Polyoxometalates covalently modified organic medicines as potential antitubercular agents: The molecular hybridization of hexamolybdate and heterocyclic hydrazides

Aimed at exploring new organically derivatized POMs, three heterocyclic hydrazides functionalized hexamolybdates (TBA)₃[Mo₆O₁₈(=N=NCOC₅H₄N)] (1) (TBA = tetrabutylammonium), (TBA)₃[Mo₆O₁₈(=N=NCOC₄H₃S)] (2) and (TBA)₃[Mo₆O₁₈(=N=NCOC₄H₃N₂)] (3) were prepared via the reflux reaction of corresponding heterocyclic hydrazides and octamolybdates in the dry acetonitrile. Their structures were determined by single crystal X-ray diffraction, all compounds can be viewed as one terminal oxo atom of hexamolybdates being substituted by corresponding heterocyclic hydrazides via the Mo=N multiple bond. There are three conformational isomers of [Mo₆O₁₈(=N=NCOC₅H₄N)]^3- with the approximate symmetry of C_s or C₁ in the asymmetric unit of compound 1, but only one anion cluster of [Mo₆O₁₈(=N=NCOC₄H₃S)]^3- with the approximate symmetry of C_s in the asymmetric unit of compound 2. Their structures are further evidenced by FT-IR, UV-Vis, ESI MS, ¹H NMR spectroscopy and so on. Antitubercular activity tests manifest that all compounds demonstrate enhanced inhibitory activities compared to that of hexamolybdates and L2, but it still inferior to L1 and L3. Considering their large molecular weight, their molar inhibitory activity is close or superior to the ligands of L1 and L3. Our current work provides a promising way to develop POMs with biological activities through the molecular hybridization of organic medicines and POMs via covalent bond.