X-ray Absorption Spectroscopic and Computational Investigation of a Possible S-S Interaction in the [Cu₃S₂]³⁺ Core

Sarangi, R.; Yang, L.; Winikoff, S. G.; Gagliardi, L.; Cramer, C. J.; Tolman, W. B.; Solomon, E. I.
J. Am. Chem. Soc. 2011, 133, 17180 (doi:10.1021/ja111323m).

The electronic structure of the [Cu₃S₂]³⁺ core of [(LCu)₃(S)₂]³⁺ (L = N,N,N´,N´-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu₃S₂]³⁺ core is best described as having all copper centers close to, but more oxidized than, Cu²⁺, while the charge on the S₂ fragment is between that of a sulfide (S^2-) and a subsulfide (S₂^3-) species. The [Cu₃S₂]³⁺ core thus is different from a previously described, analogous [Cu₃O₂]³⁺ core, which has a localized [(Cu³⁺Cu²⁺Cu²⁺)(O^2-)₂]³⁺ electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S-S σ* to the Cu and result in some bonding interaction between the two S atoms at ~2.69 Å in [Cu₃S₂]³⁺, stabilizing a delocalized S=1 ground state.