Combining theory and experiment for new insights into halogen bonds
Halogen bonds represent important intramolecular interactions for organic synthesis and organocatalysis or in biological systems and are often used in contexts such as ‘crystal engineering’ as structure-determining interactions. In our publication, we were able to gain new insights into this interaction using the example of the co-crystal of 1,4-diiodobenzene and 1,4-dinitrobenzene. It was found that the symmetrical bifurcated halogen bridge already described in the literature corresponds only to a high-temperature phase of the compound.
Using single-crystal X-ray diffraction, a rare crystal-to-crystal phase transition of this structure to a phase with an asymmetrical, non-bifurcated halogen bridge was observed. A high-resolution structure was obtained from the high-temperature phase, and the experimental electron density studies based on this structure clarified the bonding situation of the halogen bridge. This revealed a very weak halogen bridge interaction, while p-p-interactions were mainly responsible and thus explain the possibility of phase transition within the crystalline phases. Supplementary quantum chemical calculations investigated the energetic relationships between the two phases and additionally showed that the symmetrically bifurcated halogen bridge is energetically higher, so that it can be considered a transition state of the asymmetric halogen bond.
Further information can be found on the in the original publication:
Fluxional Halogen Bonds Versus Interlayer Stacking – Theory Meets Experiment
C. Y. Gao, A. Schmidt, R. Wang, C. Strohmann,* U. Englert,* S.-D. Li*
Chem. Eur. J. 2025, e02514.