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

Abstract

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.

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

“…A previous study has used triphenylsilanol [27], but polyhedral oligomeric silsesquioxanes (POSS) are considered better models since they display pK a values closer to that of silica [53]. Of the various POSSÕs, (c-C 5 H 9 ) 7 Si 7 O 12 SiOH is considered as a good model for [37,38,54].…”

Straightforward synthesis and structural characterization of the first alkoxy-zircono-silsesquioxanes — Potential models for zirconia–silica epoxidation catalysts

“…Metal derivatives of the cage silsesquioxanes, R 7 Si 7 O 12 H 3 (see Scheme 1) have been during the last decade an object of intensive structure and reactivity studies [1][2][3]. They provide, on one hand, as reliable models, a direct insight into the coordination of metal centers with a silica support for the modeling of catalyst structures [4][5][6], where, in particular, the structure of highly attractive zirconia-silica epoxidation catalysts remain insufficiently investigated [4].…”

Synthesis, characterization and ethylene polymerization by metallasilsesquioxane

“…B(C 6 F 5 ) 3 or MAO were used as co-catalyst in these systems. [14] A noteworthy feature of polyethylene obtained with catalysts 1-6 is the invariably low molecular weight of the polymer (M w = 600-3000) [carbon range~C 40 -C 100 ] of the polymer as revealed by GPC analysis. In all cases, the PEs display narrow molecular weight distribution (M w /M n = 1.5-2.1).…”