| Abstract |
Four ONO donor ligands are isolated from the condensation of 4,6-diacetyl resorcinol with isonicotinoyl hydrazide (H(2)dar-inh, I), nicotinoyl hydrazide (H(2)dar-nah, II), benzoyl hydrazide (H(2)dar-bhz, III), and 2-furoyl hydrazide (H(2)dar-fah, IV) on refluxing in MeOH. The reaction of in situ generated aqueous K[(H2VO4)-O-V] 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 [(VO)-O-IV(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 (H-1, C-13 and (51) V NMR)], and single-crystal X-ray diffraction analysis (for 1, 6, 7, and 8). X-ray analysis confirms the coordination of the ligands through O-phenolate, N-azomethine, and O-enolate to the metal center. In the molecular structure of [K(H2O)(EtOH)][(VO2)-O-V(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. |
