[Ni(P^Ph₂N^Bn₂)₂(CH₃CN)]²⁺ as an Electrocatalyst for H₂ Production: Dependence on Acid Strength and Isomer Distribution

Abstract

[Ni(PPh2NBn2)2(CH3CN)]2+ (where PPh2NBn2 is 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane), has been studied as an electrocatalyst for the production of hydrogen in acetonitrile. Strong acids, such as p-cyanoanilinium, protonate [Ni(PPh2NBn2)2(CH3CN)]2+ prior to reduction under catalytic conditions, and an effective pKa of 6.7 ± 0.4 was determined for the protonation product. Through multinuclear NMR spectroscopy studies, the nickel(II) complex was found to be doubly protonated without any observed singly protonated species. In the doubly protonated complex, both protons are positioned exo with respect to the metal center and are stabilized by an N–H–N hydrogen bond. The formation of exo protonated isomers is proposed to limit the rate of hydrogen production because the protons are unable to gain suitable proximity to the reduced metal center to generate H2. Preprotonation of [Ni(PPh2NBn2)2(CH3CN)]2+ has been found to shift the catalytic operating potential to more positive potentials by up to 440 mV, depending upon the conditions. The half-wave potential for the catalytic production of H2 depends linearly on the pH of the solution and indicates a proton-coupled electron transfer reaction. The overpotential remains low and nearly constant at 74 ± 44 mV over the pH range of 6.2–11.9. The catalytic rate was found to increase by an order of magnitude by increasing the solution pH or through the addition of water.