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

摘要

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 (2~(2+)), which has been fully characterized. Further addition of 1 equiv of CoCp~1_2 (Cp~2 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~+.