Studies of a Series of [Ni(P2RN2Ph)2(CH3CN)]2+ Complexes as Electrocatalysts for H2 Production: Substituent Variation at the Phosphorus Atom of the P2N2 Ligand

A series of [Ni((P2N2Ph)-N-R)(2)(CH3CN)](BF4)(2) complexes containing the cyclic diphosphine ligands [(P2N2ph)-N-R = 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((P2N2Ph)-N-Bn)(2)(CH3CN)](BF4)(2) and [Ni((P2n-BuN2Ph))(2)(CH3CN)](BF4)(2) have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni((P2N2Ph)-N-Bn)(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((P2N2Ph)-N-Cy)(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 H-2 in acidic acetonitrile solutions. The heterolytic cleavage of H-2 by [Ni((P2N2Ph)-N-R)(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((P2N2Ph)-N-R)(2)](BF4) 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.