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

Zeolite-Y-encapsulated Cu(salen) and Cu(5-Cl-salen) complexes, where salen is N,N'-ethylenebis(salicylidenaminato), have been synthesized and characte rized by various physicochemical techniques. "Neat" complexes showed broad EPR spectra corresponding to nearest neighbor spin-spin interactions wherea s the zeolite-encapsulated metal complexes showed well-resolved metal hyper fine features similar to spectra in dilute frozen solutions indicating the encapsulation of monomeric salen complexes in zeolite cavities. Molecules a dsorbed on the external surface exhibit spectra similar to "neat" complexes . The spectra arise from an unpaired electron occupying a "formal" 3d(x)(2) -(2)(y) orbital. Calculated groundstate molecular orbital coefficients sugg est an increase in the inplane covalency of the Cu-ligand bond and depletio n of electron density at copper on encapsulation thereby facilitating the a ttack of incoming nucleophiles like tert-butyl hydroperoxide anion at the c opper atom. In a comparitive study of the catalytic activity of the "neat" and zeolite-encapsulated Cu(salen) complexes, the activity for the decompos ition of H2O2, tert-butyl hydroperoxide as well as the oxidation of phenol and para-xylene increased on encapsulation. The observed changes in the mol ecular and electronic structure of the complexes on encapsulation are, perh aps, responsible for the enhancement in catalytic activity. (C) 1999 Academ ic Press.