X-ray Diffraction and Theoretical Studies for the Quantitative Assessment of Intermolecular Arene–Perfluoroarene Stacking Interactions

Graphical Abstract

Robust crystal synthons: Arene–perfluoroarene stacking interactions are one of the most persistent recognition modes among organic molecules. Theoretical calculations and X-ray diffraction studies on a series of (pentafluoro)phenyl (tetrafluoro)benzenedicarboxylates (see picture for an example) were performed to quantitatively assess the energies involved.

Abstract

The arene–perfluoroarene stacking interaction was studied by experimental and theoretical methods. A series of compounds with different possibilities for formation of this recognition motif in the solid state were synthesized, and their crystal structures determined by single-crystal X-ray diffraction. The crystal packing of these compounds, as well as the packing of related compounds retrieved from crystallographic databases, were analyzed with quantitative crystal potentials: total lattice energies and the cohesive energies of closest molecular pairs in the crystals were calculated. The arene–perfluoroarene recognition motif emerges as a dominant interaction in the non-hydrogen-bonding compounds studied here, to the point that asymmetric dimers formed over the stacking motif carry over to asymmetric units made of two molecules in the crystal both for pure compounds and for molecular complexes; however, inter-ring distances and angles range from 3.70 to 4.85 Å and from 5 to 21°, respectively. Pixel energy partitioning reveals that whenever aromatic rings stack, the largest cohesive energy contribution comes from dispersion, which roughly amounts to 20 kJ mol⁻¹ per phenyl ring, while the coulombic term is minor but significant enough to make a difference between the arene–arene or perfluoroarene–perfluoroarene interactions on the one hand, and arene–perfluoroarene interactions on the other, whereby the latter are favored by about 10 kJ mol⁻¹ per phenyl ring. No evidence of special interaction which can be attributed to H⋅⋅⋅F confrontation was recognizable.