Copper(ii) complexes of thioether-substituted salcyen and salcyan derivatives and their silver(i) adducts

Abstract

New syntheses are reported of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde, 5-tert-butyl-2-hydroxy-3-phenylsulfanyl-benzaldehyde, and salcyen (H₂L¹–H₂L³) and salcyan (H₂L⁴–H₂L⁶)-type ligands derived from these aldehydes and from 5-tert-butyl-2-hydroxybenzaldehyde. The complexes [CuL] (L²⁻ = [L¹]²⁻–[L⁶]²⁻) bearing sulfanyl substituents each show two distinct voltammetric ligand-based oxidations under the same conditions, the first of which is chemically reversible. The first oxidation product is much longer lived by coulometry for the salcyen than for the salcyan ligand complexes, despite the latter having a substantially lower oxidation potential. The lifetimes of all the ligand oxidation products in this system are substantially smaller than for similar compounds derived from 3,5-di(tert-butyl)-2-hydroxybenzaldehyde (Dalton Trans., 2004, 2662). Attempted chemical oxidation of the Schiff base compounds using AgBF₄ yielded instead stable silver(I) adducts. A crystal structure of one such compound showed that the Ag atom was coordinated in a slightly bent geometry by the two ligand sulfanyl groups, with two additional long-range Ag⋯O interactions to the phenoxide donors. EPR spectra showed that some of these silver adducts dimerise in CH₂Cl₂, probably through basal, apical intermolecular Cu–O⋯Cu bridging. In contrast the parent copper(II) complexes are all monomeric in this solvent by EPR.