Zeolite-encapsulated copper(II) complexes of pyridoxal-based tetradentate ligands for the oxidation of styrene, cyclohexene and methyl phenyl sulfide

Abstract

Interaction of copper(II)-exchanged zeolite-Y with N,N′- ethylenebis(pyridoxyliminato) (H2pydx-en, I) and N,N′- propylenebis(pyridoxyliminato) (H2pydx-1,3-pn, II) ligands in refluxing methanol leads to the formation of the corresponding complexes, abbreviated herein as [Cu(pydx-en)]-Y (3) and [Cu(pydx-1,3-pn)]-Y (4), in the supercages of zeolite-Y. The neat complexes, [Cu(pydx-en)] (1) and [Cu(pydx-1,3-pn)] (2), have also been prepared with these ligands. Spectroscopic studies (IR, UV/Vis and EPR), elemental analyses, thermal studies, field emission scanning electron micrographs (FESEM) and X-ray diffraction patterns have been used to characterise these complexes. The crystal and molecular structures of 1 and of 2·CH3OH, have been determined, confirming the ONNO binding mode of the ligands. The geometry around the metal ion is very slightly distorted square-planar in 1 and distorted square-pyramidal in 2. The encapsulated complexes catalyse the oxidation, by H2O 2 and tert-butyl hydroperoxide, of styrene, cyclohexene and thioanisole efficiently. Under optimised reaction conditions, the oxidation of styrene catalysed by 3 and 4 gave 23.6% and 28.0% conversion, respectively, using tert-butyl hydroperoxide as oxidant, where styrene oxide, benzaldehyde, benzoic acid and phenylacetaldehyde are the major products. Better conversions have been obtained using H2O2 as oxidant. Oxidation of cyclohexene catalysed by these complexes gave cyclohexene oxide, 2-cyclohexen-1-ol, cyclohexane-1,2-diol and 2-cyclohexen-1-one as the major products. A maximum of 90.1% conversion of cyclohexene with 3 and 83.0% with 4 was obtained under optimised conditions. Similarly, a maximum of 80.3% conversion of methyl phenyl sulfide with 3 and 81.0% with 4 was observed, where the selectivity of the major product methyl phenyl sulfoxide was found to be about 60%. Tests for the recyclability and heterogeneity of the reactions have also been carried out, and the results indicate their recyclability. A possible reaction mechanism has been proposed by titrating a methanol solution of 1 and 2 with H2O2 to identify the possible intermediates. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.