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

Abstract

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 (S2−) and a subsulfide (S23−) species. The [Cu3S2]3+ core thus is different from a previously described, analogous [Cu3O2]3+ core, which has a localized [(Cu3+Cu2+Cu2+)(O2−)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.

X‐ray Absorption Spectroscopic, Crystallographic, Theoretical (DFT) and Chemical Evidence for a Chalcogen–Chalcogen Two‐Center/Three‐Electron Half Bond in an Unprecedented “Subselenide” Se23− Ligand

“…More recently, an unusual S⋅⋅⋅S distance in a Cu 3 S 2 compound3 has led to an animated debate about whether the compound contains a singly‐bonded S 2 2− unit or two non‐bonded S 2− ligands 4. One of us proposed that the Cu 3 S 2 compound may contain a half‐bonded S 2 3− , or “subsulfide”, ligand,5 though to date experimental support for this assignment is lacking, despite extensive investigations 6. The possibility that dichalcogen E 2 n − species could be redox non‐innocent ligands suggested that a potentially large number of reported compounds may be misassigned or are poorly understood.…”

Chalcogen Impact on Covalency within Molecular [Cu3(μ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study

“…In light of ongoing electronic structure discussions in which descriptors such as oxo-copper(III) and oxylcopper(II) are frequently advanced, an alternate formalism would invoke ligand based oxidation. 4, 6 Indeed, more recent work – in particular, by Berry and coworkers 7 – provide strong evidence in support of an alternate electronic structure: 8 a bridging subsulfide or S 2 3- couples antiferromagnetically to three copper(II) cations. As alluded to above, copper-oxygen species containing formally high oxidation states (e.g., Cu III ), are invoked as reactive intermediates in biological chemistry and catalysis.…”

A Cu4S model for the nitrous oxide reductase active sites supported only by nitrogen ligands

“…Much of the available knowledge regarding copper sulphide clusters comes from studies of Cu 2 S 2 6 and Cu 3 S 2 7,8 model complexes, which feature bridging ligands with significant S-S interactions, 9 supported by nitrogen chelates. The latter category of complexes (Figure 1b), in particular, has been the subject of extensive experimental and computational characterization as well as fascinating literature discussions.…”

“…The latter category of complexes (Figure 1b), in particular, has been the subject of extensive experimental and computational characterization as well as fascinating literature discussions. 9–11 However, none of these complexes truly model the unusual μ 4 -S bridge of N 2 OR or provide insight into reduced catalytic intermediates. Phosphine 12,13 ligands have been used to stabilize “fully reduced” Cu 4 (μ 4 -S) and Cu 3 (μ 3 -S) clusters more structurally faithful to N 2 OR (Figure 1c), but the inability thus far of these systems to access open-shell oxidation states has precluded experimental determination of electron structure using typical methods of physical inorganic chemistry.…”