XAFS study of aqua (diethylenetriamine)(isonicotinato)-copper(II) complex - inference of square-pyramidal geometry

Abstract

X‐ray absorption fine structure (XAFS) of aqua (diethylenetriamine) (isonicotinato) copper(II) complex has been investigated. The crystal structure of the complex is unavailable. The X‐ray absorption near edge structure (XANES) spectrum of the complex has been compared with those of the standard compounds to estimate the coordination geometry and oxidation state of copper. This study indicates that the copper center has +2 oxidation state and that it may be penta or hexa‐coordinated. The spectrum has been further compared with those of the earlier studied complexes having square‐pyramidal geometry. Similarity has been observed between them indicating that the complex should have penta‐coordinated square‐pyramidal geometry. The same inference has been obtained from the study of the characteristic pre‐edge and XANES features. This inference has been further examined by analysis of the extended X‐ray absorption fine structure (EXAFS) spectrum. The experimental EXAFS data has been fitted with three different theoretical models for tetra‐coordination, penta‐coordination, and hexa‐coordination. The tetra model is unable to provide a satisfactory fit so it has been ignored. Although the penta and hexa models give comparable results, the parameters obtained indicate that the penta‐coordinated model fits better to the experimental data than the hexa‐coordinated model. This penta‐coordinated model has been further fitted to the experimental data by allowing the coordination number to vary. Such fitting also shows that the penta‐coordinated model is a satisfactory model for the studied complex. The obtained values of mean‐square disorder indicate that the force constants are different for the equatorial and axial bonds. Copyright © 2014 John Wiley & Sons, Ltd.

Investigating local coordination and electronic nature of Cu centers in the copper complexes having aspartic and glutamic acids

“…The feature K 1 at the absorption edge is originated by the distortion in the metal cluster, and its magnitude and position depend on the degree of the distortion. The intensity of the peak K 1 in the two complexes is nearly the same, indicating distortion in both the complexes 24 . The peak K 2 reflects the difference in the axial geometry of the Cu clusters.…”

“…The axial bond length (please see section 3.1) in 1 is 2.25 Å and in 2 , the average of the two axial bond lengths is 2.42 Å. Thus, the difference in the separation of two peaks in the two complexes is also reflected in the differences in their axial bond lengths 24 …”

“…The intensity of the peak K 1 in the two complexes is nearly the same, indicating distortion in both the complexes. 24 The peak K 2 reflects the difference in the axial geometry of the Cu clusters. The energy gap values between the peaks K 1 and K 2 should give an estimate for the destabilization of the 4p z metal orbital (z being the elongation axis).…”

“…Thus, the difference in the separation of two peaks in the two complexes is also reflected in the differences in their axial bond lengths. 24…”

“…The reason for the broadening of the white line at the Cu K-edge in the compounds having +2 oxidation state has been assigned to the transitions from Cu 1 s orbital to mainly 4p molecular orbitals. 24 The simulated XANES spectra and p-DOS for the Cu centre and the experimental XANES spectra are both given in Figure 8a,b and can be compared easily with each other. In the experimental spectra, the white line is found not as a single peak but as a broadened structure between the two arrows shown in the figure.…”