Ternary copper(II) complexes with 1,10-phenanthroline and various aminoacidates: A spectroscopic and voltammetric study in aqueous solution

Ternary copper(II) complexes with 1,10-phenanthroline and the aminoacids L-arginine, L-aspartic acid, L-histidine, L-glutamic acid, L-glutamine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-tryptophan, L-tyrosine, L-valine, were studied in aqueous solution by means of UV-Vis-NIR spectrophotometry, EPR spectroscopy either at room or at low temperatures, and Square Wave Voltammetry. From the experimental data it is possible to conclude that most of these ternary complexes show a pseudo-octahedral geometry with a CuN3O in plane chromophore and two oxygen atoms coming from water molecules perpendicularly bound to the equatorial plane. An exception to this general behaviour is given by the ternary copper(II) complex with 1,10-phenanthroline and histidine at pH value near the neutrality because of the terdentate nature of histidine when it coordinates by means of its histamine-like mode. In this case, evidence for a probable square-based pyramidal stereochemistry is given in support. At pH values around 5 the histidine behaves as bidentate ligand coordinating by its glycine-like mode, so as the copper (II) ternary complex with 1,10-phenathroline shows the pseudo-octahedral geometry found for all the ternary complexes with the other aminoacids. Moreover the ternary complex species with histidine at pH 5 and 7 are in equilibrium with each other as a function of the aqueous solution pH value and the temperature. In fact, the examination of low temperature EPR spectra at pH near 7 revealed not only a square-based pyramid complex but also products of decomposition. These results were also confirmed by the trend found in the formal redox potentials by the voltammetric measurements on many of these ternary complexes.