Complexes melamine-cyanuric acid

The complex between melamine and cyanuric acid (1 1) was reported in the literature in the late 1970s, but it was only in the early 1990s that the contributions from Whitesides and the concept of self-assembly popularized these systems [45]. Whitesides and co-workers reported the formation of tapes (Fig. 11.11), crinkled tapes and cyclic hexamers (rosettes) formed between barbituric acid and N,N -bis(p-substituted phenyl)melamine [46], In this they effectively blocked one face of melamine and, by manipulating substituents at the para position, different structures were obtained. Whiteside s putative suggestion that melamine/cyanuric acid formed an extended array (Fig. 11.12) was confirmed recently by Rao et al. with the crystal structure [47]. Hamilton and coworkers reported the crystal structure of a 5-substituted isophthalic acid derivative, which forms a cyclic aggregate held together with six pairs of hydrogen bonds, which in a way resembles the trimesic acid (Fig. 11.13) [48]. 

Secrecy surrounding the scandal has made it impossible to know for certain how many babies were poisoned—and how many died. The formation of the toxic melamine-cyanuric acid complex is the result of particularly strong attractive forces between the melamine and cyanuric acid molecules. Attractive forces between molecules (intermolecular forces) and within molecules (intramolecular forces) determine many of the physical properties of liquids and solids. 

Figures 5.2 and 6.2), the complex involving hydrogen-bonded barbiturate that should serve as sensor (Figure 6.9) and 234 have been presented earlier. The Whitesides rosette 3a [ 1 ], tapes 3b [2] and 109, crinkled tape 3c [ 1 ] (Figure 1.1) formed by melamine 1 and cyanuric acid 2 also shown earlier as well as... 

Hexagonal layer structure of the 1 1 complex of melamine and cyanuric acid. Three kinds of assembly in lower dimensions are possible (a) linear tape, (b) crinkled tape, and (c) rosette. 

Li s porphyrin rosettes (18) [22], Yagai s photoswitchable rosettes (19) [23], and Fenniri s nanotubes (20) [24] nicely illustrate the former approach. Thus, CA3M3 rosettes (Figure 3.7) endowed with a pyridine moiety at the melamine component and a strapped Zn-porphyrin (to enhance coordination from only one side of the ring) at the cyanuric acid component are more stable than the CA3 M3 model rosette initially reported by Whitesides. Even more complex versions of the same concept, involving one all-pyridine rosette and one all-porphyrin rosette to self-assemble into double rosettes were studied, although only NMR evidence was presented [22],... 

Zerkowski, J. A., MacDonald, J. C., Seto, C. T., Wierda, D. A. and Whitesides, G. M. (1994). Design of organic structures in the solid state molecular tapes based on the network of hydrogen bonds present in the cyanuric acid melamine complex. J. Am. Chem. Soc. 116, 2382. 

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...

Reaction of cyanuric acid (CA) with melamine (M) forms an insoluble 1 1 complex. This indicates directed hydrogen bonds between CA and M. ... 

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... 

Triazines are one of the oldest known compound classes in organic chemistry. First reports date back to 1776 when cyanuric acid (l,3,5-triazine-2,4,6-triol 1,3,5-triazine-2,4,6(l//,3//,5f/)-trione) was obtained by the pyrolysis of uric acid.1 The same method was also utilized in 1820 2 however, triazine compounds were probably first prepared in 1704 upon trimerization of cyanide derivatives when the Berlinerblau -complex salt, the first known cyano compound, was discovered.468 Cyanuric chloride was synthesized in 1828 from hydrogen cyanide and chlorine.3 Another method, discovered in 1834, involves treatment of potassium thiocyanate with chlorine. When heated, cyanuric chloride is obtained.4 Melamine (1,3,5-triazine-2,4,6-triamine) was also prepared in 1834 by heating potassium thiocyanate with ammonium chloride.5 Although 2,4,6-substituted 1,3,5-triazine derivatives were identified very early, the unsubstituted parent compound was not synthesized before 1895.6 At that time, however, the isolated compound was assigned to a dimeric species and not to the trimeric hydrogen cyanide. This was finally proven much later."... 

A classical two-component hydrogen-bonded system in self-assembly is represented by the melamine-barbituric/cyanuric acid complex. Whitesides and Lehn have exploited the complementarity of this interaction in the preferential formation of cyclic aggregates over the corresponding linear forms [69-71]. Cocrystals of 36 with 37, 38 and 39 indicate that the size of the substituent on the melamine component is essential in controlling the final supramolecular architecture (see Fig. 21). When relatively small substituents are present on the melamine subunit, one-dimensional tapes are formed. However, bulkier substituents enhance the formation of cyclic aggregates. 

The self-assembly behaviour and complexing properties of several strapped porphyrin-incorporated melamine-cyanuric or melamine-barbiturate-based rosette supramolecules in chloroform-d has produced some intriguing structures. The almost perfect H-bonding complementarity of the constituent units, amphfied by their multiphcity, results in particularly stable structures whose integrity is maintained in solution [42]. Strapped porphyrin cyanuric acid 40a and its Zn (II) complex 40b were designed to combine with melamine derivatives 41 to afford stable porphyrin rosettes 42. The new porphyrin rosettes 42 could efficiently complex tripyridyl derivative 43 through intermolecular, cooperative coordination between Zn (II) and pyridine to form the stable rosette 44. 

Figure 14 Aggregates in solution (a) 1 3 complex of hub B with melamines and cyanuric acid A and (b) dendritic hexamer.

Benzoic acid dimerization was also applied for the building of closed complex structures in crystalline states. For example, 5-substituted isophthalic acid derivatives form well-defined structures of cyclic hexameric aggregate 21 (Figure 18) [88]. Supramolecular cyclic hydrogen-bonded complex 22 is formed from cyanuric acid and melamine (Figure 19) [89]. A cyclic structure is formed by 1 1 complexation. 

Fig. 21 Schematic illustration of the hydrogen-bonded complexation and self-organization of azobenzene cyanuric acid 29 and a melamine 30 in water...

The use of molecular recognition was also extended to silylated melamine or cyanuric acid derivatives that formed complexes through hydrogen bonding [62]. 

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