Mechanistic, Structural, and Spectroscopic Studies on the Catecholase Activity of a Dinuclear Copper Complex by Dioxygen

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Abstract

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 their reactivity as models for tyrosinase has been investigated in comparison with that of previously reported dinuclear complexes containing similar aminobis(benzimidazole) donor groups. The complex [Cu2(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) complex [Cu2(EBA)(OH)2]2+ by addition of base (pKa1 = 7.77, pKa2 = 9.01). The latter complex can also be obtained by air oxidation of [Cu2(EBA)]2+ in methanol. The X-ray structural characterization of [Cu2(EBA)(OH)2]2+ shows that a double μ-hydroxo bridge is established between the two 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 (∼2.5 Å) with the tertiary amine donor. The small size and the quality of the single crystal as well as the fair loss of crystallinity during data collection required the use of synchrotron radiation at 100 K. [Cu2(EBA)(OH)2][PF6]2: orthorhombic Pca21 space group, a = 22.458(2) Å, b = 10.728(1) Å, c = 19.843(2) Å, R = 0.089. Besides OH-, the [Cu2(EBA)(H2O)4]4+ complex binds azide as a bridging ligand, with the μ-1,3 mode. Azide can also displace μ-OH in [Cu2(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 residue present in the previously reported dicopper(II) complexes. Electrochemical experiments show that [Cu2(EBA)(H2O)4]4+ undergoes a single, partially chemically reversible, two-electron reduction to the corresponding dicopper(I) congener at positive potential values (E0' = 0.22 V, vs SCE). Interestingly, however, coordination to azide ion makes the reduction process proceed through two separated one-electron steps. The catalytic activity of [Cu2(EBA)(H2O)4]4+ in the oxidation of 3,5-di-tert-butylcatechol has been examined in methanol/aqueous buffer, pH 5.1. The mechanism of the catalytic cycle parallels that of tyrosinase, where no hydrogen peroxide is released and dioxygen is reduced to water. Low-temperature (−80 °C) spectroscopic experiments show that oxygenation of the reduced complex [Cu2(EBA)]2+ does not produce a stable dioxygen adduct and leads to a μ-oxodicopper(II) species in a fast reaction.

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