Unusual properties of metal complexes involving acyclic polydentate thioether ligands. Copper, nickel and palladium complexes of 3,6,9,12,15-pentathiaheptadecane

Abstract

The ligand 3,6,9,12,15-pentathiaheptadecane and its complexes [CuL][X]2(X = ClO4– or CF3SO3–), [Cu2Cl3L](a mixed-valence compound), [NiL(H2O)][Y]2(Y = ClO4– or BF4–) and [PdL][ClO4]2 were prepared and X-ray crystal structure determinations of [CuL][ClO4]2 and [NiL(H2O)][ClO4]2 were carried out. The compounds have been examined by cyclic voltammetry, magnetic susceptibility, electronic spectroscopy and ESR or variable-temperature NMR spectroscopy. The [CuL][X]2 salts have the highest value of E½ for the CuII–thioether systems reported so far and an assessment of the reasons for the high redox potential suggests it to be related to both destabilization of the CuII state and more importantly to stabilization of the CuI state by the ligand. The absence of ring strain in the complexed acyclic ligand and entropy effects are considered to be significant contributors. There is evidence that the nickel co-ordination sphere remains intact in solutions of [NiL(H2O)][Y]2 and relationships between structural and spectral parameters such as Δ0 and the nephelauxetic ratio, β, have been investigated. Although Δ0 is mainly sensitive to donor type, β is found to be very sensitive to bond length. A variable-temperature NMR study of [PdL][ClO4]2 is treated as evidence for an unprecedented fluxional activity involving movement of a thioether donor from the ‘top’ to the ‘bottom’ of a co-ordination plane while inversion at five thioether donors is also occurring.