Catalytic proton reduction with transition metal complexes of the redox-active ligand bpy2PYMe

A new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe²⁺ (1), Co ²⁺ (2), Ni²⁺ (3), Cu²⁺ (4), and Zn²⁺ (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 H₂ 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 CF₃ group into the pyridine moiety of the ligand as in bpy2PYMe-CF₃ 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 Co¹⁺ 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.