Ethanediyl bridged

The bis-Gp titanium compound (C2H4)2(C5H3)2TiCl2 (Scheme 656) containing two ethanediyl bridges between the Gp rings is obtained by the reaction of the dilithium salt with TiCl3(THF)3 after appropriate workup. Its molecular... 

Dorer, B. Prosenc, M. -Ft. Rief, U. Brintzinger, H. H. Synthesis and structure of titanocene, zireono-cene, and vanadocene dichloride complexes with two ethanediyl bridges. Organometallics 1994,13,... 

Fig. 8 R, S, and meso forms of ethanediyl-bridged bis (indenyl)zirconium dichloride...

In the three decades that have passed since the first syntheses of chiral ethanediyl-bridged bis(indenyl)titanium and bis(indenyI)zirconium complexes [1, 2] and the first reports on their use - together with methylalumoxane as activator [3] - as catalysts for isotactic olefin polymerization [4,5], this field of catalysis research has seen many structural variations on the ansa-metaUocene theme. 

The first isospecific propene polymerization with an ethanediyl-bridged bis (indenyl)-titanium complex demonstrated in 1983 the capability of ansa-metaUocenes to control the tacticity of a growing polypropylene chain [4, 48], thus creating a strong interest in exploring the potential use of ansa-metallocenes for industrial polypropylene production. A first step towards industrially suitable systems was the use of much more stable zirconium complexes, which when activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization temperatures and thus higher activities [5]. 

Freedom of rotation about the double methylene bridge in the compound (7) (dimethyl 4,4 -(l,2-ethanediyl)bisben2oate [797-21-7]) destroys the rod shape of the molecule and prevents Hquid crystal formation. The stilbene derivative (8) (dimethyl 4,4 -(l,2-ethenediyl)bisben2oate [10374-80-8]) however, is essentially linear and more favorable for Hquid crystal formation. 

The spectrum obtained after two-electron oxidation of the analog with a 1,3-propanediyl spacer (i.e., n=3, shown in the inset of Fig. 32) has a prominent pair of peaks at 676 and 1,193 nm. These correspond to the formation of a radical cation % dimer the spectrum of the radical cation % dimer of the linear quinquethio-phene oUgomer obtained at low temperature has peaks at 650 and 1,084 nm. One-electron oxidation affords a spectrum that has a peak which corresponds to the presence of the neutral quinquethiophene unit (A.abs=418 nm), apair of peaks for the radical cation of the quinquethiophene unit (753 and 1,422 nm), and the pair of peaks for the radical cation n dimer. Accordingly, it appears that all three species shown in Fig. 32 are present in equilibrium. Similar sets of peaks are observed upon oxidation of the analogs with longer spacers (n=4-6). Thus, linking two quinquethiophenyl units with an alkylene chain (n=3-6) facilitates formation of the radical cation % dimer. % dimerization is hindered in the case of the 1,2-ethanediyl spacer by torsional strain within the short alkylene bridge. 

---