*2.3. Measurement*

Single-crystal X-ray di ffraction data were collected on a Rigaku AFC10 di ffractometer (Rigaku Corporation, Tokyo, Japan) equipped with a Rigaku SuperNova X-ray generator (graphite-monochromatic Mo-K α radiation, λ = 0.71073 Å). The structure of the cocrystal was solved and refined by a combination of direct methods and di fference Fourier syntheses, employing the SHELX-2014 and Olex2.0 programs [25,26]. The hydrogen atoms of the methyl groups in HMB were placed in calculated positions and refined with the riding model approximation. Anisotropic thermal parameters were assigned to the nonhydrogen atoms. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (deposition number CCDC 1996547). Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

## **3. Results and Discussion**

## *3.1. Noncovalent Interactions in the Gas Phase*

The study of the noncovalent interactions in the gas phase is significant and can provide useful information for the crystal growth and design, although in some cases the noncovalent interactions in the gas phase maybe be very different with the noncovalent interactions in the crystalline state. Figure 2 illustrates the PBE0-D3(BJ)/def2-TZVPP optimized structures and the corresponding interaction energies for the stacked complex between HMB and 1,3-DITFB, a stacked HMB dimer, a halogen-bonded complex between HMB and 1,3-DITFB, and a stacked 1,3-DITFB dimer. In fact, we also fully optimized the planar structures of the HMB dimer and the 1,3-DITFB dimer, but both of them were transformed into the stacked ones in Figure 2. This indicates that the planar structures of the HMB dimer and the 1,3-DITFB dimer are not stable in the gas phase.

**Figure 2.** The interaction energies (black numbers, in kcal/mol) for the stacked complex between HMB and 1,3-DITFB (**a**), a stacked HMB dimer (**b**), a halogen-bonded complex between HMB and 1,3-DITFB (**c**), and a stacked 1,3-DITFB dimer (**d**).

It can be clearly seen from Figure 2 that the π···π stacking interaction between HMB and 1,3-DITFB is the strongest one among all the noncovalent interactions. The π···π stacking interaction energies for the complexes C6H6···C6X6 (X = F, Cl, Br, and I) are in the range of −9.70 to −5.50 kcal/mol [22]. Thus, the π···π stacking interaction between HMB and 1,3-DITFB is much stronger than the π···π stacking interactions in the complexes C6H6···C6X6 (X = F, Cl, Br, and I). This is understandable because the van der Waals surface area of HMB is larger than that of benzene, and the minimum value of the electrostatic potential of HMB is much more negative than that of benzene. The quadrupole–quadrupole electrostatic interactions in the HMB dimer and 1,3-DITFB dimer are repulsive, and this will weaken the π···π stacking interactions in the two dimers. The π···π stacking interaction energies for the HMB dimer and 1,3-DITFB dimer are −10.32 and −7.01 kcal/mol, respectively. As a contrast, the π···π stacking interaction energy for the complex between benzene and HFB is about −6.00 kcal/mol, and the π···π stacking interaction energy for the parallel-displaced configuration of the benzene dimer is about −2.70 kcal/mol [22,27]. The π-stacked HMB dimer and 1,3- DITFB dimer can also exist in the crystal structures. The CCDC database (version 5.41) was used in a search for the structures containing HMB or 1,3-DITFB [28]. It was found that there are 8 structures containing the π-stacked HMB dimer and 27 structures containing the π-stacked 1,3-DITFB dimer.

Another focus in Figure 2 is the halogen-bonded complex between HMB and 1,3-DITFB with the interaction energy of −7.40 kcal/mol. The binding energy of the conventional C–I···N halogen bond is below 7.00 kcal/mol [29]. Here, the strength of the C–I···<sup>π</sup> halogen bond is obviously close to or even stronger than the strength of the conventional strong C–I···N halogen bond. As shown in Figure 2c, the C–I bond does not point to the centroid of HMB but points to the site which is close to the carbon atom. Tsuzuki and coworkers calculated the C–I···<sup>π</sup> interaction energies for three orientations of the complex between benzene and pentafluoroiodobenzene, and they found that the difference of the interaction energies is not very marked [30]. Bosch and coworkers performed a statistical analysis of the C–I···<sup>π</sup> halogen bonds in the crystal structures by using the Cambridge Structural Database, and their results showed that the number of the structures in which the C–I bond points to the centroid of the benzene ring is very small [31]. In other words, the C–I···<sup>π</sup> halogen bond predicted in the gas phase may also exist in the crystal structure of the complex between HMB and 1,3-DITFB.

## *3.2. Noncovalent Interactions in the Crystal Structure*

HMB and 1,3-DITFB form a 1:1 cocrystal. The cocrystal has an unexpected sandwiched-layer structure with alternating HMB layers and 1,3-DITFB layers (Figure 3). The HMB layer is corrugated, and the 1,3-DITFB layer is a 2D sheet. Crystal data for the cocrystal (*M* = 564.12 g/mol) are as follows: orthorhombic, space group *Cmcm* (no. 63), *a* = 16.3241(6) Å, *b* = 8.7254(5) Å, *c* = 13.6411(8) Å, β = 90◦, *V* = 1942.96(18) Å3, *Z* = 4, *T* = 290 K, μ(CuKα) = 3.270 mm<sup>−</sup>1, *D*calc = 1.928 g/cm3, 11066 reflections measured (7.786◦ ≤ 2Θ ≤ 56.726◦), 1219 unique (*R*int = 0.0324, *<sup>R</sup>*sigma = 0.0151), which were used in all calculations. The final *R*1 was 0.0883 (*I* > *2*σ*(I)*) and *wR*2 was 0.2298 (all data).

**Figure 3.** The side view of the sandwiched-layer structure of the cocrystal.

As expected from the gas-phase calculation, the π···π stacking interactions between HMB and 1,3-DITFB are found in the crystal structure. The interaction energy for the stacked two-body complex in the crystal structure is −11.16 kcal/mol, which is almost the same as the corresponding value of −11.27 kcal/mol) in the gas phase. In the crystal structure, the HMB and 1,3-DITFB molecules are stacked alternately in infinite columns. It is interesting to study the cooperativity of these π···π stacking interactions. Figure 4 shows the total interaction energies for the stacked two-body, three-body, and four-body complexes. Here, we use the three-body [Δ3*E*(123)] and four-body [Δ4*E*(1234)] interaction terms to assess the cooperativity of these π···π stacking interactions, such as the study of the benzene trimer and the benzene tetramer [32]. The three-body and four-body interaction terms can be defined as follows:

$$\begin{aligned} \Delta^3 E(123) &= E(123) - \sum\_i E(i) - \sum\_{ij} \Delta^2 E(ij) \\ \Delta^4 E(1234) &= E(1234) - \sum\_i E(i) - \sum\_{ij} \Delta^2 E(ij) - \sum\_{ijk} \Delta^3 E(ijk) \end{aligned}$$

**Figure 4.** The interaction energies (black numbers, in kcal/mol) for the stacked two-body complex (**a**), a three-body complex (**b**), a three-body complex (**c**), and a four-body complex (**d**) with alternating HMB and 1,3-DITFB molecules.

The three-body interaction terms for the two three-body complexes are −0.32 and −0.56 kcal/mol, respectively. The four-body interaction term for the four-body complex is−0.90 kcal/mol. The three-body and four-body interaction terms are all negative and obviously have stabilizing contributions to the total interactions. Considering that the total interaction energy is very large, it is still reasonable to estimate the total interaction energy of a large complex simply from the sum of the two-body interaction energies.

Figures 5 and 6 show the noncovalent interactions in the HMB layer and 1,3-DITFB layer. Let us add here that these noncovalent interactions do not exist in the gas phase. The HMB molecules form the corrugated layers via dispersion forces. In the corrugated HMB layer, two methyl groups of HMB along the crystallographic *a* axis are disordered, and the other four methyl groups form four H···H contacts with other HMB molecules. The disorder of the two methyl groups of the one HMB molecule indicates that the H···H contacts make negligible contribution to the stability of the cocrystal from another perspective. The 1,3-DITFB molecules form the 2D sheets via the weak C–I···F halogen bonds. One 1,3-DITFB molecule can form four C–I···F halogen bonds with four neighboring 1,3-DITFB molecules. It is the special structure of 1,3-DITFB that leads to the formation of the 2D sheet and furthers the formation of the sandwiched-layer structure of the cocrystal. A similar structure can be found in the cocrystal formed between HMB and 1,2,4,5-tetracyanobenzene [33]. This cocrystal also has a layer structure. However, the 1,2,4,5-tetracyanobenzene layer is not a 2D sheet but a corrugated layer.

**Figure 5.** The four 1,3-DITFB molecules involved in a C–I···F halogen-bonded loop. The black numbers (in kcal/mol) are the interaction energies of two neighboring molecules, and the red number (in kcal/mol) is the total interaction energy of the tetramer.

**Figure 6.** The four HMB molecules involved in a dispersion-bonded loop. The black numbers (in kcal/mol) are the interaction energies of two neighboring molecules, and the red number (in kcal/mol) is the total interaction energy of the tetramer.

Figures 5 and 6 also list the interaction energies for two neighboring monomers and the total interaction energies for the 1,3-DITFB tetramer and HMB tetramer. In the 1,3-DITFB tetramer, the interaction energy of one C–I···F halogen bond is −1.65 kcal/mol, and the interaction energy for the dimer without a C–I···F halogen bond is only −0.51 kcal/mol. The four-body interaction term for the 1,3-DITFB tetramer is about 0.05 kcal/mol, which means that the cooperativity of the noncovalent interactions in the 1,3-DITFB tetramer is negligible. The case for the HMB tetramer is quite similar. The interaction energy of two neighboring HMB molecules is a little smaller than that of two C–I···F halogen-bonded 1,3-DITFB molecules. The four-body interaction term of the HMB tetramer is also about 0.05 kcal/mol and can also be neglected.

Figure 7 lists the interaction energies for two neighboring monomers and the total interaction energies for the four-body complex formed by two HMB molecules and two 1,3-DITFB molecules. Different from the complexes in Figures 4–6, the complex in Figure 7 is formed via mixed noncovalent interactions, which include a π···π stacking interaction, a C–I···F halogen bond, and a dispersion-dominated interaction. The four-body interaction term of this complex is about 0.09 kcal/mol, which is a little larger than that of the 1,3-DITFB tetramer and HMB tetramer. However, the absolute value of the total interaction energy of this complex is over three times larger than that of the 1,3-DITFB tetramer or HMB tetramer. Again, it is reasonable to estimate the total interaction energy of a large complex simply from the sum of the two-body interaction energies.

**Figure 7.** The loop formed by two HMB molecules and two 1,3-DITFB molecules. The black numbers (in kcal/mol) are the interaction energies of two neighboring molecules, and the red number (in kcal/mol) is the total interaction energy of the four molecules.
