Vanadium complexes having [VIVO]2+ and [VVO2]+ cores with binucleating dibasic tetradentate ligands: Synthesis, characterization, catalytic and antiamoebic activities

Abstract

Binucleating hydrazones CH₂(H₂sal-bhz)₂ (I) and CH₂(H₂sal-fah)₂ (II), derived from 5,5′-methylbis(salicylaldehyde) and benzoylhydrazide or 2-furoylhydrazide, react with [V^IVO(acac)₂] to give dinuclear V^IVO-complexes [CH₂{V^IVO(sal-bhz)(H₂O)}₂] 1 and [CH₂{V^IVO(sal-fah)(H₂O)}₂] 4, respectively. In the presence of KOH or CsOH·H₂O, oxidation of 1 and 2 results in the formation of dioxidovanadium(V) complexes, K₂[CH₂{V^VO₂(sal-bhz)}₂]·2H₂O 2, K₂[CH₂{V^VO₂(sal-fah)}₂]·2H₂O 5, Cs₂[CH₂{V^VO₂(sal-bhz)}₂]·2H₂O 3 and Cs₂[CH₂{V^VO₂(sal-fah)}₂]·2H₂O 6. These complexes have also been prepared by aerial oxidation of in situ prepared oxidovanadium(IV) complexes 1 and 4. The compounds were characterized by IR, electronic, EPR, ¹H, ¹³C and ⁵¹V NMR spectroscopy, elemental analyses and thermogravimetric patterns. Single crystal X-ray analysis of 3 confirms the coordination of the ligand in the dianionic (ONO²⁻) enolate tautomeric form. The V^VO₂-complexes were used to catalyze the oxidative bromination of salicylaldehyde, therefore acting as functional models of vanadium dependent haloperoxidases, in aqueous H₂O₂/KBr in the presence of HClO₄ at room temperature. It is shown that the V^IVO-complexes [CH₂{V^IVO(sal-bhz)(H₂O)}₂] 1 and [CH₂{V^IVO(sal-fah)(H₂O)}₂] 4 are catalyst precursors for the catalytic oxidation of organic sulfides using aqueous H₂O₂. Plausible intermediates involved in these catalytic processes are established by UV-Vis, EPR and ⁵¹V NMR studies. The vanadium complexes along with ligands I and II are also screened against HM1:1MSS strains of Entamoeba histolytica, the results showing that the IC₅₀ values of compounds 3 and 6 are lower than that of metronidazole. The toxicity studies against human cervical (HeLa) cancer cell line also showed that although compounds 3 and 6 are more toxic than metronidazole towards this cell line, the corresponding IC₅₀ values are relatively high, the cell viability therefore not being much affected.