Spectroscopic and Theoretical Studies of Oxygenated Dicopper(I) Complexes Containing Hydrocarbon-Linked Bis[2-(2-pyridyl)ethyl]amine Units: Investigation of a Butterfly [Cu2(μ-η2:η2)(O2)]2+Core

A series of side-on peroxide-bridged binuclear copper complexes [Cu2(NnPY2)(O2)]2+ (where n = 3−5), which have been proposed to adopt a butterfly Cu2O2 geometry due to the constraints placed on the Cu−Cu distance by the alkyl chain linker of length −(CH2)n−, have been studied using absorption and resonance Raman spectroscopy and theoretical techniques. The four components of the peroxide to copper(II) charge-transfer transitions have been identified for the first time in the [Cu2(NnPY2)(O2)]2+ (where n = 3−5) complexes. The observed shift of the peroxide O−O stretch by 25 cm-1 to higher energy and the changes observed in the energy and intensity of absorption bands including the presence of an additional band at 23 800−20 400 cm-1 (420−490 nm) (not seen in planar side-on peroxide-bridged dicopper cores) are correlated to the butterfly structure using transition dipole vector coupling and valence bond configuration interaction models. The identification of an absorption band at 23 800−20 400 cm-1 (420−490 ...