Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P^Cy₂N^t-Bu ₂)₂](BF₄)₂ (P^Cy ₂N^t-Bu₂ = 1,5-di(tert-butyl)-3,7-dicyclohexyl- 1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni^II complex gives three isomers of the doubly protonated Ni⁰ complex [Ni(P^Cy ₂N^t-Bu₂H)₂](BF₄) ₂. Using the pK_a values and Ni^II/I and Ni ^I/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P^Cy₂N^t-Bu ₂)₂]²⁺ was determined to be -7.9 kcal mol ^-1. The catalytic rate observed in dry acetonitrile for the oxidation of H₂ depends on base size, with larger bases (NEt₃, t-BuNH₂) resulting in much slower catalysis than n-BuNH₂. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt₃ and t-BuNH₂. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H₂ using n-BuNH₂ as a base and with added water, a turnover frequency of 58 s^-1 is observed at 23 C.