Polar Substituents Enable Efficient Catalysis for a Class of Cobalt Polypyridyl Hydrogen Evolving Catalysts

A series of structurally related, acyclic cobalt tetrapyridyl hydrogen evolving catalysts (HEC) were prepared and characterized. The common motif, di(2,2 '-bipyridin-6-yl)-methane, was derivatized at the bridging methylene to include a carbonyl group (L1), a hydroxy (L2), a methyl and a hydroxy (L3), a 1,1 '-biphenyl-2,2 '-diyl (L4), a hydroxy and a phenyl (L5) or a hydroxy and a pyrid-6-yl group. These catalysts were compared with the known HEC [Co(appy)Br]Br. Photo- and electrochemistry showed a distinct influence of the bridging position on rates and stabilities of the hydrogen evolution reaction (HER). Apolar ligands resulted in inferior catalytic performance as compared to HECs with polar substituents. Electrochemically, [Co(L1)Br-2] was shown to be converted to [Co(L2)Br-2] in catalysis. The best catalyst made more than 10'000 turnovers, albeit at an overpotential of 600 mV. Additional pH dependent mechanistic aspects were elucidated by cyclic voltammetry.