Cooperativity and Feedback Mechanisms in the Single-Crystal-to-Single-Crystal
Solid-State Diels–Alder Reaction of 9‑Methylanthracene
with Bis(<i>N</i>‑cyclobutylimino)-1,4-dithiin
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Abstract
Electron donor-to-acceptor interactions
between 9-methylanthracene
and bis(<i>N</i>-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 °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 <i>b</i> 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