Ferrocenes silicon-bridged

Silicon-bridged [l,l]metallocenophanes of type 40 in Scheme 14 represent another class of dinuclear compounds with the ligand system (30). The synthesis of the 1,1,12,12-tetramethyl [1,1] silaferrocenophane was reported by two groups only recently. In one procedure, the dilithium salt of dicyclopentadienyldimethylsilane was reacted with ferrous chloride to give a mixture of the metallocenophane with poly(ferrocenyldimethylsilane) (equation 55)124. In the other procedure, this compound was prepared in a multistep synthesis starting from dilithio ferrocene, as shown in equation 56125... 

Substantial differences in the electrochemical behaviour have been observed for trifer-rocenes bridged by methylene (compound 80) and by dimethylsilyl (compound 81) units (Scheme 21). Cyclic voltammetry experiments reveal that 80 exhibits three reversible redox processes175, whereas for 81 only two redox events are observed176. The three reversible waves in the carbon-bridged ferrocene (80) are in the ratio 1 1 1 and can be assigned to successive redox events at each iron center, as described in Scheme 21. The two reversible waves in the silicon-bridged ferrocene (81) are in the ratio 2 1 and are consistent with oxidation of alternate ferrocene units, with subsequent oxidation of the intervening iron site (see Scheme 21). 

Comparable effects have been described for silicon-bridged species with up to eight ferrocenyl units176 and also for poly(ferrocenylsilanes) of type 68163. The difference in the electrochemical behaviour of carbon and silicon substituted multiferrocenes may be explained by the different electron donating or withdrawing effects exerted by these substituents on a ferrocene unit. The phenomena described show that a proper choice of the bridges allows one to determine whether a molecule with three or more ferrocenyl units is first oxidized in the center or at the periphery151. 

Prior to 1992 no efforts to prepare polymers with main chains consisting of ferrocene and organosilane units via ring-opening methods had been reported. The first developments in this area involved attempts to polymerize silicon-bridged ferrocenophanes and related species (23,41). The first successful ROPs involved the use of [l]silaferrocenophanes as cyclic monomers, and the progress in this area to date together with relevant previous work is now reviewed. 

Gomez-Elipe, P., Resendes, R., Macdonald, P.M., and Manners, I. (1998) Transition metal catalyzed ring-opening polymerization (ROP) of silicon-bridged [l]ferrocenophanes facile molecular weight control and the remarkably convenient synthesis of poly(ferrocenes) with regioregular, comb, star, and block architectures. Journal of the American Chemical Society, 120,8348. 

These ferrocenes containing the silicon-silicon bond are very stable under basic conditions, e.g., at reflux with 0.14 M sodium methoxide in methanol, with the exception of the l,2-(l,l -ferrocenylene)disilane with bridged structure, which in part undergoes cleavage. Cyclopentadienylpentamethyl-disilane itself is stable to the acid-catalyzed cleavage under the same conditions as used for the ferrocenes mentioned above. 

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