Synthesis, Characterization, and Biological Activity of Oxidovanadium(IV) Hydroxamate Complexes Supported by Density Functional Theory

The oxidovanadium(IV) complex [VO(HL)₂] (1) (where, HL = 4-nitrocinnamohydroxamate; 4-NO₂C₆H₄CH=CHCONHOH) has been synthesized by the condensation reaction of VOSO₄∙5H₂O and potassium 4-nitrocinnamohydroxamate in methanol-water medium. The complex is characterized by elemental analysis, molar conductivity, magnetic susceptibility measurement, FTIR, UV-Vis, Electron Paramagnetic Resonance (EPR) spectral techniques and mass spectrometry. The bidentate linkage of hydroxamate ligand involving O,O-coordination through hydroxamic and carbonyl oxygen atoms has been deduced. The magnetic susceptibility, EPR and mass spectra (ESI-MS) indicate that the complex exists as monomer and a distorted square pyramidal geometry around vanadium is proposed. The electrochemical study of 1 has shown it is to be electrochemically active exhibiting VO^V/VO^IV quasi-reversible redox couple. The thermal study of the complex yielded VO₂ as sole decomposition product. The coordination compounds 2 and 3 have been isolated from the reaction of complex 1 with 2-cyanopyridine (2-CNPy) and 4-aminobenzonitrile (4-CNAn) respectively and characterized by physicochemical and IR spectral study. The biological activity of 1-3 has been studied against various pathogenic bacteria E. coli, S. aureus, S. typhi, S. paratyphi, S. epidermidis, K. pneumonia and fungi C. albicans, B. fulva, and F. oxysporum by minimum inhibitory concentration (MIC) method. The complexes exhibit enhanced antimicrobial activity relative to both the free ligand and vanadyl sulphate. The cytotoxicity of 1-3 has been studied on mammalian transformed cell line Hep2c, a derivative of human cervix carcinoma HeLa cells by MTT assay. 2 and 3 exhibit higher cytotoxic activity than 1 and reveal a marked effect of the coordination of nitrogen bases. Density functional theory studies have been carried out to determine the relative free energy of formation and stable molecular structures of 1-3. Time dependent density functional theory (TD-DFT) based calculation have been performed to find out the frontier molecular orbitals and to corroborate with the experimentally observed UV-Vis spectrum of 1. Other parameters like HOMO, and LUMO energies, density of state (DOS), and global reactivity descriptors clearly support higher biological activity of 2 and 3 than 1.