219868-13-0Relevant academic research and scientific papers
New syntheses of ansa-metallocenes or unbridged substituted metallocenes by the respective reductive dimerization of fulvenes with Group 4 metal divalent halides or with Group 4 metal dichloride dihydrides
Eisch, John J.,Owuor, Fredrick A.,Shi, Xian
, p. 1325 - 1339 (2008/10/09)
Two unprecedented syntheses of Group 4 metallocenes from 6-substituted fulvenes have been discovered and developed into high-yielding processes. In the first route the di-n-butylmetal dichlorides of Ti, Zr and Hf are generated in toluene suspensions of LiCl at -78°C from the metal tetrachlorides and 2 equiv. of n-butyllithium. Bringing the Bun2MCl2 to 25°C and then heating at reflux for several hours gave complete conversion to slurries of MCl2 (M = Ti, Zr, Hf). Heating such slurries of MCl2 with 2 equiv. of 6-substituted or 6,6-disubstituted fulvenes gave high yields of ansa-metallocenes or substituted ethylene-bis(cyclopentadienyl)metallocene dichlorides (fulvenes: 6,6-dimethyl-, 6-phenyl-, 6-(1-naphthyl)-, 6-(9-anthryl)-). For 6-substituted fulvenes, both racemic- and meso-1,2-disubstituted ethylene-ansa-metallocene dichlorides are expected to form, but with M = Zr (or Ti), the actual racemic- to meso-ansa-metallocene dichloride ratios observed were: phenyl, 50:50; 1-naphthyl, 83:17; 9-anthryl, 100:0. Apparently for steric reasons 6,6-diphenylfulvene underwent no ansa-metallocene dichloride formation with ZrCl2 but rather produced bis(diphenylmethyl(cyclopentadienyl))zirconium dichloride. The second route to novel metallocenes involves generating Bun2MCl2 at -78°C in toluene slurry, as in the foregoing method, but then adding 2 equiv. of the 6-substituted or 6,6-disubstituted fulvene immediately thereafter at -78°C. Except with Bun2TiCl2, warming the reaction mixture to 25°C and then further heating at 65°C cause a smooth bis-hydrometallation by transfer to occur, giving good to very good yields of bis(substituted cyclopentadienyl)metal dichlorides (M = Zn, Hf). The instability of Bun2TiCl2, even at -78°C, rapidly led to a mixture of TiCl2 and Bun2TiCl2and hence to a mixture of ansa-titanocene dichlorides and unbridged, bis(substituted cyclopentadienyl) titanocene dichlorides. With a detailed study of the attainment and the stereochemistry of the formation of ansa-bridged complexes or metallocenes with acetophenone, benzylideneaniline and 6-arylfulvenes, a mechanistic model is developed involving either a three-membered metallocycle formed from MCl 2 or an open-face sandwich complex of the fulvene and MCl2. Such intermediates offer a rational steric explanation for the observed stereochemistry of ansa-bridge C-C bond formation. Finally, in comparative polymerizations of ethylene by such metallocenes, cocatalyzed by MAO, the superior catalytic activity of ansa-metallocenes in the order, Ti > Zr > Hf and of ansa-metallocenes over unbridged substituted metallocenes is attributed to the hyperconjugative stabilization afforded by the ansa σ C-C bond to the metallocenium cation at the active olefin-polymerization site.
Cationic Zirconocene Complexes with Benzyl and Si(SiMe3)3 Substituted Cyclopentadienyl Ligands
Bochmann, Manfred,Green, Malcolm L. H.,Powell, Annie K.,Sasmannshausen, Joerg,Triller, Michael,Wocadlo, Sigrid
, p. 43 - 50 (2007/10/03)
Alkyl zirconocenes (where R = CH2Ph, X = Cl 1a or Me 1b; R = CHPh2, X = Cl 2a or Me 2b; R = Si(SiMe3)3, X = Cl 4a or Me 4b) and for comparison 3a were prepared and characterised. The reactions of these compounds with the methide abstracting reagents B(C6F5)3, B(o-C6F5C6F4)3 and +- were investigated by low temperature NMR spectroscopy. Compound 1b reacts with +- to form the homodinuclear complex 2(μ-Me)>+-. The related compound 2(μ-Me)>+- 5a was formed from the reaction of 1b with 0.5 equivalent of B(C6F5)3. Reaction between 1b and 1 equivalent B(C6F5)3 gave 6a and the ion pair 6b which are in equilibrium with each other. A similar observation was made when 2b was used instead of 1b. The sterically more demanding 4b does not show this behaviour. The role of the ligands in ethylene polymerisation was investigated.
