Liquid phase versus microwave assisted selective oxidation of volatile organic compounds involving dioxidomolybdenum(VI) and oxidoperoxidomolybdenum(VI) complexes as catalysts in the presence/absence of an N-based additive

Abstract

Four dioxidomolybdenum(VI) complexes, [MoVIO2(dha-bhz)(MeOH)]·(H2dha-bhz) (1), [MoO2(dha-inh)(MeOH)] (2), [MoO2(dha-nah)(MeOH)] (3) and [MoO2(dha-fah)(MeOH)] (4), and their corresponding oxidoperoxidomolybdenum(VI) complexes, [MoO(O2)(dha-bhz)(MeOH)] (5), [MoO(O2)(dha-inh)(MeOH)] (6), [MoO(O2)(dha-nah)(MeOH)] (7) and [MoO(O2)(dha-fah)(MeOH)] (8), with ONO tridentate Schiff base ligands derived from 3-acetyl-6-methyl-(2H)-pyran-2,4(3H)-dione (dehydroacetic acid, Hdha) and aromatic hydrazides, {benzoyl hydrazide (bhz), isonicotinoyl hydrazide (inh), nicotinoyl hydrazide (nah) and furoyl hydrazide (fah)} have been synthesized and characterized by elemental analysis, spectroscopic techniques (infrared, UV-Vis, 1H and 13C NMR) and thermogravimetric analyses. The structures of complexes 1 and 2, confirmed by single crystal X-ray study, reveal that the tridentate ligand binds to the metal centers through the enolic oxygen (of the pyrone group), azomethine nitrogen and enolic oxygen (of the hydrazide group) atoms; methanol is coordinated at one site through the oxygen atom. The dioxidomolybdenum(VI) complexes have been tested as catalysts for the homogeneous oxidation of secondary alcohols (1-phenyl ethanol, 2-propanol and 2-butanol), using 30% H2O2 as an oxidant. Both microwave and conventional liquid phase oxidation methods have been tested for catalytic reactions. Various parameters, such as amount of catalyst, oxidant, solvent and temperature, of the reaction mixture have been taken into consideration for the maximum conversion of the substrates. Under the atmospheric optimized reaction conditions, secondary alcohols give high yields of the respective ketones selectively. Addition of an N-based additive reduces the time and increases the conversion of alcohols. Amongst the two methods studied, the microwave technique proves to be a time efficient system. © 2015 Elsevier Ltd. All rights reserved.