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At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO2 and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO2 reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue. Interestingly, quasi-identical catalytic performances are obtained when one of the two iron centers was replaced by a redox inactive Zn metal, questioning the cooperative action of the two metals. Snapshots of X-ray structures indicate that the two metalloporphyrin units tethered by a urea group is a good compromise between rigidity and flexibility to accommodate CO2 capture, activation, and reduction.

International audience; Coupling a photoredox module and a bio-inspired non-heme model to activate O$_2$ for oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O$_2$ at the catalytic unit. We found that the efficient oxidative quenching mechanism between [Ru(bpy)$_3$]$^{2+}$ chromophore and a reversible electron mediator, methyl viologen (MV$^{2+}$), to form the reducing species methyl viologen radical (MV$^{•+}$) can convey an electron to O$_2$ to form the uperoxide radical and resetting an Fe(III) species in a catalytic cycle to the Fe(II) state in an aqueous solution. The formation of the Fe(III)-hydroperoxo (Fe$^{III}$ OOH) intermediate therefrom to evolve to highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O$_2$. The Fe$^{III}$-OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining a metal assisted catalysis. Importantly, we found that the substrate itself can act as the electron donor to reset the photooxidzed chromophore in the initial state closing the photocatalytic loop hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.

International audience; Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO$_2$ reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO$_2$ substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h$^{-1}$ , respectively. While the Fe$^0$ redox state has been widely accepted as the catalytically active species, we show here that the Fe(I) species is already involved in the CO$_2$ activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO$_2$ upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO$_2$

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with Mn

International audience; The [Re(bpy)(CO)3Cl] catalyst pioneered by Lehn for the two-electron reduction of CO2 has constantly revealed unique facets in the mechanistic understanding of the selective transformation of CO2. A novel triazole-linked ruthenium photosensitizer and a rhenium catalyst dyad was synthesized and investigated for photo-induced charge accumulation using time-resolved absorption spectroscopy. The triazole bridging ligand promoted weak electronic communication between the two units, resulting in an anodic shift of the reduction potentials of the Re moiety. Upon excitation of the photosensitizer, the first reduction of the catalyst occurred with a fast apparent rate of >5×107 s−1. Using a double-excitation nanosecond pump-pump-probe setup to track the second electron accumulation on the catalytic unit was not conclusive as no observable absorption changes occurred upon the second excitation, suggesting a pathway for an efficient intramolecular reverse electron transfer preventing the two-electron accumulation at the catalyst under our experimental conditions. Nevertheless, under continuous irradiation and with the use of sacrificial electron donors, photocatalytic CO2 reduction assays showed good turnover numbers, hinting at the non-innocent role of byproducts in solution.

International audience; In his pioneering work to unravel the catalytic power of enzymes, A. Warshel has pertinently validated that electrostatic interactions play a major role in the activation (bond making and breaking) of substrates. Implementing such chemical artifice in bio-inspired molecular-based catalysts may help in improving their catalytic properties. In this study, we have designed a series of tetra-, di-and mono-substituted iron porphyrins with cationic imidazolium functions. The presence of a cationic module in the second coordination sphere could help to stabilize the [Fe-CO2] intermediate upon electrocatalysis through an electrostatic interaction. We found herein that the overpotential of these catalysts is a function of the number of embarked imidazolium units ranging from 230 to 620 mV compared to 680 mV for the parent nonfunctionalized tetra-phenyl iron porphyrin. Importantly, we evidenced a gain of six orders of magnitude for the turnover frequencies going from the tetra-to the mono-substituted catalyst. The comparative study nails the fact that the electrocatalytic performance trend of through-space electrostatic interaction models outperforms the classic throughstructure electronic effect strategy. Henceforth, including controlled topological electrostatic interaction may be an invaluable chemical tool in the design of molecular catalysts in the activation of small molecules.

International audience

International audience; The mass scale catalytic transformation of carbon dioxide (CO2) into reduced forms of carbon is an imperative to address the ever-increasing anthropogenic emission. Understanding the mechanistic routes leading to the multi-electron-proton conversion of CO2 provides handles for chemists to overcome the kinetically and thermodynamically hard challenges and further optimize these processes. Through extensive electrochemical investigations, Prof. J-M. Savéant and coworkers have made accessible to chemists invaluable electro-analytical tools to address and position the electrocatalytic performance of molecular catalysts grounded on a theoretical basis. Furthermore, he has bequeathed lessons to future generations on ways to improve the catalytic activity and on the electrocatalytic zone we must target. As a tribute to his accomplishments, we recall here a few aspects on the tuning of iron porphyrin catalysts by playing on electronic effects, proton delivery, hydrogen bonding and electrostatic interactions and its implications to other catalytic systems.

In his pioneering work to unravel the catalytic power of enzymes, Warshel has pertinently validated that electrostatic interactions play a major role in the activation of substrates. Implementing such chemical artifice in molecular catalysts may help improve their catalytic properties. In this study, a series of tetra-, di-, and mono-substituted iron porphyrins with cationic imidazolium groups were designed. Their presence in the second coordination sphere helped stabilize the [Fe-CO

International audience; Research in the development of new molecular catalysts for the selective transformation of CO2 to reduced forms of carbon is attracting enormous interest from chemists. Molecular catalyst design hinges on the elaboration of ligand scaffolds to manipulate the electronic and structural properties for the fine tuning of the reactivity pattern. A cornucopia of ligand sets have been designed along this line and more and more are being reported. In this quest, the porphyrin molecular platform has been under intensive focus due to the unmatched catalytic properties of metalloporphyrins. There have been rapid advances in this particular field during the last few years wherein both electronic and structural aspects in the second coordination spheres have been addressed to shift the overpotential and improve the catalytic rates and product selectivity. Metalloporphyrins have also attracted much attention in terms of the elaboration of hybrid materials for heterogeneous catalysis. Here too, some promising activities have made metalloporphyrin derivatives serious candidates for technological implementation. This review collects the recent advances centred around the chemistry of metalloporphyrins for the reduction of CO2.

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N2S3 ligand was synthesized. When coupled with a [Ru(bpy)3] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N2S2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

International audience; The manipulation of the second coordination sphere for improving the electrocatalytic CO2 reduction has led to amazing breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron porphyrin complex holding two urea functions acting as multiple hydrogen bonding tweezers to lock the metal bound CO2 in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. We found that the  topological isomer with the two urea groups on the same side of the porphyrin platform provides a stronger binding affinity to tether the incoming CO2 substrate in comparison to the  disposition. However, the electrocatalytic activity of the  atropisomer outperforms its congener with one of the highest reported turnover frequency at low overpotential. The strong H/D KIE observed for the system indicates the existence of a tight water hydrogen bonding network for proton delivery which is disrupted upon addition of exogenous acid source. While the small H/D KIE for the  isomer and the enhanced electrocatalytic performance upon addition of stronger acid pertain the free access of protons to the bound CO2 on the opposite side of the urea arm.

Inspired by the sulfur-rich environment found in active hydrogenase enzymes, a Ni-based proton reduction catalyst with pentadentate N

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. 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.; 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 cœur 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.

International audience; Inspired by nature's orchestra of chemical subtleties to activate and reduce CO2, we have developed a family of iron porphyrin derivatives in to which we have introduced urea groups functioning as multipoint hydrogen‐bonding pillars on the periphery of the porphyrinic ring. This structure closely resembles the hydrogen‐bond stabilization scheme of the carbon dioxide (CO2) adduct in the carbon monoxide dehydrogenase (CODH). We found that such changes to the second coordination sphere significantly lowered the overpotential for CO2 reduction in this family of molecular catalysts and importantly increased the CO2 binding rate while maintaining high turnover frequency (TOF) and selectivity. Entrapped water molecules within the molecular clefts were found to be the source of protons for the CO2 reduction.

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N 2 S 3 ligand was synthesized. When coupled with a [Ru(bpy) 3 ] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N 2 S 2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

Inspired by nature's orchestra of chemical subtleties to activate and reduce CO

In this study we report a strategy to attach methylimidazolium fragments as ionic liquid units on an established iron porphyrin catalyst for the selective reduction of CO2 to CO. Importantly, we found that the tetra-methylimidazolium containing porphyrin exhibits an exalted electrocatalytic activity at low overpotential in water precluding the need for an external proton donor.

WOS:000441548500003; International audience; A convenient and safe approach in valorizing carbon monoxide (CO) produced from the photocatalytic reduction of carbon dioxide (CO2) has been investigated. Visible light was used to drive an optimized photocatalytic reduction using a ruthenium trisbipyridine complex as a sensitizer and a rhenium bipyridyl carbonyl complex as a catalyst to perform an efficient reduction of CO2 to CO, which was then simultaneously utilized in a palladium-catalyzed aminocarbonylation reaction at room temperature. This approach provides safe handling of the produced CO which also opens the way for a more efficient application of C-13-isotope and C-14-radioisotope-labeled CO2 in pharmaceutically relevant drug labeling.

International audience; In this study we report a strategy to attach methylimidazolium fragments as ionic liquid units on an established iron porphyrin catalyst for the selective reduction of CO2 to CO. Importantly, we found that the tetra-methylimidazolium containing porphyrin exhibits an exalted electrocatalytic activity at low overpotential in water precluding the need for an external proton donor.