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.

CONSPECTUS: Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world's energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production−consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earthabundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal−polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH 3 CN) 2 ] 2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me 2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me 2 )MoO] 2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H 2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H 2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me 2 )MoS 2 ] 2+ , which bears a molecular MoS 2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me 2 )MoS 2 ] 2+ onsets at 400 mV of overpotential, whereas [(PY5Me 2 )MoO] 2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me 2 ligand with an appropriate Co(II) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me 2 )Co(H 2 O)] 2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF 3 SO 3 )] 1+ system, we showed that introducing a redox-active moiety can facilitate the electro-and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we...