Spectroscopic and Theoretical Studies of Mononuclear Copper(II) Alkyl- and Hydroperoxo Complexes: Electronic Structure Contributions to Reactivity

Spectroscopic studies combined with calculations are used to describe the electronic structure and vibrational properties of mononuclear four-coordinate end-on alkylperoxo and hydroperoxo Cu(II) complexes. EPR defines a Cu x2−y2 ground state with ∼62% Cu character. From absorption, MCD, and resonance Raman spectroscopies, the main bonding interaction between the alkyl(hydro)peroxide and Cu(II) is found to involve the π-donation of the alkyl(hydro)peroxide π*v into the Cu x2−y2 orbital, which dominates the observed spectroscopic features, producing an intense absorption band at ∼16 600 cm-1 (∼600 nm). On the basis of the vibrational frequencies, isotope shifts, and normal coordinate analyses, the dominant vibrations of the alkyl(hydro)peroxo complexes are assigned and the Cu−O and O−O force constants are determined. The observed strong Cu−O bond and the large alkyl(hydro)peroxide-to-Cu(II) charge donation are ascribed to the low coordination number of Cu and the distorted Td ligand field. The observed stro...