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

Abstract

Reaction of the tricopper(I)-dinitrogen tris(β-diketiminate) cyclophane, Cu₃(N₂)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu₃(μ₃-O)L (2) or the corresponding Cu₃(μ₃-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L_2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu₃(μ₃-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu₃(μ-E)]³⁺ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., Cu^III + O^2- vs Cu^II + O^-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu₃^II/II/I + E^2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.