A NEW MODEL FOR DIOXYGEN BINDING IN HEMOCYANIN - SYNTHESIS, CHARACTERIZATION, AND MOLECULAR-STRUCTURE OF THE MU-ETA-2-ETA-2 PEROXO DINUCLEAR COPPER(II) COMPLEXES, [CU(HB(3,5-R2PZ)3)]2(O2) (R = ISO-PR AND PH)

The synthesis and characterization of mu-eta-2:eta-2 peroxo dinuclear copper(II) complexes which show many similarities to oxyhemocyanin (or oxytyrosinase) in their physicochemical properties are presented. The low-temperature reaction of a di-mu-hydroxo copper(II) complex [Cu(HB(3,5-i-Pr2pz)3)]2(OH)2 (8) with H2O2 gave a mu-peroxo complex [Cu(HB(3,5-i-Pr2pz)3)]2(O2) (6). Complex 6 was also prepared by dioxygen addition to a copper(I) complex Cu(HB(3,5-i-Pr2pz)3) (9). The preparation of an analogous peroxo complex [Cu(HB(3,5-Ph2pz)3)]2(O2) (7) was accomplished by the similar dioxygen treatment of a copper(I) acetone adduct Cu(Me2CO)(HB(3,5-Ph2pz)3) (10). The reaction of 6 with CO or PPh3 causes release of dioxygen, resulting in formation of the corresponding copper(I) adduct, Cu(CO)(HB(3,5-i-Pr2pz)3) (11) or Cu(PPh3)(HB(3,5-i-Pr2pz)3) (12). Crystallography was performed for 6-6(CH2Cl2), 8-1.5(CH2Cl2), and 11. Compound 6-6(CH2Cl2) crystallizes in the monoclinic space group C2/c with a = 26.36 (2) angstrom, b = 13.290 (4) angstrom, c = 29.29 (2) angstrom, beta = 114.59 (6)-degrees, V = 7915 (9) angstrom 3, and Z = 4. The refinement converged with the final R (R(w)) value, 0.101 (0.148), for 3003 reflections with F greater-than-or-equal-to 3-sigma(F(o)). Compound 8-1.5(CH2Cl2) crystallizes in the triclinic space group P1BAR with a = 16.466 (4) angstrom, b = 16.904 (5) angstrom, c = 14.077 (3) angstrom, alpha = 112.92 (2)-degrees, beta = 99.21 (2)-degrees, gamma = 90.76 (2)-degrees, V = 3550 (2) angstrom 3, Z = 2, and the final R (R(w)) factor, 0.083 (0.105), for 7226 reflections with F greater-than-or-equal-to 3-sigma(F(o)). Compound 11 crystallizes in the monoclinic space group, P2(1/a) with a = 16.595 (4) angstrom, b = 19.154 (4) angstrom, c = 10.359 (2) angstrom, beta = 106.65 (2)-degrees, V = 3155 (1) angstrom 3, Z = 4, and the final R (R(w)) value 0.083 (0.074) for 5356 reflections with F greater-than-or-equal-to 3-sigma(F(o)). The X-ray analysis of 6-6(CH2Cl2) definitely established the mu-eta-2:eta-2 coordination structure of the peroxide ion for the first time. This unusual side-on structure is entirely novel for a d-block element transition-metal-dioxygen complex. Both 6 and 7 show remarkable characteristics which are very similar to those known for oxyhemocyanin and oxytyrosinase. Complex 6: diamagnetic; nu(O-O), 741 cm-1; UV-vis, 349 nm (epsilon, 21 000), 551 nm (epsilon, 790); Cu-Cu, 3.56 angstrom. Complex 7: diamagnetic, nu(O-O), 759 cm-1; UV-vis, 355 nm (epsilon, 18 000), 542 nm (epsilon, 1040). These properties are all consistent with those of an analogous complex, [Cu(HB(3,5-Me2pz)3)]2(O2) (5), of which the characterization and reactivities were reported already (Kitajima, N., et al. J. Am. Chem. Soc. 1990, 112, 6402; 1991, 113, 5664). The magnetic and spectroscopic features of 5-7 and their biological relevance are discussed in detail. Furthermore, simple interpretation of the electronic state of the N3Cu(O22-)CuN3 chromophore is provided based on extended Huckel MO calculations. The close resemblance between the properties of mu-eta-2:eta-2 peroxo complexes 5-7 and oxyhemocyanin led us to propose a new model for dioxygen binding in hemocyanin; dioxygen is simply bound between the two copper ions in the mu-eta-2:eta-2 mode. With this structural model; the existence of an endogenous bridging ligand, which has been generally supposed to account for the diamagnetism of oxyhemocyanin, is no longer necessary.