Ferrocene dendrimers

Recently, Astruc et al. [189] reported novel amido-ferrocene dendrimers (e.g., 91) which were shown to act as supramolecular redox sensors for the recognition of small inorganic ions (Fig. 41). It was further observed that as the den-drimer generation number increased the sensitivity to the guest molecules also increased as observed by cyclic voltammetry experiments. 

The appearance of a single multielectron oxidation is a quite common response for most ferrocene dendrimers. For instance, all the derivatives shown in Scheme 7, which contain 5, 6, 9, 18, 27, 54, as well as similar derivatives containing 81 and 243 ferrocene subunits, respectively,45 ... 

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

The microstructuie of films of the ferrocene dendrimers electrochemically deposited on platinum wire working electrodes was examined by scanning electron microscopy (SEM). The SEM micrograph in Figure 7 corresponding to a film of the octanuclear dendrimer 2 shows a sheet-like compact morphology and exhibits small agglutinations and some porosity. 

Figure 28 Plot of ln(///0) versus (in arbitrary units) for the first, second, and third generation ferrocene dendrimers based on A. The largest compound shows the smallest slope a D. (Reprinted with permission from D. Valentini, M. Pregosin, P. S. Ruegger, H (2000) Organometallics 19 2551. 2000 American Chemical Society)...

A mixed [60]fullerene-ferrocene dendrimer (Fig. 65) was also found to be mesomorphic, showing a room temperature smectic A phase which cleared at 157 °C (no glass transition temperature was detected) [294,295]. The mesophase was only slightly destabilized upon the addition of Ceo when com-... 

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

Valerio C, Fillaut J-L, Ruiz J, Guittard J, Blais J-C, Astruc D (1997) The dendritic effect in molecular recognition ferrocene dendrimers and their use as supramolecular redox sensors for the recognition of small inorganic anions. J Am Chem Soc 119 2588-2589... 

Fig. 6.18. The structure of CB[7] and the first, second and third generation ferrocene-dendrimers (FCl, FC2 and FC3, respectively) and a schematic representation of their complexes along with their diffusion coefficients at different pH [44a]. (Adapted with permission from ref. [44a]. Copyright 2005 Royal Society of Chemistry.)...

Figure 34 Cyclic voltammograms for ferrocene dendrimer 239 in (a) CH2CI2 (b) CH2Cl2/MeCN 5 1, and (c) as a film on a Pt electrode. Supporting electrolyte 0.1 M [Bu4N][PFe] scan rate lOOmVs (Reproduced with permission of The American Chemical Society from Cuadrado, C. M. etal., J. Am. Chem. Soc., 1997, 119, 7613.)...

Another interesting class of ferrocene dendrimers 241 possesses electrochemical properties which can be tuned by A-methylation or protonation of the nitrogen sites. For example, a shift in E of 100-200 mV for the ferrocene groups is effected on protonation of the adjacent nitrogen centres. " ... 

Ferrocene dendrimers are also of interest for reasons other than their redox activity. For example, metalloden-drimer 242 possesses planar chiral ferrocene units that make the bidentate phosphine ligation sites of potential interest for applications in asymmetric catalysis. Indeed, asymmetric hydrogenations of dimethyl itaconate catalyzed by Rh complexes of 242 showed impressive ee values of 98%. " ... 

The versatile chemistry of the ferrocenyl moiety has also allowed for the preparation of a large number of dendrimeric structures with this group at the periphery (116-118). A series of dendrimers based upon polypropylenimine cores have been reported (119,120) and star-shaped macromolecules with ferrocene units at the periphery have been produced (121), as well as amido-ferrocene dendrimers (eg, 38) (122). A convergent approach has allowed for the synthesis of dendrimers (39) for which the ferrocene units at the periphery display electronic interaction (123). 

A review article examines the literature up to 1999 on silicon- and amine-based ferrocene dendrimers and polymers and those immobilized on electrodes. In these dendrimers the ferrocene itself may be at the cote, the periphery, or be used as a spacer molecule. Some examples of these molecules are shown as 15-17. ... 

Ferrocene has also been encapsulated inside a symmetric ether-amide shell—the synthesis again begins with the use of l,l -bis(chlorocarbonyl)ferrocene at the core/ and these dendrimers have been examined in relation to their anion-binding ability. " Ferrocene dendrimers have also been prepared with cyclotriphosphazene cores/ trisub-stituted benzene cores/ and oligo-alkylamine cores. Ferrocene has been used as the central subunit in a number of dendritic structures such as the series of dendrimers shown as 20 again, the initial synthesis begins with chlorocarbonylferrocene. It has also been used in many cases as the peripheral group on molecules such as those shown in... 

Thin hydrogel films made of ferrocene-modified poly(ethylene imine) with embedded enzymes can act as biosensors for glucose [600, 601]. Similarly, other glucose-dependent biosensors have been constructed [602-604] e.g., a poly (vinylferrocene)-enzyme composite deposited on an electrode acted as a mediator for glucose oxidation [605-610]. Immunosensors were realized on glassy carbon electrodes by ferrocene-dendrimer-modified antibodies, which quench electrochemiluminescence [611]. 

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