Self-assembly anion templating

The size-exclusion and ion-exchange properties of zeoHtes have been exploited to cause electroactive species to align at a zeoHte—water interface (233—235). The zeoHte thus acts as a template for the self-organization of electron transfer (ET) chains that may find function as biomimetic photosynthetic systems, current rectifiers, and photodiodes. An example is the three subunit ET chain comprising Fe(CN)g anion (which is charge-excluded from the anionic zeoHte pore stmcture), Os(bipyridine)3 (which is an interfacial cation due to size exclusion of the bipyridine ligand), and an intrazeoHte cation (trimethylamino)methylferrocene (F J ). A cationic polymer bound to the (CN) anion holds the self-assembled stmcture at an... 

One of the most successful approaches to the synthesis of metalla-supramolecular systems has been by templated self-assembly processes. Most of the templating agents used by synthetic chemists are either cationic or neutral species. Anions, on the other hand, have only fairly recently been successfully utilized as templates. The new metalla-macrocycles [Pd2Ni2(atu)4(PPh3)4X]3+ (atu = deprotonated amidinothiourea, X = C1, Br, I) have been prepared using anion templated syntheses. Only in the presence of each of these three halides are the metalla-macrocycles... 

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

Further studies by the same authors have demonstrated that PFj acts a hydrogen bond-acceptor template in the assembly of several other interwoven structures. In an extensive study aimed at using a combination of hydrogenbonding motifs to self-assemble pseudorotaxanes into more complex structures it was discovered that PFj assists on the organization of the components that yield the final superstructure [74]. Particularly, it was found that this anion dictates the orientation of the two carboxylic acid groups of the [3]pseudorotaxanes 58 and 59 (see Schemes 27 and 28) when these groups are co-directional with respect to each other the formation of discrete hydrogen-bonded dimers is observed. 

De Mendoza reported the first example of anion-directed helix formation in 1996 [91]. The assembly of this helical structure relies, not only on electrostatic interactions between the anionic template and the positively charged strands, but also on hydrogen bonding. The tetraguanidinium strand 69 (see Scheme 34) self-assembles around a sulfate anion via hydrogen bonding to produce a double helical structure. The formation of this assembly and its anion-dependence was proposed on the basis of NMR and CD spectroscopic studies. 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

Anion templating is a formidable approach to the construction of large coordination complexes, cages and polyhedra by self-assembly using relatively simple ligands and we will examine some anion templated systems as part of our discussion of self-assembly in Chapter 10. For example, a... 

Scheme 5.4 Anion templated self-assembly of gold(I) phosphine macrocycles.34...

For a number of years the research interests of the Beer group have covered many areas of macrocychc coordination and supramolecular chemistry. This article reviews our latest results of current research by focusing on three major sections sensing of cations and anions anion templated assembly of pseudorotaxanes and rotaxanes, and metal-directed self-assembly using the dithiocaibamate ligand. 

Figure 4. Anion-templated self-assembly ofpseudorotaxanes. Recognition of the anion by macrocycle results in the formation of an interpenetrated structure.

The cyanometalate box Cs c [Cp Rh(CN)3]4[Mo(CO)3]4 3 is formed in low yield from the reaction of [Cp Rh(CN)3] (Cp = CsMes) and (/ -CeHtMet )Mo(CO)3 in the presence of cesium ions, and it can be crystallized as a Et4N+ salt. The Cs+ ion serves as a template in the self-assembly of the anionic molecular box, which has a cubic Rh4Mo4(/r-CN)i2 core with three exterior carbonyl ligands attached to each Mo and a Cp group to each Rh. The encapsulated Cs+ ion has a formal coordination number of 12 if interaction with the centers of cyano groups is considered (Fig. 20.3.15). 

It is also possible to prepare chiral PANI by in situ polymerisation with CSA, and in this case the reaction can afford chiral nanotubes [63]. The optically active materials contain nanotubes with 80 to 200 nm outer diameter and an internal diameter of between 20 and 40 nm, as revealed through microscopy images. A self-assembly process was proposed in which anilinium cations and CSA anions form micelles which act as templates for the growing polymer chains. Nanotubes are also formed when (R)- or (S)-2-pyrrolidone-... 

Anions have also been used as templates in the synthesis of catenanes and rotaxanes, as recently described by Beer and coworkers [15]. They used the self-assembly of diamides, known as anion receptor molecules, around a chloride anion to produce, for example, [2]- and [3]catenanes by olefin metathesis [16] (Scheme 5.4). In this special example, the template effect was increased by hydrogen bonding between the N-methyl group to the crown ether chain and by n-n stacking interactions. 

Another use of the DNA-lipid complex was reported by Wong and coworkers [33]. The DNA molecule structural features can be imprinted into CdS nanostructures by using self-assembled DNA-membrane templates. The initial application of the DNA-lipid multilamellar structure was the use as gene carriers in gene therapy, and many reports [34-39] were published (Fig. 6b). To prepare the DNA templates, anionic DNA was reacted with a mixture of... 

Fig. 5 Anion-templated self-assembly of two coordination squares and a pentagon, and their dynamic interconversion...

Campos-Femandez C, Schottel BL, Chifotides HT et al (2005) Anion template effect on the self-assembly and interconversion of metallacyclophanes. J Am Chem Soc 127 12909-12923... 

Since ring formation is greatly favored over oligomerizition in the previous examples, the synthesis of mercura-carborand occurs via a preferred self-assembly process, and it is clear that the ring size is dictated by the associated mercury anion. The noncoordinating acetate anion leads to the less-strained and presumably more-preferred trimeric macrocycle 5. In contrast, halide anions produce the tetramer macrocycles 9-11. It is postulated that the halide ions act as kinetic template by stabilizing a trimer intermediate, which subsequently reacts further to form the tetrameric macrocycles <1997ACR267>. 

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