**4. Conclusions**

In this study, the cocrystal formed by HMB with 1,3-DITFB was successfully synthesized, and the noncovalent interactions in the crystal structure were calculated at the PBE0-D3(BJ)/def2-TZVPP level of theory. Unexpectedly, the cocrystal has a sandwiched-layer structure with alternating HMB layers and 1,3-DITFB layers. In the corrugated HMB layer, the HMB molecules attract each other mainly via the dispersion forces. In the 1,3-DITFB layer, the 1,3-DITFB molecules form a 2D sheet via the C–I···F halogen bonds, and one 1,3-DITFB molecule can form four C–I···F halogen bonds with four neighboring 1,3-DITFB molecules. The alternating HMB layers and 1,3-DITFB layers are stacked together by strong π···π stacking interactions between HMB and 1,3-DITFB molecules. No C–I···<sup>π</sup> halogen bonds and π···π stacking interactions between the HMB molecules or between the 1,3-DITFB molecules were found in the crystal structure. It was also found that the cooperativity of the noncovalent interactions in each layer is not very obvious. However, the cooperativity of the π···π stacking interactions in the sequence of alternating HMB and 1,3-DITFB molecules is considerable.

The formation of the unexpected sandwiched-layer structure of the molecular cocrystal is attributed to the special geometry of 1,3-DITFB. Other perfluoroiodobenzenes such as the 1,2-diiodotetrafluorobenzene, 1,4-diiodotetrafluorobenzene, and 1,3,5-trifluoro-2,4,6-triiodobenzene do not have such geometries and cannot form 2D sheets via the weak C–I···F halogen bonds. Hence, we predict that the cocrystals formed by HMB with these molecules will not have such a sandwiched-layer structure. The controlled experiments are in progress in our laboratory. The preliminary results show that these predicted cocrystals are a little more difficult to be synthesized than the cocrystal reported in this study.

**Author Contributions:** Y.Z. grew the cocrystals and performed all the quantum chemical calculations; J.-G.W. carried out the X-ray single diffraction studies; W.W. designed and supervised this project; Y.Z. and W.W. jointly wrote and revised the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Foundation of China, gran<sup>t</sup> number 21773104.

**Acknowledgments:** We thank the National Science Foundation of China for the financial support. W.W. thanks the National Supercomputing Center in Shenzhen for the computational support.

**Conflicts of Interest:** The authors declare no conflicts of interest.
