Mechanistic, structural, and spectroscopic studies on the catecholase activity of a dinuclear copper complex by dioxygen

Dinuclear copper(II) complexes with the new ligand 1,6-bis[[bis(1-methyl-2- benzimidazolyl)methyl]amino]-n-hexane (EBA) have been synthesized, and thei r reactivity as models for tyrosinase has been investigated in comparison w ith that of previously reported dinuclear complexes containing similar amin obis(benzimidazole) donor groups. The complex [Cu-2(EBA)(H2O)(4)](4+), five -coordinated SPY, with three nitrogen donors from the ligand and two water molecules per copper, can be reversibly converted into the bis(hydroxo) com plex [Cu-2-(EBA)(OH)(2)](2+) by addition of base (pK(a1) = 7.77, pK(a2) = 9 .01). The latter complex can also be obtained by air oxidation of [Cu-2(EBA )](2+) in methanol. The X-ray structural characterization of [Cu-2(EBA)(OH) (2)](2+) shows that a double mu-hydroxo bridge is established between the t wo Cu(II) centers in this complex. The coordination geometry of the coppers is distorted square planar, with two benzimidazole donors and two hydroxo groups in the equatorial plane, and an additional, lengthened and severely distorted axial interaction (similar to 2.5 Angstrom) with the tertiary ami ne donor. The small size and the quality of the single crystal as well as t he fair loss of crystallinity during data collection required the use of sy nchrotron radiation at 100 K. [Cu-2(EBA)(OH)(2)][PF6](2): orthorhombic Pca2 (1) space group, a = 22.458(2) Angstrom, b = 10.728(1) Angstrom, c = 19.843 (2) Angstrom, R = 0.089. Besides OH-, the [Cu-2(EBA)(H2O)(4)](4+) complex b inds azide as a bridging ligand, with the mu-1,3 mode. Azide can also displ ace mu-OH in [Cu-2(EBA)-(OH)(2)](2+) as a bridging ligand. In general, the binding constants indicate that the long alkyl chain of EBA is less easily folded in the structures containing bridging ligands than the m-xylyl resid ue present in the previously reported dicopper(II) complexes. Electrochemic al experiments show that [Cu-2(EBA)(H2O)(4)](4+) undergoes a single, partia lly chemically reversible, two-electron reduction to the corresponding dico pper(I) congener at positive potential values (E-0' = 0.22 V, vs SCE). Inte restingly, however, coordination to azide ion makes the reduction process p roceed through two separated one-electron steps. The catalytic activity of [Cu-2(EBA)(H2O)(4)](4+) in the oxidation of 3,5-di-tert-butylcatechol has b een examined in methanol/aqueous buffer, pH 5.1. The mechanism of the catal ytic cycle parallels that of tyrosinase, where no hydrogen peroxide is rele ased and dioxygen is reduced to water. Low-temperature (-80 degrees C) spec troscopic experiments show that oxygenation of the reduced complex [Cu-2(EB A)](2+) does not produce a stable dioxygen adduct and leads to a mu-oxodico pper(II) species in a fast reaction.