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 (+).