Nickel thioether chemistry: a re-examination of the electrochemistry of [Ni([9]aneS3)2]2+. The single-crystal X-ray structure of a nickel(III) thioether complex, [NiIII([9]aneS3)2][H5O2]3[ClO4]6([9]aneS3= 1,4,7-trithiacyclononane)

Abstract

Cyclic voltammetry of [Ni([9]aneS₃)₂][PF₆]₂([9]aneS₃= 1,4,7-trithiacyclononane) in MeCN (0.1 mol dm^–3 NBu₄PF₆) at 293 K at platinum electrodes shows the previously reported chemically reversible oxidation at E_½=+0.98 V, ΔE_p= 82 mV and a quasi-reversible reduction at E_½=–1.11 V vs. ferrocene–ferrocenium, ΔE_p= 90 mV. These redox processes have been investigated spectroelectrochemically using ESR and electronic spectroscopy which indicate [Ni([9]aneS₃)₂]³⁺ to be a genuine d⁷ nickel(III) complex. The [Ni([9]aneS₃)₂]³⁺ cation can be generated quantitatively and stabilised for extended periods by dissolution of [Ni([9]aneS₃)₂]²⁺ in HClO₄ solution. The complex [Ni^III([9]aneS₃)₂][H₅O₂]₃[ClO₄]₆ crystallises in the rhombohedral space group Rc. The single-crystal X-ray structure confirms octahedral homoleptic thioether co-ordination with Ni–S bond lengths of 2.3129(25)Å. These Ni–S distances are significantly shorter than in the nickel(II) analogue [Ni([9]aneS₃)₂]²⁺ and confirm the formulation of a mononuclear nickel(III)–thioether complex cation. Cyclic voltammetry of [Zn([9]aneS₃)₂][PF₆]₂ reveals one irreversible oxidation and one irreversible reduction at E_pa=+1.30 and E_pc=–1.77 V respectively at a scan rate of 400 mV s^–1. Significantly, the redox potential for the oxidation of the zinc(II) species is more anodic than for the nickel(II) analogue.