Ligand structural effects on Cu2S2 bonding and reactivity in side-on disulfido-bridged dicopper complexes

To assess supporting ligand effects on S-S bond activation, a series of [Cu-2(mu-eta(2):eta(2)-S-2)](2+) complexes supported by various beta-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH3CN) with S-8. For the cases where L = beta-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)](4), where SR represents the ligand functionalized by sulfur at the central methine position. The [Cu-2(mu-eta(2):eta(2)-S-2)](2+) products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu-S, Cu-Cu, and S-S distances and the nu(S-S) values were observed and interpreted within the framework of a previously described bonding picture (Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 6394). Comparison of these data to those for other relevant species revealed a remarkable degree of S-S bond activation in the compounds supported by the beta-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S sigma* orbital and cause S-S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh3 and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh3 but only displacement of sulfur to yield a LCu(I)-CNAr (Ar = xylyl) complex with the isocyanide.