The rotational movement in solution of fumaramide- vs. succinamide [2]rotaxanes: The influence of intercomponent interactions

Abstract

Due to their intrinsic topological features, most artificial molecular devices are based on rotaxane molecules. One of the most important characteristics of this class of compounds is the likelihood of performing molecular movements, which are directly correlated with the intercomponent interactions present in these molecules. The tetralactam macrocycle is quite versatile and used in numerous molecular threads, which have stations that work as fundamental parts in the formation, molecular stabilization, and molecular movements performed. Inspired by how these stations may influence molecular movements (e.g., rotational ones), we sought to study two diamides commonly used as rotaxane stations: fumaramide and succinamide. In this context, we explored the change of station in six pairs of [2]rotaxanes. Studies were carried out in solution through NMR experiments with temperature variation techniques. In the crystalline solid state, the quantum theory of atoms in molecules analysis was performed through quantum mechanics calculations using the density functional theory. The rotational movement found for fumaramide-based [2]rotaxanes is, on average, over 14.91 kcal mol−1. In comparison, succinamide-based [2]rotaxanes have values of 12.78 kcal mol−1 (a 14% difference). In solution, fumadamide-based rotaxanes better retain the macrocycle due to the rigidity of the double bond, making its rotation more difficult and consequently leading to higher rotational energy barriers. In the crystalline solid state, the succinamide station, which has greater structural freedom due to the simple carbon–carbon bond, promotes more interactions, having a greater “complementary adjustment” and a higher stabilization energy. Lastly, our findings demonstrated how intercomponent interactions present in the studied stations can be changed to be used as a “lubricant” in solution for these molecular machine prototypes.