Potentiometric, spectroscopic, electrochemical and DFT characterization of oxovanadium(iv) complexes formed by citrate and tartrates in aqueous solution at high ligand to metal molar ratios: the effects of the trigonal bipyramidal distortion in bis-chelated species and biological implications

Abstract

The complexation of VO(IV) ion with citrate (L³⁻), D-, L- and DL-tartrate (L²⁻) at high ligand to metal molar ratios was studied in aqueous solution through the combined application of potentiometric, spectroscopic (UV-vis and EPR) and electrochemical (cyclic voltammetry) techniques. Unlike in equimolar solution, mononuclear and not dinuclear species are formed with the binding of carboxylate-COO⁻ and alcoholate-O⁻ donors yielding mono- and bis-chelated species with VOLH, VOL, VOLH₋₁ and VOL₂H₋₂ composition; for tartrates also the “sugar-like” (O⁻, O⁻) coordination is involved in the vanadium binding at basic pH values giving rise to the formation of VOL₂H₋₃ and VOL₂H₋₄ complexes. Among the species formed, VOL₂H₋₂ is characterised by a strong distortion towards the trigonal bipyramid with the two V–O(alcoholate) bonds in the equatorial and the two V–O(carboxylate) bonds in the axial positions. The geometry and electronic absorption spectra of such complexes were simulated by DFT methods and it was found that in aqueous solution the distortion follows the steric hindrance of the substituents on the α-carbon atom and the hydrophobicity of the ligands. The results were compared with those displayed by simple α-hydroxycarboxylates (glycolate, 2-hydroxyisobutyrate, 2-ethyl-2-hydroxybutyrate and benzilate). The trigonal bipyramidal distortion was correlated with the values of: i) Δλ = λ₂ − λ₃, where λ₂ and λ₃ are the central bands in the electronic absorption spectrum; ii) |A_x − A_y|, where A_x and A_y are the ⁵¹V hyperfine coupling constants along the x and y axes in the anisotropic EPR spectrum; iii) the half-wave potential E_1/2 of oxidation of VO(IV) to the corresponding VO₂(V) species in the cyclic voltammogram. Finally, a discussion on the possible form of VO(IV)–citrate complexes in blood serum is presented, where it is found that the most relevant species under physiological conditions should be [VO(citrH₋₁)]²⁻.