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(PCy 2N t‑Bu 2)2](BF4)2 (PCy 2N t‑Bu 2 = 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 NiII complex gives three isomers of the doubly protonated Ni0 complex [Ni(PCy 2N t‑Bu 2H)2](BF4)2. Using the pK a values and NiII/I and NiI/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(PCy 2N 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.