EFFECT OF PROTONATION ON PEROXO-COPPER BONDING - SPECTROSCOPIC AND ELECTRONIC-STRUCTURE STUDY OF [CU-2((UN-O-)(OOH)](2+)

Spectroscopic studies of a mu-1,1-hydroperoxo-bridged copper dimer are combined with SCF-X alpha-SW molecular orbital calculations to descri be the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper spe cies. Four vibrational modes of the Cu2OOB unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: v(O -O) = 892 cm(-1) v(as)(Cu-O) = 506 cm(-1), v(s)(Cu-O) = 322 cm(-1), an d v(O-H) = 3495 cm(-1). The 892 cm(-1) O-O stretch of the mu-1,1-hydro peroxo-bridged copper dimer is 89 cm(-1) higher than that of the unpro tonated complex. Resonance Raman profiles of the 892 cm(-1) O-O stretc h are used to assign an electronic absorption band at 25200 cm(-1) (ep silon = 6700 M-1 cm(-1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is similar to 5000 cm(-1) higher in energy than the corresponding transition in the unprotonated complex. The pi -to-Cu CT transition intensity defines the degree of hydroperoxide-to- copper charge donation, which is lower than in the unprotanated comple x due to the increased electronegativity of the peroxide with protonat ion. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to -Cu charge donation but rather reflects the direct effect of protonati on on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonati on of the peroxo-copper complex and to relate these changes to changes in reactivity.