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Issue 26, 2011
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Synthesis, characterization, reactivity and catalytic activity of oxidovanadium(IV), oxidovanadium(V) and dioxidovanadium(V) complexes of benzimidazole modified ligands

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Abstract

The reaction between [VIVO(acac)2] and the ONN donor Schiff base obtained by the condensation of pyridoxal and 2-aminoethylbenzimidazole (Hpydx-aebmz, I) or 2-aminomethylbenzimidazole (Hpydx-ambmz, II) in equimolar amounts results in the formation of [VIVO(acac)(pydx-aebmz)] 1 and [VIVO(acac)(pydx-ambmz)] 2, respectively. The aerobic oxidation of the methanolic solution of 1 yielded [VVO2(pydx-aebmz)] 3 and its reaction with aqueous H2O2 gave the oxidoperoxidovanadium(V) complex, [VVO(O2)(pydx-aebmz)] 4. The formation of 4 in solution is also established by titrations of methanolic solutions of 1 with H2O2. By titrating solutions of 3 and of 4 with aqueous H2O2 several distinct VV-pydx-aebmz species also containing the peroxido ligand are detected. The full geometry optimization of all species envisaged was done using DFT methods for suitable model complexes. The 51V NMR chemical shifts (δV) have also been calculated, the theoretical data being used to support assignments of the experimental chemical shifts. The 51V hyperfine coupling constants are calculated for 1, the obtained values being in good agreement with the experimental EPR data. Reaction between the VIVO2+ exchanged zeolite-Y and Hpydx-aebmz and Hpydx-ambmz in refluxing methanol, followed by aerial oxidation results in the formation of the encapsulated VVO2-complexes, abbreviated herein as [VVO2(pydx-aebmz)]-Y 5 and [VVO2(pydx-ambmz)]-Y 6. The molecular structure of 1, determined by single crystal X-ray diffraction, confirms its distorted octahedral geometry with the ONN binding mode of the tridentate ligand, with one acetylacetonato group remaining bound to the VIVO-centre. Oxidation of styrene is investigated using some of these complexes as catalyst precursors with H2O2 as oxidant. Under optimised reaction conditions for the conversion of styrene in acetonitrile, a maximum of 68% conversion of styrene (with [VVO2(pydx-aebmz)]-Y) and 65% (with [VVO2(pydx-ambmz)]-Y) is achieved in 6 h of reaction time. The selectivity of the various products is similar for both catalysts and follows the order: benzaldehyde (ca. 55%) > 1-phenylethane-1,2-diol > benzoic acid > styrene oxide > phenyl acetaldehyde. Speciation of the systems and plausible intermediates involved in the catalytic oxidation processes are established by UV-Vis, EPR, 51V NMR and DFT studies. Both non-radical (Sharpless) and radical mechanisms of the olefin oxidations were theoretically studied, and the radical pathway was found to be even more favorable than the Sharpless mechanism.

Graphical abstract: Synthesis, characterization, reactivity and catalytic activity of oxidovanadium(iv), oxidovanadium(v) and dioxidovanadium(v) complexes of benzimidazole modified ligands

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Publication details

The article was received on 16 Feb 2011, accepted on 03 May 2011 and first published on 07 Jun 2011


Article type: Paper
DOI: 10.1039/C1DT10261C
Dalton Trans., 2011,40, 6968-6983

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    Synthesis, characterization, reactivity and catalytic activity of oxidovanadium(IV), oxidovanadium(V) and dioxidovanadium(V) complexes of benzimidazole modified ligands

    M. R. Maurya, M. Bisht, A. Kumar, M. L. Kuznetsov, F. Avecilla and J. C. Pessoa, Dalton Trans., 2011, 40, 6968
    DOI: 10.1039/C1DT10261C

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