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 : I 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 D4h 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,5-2,3-diaminopropionic acid and R,5-2,4-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 5-asparagine above pH ~ 9, the first deprotonated amide nitrogen is bound axially, and the second one equatorially. In the 1: 2 complexes of 5-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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Materials Chemistry