SPECTROSCOPIC STUDIES OF SIDE-ON PEROXIDE-BRIDGED BINUCLEAR COPPER(II) MODEL COMPLEXES OF RELEVANCE TO OXYHEMOCYANIN AND OXYTYROSINASE

Spectrosopic studies on two side-on mu-eta2:eta2 peroxide-bridged cupric dimers, [Cu(HB(3,5-R2pz)3)]2(O2), where (HB(3,5-R2pz)3) is a tris(pyrazolyl)borate ligand and R = i-Pr or Ph, are presented and compared to our previous studies of end-on bound(eta1) peroxide-coppper monomer and bridged dimer (trans-mu-1,2) complexes. Two transitions at 350 nm (epsilon = 26 000 M-1 cm-1) and 538 nm (epsilon = 2000 M-1 cm-1) are assigned as peroxide-to-copper charge-transfer transitions. A method has been developed to quantitate electron density donation from the ligand to the metal center using the intensity of the ligand-to-metal charge-transfer transitions. It is found that the side-on bridging peroxide donates significantly more electron density from the peroxide pi* orbitals to the coppers than does,peroxide in end-on bound monomer and dimer complexes. The amount of charge donation is proportional to the number of sigma bonds between the peroxide and the coppers (side-on bridged dimer = 4, end-on bridged dimer = 2, end-on monomer = 1). The increased charge donation results in a less negative peroxide in the side-on complex and should in principle produce a stronger O-O bond. Four vibrational modes are assigned in the resonance Raman and infrared spectra on the basis of isotopic shifts at 763 (rR, 723 cm-1 with O-18(2)), 331 (IR, 321 cm-1 with O-18(2)), 284 (rR, no shift with O-18(2)), and 572 cm-1 (based on an overtone at 1144 cm-1 in rR, 1098 cm-1 with O-18(2)) in the R = Ph complex, with corresponding peaks at 749, 285, and 1055 cm-1 in the resonance Raman spectra of the R = i-Pr complex. A normal coordinate analysis shows that the oxygen-oxygen force constant, k(O-O), is smaller in the side-on bridged complex (2.4 mdyn/angstrom) than in the end-on monomer (2.9 mdyn/angstrom) or end-on bridged dimer (3.1 mdyn/angstrom), and thus the O-O bond is weaker in the side-on complex than in the end-on, despite its greater peroxide charge donation. This is direct evidence for the pi acceptor ability of the peroxide in this side-on bridged structure, which involves some peroxide sigma* character mixing into the highest energy occupied molecular orbital, as predicted by Xalpha calculations [Ross, P. K.; Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 3246-3259]. Using the correlation of charge-transfer intensities to ligand charge donation, the charge-transfer intensity of oxyhemocyanin indicates that it likely has four copper-peroxide bonds and thus a side-on peroxide bridging structure. The electronic structure of this side-on bridging geometry in oxyhemocyanin explains its unique spectroscopic features, including the high intensity and energy of the 345-nm absorption band, the low O-O stretching frequency, and the lack of a symmetric Cu-O stretch in the expected energy range of the resonance Raman spectrum. This electronic structure also provides insight into the mechanisms of oxygen binding and activation in hemocyanin and tyrosine.