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

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AbstractA series of cyclic silylene‐bridged (amidocyclopentadienyl)dichlorotitanium(IV) complexes [TiCl2{η5‐1‐(CySitBuN‐κN)‐2,3,4,5‐R4‐C5}] was prepared, where CySi = silacyclobutyl (a), silacyclopentenyl (b), silacyclopentyl (c), and silacyclohexyl (d); R = H (4), Me (5). The starting silane, dichlorosilacycloalkane CySiCl2 (1), was treated with NaCp (LiCp*), followed by LiNHtBu to yield the cyclic silylene‐bridged ligands (R4C5)CySi(NHtBu) [R = H (2); Me (3)]. Subsequent deprotonation with n‐butyllithium, followed by transmetalation with TiCl4 yielded the desired constrained geometry complexes (CGCs) (CpCySiNtBu)TiCl2 (4) and (Cp*CySiNtBu)TiCl2 (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 TiIV with a six‐membered silylene‐bridged ligand (5d) produced active catalytic species for the formation of polyethylene with Mw = 42.7 × 10–4 g mol–1 and Mw/Mn = 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)