ESR STUDY OF COPPER(II) COMPLEXES OF α-AMINO ACIDS. COORDINATION MODES AND METAL-LIGAND BONDS IN FROZEN AQUEOUS SOLUTIONS

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Abstract
Various copper(II) complexes of potentially terdentate α-amino acids and, for comparison, those of ethylenediamine and histamine have been investigated. The ESR spectra at 1 : 1 and 1 : 4 metal-ligand concentration ratios over different pH regions have been recorded in frozen aqueous solutions. The parallel and perpendicular components of the g and A tensors, the nitrogen superhyperfine coupling constants in the different directions, and the numbers of equatorial nitrogen donor atoms have been determined by computer simulation of the spectra. The energies of the d-d electronic transitions have been determined by Gaussian analysis of the visible absorption bands. Molecular orbital coefficients characteristic of the metal-ligand bonds have been derived for an effective D 4h local symmetry. Near the freezing point, the equilibrium for the complexes involving bis(glycinato) coordination is shifted towards the trans isomer. The amino groups of R,S-2,3-diaminopropionic acid and R,S-2,4-diaminobutyric acid (Daba), which are protonated in acidic media, bind equatorially to copper(II) with increasing pH, except for the 1 : 2 complex of Daba above pH ∼ 8, where the fourth nitrogen is coordinated axially. In the 1 : 2 complexes of S-asparagine above pH ∼ 9, the first deprotonated amide nitrogen is bound axially, and the second one equatorially. In the 1 : 2 complexes of S-histidine, the imidazole nitrogens enter the equatorial plane when they lose the protons attached to them at low pH. A competition can be observed between the σ-bonds with different symmetries. The influence of the coordination modes on the covalent character of the metal-ligand bonds is discussed.

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