INTERACTION OF CHROMIUM(II) COMPLEXES WITH MOLECULAR-OXYGEN - SPECTROSCOPIC AND KINETIC EVIDENCE FOR ETA(1)-SUPEROXO COMPLEX-FORMATION

Two chromium adducts with molecular oxygen, (H2O)(5)CrO22+ and (H2O)(cyclam)CrO22+ (cyclam = [14]-aneN(4) = 1,4,8,11 -tetraazacyclotetradecane), have been investigated by UV resonance Raman and transient absorption spectroscopic methods. The Raman bands at 1166 and 503 cm(-1) for (H2O)(5)CrO22+ are the first such data for an aqua-transition-metal-superoxo complex. These bands shift to 1098 and 491 cm(-1) upon O-18(2) substitution and are assigned to the nu(O-O) and nu(Cr-OO) [stretching] vibrations, respectively. The corresponding vibrations in (H2O)(cyclam)CrO22+ were observed at lower frequencies, 1134/1145 (doublet) and 489 cm(-1). Both complexes are photolabile upon UV irradiation, yielding the respective Cr(II) complexes and O-2. The quantum yield for photohomolysis at 290 nm (LMCT) is greater than that at 245 nm (superoxide-based pi-pi(*) transition). These vibrational and transient absorption data are consistent with electronic and kinetic behavior of these complexes and support assignment of both species as eta(1) (end-on) superoxo complexes of Cr(III). Effects of the macrocyclic cyclam ligand on Cr-O-2 and O-O bonding are discussed. The O-O bond dissociation energy in (H2O)(5)CrO22+ is comparable to that in O-2(.-) (393 kJ/mol).