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

A cobalt complex bearing a kappa-N3P2 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 pi-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(+).