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

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