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-((P2N2t-Bu)-N-Cy)(2)](BF4)(2) ((P2N2t-Bu)-N-Cy = 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 Ni-0 complex [Ni((P2N2t-BuH)-N-Cy)(2)](BF4)(2). Using the pK(a) values and Ni-II/I and Ni-1/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni((P2N2t-Bu)-N-Cy)(2)](2+) was determined to be -7.9 kcal mol(-1). The catalytic rate observed in dry acetonitrile for the oxidation of H-2 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 H-2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 degrees C.