Electrochemical, electron spin resonance and nuclear magnetic resonance investigations of tertiary phosphine complexes of nickel. Part 2. [NiIL4]+, [NiI(L–L)2]+ and their precursors (L = monodentate phosphine, L–L = bidentate diphosphine)

Abstract

The complexes [Ni(L–L)₂]²⁺ and [NiX(L–L)₂]⁺[L–L =cis-C₂H₂(PPh₂)₂-1,2, C₆H₄(PPh₂)₂-1,2, C₆H₄(PMePh)(PPh₂)-1,2 or C₆H₄(PBuⁿPh)(PPh₂)-1,2; X = Cl or Br] have been prepared and characterized. Phosphorus-31 NMR spectroscopy reveals the presence of stereoisomers of [Ni(L–L)₂]²⁺ when L–L is unsymmetrically substituted, and the results for the case L–L = C₆H₄(PBuⁿPh)(PPh₂)-1,2 are presented and analysed. Electrochemical studies of the above complexes in CH₂Cl₂ and of CH₂Cl₂ solutions containing nickel(II) and an excess of ligand L = PMePh₂ or PBuⁿPh₂ or L–L = C₆H₄(PMePh)₂-1,2 have been made. Two successive one-electron metal-based reductions were observed for the [Ni(L–L)₂]²⁺ complexes. The halide-containing complexes showed complex redox behaviour involving loss of halide upon one-electron reduction to form the stable [Ni^I(L–L)₂]⁺ species. Electron spin resonance studies of nickel(I) complexes obtained upon controlled-potential reduction of these systems have been made. Data from both electrochemical and ESR studies show effects due to variations in the structure of the phosphine ligands. The most important factor is the nature of the interphosphorus linkage, but effects due to changes in the substituents at phosphorus are also observed. The electrochemical and ESR properties of [Ni(PPh₂R)₄]⁺ are very sensitive to the nature of the alkyl substituent R.