Structure-catalytic activity relationship in bridging silacycloalkyl ring conformations of constrained geometry titanium complexes

Abstract

A series of cyclic silylene-bridged (amidocyclopentadienyl)-dichlorotitanium(IV) complexes [TiCl2{eta(5)-1-(CySitBuN-kappa 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, dichloro-silacycloalkane 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 Ti-IV with a six-membered silylene-bridged ligand (5d) produced active catalytic species for the formation of polyethylene with M-W = 42.7 X 10(-4) g mol(-1) and M-W/M-n = 2.1 with excellent catalytic activities (20.9 kgpolymer 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim., Germany, 2008).