Oxidomolybdenum Complexes of Mono- and Dibasic Polydentate Ligands: Characterization, Biological and Catalytic Evaluation

Abstract

Chapter 1: In this chapter the scope of the present investigation is portrayed briefly along with the aim of the work.

Chapter 2: Four novel dimeric bis-μ-imido bridged metal-metal bonded oxidomolybdenum(V) complexes [MoV2O2L21-4] (14) (where L1-4 are rearranged ligands formed in situ from H2L1-4) and a new mononuclear dioxidomolybdenum(VI) complex [MoVIO2L5] (5) synthesized from salen type N2O2 ligands are reported. This rare series of imido-bridged complexes (14) have been furnished from rearranged H3Lʹ1-4 ligands, containing an aromatic diimine (o-phenylenediamine) „linker‟, where Mo assisted hydrolysis followed by −C=N bond cleavage of one of the arms of the ligand H2L1-4 took place. A monomeric molybdenum(V) intermediate species [MoVO(HL1-4)(OEt)] (Id1-4) was generated in situ and finally formation of bis-μ-imido bridge between the two molybdenum centers of [MoV2O2L21-4] (14) took place. The mechanism of formation of 14 has been discussed and one of the rare intermediate monomeric molybdenum(V) species Id4 has been isolated in the solid state and characterized. The monomeric dioxidomolybdenum(VI) complex [MoVIO2L5] (5) was prepared from the ligand H2L5 containing an aliphatic diimine (1,2-diaminopropane). All the ligands and complexes have been characterized by elemental analysis, IR, UV–Vis spectroscopy, NMR, ESI–MS and cyclic voltammetry, and the structural features of 1, 2, 4 and 5 have been solved by X-ray crystallography. All the complexes (1–5) were tested for their ability to exhibit DNA binding and cleavage activity. Moreover, in vitro cytotoxic activity of all the complexes was evaluated by MTT assay, which reveals the complexes induce cell death in MCF-7 (human breast adenocarcinoma) and HCT-15 (colon cancer) cell lines.

Chapter 3: Seven new dioxidomolybdenum(VI) complexes [MoO2L17(X)] (1–7) [Where X= EtOH in case of 1 and 5 and X=DMSO in case of 24 and 6, 7] of aroylazines containing a bulky 3-hydroxy-2-naphthoic substituent, were isolated and structurally characterized. The aroylazine ligands H2L1-7 were derived from the condensation of 3-hydroxy-2-naphthoic acid hydrazide with several substituted aromatic aldehydes/ketones. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV–Vis and NMR spectroscopy. X-ray structures of 16 revealed that the ligands coordinate to the metal center as a dibasic tridentate ligand. Cyclic voltammetry of the complexes shows two irreversible reductive responses within the potential window 0.50 to 1.36 V, due to MoVI/MoV and MoV/MoIV processes. The synthesized complexes 1–7 were used as catalysts for the oxidation of benzoin, and for the oxidative bromination of salicylaldehyde, as a functional mimic of haloperoxidase. It was found that the percentage of conversion increased significantly in the presence of catalysts 1–7 which contained bulky substituents, and showed high percentage of conversion (>90%) with high turnover frequency (>1100 h-1) than previously reported catalysts. Benzil, benzoic acid and benzaldehyde-dimethylacetal were formed selectively
for the oxidation of benzoin. Formation of 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde took place during the oxidative bromination of salicylaldehyde in presence of H2O2 as an oxidant and therefore 1–7 act as functional models of vanadium dependent haloperoxidases.

Chapter 4: Four novel mononuclear oxido-imido molybdenum(VI) complexes [MoVIOL′1-4L′′] (1−4), and a dimeric oxidomolybdenum(V) complex [Mo2VO3L′′2] (5) containing rearranged ligand fragments L′1−4 and L′′, are reported. H2L1−4′ and H2L′′, were formed by in situ ligand rearrangement of the ligands HL1-4 which gave rise to the complexes 1−4 during the first 15 mins of the reaction while complex 5 was obtained from the same reaction mixture after 24 h. All the complexes as well as the ligands were characterized by various spectroscopic techniques (IR, UV-Vis, NMR) and ESI-MS. The structures of 1, 3 and 4 were solved by single crystal X-ray crystallography. The complexes 1−4 underwent changes in solution, and time dependent UV-Vis and EPR spectroscopy studies revealed that 1−4 formed a mixed valence molybdenum(V,VI) species [MoVMoVIO3L′′2] (6) in solution.

Chapter 5: The synthesis of three new dioxidomolybdenum(VI) complexes [MoO2L1-3(X)] (1−3) of aroylhydrazone ligands (H2L1-3) containing azobenzene moiety have been reported. The ligands H2L1-3 have been synthesized from the condensation of 5-(Arylazo) salicylaldehyde derivatives with corresponding aroyl hydrazides. The azobenzene functionality was incorporated in the aroylhydrazone systems containing benzoyl, naphthyl and furyl moieties in order to explore their influence if any, on the DNA/BSA interactions of the corresponding molybdenum complexes. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV–Vis and NMR spectroscopy. The redox properties of the complexes were studied by cyclic voltammetry. Molecular structures of all the complexes (1−3) have been determined by X‒ray crystallography. The complexes interact with calf-thymus DNA (CT-DNA) with binding constants ~104 M-1. The cytotoxicity of the complexes against A-549 cell line has also been explored.

Chapter 6: The synthesis of four new dioxidomolybdenum(VI) complexes [MoO2L1-4(S)] (1−4) of Schiff base ligands (H2L1-4) containing substituted azobenzene derivatives have been reported. The ligands H2L1-4 have been synthesized from the condensation of substituted 5-(Arylazo) salicylaldehyde derivatives with 2-aminophenol. The substituted azobenzene functionalities were incorporated in order to explore their influence if any, on the biological activities of the corresponding molybdenum complexes. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV–Vis and NMR spectroscopy. The redox properties of the complexes were studied by cyclic voltammetry. Molecular structures of all the complexes (1−4) have been determined by X‒ray crystallography. The complexes have also been tested for their antibacterial activity against Pseudomonas aeruginosa , Vibrio cholerae, Escherichia Coli and Bacillus.

Chapter 7: Summary of the work embodied in the research work and the scope of further research study has been discussed