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

Dimeric copper acetate and chloroacetate complexes (CuAc and CuClAc, respec tively), were encapsulated in zeolite-Y by the flexible ligand synthesis me thod. 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 ca talytic properties are examined. The separation (av) between nu(as)(COO-) a nd nu(s)(COO-) bands in the FT-IR spectra changes from 182 to 213 cm(-1) fo r 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. Va riable temperature EPR studies (77-298 K) indicate an antiferromagnetic int eraction between the two Cu(II) ions in the dimers. The magnitude of the ex change interaction between the two Cu(II) ions and molecular symmetry for b oth the complexes change as a consequence of encapsulation. The Cu-Cu separ ation 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 convers ion increases significantly upon encapsulation. Due to the enhanced Cu-Cu b inding upon encapsulation, the strength and lability of the Cu-phenolate an d Cu-dioxygen bonds are modified by a trans-axial ligand effect accounting for the enhanced reactivity of the encapsulated complex. (C) 2000 Academic Press.