Silicon-bridged ferrocenophanes

The silicon-bridged [l]ferrocenophane LXXXV undergoes polymerization at 120-150°C to yield polymers of number-average molecular weights up to 105 with PDI 1.5-2.5... 

Braunstein and coworkers have shown (vide supra) that in the bimetallic Fe—Pt complex (CO)3(SiR3)Fe(yU.-PPh2)Pt(PPh3)CO(Fe—Pt), a silyl migration occurred from the Fe to the Pt center (Scheme 17)102. Remarkably, the first example of a well-characterized insertion of a transition-metal fragment into a strained silicon-carbon bond of a silicon-bridged [l]ferrocenophane was recently reported by Sheridan, Lough and Manners. The... 

Another impressive example is the facile cleavage of Si—C(Cp) bonds in strained [1]-ferrocenophanes with bridging silicon. This behaviour is the basis for the synthesis of high-molecular-weight poly(ferrocenylsilanes) (equation 47) and will be described in more detail in Section II.E.l. 

There are very few examples of silicon-bridged bent-metallocenes of type 42 in Scheme 14 described in the literature. The class of silicon-bridged [l]-ferrocenophanes is most important due to the exceptional structure and reactivity, and representative examples 43-52 are listed in Scheme 15. 

Organometallic macro cycles and cyclic polymers were prepared by the photolytic ring opening of a silicon-bridged ferrocenophane with a bipyridine initiator. The relative amounts of cyclic oligomers and cyclic polymer, as well as the molecular weight of the cyclic polymer, can be controlled by the reaction temperature [228]. 

Both PFS-PI and PFS-PDMS are synthesized by two-step anionic polymerization. The synthetic approach for the preparation of PFS-PDMS is shown in Scheme 1 [8,9] n-butyllithium was used to initiate the polymerization of the strained silicon-bridged ferrocenophane in THF solution, while the second block was built by the subsequent addition of hexamethyltrisiloxane (D3). The reaction was terminated with chlorotrimethylsilane. To obtain PFS-PI, the PI block was initiated with butyllithium, followed by the addition of the silicon-bridged ferrocenophane. 

RING-OPENING POLYMERIZATION OF SILICON-BRIDGED [1]FERROCENOPHANES... 

B. Synthesis, Properties, and Ring-Opening Polymerization of Silicon-Bridged 1 ]Ferrocenophanes... 

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. 

Figure 8 Molecular structure of silicon-bridged [1 ]ferrocenophane 72 (R = R = Me) as determined by single crystal X-ray diffraction the planes of the Cp ligands are tilted by 20.8°. (Reproduced with permission of The American Chemical Society from Finckh, W. etal., Organometallics, 1993, 12, 823.)...

The mechanism of the thermal ROP of silicon-bridged [l]ferrocenophanes is not yet known in definitive detail. However, studies of the polymerization of monomers with unsymmetrically substituted Cp ligands indicate that ROP proceeds via cleavage of the Cp-Si bond. It is possible that traces of nucleophilic impurities initiate the polymerization, although a mechanistic process involving radicals cannot be completely ruled out. " ... 

The thermal ROP of silicon-bridged [l]ferrocenophanes requires moderately high temperatures (100-250°C) and there is little or no molecular weight control. As a result, the molecular weight distributions are quite broad (d/w/47n E5-2.5). This polymerization method has now been superseded in many ways by ambient-temperature ROP methods that involve the use of anionic initiators (see Section 12.06.3.3.4) or transition metal catalysts (see Section 3.3.4). 

The thermal ROP methodology established for silicon-bridged [I]ferrocenophanes has been extended to many other strained metallocenophanes, which allows access to high molecular weight polyferrocenes with different spacer groups. As discussed later (see Section 12.06.3.3.6), this permits modification of the properties of the polymer, such as the extent of the metal-metal interactions. 

In addition, random co-polymers 90 derived from [l]ferrocenophanes and silicon-bridged bis(benzene)chromium complexes have been synthesized (Equation (36)). ... 

Anionic ROP reactions of metallocenophanes were first reported in Silicon-bridged [I]ferrocenophanes 72... 

The discovery of the anionic ring-opening polymerization of silicon-bridged[l]ferrocenophanes enabled the synthesis of well-defined, near-monodisperse poly(ferrocenylsilane) homo- and block copolymers. PFS was combined with polystyrene [38], polyisoprene [39], poly(dimethylsilox-ane) [40], poly(ethylene oxide) [41], poly(ferrocenylphenylphosphine) [42], poly(aminoalkyl methacryate) [43] and recently with poly(methyl methacrylate) [44,45] blocks. 

The first examples of the use of ROP of strained metallocenophanes to prepare high molecular weight polymetallocenes (Mn>10 ) involved silicon-bridged [Ijferrocenophane monomers 3.21 [61]. Specifically, polyferrocenylsilanes (PFSs) 3.22 (R,R = Me or Ph) were prepared by the thermal ROP of strained, ring-tilted silicon-bridged [l]ferrocenophanes 3.21 (R,R = Me or Ph) in the melt, at 130-... 

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