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

Abstract

The reaction between [V(IV)O(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 [V(IV)O(acac)(pydx-aebmz)] 1 and [V(IV)O(acac)(pydx-ambmz)] 2, respectively. The aerobic oxidation of the methanolic solution of 1 yielded [V(V)O(2)(pydx-aebmz)] 3 and its reaction with aqueous H(2)O(2) gave the oxidoperoxidovanadium(v) complex, [V(V)O(O(2))(pydx-aebmz)] 4. The formation of 4 in solution is also established by titrations of methanolic solutions of 1 with H(2)O(2). By titrating solutions of 3 and of 4 with aqueous H(2)O(2) several distinct V(V)-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 (51)V NMR chemical shifts (δ(V)) have also been calculated, the theoretical data being used to support assignments of the experimental chemical shifts. The (51)V hyperfine coupling constants are calculated for 1, the obtained values being in good agreement with the experimental EPR data. Reaction between the V(IV)O(2+) exchanged zeolite-Y and Hpydx-aebmz and Hpydx-ambmz in refluxing methanol, followed by aerial oxidation results in the formation of the encapsulated V(V)O(2)-complexes, abbreviated herein as [V(V)O(2)(pydx-aebmz)]-Y 5 and [V(V)O(2)(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 V(IV)O-centre. Oxidation of styrene is investigated using some of these complexes as catalyst precursors with H(2)O(2) as oxidant. Under optimised reaction conditions for the conversion of styrene in acetonitrile, a maximum of 68% conversion of styrene (with [V(V)O(2)(pydx-aebmz)]-Y) and 65% (with [V(V)O(2)(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, (51)V 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.