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

Abstract

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 1 H 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.

[2]Rotaxane Solvates: Insights into the Presence of Solvents in the Crystal Lattice

“…Thus, higher RMS values might imply a more pronounced supramolecular crystalline organization. Both compounds 2 and 3 exhibit nearly identical average RMS values, underscoring their molecular and supramolecular similarities, as previously discussed elsewhere …”

“…Studies by our research group have consistently demonstrated solvate formation in rotaxane molecules. ,,− ,− As part of our ongoing projects, we came across an interesting case through crystallization assays: two crystalline structures were found for a rotaxane molecule ( 1 ), one as a hydrate form ( 1I ) and the other in an anhydrous form ( 1II ) (Figure ). Our recent investigation exploring the crystalline diversity in halogenated [2]­rotaxanes revealed similar cases featuring pairs and trios of anhydrous and solvate forms of [2]­rotaxanes . Instigated by these results, we sought to understand the intricacies surrounding solvate formation in rotaxane molecules.…”

“…This phenomenon is common and occurs with numerous molecules; statistical data have shown that roughly a third of all organic compounds may crystallize in the solvate form . In particular, rotaxane molecules exhibit a propensity to crystallize as solvates, and despite its significance, this topic remains underexplored, with only a handful of studies addressing the topic. − Most articles report that solvates were obtained incidentally or as byproducts of crystallization, and others use solvate formation merely as evidence for the successful synthesis of the rotaxane molecule. − …”

Silver and Gold Pillarplex Pseudorotaxanes from α,ω‐Dicarboxylic Acids

“…By inspecting the Hirshfeld surface analysis of 3 and 4 , mainly potential hydrogen bonding sites at the rim are observed, therefore 3 and 4 resemble hydrogen‐bonded polypseudorotaxanes [34–35] in the solid state. These new findings, that the variation of guest molecules to encapsulate into pillarplexes can strongly influence the packing and the occurring (non–)covalent interactions, makes those mechanically‐interlocked molecules interesting in the context of recently introduced hydrogen‐bonded frameworks (HOFs) [36–39] and in the context of crystal engineering [40] …”

“…These new findings, that the variation of guest molecules to encapsulate into pillarplexes can strongly influence the packing and the occurring (non-)covalent interactions, makes those mechanically-interlocked molecules interesting in the context of recently introduced hydrogen-bonded frameworks (HOFs) [36][37][38][39] and in the context of crystal engineering. [40]…”