Abstract

The interaction of the VO²⁺ ion with pyrone derivatives and tropolone, which form very effective antidiabetic compounds, is critically re-examined. The binary systems with ethylmaltol (Hema) and tropolone (Htrop) were studied in aqueous solution and in the solid state through the combined application of spectroscopic (EPR, UV/Vis and IR) and pHpotentiometric techniques. The results were compared with those of the systems with maltol (Hma) and kojic acid (Hkoj) and rationalized on the basis of DFT simulations. All the ligands L^– form [VOL]⁺, cis-[VOL₂(H₂O)] and cis-[VOL₂(OH)]– species in aqueous solutions and a square-pyramidal [VOL₂] complex in the solid state, which transforms into cis-[VOL₂-(solvent)] when it is dissolved in water or in a coordinating solvent. The coordinating properties of the ligands studied were compared with those of pyridinone [3-hydroxy-1,2-dimethyl- 4(1H)pyridinone (Hdhp), and 1,2-diethyl-3-hydroxy- 4(1H)pyridinone (Hdepp)] derivatives and catechol (H₂cat), and were explained by postulating that from pyrones to pyridinones and to catechol the donor set changes progressively from (CO, O^–) to (O^–, O^–). DFT calculations allowed us to determine the relative stability of the four possible structures (square pyramidal, trans- and two cis-octahedral) of the bischelated species and the aromaticity of the protonated, neutral and deprotonated form of the ligands through the calculation of the HOMA (harmonic oscillator model of aromaticity) index. The relationship between the electric charge on the oxygen donors, the mean distances and the difference between the lengths of C–O_ket and C–O_phen bonds with (i) the pK_a of the ligands, (ii) the pK of deprotonation of the equatorially coordinated water molecule in cis-[VOL₂(H₂O)], and (iii) the ⁵¹V hyperfine coupling constant (A_z) of [VOL₂], cis-[VOL₂(H₂O)] and cis-[VOL₂(OH)]^– was also found and discussed.

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