11B NMR Analysis of Catalyst Solutions Obtained from Bis(boratabenzene)zirconium Dichloride Complexes and Methylaluminoxane

Abstract

Boratabenzene complexes of the type [C5H5B-R]2ZrCl2 are activated by methylaluminoxane (MAO) to form catalyst mixtures that polymerize (R = dialkylamino) or oligomerize (R = alkyl, alkoxy) ethylene. The selectivity for specific olefin products is influenced by the exocyclic substituent on the boratabenzene ring. When [C5H5B-OEt]2ZrCl2(1)/MAO is exposed to 1 atm of ethylene, α-olefins are produced in greater than 99% purity, while [C5H5B-Me]2ZrCl2(4)/MAO/C2H4 gives a mixture of α-olefins, internal olefins, and vinylidenes. Other catalysts, such as [C5H5B-OCH2Ph]2ZrCl2(2)/MAO and [C5H5B-OCy]2ZrCl2(3)/MAO, oligomerize ethylene with selectivities for α-olefin production that are between those of 1/MAO and 4/MAO. The isomerization and dimerization of 1-decene by 1−4 with MAO agree well with the ethylene data. More selective catalysts (1/MAO) exhibit slow rates of isomerization, while less selective catalysts (4/MAO) rapidly convert 1-decene to 2-decene and 2-octyl-1-dodecene. The presence of boron in the ancillary ligands of these catalyst mixtures allows their reaction chemistry to be examined using 11B NMR spectroscopy. Signals are observed at 35 and 42 ppm for 1/MAO and 4/MAO, respectively. These signals remain unchanged over a 3 h period. For 2/MAO the initial resonance at 35 ppm diminishes with time and gives rise to a signal at 42 ppm. Two new zirconium complexes, [C5H5B-OMe]2ZrCl2(5)/MAO and [C5H5B-OCMe3]2ZrCl2(6)/MAO, were synthesized to further examine steric effects. With 5/MAO, no exchange was observed. A rapid exchange of exocyclic substituent occurs for 6/MAO. Ethylene oligomerization and 1-decene isomerization reactions with 5/MAO and 6/MAO support these observations. Different commercially available samples of MAO were also compared.