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 STAR) OR DIAMINES (DONOR SET N2S2)

A series of tetradentate ligands derived from the condensation of (1R)-3-hydroxymethylenebornane-2-thione and several 2-aminothio-alkylbenzimidazoles were synthesized together with their cationic Zn(II) and Cu(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 Cu(II) complexes exhibit medium-intensity absorptions between 410 and 435 and between 520 and 535 nm attributable to enethiolate σ(S-) → CuII and π(S-) → CuII ligand-to-metal charge transfer (LMCT) transitions, resp., and relatively intense ligand-field transitions at 610-650 nm. All the bands undergo progressive red shifts as the length of the alkyl chain connecting the imine and thioether functions increases from 2-4 atoms. The frozen-soln. EPR spectra are indicative of progressive distortion towards a pseudo-tetrahedral geometry for Cu(II) as the Cu chain length increases. A series of neutral Cu(II) complexes derived from the condensation of (1R)-3-hydroxymethylenebornane-2-thione and sym. diamines was also synthesized for comparison. Here too the length of the alkyl chain connecting the imine N atoms was varied from 2-4 atoms. These complexes exhibit the same spectroscopic trends but with less marked distortion of the Cu(II) chromophores as the length of the C chain increases. The cationic Cu(II) complexes from (1R)-3-hydroxymethylenebornane-2-thione and 2-aminothia-alkylbenzimidazoles display, in MeCN soln. at -15°, easy access to the corresponding Cu(I) congeners (E0 = -0.02 V vs. SCE). By contrast, the neutral Cu(II) species derived from (R)-3-hydroxymethylenebornane-2-thione and diamines are reduced to the corresponding Cu(I) complexes at potentials more neg. by 0.7-1.0 V, depending upon the extent of stereochem. distortion around the Cu(II) center induced by the length of the imine-N-linking C chain. The planar complex bearing an ethylene bridging chain can be reversibly oxidized to the corresponding Cu(III) species (E0 = +0.37 V).