Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst.

A cobalt complex bearing a κ-N 3 P 2 ligand is presented (1 + or Co I (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1 + is stable under air at oxidation state Co I thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1 + reveals a two-electron Co I /Co III oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at E onset =-1.6 V vs Fc/Fc + . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1 + forms the cobalt hydride derivative Co III (L)-H (2 2+ ), which has been fully characterized. Further addition of 1 equiv of CoCp* 2 (Cp* is pentamethylcyclopentadienyl) affords the reduced Co II (L)-H (2 + ) species, which rapidly forms hydrogen and regenerates the initial Co I (L) (1 + ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1 + .