Lanthanide Template Synthesis of Trefoil Knots of Single Handedness

Abstract

We report on the assembly of 2,6-pyridinedicarboxamide ligands (1) with point chirality about lanthanide metal ion (Ln(3+)) 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.

Securing a Supramolecular Architecture by Tying a Stopper Knot

“…Tris(2,6‐pyridinedicarboxamide) ligands can be tied5d into overhand knots15 through coordination to lanthanide(III) ions 14, 16, 17. We found it convenient to assemble the complete axle as a rotaxane (Scheme 2; see also the Supporting Information).…”

“…In MeCN‐ d 3 , the macrocycle samples much of the axle as evidenced by modest shifts in the 1 H NMR resonances of protons all along the length of the thread (Figure S3). X‐ray crystal structures of knotted tris(2,6‐pyridinedicarboxamide) lanthanide complexes5d, 16 indicate that the tied knot has a diameter of approximately 2 nm, whereas the aperture of DB24C8 is <1 nm. Accordingly, with the binding site for the crown ether deactivated, the rotaxane architecture of 1 is still kinetically stable; at room temperature, 1 showed no signs of dethreading over several weeks in CD 2 Cl 2 or MeCN‐ d 3 solution.…”

Molekulare Knoten

“…[65] c) Rçntgenstruktur des verwandten Kleeblattknoten Cu 2 5c.D ieses frühere Design wurde in geringerer Ausbeutee rhalten,d adie flexible Alkylkette einen geringeren Grad an Präorgani-sation aufweist als die m-Phenylen-Einheit späterer Designs. [63] [73] (Schema 4a). Die Rçntgenkristallstruktur des enantiomerenreinen Knotens ist in Schema 14 bdargestellt.…”

“…Die achirale Vorstufe 14 a gibt ein racemisches Gemisch der beiden Enantiomere des Kleeblattknotens Ln15 a.Die Verwendungdes C 2 -symmetrischen Liganden 14 b gibt den enantiomerenreinen Kleeblattknoten Ln15 b.b )Rçntgenkristallstruktur des enantiomerenreinen KleeblattknotensL n 15 b. [72,73] Angewandte Chemie Aufsätze dass die Verbindung tatsächlich ein Kleeblattknoten (22)war (Schema 7, oben). [82] Später wurde erfolgreich eine Racematspaltung der Knoten-Enantiomere durchgeführt.…”

“…Einige Beispiele fürt opologische Chiralitäti nK leinmolekülknoten wurden eingehender studiert, da es mçglich war, [68,73,74,84,87,99] oder die racemischen Knotengemische in die Enantiomere aufzutrennen. [64,67,83,107,110] Knotens einen signifikanten Effekt auf die Asymmetrie der Chromophor-Umgebung hat.…”