Vanadium-induced formation of thiadiazole and thiazoline compounds. Mononuclear and dinuclear oxovanadium(V) complexes with open-chain and cyclized thiosemicarbazone ligands

Reactions of the salicylaldehyde 4-phenylthiosemicarbazone (H₂L) with selected vanadium(IV) and vanadium(V) precursors ([VO(acac)₂], [VO(OAc)₂], VOSO₄, [V₂O₄(acac)₂]) were investigated under aerobic conditions in different alcohols (methanol, ethanol, propanol). In all examined cases mononuclear alkoxo vanadium(V) complexes [VOL(OR)] (1) (OR = OMe, OEt, OPr) were isolated as major products. On prolonged standing, mother liquids afforded dinuclear vanadium(V) complexes [V₂O₃(L_cycl)₂(OR)₂] (3) (OR = OMe, OEt, OPr), where L_cycl⁻ represents 1,3,4-thiadiazole ligand, formed by vanadium-induced oxidative cyclization of H₂L. When [VO(acac)₂] or [V₂O₄(acac)₂] were used as precursors, in addition to products 1 and 3, a thiazoline derivative HL(acac)_cycl (2) was isolated. This compound, formed by a reaction between acetylacetone and H₂L, represented the second type of cyclic product. The products were characterized by IR and NMR spectroscopies, TG analysis, and in some cases by single-crystal X-ray diffraction. To the best of our knowledge, compounds [V₂O₃(L_cycl)₂(OR)₂] represent the first structurally characterized dinuclear vanadium(V) complexes with a thiadiazole moiety acting as a bridging ligand. Complexes 1 and 3, when dissolved in an appropriate alcohol, underwent substitution of the alkoxo ligand as confirmed by XRPD. The kinetics of reactions in methanolic solutions was qualitatively studied by UV-Vis and ESMS spectrometries. Under the experimental conditions applied, a relatively slow formation of the mononuclear complex [VOL(OMe)] and an even slower formation of the cyclic species 2 were observed, whereas the presence of dinuclear compound [V₂O₃(L_cycl)₂(OMe)₂] in the reaction mixture could not be detected.