Self-Sorting Assembly of Molecular Trefoil Knots of Single Handedness

We
report on the stereoselective synthesis of trefoil knots of
single topological handedness in up to 90% yield (over two steps)
through the formation of trimeric circular helicates from ligand strands
containing either imine or, unexpectedly, amide chelating units and
metal ion templates of the appropriate coordination character (zinc­(II)
for imines; cobalt­(III) for amides). The coordination stereochemistry
of the octahedral metal complexes is determined by asymmetric carbon
centers in the strands, ultimately translating into trefoil knots
that are a single enantiomer, both physically and in terms of their
fundamental topology. Both the imine–zinc and amide–cobalt
systems display self-sorting behavior, with racemic ligands forming
knots that individually contain only building blocks of the same chirality.
The knots and the corresponding trimeric circular helicate intermediates
(Zn­(II)3 complex for the imine ligands; Co­(III)3 complex for the amide ligands) were characterized by nuclear magnetic
resonance spectroscopy, mass spectrometry, and X-ray crystallography.
The latter confirms the trefoil knots as 84-membered macrocycles,
with each of the metal ions sited at a crossing point for three regions
of the strand. The stereochemistry of the octahedral coordination
centers imparts alternating crossings of the same handedness within
each circular helicate. The expression of chirality of the knotted
molecules was probed by circular dichroism: The topological handedness
of the demetalated knots was found to have a greater effect on the
CD response than the Euclidean chirality of an individual chiral center.