The structural basis for the enhanced oxygenase activity of copper acetatedimers encapsulated in zeolites

The oxygenase mimicking activity of copper acetate dimers in the regioselec tive ortho-hydroxylation of L-tyrosine to L-dopa is enhanced on encapsulati on in zeolite Y. The structure and magnetic properties of the catalytic act ive site were characterized by EPR spectroscopy. The spectra of this "zeozy me" reveal the presence of (1) copper acetate dimers in the supercages and (2) isolated Cu(II) ions in the sodalite cages of the zeolite. There are si gnificant differences in the EPR spectra of the "neat" and encapsulated com plexes: on encapsulation in zeolite, the Cu-Cu exchange coupling constant, -J, increases to 310 from 259 cm(-1) for the "neat" complex (i.e., by about 19.7%). Simultaneously the Cu-Cu separation in the dimer, estimated indire ctly from the exchange coupling constant, shortens to 2.40 Angstrom in the encapsulated state from 2.64 Angstrom in the "neat" complex. There is, henc e, a relatively greater overlap of the metal orbitals of the dimer copper a toms inside the restricted confines of the zeolite cages. The consequent, e nhanced, trans axial lability of the phenolate and dioxygen ligands promote s the catalytic oxygenase activity of copper acetate dimers on encapsulatio n in zeolites. A causal relationship between changes in the structural feat ures of an active site on encapsulation in the zeolite and the correspondin g catalytic activity has, thus, been established.