X-Ray absorption spectroscopy quantitative analysis of biomimetic copper(ii) complexes with tridentate nitrogen ligands mimicking the tris(imidazole) array of protein centres

Abstract

In this study copper(II) complexes with the tridentate nitrogen ligand bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB) are considered as model compounds for the Cu-tris(imidazole) array found in several copper proteins. 2-BB chelates copper(II) forming two six-membered rings and the complexes contain methanol, nitrite, azide and water as ancillary ligands; both the coordination numbers and stereochemistries differ in these complexes. Their key structural features were investigated by using full multiple-scattering theoretical analysis of the copper K-edge X-ray absorption spectrum with the MXAN code. We showed that using cluster sizes large enough to include all atoms of the ligand, the analysis of the XANES region can give both a structural model of the metal centre and map the structure of the 2-BB complexes. Complex [Cu(2-BB)(N₃)]⁺ provided a critical test through the comparison of the XANES simulation results with crystallographic data, thus permitting the extension of the method to the complex [Cu(2-BB)(H₂O)_n]⁺ (n = 1 or 2), for which crystallographic data are not available but is expected to bear a five-coordinated Cu(3N)(2O) core (n = 2). The structural data of [Cu(2-BB)(MeOH)(ClO₄)]⁺ and [Cu(2-BB)(NO₂)]⁺, both with a Cu(3N)(2O) core but with a different stereochemistry, were used as the starting parameters for two independent simulations of the XANES region of the [Cu(2-BB)(H₂O)₂]⁺ cation. The two structural models generated by simulation converge towards a structure for the aqua-cation with a lower coordination number. New calculations, where four-coordinated Cu(3N)(O) cores were considered as the starting structures, validated that the structure of the aqua-complex in the powder state has a copper(II) centre with a four-coordinated Cu(3N)(O) core and a molecular formula [Cu(2-BB)(H₂O)](ClO₄)·(H₂O). A water solvation molecule, presumed to be disordered from the simulations with the two Cu(3N)(2O) cores, is present. The successful treatment of this Cu-2-BB complex system allows the extension of the method to other biomimetic compounds when a structural characterization is lacking.