Molecular Russian dolls

Abstract

The host-guest recognition between two macrocycles to form hierarchical non-intertwined ring-in-ring assemblies remains an interesting and challenging target in noncovalent synthesis. Herein, we report the design and characterization of a box-in-box assembly on the basis of host-guest radical-pairing interactions between two rigid diradical dicationic cyclophanes. One striking feature of the box-in-box complex is its ability to host various 1,4-disubstituted benzene derivatives inside as a third component in the cavity of the smaller of the two diradical dicationic cyclophanes to produce hierarchical Russian doll like assemblies. These results highlight the utility of matching the dimensions of two different cyclophanes as an efficient approach for developing new hybrid supramolecular assemblies with radical-paired ring-in-ring complexes and smaller neutral guest molecules.

Fig. 1

Design and structures. a Examples of CBPQT⁴⁺ and CBPQT^2(+•) as hosts. b Proposed superstructures of the tetraradical tetracationic box-in-box complex and Russian doll assemblies

Fig. 2

Synthesis and crystal structure. a Synthesis of 1·4PF₆ and b solid-state structure of 1⁴⁺

Fig. 3

UV–Vis–NIR characterization of [CBPQT ⊂ 1]^4(+•). a UV–Vis–NIR spectra (0.50 mM in MeCN, 1 mm path cuvette) of CBPQT^2(+•) (blue), 1^2(+•) (red), and a 1:1 molar ratio of CBPQT^2(+•) and 1^2(+•) (purple); b Vis/NIR Spectra (MeCN, 2 mm cuvette) on titrating CBPQT^2(+•) into 1^2(•+) (0.10 mM). Initial and final spectra are highlighted in blue and purple, respectively. The inset shows the change in absorption at 910 nm on titration of 1^2(+•) with CBPQT^2(+•). Curve fitting is highlighted in red

Fig. 4

VT-NMR studies of [CBPQT ⊂ 1]^4(+•). ¹H NMR Spectra recorded from −40 to +25 °C for a 1:1 molar mixture of CBPQT^2(+•) and 1^2(•+) (1.0 mM each) in CD₃CN

Fig. 5

EPR characterization. a EPR Spectra recorded in 10 degree interval on a 1:1 molar mixture of CBPQT^2(+•) and 1^2(+•) (0.50 mM each) in MeCN. Cyclic voltammograms of b 1⁴⁺ (0.20 mM), c CBPQT⁴⁺ (0.20 mM), and d a 1:1 molar mixture of 1⁴⁺ and CBPQT⁴⁺ (0.20 mM each)

Fig. 6

Solid-state superstructures of [CBPQT ⊂ 1]^4(+•). a Perspective and b plan views depicted as tubular and space-filling representations. c, d Different side-on views, depicted as tubular representations. Hydrogen atoms in c and d are omitted for the sake of clarity

Fig. 7

VT-NMR studies of [p-C₆H₄Cl₂ ⊂ CBPQT ⊂ 1]^{4(+•) 1}H NMR Spectra recorded from −40 to +25 °C for a 1:1:1 molar mixture of CBPQT^2(+•), 1^2(+•) and 1,4-dichlorobenzene (1.0 mM each) in CD₃CN

Fig. 8

Solid-state superstructure of Russian doll assemblies with different guests inside the [CBPQT ⊂ 1]^4(+•) complex. a Structural formulas of the guest molecules. b Perspective views depicted as tubular and space-filling representations. c Side-on views depicted as tubular representations. Hydrogen atoms are omitted for the sake of clarity