A Stimuli-Responsive Molecular Capsule with Switchable Dynamics, Chirality, and Encapsulation Characteristics.

Abstract

In this study, we report the preparation, conformational dynamics, and recognition characteristics of novel molecular capsule 1 comprising a bowl-shaped framework conjugated to a tris(2-pyridylmethyl)amine (TPA) lid. With the assistance of experiment (1H NMR spectroscopy) and theory (MM and DFT) we found that C3 symmetric 1 is poorly preorganized with three pyridines at the rim adopting a propeller-like orientation and undergoing P-to- M (or vice versa) stereoisomerization (Δ G⧧ < 8 kcal/mol, VT 1H NMR). Capsule 1 binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 with Ka < 7 M-1. Protonation of 1 with HCl, however, gives [1·H]-Cl, with the solid-state structure showing the TPA lid being "flattened" and the +N-H---Cl hydrogen-bonded group residing outside. Importantly, the P-to- M stereoisomerization would for [1·H]-Cl occur with Δ G⧧ = 11 kcal/mol (VT 1H NMR). Less dynamic and more preorganized [1·H]-Cl binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 guests with a greater affinity ( Ka = 100-400 M-1) than 1. The results of our studies suggest that the complexation of increasingly larger guests takes place in an induced-fit fashion, with [1·H]-Cl (a) elongating along its vertical axis and concurrently potentially (b) twisting pyridines from P into M (and vice versa) orientation. The addition of Et3N to [1·H]-Cl⊂CH2Cl2 causes deprotonation of the capsule and the release of CH2Cl2 with the process being fully reversed after the addition of HCl. Allosteric capsule 1 with unique structural and dynamic characteristics is expected to, in the future, assist the construction of complex molecular machines and smart functional materials.