Utilization of Thermodynamic Scaling Relationships in Hydricity To Develop Nickel Hydrogen Evolution Reaction Electrocatalysts with Weak Acids and Low Overpotentials

Two heteroleptic nickel­(II) complexes of the type [Ni­(bis-NHC)­(dRpe)]2+ (1, R = phenyl; 2, R = methyl; bis-NHC = 1,l′:3,3′-bis­(1,3-propanediyl)­dibenzimidazolin-2,2′-diylidene) have been synthesized and characterized. The complexes exhibit reversible, two-electron reductions at −1.53 and −1.87 V versus Fc+/0, respectively. Through the use of thermodynamic scaling relationships, hydricities for the corresponding nickel­(II) hydride complexes 1H and 2H are estimated to be 45.6 and 37.8 kcal mol–1, respectively. Experimental estimation of an upper bound for the hydricities (ΔG H– °(1H) < 50.3 kcal mol–1 and ΔG H– °(2H) < 40.6 kcal mol–1) was determined by selection of an added weak acid such that the hydrogen evolution reaction (HER) becomes exergonic vs endergonic. Further electrochemical studies establish 2 as an efficient hydrogen evolution electrocatalyst, operating at low overpotential (η = 0.4 V) and reasonable rates (TOF ∼ 1000 s–1) using phenol as the Brønsted acid source. This demonstrates the utility of using ligand donor effects to impart noble-like, two-electron redox behavior at very negative potentials and increased hydride donor ability at nickel. This expands consideration of substrates suitable for hydrogen evolution at low overpotentials.