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

Abstract

Synthesis, structure and solution chemistry of dicopper(II) macrocyclic complexes which mimic the Tyr-enzyme have been investigated. These complexes are formulated as [BDBPHCu 2(II)Br]Br ( 1), [BDBPHCu 2(II)(μ-Im)]Br 2 ( 2) and [BDBPHCu 2(II)(μ-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 1 crystallizes in the monoclinic space group P2(1)/ n, with cell constants a=13.917(8) Å, b=15.832(10) Å, c=15.390(9) Å, α=90°(10), β=110.466(12)°, γ=90°, and Z=4. Elemental analysis, spectra and potentiometric titration indicate that complex 2 and 3 have cyclic structures with μ-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.