**Unexpected Sandwiched-Layer Structure of the Cocrystal Formed by Hexamethylbenzene with 1,3-Diiodotetrafluorobenzene: A Combined Theoretical and Crystallographic Study**

## **Yu Zhang, Jian-Ge Wang and Weizhou Wang \***

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, China; yzhpaper@yahoo.com(Y.Z.);wang\_jiange2@126.com(J.-G.W.)

 **\***Correspondence:wzw@lynu.edu.cn;Tel.:+86-379-686-18320

Received: 15 April 2020; Accepted: 5 May 2020; Published: 7 May 2020

**Abstract:** The cocrystal formed by hexamethylbenzene (HMB) with 1,3-diiodotetrafluorobenzene (1,3-DITFB) was first synthesized and found to have an unexpected sandwiched-layer structure with alternating HMB layers and 1,3-DITFB layers. To better understand the formation of this special structure, all the noncovalent interactions between these molecules in the gas phase and the cocrystal structure have been investigated in detail by using the dispersion-corrected density functional theory calculations. In the cocrystal structure, the theoretically predicted π···π stacking interactions between HMB and the 1,3-DITFB molecules in the gas phase can be clearly seen, whereas there are no π···π stacking interactions between HMB molecules or between 1,3-DITFB molecules. The attractive interactions between HMB molecules in the corrugated HMB layers originate mainly in the dispersion forces. The 1,3-DITFB molecules form a 2D sheet structure via relatively weak C–I···F halogen bonds. The theoretically predicted much stronger C–I···<sup>π</sup> halogen bonds between HMB and 1,3-DITFB molecules in the gas phase are not found in the cocrystal structure. We concluded that it is the special geometry of 1,3-DITFB that leads to the formation of the sandwiched-layer structure of the cocrystal.

**Keywords:** molecular cocrystal; sandwiched-layer structure; C–I···F halogen bonds; π···π stacking interactions; PBE0-D3(BJ) calculations
