New Insights into the Coordination Chemistry and Molecular Structure of Copper(II) Histidine Complexes in Aqueous Solutions

Abstract

Aqueous solutions of Cu 2+ /histidine (his) (1:2) have been analyzed in parallel with infrared, Raman, ultraviolet/ visible/near-infrared, electron spin resonance, and X-ray absorption spectroscopy in the pH range from 0 to 10. Comprehensive interpretation of the data has been used to extract complementary structural information in order to determine the relative abundance of the different complexes. O c ,N am ,N im )] 2 is the major species with the N atoms in the equatorial plane and the O atoms in the axial position. This complex decomposes at pH > 10 into a copper oxide/hydroxide precipitate. The overall results provide a consistent picture of the mechanism that drives the coordination and complex formation of the Cu 2+ /his system.

Host–Guest Chemistry of Copper(II)–Histidine Complexes Encaged in Zeolite Y

“…A better signal to noise ratio was obtained particularly for sample d with the highest Cu 2 + /His concentration, but this spectrum appeared to be virtually identical to that of the reference compound His/Y. To illustrate this, the spectrum of sample d is presented in Figure 11, together with the spectra of the reference compound His/NaY and His in aqueous solution recorded at pH 5 (for the assignments of the bands we refer to earlier work [14,15] ). Because of interfering bands of the zeolite support only the range 1800-1200 cm À1 is shown.…”

“…Firstly, as follows from the IR and Raman spectra, the "pH" inside the zeolite is around 5 and as we recently demonstrated, a bisglycine-like NONO coordination and a combined glycine/histamine-like NNNO chelation is much more likely under these circumstances ( Figure 13, complex B'). [15] Secondly, the corresponding seven-membered ring coordination (N im , O c ) may not very stable as it is associated with a considerable ring tension regarding the N···O distance in this structure. This should affect the EXAFS data in a similar way to that we observed for the Cu 2 + /His complexes in aqueous solution.…”

“…[6] Consequently, the IR and Raman spectra represent mainly these non-complexed His species. However, as we reported recently, [14,15] IR and Raman can be used to determine the state of protonation of the His molecules. This is dependent on the "pH" of the local environment and thus, the vibrational spectra of c and d can be used as an indirect tool to establish the "pH" within the zeo- Table 1).…”

“…This should affect the EXAFS data in a similar way to that we observed for the Cu 2 + /His complexes in aqueous solution. [15] However, the EXAFS data are similar and for that reason we propose the structures A and B, presented in Figure 13 as refined modifications of the complexes proposed by Baute et al [8] and Grommen et al [11] Complex formation: Based on the preceding results, we have developed the following mechanism for the formation of the Cu 2 + /His complexes A and B. The pH in the ion-exchange solution is kept at 7.3, which implies that in the aqueous phase Cu 2 + will be mainly present as a complex with two His ligands that coordinate through the nitrogen atoms (NNNN).…”

“…To obtain insight into the host-guest chemistry of the Cu 2 + /His/zeolite system, we decided to carry out a model study on a series of these solid samples as a logical continuation of the research on the Cu 2 + /His complexes in aqueous solution as described in two recent papers. [14,15] It has been shown that X-ray absorption (XAFS), infrared (IR), and Raman spectroscopy can be valuable tools to obtain complementary information not only on the local structure around the central copper ion, but also on the state of protonation of the potential sites for coordination of the His ligand. For that reason we decided to use these techniques in addition to UV/Vis/NIR and ESR spectroscopy to study the structural behavior of Cu 2 + /His complexes on immobilization in the zeolite pores.…”