X-ray Absorption Spectroscopic and Computational Investigation of a Possible S···S Interaction in the [Cu_3S_2]~(3+)Core

摘要

The electronic structure of the [Cu_3S_2]~(3+) core of [(LCu)_3(S)_2]~(3+) (L = N,N,N',N'-tetramethyl-2R,3R-cy-clohexanediamine) 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 σ* to the Cu and result in some bonding interaction between the two S atoms at ～2.69 A in [Cu_3S_2]~(3+), stabilizing a delocalized S = 1 ground state.