Iron dendrites

Figure 2-9. (a) Electrolytically deposited copper, magnification x 1000. Courtesy of Maria Kazinets, Beer Sheva (b) Directionally solidified iron dendrites from an iron-copper alloy after dissolving away the copper, magnification x 2600. Courtesy of J. Morral, Storrs, Connecticut. 

The occurrence of radial type symmetries rather than more restricted ones necessitates a spatial freedom in all relevant directions. Thus, for example, the copper formation in Figure 2-10a has a tendency to form cylindrically symmetric structures. On the other hand, the solidified iron dendrites obtained from iron-copper alloys, after dissolving away the copper, display bilateral symmetry in Figure 2-lOb. 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

Scheme 13 Use of para-hydroxy 1 as a means of linking to dendritic wedges and complexation with iron(II) chloride...

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

The electrochemical properties of the two types of iron-cluster-derivatized AuNPs resemble that of the monomeric tetrairon cluster, except that they additionally show adsorption due to their large size, and evidence that all the Fe4 clusters are active at about the same potential, thereby indicating that they are sufficiently remote from one another to behave independently. The changes in the cyclic voltammetric pattern caused by the addition of H2P04 and ATP2- oxoanions evidence recognition features that are very different from those obtained with dendritic ferrocene exoreceptors. In particular, the results show that with these iron-cluster-derivatized AuNPs... 

Further electrochemical oxidation to FeO(OH) may also take place. Dendrites are not formed on charging since the solubility of Fe(OH)2 is low, but considerable hydrogen evolution takes place which lowers the cycle efficiency. The iron electrode also suffers from very high self-discharge ( 2% per day at 25°C), due to the reactions... 

Similar dendritic architectures were formed by fixing polyether dendrons to a terpyridine [132] core unit by means of iron(II) ions [133]. 

Diederich et al. had postulated that the highly reactive iron-oxo species, arising from oxygen transfer from the oxidant to the Fem site [87], should be greatly stabilised by enclosure within a dendritic superstructure. The catalytic potential of the dendrimers 6 a-c was determined in the epoxidation of alkenes [83 a, 88] (1-octene and cyclooctene) and the oxidation of sulphides [83 a] ((methylsulphanyl)benzene and diphenyl sulphide) to sulphoxides - in dichloro-methane with iodosylbenzene as oxidising agent. Compared to the known metal-porphyrin catalysts, 6a-c exhibit only low TON (7 and 28, respectively, for... 

Modification of the electrochemical properties of a redox centre surrounded by dendritic fragments [93] can lead to two different dendritic effects. The first one is manifested in a shift of the redox potentials, the extent and direction depending upon the dendritic architecture and the solvent. Such behaviour was observed in dendritic iron-porphyrins [94]. The second effect is apparent in a delay of redox transfer kinetics and is characterised by a stepwise increase in the distance between the peaks in a cyclovoltammogram with increasing dendrimer generation number. 

Diederich et al. produced a mimic of the electron-transfer protein cytochrome C. Divergent synthesis afforded a water-soluble iron-porphyrin which possesses a covalently attached dendritic shell surrounding the electrically active heme core which is reminiscent of a protein shell (Fig. 8.7). In contrast to the dendri-... 

Dandliker, P. J., Diederich, F., Gisselbrecht, J. P., Louati, A., Gross, M., Water-soluble dendritic iron porphyrins Synthetic models of globular heme proteins. Angew. Chem., Int. Ed. Engl. 1996, 34, 2725-2728. 

In other work, Dandliker et al. have reported the inclusion of iron porphyrins within dendrimers to serve as functional mimics of redox-based proteins (Dandliker et al., 1994, 1995, 1997). These redox-switchable porphyrins show that the Fe3+/Fe2+ redox couple can be altered by the polarity of the surrounding environment. By changing the polarity imposed by the tightly packed branches of the dendritic core, the authors have illustrated that electrochemical behavior can be controlled by slight and subtle through-space environmental factors. These mimics may potentially model a wide variety of redox-driven enzymes and possibly provide mechanistic insights into their function. 

A typical method for fabricating multiple complex layers is illustrated in Figure 2.11,12 First, an Au/mica or Au/ITO plate is immersed in a chloroform solution of tpy-AB-SS-AB-tpy (tpy=2,2 6, 2" -terpyridyl), providing Au-S-AB-tpy SAM on the plate. In the case of connecting the Fe(II) ion, the tpy-terminated plate is immersed in 0.1 M Fe(BF4)2 aq or (NH4)2Fe(S04)2 aq to form a metal complex. Subsequently, the metal-terminated surface is immersed in a chloroform solution of the ligand Lj or L2 to form a bis(tpy)iron structure (Fig. 2b). The latter two processes are repeated for the preparation of multilayered bis(tpy)iron (II) complex films with linear structures. When L3 is used instead of Lj or L2, the resulting molecular wires have a dendritic structure (Fig. 2c). 

Dendritic Cationic Aminoarene Iron-sandwich Complexes... 

Dendritic Cationic Aminoarene Iron-sandwich Complexes Selectively Recognize Chloride and Bromide Anions... 

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