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 = 1,1':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 (Delta G(H)degrees-(1H) < 50.3 kcal mol(-1) and Delta G(H)degrees-(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 (eta = 0.4 V) and reasonable rates (TOF similar to 1000 s(-1)) using phenol as the Bronsted 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.