Supramolecular Inhibition of [4 + 2] Diels–Alder Reactions in Charge-Transfer Crystals

Heteromolecular charge-transfer (CT) crystals formed using 1,4-dithiintetracarboxydiimide type compounds and anthracene derivatives are typically capable of undergoing [4 + 2] Diels–Alder (DA) reactions in the solid state. Several examples have highlighted the single-crystal-to-single-crystal (SCSC) reactivity of these types of CT crystals at temperatures around 20–50 °C. In furthering these studies, CT crystals with bis­(N-benzylimino)-1,4-dithiin and various anthracenes were grown, and their structures were elucidated through single crystal X-ray diffraction (six structures in total). The benzyl groups help facilitate CT formation and crystallization through C–H···π interactions between included anthracene molecules (the electron donor) and the bis­(N-benzylimino)-1,4-dithiin (DBn; the electron acceptor molecule), but their subsequent orientations within the stacks in the crystal structure inhibit product formation by preventing the molecular motion required for product formation. Assessment of their solid-state structures showed that the majority of reactive sites comply with Schmidt’s criteria. However, two modes of reaction were discovered. If the reaction was carried out quickly (less than an hour), the crystals were stable until their solid-state DA reaction temperature (above 130 °C for all six structures) leading to amorphous crystals of the product where the crystal habit was maintained. However, if the reaction was carried out at 50–70 °C for an extended time of 5 days, the crystals appeared the same as the starting crystals for up to 2 days and steadily discolored until the complete reaction in 5 days enabled by defects that become more pronounced with the reaction time. The crystals were maintained during the process but were amorphous. These results show that although the alignment and separation between molecules are important for solid-state reactivity, so too is the nature and size of the groups in close proximity to the sites of reaction. This opens up the possibility of controlling the direction of a reaction or stopping a reaction altogether through the use of appropriate secondary groups not involved in the reaction itself.