Dioxovanadium(V) and μ-oxo bis[oxovanadium(V)] complexes containing thiosemicarbazone based ONS donor set and their antiamoebic activity

Neutral dioxovanadium (V) complexes [VO2(HL)] (H2L = I: 1, H2L = II: 2 and H2L = III: 3; H2L are the thiosemicarbazones H2pydx-tsc (I), H2pydx-chtsc (II) and H2pydx-clbtsc (III); pydx = pyridoxal, tsc = thiosemicarbazide, chtsc = N4-cyclohexylthiosemicarbazide, clbtsc = N4-(2-chloro)benzylthiosemicarbazide) have been isolated and characterised on the basis of elemental and electrochemical analyses, spectroscopic (IR, UV–Vis, 1H and 51V NMR) data, thermogravimetric studies and reactivity patterns. These complexes are stable in solution at ambient temperature but heating of the methanolic solutions yields the μ-oxo binuclear complexes [(VOL)2μ-O] (H2L = I: 4, H2L = II: 5, H2L = III: 6). Treatments of dioxo species with H2O2 yield oxoperoxo species, the formations of which have been established spectrophotometrically. Similarly, the formations of oxohydroxo species, an intermediate proposed during the catalytic action of haloperoxidases, have also been established in solution by treating 1, 2 and 3 with a methanolic solution of HCl. The antiamoebic activities were carried out to ascertain the effectiveness of dioxovanadium(V) and μ-oxobis(oxovanadium(V)) complexes in comparison to their corresponding thiosemicarbazones. These complexes possess noteworthy potencies against HM1:1MSS strain of Entamoeba histolytica. Complexes 2, 3, 5 and 6 showed less IC50 values than metronidazole, indicating that these metal thiosemicarbazones may be the lead molecules to inhibit the growth of E. histolytica. Within the series, complex 5 showed the most promising amoebicidal activity (IC50 = 0.5 μM versus IC50 = 1.8 of metronidazole).