Impact of Weak Agostic Interactions in Nickel Electrocatalysts for Hydrogen Oxidation

To understand how H2 binding and oxidation is influenced by [Ni­(PR2NR′2)2]2+ catalysts with H2 binding energies close to thermoneutral, two [Ni­(PPh2NR′2)2]2+ (R = Me, C14H29) complexes with phenyl substituents on phosphorus and varying alkyl chain lengths on the pendant amine were studied. In the solid state, [Ni­(PPh2NMe2)2]2+ exhibits a weak agostic interaction between the Ni­(II) center and the a C–H bond of the pendant N–CH3 group. DFT computations and variable-temperature 31P NMR experiments suggest that the agostic interaction persists in solution. The equilibrium constants for H2 addition to these complexes were measured by 31P NMR spectroscopy, affording free energies of H2 addition (ΔG°H2) of −0.8 kcal mol–1 in benzonitrile and −1.7 to −2.7 kcal mol–1 in THF. The agostic interaction contributes to the low driving force for H2 binding by stabilizing the four-coordinate Ni­(II) species prior to binding of H2. The pseudo-first-order rate constants for addition of H2 (1 atm) were measured by variable-scan rate cyclic voltammetry and were found to be similar for both complexes, with rate constants of 3–6 s–1 in THF and less than 0.2 s–1 in benzonitrile. In the presence of exogenous base and H2, turnover frequencies of electrocatalytic H2 oxidation were measured to be less than 0.2 s–1 in benzonitrile and 4–6 s–1 in THF. These complexes are slower electrocatalysts for H2 oxidation in comparison to previously studied [Ni­(PR2NR′2)2]2+ complexes because of a competition between H2 binding and formation of the agostic bond. However, the decrease in catalytic rate is accompanied by a beneficial 130 mV decrease in overpotential.