Cooperativity and Feedback Mechanisms in the Single-Crystal-to-Single-Crystal Solid-State Diels-Alder Reaction of 9-Methylanthracene with Bis(N-cyclobutylimino)-1,4-dithiin

Electron donor-to-acceptor interactions between 9-methylanthracene and bis(N-cyclobutylimino)-1,4-dithiin lead to the formation of chiral charge-transfer (CT) crystals. The structure consists of charge-transfer stacks where these two molecules arrange in a 1:1 alternating arrangement. These undergo a topochemical thermal single-crystal-to-single-crystal (SCSC) [2 + 4] Diels-Alder reaction in the solid state. CT crystals were reacted at 40 degrees C, their structures were determined by X-ray diffraction at various degrees of conversion, and they were examined using Hirshfeld surfaces and lattice energy calculations to find evidence of reaction cooperativity and feedback mechanisms. The results show that steric effects between product molecules and reactant molecules during the SCSC reaction influence the formation of products along the b axis, resulting in a more ordered structure than initially suggested by the crystal structure analysis. A maximum reaction conversion of around 96% was obtained, which indicates that the reaction is also nonrandom within the charge-transfer stacks. Lattice and intramolecular energy calculations show that the energy of an inherently metastable crystal obtained via the SCSC reaction is slightly higher compared to that of the recrystallized product crystal. Finally, structural analysis using CrystalExplorer shows that the shape, size, and surface curvature of the Hirshfeld surface are not much changed by the reaction, indicating that the reaction cavity remains relatively constant and that the reaction is under topochemical control.