Ethylene Polymerization with Dimeric Zirconium and Hafnium Silsesquioxane Complexes

Treatment of the silanol ( c -C 5 H 9 ) 7 Si 8 O 12 (OH) with Cp‘‘Ti(CH 2 Ph) 3 (Cp‘‘ = 1,3-C 5 H 3 (SiMe 3 ) 2 ) or TiCl 4 selectively affords the mono(silsesquioxane) complexes Cp‘‘[( c -C 5 H 9 ) 7 Si 8 O 13 ]Ti(CH 2 Ph) 2 ( 1 ) and [( c -C 5 H 9 ) 7 Si 8 O 13 ]TiCl 3 ( 2 ), respectively, while with M(CH 2 Ph) 4 (M = Ti, Zr, Hf) mixtures of products were obtained. When the disilanol ( c -C 5 H 9 ) 7 Si 7 O 9 (OSiMe 3 )(OH) 2 is reacted with M(CH 2 Ph) 4 (M = Ti, Zr), the bis(silsesquioxane) complexes [( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMe 3 )] 2 M (M = Ti ( 3 ), Zr ( 4 ), Zr·2THF ( 5 )) are formed exclusively. With (PhCH 2 ) 2 ZrCl 2 ·OEt 2 as precursor, the mono(silsesquioxane) complex [( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMe 3 )]ZrCl 2 ·2THF ( 6 ) can be isolated. M(CH 2 Ph) 4 (M = Ti, Zr, Hf) reacts smoothly with the tris(silanol) ( c -C 5 H 9 ) 7 Si 7 O 9 (OH) 3 ( III ), giving the metallasilsesquioxane benzyl species, {[( c -C 5 H 9 ) 7 Si 7 O 12 ]MCH 2 Ph} n (M = Ti, n = 1 ( 7 ); M = Zr, n = 2 ( 8 ); M = Hf, n = 2 ( 9 )). Compounds 5 and 8 have been characterized by X-ray analysis. Dimer 8 consists of a zwitterionic-like structure with two electronically different metal sites. M−C bond hydrogenolysis of 8 and 9 affords the corresponding hydrides, which are active α-olefin hydrogenation catalysts. Without cocatalyst, the neutral dimers 8 and 9 are poor, though active ethylene polymerization catalysts (activity: (5−10) × 10 3 g PE/(mol·h)). Addition of B(C 6 F 5 ) 3 affords the cationic, mono(benzyl) complexes {[( c -C 5 H 9 ) 7 Si 7 O 12 ] 2 M 2 (CH 2 Ph)} (+) (M = Zr, Hf): single-site catalysts (activity: (2−8) × 10 6 g PE/(mol·h)) that are considerably more active than the neutral 8 and 9 . Whereas titanasilsesquioxanes 3 and 7 do not react with THF, the corresponding zirconasilsesquioxanes 4 and 8 form bis(THF) adducts, [( c -C 5 H 9 ) 7 Si 7 O 11 (OSiMe 3 )] 2 Zr·2THF ( 5 ) and [( c -C 5 H 9 ) 7 Si 7 O 12 ]ZrCH 2 Ph·2THF ( 10 ), which suggests that the titanium complexes are less electrophilic than the zirconium ones. Accordingly, the titanium complex 7 does not react with dihydrogen and is inactive in ethylene polymerization.