Amplification of weak chiral inductions for excellent control over the helical orientation of discrete topologically chiral (M3L2) n polyhedra

Abstract

Superb control over the helical chirality of discrete (M₃L₂) _n polyhedra (n = 2,4,8, M = Cu^I or Ag^I) created from the self-assembly of propeller-shaped ligands (L) equipped with chiral side chains is demonstrated here. Almost perfect chiral induction (>99 : 1) of the helical orientation of the framework was achieved for the largest (M₃L₂)₈ cube with 48 small chiral side chains (diameter: ∼5 nm), while no or moderate chiral induction was observed for smaller polyhedra (n = 2, 4). Thus, amplification of the weak chiral inductions of each ligand unit is an efficient way to control the chirality of large discrete nanostructures with high structural complexity.

Fig. 1. (a) Chemical structure of propeller-shaped ligand 1 and its P- and M-forms. Unless otherwise noted, the (S)-2-alkoxy isomers of 1b–e were used in the main study. (b) Schematic representation of the self-assembly of the (M₃L₂)_n coordination polyhedra 2–4 (n = 2, 4, 8) with chiral side chains. Each assembly is formed by the oligomerization of n units of M₃L₂, a hypothetical intermediate stabilized by metal–acetylene π-coordination. Illustrations in the second column from the left were generated by combining 2-butoxy side chains (molecular-mechanics (MM) models) with polyhedral frameworks based on the reported crystal structures. Cartoon illustrations in the third column exhibit the entangled structures of the (M₃L₂)_n polyhedra, while metal–acetylene π-coordination is omitted for clarity.

Fig. 2. Self-assembly of complex 2b and tetrahedron 3b. Partial ¹H NMR spectra of (a) ligand 1b (500 MHz, 300 K, CDCl₃), (b) complex 2b (700 MHz, 300 K, CD₃NO₂), and (c) tetrahedron 3b (500 MHz, 300 K, toluene-d₈). Red circles and blue triangles indicate two diastereomeric complexes, respectively. A ¹H DOSY spectrum is shown in addition (for details, see Fig. S5†). (d) X-ray crystal structure of 2b; 2-butoxy chains are shown as space-filling models.

Fig. 3. Self-assembly of cube 4 with ligands that contain chiral side chains. (a) Schematic representation of the nitrate-templated self-assembly of 4 (depicted with R = 2-butoxy for simplicity); partial ¹H NMR spectra (500 MHz, 287 K, CDCl₃/CD₃OD (94/6, v/v)) of ligand 1d and cube 4 showing (b) time-dependent redistribution of the diastereomers, (c) cube 4 with ligand 1d in CDCl₃/CD₃OD (97/3, v/v, t = 18 h), and (d) cube 4 with ligand 1b in CDCl₃/CD₃OD (97/3, v/v). (e) Changes in the CD (293 K, CHCl₃/MeOH (94/6, v/v)) spectrum upon formation of cube 4d. (f) Changes in the CD (293 K, CHCl₃/MeOH (97/3, v/v)) spectrum upon formation of cube 4b.