Isolation of a Transition-State Geometry via Intermolecular Stabilization in the Solid State

Conformers can interconvert when the transition state
lies sufficiently
low in energy. Nevertheless, the transition-state geometry is minimally
populated or intrinsically unpopulated in solution, making its isolation
in the solid state extremely challenging. A tetraaza derivative of
anthraquinodimethane bearing vicinal dimethyl groups exhibits high
conformational flexibility. Owing to the very small energy difference
between the twisted form and the stable folded conformer, the twisted
form was successfully isolated by recrystallization and structurally
characterized by single-crystal X-ray analysis, despite being computationally
identified as the transition-state geometry for the ring-flip motion
of the folded form. The stabilization of the twisted form is attributed
to the formation of a one-dimensional columnar stack, accompanied
by π–π and C–H···π
interactions.