Impact of Weak Agostic Interactions in Nickel Electrocatalysts for Hydrogen Oxidation

To understand how H₂ binding and oxidation is influenced by [Ni(P^R ₂N^R′ ₂)₂]²⁺ catalysts with H₂ binding energies close to thermoneutral, two [Ni(P^Ph ₂N^R′ 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(P^Ph ₂N^Me ₂)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(P^R ₂N^R′ 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.