Structure and catalytic properties of dimeric copper(II) acetato complexes encapsulated in zeolite-Y

Dimeric copper acetate and chloroacetate complexes (CuAc and CuClAc, respectively), were encapsulated in zeolite-Y by the flexible ligand synthesis method. Spectroscopic techniques (FT-IR, diffuse reflectance UV-visible, and EPR) and thermal analysis provide convincing evidence for the formation of acetato-bridged dimeric copper(II) complexes in the supercages of zeolite-Y. The effects of encapsulation on the geometric, magnetic, spectral, and catalytic properties are examined. The separation (av) between nu(as)(COO-) and nu(s)(COO-) bands in the FT-IR spectra changes from 182 to 213 cm(-1) for CuAc and 185 to 205 cm(-1) for CuClAc upon encapsulation and corresponds to the syn-syn mode of coordination for the bridging carboxylato groups. Variable temperature EPR studies (77-298 K) indicate an antiferromagnetic interaction between the two Cu(II) ions in the dimers. The magnitude of the exchange interaction between the two Cu(II) ions and molecular symmetry for both the complexes change as a consequence of encapsulation. The Cu-Cu separation in the dimer decreases upon encapsulation, from 2.64 to 2.40 Angstrom for CuAc and from 2.92 to 2.73 Angstrom for CuClAc. The complexes catalyze the o-hydroxylation of phenols to catechols and additional oxidation to o-benzoquinone by molecular oxygen. The turnover frequency for phenol conversion increases significantly upon encapsulation. Due to the enhanced Cu-Cu binding upon encapsulation, the strength and lability of the Cu-phenolate and Cu-dioxygen bonds are modified by a trans-axial ligand effect accounting for the enhanced reactivity of the encapsulated complex. (C) 2000 Academic Press.