[Ni(PPh2NAr2)2(NCMe)][BF4]2 as an electrocatalyst for H2 production: PPh2NAr2 = 1,5-(di(4-(thiophene-3-yl)phenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane)

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 confirm that 3 is a relatively active catalyst for H2 production in acetonitrile solutions. Oxidation of the pendant thiophene substituents of 3 results in the formation of films on glassy carbon electrode surfaces. However these films are not electroactive, and electrocatalysis of proton reduction is not observed with these modified electrodes.

Graphical abstractA thiophene modified nickel complex containing a cyclic diphosphine ligand with pendant nitrogen bases is shown to be an effective catalyst for the electrochemical reduction of protons to hydrogen.Figure optionsDownload full-size imageDownload as PowerPoint slide