Methyl Aluminosilsesquioxanes, Models for Lewis Acidic Silica-Grafted Methyl Aluminum Species

The hydroxysilsesquioxanes ( c -C 5 H 9 ) 7 Si 8 O 12 (OH) ( I ) and ( c -C 5 H 9 ) 7 Si 7 O 9 (OH) 2 OSiMePh 2 ( II ) have been studied as model supports for silica-grafted aluminum alkyl species. Treatment of AlMe 3 with I gave polymeric {[( c -C 5 H 9 ) 7 Si 8 O 13 ]AlMe 2 } n ( 1a ), which is readily transformed into the corresponding monomeric pyridine adduct, {[( c -C 5 H 9 ) 7 Si 8 O 13 ]AlMe 2 ·Py ( 1b ). When AlMe 3 was reacted with II, noticeable amounts of the 2:1 product {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )](AlMe 2 ) 2 } 2 ( 2 ) and the Brønsted acidic 1:2 product {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 Al - }{H + } ( III ) were obtained besides the main product of the reaction, {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )]AlMe} 2 ( 3a − c ). The main product is a mixture of three dimeric conformational isomers all with the aluminum methyls trans to each other. The difference of the conformers originates from the different orientation of the silsesquioxane ligands. Reaction of the Brønsted acid III with AlMe 3 yielded the kinetic product {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 Al 2 Me 2 ( 4 ). The kinetic and thermodynamic stability of the three conformeric methyl aluminosilsesquioxanes {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )]AlMe} 2 ( 3a − c ) and their chemical isomer {[( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 Al 2 Me 2 ( 4 ) has been investigated. Isomerization experiments showed that 3a isomerizes to 3b, which subsequently isomerizes to 3c, affording the thermodynamically most stable mixture with a 3a: 3b: 3c ratio of 1:4:4 after 400 h at 76 °C. Isomerization of 3a to 3b is considerably faster than from 3b to 3c . Direct conversion of 3a into 3c was not observed. Complex 4 slowly isomerizes into 3c, which consecutively isomerizes into the thermodynamic most stable isomeric mixture (1000 h at 76 °C, E a = 117 kJ·mol - 1 ). Treating Cp 2 ZrMe 2 with the Brønsted acid III gave clean transfer of a silsesquioxane ligand to zirconium, yielding [( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )]ZrCp 2 ( 5 ). The methyl aluminosilsesquioxanes 1a and 2 − 4 are not Lewis acidic enough to effectively abstract a substituent X from Cp 2 ZrX 2 (X = Me, CH 2 Ph, Cl). Though, 3a − c and 4 definitely interact with Cp 2 ZrX 2 . Dependent on the substituent X, the zirconocene can accelerate the rate of isomerization over 2 orders of magnitude ( 3a, 1.5 h; 4, 8 h at 76 °C). Surprisingly, complex 4 also reacts with the strongly Lewis acidic B(C 6 F 5 ) 3 . As soon as all 4 has been converted into 3a − c, the accelerating effect stops, which demonstrates that Lewis acids have no effect on the isomerization of 3a − c . Complexes 2, 3a, 3c, 4, and 5 have been structurally characterized.

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