.84 (3H, s), 3.43 (2H, td), 1.60 (2H, q), 1.32 (6H, m), 0.89 (3H, t) 13 C NMR (CDCl3, 298 K, 250 MHz,): δC 167.0, 162.0, 128.6 (2C), 127.2, 113.7 (2C), 55.4, 40.0, 31.5, 29.7, 26.7, 22.6, 14.0 ESI-HRMS: calculated m/z = 258.1464 [M+Na + ] for C14H21NO2, found 258. 1466

1 H

  NMR (CDCl3, 298 K, 400 MHz,): δH 7.77 (2H, d), 6.91 (2H, d), 5.96 (1H, br s), 3.78 (3H, s), 3.37 (2H, td), 1.53 (2H, q), 1.26 (6H, m), 0.82 (3H, t) 13 C NMR (CDCl3, 298 K, 400 MHz,): δC 166.0 ( 13 C-enriched), 161.0, 127.6 (2C), 126.3, 112.7(2C), 54.4, 39.0, 30.5, 28.7, 25.7, 21.6, 13.0 LC-MS: calculated m/z = 237.2 [M+H + ] for C13 13 CH21NO2, found 237.4

	Annex
	1 H
	202

  NMR (CDCl3, 298 K, 300 MHz,): δH 7.68 (2H, d), 7.36 (2H, d), 6.84 (1H, br s), 3.68 (4H, t), 3.48 (2H, q), 2.54 (2H, t), 2.45 (4H, t) 13 C NMR (CDCl3, 298 K, 300 MHz,): δC 166.5, 137.7, 133.1, 128.9 (2C), 128.5 (2C), 67.1 (2C), 57.0, 53.4 (2C), 36.2. ESI-HRMS: calculated m/z = 269.1051 [M+H + ] for C13H18ClN2O2, found 269.1055

1 H

  NMR (CDCl3, 298 K, 300 MHz): δH 7.70 (2H, d), 7.38 (2H, d), 6.78 (1H, br s), 3.70 (4H, t), 3.5 (2H, q), 2.58 (2H, t), 2.48 (4H, t) Stratégies bio-inspirées pour la réduction catalytique et la valorisation du dioxyde de carbone Mots clés : photosynthèse artificielle, réduction de CO2, bio-inspiré, photocatalyse, électrocatalyse, valorisation Résumé : La criticité du réchauffement climatique incite à chercher des solutions pour réduire les émissions de dioxyde de carbone (CO2). Le développement de catalyseurs qui peuvent aider à capturer, activer, réduire et valoriser le CO2 est au coeur de ce défi. Cette thèse a répondu à cet appel en développant des mimétismes moléculaires inspirés de la Nature, dans le cadre plus large de la photosynthèse artificielle. Au début il s'agissait de suivre le parcours d'un photon de lumière visible et de déterminer comment il peut réduire la molécule de CO2.Ensuite afin de réaliser des catalyseurs plus efficaces, de nouvelles molécules ont été synthétisées en s'inspirant de l'enzyme CO déshydrogénase (CODH) qui présente des performances exceptionnelles pour la réduction du CO2. Enfin, une autre propriété du CODH a conduit à une validation de principe pour la valorisation immédiate du CO photo-produit dans la synthèse des liaisons amides marqués, une caractéristique courante des médicaments.Title : Bio-inspired Strategies for the Catalytic Reduction and Valorization of Carbon DioxideKeywords : artificial photosynthesis, CO2 reduction, bio-inspired, photocatalysis, electrocatalysis, valorization Abstract : The criticality of global warming urges for the advancement of science to reduce carbon dioxide (CO2) emissions in the atmosphere. At the heart of this challenge is the development of sustainable catalysts that can help capture, activate, reduce, and eventually valorize CO2. This PhD work tried to respond to this call by developing molecular mimics inspired by natural systems in the larger scheme of artificial photosynthesis. Firstly, it involved tracking the journey of a photon of visible light and how it is transformed to a reducing power able to reduce CO2.

	Secondly, in search for more efficient and
	stable catalysts, new mimics were synthesized
	inspired by the exceptional performance of CO
	dehydrogenase enzymes (CODH) in reducing
	CO2.

1 H 13 C NMR (CDCl3, 298 K, 300 MHz,): δC 166.5 ( 13 C-enriched), 137.8, 133.5, 128.9 (2C), 128.5 (2C), 67.1 (2C), 57.0, 53.5 (2C), 36.2. ESI-HRMS: calculated m/z = 270.1085 [M+H + ] for C12 13 CH18ClN2O2, found 270.1084 Bibliography Université Paris-Saclay Espace Technologique / Immeuble Discovery Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France Titre :

Chapter 6

Conclusions and Perspectives

Annex

Supplementary Information

General materials and methods

Dimethylformamide (DMF, Aldrich 99.9%), acetonitrile (ACN, Aldrich 99.9%), tetra-N-butylammonium hexafluorophosphate (TBAP, Aldrich 99%) were used as received. All other chemical reagents used in the synthetic routes were obtained from commercial sources as guaranteed-grade reagents and used without further purification. The water utilized in most measurements and synthetic procedures was Milli-Q filtered (18.2 MΩ•cm at 25 °C). Proton and carbon nuclear magnetic resonance ( 1 H and 13 C NMR) spectra were recorded at room temperature on Bruker Advance 400 MHz spectrometers. Two-dimensional NMR (COSY, HSQC and HMBC) experiments were also performed in order to confirm the attribution of onedimensional 1 H NMR signals. The electrospray ionization mass spectrometry (ESI-HRMS) experiments were performed on TSQ (Thermo Scientific. 2009) with an ESI + method. Ground state absorption measurements were measured in a Specord double beam spectrophotometer (Analytik Jena), consisting of halogen and deuterium lamps as light sources, monochromator with holographic concave grating, and two photodiodes as radiation detectors. For all optical experiments sealed quartz cuvettes (1 cm x 1 cm) were used and samples purged with argon before measurements.

X-ray diffraction

X-ray diffraction data for compounds were collected by using a VENTURE PHOTON100 CMOS Bruker diffractometer with Micro-focus IuS source Cu Kα radiation. All crystals were mounted on a CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as cryoprotectant and then flash-frozen in a nitrogen-gas stream at 100 K. For compounds, the temperature of the crystal was maintained at the selected value by means of a N-Helix cooling device within an accuracy of ±1 K. The data were corrected for Lorentz polarization and absorption effects. The structures were solved by direct methods using SHELXS-97 [1] and refined against F 2 by full-matrix least-squares techniques using SHELXL-2018 [2] with anisotropic displacement parameters for all non-hydrogen atoms. All calculations were performed by using the Crystal Structure crystallographic software package WINGX. [3]

Pump-Pump-Probe Measurements

Pump-pump-probe measurements were measured in collaboration with Thu Trang Than, Dr. Minh Huong Ha Thi, and Dr. Thomas Pino from the Laboratory of Molecular Systems, Astrophysics, and Environment in Institut des Sciences Moléculaires d'Orsay (ISMO). The setup was described in a recently published report. [14] Two Nd:YAG pumped optical parametric oscillator (OPO) lasers are used for sample excitation at 460 nm with an energy of ~ 5 mJ/pulse. The first pump operates at 20 Hz, and the second one has a repetition rate of 10 Hz. After excitation the sample is probed by a white light continuum laser (LEUKOS) with a repetition rate of 20 Hz. The probe beam is split into two arms, one for probing sample and the other for reference in order to compensate for energy fluctuation.

The probing arm after passing the sample is coupled into an optical fiber bundle before being analyzed by a spectrograph SPEX 270M (Jobin Yvon). Detection of the dispersed white light is performed by an intensified CCD (ICCD) detector (PIMAX 4, Princeton Instruments). The setup is operated in online subtraction mode in which the transient absorption after the first excitation is inherently removed by running the first pump at 20 Hz which is the same as the repetition rate of the probe.

Therefore, in principle we obtain a relatively pure signal from molecules that are excited twice, as the off signal then contains contributions from the singly pumped solution. It is noteworthy that the double pumped transient signal contains contributions from singly and doubly excited molecules because excitation efficiency by the first pump laser cannot reach 100%.

Photocatalysis set-up

Photocatalytic experiments were made in a modified quartz cuvette with a total volume of 6.85 mL, thermostated at 20°C, and sealed with a rubber septum. About 3.0 mL solutions of Ru(bpy)3Cl2 photosensitizer, catalyst, proton source, and sacrificial electron donor were dissolved in appropriate solvents and were purged with CO2 for 15 minutes for CO2 reduction experiments or with Ar gas for proton reduction experiments. To excite only the ruthenium photosensitizer's MLCT band and to prevent the sample from excessive heating, a SugarCUBE high intensity LED fiber optic illuminator was utilized with a blue light output centered at 463 nm with a full width at half maximum (FWHM) characteristic of about 50 nm. The source was adjusted to produce the lowest light intensity of 180 W/m 2 (18 mW/cm 2 ).

Product quantification by gas chromatography

Quantification of products during the photo-and electrocatalytic reactions was performed by withdrawing 50 µL gas aliquots from the headspace of the reaction vessel with a gas-tight syringe and injected into a gas chromatography (GC -TraceGC Ultra, ThermoScientific) equipped with a 30 m molecular sieve porous layer open tubular (PLOT) column having an internal diameter of 0.53 mm, helium carrier gas, and a thermal conductivity detector (TCD). Peak separation between H2, CO, and CO2 were achieved by programming the oven temperature from an initial 40 °C for 2 min, ramped by 50 °C/min until a final temperature of 250 °C held for 2 min. H2 was detected at 2.16 min, CO was detected at 3.13 min, and CO2 was detected at 6.10 min. For more sensitive quantification of H2 for proton reduction experiments, an argon gas was utilized as the carrier gas due to the bigger difference in the thermal conductivity between Ar and H2. A splitless injector line was utilized to maximize the injection volume and become sensitive to possible trace amounts. A calibration curve relating peak area and concentration of CO (or H2) was established by injecting known amounts of pure CO (or H2) into the reaction vessel and after 10 min equilibration time, 50 μL gas aliquots were drawn from the headspace and injected into the GC. This calibration method accounts for the headspace volume of the set-up and any CO dissolved in the solution and as such, it can be used to determine actual amounts of CO (or H2) produced.

Photo-accumulation studies

Photo-accumulation studies were performed by recording the spectra of the solution during photo-irradiation, in which the photocatalytic cell is directly mounted inside an Agilent Cary 60 UV-Vis spectrophotometer. A scanning kinetics mode is applied with a scanning rate of 9600 nm/min and a resolution of 1 nm, for a cycle time of 0.1 min for the duration of the desired photo-irradiation. This was performed with the same conditions and light source as previously described in the photocatalytic set-up, optimizing the concentrations and light intensity to achieve significant observable changes in the spectra.

The time-resolved spectra were then fitted with the fitting software Kintek Global Kinetic Explorer TM . This dynamic kinetic simulation program allows for multiple data sets to be fit simultaneously to a single model based on numerical integration of the rate equations describing the reaction mechanism. [16] The experimental spectra were input in the software as well as a proposed reaction model, based on initial kinetic studies by nanosecond pump-probe measurements and literature know-how on proposed mechanisms. The output would include the reconstruction of the experimental data, rate constants of the elementary steps proposed in the reaction model, concentration profile of the observable intermediates, and the corresponding spectra of the intermediates.

DFT Calculations

DFT optimization calculations were performed using the ORCA program package developed by Neese and co-workers [20] The DFT optimizations reported here were carried out using ORCA, B3LYP [21,22] as functional, the Zeroth Order Regular Approximation (ZORA) as previously reported [23] with def2-TZVP triple-zeta basis sets, [24] and the atom-pairwise Grimme dispersion correction with the Becke-Johnson damping scheme (D3BJ). [25,26]

Optical and XANES simulations

Optical and time-dependent DFT (TD-DFT) calculations for the UV-Visible absorption spectra and XANES spectra of the proposed Ni II and Ni I geometries were performed using the same functional, basis sets and dispersion correction effects with dense integration grids, and the fully decontracted def2-TZVP/J [24] auxiliary basis set. Up to 150 roots were calculated. A broadening of 2500 cm -1 was applied to all calculated optical spectra whereas that of 2.0 eV was applied to all calculated XANES spectra. The donor orbitals for XAS calculations were chosen as Ni 1s and all virtual orbitals were selected as acceptor orbitals. The calculated Ni XANES spectra contain contributions from electric quadrupole, electric dipole, and magnetic dipole transitions.

Time-resolved XAS measurements

Time-resolved X-ray absorption spectra were collected at 11 ID-D 1 beamlines at the Advanced Photon Source using undulator radiation at electron energy 8.43

keV. The experiments were carried out using the 24-bunch timing mode of APS (in top up mode with a constant 102 mA ring current) which consists of a train of Xrays separated by 153 ns. This mode easily allows for gateable detectors that selects X-ray pulses. This timing mode was suitable for this type of experiments in which a fresh sample was required for every incoming X-ray pulse as well as for the time resolution of the avalanche Photodiode (APD) detectors.

The samples were pumped at 400 nm wavelength using a regenerative amplified laser with 10 kHz repetition rate 5ps-FWHM pulse length and laser power of 630 mW. The sample was circulated through a stainless-steel nozzle into a freeflowing 550 µm cylindrical jet inside an airtight aluminium chamber, and continuously degassed with nitrogen. The X-ray and laser beams were spatially

Synthesis of TPPAm

To a solution of TAPP  in a dry mixture 1:1 of CHCl3 and THF was added 3,5-bis(trifluoromethyl)benzoyl chloride (240 µL, 1.304 mmol) in presence of triethylamine (400 µL, 2.964 mmol) at room temperature under an argon atmosphere. After an overnight stirring, the solvent was removed under vacuum and the crude was chromatographed on a silica gel and a mixture methanol:dichloromethane from 0:100 to 0.5:99.5 . The expected compound was precipitated by dissolving it in a minimum amount of dichloromethane and adding a large amount of pentane. After a filtration, washing with pentane and drying under vacuum, compound TPPAm was obtained as purple solid with 85 % yield (412 mg). Due to the D2d symmetry of TPPAm porphyrin, the four amido arms are magnetically equivalent. 2D NMR (COSY, HSQC and HMBC) experiments were also performed in order to confirm the attribution of 1D 1 H NMR signals.

Synthesis of FeTPPAm

A solution of porphyrin TPPAm (200 mg, 0.188 mmol) and lutidine (546 µL, 4.720 mmol) in dry CHCl3 was degassed by argon bubbling for 30 min. Then, FeBr2

(356 mg, 1.652 mmol) was added and the reaction mixture was stirred overnight at 50° C under argon. The reaction mixture was then filtered to remove the excess of iron salts and the solvent was evaporated under vacuum. The crude was dissolved in 50 mL of CHCl3 and 50 mL aqueous solution of 0.5 M HCl was added. After, 3

hours stirring the two phases were separated and the organic phase containing the iron complex was evaporated under vacuum. Chromatographic purification on a silica gel using a mixture of methanol:CHCl3 (from 0:100 to 3:97) gives FeTPPAm (with a Cl -as axial coordinating anion) in 78 % yield (165 mg). Single crystals suitable for X-ray diffraction analysis were obtained by a slow evaporation a FeTPPAm saturated solution in a 1:6 mixture of methanol:H2O, resulting to a crystal structure having a methoxide axial ligand.

Crystallographic data of modified iron porphyrins

DFT calculations for FeTPPAm and FeTPPUr

DFT calculations were performed using the Gaussian 09 program package. [33] Unrestricted BP86 functional was selected based on previous calculations. [34] Def2-SVP and 6-31G(d,p) polarized basis sets were chosen for Fe and main group atoms, respectively. Diffuse basis functions were added to p-type orbitals of main group elements to take into account the dianionic character of the molecules. Geometries of all Fe porphyrin models were optimized in the gas phase based on experimental X-ray structures. To speed up the calculations, CF3 substituents and urea arms opposite to the CO2 coordination side were replaced with protons. Optimizations were followed by vibrational frequency calculations to confirm the nature of all computed stationary points as minima. Free enthalpy energies were calculated at 298 K, 1 atm. Figures and mapped electrostatic potential were generated using ChemCraft program. [35] The geometry of the triply reduced [FeTPPUr-CO2] 2-active species was optimized as singlet, triplet, and quintuplet states under unrestricted open-shell calculation. The singlet state was found as the ground state, while triplet and quadruplet state lie 8 and 23 kcal mol -1 higher in energy. Consequently [FeTPPAm-CO2] 2-was only optimized as a singlet. The following parameters were used: n (electron transferred) = 1, F = 96485 C mol -1 , R = 8.314 J K -1 mol -1 , T = 298 K, A = 0.07065 cm 2 (glassy carbon electrode with diameter of 3 mm), C = 1 x 10 -6 mol cm -3 . The calculated average diffusion coefficient of catalyst FeTPPUr in DMF (0.1 M TBAP) is 1.40 x 10 -6 cm 2 s -1 .

The electrolysis is described by the equation [8,36] :

Calculations were made from i = -1.12 mA (taking into account 91% FE for CO production) and the following parameters: A (electrode surface) = 1.60 cm 2 , C 0 cat (catalyst concentration) = 5 x 10 -7 mol cm -3 , Dcat (catalyst diffusion coefficient) = 1.40

x 10 -6 cm 2 s -1 , Eelectrolysis = -1.076 V vs. NHE, Ecat = -1.118 V vs NHE, and t (electrolysis time) = 7200 s. The turnover frequency (TOF) and turnover number (TON) were then calculated using:

Bulk electrolysis of FeTPPIm in water

Bulk electrolysis was performed in a CO2-saturated water containing 0.1 M KCl as supporting electrolyte at pH = 6.

Control experiments for chamber 1

Control experiments on Chamber 1 were done to determine the amount of 12/13 CO produced using the two-chamber system. For 12 CO2 manipulations, the COware® system in Figure 0-19 was utilized. Chamber 1 was loaded according to "General protocol for chamber 1" and Chamber 2 was loaded with 4 mL DMF. The chambers were sealed with a screwcap fitted with a Teflon® seal. Chamber 2 solution was purged with argon gas for 10 minutes (keeping 1 atm pressure by putting an outlet syringe), after which Chamber 1 was purged with carbon dioxide gas for 15 minutes (keeping 1 atm pressure by putting an outlet syringe). From this