Bifunctional Lewis Acids. Synthesis and Olefin Polymerization Chemistry of the 1,1-Di[bis(perfluorophenyl)boryl]alkenes RCHC[B(C₆F₅)₂]₂ (R = <i>t</i>-Bu, C₆H₅, C₆F₅)

Abstract

The synthesis and characterization, including crystallographic analysis, of the bifunctional boranes RCH C[B(C 6 F 5 ) 2 ] 2 (R = t -C 4 H 9, 1a; C 6 H 5, 1b; C 6 F 5, 1c ) by regioselective hydroboration of the corresponding 1-boraalkynes using HB(C 6 F 5 ) 2 are reported herein. Compounds 1a and 1b have been screened as cocatalysts for ethylene polymerization in the presence of Cp 2 ZrMe 2 ( 3 ) under a variety of conditions. NMR spectroscopic studies indicate that Cp 2 Zr[η 2 -Bu t C⋮CB(C 6 F 5 ) 2 ] ( 4a ), Cp 2 ZrMe(C 6 F 5 ), the organoborane Me 2 BC 6 F 5, 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(C 6 F 5 ) 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 Cp 2 ZrMe 2 and 7 gives rise to the species [Cp 2 ZrMe] + [Me 2 B(C 6 F 5 ) 2 ] - ( 8 ), which although unstable at room temperature in solution (decomposing over a period of 60 min to give Cp 2 ZrMe(C 6 F 5 ) and the organoborane Me 2 BC 6 F 5 ), 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 .