Self-assembly via hydrogen bonding

Consider the various forces of molecular recognition involved in these systems, as well as the other design principles that favor controlled assembly. One key issue is directionality. 

The creation of a hexameric melamine compound gives very high positive cooperativity for the creation of a supramolecule containing six isocyanuric acids. Mathias, J. P., Seto, C. T., Simanek, E. E., and Whitesides, G. M. Self-Assembly through Hydrogen Bonding Preparation and Characterization of Three New Types of Supramolecular Aggregates Based on Parallel Cyclic CA3-M3 Rosettes. /. Am. Chem. Soc., 116,1725-1736(1994). 

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FIGURE 15 The Rebek group s self-replicating system a naphthalene ester and a heterocyclic amine self-assemble via hydrogen bonding and 7t-7t stacking (53,54). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Whereas Alajarin and Steed demonstrated the ability of ort/zo-substituted tris (o-ureido-benzyl)amine to form a homodimer in the solid state,13 the same group later revealed the ability of the meta-derivative, tris(m-ureido-benzyl)amine, to give a dimeric capsule (Fig. 21).41 In particular, the amines self-assembled via hydrogen bonds involving the urea groups to form a dimer larger than the ortho-structure. The dimer possessed a central cavity that hosted an ordered dichloromethane molecule as a guest. 

Nature itself is full of supramolecular systems, for example, deoxyribonucleic acid (DNA) is made up from two strands which self-assemble via hydrogen bonds and aromatic stacking interactions to form the famous double helical structure (see Chapter 3, Section 3.2.4). The inspiration for many supramolecular species designed and developed by chemists has come from biological systems. 

Figure 26 Structures of monodentate phosphine ligands 96-100, which self-assemble via hydrogen-bonding interactions to genante bidentate ligands that mediate hydroformylation reactions.

Coordination chains and helices self-assembled via hydrogen-bond interactions... 

Aoyama, Y. Endo, K. Anzai, T. Yamaguchi, Y. Sawaki, T. Kobayashi, K. Kane-hisa, N. Hashimoto, H. Kai, Y. Masuda, Y. Crystal Engineering of Stacked Aromatic Columns. Three-Dimensional Control of the Alignment of Orthogonal Aromatic Triads and Guest Quinones via Self-Assembly of Hydrogen-Bonded Networks. J. Am. Chem. Soc. 1996, 118, 5562-5571. 

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. 

Fig. 40. Self-assembly of the polymolecular supramolecular species (TP2, TU2) 174 from the complementary chiral components TP2 and TU2 via hydrogen bonding T represents L-, D- or meso(M)-tartaric acid R = Ci2H25.

A relatively new field called supramolecular chemistry has been developed over the last three decades. Supramolecular assemblies and supramolecular polymers differ from macromolecules, where the monomeric units are covalently linked. In a supramolecular polymer, the monomeric units self-assemble via reversible, highly directional, noncova-lent interactions. These types of bonding forces are sometimes called secondary interactions. Hydrogen bonding is the secondary force most utilized in supramolecular chemistry, but metal coordination and aromatic tt-tt electronic interactions have also been used. From a materials standpoint, supramolecular assemblies are promising because of the reversibility stemming from the secondary interactions. The goal is to build materials whose architectural and dynamical properties can respond reversibly to external stimuli. Solid phases are prepared by self-assembly from solution. In the solid-state, supramolecular polymers can be either crystalline or amorphous. 

Fig. 9 Self assembly via amidic hydrogen bonds and fixation of the assembly via a sol/gel process...

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