Skeletal ferrocene

The unique and appealing conformations led us to propose a unique structure for a novel organometallic non-natural polypeptide with skeletal ferrocenes as shown in Fig. 3.35(a). When the polymer is dissolved in polar solvent, the secondary structure is probably changed to a turn or random structure (Fig. 3.35(b)). The folding structure reminds us of secondary structures such as the a-helix and P-sheet of polypeptides, and the conformational change calls to mind the helix-coil transition of natural peptides. 

Table 9.1. Observed and calculated vibrational frequencies of the skeletal modes of ferrocene 981.

Although vibrational frequencies have been calculated and compared with experimental data since the early days of molecular mechanics refinements using full-matrix second-derivative procedures [13 73], there are few reports on the application of this method in the field of transition metal chemistry. One of the examples is a study on linear metallocenes[108]. Here, the molecular mechanics force constants were obtained by adjusting starting values by fitting the calculated vibrations to thoroughly analyzed experimental spectra. The average difference (rms) between experimental and calculated vibrations was of the order of ca. 30 cm-1. Table 10.1 shows the observed and calculated frequencies associated with the skeletal modes of ferrocene (see Fig. 10.1 for the representation of these modes). 

The same reasons for the interest in incorporating ferrocene units into the side group structure have also provided motivation for the synthesis of polymers in which ferrocenyl moieties are part of the main chain. An important additional reason for the synthesis of polymers with skeletal ferrocenyl units is provided by the observation that in molecular species in which two ferrocene units are linked together in close proximity the iron atoms can often interact to yield delocalized, mixed-valent species on one-electron oxidation. For example, the mixed-valence cations derived... 

Finally, the polymers produced from the ROP reactions, which are very rare examples of high molecular weight, well-defined polymers with skeletal transition metal atoms held in close proximity to one another, are of considerable interest with respect to their properties and possible applications (93). The most promising materials derived from [1]- or [2]metallo-cenophanes appear to be the poly(ferrocenylsilanes) which are easy to prepare from the readily available starting materials, ferrocene and di-chloroorganosilanes, of which many are provided by the silicone industry. Although the main features of these polymers appear to have been elucidated, much detailed work remains to be done to fully understand the characteristics of these unusual materials. It is expected that many interesting developments will result in the near future from studies of the... 

The target polymers 10-20, upon dialysis in aqueous solution and freeze-drying, were obtained as water-soluble solids in yields of 35-90%. Inherent viscosities were typically in the range of 10-15 ml g l, and the degree of ferrocene incorporation, assessed by elemental analysis and NMR data, ranged from about 20 to 75% of theory, depending critically on reactant ratio and type of anchoring function. The polymers showed the characteristic ferrocene CH out-of-plane deformation and skeletal bands at 810 and 500 90 cm"i in the IR spectra. The... 

The development of processible high-molecular-weight polymers with skeletal transition metal atoms, as present in ferrocenes, represents a synthetic challenge, and a variety of materials with novel electrical, optical and magnetic properties can be expected . In this context, polymers containing repeating ferrocenyl units are of great interest. Some representative examples are listed in Scheme 18. 

Some examples include rather unusual species as well as traditional molecules viewed in a rather different light. CpMn(CO)3 [48], 22.44 is isolobal with ferrocene and CsMeg with an apical CMe unit along with 21.74 and 21.75. All can be regarded simply as a nido pentagonal bipyramid (number of skeletal pairs = I /2[(6 x 3) — 2] = 8) ferrocene contains the isolobal FeCp unit in its place. 

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