Spectroscopic and theoretical studies of oxygenated dicopper(I) complexes containing hydrocarbon-linked bis[2-(2-pyridyl)ethyl]amine units: Investigation of a butterfly [Cu-2(mu-eta(2):eta(2))(O-2)](2+) core

A series of side-on peroxide-bridged binuclear copper complexes [CU2(NnPY2) (O-2)](2+) (where n = 3-5), which have been proposed to adopt a butterfly C u2O2 geometry due to the constraints placed on the Cu-Cu distance by the al kyl chain linker of length -(CH2)(n)- have been studied using absorption an d resonance Raman spectroscopy and theoretical techniques. The four compone nts of the peroxide to copper(II) charge-transfer transitions have been ide ntified for the first time in the [Cu-2(NnPY2)(O-2)](2+) (where n = 3-5) co mplexes. The observed shift of the peroxide O-O stretch by 25 cm(-1) to hig her energy and the changes observed in the energy and intensity of absorpti on 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 identificat ion of an absorption band at 23 800-20 400 cm(-1) (420-490 nm) associated w ith the butterfly side-on peroxide-bridged dicopper core is important since the isomeric, bis(mu-oxo) core is also characterized by an absorption band in this region. The changes in bonding associated with a butterfly distort ion of the Cu2O2 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 dic opper core using the frontier molecular orbital description.