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

The cuboctameric hydroxysilsesquioxane (c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>12</INF>(OH) (<BO>2</BO>), obtained after hydrolysis of (c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>12</INF>Cl (<BO>1</BO>), 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‘ ‘MCl<INF>3</INF> (M = Ti, Zr; Cp‘ ‘ = 1,3-C<INF>5</INF>H<INF>3</INF>(SiMe<INF>3</INF>)<INF>2</INF>) with silsesquioxane and siloxylithium or -thallium salts, [(c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF>]M‘ (M‘ = Tl (<BO>3</BO>), Li (<BO>4</BO>), Li·TMEDA (<BO>5</BO>)) or Ph<INF>3</INF>SiOTl gave either the dichloride complexes Cp‘ ‘[(c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF>]MCl<INF>2</INF> (M = Ti (<BO>6a</BO>), Zr (<BO>7a</BO>)) and Cp‘ ‘[Ph<INF>3</INF>SiO]TiCl<INF>2</INF> (<BO>8a</BO>) or the monochloride species Cp‘ ‘[(c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF>]<INF>2</INF>MCl (M = Ti (<BO>6b</BO>), Zr (<BO>7b</BO>)) and Cp‘ ‘[Ph<INF>3</INF>SiO]<INF>2</INF>MCl (M = Ti (<BO>8b</BO>), Zr (<BO>9</BO>)). Similarly, protonolysis of Cp‘ ‘MR<INF>3</INF> with the silanols <BO>2</BO> and Ph<INF>3</INF>SiOH yielded the corresponding silsesquioxane bis(alkyl) complexes Cp‘ ‘[(c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF>]TiR<INF>2</INF> (R = CH<INF>2</INF>Ph (<BO>10a</BO>), Me (<BO>10b</BO>)) and triphenylsiloxy bis(alkyl) compounds Cp‘ ‘[Ph<INF>3</INF>SiO]MR<INF>2</INF> (M = Ti, R = CH<INF>2</INF>Ph (<BO>11a</BO>), Me (<BO>11b</BO>); M = Zr, R = CH<INF>2</INF>Ph (<BO>12a</BO>)) and the monobenzyl complex Cp‘ ‘[Ph<INF>3</INF>SiO]<INF>2</INF>ZrCH<INF>2</INF>Ph (<BO>12b</BO>). When activated with MAO, not only the dichloride complexes (<BO>6a</BO>, <BO>7a</BO>, <BO>8a</BO>) but also the monochlorides (<BO>6b</BO>, <BO>7b</BO>, <BO>8b</BO>, <BO>9</BO>) yield active ethylene polymerization catalysts. The observation that even complexes containing a tridentate silsesquioxane ligand, [(c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>12</INF>]MCp‘ ‘ (M = Ti (<BO>13</BO>), Zr (<BO>14</BO>)), 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 (<BO>10</BO>, <BO>11</BO>, <BO>12a</BO>) form active olefin polymerization catalysts when activated with B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>, which leaves the M−O bond unaffected. Although the different cone angles of (c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF> (155°) and Ph<INF>3</INF>SiO (132°) suggest otherwise, the effective steric congestion around the metal center of (c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>13</INF>- and Ph<INF>3</INF>SiO-stabilized complexes was found to be reasonably comparable. The electronic differences between (c-C<INF>5</INF>H<INF>9</INF>)<INF>7</INF>Si<INF>8</INF>O<INF>12</INF>(OH) (<BO>2</BO>) and Ph<INF>3</INF>SiOH are more pronounced. pK<INF>a</INF> measurements and DFT calculations indicate that <BO>2</BO> is notably more Brønsted acidic and electron withdrawing than Ph<INF>3</INF>SiOH.