Stabilisation of trivalent nickel through 1 : 2 co-ordination by cyclic terdentate ligands CH2CH2NH(CH2)2NH(CH2)2X (X = NH, O, or S)

Abstract

The octahedral six-co-ordinated complexes [Ni(L²)₂]²⁺ and [Ni(L³)₂]²⁺(L² and L³= 1 -oxa- and 1 -this-4,7-diazacyclononane, respectively)(donor set: N₄O₂ and N₄S₂, respectively) undergo a reversible one-electron oxidation process in MeCN and dimethyl sulphoxide (dmso) solution to give authentic Ni^III species. These Ni^III complexes present a distorted stereochemical arrangement, with the O and S heteroatoms occupying the axial sites of an elongated octahedron, and persist in solution for hours. Comparison with the corresponding complexes of 1,4,7-triazacyclonanane (L¹)(donor set: N₆) indicates that the relative stability of the trivalent species, expressed by the value of E_½(Ni^III–Ni^II), decreases along the series: L¹ > L³ > L², the trend reflecting the decrease in the strength of the Ni–X interactions (X = N, S, or O). The corresponding Ni^III complexes of the open-chain analogues of the macrocycles L¹ or L³ form at comparable potentials, but persist in solution only in the time-scale of the voltammetry experiment, thus demonstrating the existence of a purely kinetic macrocyclic effect in the stabilisation of high oxidation states of metals.