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 CoI(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 CoI thanks to the π‐acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two‐electron CoI/CoIII oxidation process and an additional one‐electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset=−1.6 V vs Fc/Fc+. In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII(L)‐H (22+), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII(L)‐H (2+) species, which rapidly forms hydrogen and regenerates the initial CoI(L) (1+). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+. [ABSTRACT FROM AUTHOR]