Halogen-Bonding-Driven Self-Assembly of Solvates of Tetrabromoterephthalic Acid

: Halogen bonding is one of the most interesting noncovalent attractions capable of self-assembly and recognition processes in both solution and solid phase. In this contribution, we report on the formation of two solvates of tetrabromoterephthalic acid (H 2 Br 4 tp) with acetonitrile (MeCN) and methanol (MeOH) viz. H 2 Br 4 tp · 2MeCN ( 1 MeCN ) and H 2 Br 4 tp · 2MeOH ( 2 MeOH ). The host structures of both 1 MeCN and 2 MeOH are assembled via the occurrence of simultaneous Br ··· Br, Br ··· O, and Br ··· π halogen bonding interactions, existing between the H 2 Br 4 tp molecular tectons. Among them, the cooperative effect of the dominant halogen bond in combination with hydrogen bonding interactions gave rise to different supramolecular assemblies, whereas the strength of the halogen bond depends on the type of hydrogen bond between the molecules of H 2 Br 4 tp and the solvents. These materials show a reversible release/resorption of solvent molecules accompanied by evident crystallographic phase transitions.


Introduction
Supramolecular interactions have been extensively investigated due to their importance in governing various interesting physical properties as well as chemical and biological assemblies [1][2][3]. Halogen bonding (XB) is emerging as one of the prominent intermolecular interactions that takes place between the sigma (σ)-hole of the polarizable halogen atom (Lewis acid, XB donor) and the electron-rich atom or π-electron system (Lewis base, XB acceptor) [4,5]. Generally, the XB interaction is presented as D−Y···A, where D−Y and A are the XB donor and XB acceptor, respectively. This type of interaction is highly directional and exhibits highly predictable bond geometries in the solid state. Theoretical calculations have suggested that the strength of the halogen bonding interaction energies is comparable to that of the ubiquitous hydrogen bond [6] and that the strength of the XB donor increases in the following order as the XB donor ability increases: F < Cl < Br < I [7]. These features suggest that the halogen bonding interactions could be used as a crystal engineering tool for designing and developing novel functional materials in the crystalline state [8].
Recently, halogen bond-based supramolecular synthons have been used to construct an exciting class of porous organic materials named halogen-bonded organic frameworks (XBOFs), which are self-assembled from pure organic building blocks (tectons) [9][10][11]. Compared with analogous materials such as covalent organic frameworks (COFs) and

Materials and Methods
All chemicals and solvents, i.e., H 2 Br 4 tp, MeCN, and MeOH, were reagent grade and were used without further purification. Elemental (C, H, and N) analysis was determined with a LECO CHNS 932 elemental analyzer. Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker D8 ADVANCE X-ray powder diffractometer using Cu Kα (λ = 1.5418 Å). FT-IR spectra were recorded on a Perkin-Elmer model Spectrum GX FT-IR spectrometer using attenuated total reflection (ATR) mode in the range of 650-4000 cm −1 . Thermogravimetric analyses (TGA) were carried out using a Metter Toledo TGA/DSC3+ from 30-500 • C with a heating rate of 10 • C min −1 , under nitrogen atmosphere.

Synthesis and Crystallization of H 2 Br 4 tp·2MeCN (1 MeCN )
A mixture of H 2 Br 4 tp (5 mg) and MeCN (2 ml) was added into a 25 mL Teflon-lined reactor, sealed in a stainless-steel autoclave, and placed in an oven. The mixture was heated from room temperature to 110 • C under autogenous pressure for 1 h and then cooled down to room temperature. Colorless block-shaped crystals of 1 MeCN

Synthesis and Crystallization of H 2 Br 4 tp·2MeOH (2 MeOH )
Colorless block-shaped crystals of 2 MeOH were synthesized by a similar procedure as 1 MeCN

X-ray Crystallography
Suitable crystals of 1 MeCN and 2 MeOH were carefully selected and mounted on MiTeGen micromounts using paratone-N oil. X-ray diffraction data were collected using a Bruker D8 QUEST CMOS PHOTON II with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation at 296(2) K. Data reduction was performed using SAINT, and the SADABS scaling algorithm [18] was used for absorption correction. The structure was solved with the ShelXT structure solution program using combined Patterson and dual-space recycling methods [19]. The structure was refined by least squares using ShelXL [20]. All non-H atoms were refined anisotropically. The H atoms of solvent molecules were positioned geometrically with C-H = 0.96 Å and refined using a riding model (AFIX137 for methyl H atom in ShelXL program) with fixed displacement parameters U iso (H) = 1.5U eq (C). The O-H hydrogen atoms were located on difference Fourier maps but refined with O-H = 0.82 ± 0.01 Å with U iso (H) = 1.5U eq (O). A summary of crystal data and structural refinement parameters for 1 MeCN and 2 MeOH is given in Table 1.

Structural Description
Colorless block-shaped crystals of 1 MeCN and 2 MeOH were obtained upon the crystallization of H 2 Br 4 tp from the solvents MeCN and MeOH at 110 • C for 1 h in a 25 mL Teflon-lined stainless-steel container. Alternatively, single crystals of these two solvates can also easily be grown by dissolving H 2 Br 4 tp in each respective solvent and by allowing them to crystallize by slow evaporation at room temperature for 24 h. The single-crystal X-ray diffraction analysis revealed that the solvates crystallize in the centrosymmetric system with space groups P-1 and P2 1 /c for 1 MeCN and 2 MeOH , respectively. These solvates have a similar 1:2 stoichiometric ratio of H 2 Br 4 tp and solvent molecules. Figure 1 shows the molecular structure with the atomic numbering schemes of the solvates. The asymmetric unit of 1 MeCN contains half a molecule of H 2 Br 4 tp located at a center of inversion and one MeCN molecule. In contrast, there are one H 2 Br 4 tp molecule and two MeOH molecules in the asymmetric unit of 2 MeOH . easily be grown by dissolving H2Br4tp in each respective solvent and by allowing them to crystallize by slow evaporation at room temperature for 24 h. The single-crystal X-ray diffraction analysis revealed that the solvates crystallize in the centrosymmetric system with space groups P-1 and P21/c for 1MeCN and 2MeOH, respectively. These solvates have a similar 1:2 stoichiometric ratio of H2Br4tp and solvent molecules. Figure 1 shows the molecular structure with the atomic numbering schemes of the solvates. The asymmetric unit of 1MeCN contains half a molecule of H2Br4tp located at a center of inversion and one MeCN molecule. In contrast, there are one H2Br4tp molecule and two MeOH molecules in the asymmetric unit of 2MeOH. In the crystal of 1MeCN, the H2Br4tp molecules are linked by Br···O halogen bond and weak van der Waals (vdW) O···O interactions to generate a two-dimensional (2D) sheet structure along the a axis ( Figure 2a). The observed Br···O halogen bonding interaction (3.270(3) Å) between bromine atom and the oxygen carbonyl atom in 1MeCN is ≈3% shorter than the sum of the vdW radii of the Br and O atoms (3.37 Å) [21], and the C−Br···O bond angle (155.3°) is slightly bent. In contrast, the non-bonded O···O distance (3.038(6) Å) between the oxygen atoms of the carboxyl groups is almost the same as the sum of the vdW radii of the two oxygen atoms (3.04 Å). Apparently, the solvent MeCN molecules are located between the 2D layered sheets and participate in O−H···N and C−H···O hydrogen bonding interactions (Table S1) in the tetrameric motif with graph set notation R 4 4(16) [22] ( Figure 2b). Such interactions along with additional weak type I Br···Br halogen bonds and Br···π contacts (Br···C ≈3.5 Å) [23] link the 2D sheets into a 3D supramolecular architecture. Further investigation of the packing structure found that the centroid-centroid distances between the stacked C≡N group of the MeCN molecule and the aromatic ring of the H2Br4tp molecule is 3.997(5) Å, which indicates a weak π−π interaction [24]. This also contributes to the packing stabilization of the solvate 1MeCN. The geometric parameters for the halogen bonds and the hydrogen bonds of the solvates are provided in Table 2 and Table  S1 (Supplementary Materials), respectively. In the crystal of 1 MeCN , the H 2 Br 4 tp molecules are linked by Br···O halogen bond and weak van der Waals (vdW) O···O interactions to generate a two-dimensional (2D) sheet structure along the a axis ( Figure 2a). The observed Br···O halogen bonding interaction (3.270(3) Å) between bromine atom and the oxygen carbonyl atom in 1 MeCN is ≈3% shorter than the sum of the vdW radii of the Br and O atoms (3.37 Å) [21], and the C-Br···O bond angle (155.3 • ) is slightly bent. In contrast, the non-bonded O···O distance (3.038(6) Å) between the oxygen atoms of the carboxyl groups is almost the same as the sum of the vdW radii of the two oxygen atoms (3.04 Å). Apparently, the solvent MeCN molecules are located between the 2D layered sheets and participate in O-H···N and C-H···O hydrogen bonding interactions (Table S1) in the tetrameric motif with graph set notation R 4 4 (16) [22] (Figure 2b). Such interactions along with additional weak type I Br···Br halogen bonds and Br···π contacts (Br···C ≈3.5 Å) [23] link the 2D sheets into a 3D supramolecular architecture. Further investigation of the packing structure found that the centroid-centroid distances between the stacked C≡N group of the MeCN molecule and the aromatic ring of the H 2 Br 4 tp molecule is 3.997(5) Å, which indicates a weak π−π interaction [24]. This also contributes to the packing stabilization of the solvate 1 MeCN . The geometric parameters for the halogen bonds and the hydrogen bonds of the solvates are provided in Table 2 and   (Table 2) are much shorter than those observed for 1 MeCN . This is possibly due to the influence of the geometry of the MeOH molecules on packing. H 2 Br 4 tp and the MeOH molecules can behave as either hydrogen bond donor or acceptor sites similar to that of 1 MeCN and interact with each other via the O-H···O and C-H···O hydrogen bonding interactions (Table S1), leading to the formation of the Crystals 2021, 11,198 5 of 12 tetrameric hydrogen bonding motif with an R 4 4 (12) graph set ( Figure 3c). These interactions serve to connect the sheets into a 3D architecture.
3.7233 (7) 89.77(13)   (3) 155.24 (11) x, y, z − 1 2MeOH  (Table S1), leading to the formation of the tetrameric hydrogen bonding motif with an R 4 4(12) graph set ( Figure 3c). These interactions serve to connect the sheets into a 3D architecture.   a and b axes for 1MeCN and 2MeOH, respectively. Although both solvates have the same composition and stoichiometry, it can clearly be seen that different solvents lead to differences in molecular orientation within the crystal packing. This can be attributed to  Figure 4 depicts the packing diagrams and the contact surface of the channels viewed along the a and b axes for 1 MeCN and 2 MeOH , respectively. Although both solvates have the same composition and stoichiometry, it can clearly be seen that different solvents lead to differences in molecular orientation within the crystal packing. This can be attributed to the nature (size, shape, and intermolecular interaction capabilities) and the different Crystals 2021, 11,198 7 of 12 roles of solvent molecules during supramolecular framework assembly. It appears that the space accommodating the MeCN molecule in 1 MeCN has a cylinder-like geometry (1D channel along the a axis) with discontinuous voids (Figure 4a). For 2 MeOH , the space occupied by the MeOH molecules shows continuous in-void volume maps (Figure 4b) that propagate in two directions (the ac plane). Despite the different packing arrangements, either solvent MeCN or MeOH molecules are involved in the hydrogen bonding with similar tetrameric hydrogen-bonding motifs as described above. A comparison of the packing efficiency (Ck) using PLATON [25] revealed that 1 MeCN (Ck = 46.9%) possesses a lower packing efficiency than that of 2 MeOH (Ck = 52.1%). This result demonstrates that the components in the solvate 2 MeOH pack more tightly, which may be attributed to the presence of numerous Br···O, Br···Br, and Br···π halogen bonding and classical O-H···O hydrogen bonding interactions. Furthermore, the potential solvent-accessible void space after the removal of solvent molecules, also calculated using PLATON, was estimated to be ≈34.8% and 30.4% for 1 MeCN and 2 MeOH , respectively. In this regard, the 2D supramolecular frameworks with visualized surfaces of void structures of these solvates may potentially serve as a stable soft host framework for polar organic molecules.
Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 the nature (size, shape, and intermolecular interaction capabilities) and the different roles of solvent molecules during supramolecular framework assembly. It appears that the space accommodating the MeCN molecule in 1MeCN has a cylinder-like geometry (1D channel along the a axis) with discontinuous voids (Figure 4a). For 2MeOH, the space occupied by the MeOH molecules shows continuous in-void volume maps (Figure 4b) that propagate in two directions (the ac plane). Despite the different packing arrangements, either solvent MeCN or MeOH molecules are involved in the hydrogen bonding with similar tetrameric hydrogen-bonding motifs as described above. A comparison of the packing efficiency (Ck) using PLATON [25] revealed that 1MeCN (Ck = 46.9%) possesses a lower packing efficiency than that of 2MeOH (Ck = 52.1%). This result demonstrates that the components in the solvate 2MeOH pack more tightly, which may be attributed to the presence of numerous Br···O, Br···Br, and Br···π halogen bonding and classical O−H···O hydrogen bonding interactions. Furthermore, the potential solvent-accessible void space after the removal of solvent molecules, also calculated using PLATON, was estimated to be ≈34.8% and 30.4% for 1MeCN and 2MeOH, respectively. In this regard, the 2D supramolecular frameworks with visualized surfaces of void structures of these solvates may potentially serve as a stable soft host framework for polar organic molecules. Additionally, given the abundance of bromine atoms in the H2Br4tp molecular tectons, perhaps it is not surprising that Br···O, Br···Br, and Br···π synthons were the most frequently found halogen bonding motifs in 1MeCN and 2MeOH. Our previous studies also indicated that this type of interaction was common in H2Br4tp solvates with acetone, ethanol, dimethyl sulfoxide, and ethylene glycol solvents [23] and is mainly responsible for the formation of their layered sheets. Despite this, each solvate exhibits subtle differences in overall packing due to different hydrogen bonding and the orientation of the solvent molecules. Notably, H2Br4tp can selectively accommodate MeOH molecules relative to other solvents. This is probably related to the molecular shape and size, the acidity scale, as well as some specific intermolecular interactions. Additionally, given the abundance of bromine atoms in the H 2 Br 4 tp molecular tectons, perhaps it is not surprising that Br···O, Br···Br, and Br···π synthons were the most frequently found halogen bonding motifs in 1 MeCN and 2 MeOH . Our previous studies also indicated that this type of interaction was common in H 2 Br 4 tp solvates with acetone, ethanol, dimethyl sulfoxide, and ethylene glycol solvents [23] and is mainly responsible for the formation of their layered sheets. Despite this, each solvate exhibits subtle differences in overall packing due to different hydrogen bonding and the orientation of the solvent molecules. Notably, H 2 Br 4 tp can selectively accommodate MeOH molecules relative to other solvents. This is probably related to the molecular shape and size, the acidity scale, as well as some specific intermolecular interactions.

Hirshfeld Surface Analysis
The nature of the intermolecular interactions between the molecules within the crystal structure of the solvates 1 MeCN and 2 MeOH was further quantified and visualized by Hirschfeld surfaces [26] and their associated 2D fingerprint plots [27] performed with CrystalExplorer [28]. The shorter and longer contacts on the Hirschfeld surfaces are indicated as red and blue spots, respectively, while white spots indicate contacts with distances approximately equal to the sum of the vdW radii. The function d norm (normalized distance) is a ratio enclosing the distances of any surface point to the nearest interior (d i ) and exterior (d e ) atom and the vdW radii of the atoms. As can be seen from the structural analysis above, the bromine atoms of the host H 2 Br 4 tp molecules are involved in Br· · · Br, Br· · · O, and Br· · · π halogen bonding interactions. The contributions of such interatomic contacts to the d norm surface in these solvates are compared and shown in Figure 5.

Hirshfeld Surface Analysis
The nature of the intermolecular interactions between the molecules within the crystal structure of the solvates 1MeCN and 2MeOH was further quantified and visualized by Hirschfeld surfaces [26] and their associated 2D fingerprint plots [27] performed with CrystalExplorer [28]. The shorter and longer contacts on the Hirschfeld surfaces are indicated as red and blue spots, respectively, while white spots indicate contacts with distances approximately equal to the sum of the vdW radii. The function dnorm (normalized distance) is a ratio enclosing the distances of any surface point to the nearest interior (di) and exterior (de) atom and the vdW radii of the atoms. As can be seen from the structural analysis above, the bromine atoms of the host H2Br4tp molecules are involved in Br⋯Br, Br⋯O, and Br⋯π halogen bonding interactions. The contributions of such interatomic contacts to the dnorm surface in these solvates are compared and shown in Figure 5. It is evident that the 2D fingerprint plots of all contacts among these solvates differ significantly due to the differences in packing arrangements and intermolecular interac- It is evident that the 2D fingerprint plots of all contacts among these solvates differ significantly due to the differences in packing arrangements and intermolecular interactions in the solid state. Specifically, there is major significant difference in the Br···Br contact, which comprises 4.4% and 12.1% of the d norm surface for 1 MeCN and 2 MeOH , respectively, while the Br···O contacts show quite similar contributions to the surface (9.3% for 1 MeCN and 8.5% for 2 MeOH ). Apparently, both solvates feature Br···C/C···Br contacts (9.9% for 1 MeCN and 10.8% for 2 MeOH ), which are manifested as weak Br···π contacts. The Br···H/H···Br contacts also have a significant contribution towards the crystal stabilization of these solvates (22.3% for 1 MeCN and 20.3% for 2 MeOH ). It should be noted that O···O contacts for these solvates contribute a negligible percentages (1.8% for 1 MeCN and 1.1% for 2 MeOH ) towards the total surface area. Furthermore, the dominant interactions between H and O or N atoms, corresponding to the discussed hydrogen bonding interactions have also been visualized by selecting the host H 2 Br 4 tp molecules as the object. As can be clearly seen from Figure 6, these solvates exhibit red spots on the d norm surface, signifying close contacts, which originate from O-H···O or O-H···N interactions, comprising 5.0% and 20.4% of the total surface area for 1 MeCN and 2 MeOH , respectively. It is of interest to note that the contributions to the d norm surface due to H···H contacts are 5.7% in 1 MeCN  tions in the solid state. Specifically, there is major significant difference in the Br···Br contact, which comprises 4.4% and 12.1% of the dnorm surface for 1MeCN and 2MeOH, respectively, while the Br···O contacts show quite similar contributions to the surface (9.3% for 1MeCN and 8.5% for 2MeOH). Apparently, both solvates feature Br···C/C···Br contacts (9.9% for 1MeCN and 10.8% for 2MeOH), which are manifested as weak Br···π contacts. The Br···H/H···Br contacts also have a significant contribution towards the crystal stabilization of these solvates (22 .3% for 1MeCN and 20.3% for 2MeOH). It should be noted that O···O contacts for these solvates contribute a negligible percentages (1.8% for 1MeCN and 1.1% for 2MeOH) towards the total surface area. Furthermore, the dominant interactions between H and O or N atoms, corresponding to the discussed hydrogen bonding interactions have also been visualized by selecting the host H2Br4tp molecules as the object. As can be clearly seen from Figure  6, these solvates exhibit red spots on the dnorm surface, signifying close contacts, which originate from O−H···O or O−H···N interactions, comprising 5.0% and 20.4% of the total surface area for 1MeCN and 2MeOH, respectively. It is of interest to note that the contributions to the dnorm surface due to H···H contacts are 5.7% in 1MeCN and 18.5% in 2MeOH, implying vdW interactions being dominant for the supramolecular organization in 2MeOH. In addition, the small contributions of the other weak intermolecular C···C (3.2% for 1MeCN and 1.6% for 2MeOH), C···H/H···C (3.9% for 1MeCN and 0.4% for 2MeOH), C···O/O···C (3.8% 2MeOH), and H···N/N···H (8.3% for 1MeCN) contacts have negligible effects on the packing (Figure 7).

Thermal Analysis and Structural Transformation
To evaluate the thermal behaviors of the solvates of H2Br4tp, TGA was performed on crystalline samples in the temperature range of 30 to 500 °C under nitrogen atmosphere. The TGA profiles of 1MeCN, 2MeOH, and H2Br4tp are compared in Figure 8a. It was found tions in the solid state. Specifically, there is major significant difference in the Br···Br contact, which comprises 4.4% and 12.1% of the dnorm surface for 1MeCN and 2MeOH, respectively, while the Br···O contacts show quite similar contributions to the surface (9.3% for 1MeCN and 8.5% for 2MeOH). Apparently, both solvates feature Br···C/C···Br contacts (9.9% for 1MeCN and 10.8% for 2MeOH), which are manifested as weak Br···π contacts. The Br···H/H···Br contacts also have a significant contribution towards the crystal stabilization of these solvates (22 .3% for 1MeCN and 20.3% for 2MeOH). It should be noted that O···O contacts for these solvates contribute a negligible percentages (1.8% for 1MeCN and 1.1% for 2MeOH) towards the total surface area. Furthermore, the dominant interactions between H and O or N atoms, corresponding to the discussed hydrogen bonding interactions have also been visualized by selecting the host H2Br4tp molecules as the object. As can be clearly seen from Figure  6, these solvates exhibit red spots on the dnorm surface, signifying close contacts, which originate from O−H···O or O−H···N interactions, comprising 5.0% and 20.4% of the total surface area for 1MeCN and 2MeOH, respectively. It is of interest to note that the contributions to the dnorm surface due to H···H contacts are 5.7% in 1MeCN and 18.5% in 2MeOH, implying vdW interactions being dominant for the supramolecular organization in 2MeOH. In addition, the small contributions of the other weak intermolecular C···C (3.2% for 1MeCN and 1.6% for 2MeOH), C···H/H···C (3.9% for 1MeCN and 0.4% for 2MeOH), C···O/O···C (3.8% 2MeOH), and H···N/N···H (8.3% for 1MeCN) contacts have negligible effects on the packing (Figure 7).

Thermal Analysis and Structural Transformation
To evaluate the thermal behaviors of the solvates of H2Br4tp, TGA was performed on crystalline samples in the temperature range of 30 to 500 °C under nitrogen atmosphere.

Thermal Analysis and Structural Transformation
To evaluate the thermal behaviors of the solvates of H 2 Br 4 tp, TGA was performed on crystalline samples in the temperature range of 30 to 500 • C under nitrogen atmosphere. The TGA profiles of 1 MeCN , 2 MeOH , and H 2 Br 4 tp are compared in Figure 8a. It was found that no weight loss was observed before 280 • C in the TGA curve of H 2 Br 4 tp, suggesting the absence of solvent molecules in its crystal structure. Meanwhile, the TGA curves of 1 MeCN and 2 MeOH show that the solvent (MeCN or MeOH) molecules are gradually lost from room temperature to ≈80−95 • C, and then decomposition is observed beyond ≈250 • C. tained by simply immersing the desolvated samples in MeCN and MeOH solutions for 24 h. This reversible behavior can be repeated a number of times, which was confirmed by PXRD and IR spectroscopic techniques.
For a better understanding of the dynamic structural phase transition, detailed crystal structural information as well as intermolecular interactions of the desolvated crystals are very important. Although the desolvated crystals were found to possess similar morphologies to those of the original solvate forms and maintained their crystallinity, as confirmed by the PXRD experiments, they were found to diffract very poorly even at a medium resolution shell. Thus, single crystal structure determinations of these desolvated forms in this work were not possible. Despite several recrystallization attempts, regrettably, all failed to yield crystals of H2Br4tp alone. Fortunately, the solid-state structure of H2Br4tp determined from PXRD data was reported by Kumar et al. [17]. In packing, intermolecular O−H···Br hydrogen bonds are mainly responsible for the formation of a 2D sheet. Based on this evidence as well as our findings, we note that halogen bonding can be cooperative or competitive with hydrogen bonding during the desolvation-resolvation process.

Conclusions
In summary, H2Br4tp, a bromine and carboxyl-containing molecule, showed significant potential as a building block in the assembly of a 2D halogen-bonded sheet in crystalline state through a range of different halogen bonding synthons. These 2D assemblies can form 1:2 cocrystal solvates with MeCN and MeOH, in which the formation relies on very similar hydrogen bonding motifs between the respective components. Each solvate crystal showed distinct packing arrangements, which result from permutations of different halogen bonds. PXRD analysis and IR spectroscopy showed that the structural phase transitions between the solvated and the unsolvated crystals are reversible upon the release/resorption of solvent molecules. This study assessed the importance of the cooperative effect of halogen bonding in combination with hydrogen bonding interactions for engineering solvate crystals, permitting reversible release and resorption of solvent molecules.
Author contributions: conceptualization, K.C.; funding acquisition, K.C. and M.S.; investigation, K.C. and M.S.; methodology, N.C., K.S., F.K., W.D., and K.C.; validation, K.C.; visualization, N.J., According to the TGA profiles, it is interesting that the structures of the host molecules remained intact after desolvation. Notably, PXRD patterns of the desolvated samples in Figure 8b,c also clearly reveal the formation of a new phase-pure material, in which the peak positions correspond well with H 2 Br 4 tp in the monoclinic C2/m space group [17]. Furthermore, the recyclability of the solvent release and resorption experiments was also examined. The crystalline samples of each solvate (≈10 mg) were placed in a crucible and heated at 110 • C under vacuum (≈10 mbar pressure) for 1 h. Indeed, the solvate form changes to the unsolvated phase, which can be confirmed by the disappearance of PXRD peaks at 2θ = 10.40 and 11.35 • for 1 MeCN and at 2θ = 7.83 • for 2 MeOH . Additionally, the absence of the solvent molecules was also evidenced by the disappearance of the C≡N stretching vibration band of a nitrile group of MeCN (ν C≡N = 2218 cm −1 ) in the IR spectrum of 1 MeCN (Figure 8d) while only negligible changes could be observed in the IR spectrum of 2 MeOH . Both of the desolvated samples could be recovered to their original phase upon resolvation with the corresponding solvent (MeCN or MeOH) and heating (110 • C, 1 h) in a Teflon-lined stainless autoclave. Alternatively, crystals of the original phase can be obtained by simply immersing the desolvated samples in MeCN and MeOH solutions for 24 h. This reversible behavior can be repeated a number of times, which was confirmed by PXRD and IR spectroscopic techniques.
For a better understanding of the dynamic structural phase transition, detailed crystal structural information as well as intermolecular interactions of the desolvated crystals are very important. Although the desolvated crystals were found to possess similar morphologies to those of the original solvate forms and maintained their crystallinity, as confirmed by the PXRD experiments, they were found to diffract very poorly even at a medium resolution shell. Thus, single crystal structure determinations of these desolvated forms in this work were not possible. Despite several recrystallization attempts, regrettably, all failed to yield crystals of H 2 Br 4 tp alone. Fortunately, the solid-state structure of H 2 Br 4 tp determined from PXRD data was reported by Kumar et al. [17]. In packing, intermolecular O-H···Br hydrogen bonds are mainly responsible for the formation of a 2D sheet. Based on this evidence as well as our findings, we note that halogen bonding can be cooperative or competitive with hydrogen bonding during the desolvation-resolvation process.

Conclusions
In summary, H 2 Br 4 tp, a bromine and carboxyl-containing molecule, showed significant potential as a building block in the assembly of a 2D halogen-bonded sheet in crystalline state through a range of different halogen bonding synthons. These 2D assemblies can form 1:2 cocrystal solvates with MeCN and MeOH, in which the formation relies on very similar hydrogen bonding motifs between the respective components. Each solvate crystal showed distinct packing arrangements, which result from permutations of different halogen bonds. PXRD analysis and IR spectroscopy showed that the structural phase transitions between the solvated and the unsolvated crystals are reversible upon the release/resorption of solvent molecules. This study assessed the importance of the cooperative effect of halogen bonding in combination with hydrogen bonding interactions for engineering solvate crystals, permitting reversible release and resorption of solvent molecules.