Catalytic oxidation of benzyl alcohols by new Cu(II) complexes of 1,3-oxazolidine based ligand obtained from a solvent free reaction

Abstract

A new 1,3-oxazolidine based ligand, (2-pyridin-2-yl-1,3-oxazolidine-4,4-diyl)dimethanol (H3L), has been synthesized from the reaction of 2-amino-2-(hydroxymethyl)-1,3-propanediol and 2-pyridinecarboxaldehyde at 110 °C. Two mononuclear Cu(II) complexes, [Cu(H3L)Cl2]·CH3OH (1) and [Cu(H3L)2]·2Cl (2), have been synthesized with a similar procedure by the reaction of H3L and CuCl2·H2O in 1:1 and 2:1 M ratios in methanol. The ligand and complexes have been characterized by elemental analysis and spectroscopic methods. The structure of complexes has been characterized by single-crystal X-ray diffraction analysis which showed the copper ion has a distorted square pyramidal geometry in 1 and distorted octahedral geometry in 2. The 1,3-oxazolidine ligand acts as a neutral tridentate N2O-donor ligand in both 1 and 2. Two chloride anions remain coordinated to Cu(II) ion in complex 1, but they act as counter ions in complex 2. The catalytic activity of these complexes has been investigated in the oxidation of benzyl alcohol by using H2O2 or TBHP as oxidant. The effects of some influential parameters in catalytic reactions like molar ratio of oxidant-to-substrate, temperature, nature of oxidant and solvent have been studied to obtain the optimum condition. Moreover, the effects of chlorine substituent on the phenyl group (ortho-, meta- and para-positions) has also been explored. The results of these studies show that both 1 and 2 catalyze the oxidation of benzyl alcohol derivatives to the corresponding benzaldehydes and a little amount of the related benzoic acids, produced by over oxidation of benzaldehyde, are also detected in the reaction mixtures. The results indicate that the selectivity and the activity of these catalytic systems depend on the reaction conditions and the steric and electronic properties of the reagents.