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.