Silica-Grafted Borato Cocatalysts for Olefin Polymerization Modeled by Silsesquioxane−Borato Complexes

The syntheses and reactivity studies of silsesquioxane−borato complexes are described.
Treatment of B(C6F5)3 with (c-C5H9)7Si8O12(OH) and (c-C5H9)7Si7O9(OH)3 in the presence of
a Brønsted base yields the silsesquioxane−borates X+{[(c-C5H9)7Si8O13]B(C6F5)3}- (1a, X+
= PhN(H)Me2+; 1b, X+ = Et3NH+) and X+{[(c-C5H9)7Si7(OH)2O10]B(C6F5)3}- (1b, X+ = PhN(H)Me2+; 2b, X+ = Et3NH+), respectively. When the more nucleophilic base pyridine is used,
(C6F5)3B·NC5H5 (3) is formed instead, demonstrating the competition between B(C6F5)3 and
H+ to react with the amine. The dimethylaniline in 1a and 2a is readily exchanged by NEt3
to form 1b and 2b. With the nucleophilic Lewis base NC5H5, the B−O bond in 1a and 2a is
split, yielding (C6F5)3B·NC5H5 (3) and the free silsesquioxanes. Complexes 1 and 2 rapidly
undergo hydrolysis under formation of the hydroxyl complexes X+{(C6F5)3BOH}- (4a, X+ =
PhN(H)Me2+; 4b, X+ = Et3NH+). Likewise, alcoholysis of 1a and 2a with i-PrOH yields the
alkoxide {PhN(H)Me2}+{i-PrOB(C6F5)3}- (5). The B−O bond is only moderately stable toward
early-transition-metal alkyls. Nevertheless, Cp2Zr(CH2Ph)2 + 1a and Zr(CH2Ph)4 + 2a form
single-site ethylene polymerization catalysts. Detailed reactivity studies demonstrated that
both B−O and B−C bond splitting plays a crucial role, as not 1a and 2a, but their
decomposition product B(C6F5)3 is the actual cocatalyst. The solid-state structures of 1a and
4b were determined by single-crystal X-ray analysis.