Ferrocenyl units

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

SCHEME 3 The electrochemical gene sensing system based on the formation of complementary sandwich-type complex, (a) Target DNA combines the ferrocenyl ODN with the probe ODN on the electrode. Redox currents due to the surface-confined ferrocenyl units should reflect the concentration of the target, (b) Ferrocenyl units are not deposited onto the electrode using nontarget DNA. 

Alonso et al. prepared ferrocenyl silicon dendrimers [67], which could be used as mediators in glucose biosensors, based on glucose oxidase [68, 69], The ferrocenyl units are located at the end of long, flexible, silicon containing branches and serve to electrically connect the enzyme to the electrode. The flexibility of the dendrimer is proposed to play an important role in the interaction with the redox center of glucose oxidase. 

In support of the electrochemical evidence, the molecular structure of the biferrocenium ion in [( -CsF Fe -CsfLOf -CsfLOFe -CsHs)] [I3] shows that both pairs of cyclopentadienyl rings have an eclipsed conformation, Figure 16. Furthermore, the mean Fe-Cp(Centroid) distance is equivalent in both the ferrocenyl units and equal to 1.68 A. Speculatively, this value is intermediate between the values previously observed for ferrocene and ferrocenium ions, thus supporting charge delocalization between the two centres.27... 

It is reasonable to assume that the first two-electron process result in the simultaneous oxidation of the two outer ferrocenyl units of the molecule (which are quite distant from each other, and hence not electrostatically interacting), whereas the second one-electron oxidation should be centred on the central ferrocenyl group (that can interact electrostatically with the now positively charged lateral subunits). 

The dendrimers containing more ferrocene units are more interesting from the electrochemical viewpoint. Scheme 6 shows the 1st and 2nd generation members of a silicon-centred series containing four and eight ferrocenyl units, respectively.48... 

Both the complexes display, in dichloromethane solution, a single oxidation process which involves four ( ° = + 0.40 V, vj. SCE) and eight (jE °/ = -1- 0.44 V, vj. SCE) electrons, respectively. This means that no electronic communication exists among the ferrocenyl units. 

In this connection it is useful to look at the voltammetric response of the dendrimer illustrated in Figure 30, which is constituted by a central ferrocene core branched to ten outer ferrocenyl units.54... 

Exhaustive oxidation at the first process makes the original orange solution assume an intense green colour typical of ferrocenium species. One may thus assume that the first oxidation is centred on the ferrocenyl unit and, therefore, the second process must be attributed to the Mo(II)/ Mo(III) oxidation. 

As illustrated in Figure 5, the complex exhibits a reversible one-electron oxidation (Eof = +0.36V), easily assigned to the ferrocenyl unit, as well as two sequential one-electron reductions showing features of chemical reversibility, which are centred on the maleic anhydride capped tricobalt cluster.4... 

Let us now consider a few complexes in which a central metal fragment is interposed between two ferrocenyl units. 

UV-photoirradiation causes trans-to-cis isomerization and the resulting solution exhibits a new voltammetric profile, Figure 2b, in which traces of the original waves are preceded by an anodic process (Eof = +0.03 V), which has been attributed to the single-stepped two-electron oxidation of cw-[Fc—N=N—Fc].3b This suggests that the apparent simple isomerization really involves important electronic effects in that (based on the discussions in Section 1.3, Chapter 4) one must conclude that in trans-[Fc—N=N—Fc] the two ferrocenyl units are interacting with each other, while in cz>[Fc—N=N—Fc] they do not interact. 

The first iron-containing silsesquioxanes which appeared in the literature were compounds containing ferrocenyl units as side-groups.However, these are not within the scope of this review as iron is not part of the metallasilsesquioxane skeleton. Meanwhile, several ferrasilsesquioxane complexes have been synthesized. The first iron(III) compound of this type was prepared in our laboratory according to Scheme 56. ° In 161, the coordination sphere of iron is completed by TMEDA (N,N,N, N -tetramethylethylenediamine) as a chelating amine ligand. Pale yellow, crystalline 161 was isolated in 80% yield and structurally characterized by X-ray diffraction. This compound was later used by Maxim et to prepare iron... 

Cuadrado et al. reported that hydrosilylation of 1,3,5,7-tetramethylcy-clotetrasiloxane (1) with four equivalents of vinylferrocene (2) in the presence of catalytic bis(divinyltetramethyldisiloxane)platinum(0) yielded tetraferrocenyl compound 3 in 92% yield (Scheme 1). The cyclic voltammogram of 3 exhibited a single reversible oxidation wave and coulometry established that this wave corresponded to the removal of four electrons per molecule, suggesting that the four ferrocenyl units act as independent, non-interacting redox centers. ... 

The same reasons for the interest in incorporating ferrocene units into polymers also provided motivation for the synthesis of dendritic macromolecules of well-defined size and structure containing ferrocenyl units. An important additional rationale for the construction of ferrocenyl dendrimers is provided by the fact that such macromolecules raise the possibility of combining the unique and valuable redox properties associated with the ferrocene nucleus with the highly structured macromolecular chemistry. This may provide access to materials of nanoscopic size possessing unusual symmetrical architectures, as well as specific physical and chemical properties which would be expected to differ from those of the ferrocene-based materials prepared to date. 

On the other hand, we have also isolated and characterized the oxidized forms of some of the synthesized polyferrocene dendrimeric systems. Chemical oxidation of the ferrocenyl units in 1,2,5, and 6 has been achieved by reaction with NOPF in CH2CI2 solution, and affords the blue tetra- and octanuclear dendritic cations [I ltPF l4.[2 ][PFj ]g,[5 ][PF4 ]4,andI6 ][PF- TheUV-visiblespectra... 

Dendrimers are large molecules, and their solubility is often an issue. Thus, it is not surprising that dendrimers can be good candidates to precipitate upon and thus modify the surface of an electrode. Dendrimers with ferrocenyl peripheral units were shown to display well-defined redox waves with the ferrocenyl units oxidizing independently.176-178 These molecules oxidatively precipitated onto the electrode surface and were characterized electrochemically and by atomic force microscopy.44 It was later shown that cyclodextrin complexation increased the solubility of these molecules.179 Similar results were obtained with dendrimers containing pendant ruthenium tris (bipyridine) and bis(terpyridine) groups.180... 

Silicon-based dendrimers 8 and 9 (Fc = ferrocenyl) also showed oxidative precipitation onto electrodes to give idealized electrochemistry as films.181 Specifically, the peak current was linear with scan rate and the potential difference between the anodic and cathodic waves was small (AE = 10 mV at a scan rate of 100 mV/s).182 This latter observation indicated that the rate of electron transfer was rapid. For molecule 9, the surface coverage was measured as = 2 x 10 10 mol/cm2. These molecules were also explored as mediators in amperometric biosensors.183 In contrast, molecule 10 showed two redox peaks, indicative of interaction between the two ferrocenyl units at each peripheral site. 181 When oxidation of one of the two interacting redox units results in some electron sharing between the two units (Robin-Day class II mixed valence species), the second oxidation is naturally... 

Similarly, the fluorescence intensity of the 1,4-disubstituted azine with ferrocene and pyrene units (17) can be reversibly modulated by sequential redox reactions of ferrocene moiety. In the neutral state, compound 17 displays weak fluorescence owing to the electron transfer from the ferrocenyl group to the excited pyrene unit or by energy transfer from the excited pyrene unit to the ferrocenyl unit. Oxidation of the ferrocenyl unit, however, leads to remarkable fluorescence enhancement. This is because the ferrocenium cation shows weak electron donating ability and also the corresponding spectral overlap becomes small.27... 

Photochromic dithenylmaleimide 32 contains two ferrocene units.41 Electrochemical absorption spectral studies indicate that oxidation of ferrocenyl units can induce ring-opening reaction for the ring-closed isomer of 32. 

The preparation of a bis-O-ferrocenoyl-L-tyrosine peptide 106 has been reported (Scheme 31 ).t69i This compound was rationally designed to be used as a model peptide to study the binding ability of a fullerene-modified silica gel. The attachment of the ferrocenyl units was performed presumably in solution using the free hydroxy tyrosine peptide and ferrocenoyl chloride. 

SCHEME 18. Polymers containing repeating ferrocenyl units... 

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. 

Higher molecular weight polymers (40,000-50,000) have been reported from condensation reactions between monomer 6.15 and various silanediols, as illustrated in reaction (6).2 Polymer 6.16 can be melt-fabricated into films or fibers, which probably reflects the presence of roughly 100 repeating units per chain. However, the separation of the ferrocenyl units by the organic linker groups reduces the influence of the organometallic component. 

Finally, ferrocenyl units have been incorporated into the perimeter of dcndrimcric polymers by Astruc and coworkers.40 Gold nanoparticles form the core of the dendrimers, with the branched units with terminal ferrocene units coordinated to the gold surface via... 

Scheme 8.8. Silane-based dendrimers have been used for scaffolding of ferrocenyl units described as noninteracting redox centers.

Fe(III) is observed and revealed by Mossbauer spectroscopy. This unique behaviour is indicative of a particular substructure in the gel where ferrocene units are mainly located at the surface of the material. A completely different situation is observed for the bi-silylated xerogel obtained from 102 for which a Cottrelian charge transfer is observed and it exhibits a classical reversible voltammogram the peak current scales linearly with the square root of scan rate. Moreover, it is impossible to achieve a complete Fe(II)/Fe(ni) oxidation although the process is still reversible. Such behaviour demonstrates that some of the ferrocenyl units are not accessible and do not participate in the electrochemical process103. 

Vessieres A, Spera D, Top S, Misterkiewicz B, Heldt JM, Hillard EA, Huche M, Plamont MA, Napolitano E, Fiaschi R, Jaouen G (2006) The presence of a ferrocenyl unit on an estrogenic molecule is not always sufficient to generate in vitro cytotoxicity. ChemMed-Chem 1 1275-1281... 

Chart 1. Third-generation polyallyl dendrimer with was hydrosilylated using dimethylferrocenylsilane a theoretical number of 243 allyl branches syn- Fc(Me)2SiH to 243-Fc , a molecular battery thesized according to Scheme 20. This 243-allyl dendrimer with a theoretical number of 243 dendrimer (like the 9-allyl, 27-allyl, and 81 -allyl peripheral ferrocenyl units,... 

Cyclic voltammetry of all the ferrocenyl dendrimers on a Pt anode showed all the ferrocenyl centers to be equivalent as only one wave was observed. It was possible to avoid adsorption even using CH2CI2 for the small ferrocenyl dendrimers, but the use of MeCN was necessary for the medium-sized ones (27-Fc, 54-Fc, and 81-Fc). Finally, adsorption could not be avoided even with MeCN for the 243-Fc dendrimer. From the intensity of the wave, the number of ferrocenyl units could be estimated using the Anson-Bard equation [75], and the numbers found were within 5 % of the branch numbers except in the case of the 243-Fc dendrimer, for which the experimental number was too high (250) because of the adsorp-... 

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