μ-Imidazole-bridged dicopper(II) macrocyclic complex:: synthesis, structure, stability and Tyr-like activity

Synthesis, structure and solution chemistry of dicopper(II) macrocyclic complexes which mimic the Tyr-enzyme have been investigated. These complexes are formulated as [BDBPHCu2(II)Br]Br (1), [BDBPHCu2(II)(mu-Im)]Br-2 (2) and [BDBPHCu2(II)(mu-mIm)]Br-2 (3). (BDBPH = 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyl-tricyclo[23,3,1,1(11,15)]- triacontal-(28),11,13,15(30),25,26,-hexaene; Im denotes imidazole and mIm denotes 2-methyl-imidazole, respectively). Complex I crystallizes in the monoclinic space group P2(1)/n, with cell constants a = 13.917(8) Angstrom, b = 15.832(10) Angstrom, c = 15.390(9) Angstrom, alpha = 90degrees(10), beta = 110.466(12)degrees, gamma = 90degrees, and Z = 4. Elemental analysis, spectra and potentiometric titration indicate that complex 2 and 3 have cyclic structures with mu-Im or mIm as the bridging ligands. Potentiometric titration indicates that mIm bridged dicopper(II) species are dominant in solution over the range of pH 8-12. Binuclear species is the main species in complex 2 and it is the active complex for oxidation of catechol in methanol solution. The observed steady-state for complex 2 shows that the reaction rate is first order in ternary complex 2 and zero order in catechol. UV-Vis spectral and ESR studies support a mechanism that involves Cu(I)-O-2-Cu(I) as the intermediate. Based on these results, the function of the bridged imidazole ligand in the catalytic reaction is illustrated and a novel reaction mechanism is proposed. (C) 2002 Published by Elsevier Science B.V.