Bifunctional Lewis Acids. Synthesis and Olefin Polymerization Chemistry of the 1,1-Di[bis(perfluorophenyl)boryl]alkenes RCHC[B(C6F5)2]2 (R = t-Bu, C6H5, C6F5)
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Abstract
The synthesis and characterization, including crystallographic analysis, of the bifunctional boranes RCHC[B(C6F5)2]2 (R = t-C4H9, 1a; C6H5, 1b; C6F5, 1c) by regioselective hydroboration of the corresponding 1-boraalkynes using HB(C6F5)2 are reported herein. Compounds 1a and 1b have been screened as cocatalysts for ethylene polymerization in the presence of Cp2ZrMe2 (3) under a variety of conditions. NMR spectroscopic studies indicate that Cp2Zr[η2-ButC⋮CB(C6F5)2] (4a), Cp2ZrMe(C6F5), the organoborane Me2BC6F5, and methane gas are the final products formed from reaction of 1a with 3 in toluene solution at room temperature. The stoichiometric mechanism for this transformation has been elucidated through variable-temperature NMR studies. Complex 4a and MeB(C6F5)2 (7) were prepared independently and screened as ethylene polymerization catalysts and cocatalysts, respectively. Compound 4a is inactive for ethylene polymerization, either alone or in the presence of additional 1a. However, the combination of Cp2ZrMe2 and 7 gives rise to the species [Cp2ZrMe]+[Me2B(C6F5)2]- (8), which although unstable at room temperature in solution (decomposing over a period of 60 min to give Cp2ZrMe(C6F5) and the organoborane Me2BC6F5), is active for ethylene polymerization. From a comparison of activity and MW data, it is concluded that the putative ion pairs formed from 1a (or 1b) and 3 lack sufficient thermal stability at conventional polymerization temperatures and that the polymerization activity observed can be interpreted as arising from species 8.
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