Molecular Meccano, 27: A template-directed synthesis of a molecular trefoil knot

Abstract

A design logic for the self-assembly of a molecular trefoil knot and of its isomeric trivial knot is described. The approach relies on the formation of a double-stranded supramolecular complex between an acyclic π-electron rich 1,5-dioxynaphthalene-based polyether and an acyclic π-electron deficient bipyridinium-based tetracation. The reversible complexation of the two acyclic complementary components in solution is followed by the irreversible formation of covalent bonds connecting the termini of these two precursors within the supramolecular complex, affording both a trefoil knot and a trivial knot, though only in extremely low yields. The isomers were separated by preparative high-performance liquid chromatography and characterized by liquid secondary ion mass spectrometry which revealed the binding of up to two ammonium ions in the case of only one of the two isomers -namely, the trivial knot. In addition, we have performed binding studies between some model acyclic and macrocyclic 1,5-dioxynaphthalene-based polyethers and the bipyridinium based precursor of the knots. One of the complexes was also characterized in the solid state by X-ray crystallography which revealed the formation of a [2]pseudorotaxane in the solid state. © Wiley-VCH Verlag GmbH, 1997.