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

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[H 2 V V O 4 ] with ligands I – IV at neutral pH gives complexes [K(H 2 O) 2 ][VO 2 (dar‐inh)] ( 1 ), [K(H 2 O) 2 ][VO 2 (dar‐nah)] ( 2 ), [K(H 2 O) 2 ][VO 2 (dar‐bhz)] ( 3 ), and [K(H 2 O) 2 ][VO 2 (dar‐fah)] ( 4 ), respectively. The reaction of [V IV O(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 ( 1 H, 13 C 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(H 2 O)(EtOH)][V V O 2 (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, HClO 4 , and H 2 O 2 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 H 2 O 2 in the presence of NaHCO 3 as promoter, giving the corresponding epoxides selectively.