Oxidovanadium(iv) and dioxidovanadium(v) complexes of hydrazones of 2-benzoylpyridine and their catalytic applications

Abstract

Reactions between the tridentate ONN donor ligands, Hbzpy-tch (I) and Hbzpy-inh (II), with [V^IVO(acac)₂] in dry methanol give two different types of complexes, [V^IVO(acac)(bzpy-tch)] (1) and [V^IVO(OMe)(bzpy-inh)] (2), respectively. Irrespective of their nature in methanol upon aerial oxidation and precipitation both yield dinuclear [{V^VO(bzpy-tch)}₂(μ-O)₂] (3) and [{V^VO(bzpy-inh)}₂(μ-O)₂] (4). Treatment of 1 or 2 in methanol with H₂O₂ yields the oxidomonoperoxidovanadium(V) complexes [{V^VO(bzpy-tch)}₂(μ-O₂)₂] (5) and [V^VO(O₂)(bzpy-inh)] (6). Reactions of 3 and 4 with imidazolomethylpolystyrene cross-linked with 5% divinylbenzene (PS-im) in DMF give the polymer-supported PS-im[V^VO₂(bzpy-inh)] (7) and PS-im[V^VO₂(bzpy-tch)] (8). The compounds are characterized by various spectroscopic techniques and compounds 7 and 8 were also analyzed by thermal, atomic force microscopy (AFM), field-emission scanning electron micrographs (FE-SEM) as well as energy dispersive X-ray (EDAX) studies. The molecular structures of 3–5 and of [V^VO(O₂)(bzpy-inh)(H₂O)]·0.5MeOH (6a) were determined by single-crystal X-ray diffraction, confirming the μ-bis(O) and ONN binding mode in the dinuclear complexes 3 and 4, as well as the side-on coordination of the peroxido ligand in 5 and 6a. The polymer-grafted compounds 7 and 8 were used for the catalytic oxidation of isoeugenol using aqueous H₂O₂ as an oxidant. The intermediate peroxido species, expected to be involved during catalytic action, were also generated from solutions of 1–4 and studied by UV-Vis and ⁵¹V NMR. The catalytic activity of the several systems was tested and the polymer-supported versions showed higher conversions than their neat counterparts. The polymer-supported complexes allow for recyclable catalytic systems, thus providing additional advantages over their homogeneous counterparts in terms of increased catalyst lifetime and easier separation from the reaction mixture.