Axial co-ordination and ion-pair formation for R,S,R,S-1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecanenickel(II) perchlorate in various mixed solvents with nitrobenzene

Abstract

Conductance measurements of the complex R,S,R,S-[NiL][ClO4]2(L = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane), in mixed solvents of N,N-dimethylacetamide (dma) and acetonitrile with nitrobenzene, which are of iso-relative permittivity, have shown that the first ion-pair formation constant (1 190 dm3 mol–1) in nitrobenzene of much lower basicity decreases with increasing fraction of dma and MeCN, although MeCN is axially bonded to the nickel but dma is hardly co-ordinated. The axial co-ordination constant, KNiS=[NiL(S)2+]/[NiL2+][S], of twenty donor solvents (S) in nitrobenzene as a non-co-ordinating diluent was also determined spectrometrically. It was found that the oxygen-donor co-ordinating solvents such as acetone and water for which Gutmann's donor number (DN) is less than 20 are not co-ordinated to the nickel, and the nitrogen-donor co-ordinating solvents with a nitrile group such as acetonitrile tend to be strongly co-ordinated in spite of their lower DN. These results show no correlation of KNiS with DN and are discussed in terms of the π-acceptor properties of the C–N bond, steric hindrance due to the four methyl groups, and self-interactions of the donor solvents.