Supramolecular rosette

Design of supramolecular rosette tape and linker. From T. C. W. Mak and... 

Each supramolecular rosette comprises a quasihexagonal assembly of two GM+ and four HC units connected by strong Nqm-H- -Ohc and Ohc-H- -Ohc hydrogen bonds. The (HC )2 dimer is shared as a common edge of adjacent rosettes and makes full use of its remaining acceptor sites in linking with GM+. However, each GM+ in... 

Mak TCW, Xue F (2000) Supramolecular rosette ribbon construded from guanidium and hydrogen carbonate ions in the crystal engineering of hydrogen-bonded networks. J Am Chem Soc 122 9860... 

Fig. 7. Formation of a supramolecular aggregate composed of a compound containing nine melamine rings (the three-layered nonamelamine derivative A) and nine molecules of neohexylisocyanurate (B). Of the 16 possible conformers that can result, two are shown The first has the nine molecules of B arranged in three rosettes of three molecules each, stacked atop each other in the second, the rosettes are staggered with respect to each other such that the rosettes in the first and third layers of A are aligned with each other, but not with the rosette in the second layer. The supramolecular assembly is stabilized...

The design and synthesis of supramolecular architectures with parallel control over shape and dimensions is a challenging task in current organic chemistry [13, 14], The information stored at a molecular level plays a key role in the process of self-assembly. Recent examples of nanoscopic supramolecular complexes from outside the dendrimer held include hydrogen-bonded rosettes [15,16], polymers [17], sandwiches [18, 19] and other complexes [20-22], helicates [23], grids [24], mushrooms [25], capsules [26] and spheres [27]. 

More detailed information about the characterization of double rosette assemblies can be found in D. N. Reinhoudt, P. Timmerman, F. Cardullo, and M. Crego-Calama, in Supramolecular Science Where it is and Where it is Going, edited by R. Ungaro and E. Dalcanale (Kluwer Academic Publishers, Dordrecht, 1999), pp. 181-195. 

As one example, the observation of hydrogen bonding [7] in natural systems such as peptide helices and DNA base pairs led to a theoretical understanding of this phenomenon. This understanding has permitted the use of hydrogen bonding in synthesis, leading to the preparation of such diverse structures as Rebek s capsules [8], Lehn s supramolecular polymers [9], and Whitesides rosettes [10]. 

Similar chiral supramolecular assemblies are also observed from achiral calix[4]arene dimelamine derivatives with chiral cyanurates thus leading to D3 symmetry species [21]. Analogously to chiral dimelamines, predominantly one of the two possible diastereomeric double-rosette assemblies was observed in most cases and it is noteworthy that the same (R) chiral moiety when mounted in the dimelamine is located on the left-hand side of the calix[4]arene and induces (M) chirality, while when implemented in the cya-nurate is placed on the right-hand side and therefore promotes P chirality (Fig. 8). 

Fig. 24 Enantioselective non covalent synthesis of double rosettes exploiting the chiral memory effect. Exchange of chiral barbiturate with the achiral cyanurate occurs faster then racemization by dissociation of the supramolecular structure and allows temporary preservation of the asymmetry instructed in the first diastereoselective step...

Fig. 13 Schematic illustration of supramolecular self-assembly of 27 forming a left-handed rosette nanotube with L-alanine...

The opposite supramolecular chiral motif is always observed for opposite enantiomers, but which handedness will be expressed in the supramolecular assembly of a particular enantiomer is difficult to predict. An example for opposite-handed supramolecular structures built by equally handed monomers is the formation of chiral rosettes with oligp(p-phenylene vinylene) derivatives on hopg (Fig. 17). Again, the interplay of lateral hy-... 

Finally, we mention a remarkable example of lateral resolution reported for supramolecular nanostructures on hopg [96]. Held together by 72 hydrogen bonds, the molecular nanostructure is formed from three melamine-substituted calix[4]arene units and twelve 5,5-diethylbarbiturate molecules (Fig. 31a). The nanostructure, basically a stack of four rosettes, has chiral symmetry. With its components all being achiral, both enantiomers are formed upon self-assembly in solution. Deposition of the tetrarosettes on hopg leaves this nanostructure intact and allows surface self-assembly. AFM studies revealed close-packed 2D lattices formed by the tetrarosettes on hopg... 

Strategies to Build up Supramolecular Macrocycles Based on Hydrogen Bonds 77 Table 3.1 Stability indices for hydrogen-bonded rosettes. 

Figure 36 Supramolecular assemblies of finite size (a) rosettes from melamine and cyanuric acid [272,405]. (b) Cyclic hexamers from isophthalic acid bearing bulky substituents in 5-position, (c) 3 x 3 grid from metal-complexed biphenyl phenanthrolines [407]. With permission of Wiley-VCH...

H. Schonherr, V. Paraschiv, S. Zapotoczny, M. Crego-Calama, P. Timmerman, C.W. Frank, G.J. Vancso, D.N. Reinhouldt, Unraveling the Nanostructime of Supramolecular Assemblies of Hydrogen-Bonded Rosettes on Graphite An Atomic Force Microscopy Study , Proc. Natl. Acad. Sci., USA, 99,5024 (2002)... 

Three equivalents of a calix[4]arene derivative, diametrically substituted at its upper rim with two melamine units, have also been demonstrated to yield well-defined box-like assemblies on interaction with six equivalents of 5,5-diethylbar-bituric acid. The product, which is based on an extended rosette motif, consists of nine different components held together by 36 hydrogen bonds. It is stable in apolar solvents and remains so even at very low concentrations. Further, the structure remains intact in the solid state, as confirmed by an X-ray diffraction study. In an extension of this study, three enantio-pure calix[4]arene-dimelamine derivatives were employed for a parallel study. In this instance, the chiral information in the individual calixarene derivatives proved successful in controlling the conformation of the final assembly - a homochiral supramolecular entity was the result. 

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