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 [Cu₂(μ-η²:η²-S₂)] ²⁺ complexes supported by various β-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH₃CN) with S₈. For the cases where L = β-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 [CU₂(μ-η²: η²-S₂)]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 ν(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 β-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S σ* 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.