A series of side-on peroxide-bridged binuclear copper complexes [Cu 2 (N n PY2)(O 2 )] 2+ (where n = 3−5), which have been proposed to adopt a butterfly Cu 2 O 2 geometry due to the constraints placed on the Cu−Cu distance by the alkyl chain linker of length −(CH 2 ) 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 [Cu 2 (N n PY2)(O 2 )] 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 nm) associated with the butterfly side-on peroxide-bridged dicopper core is important since the isomeric, bis(μ-oxo) core is also characterized by an absorption band in this region. The changes in bonding associated with a butterfly distortion of the Cu 2 O 2 core are defined, and the reactivity of the butterfly core with respect to electrophilic aromatic substitution and H atom abstraction reactions is compared with that of the planar side-on peroxide-bridged dicopper core using the frontier molecular orbital description.
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Pidcock et al. (1999) studied this question.