Half-Sandwich Group 4 Metal Siloxy and Silsesquioxane Complexes:  Soluble Model Systems for Silica-Grafted Olefin Polymerization Catalysts

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)7Si8O13M‘ (M‘ = Tl (3), Li (4), Li·TMEDA (5)) or Ph3SiOTl gave either the dichloride complexes Cp‘ ‘(c-C5H9)7Si8O13MCl2 (M = Ti (6a), Zr (7a)) and Cp‘ ‘Ph3SiOTiCl2 (8a) or the monochloride species Cp‘ ‘(c-C5H9)7Si8O132MCl (M = Ti (6b), Zr (7b)) and Cp‘ ‘Ph3SiO2MCl (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)7Si8O13TiR2 (R = CH2Ph (10a), Me (10b)) and triphenylsiloxy bis(alkyl) compounds Cp‘ ‘Ph3SiOMR2 (M = Ti, R = CH2Ph (11a), Me (11b); M = Zr, R = CH2Ph (12a)) and the monobenzyl complex Cp‘ ‘Ph3SiO2ZrCH2Ph (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)7Si8O12MCp‘ ‘ (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. pKa measurements and DFT calculations indicate that 2 is notably more Brønsted acidic and electron withdrawing than Ph3SiOH.