TitleCatalytic proton reduction with transition metal complexes of the redox-active ligand bpy2PYMeA new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe 2+ (1), Co 2+ (2), Ni 2+ (3), Cu 2+ (4), and Zn 2+ (5) have been investigated for electro-and photo-catalytic proton reduction in acetonitrile and water, respectively. Under weak acid conditions, the Co complex displays catalytic onset at potentials similar to those of the ligand centered reductions in the absence of acid. Related Co complexes devoid of ligand redox activity catalyze H 2 evolution under similar conditions at significantly higher overpotentials, showcasing the beneficial effect of combining ligand-centered redox activity with a redox-active Co center. Furthermore, turnover numbers as high as 1630 could be obtained under aqueous photocatalytic conditions using [Ru(bpy) 3 ] 2+ as a photosensitizer. Under those conditions catalytic hydrogen production was solely limited by photosensitizer stability. Introduction of an electron withdrawing CF 3 group into the pyridine moiety of the ligand as in bpy2PYMe-CF 3 renders its corresponding Co complex 6 less active for proton reduction in electro-and photocatalytic experiments. This surprising effect of ligand substitution was investigated by means of density functional theory calculations which suggest the importance of electronic communication between Co 1+ and the redox-active ligand. Taken together, the results provide a path forward in the design of robust molecular catalysts in aqueous media with minimized overpotential by exploiting the synergy between redox-active metal and ligand components. † Electronic supplementary information (ESI) available: Molecular structures of 1, 3, 4, 5; crystallographic information and bond lengths; UV/Vis and EPR spectra; DC magnetic susceptibility data for 2; pH, concentration dependence of [Ru(bpy) 3 ] 2+ and ascorbic acid of photocatalytic H 2 production; photoluminescence quenching of [Ru(bpy) 3 ] 2+ ; computational information. CCDC 943488-943492. For ESI and crystallographic data in CIF or other electronic format see Scheme 1 Relation of acid strength to redox potentials of Co based HER catalyst.Scheme 2 Redox active ligands utilized in HER catalysis.

The electrocatalytic reduction of carbon dioxide (CO2) could be a powerful tool for generating chemical fuels and feedstock molecules relevant to the chemical industry. One of the major challenges for molecular catalysts remains the necessity of high overpotentials, which can be overcome by identifying novel routes that improve the energetic reaction trajectory of critical intermediates during catalysis. In this combined experimental and computational study, we show that imidazolium functionalization of molecular fac-Mn­(CO)3 bipyridine complexes results in CO2 reduction at mild electrochemical potentials in the presence of H2O. Importantly, our studies suggest that imidazolium groups in the secondary coordination sphere promote the formation of a local hydration shell that facilitates the protonation of CO2 reduction intermediates. As such, we propose a synergistic relationship between the functionalized catalyst and H2O, which stands in contrast to other systems in which the presence of H2O frequently has detrimental effects on catalysis.

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