Zeolite‐Encapsulated Copper(II) Complexes of Pyridoxal‐Based Tetradentate Ligands for the Oxidation of Styrene, Cyclohexene and Methyl Phenyl Sulfide

Interaction of copper(II)‐exchanged zeolite‐Y with N , N′ ‐ethylenebis(pyridoxyliminato) (H 2 pydx‐en, I ) and N , N′ ‐propylenebis(pyridoxyliminato) (H 2 pydx‐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 (FE‐SEM) and X‐ray diffraction patterns have been used to characterise these complexes. The crystal and molecular structures of 1 and of 2· CH 3 OH, 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 H 2 O 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 H 2 O 2 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 H 2 O 2 to identify the possible intermediates.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)