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

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 NiIII species. These NiIII 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 E½(NiIII–NiII), 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 NiIII 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. bottom