The cuboctameric hydroxysilsesquioxane (c-C5H9)7Si8O12(OH) (2), obtained after hydrolysis
of (c-C5H9)7Si8O12Cl (1), and triphenylsilanol have been applied as model supports for silica-grafted olefin polymerization catalysts. The ligands were introduced on group 4 metals by
either chloride metathesis or protonolysis. Treatment of Cp‘ ‘MCl3 (M = Ti, Zr; Cp‘ ‘ = 1,3-C5H3(SiMe3)2) with silsesquioxane and siloxylithium or -thallium salts, [(c-C5H9)7Si8O13]M‘
(M‘ = Tl (3), Li (4), Li·TMEDA (5)) or Ph3SiOTl gave either the dichloride complexes Cp‘ ‘[(c-C5H9)7Si8O13]MCl2 (M = Ti (6a), Zr (7a)) and Cp‘ ‘[Ph3SiO]TiCl2 (8a) or the monochloride
species Cp‘ ‘[(c-C5H9)7Si8O13]2MCl (M = Ti (6b), Zr (7b)) and Cp‘ ‘[Ph3SiO]2MCl (M = Ti (8b),
Zr (9)). Similarly, protonolysis of Cp‘ ‘MR3 with the silanols 2 and Ph3SiOH yielded the
corresponding silsesquioxane bis(alkyl) complexes Cp‘ ‘[(c-C5H9)7Si8O13]TiR2 (R = CH2Ph (10a),
Me (10b)) and triphenylsiloxy bis(alkyl) compounds Cp‘ ‘[Ph3SiO]MR2 (M = Ti, R = CH2Ph
(11a), Me (11b); M = Zr, R = CH2Ph (12a)) and the monobenzyl complex Cp‘ ‘[Ph3SiO]2ZrCH2Ph (12b). When activated with MAO, not only the dichloride complexes (6a, 7a, 8a) but also
the monochlorides (6b, 7b, 8b, 9) yield active ethylene polymerization catalysts. The observation that even complexes containing a tridentate silsesquioxane ligand, [(c-C5H9)7Si8O12]MCp‘ ‘ (M = Ti (13), Zr (14)), form active ethylene polymerization catalysts when activated
with MAO indicates that silsesquioxane and siloxy ligands are easily substituted by MAO.
The silsesquioxane and siloxy bis(alkyl) complexes (10, 11, 12a) form active olefin polymerization catalysts when activated with B(C6F5)3, which leaves the M−O bond unaffected.
Although the different cone angles of (c-C5H9)7Si8O13 (155°) and Ph3SiO (132°) suggest
otherwise, the effective steric congestion around the metal center of (c-C5H9)7Si8O13- and
Ph3SiO-stabilized complexes was found to be reasonably comparable. The electronic
differences between (c-C5H9)7Si8O12(OH) (2) and Ph3SiOH are more pronounced. pK
a
measurements and DFT calculations indicate that 2 is notably more Brønsted acidic and
electron withdrawing than Ph3SiOH.