A Triphenylsiloxy Complex of Chromium(II) as a Switchable Catalyst for Ethylene Polymerization and Nonselective Oligomerization

A novel dinuclear oxygen-bridged chromium(II) complex of [(Ph3SiO)Cr center dot(THF)](2)(mu-OSiPh3)(2) (1) was successfully synthesized and structurally characterized. The crystal structure of complex 1 shows that the chromium centers adopt a tetrahedrally distorted square-planar coordination geometry. Complex 1 was tested for ethylene polymerization upon activation with and without aluminum alkyl cocatalysts. No ethylene polymerization activity was observed in the absence of aluminum alkyl cocatalysts; however, in the presence of methylaluminoxane (MAO), complex 1 demonstrates an unexpected transformation from ethylene polymerization to ethylene oligomerization, with the molar ratio of Al to Cr increasing from 50 to 1000. The activation of complex 1 with MAO was investigated by ESR and Si-29 NMR spectroscopy. It was found that the Ph3SiO- groups could be transferred from the chromium centers of complex 1 to the aluminum of MAO.