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

Abstract

Abstract A cobalt complex bearing a κ ‐N 3 P 2 ligand is presented ( 1 + or Co I (L), where L is (1 E ,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 + .