Polystyrene bound dioxidovanadium(V) complexes of 2-acetylpyridine derived ligands for catalytic oxidations

Three neat complexes [VVO2(acpy-bhz)] (1) [VVO2(acpy-inh)] (2) and [VVO2(acpy-nah] (3) and the corresponding polymer-supported (PS) dioxidovanadium(V) complexes having monobasic tridentate ONN donor ligands, abbreviated herein as PS-im[VVO2(acpy-bhz)] (4) PS-im[VVO2(acpy-inh)] (5) and PS-im[VVO2(acpy-nah)] (6) are isolated through covalent bonding of imidazolomethylpolystyrene, obtained by reacting chloromethylated polystyrene cross-linked with 5% divinylbenzene with imidazole, with the corresponding neat complexes 1–3. All compounds are characterized in solid state and in solution, namely by spectroscopic techniques (IR, UV–Vis, 51V NMR, thermal and scanning electron micrograph studies). The monomeric form {[VO2(acpy-nah)]·DMSO (3·DMSO)} (3a) of complex 3 is also isolated from its solution in DMSO and its molecular structure is confirmed by single crystal X-ray diffraction. Polymer-supported as well as neat complexes are used as catalyst precursors for the oxidative bromination of styrene and trans-stilbene using 30% aqueous H2O2 as an oxidant, the compounds acting as functional models of vanadium dependent haloperoxidases. 1-Phenylethane-1,2-diol, 2-bromo-1-phenylethane-1-ol (bromohydrin) and 1,2-dibromo-1-phenylethane are the reaction products of styrene after 1 h of reaction, while those of trans-stilbene are: 2,3-diphenyloxirane (trans-stilbene oxide), 2-bromo-1,2-diphenylethanol and 1,2-dibromo-1,2-diphenylethane. It is also shown that all these compounds are catalyst precursors for the catalytic oxidation of benzoin by peroxide, the products being benzil, methylbenzoate, benzoic acid and benzaldehyde-dimethylacetal. An outline of the mechanism is proposed and plausible intermediates involved in the catalytic processes are proposed/established by UV–Vis and 51V NMR studies.