Spectroscopic and Electronic Structural Studies of the Cu(III)₂Bis-μ-oxo Core and Its Relation to the Side-On Peroxo-Bridged Dimer

Recently a Cu(III)2 bis-μ-oxo dimer ligated by peralkylated amines has been found to interconvert with the side-on peroxo-bridged, μ-η2:η2 isomer. The Cu(III)2(μ-O)2 dimer exhibits two intense charge transfer (CT) features in the near-UV region of the electronic absorption spectrum. Laser excitation into the lower-energy CT absorption band at 25 000 cm-1 results in intense resonance enhancement of the Raman peaks at 609 and 118 cm-1 which profile this band and give overtone and combination progressions. The combined application of a normal coordinate analysis of the Raman features and a time-dependent Heller theory analysis of the electronic absorption spectrum and resonance Raman profiles provide the excited-state geometry. As this transition corresponds to an oxo-to-Cu(III) CT, this excited state is formally an oxyl−Cu(II) species. Density functional calculations correlated to these data (including the excited-state geometry and the relative CT intensities) allow for an unambiguous assignment of the observed charge-transfer transitions. This assignment shows that one of the CT features involves the same orbital origin as a corresponding transition in the side-on peroxo dimer, while the new, low-energy band (∼25 000 cm-1) only observed for the bis-μ-oxo species corresponds to an oxo σu* to Cu(III) dxy CT transition which is present when the O−O bond is cleaved. This study provides electronic structural insight into the relationship between the bis-μ-oxo and side-on peroxo-bridged Cu species and their relative reactivities.