SPECTROSCOPIC AND THEORETICAL-STUDIES OF AN END-ON PEROXIDE-BRIDGED COUPLED BINUCLEAR COPPER(II) MODEL COMPLEX OF RELEVANCE TO THE ACTIVE-SITES IN HEMOCYANIN AND TYROSINASE

Spectroscopic studies have been combined with broken-symmetry SCF-X-alpha-SW calculations to determine the vibrational and electronic structure of peroxide-copper bonding in the end-on trans-mu-1,2 peroxide-bridged copper dimer, [{Cu(TMPA)}2(O2)]2+. This study provides a detailed understanding of the electronic structure generally associated with an end-on peroxide-bridging mode and additionally shows important dimer interactions which are not present in the copper-peroxide monomer. The X-alpha calculations show that the electronic structure is dominated by the interaction of the peroxide-tau* sigma-orbital with the half-occupied copper d(z)2 orbitals. This copper-peroxide bonding is probed experimentally by a peroxide-to-copper charge-transfer spectrum which contains three transitions at 615 (epsilon = 5800 M-1 cm-1), 524 (epsilon = 11 300 M-1 cm-1), and 435 nm (epsilon = 1700 M-1 cm-1) assigned as the electric dipole allowed singlet transitions from tau*v, and tau*sigma (the HOMO orbitals of peroxide) and the spin-forbidden triplet from tau*v, respectively. The differences between the observed and calculated transition energies, which are present in the dimer but not the monomer, are shown to arise from excited-state splittings which derive from excitation transfer in the dimer and from excited-state exchange interactions much larger than those in the dimer ground state [-2J(es)(tau*v) almost-equal-to 7600 cm-1]. The resonance Raman spectra show two enhanced vibrations which shift upon isotopic substitution of O-16(2) by O-18(2) at 832 (788 cm-1 with O-18(2)) and 561 cm-1 (535 cm-1 with O-18(2)). They are assigned as the intraperoxide stretch and the symmetric copper-oxygen stretch, respectively. A normal-coordinate analysis has been performed using these vibrational data to compare this trans-mu-1,2 dimer to a previously studied monomer. This analysis shows that the O-O bond force constant, k(o-o), increases upon going from the monomer to the dimer due to increased donation of electron density from the antibonding tau*sigma-orbital to the second copper in the dimer. The resonance enhancement of the intraperoxide stretch is similar to that of the monomer; however, the enhancement behavior of the copper-oxygen stretch is significantly different from that in the monomer, showing enhancement only from the tau*v transitions. This enhancement behavior is discussed with respect to excited-state distortions of the dimer which can be very different from the monomer depending on the extent of delocalization (i.e. electron coupling) of the charge-transfer excitation over the two halves of the dimer.