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

A cobalt complex bearing a κ-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 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 (2²⁺), 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⁺.