PNNL

Abstract

A series of [Ni(PR2NPh2)2(CH3CN)](BF4)2 complexes containing the cyclic diphosphine ligands (PR2NPh2 = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), n-butyl (n-Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)) have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(PBn2NPh2)2(CH3CN)](BF4)2 and [Ni(Pn-Bu2NPh2)2(CH3CN)](BF4)2 have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(PBn2NPh2)2 (CH3CN)] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(PCy2NPh2)2(CH3CN)](BF4)2, all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H2 in acidic acetonitrile solutions. The heterolytic cleavage of H2 by [Ni(PR2NPh2)2(CH3CN)](BF4)2 complexes in the presence of p-anisidine or p-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(PR2NPh2)2](BF4) complexes. However, the failure to observe a strong correlation between the turnover frequencies for H2 production and the hydride donor abilities, along with structural features of [Ni(PBn2NPh2)2(CH3CN)], suggest that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate. This material is based upon work supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.