X-ray absorption spectroscopic and computational investigation of a possible S⋯S interaction in the [Cu ₃S ₂] ³⁺ core

The electronic structure of the [Cu ₃S ₂] 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 [Cu ₃S ₂] ³⁺ core is best described as having all copper centers close to but more oxidized than Cu ²⁺, while the charge on the S ₂ fragment is between that of a sulfide (S ^2-) and a subsulfide (S ₂ ^3-) species. The [Cu ₃S ₂] ³⁺ core thus is different from a previously described, analogous [Cu ₃O ₂] ³⁺ core, which has a localized [(Cu ³⁺Cu ²⁺Cu ²⁺)(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 θ* to the Cu and result in some bonding interaction between the two S atoms at ∼2.69 Å in [Cu ₃S ₂] ³⁺, stabilizing a delocalized S = 1 ground state.