Ferrocenyl dendrimers

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. 

The structures of the novel ferrocenyl dendrimers 1-6 were straightforwardly established by a variety of spectroscopic and analytical techniques including H, C, and Si NMR and IR spectroscopies, fast atom bombardment mass spectrome-... 

Attempts to isolate the ferrocenyl dendrimers described above in a crystalline form suitable for X-ray structural determination have so far been unsuccessful. For this reason, we have used computer-generated molecular models in order to gain further information about the structural features of these materials. Figure 3 illustrates an energy-minimized structure determined from CAChe molecular mechanic calculations of the ferrocenyl dendrimer 2. From these studies, we have measured approximate diameters of 2 run for the first-generation dendrimers 1,3, and 5, and 3 nm for the second-generation dendrimers 2,4, and 6. 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

Ferrocenyl dendrimers also afford electroactive films on indium tin oxide (ITO) electrodes in the same manner as described above. UV-visible spectroelectro-chemical measurements of this modified electrodes on oxidation show changes characteristic for the formation of fenocenium cations. Thus, Figure 8 shows the UV-visible absorption spectrum of a film of 2 electrodeposited on a transparent ITO electrode, which exhibits a strong band at 260 nm and a weak absorption band centered at 600 nm, which agree with those observed for the cationic dendrimer [2 KPF j ]g in solution described above. 

The molecular recognition of anionic guest species by positively chaiged or neutral receptors is a relatively new area of research of growing interest in view of the key roles that these anions play in biochemical and chemical processes. For this reason, as part of the electrochemical studies, we decided to examine the use of the redox-active ferrocenyl dendrimers 3 and 4 that contain multiple N-H linkages capable of participating in H-bonding, as well as characteristic internal cavities,... 

Reversible attachment of nanostructures at molecular printboards was exemplified by the adsorption and desorption of CD-functionalized nanoparticles onto and from stimuli-responsive pre-adsorbed ferrocenyl-dendrimers at a CD SAM (Fig. 13.7).65 Electrochemical oxidation of the ferrocenyl endgroups was employed to induce desorption of the nanostructure from the CD SAM. An in situ adsorption and desorption of ferrocenyl dendrimers and CD-functionalized Au nanoparticles (d 3 nm) onto and from the molecular printboard was observed by a combination of surface plasmon resonance spectroscopy (SPR) and electrochemistry. Similar behavior was observed when larger CD-functionalized silica nanoparticles (d 60 nm) were desorbed from the surface with the aid of ultrasonication. 

Figure 13.7 The adsorption and desorption of CD-functionalized nanoparticles onto and from a CD SAM with ferrocenyl dendrimers as a reversible supramolecular glue.

The first ferrocenyl dendrimers suitable for redox recognition contained amido groups attached to the ferrocenyl moiety [23,24], so that the H-bonding interaction with oxo-anions, known from Beer s work with endo-receptors [8-14], would be efficient. At this point, it is essential to point out that mononuclear amidoferrocenes that contain a linear substituent without any special topological requirement for anion recognition display no specific effect [23,24],... 

Scheme 2 Syntheses of third-generation amidoferrocenyl [34-41] and pentamethylamido-ferrocenyl dendrimers [64,65] from the corresponding DSM polyamine dendrimer G3-dend-(NH2)i6- For analysis and comparison of their behavior as hosts of anions, see [64,65]...

With the dendrimers series shown in Fig. 1, the order of AE° values was H2PO4- > HS04 > Cl" > NO3-. This order can, however, vary greatly from one dendrimer series to the other, and the same can be said of the dendritic effect some dendrimer families (such as the above one) give a strong dendritic effect for a given anion, and others do not. For instance, an amido-ferrocenyl dendrimer with an octabenzylated durene core and 24 redox termini underwent values of the order of what was found for the dendrimer 9-Fc of Fig. 1, far less than 18-Fc of Fig. 1. 

Scheme 13. Synthetic scheme for 24-CN and octa-ferrocenyl dendrimers.

Convergent and Divergent Syntheses of Large Ferrocenyl Dendrimers with Good Redox Stabilities 421... 

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-... 

Scheme 26. Divergent construction of ferrocenyl dendrimers from the 9-allyl, 27-allylf 81-allyl and 243-allyl dendrimers.

Scheme 27. Schematic strategy for the divergent construction of ferrocenyl dendrimers exemplified in Schemes 23.

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