Blue copper models. Synthesis and characterization of copper(II) enethiolate complexes derived from (1R)-3-hydroxymethylenebornane-2-thione and 2-aminothia-alkyl-1-methylbenzimidazoles (donor set N2SS*) or diamines (donor set N2S2)

Abstract

A series of tetradentate ligands derived from the condensation of (1R)-3-hydroxymethylenebornane-2-thione and several 2-aminothia-alkylbenzimidazoles were synthesized together with their cationic zinc(II) and copper(II) complexes. While the free ligands exist in the thioxoenamine tautomeric form, binding to the metals occurs in their enethiolateimine form, as deduced from cumulative spectral evidence. The copper(II) complexes exhibit medium-intensity absorptions between 410 and 435 (ε≳ 3 500) and between 520 and 535 nm (ε≳ 1 200 dm³ mol^–1 cm^–1) attributable to enethiolate σ(S^–)→ Cu^II and π(S^–)→ Cu^II ligand-to-metal charge transfer (I.m.c.t.) transitions, respectively, and relatively intense ligand-field transitions in the range 610–650 nm (ε≳ 1 500 dm³ mol^–1 cm^–1). All the bands undergo progressive red shifts as the length of the carbon chain connecting the imine and thioether functions increases from two to four atoms. The frozen-solution e.p.r. spectra are indicative of progressive distortion towards a pseudo-tetrahedral geometry for copper(II) as the carbon chain length increases. A series of neutral copper(II) complexes derived from the condensation of (1R)-3-hydroxymethylenebornane-2-thione and symmetric diamines was also synthesized for comparison. Here too the length of the carbon chain connecting the imine nitrogen atoms was varied from two to four atoms. These complexes exhibit the same spectroscopic trends but with less marked distortion of the copper(II) chromophores as the length of the carbon chain increases. The cationic copper(II) complexes from (1R)-3-hydroxy-methylenebornane-2-thione and 2-aminothia-alkylbenzimidazoles display, in acetonitrile solution at –15 °C, easy access to the corresponding copper(I) congeners (E⁰=–0.02 V vs. s.c.e.). By contrast, the neutral copper(II) species derived from (1R)-3-hydroxymethylenebornane-2-thione and diamines are reduced to the corresponding copper(I) complexes at potentials more negative by 0.7–1.0 V, depending upon the extent of stereochemical distortion around the copper(II) centre induced by the length of the imine-nitrogen-linking carbon chain. The planar complex bearing an ethylene bridging chain can be reversibly oxidized to the corresponding copper(III) species (E⁰=+0.37 V).