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meso-Tetrakis(3,5-dimethoxyphenyl)porphyrinatooxidovanadium (1) and β-octabromo-meso-tetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphyrinatooxidovanadium (2) were synthesized in excellent yields. The synthesized oxidovanadium(IV) porphyrin complexes were characterized by various spectroscopic methods such as UV-Vis, FT-IR, EPR and MALDI-TOF mass spectrometry, as well as by single crystal X-ray crystallography. The acetonitrile coordinated porphyrin 1 is relatively planar whereas 2 is highly nonplanar as shown by their crystal structures and also by electrochemical studies. Selective epoxidation studies of alkenes were successfully carried out in CH3CN/H2O as the solvent mixture at ambient temperature resulting in very high TOF numbers (12 227–14 347 h−1 for 2) even with low catalyst loadings. Remarkably, 2 biomimics the vanadium bromoperoxidase (VBrPO) enzyme with extremely high TOF values (83 333–87 719 h−1) for the oxidative bromination of thymol and some other phenols. Both the catalysts were successfully recovered at the end of the reactions, indicating their viability and industrial applicability.

The reaction of dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine and 2,4-di-tert-butylphenol (H2L1I), 2,4-di-methylphenol (H2L2II), 2-tert-butyl-4-methylphenol (H2L3III), 2,4-di-chlorophenol (H2L4IV) and 2-naphthol (H2L5V) with [MoVIO2(acac)2] (Hacac = acetylacetone) in a 1 : 1 molar ratio in refluxing MeOH gave the corresponding cis-dioxidomolybdenum(VI) complexes [MoVIO2(L1)] 1, [MoVIO2(L2)] 2, [MoVIO2(L3)] 3, [MoVIO2(L4)] 4, and [MoVIO2(L5)] 5, respectively. All complexes were characterized by elemental analysis, various spectroscopic (FT-IR, UV/Vis, 1H and 13C NMR) techniques and single-crystal X-ray analysis (of 1, 2, 3 and 5). These complexes adopt a distorted six-coordinated octahedral geometry where the ligands act in a tetradentate manner, coordinating through the two Ophenolate and two Namine atoms in a cis-α type binding mode involving coordination with one of the Namine atoms in the apical position and one Ooxido terminal oxygen atom in the equatorial position. These complexes catalyze oxygen atom transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C. The formation of benzil could easily be monitored by HPLC. The electronic effect caused by the para substituent and the steric effect caused by the ortho substituent influence the formation of benzil and therefore the conversion varied between 66 and 99% in 18 h of reaction time under optimized conditions; the most active catalyst was [MoVIO2(L5)]. A similar trend has also been obtained with 4-chlorobenzoin. The pseudo first order rate constant for [MoVIO2(L5)] was found to be 0.0998 h−1. During catalytic reaction, the formation of the binuclear intermediate and its fast decay into the initial dioxidomolybdenum(VI) complex were established by time dependent UV/Vis studies.

The reaction of dibasic tripodal tetradentate ligands H2L1-4 (I-IV) (prepared from the reaction of aminoethylmorpholine and 2,4-disubstituted phenols) with [VIVO(acac)2] or VIVOSO4*5H2O gave the corresponding mononuclear oxidoethoxidovanadium (V) complexes [VVO(OEt)(L1-4)] (1-4) in good yields. Similar reaction with H2L2 (II) and H2L4 (IV) in acetonitrile resulted in the formation of the corresponding dinuclear complexes [{VVO(L2)}2µ-O] (5) and [{VVO(L4)}2µ-O] (6). Single crystal X-ray diffraction analysis of 1, 2 and 4 confirmed the the distorted octahedral geometry. The molecular structures of 1, 2 and 5 were calculated by DFT, as well as the corresponding 51V NMR chemical shifts (in methanol and chloroform) and IR frequencies. The V-compounds depict time dependent solution behaviour which was studied and rationalized through the combined interpretation of data from spectroscopic techniques (mainly UV-Vis and 51V NMR), mass spectrometry and DFT calculations. They exhibit distinct speciation in different solvents and it was confirmed that in acetonitrile they form -O-oxido dinuclear species. The alkoxido ligand of complexes [VVO(OR)(L)] in CDCl3, are exchanged upon addition of alcohols; namely the 51V NMR chemical shift values of [VVO(OR)(L2)] depict an increasing shielding effect of the OR moiety in the order: OMe < OEt < OiPr < OtBu. Compounds 1-4 were employed as catalyst precursors in the oxidation of dopamine to aminochrome in MeOH using dioxygen as the oxidant.

The reactions of the tripodal tetradentate ONNO donor ligands 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis(2,4-di-tert-butylphenol) (H2L1), 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis(2,4-dimethylphenol) (H2L2) and 6,6′-{[(2-morpholinoethyl)azanediyl]bis(methylene)}bis[2-(tert-butyl)-4-methylphenol] (H2L3) with [MoVIO2(acac)2] (acac = acetylacetonato) in a 1:1 molar ratio in MeOH gave the corresponding cis-dioxidomolybdenum(VI) complexes [MoO2(L1)], [MoO2(L2)] and [MoO2(L3)], respectively, in excellent yields. These complexes were characterized by various spectroscopic (IR, UV/Vis, 1H and 13C NMR), electrochemical, thermogravimetric, single-crystal XRD, and powder XRD (PXRD) studies. In these complexes, the geometry around the cis-[MoO2]2+ core is distorted octahedral, and the ligands are tetradentate and coordinate through two Ophenolate, one Ntripodal, and one Nmorpholine atoms. One of the oxido groups and the morpholine nitrogen atom occupy the axial sites. These complexes were used for catalytic oxygen atom transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C, and the formation of benzil was followed by HPLC. Detailed kinetic studies revealed a first-order rate in benzoin and catalyst, and the rate constant for the second-order oxygen atom transfer reaction was 0.0162 m–1 h–1. The formation of the dinuclear intermediates [LMoV–µ-O-MoVL] was established by MALDI-TOF MS and UV/Vis spectroscopy. Its reversible nature was further supplemented by UV/Vis spectroscopy. These complexes also catalyze the oxidation of pyrogallol in a fashion similar to that of transhydroxylases. Under aerobic conditions, the initially formed oxidation product phloroglucinol undergoes further oxidative coupling in the presence of H2O2 to give purpurogallin as the final product. This process follows Michaelis–Menten-type kinetics with respect to pyrogallol; the kcat values obtained were 394, 300 and 247 h–1 for [MoVIO2(L1)], [MoVIO2(L2)] and [MoVIO2(L3)], respectively.

The tridentate ONN donor ligand, Hbzpy-fah (I) (Hbzpy-fah = Schiff base derived from 2-benzoylpyridine and 2-furoylhydrazide) upon reaction with [VIVO(acac)2] in methanol followed by aerial oxidation give a dinuclear complex, [{VVO(bzpy-fah)}2(μ-O)2] (1) having [{VVO}2(μ-O)2]2+ core. Treatment of 1 with H2O2 in methanol results in the formation of dinuclear peroxido complex, [{VVO(bzpy-fah)}2(μ-O2)2] (2). The molecular structure of 2 was determined by single-crystal X-ray diffraction, confirming the ONN binding mode of I and μ-bis(peroxido) with side-on coordination. Reaction of 1 with imidazolomethylpolystyrene cross-linked with 5% divinylbenzene (PS-im) in DMF gives the polymer-supported complex 3 (abbreviated as PS-im[VVO2(bzpy-fah)]) which was characterized by field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDAX), atomic force microscopy (AFM) and thermal analysis along with usual spectroscopic techniques. Complexes 1 and 3 have been used as catalyst precursors for the oxidation of isoeugenol in the presence of aqueous H2O2 as oxidant and gave vanillin, vanillic acid and dehydrodiisoeugenol. The polymer-supported complex showed higher conversion than its neat counterpart. This has also allowed for recyclable catalytic system with increased catalyst lifetime and thus proved to be better over the homogeneous counterpart.