Lanthanide Template Synthesis of Trefoil Knots of Single Handedness

We
report on the assembly of 2,6-pyridinedicarboxamide ligands
(1) with point chirality about lanthanide metal ion (Ln3+) templates, in which the helical chirality of the resulting
entwined 3:1 ligand:metal complexes is covalently captured by ring-closing
olefin metathesis to form topologically chiral molecular trefoil knots
of single handedness. The ligands do not self-sort (racemic ligands
form a near-statistical mixture of homoleptic and heteroleptic lanthanide
complexes), but the use of only (R,R)-1 leads solely to a trefoil knot of Λ-handedness,
whereas (S,S)-1 forms
the Δ-trefoil knot with complete stereoselectivity. The knots
and their isomeric unknot macrocycles were characterized by NMR spectroscopy,
mass spectrometry, and X-ray crystallography and the expression of
the chirality that results from the topology of the knots studied
by circular dichroism.