Synthesis and characterization of copper(II) and nickel(II) complexes with ligands having N₂S*₂S₂ (S* = thioether) donors: pseudo-reversible Cu^III/Cu^II couple

Abstract

Two new hexadentate ligands having thioether and thiolate groups in pairs along with two amide nitrogens have been synthesized. The thiols are protected as their tertiary butyl derivatives. On refluxing the ligands with a copper(II) or a nickel(II) salt in isopropanol, the S-tertiary butyl group was cleaved to form the corresponding hexa-coordinated neutral complex with the chromophore M^IIN₂S^∗ ₂S₂, where the four sulphurs are equatorial and the amide nitrogens are axial. Both the copper(II) complexes show pseudo-reversible responses (Δ E_p = 100 mV) in cyclic voltammograms with E _½ = 0.5 V vs S.C.E. at room temperature in acetonitrile, attributable to a Cu^III/Cu^II couple. The corresponding nickel(II) complex shows an irreversible response in the region −1.0 to +1.0 V. In their electronic spectra the copper(II) complexes show a strong band near 600 nm, attributable to a σ (thiolate) → Cu^II LMCT transition. Each of the copper(II) complexes show ligand field bands typical of a tetragonally distorted octahedral complex. The nickel(II) complexes show three ligand field bands typical of octahedral geometry.