Hydrogen-bond-based Assemblies

1 Homomeric 0-D Assemblies Constructed from Hydrogen Bonds 

Carboxylic acids, oximes, diones and amino-pyridines can all form homomeric 0-D motifs through self-complementary and, frequently, symmetry-related hydrogen-bond interactions, 

Amides frequently combine two unique structural motifs, dimers and catemers, into a well-known 1-D ribbon-like structure. The dimer is constructed from self-complementary N-H O hydrogen bonds, while the catemeric N-H O hydrogen bonds result from the anti-proton of the amine and the bifurcated carbonyl oxygen atom. 

Symmetric dipyridones form 1-D arrays through pairs of N-H O interactions. Similarly, diacids, both aliphatic and aromatic, commonly produce infinite 1-D chains in the absence of strong competitive hydrogen-bond acceptors. 

Oxime functionalities have the capability of forming either dimers or catemers both of which result from O-H N hydrogen bonds. The balance between the two types of motifs is very subtle, which is illustrated by the fact that 4-chloropheny- 

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Lehn has also reported the hydrogen-bonding templated assembly of receptors based on bipyridine copper and palladium complexes [102]. A mixture of substituted bipyridines (76, 77) (see Scheme 39) with copper(I) triflate generates a mixture of tetrahedral complexes and uncoordinated ligands. 

Figure 6.7 Illustration of multipoint hydrogen bonding based self-assembly (a) hydrogen bond formation between barbituric acid functionalized gold nanoparticles and Hamilton receptor functionalized block copolymers and (b) selective deposition of nanoparticles on a microphase-separated block copolymer film. Reprinted with permission fi om Binder et al. (2005). Copyright 2005 American Chemical Society.

As stated, hydrogen bonds have been used to construct the majority of finite molecular assemblies. Thus, most synthons used to form finite assemblies in the solid state have been based on hydrogen bonds. Many such synthons have also been used to form networks.2 Examples include single-point hydrogen bonds based on phenols and imidazoles, as well as multi-point hydrogen bonds based on carboxylic acid dimers, pyridone dimers, urea dimers, cyanuric acid-melamine complexes, and pyridine-carboxylic acid complexes.2... 

Early work of Whitesides demonstrated the formation of heteromeric cyclic assemblies of barbital and ArW,-bis(4-tert-butylphenyl)-melamine that formed a 2D hydrogen-bonded rosette , of composition 3(barbital) 3(melamine), in the solid state.24 The components assembled via 18 hydrogen bonds based on alternating ADA and DAD sequences of a cyanuric acid-melamine lattice. Covalent... 

Hydrogen-bond-based strategies have also been successfully employed in the supramolecular assembly of metal clusters. The reaction of [Mo6C18]4+ ions with six equivalents of 4-hydroxy-benzamide yielded six octahedrally arranged oxobenzamide groups per cluster. Amide- amide hydrogen bonds produced infinite 2D networks separated by counterions.57... 

At the same time that Rebek s group began investigation of the hydrogen-bond self-assembly of glycoluril-based... 

Ranganathan. D. Lakshmi. C. Karle. I.L. Hydrogen-bonded self-assembled peptide nanotubes from cystine-based macrocyclic bisureas. J. Am. Chem. Soc. 1999. 121 (26), 6103-6107. 

Rebck and coworkers extended their studies on the hydrogen-bond-based self-assembly of tetrameric capsules around guest molecules using a combinatorial approach. Starting from two different monomers, a small dynamic library of guest-filled capsules was produced. The distribution of capsule compositions was analyzed by MS and found to depend on the nature of the guest, with methylquinuclidinium cation as the most... 

Similar to hydrogen-bond-based networks, the halogen-bonded 2D and 3D networks described above often contain large cavities, as is the case for similarly sized networks assembled through HB. " The voids are filled by solvent molecules or through interpenetration in the overall crystal packing. ... 

An intriguing class of hydrogen-bonded self-assembled molecular capsule is that of the Rebek dimeric capsules based on deep cavitands prepared from resorcin[4] arene derivatives [3,12,22], The importance of this class of dimeric capsules arises from their elongated shape which resulted in anisotropic imsymmetrical cavities which gave birth to several phenomena such as social and constellational isomerism, [23a-b] which could not be observed in the bulk or in other capsules... 

Limitations on the application of hydrogen bonded capsular assemblies come from the poor stability of these assemblies in polar media compared to those held by covalent bonds. In an attempt to overcome these Umitations, Bohmer described the synthesis of a mechanically locked chiral bis-catenane based on calixarene capsules [55]. Inspired by this pioneering work, our group produced a mechanically locked hybrid tetraurea calix[4]arene-calix[4]pyrrole capsule 30 (Fig. 32.14) [56]. This catenated capsule displayed reversible encapsulation (i.e. reversible assembly of the urea belt). The two hemispheres of the capsule never dissociate completely due to the bis-catenated topology. 

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