Structural and functional models for the dinuclear copper active site in catechol oxidases: Syntheses, X-ray crystal structures, magnetic and spectral properties, and X-ray absorption spectroscopic studies in solid state and in solution

Two novel tridentate dinucleating ligands containing benzimidazole were prepared, 1,3-bis(2-benzimidazolyl)-2-propanol (Hbbp, 1) and 1,5-bis(2-benzimidazolyl)-3-pentanol (Hbbpen, 2). Their complexing properties toward copper were studied in order to obtain structural and functional models for catechol oxidases. Syntheses and crystal structures of dinuclear Cu(II) complexes derived from these ligands are reported. [Cu(2)bbp(2)](ClO4)(2) . 2MeOH, 3, crystallizes in the triclinic space group <P(1)over bar> with the following unit cell parameters: a = 7.702(3) Angstrom, b = 10.973(6) Angstrom, c = 12.396(6) Angstrom, alpha = 100.59(4)degrees, beta = 99.02(4)degrees, gamma = 98.90(4)degrees, V = 998.7(8) Angstrom(3), and Z = 1. [Cu(2)bbpen(2)](ClO4)2 . 3MeOH, 4, crystallizes in the orthorhombic space group Peen, with the following unit cell parameters: a = 17.478(9) Angstrom, b = 18.795(8) Angstrom, c = 13.888(6) Angstrom, V = 4562.2(4) Angstrom(3), and Z = 4. Magnetic susceptibility measurements in the temperature ranges 4.6-459 K (3) and 4.6-425 K (4) indicate an antiferromagnetic coupling between the Cu(II) centers of both complexes. In order to determine the structures of the complexes in solution, XAS spectra (EXAFS and XANES) were recorded in the solid state and in solution. The interpretation of these data, including multiple scattering calculations, together with UV-vis titrations, shows that the complexes have the same structure in the crystalline state as well as in methanolic solution. Complex 4 is able to oxidize 3,5-di-tert-butylcatechol (3,5-DTBC) to the quinone (catecholase activity). This reaction was also studied by XAS and UV-vis spectroscopy. These measurements reveal the reduction of Cu(II) to Cu(I) accompanied by a decrease of the coordination number.