Studies of a series of [Ni(P(R)2N(Ph)2)2(CH3CN)]2+ complexes as electrocatalysts for H2 production: substituent variation at the phosphorus atom of the P2N2 ligand.

A series of Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)(2) complexes containing the cyclic diphosphine ligands [P(R)(2)N(Ph)(2) = 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(P(Bn)(2)N(Ph)(2))(2)(CH(3)CN)(2) and Ni(P(n-Bu)(2)N(Ph)(2))(2)(CH(3)CN)(2) have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P(Bn)(2)N(Ph)(2))(2)] 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(P(Cy)(2)N(Ph)(2))(2)(CH(3)CN)(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 H(2) in acidic acetonitrile solutions. The heterolytic cleavage of H(2) by Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)(2) complexes in the presence of p-anisidine or p-bromoaniline was used to determine the hydride donor abilities of the corresponding HNi(P(R)(2)N(Ph)(2))(2) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H(2), suggesting 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.