Structure–Catalytic Activity Relationship in Bridging Silacycloalkyl Ring Conformations of Constrained Geometry Titanium Complexes

A series of cyclic silylene‐bridged (amidocyclopentadienyl)dichlorotitanium(IV) complexes [TiCl 2 {η 5 ‐1‐(CySi t BuN‐ κ N )‐2,3,4,5‐R 4 ‐C 5 }] was prepared, where CySi = silacyclobutyl ( a ), silacyclopentenyl ( b ), silacyclopentyl ( c ), and silacyclohexyl ( d ); R = H ( 4 ), Me ( 5 ). The starting silane, dichlorosilacycloalkane CySiCl 2 ( 1 ), was treated with NaCp (LiCp*), followed by LiNH t Bu to yield the cyclic silylene‐bridged ligands (R 4 C 5 )CySi(NH t Bu) [R = H ( 2 ); Me ( 3 )]. Subsequent deprotonation with n ‐butyllithium, followed by transmetalation with TiCl 4 yielded the desired constrained geometry complexes (CGCs) (CpCySiN t Bu)TiCl 2 ( 4 ) and (Cp*CySiN t Bu)TiCl 2 ( 5 ). The structures of the resulting cyclopentadienyl‐ ( 4b and 4c ) and tetramethylcyclopentadienyl(silacycloalkyl)amidotitanium(IV) dichloride ( 5a , 5c , and 5d ) species were studied by using X‐ray crystallography to obtain geometrical information on cyclic silylene‐modified CGCs. The ethylene polymerization by the cyclic silylene‐bridged CGCs 4 and 5 was examined to verify the structure–catalytic activity relationship derived from variation of the size of a cyclic silylene ring. Indeed, the size of the cyclic silylene ring at the 1,1′‐position of 4 and 5 affected the catalytic activities through the ethylene polymerization. Systematic increase in the catalytic activities was observed as the cyclic silylene‐bridging unit was expanded from a four‐ to six‐membered ring. In the present study, we found that CGCs of Ti IV with a six‐membered silylene‐bridged ligand ( 5d ) produced active catalytic species for the formation of polyethylene with M w = 42.7 × 10 –4 g mol –1 and M w / M n = 2.1 with excellent catalytic activities (20.9 kg polymer per mmol of Ti). When titanium(IV) dimethyl complex ( 6d ) was applied in continuous polymerization process, impressive high catalytic activity on copolymerization with 1‐octene was observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)