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.
[Cu2bbp2](ClO4)2·2MeOH, 3, crystallizes in the triclinic space group
P1̄ with the following unit cell parameters: 
a = 7.702(3) Å,
b = 10.973(6) Å, c = 12.396(6) Å,
α = 100.59(4)°, β = 99.02(4)°, γ =
98.90(4)°, V = 998.7(8) Å3, and
Z =
1.
[Cu2bbpen2](ClO4)2·3MeOH,
4, crystallizes in the orthorhombic space group
Pccn, with the following unit
cell parameters:  a = 17.478(9) Å, b =
18.795(8) Å, c = 13.888(6) Å, V =
4562.2(4) Å3, 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.