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 E_½ for the Cu^II–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 Cu^II state and more importantly to stabilization of the Cu^I 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.