Electrochemical, electron spin resonance and nuclear magnetic resonance investigations of tertiary phosphine complexes of nickel. Part 1. Mono- and di-nuclear nickel dithiolene and nickel dihalide species

Abstract

The synthesis, characterisation and ³¹P NMR parameters of [Ni{S₂C₂(CN)₂}(PR₃)₂](PR₃= PMePh₂ or PBuⁿPh₂) and of [{Ni(S₂C₂R₂)(L–L)}_n] complexes (L–L = ditertiary phosphine containing vinylene or ortho-phenylene interphosphorus linkages, n= 1 or 2, R = CN or Ph) are reported. The ³¹P NMR data for the [{Ni(S₂C₂R₂)(L–L)}_n] complexes show that the ring contribution to the chemical shift is large and positive for the mononuclear species (n= 1) and near zero or negative for the dinuclear species (n= 2). Electrochemical studies of the complexes and ESR studies of their electrogenerated products have shown variations in electrochemical and ESR parameters due to variations in (i) co-ordination ring size, (ii) interphosphorus linkage and (iii) substituent at phosphorus. The ESR spectrum of the redox product identified as [Ni{S₂C₂(CN)₂}{C₆H₄(PMePh)(PPh₂)-1,2}]⁺ exhibited two chemically inequivalent ³¹P nuclei. The dinuclear complexes (n= 2) exhibited multiple redox processes. The ESR spectra show that the reduction products involve metal-based electron addition yield the corresponding nickel(I) species, while the oxidation products involve removal of a primarily ligandbased electron. The one-electron reduction product of the complex [Ni(S₂C₂Ph₂){C₆H₄(PMePh)₂-1,2}] shows a spectrum characteristic of a nickel(I) monomer (S=½) at ca. 298 K, but is ESR inactive at ca. 90 K. This is attributed to the reversible formation of a diamagnetic dimer (S= 0) upon freezing. The reduction products of the complexes [NiX₂(trans-Ph₂PCHCHPPh₂)}₂](X = Cl, Br, l or SCN) exhibit no solution ESR spectra at ca. 298 K, but for X = l a triplet state (S= 1) spectrum at ca. 90 K is observed, characteristic of a dinuclear Ni^t–Ni^t species.