Calix arene bismelamine

The group of Reinhoudt has reported the construction of a >3 hydrogen-bonded assembly of three calix[4]arene bismelamine and six barbituric acid derivatives (Fig. 20.2.7). 

Calix[4]arene 1, diametrically substituted at the upper rim with two melamine units is a perfect U-turn which has the correct structure to spontaneously form a well-defined 3D box-like assembly (X)3 (DEB)6 in the presence of two equivalents of 5,5-diethylbarbituric acid (DEB) (Figure 1). The assembly comprises three bismelamine and six barbiturate units and is held together by a total number of 36 hydrogen-bonds. The top and bottom of this box-like assembly consist of the cyclic hydrogen-bonded platform whereas the calix[4]arene units act as side walls. The assembly is stable in apolar solvents like CHCI3 or toluene even at 0.1 mM concentration, but the stability significantly decreases when competitive solvents like Me2SO or MeOH (10-20%) are added [8]. 

Mixing of 5 mM solutions of (1)3 (DEB)6 and (2)3 (DEB)6at 0 C (ratio 2 1) in toluene- /g gives only a mixture of the two separate homomeric assemblies. Exchange of the bismelamine calix[4]arenes is extremely slow at this temperature and, as a consequence, formation of the heteromeric assemblies is not observed. When the mixture is warmed to temperatures over 15 C the heteromeric assemblies start to form slowly. This reflects the relatively high kinetic stability of the separate assemblies in this apolar solvent. After 2.5 hours at 25 C the system reaches thermodynamic equilibrium (Figure 8). In the more polar solvent CDCI3 the exchange of components 1 and 2 is much faster. The mixture equilibrates within seconds at 25 C. Only below 50 ° C are the homomeric assemblies kinetically inert. 

Figure 10. Bismelamine derivatives 16-18 comprising dimethyl calix[4]arene-based spacers...

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