Synthesis, characterization, and cycloaddition reaction studies of zinc(II) acetate complexes containing 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine ligands

Abstract

The complexes [Py(Pz)2]Zn(OC(O)Me)2 (1) and [Py(Me2Pz)2]Zn(OC(O)Me)2 (2), where ligands Py(Pz)2 and Py(Me2Pz)2 are tridentate 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine, respectively, have been synthesized and characterized. The single crystal X-ray diffraction analysis confirmed compound 2 to be monomeric with six-coordinate zinc center. In addition to tridentate ligand Py(Me2Pz)2, both κ1-acetate and κ2-acetate ligands are ligated to zinc metal atom in 2. The synthesized complexes 1 and 2 were used as effective catalysts for the cycloaddition between CO2 and epoxides in the presence of various kinds of cocatalysts such as n-Bu4PBr, n-Bu4NI, n-Bu4NBr, n-Bu4NCl, PPNCl, and DMAP under the condition of 75°C, 10bar CO2 pressure, 0.1mol% catalyst loading, and 24h. The reaction temperature, CO2 pressure, and catalyst loading ratio applied in this study are somewhat milder condition than those for other reported zinc-based catalysts. In addition, 1/n-Bu4PBr system showed the best catalytic activity for the cycloaddition of CO2 to propylene oxide, which showed the highest reactivity among seven other kinds of epoxides.