Mechanical Bonding as a Promoter of Crystalline Diversity in Halogenated [2]Rotaxanes

Understanding the nature of the mechanical bond is critical to advancing the design of novel mechanically interlocked molecules. Given that numerous applications of new materials are related to solid-state properties, the behavior of the mechanical bond in the crystalline solid state is a key issue to be explored. Hence, this study sought to address how the freedom degree of the interlocked components can promote the formation of different crystalline phases. A series of rotaxanes substituted with different halogen atoms in the same position of the macrocycle were chosen as the model for study. Crystallization experiments were carried out in different solvents, obtaining 15 crystalline phases for the five studied compounds. The intercomponent interactions were evaluated in solution through variable-temperature 1H NMR experiments and in the crystalline solid state through quantum mechanics calculations. All rotaxanes presented the same type of interaction in solution and the solid state, although the distribution of interaction energies was modified depending on the crystalline phase. Selecting the substituent at the macrocycle and using different crystallization solvents at different temperatures formed these different crystalline phases. At the supramolecular crystalline level, four similar comparisons were found correlating the different compounds, in which one was the anhydrous phase and the other three phases were solvates of chloroform, tetrahydrofuran, and toluene. The key to understanding the formation of the different crystalline forms is the presence of the mechanical bond, providing the flexibility and mobility of the entwined components for keeping the most favorable intermolecular interactions in each experimental condition assayed.

Automated summary

Understanding the nature of the mechanical bond is crucial for the design of new mechanically interlocked molecules. This study investigated the formation of different crystalline phases by examining the freedom degree of interlocked components in rotaxanes. Through crystallization experiments and various solvents, 15 crystalline phases were obtained for five studied compounds. The intercomponent interactions were evaluated in solution and the solid state, and it was found that all rotaxanes had the same type of interaction, although the distribution of interaction energies varied depending on the crystalline phase.