A spectroscopic study of oxovanadium(IV) complexes with hydroxylated carboxylic ligands in aqueous solution

This study is an enhancement of previous investigations on the vanadyl chemistry in water solution where our attention is addressed to the study of coordination compounds between vanadyl oxocation and carboxylic acids and, in particular, to their structural description by means of spectroscopic characterization of complexes. We report the investigation on coordination compounds of vanadyl ion with citric, D(+)-threo-isocitric, L-malic and L-tartaric acids in aqueous solution. The four acids were chosen in order to characterize the role of hydroxo groups and the effect of their position in the ligand molecule on the vanadyl coordination capability and on the structure of complexes. The different binary systems have been studied by potentiometric and spectroscopic techniques at t = 25 °C and I = 0.1 mol dm-3. Electronic paramagnetic resonance spectroscopy (EPR) at room temperature and molecular absorption spectrophotometry were employed. A speciation model was proposed for all the metal/ligand systems from potentiometric data. Moreover, the joint elaboration of potentiometric and spectroscopic data obtained on the vanadyl-ligand containing solutions allowed us to strengthen the speciation model proposed and to achieve a deeper knowledge of the structure of complexes in solution. The interpretation of spectrophotometric and EPR data by the application of solution speciation models allowed us to calculate the visible absorption spectra and the values of theoretical isotropic hyperfine coupling constant A0 of each complex species. For all the systems studied the chemical models assume the formation of monomeric species MLHr and binuclear species M2LH-r. All the ligands form ML complex, except L-tartaric acid, and dimeric complexes with general formula M2L2H-r. The dimeric complexes of citric, D(+)-treo-isocitric and L-malic acids are EPR silent, while the dimeric species of L-tartaric acid presents a characteristic EPR signal. These results suggest a magnetic coupling of two metal center due to the formation of an alcoholate-bridged dinuclear structure, but different complex structures for tartrate dimers with respect to the others hydroxylated ligands. The spectrophotometric study of metal-ligand systems provide interesting information about the structure of complexes. The spectrophotometric behaviour of citrate and L-tartrate systems differs significantly from that of D(+)-treo-isocitric and L-malic acids. For both the ligands the two principal absorption bands (765 and 635 nm for aquoion) present an opposite shift suggesting an increase of tetragonal distortion for the dimeric species M2L2H-r. The spectra of dimeric complexes of L-tartrate, moreover, show the electronic transition band at ~395 nm, often hidden under a charge transfer band. The results acquired are in accordance with the literature and highlight the weight of the location of alcoholic group in relation to the coordination capability and the complex structure.