EPR and catalytic investigation of Cu(Salen) complexes encapsulated in zeolites

Abstract

Zeolite-Y-encapsulated Cu(salen) and Cu(5-Cl-salen) complexes, where salen is N,N′-ethylenebis(salicylidenaminato), have been synthesized and characterized by various physicochemical techniques. “Neat” complexes showed broad EPR spectra corresponding to nearest neighbor spin–spin interactions whereas the zeolite-encapsulated metal complexes showed well-resolved metal hyperfine features similar to spectra in dilute frozen solutions indicating the encapsulation of monomeric salen complexes in zeolite cavities. Molecules adsorbed on the external surface exhibit spectra similar to “neat” complexes. The spectra arise from an unpaired electron occupying a “formal” 3dx2−y2 orbital. Calculated ground-state molecular orbital coefficients suggest an increase in the in-plane covalency of the Cu–ligand bond and depletion of electron density at copper on encapsulation thereby facilitating the attack of incoming nucleophiles like tert-butyl hydroperoxide anion at the copper atom. In a comparitive study of the catalytic activity of the “neat” and zeolite-encapsulated Cu(salen) complexes, the activity for the decomposition of H2O2, tert-butyl hydroperoxide as well as the oxidation of phenol and para-xylene increased on encapsulation. The observed changes in the molecular and electronic structure of the complexes on encapsulation are, perhaps, responsible for the enhancement in catalytic activity