4,6-Diacetyl Resorcinol Based Vanadium(V) Complexes: Reactivity and Catalytic Applications

Abstract

Four ONO donor ligands are isolated from the condensation of 4,6-diacetyl resorcinol with isonicotinoyl hydrazide (H2dar-inh, I), nicotinoyl hydrazide (H2dar-nah, II), benzoyl hydrazide (H2dar-bhz, III), and 2-furoyl hydrazide (H2dar-fah, IV) on refluxing in MeOH. The reaction of in situ generated aqueous K[H2VVO4] with ligands I–IV at neutral pH gives complexes [K(H2O)2][VO2(dar-inh)] (1), [K(H2O)2][VO2(dar-nah)] (2), [K(H2O)2][VO2(dar-bhz)] (3), and [K(H2O)2][VO2(dar-fah)] (4), respectively. The reaction of [VIVO(acac)2] (acac = acetylacetonato) with these ligands (I–IV) under aerobic conditions in methanol yields oxidomethoxidovanadium(V) complexes [VO(OMe)(MeOH)(dar-inh)] (5), [VO(OMe)(MeOH)(dar-nah)] (6), [VO(OMe)(MeOH)(dar-bhz)] (7), and [VO(OMe)(MeOH)(dar-fah)] (8). All the isolated complexes are characterized by elemental, thermal, electrochemical, and spectroscopic techniques [FTIR, UV/Vis, NMR (1H, 13C and 51V NMR)], and single-crystal X-ray diffraction analysis (for 1, 6, 7, and 8). X-ray analysis confirms the coordination of the ligands through Ophenolate, Nazomethine, and Oenolate to the metal center. In the molecular structure of [K(H2O)(EtOH)][VVO2(dar-inh)] (abbreviated as 1a where one molecule of water is replaced by EtOH), water molecules act as bridges between two K+ ions and the complex shows a dimeric structure due to the presence of electrostatic interactions between V=O oxygen atoms with K+ ions. These complexes are active catalysts for the oxidative bromination of thymol in the presence of KBr, HClO4, and H2O2 and give 2-bromothymol, 4-bromothymol, and 2,4-dibromothymol as major products. Complexes 1–4 were also tested as catalysts for the epoxidation of various alkenes (namely styrene, cyclohexene, cis-cyclooctene, 1-hexene, 1-octene, cyclohexenone, and trans-stilbene) with H2O2 in the presence of NaHCO3 as promoter, giving the corresponding epoxides selectively. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim