Anionic templates

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... 

Another series of macrocycles prepared by hydrogen bonding to anion templates are the [ljimidazoliophanes recently reported by Alcalde [27]. The preparation of such macrocycles is based on a 3+1 convergent maerocyclization reaction which is highly dependent on the presence of specific anions (see Scheme 4). The yields of this reaction vary between 42%, when no anion is pre-... 

Scheme 5 One of the first anion-templated reactions was the macrocyclisation of 5 reported by Sessler...

The spherical metalla-cages [M2Ni4(atu)8X]3+ (M=Ni, 8a,b Pd, 9a,b X=C1, Br) have also been prepared by an anion templated process. These species can be obtained by reacting Ni(atu)2 with NiX2 and [PdX2(PhCN)2] respectively. Once again the formation of these metalla-assemblies is highly dependent on the pres-... 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

The anion-templated synthesis of rotaxanes first introduced by Vogtle has now been employed by others to produce other interlocked assemblies. Schalley for example [68], has reported an improved yield for the synthesis of rotaxanes by... 

Scheme 21 Anion-templated synthesis of [2]rotaxanes first introduced by Vogtle...

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. 

This seems to be the first rotaxane synthesis based on an anionic template and latest results indicate that this new strategy can be extended to ester- and acetal-based axles [70]. Hence the anionic procedure represents a powerful alternative to the rotaxane synthesis assisted by a neutral template. 

Figure 51. Anionic template synthesis of rotaxane 118 with a bis(phenyl ether) axle. The mechanical bond is formed by reaction of the phenolate-macrocycle complex [32-115] (supramolecular nucleophile) with the semiaxle intermediate 117. 

Scheme 15 A new anion template effect for the syntheses of ether rotaxanes.

Scheme 16 anion-templated rotaxane synthesis via a Michael addition reaction. 

SCHEME 17 The yields of rotaxanes synthesized via the anion template strategy depend much on... 

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...

Figure 5.14 Chloride anion templated metallacages based on amidothiourea (Hatu) (M = Ni, Pd).35...

Beer, P. D., Sambrook, M. R., Curiel, D., Anion-templated assembly of interpenetrated and interlocked structures. Chem. Commun. 2006, 2105-2117. 

The procedure of the anion-templated synthesis was later used for the preparation of various rotaxanes by the groups of Smith [41] and Schalley [34-36], The anion-templation technique was carefully tested by Schalley et al. Altering the length of the axle-center pieces and the size of stopper groups they synthesized a variety of rotaxanes and analysed their yields and de-slipping rates [34, 35],... 

Schalley, C.A, Silva, G., Nising, C.F., Linnartz, P. Analysis and improvement of an anion-templated rotaxane synthesis, Helv. Chim. Acta, 85 (2002), 1578-1596. 

To overcome this difficulty, a new solution is developed that involves spatial separation of the functional group for stopper attachment from the phenolate that provides the basis for anion template. Rotaxanes can be synthesized with yields up to 45% [32] (Figure 11). 

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