Azooximates of bi- and tri-valent nickel

Abstract

The reaction of arylazooximes, RC(NOH)NNPh (HL^R, R = Me or Ph), with nickel(II) acetate tetrahydrate in methanol under anaerobic conditions afforded [NiL^R₃]^- isolated as the NEt₄⁺ salt. One (L^Ph)^- ligand in [NiL^Ph₃]^- underwent facile displacement by L–L ligands like 2,2′-bipyridine (bipy) furnishing [NiL^Ph₂ (bipy)]. The Ni^III–Ni^II reduction potential of [NiL^R₃]^- in acetonitrile is ≈ 0.1 V vs. saturated calomel electrode. The trivalent complex [NiL^R₃] was quantitatively isolated via constant-potential electrolysis at 0.3 V. The Ni^IV–Ni^III couple of the tris chelate was observed near 0.9 V, but the nickel(IV) complex could not be isolated in the solid state. The relatively low metal reduction potential allowing facile preparation of the stable [NiL^R₃] system is attributed to the strong-field nature of the oximato-N atom. In going from [NiL^Ph₃]^- to [NiL^Ph₂ (bipy)] the Ni^III–Ni^II reduction potential increases by ≈ 0.3 V showing that (L^Ph)^- is a much better stabiliser of Ni^III than is bipy. The crystal structures of [NEt₄][NiL^Ph₃] and [NiL^Ph₂ (bipy)] have been determined. The geometry of [NiL^R₃] (S = ½) was studied with the help of its EPR spectrum (d_z2 ground state) in the [CoL^R₃] lattice. Both [NiL^R₃]^- and [NiL^R₃] have exclusive meridional geometry consistent with steric and angular-overlap considerations. In [NiL^Ph₂ (bipy)] the two anionic oximato functions are placed in mutually trans positions. The oximato-N ligand displays substantial trans influence. Thus in [NiL^Ph₃]^- the Ni–N (azo) bond lying trans to Ni–N (oxime) is ≈ 0.05 Å longer than the other two mutually trans Ni–N (azo) bonds. The average Ni–N (azo) distance in [NiL^Ph₂ (bipy)] is ≈ 0.04 Å shorter than that in [NiL^Ph₃]^- because none of the Ni–N (azo) bonds in the former complex is subject to the trans influence of Ni–N (oxime). In both complexes the Ni–N (oxime) lengths are significantly shorter than the Ni–N (azo) lengths, consistent with stronger Ni–N (oxime) σ bonding which is also a reason behind the strong-field nature of the oximate ligand.