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.