Simple Host-Guest Assemblies

One of the simplest expressions of self-assembly is the 1 1 complexation of a host and guest. The investigation of such systems over many years has provided a cornerstone for the development of supramolecular chemistry involving larger molecular aggregates. It is appropriate that we look first at a few simple systems. 

Self-assembly, involving dimerisation of a single suitable monomer, corresponds to the simplest type of host-guest complexation. Thus, the dithiourea derivative 1, 

In a subsequent study, the synthesis of urea and thiourea derivatives of types 3a and 3b were undertaken in an attempt to obtain new hosts that might yield more stable complexes with glutamic acid (of types 4a and 4b) and that might, in fact, hold together in polar solvents. The rationale for the design was based on the previously documented observation that both 1,3-dimethylurea and 1,3-dimethyl-thiourea form stable host-guest complexes with acetate in deutero-dimethyl sulfoxide the K values for the latter are 45 dm moF and 340 dm moF , respectively. 

In contrast to 2, the above derivatives incorporate four hydrogen bond donors. As a consequence, a favourable bonding situation is present in which the four secondary amine protons of each host can interact with the two charged carboxylate groups of the guest - see 4a and 4b. The proposed structure of the complex of the bis-urea derivative was supported by the existence of large NMR downfield shifts for both the inner and outer urea NH resonances in deutero-dimethyl sulfoxide and the observation of intramolecular H NOEs between the receptor aryl and the guest CH2 resonances in this solvent. A Job s plot confirmed the 1 1 stoichiometry of the product. 

Structure 5 is one example of a number of dipyridones that incorporate different spacer groups.Since 5 was designed to be self-complementary, it was anticipated that it would self-associate to produce a dimer of type 6. Indeed, this was shown to be so in chloroform ( 90% dimer) by means of vapour pressure osmometry. X-Ray crystallography also confirmed that the dimer persists in the solid state. The behaviour of 5 contrasts markedly with that of 7 which was designed to be complementary only in an offset manner, such that linear polymerisation might be promoted. Under the dilute conditions of measurement, vapour pressure osmometry indicated that this species remains predominantly monomeric in chloroform however. X-ray diffraction confirmed that 7 adopts the linear polymeric structure illustrated by 8 in the solid state. As anticipated, since self-association involves hydrogen bonding, both 5 and 7 were shown to exist only as monomers in the pro-tic solvent methanol. 

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The field of supramolecular chemistry is concerned with a large number of systems ranging from simple host-guest complexes to more complicated solution assemblies, as well as two-dimensional (organized monolayers) and three-dimensional assemblies (crystalline solids). Nonco-valent interactions play an important role in the kinetic assembly and thermodynamic stabilization of all these systems and constitute their most distinctive feature. Electron-transfer reactions can obviously be affected by supramolecular structures, but the reverse is also true. It is possible to alter the structure and the thermodynamic stability of supramolecular assemblies using electrochemical (redox) conversions. In other words, electron-transfer reactions can be utilized to exert some degree of control on supramolecular aggregates. Provided in this article is an overview of the interplay between supramolecular structure and electron-transfer reactions. 

Detection of pyrophosphate has also been demonstrated by a simple selfassembling system A-10 (Figure 10.32) with a pyrene-functionalized monoguani-dinium receptor. This receptor was found to self-assemble to form a 2 1 (host -guest) complex with high selectivity for biologically important pyrophosphate ions in methanol. A sandwich-like ground-state pyrene dimer is formed. The character-... 

X-ray crystallography provides the clearest structural evidence of the often-subtle intermolecular interactions. Combination of solution-state methods (NMR spectroscopy, mass spectrometry) and soUd-state methods (X-ray structure analysis) is necessary for an overall understanding of the host-guest behavior. In the case of self-assembled NRs, mechanisms of guest inclusion, guest exchange, and host-guest interactions are not simple, due to the dynamic, flexible nature of self-assembled NRs [13]. 

Because of their intellectual and practical importance, and their aesthetic beauty, chemists have longed to mimic such large, multimolecular complexes. The phrase supramolecular chemistry—chemistry beyond the molecule—has been coined to describe this field. Certainly, the host-guest systems we discussed above are simple examples of supramolecules, and the molecular recognition forces we ve enumerated will provide the driving force for assembly. But, when a large number of molecules assembles into one complex system, new issues arise. Here we will summarize some of the general issues associated with supramolecular chemistry, and then we will present some examples of especially successful or informative systems. 

Class B. This class includes self-assembled structures based on supramolecular binding of monofunctional unimers. These unimers cannot undergo open snpramolecular polymerization, but can form closed assemblies involving low- or high-MW species. Classical host-guest complexes, base pairing of simple nucleosides, and supermolecules are low-MW examples. Polymeric examples include side-chain binding of... 

In initial studies the 2,5-diaminoquinone system 19 was prepared [33]. The X-ray crystal structure of this compound demonstrated that the 2,5-diaminoquinone assembled with a molecular repeat distance of 7.46 A, which is close to the dm value of 7.4A believed to be suitable for a topochemical polymerization (Fig. 5.8). Unfortunately, these simple 2,5-diaminoquinones proved to be very insoluble and not suitable for host-guest studies. However, the sub-structural feature responsible for the repeat distance is the vinylogous amide. Simple vinylogous amides such as 20 should have better solubility properties although the persistency of this functional group to assemble into the required motif is more problematical [35] . Among the... 

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