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

Peter Timmerman, Remko H. Vreekamp, Ron Hulst, Willem Verboom, David N. Reinhoudt, Kari Rissanen, Konstatin A. Udachin, John Ripmeester

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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.
Original languageEnglish
Pages (from-to)1823-1832
JournalChemistry: a European journal
Volume3
Issue number11
DOIs
Publication statusPublished - 1997

Keywords

  • Molecular boxes
  • Supramolecular Chemistry
  • Noncovalent assembly
  • Hydrogen bonds
  • Calixarenes

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