Oxidative bromination of monoterpene (thymol) using dioxidomolybdenum(VI) complexes of hydrazones of 8-formyl-7-hydroxy-4-methylcoumarin

Abstract

Condensation of equimolar amount of 8-formyl-7-hydroxy-4-methylcoumarin (fhmc) and hydrazides [benzoylhydrazide (bhz), isonicotinoylhydrazide (inh), nicotinoylhydrazide (nah) and furoylhydrazide (fah)] in methanol results in the formation of potential ONO tridentate ligands H2fhmc-bhz (I), H2fhmc-inh (II), H2fhmc-nah (III), and H2fhmc-fah (IV), respectively. Reaction of these ligands with [MoVIO2(acac)2] in refluxing methanol gives dioxidomolybdenum(VI) complexes, [MoVIO2(fhmc-bhz)(MeOH)] (1), [MoVIO2(fhmc-inh)(MeOH)] (2), [MoVIO2(fhmc-nah)(MeOH)] (3) and [MoVIO2(fhmc-fah)(MeOH)] (4), respectively. The structures of the obtained ligand and their respective metal complexes were elucidated by elemental analyses, spectroscopic techniques (IR, electronic, 1H and 13C NMR) and thermogravimetric analyses. Single crystal X-ray analysis of complex 1a confirms the chelation of the ligand to the metal ion through two oxygen and a nitrogen atoms. These metal complexes have been tested against oxidative bromination of monoterpene (thymol) to yield 2-bromothymol, 4-bromothymol and 2,4-dibromothymol using H2O2 as an oxidant therefore they act as functional models of vanadium dependent haloperoxidases. The effects of various factors, such as amounts of catalyst, oxidant, KBr, HClO4 and different solvents have been considered to optimize the reaction conditions for the maximum brominated products.