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

Abstract

The reaction between [V^IVO(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^IVO(acac)(pydx-aebmz)] 1 and [V^IVO(acac)(pydx-ambmz)] 2, respectively. The aerobic oxidation of the methanolic solution of 1 yielded [V^VO₂(pydx-aebmz)] 3 and its reaction with aqueous H₂O₂ gave the oxidoperoxidovanadium(V) complex, [V^VO(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 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 ⁵¹V NMR chemical shifts (δ^V) 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 V^IVO²⁺ exchanged zeolite-Y and Hpydx-aebmz and Hpydx-ambmz in refluxing methanol, followed by aerial oxidation results in the formation of the encapsulated V^VO₂-complexes, abbreviated herein as [V^VO₂(pydx-aebmz)]-Y 5 and [V^VO₂(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^IVO-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 [V^VO₂(pydx-aebmz)]-Y) and 65% (with [V^VO₂(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.