X-ray Absorption Study of Copper(II)−Glycinate Complexes in Aqueous Solution

Abstract

The structures of the mono-, bis-, and tris(glycinato)copper(II) complexes in aqueous solution have been determined by X-ray absorption spectroscopy. Four solutions with different complex ratios have been examined, and the species concentrations have been determined on the basis of complex stability constants. An advanced data analysis including multiple-scattering effects and multielectron excitation processes produced quantitative information on the Cu(II)-glycinate complexes present in aqueous solution. The structure of the bis(glycinato)copper(II) complex, which has a low solubility in water, has been determined for the first time. It has been found to have a distorted octahedral geometry with two bidentate glycine ligands coordinating to the Cu 2+ ion in the equatorial plane and with the axial sites occupied by two additional water molecules at 2.40 ( 0.06 Å. Analysis of the X-ray absorption data allowed a detailed description of the structures of the mono-and tris(glycinato)copper(II) species. The former complex has an axially elongated octahedral structure with a glycine bidentate ligand and two water molecules placed at the equatorial and two water molecules at the axial positions (2.44 ( 0.08 Å). Experimental data for the latter complex were also explained in terms of a distorted octahedral model with two glycine molecules in the equatorial plane and with the amino nitrogen of the third glycine coordinating to the Cu 2+ ion at the axial site at a distance of 2.33 ( 0.05 Å. This result conflicts with the regular octahedral geometry previously determined by XRD investigations. The axial bonds of the three complexes are significatively longer than the average within the hexaaquocopper-(II) ions. This finding indicates that the axial bonds are lengthened upon the formation of the Cu-glycine complexes and the Cu(II)-water interaction at the axial site is weakened.

Ab initiox-ray absorption study of copperK-edge XANES spectra in Cu(II) compounds

“…It has to be stressed that the splitting of the main resonance in peaks B and C is a common feature of several Cu͑II͒ compounds. 23,25,26,[29][30][31][32]34 Then the following tests have been performed to improve the calculation: ͑i͒ more realistic self-consistency field ͑SCF͒ potentials and ͑ii͒ the incorporation of many-body processes to compute the absorption spectrum.…”

“…1, are in agreement with previously published data. [29][30][31][32] Both FEFF8 9 and CONTINUUM 4 codes were used to calculate the Cu K-edge XANES spectra. No significant differences were found for the standard single-channel MS computations obtained by using the two different codes.…”

The nature of Cu bonding to natural organic matter

“…5e). However, detecting Cu-N bonds in the presence of Cu-O interactions is generally difficult, because O and N have similar sizes and phase and amplitude functions in EXAFS (D'angelo et al, 1998;Carrera et al, 2004). It is possible in histidine because the EXAFS spectrum is from a single bonding environment, but NOM has multiple binding configurations with a predominance of O ligands, at least at intermediate and high Cu concentration, which makes Cu-N bonds undetectable.…”

The cis/trans isomerization of Cu(II)–bis‐(glycinato) complex in solution: a computer aided multifrequency EPR and DFT/PCM calculation study

“…19 An X-ray absorption study was also performed on the Cu(II)-glycinate complexes in aqueous solution, but the applied EXAFS technique could not give conclusive information about the presence of the cis or trans isomers of the Cu(II)-bis-glycinato Ð 2H 2 O complex. 23 Moreover, the geometry and the detailed electronic structure of the solid state complex do not need to be identical to the structure of the complex in solution because of the difference between the crystal lattice and solute-solvent interactions.…”