Noncovalent assembly of functional groups on calix[4]arene molecular boxes


Timmerman, Peter and Vreekamp, Remko H. and Hulst, Ron and Verboom, Willem and Reinhoudt, David N. and Rissanen, Kari and Udachin, Konstatin A. and Ripmeester, John (1997) Noncovalent assembly of functional groups on calix[4]arene molecular boxes. Chemistry: a European journal, 3 (11). pp. 1823-1832. ISSN 0947-6539

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Abstract:Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3-bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen-bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen-bonded assembly, but this effect very much depends on the exact positions of the groups.
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Copyright:© 1997 WILEY-VCH Verlag
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