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International audience; Orange Protein (Orp) is a small bacterial metalloprotein of unknown function that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [S2MoS2CuS2MoS2]3−. In this paper, the performance of Orp as a catalyst for the photocatalytic reduction of protons into H2 has been investigated under visible light irradiation. We report the complete biochemical and spectroscopic characterization of holo-Orp containing the [S2MoS2CuS2MoS2]3− cluster, with docking and molecular dynamics simulations suggesting a positively charged Arg, Lys-containing pocket as the binding site. Holo-Orp exhibits an excellent photocatalytic activity, in the presence of ascorbate as the sacrificial electron donor and [Ru(bpy)3]Cl2 as the photosensitizer, for hydrogen evolution with a maximum turnover number of 890 after 4 hours irradiation. DFT calculations were used to propose a consistent reaction mechanism in which the terminal sulfur atoms are playing a key role in promoting H2 formation. A series of dinuclear [S2MS2M’S2MS2](4n)− clusters, with M = MoVI, WVI and M’(n+) = CuI, FeI, NiI, CoI , ZnII, CdII were assembled in Orp, leading to different M/M’-Orp versions which are shown to display catalytic activity, with the Mo/Fe-Orp catalyst giving a remarkable TON of 1150 after 2.5 hours reaction and an initial turnover frequency (TOF°) of 800 h-1 establishing a record among previously reported artificial hydrogenases.

Closing the carbon dioxide (CO 2 ) cycle by transforming it into fuel in order to mitigate its accumulation in the atmosphere is one of the most important challenges of our fossil-fuel-dependent society. [1-6] This solution was inspired by nature itself where green plants efficiently capture sunlight to convert CO 2 and water into various hydrocarbons and O 2 by photosynthesis. [7,8] After eons of evolution, the different steps of capture, activation, and transformation of the thermodynamically very stable CO 2 molecule were optimized in sophisticated metalloenzymes. [9-12] Similarly, an artificial transformation of CO 2 requires a catalyst that can capture and activate efficiently CO 2 before reducing it in proton-coupled multi-electron processes to skirt the formation of thermodynamically unfavorable high-energy intermediates. [13] Principally, the search for homogenous catalysts for CO 2 conversion encompasses noble metals- (Ru, Rh, Pd, Re and Ir) and first-row transition metals- (Mn, Fe, Co, Ni and Cu) based complexes with mainly nitrogen-based ligands. [14-18].

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.