Electronic ‎effects of ‎aromatic rings ‎on the catalytic ‎activity of dioxidomolybdenum(VI)-‎ hydrazone ‎complexes

Nine dioxidomolybdenum(VI) complexes were synthesized by the reaction of MoO3 with tridentate hydrazone Schiff base ligands obtained from the reaction of aromatic acid hydrazides (3-hydroxy-2-naphthoic acid hydrazide, 4-pyridine carboxylic acid hydrazide or 2-furane carboxylic acid hydrazide) and ortho-hydroxy aldehyde derivatives (5-iodo-2-hydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde or 2-hydroxy-3-methoxybenzaldehyde). All ligands and complexes were characterized by elemental analysis and spectroscopic methods. The structures of seven complexes were further elucidated by single-crystal X-ray diffraction analysis which indicated a distorted octahedral geometry at the metal centre. Spectroscopic and X-ray analyses indicated that the ligands are coordinated to the molybdenum(VI) ion as dinegative ligands due to deprotonation of phenolic OH and amidic NH groups upon complexation. These complexes were used as catalysts in the oxidation of cyclooctene and thioanisole in the presence of hydrogen peroxide as environmental friendly oxidant. In order to achieve the highest catalytic activity, the effects of important parameters such as solvent, temperature and the molar ratio of oxidant to substrate were optimized. The results indicate that electron-withdrawing substituents on the ligands increase the catalytic activity of dioxidomolybdenum(VI)-hydrazone complexes.