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

The complexes [Py(Pz)₂]Zn(OC([dbnd]O)Me)₂(1) and [Py(Me₂Pz)₂]Zn(OC([dbnd]O)Me)₂(2), where ligands Py(Pz)₂and Py(Me₂Pz)₂are 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 CO₂and 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, 10 bar CO₂pressure, 0.1 mol% catalyst loading, and 24 h. The reaction temperature, CO₂pressure, 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 CO₂to propylene oxide, which showed the highest reactivity among seven other kinds of epoxides.