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

Abstract

During synthesis and purification, compounds are exposed to various solvents. These solvents can, at times, assemble in the solid state. This association can arise from intermolecular interactions, allowing solvent molecules to bind with chemical substances and resulting in a unique crystalline structure known as a solvate. Nevertheless, such an association with solvents can substantially alter the properties of the compounds. In the crystalline solid state, the occurrence of solvates is recurrent among mechanically interlocked molecules (MIMs), including rotaxanes; however, solvates are frequently considered incidental or inconsequential. In this context, we sought to investigate the role and relevance of solvent molecules in the crystallization process of rotaxanes by systematically analyzing 24 models, employing the supramolecular cluster as a demarcation tool and using tetralactam-macrocycle based [2]rotaxanes. In general, the most frequently occurring [2]rotaxane solvates are with chloroform and water molecules (∼66%). The occurrence of solvates does not follow any rule or pattern, implying that similar structures can and do contain different solvates in varying proportions. By the crystallization mechanisms proposed, it was feasible to determine at which stage of the crystallization process the solvates are formed, and the energetic and topological contributions of the solvent molecules during crystallization were assessed. The most rotaxanes "trap" solvent molecules during crystallization, and interestingly, in certain instances, these solvent molecules are essential for crystallization, contributing up to 40% of the total stabilization energy of the supramolecular cluster.