Solvent-driven selective π-cation templating in dynamic assembly of interlocked molecules

Abstract

Both bispyridinium (BPY) and trispyridinium (TPY) have been used to template the formation of linear or triply threaded [2]rotaxanes through imine-based dynamic clipping reactions. In this paper, we report contrasting solvent dependence between these two templated clipping reactions when two different solvents, namely CDCl₃ and CD₃CN, are used. The solvent dependence is elucidated based on ¹H NMR studies, and structural features are revealed by single crystal X-ray analyses of the respective linear and triply threaded interlocked molecules. We have shown that although both clipping reactions are affected by hydrogen-bonding and aromatic–aromatic interactions in general, the nature of the aromatic–aromatic interactions is quite different, which is responsible for the different solvent response. The BPY-based clipping reaction is driven by electrostatic interactions between aromatic surfaces, while the TPY-based reaction is mainly governed by the solvation/desolvation effect (solvophobic interactions). These findings led us to design a rare solvent switchable system. In competition clipping experiments employing both BPY and TPY as the templates, exclusive formation of the BPY-based linear [2]rotaxane can be achieved in pure CDCl₃, while in pure CD₃CN, a 6.7 : 1 selectivity is achieved in favor of the TPY-based triply threaded [2]rotaxane. The detailed structural analysis of the two [2]rotaxanes as well as the solvent-dependent selectivity, may encourage more integrated approaches for the design of complex molecular architectures.