Raman spectroelectrochemical studies and crystal structure of a binuclear copper(I) complex with a bridging diimine ligand

Abstract

Raman spectroelectrochemical and X-ray crystallographic studies have been made for the binuclear copper(I) complex, [(Ph₃P)₂Cu(dpq)Cu(PPh₃)₂][BF₄]₂, where dpq is the bridging ligand 2,3-di(2-pyridyl)quinoxaline. The X-ray data show that the pyridine rings are twisted out of plane with respect to the quinoxaline ring which is itself non-planar. The UV/VIS spectra of the metal-to-ligand charge-transfer excited state and those of the electrochemically reduced complex are similar. The resonance-Raman spectrum of the latter species exhibits little change in the frequency of the pyridinylquinoxaline inter-ring C–C bond stretching mode, compared to the ground elecronic state. This suggests minimum change in the inter-ring C–C bond order in the electrochemically or charge-transfer generated radical anion. Semiempirical molecular-orbital calculations on both the neutral dpq and radical anion show two near-degenerate lowest unoccupied orbitals in the neutral species. One is strongly bonding across the inter-ring C–C bond while the other is almost non-bonding. The Raman data suggest that it is this latter orbital which is populated in the transient and electrochemical experiments.