PNNL

Abstract

A new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligand was prepared with phenyl substituents on phosphorus and (thiophene-3-yl)phenyl substituents on nitrogen. This ligand reacts with [Ni(CH3CN)6][BF4]2 to form the corresponding [Ni(PPh2NAr2)2(NCMe)][BF4]2 complex, 3, which is an active electrocatalyst for H2 production. Kinetic studies indicate that the catalytic rate is first order in catalyst and second order in acid at low concentrations of acid, but at higher acid concentrations the catalytic rate becomes independent of acid concentration. The rate-determining step at high acid concentrations is attributed to the elimination of H2 from a reduced Ni species. The modest overpotential of 280 mV and a turnover frequency of 56 s-1 confirms that high activity observed is a general feature of this class of complexes and not an exception. Oxidation of the pendant thiophene substituents of 3 results in the formation of films on the electrode surface. However these films are not electroactive and electrocatalysis of proton reduction is not observed with these modified electrodes. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.