Fluxional Halogen Bonds Versus Interlayer Stacking - Theory Meets Experiment.

Halogen bonds (XBs) represent strongly directional interactions, relevant for bond activation and often quoted in the context of Crystal Engineering. Bifurcated XBs, albeit rare, are unambiguously established in crystalline solids but do not correspond to an energetically favorable arrangement at the molecular level. Here the dynamic behavior of such a system, jointly described by experiment and theory, reconciles both aspects: Experiment shows that the cocrystal of 1,4-diiodobenzene and 1,4-dinitrobenzene with a symmetric bifurcated XB undergoes a rarely observed reversible crystal-to-crystal transition at 130 K. The resulting low- temperature phase features an asymmetric bifurcated XB, a first step toward a more linear arrangement. Theoretical calculations based on revised van der Waals density functional theory and diffraction experiments conducted at high resolution agree remarkably well in their interpretation of transition temperature and relevance of interaction types. The XB is fluxional and entropy-favored in the symmetric high- temperature phase and locks into an asymmetric geometry with slightly more favorable Gibbs free energy at low temperature; the energy differences involved are extremely small. Theoretically derived and experimentally observed electron densities agree that the perceived directional interactions are weak rather than decisive synthons. Stacking of alternating diiodobenzene and dinitrobenzene molecules represents the most relevant contact.