Aggregate melamine

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...

Fig. 12 Neat melamine Hekates 40, their 1 1 host-guest complexes 41,42 and the 1 3 aggregates of a melamine core with three arms 43 and 44. The disk-shaped aggregates 41 stack in a Colrd phase [99]. The Janus-type Hekate 42 arrange in a Colh phase with hexagonal superstructure of the nanosegregated semi-perfluorinated chains [105]. The supramolecular Hekate discs 43-45 self-assemble by hydrogen bonding of the three arms around a melamine core...

A broad variety of sacrificial colloidal cores have been used for hollow capsule fabrication. They are inorganic or organic particles from tens of nanometers and up to tens of micrometerss, like melamine formaldehyde (MF), polysterene spheres, CaCC>3 and MgCQ3 particles, protein and DNA aggregates, small dye... 

One of the earliest examples of chiral melamine-cyanurate was reported by Whitesides and collaborators, who studied the hydrogen-bonded aggregate between three molecules of chiral cyanurate and a complementary achiral fr/ s-melaminc unit based on a 1,3,5 trisubstituted benzene with a melamine unit appended at each arm (Fig. 5) [20]. The fris-mclaminc unit is not chiral itself, but in the complex with cyanurate forms a tightly bonded hydrogen bond platform with C3 symmetry which is chiral at a supramolecular level due to the clockwise (P) and counterclockwise (M) conformation of the pendants (Fig. 5). 

Dyes can also be incorporated into hydrogen-bonded superstructures using the complementarity of the hydrogen bond donors and acceptors. The addition of a perylene bisimide dye to chiral dialkyl melamine derivatives leads to aggregates (Fig. 26) which show induced circular dichroism in the dye part of the assembly in methylcyclohexane as well as the formation of mesoscopic fibres when the solvent is evaporated [227]. The melamine compounds... 

Scheme 3.4 Tape vs. hexagonal rosette formation in Whitesides melamine-cyanurate aggregates [10].

The most stable aggregate of Whitesides rosettes family shown in Table 3.1 uses a combination of preorganization (sideways connection of the cyanurate subunits) and peripheral crowding of the melamine subunits for the preferential formation of... 

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]. 

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

In order to form such an aggregate, the bis-melamine calix[4]arene 135 may assume either a C2-symmetrical (staggered) conformation, in which the residues R1 are in an identical environment, or a Cs-symmetrical (eclipsed) conformation with two different environments for the residues R1. As illustrated in Figure 35, three different diastereomeric boxes may be formed by combination of three molecules of 135 with six molecules 136.281... 

Usually the H NMR spectra of the aggregates (135)3-(136)6 are simple, exhibiting for instance only two sharp singlets for the imide protons of the barbiturate (at 13-14 ppm in CDC13). This strong downfield shift is characteristic for the melamine-barbiturate hexamer, while the simple pattern in general is only in agreement with the formation of the chiral assembly with an overall D3 symmetry. However, the simultaneous formation of all three supramolecular diastereomers was detected by H NMR spectroscopy in the case of sterically hindered cyanurates... 

Figure 35. Three possible diastereomeric boxes and their symmetry. The combination of C2- and Cs-symmetrical bis-melamines is not possible in one aggregate.

It should be mentioned, that larger assemblies consisting of nine calixarenes are available with the calixarene-monocyanurate 139,208a while the double-calixarene 140 forms a double-box with 136 consisting of 15 molecules (3 x 140 and 9 x 136).283 Polymeric aggregates are formed between bis-melamine and bis-cyanurate calix[4]arenes.284 In all these cases, chiral induction still remains to be explored. 

Figure 2 The structure of the hydrogen-bonded CA M lattice. Three classes of aggregates that we have observed in solution and the solid state are highlighted the rosette, crinkled tape, and linear tape motifs. The molecular strucmres of melamine and cyanuric acid and their abbreviated representation as dark and light disks are shown...

Preorganization Preorganization is a qualitative concept introduced by Cram [4] to describe the conformational similarity of a molecule in its uncomplexed relative to its complexed state. We have covalently linked the three melamines of the CA M rosette by spokes to a central benzene-based hub this linkage results in preorganization by reducing unfavorable changes in translational, rotational, and conformational entropy upon formation of the aggregate. 

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