Observation of Rapid Desolvation of Hexafluorobenzene Involving

Nov 20, 2017 - Synopsis. A very fast and facile single-crystal-to-single-crystal phase transition was observed at room temperature which is driven by ...
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Observation of rapid desolvation of hexafluorobenzene involving singlecrystal-to-single-crystal phase transition in a non-porous organic host Subhrajyoti Bhandary, and Deepak Chopra Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01309 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Observation of Rapid Desolvation of Hexafluorobenzene Involving Single-Crystal-to-Single-Crystal Phase Transition in a Non-porous Organic Host Subhrajyoti Bhandary,[a] and Deepak Chopra*[a] Crystallography and Crystal Chemistry Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal By-Pass Road, Bhopal, Madhya Pradesh, India-462066. Email: [email protected]; Fax: +91-0755-6692392

ABSTRACT:We report an unusual occurrence of an extremely fast Single-Crystal-to-Single-Crystal phase transition induced by rapid desolvation of hexafluorobenzene at room temperature mediated by a subtle interplay of π···π stacking interactions in N-(3-Ethynylphenyl)-4-fluorobenzamide. The nature of the host-guest stacking interaction has been explored in terms of interaction energy, electrostatic complementarity and topological analysis with the inputs from RDGNCI fingerprint descriptor.Furthermore, the compound also exists in two other non-solvated polymorphic forms.

The structural diversity which exists in crystalline solids is a key attraction in supramolecular chemistry.12 Particularly, the occurrence of multiple crystalline phases of a given compound (polymorphs), including crystallization with guest solvent molecules (pseudo-polymorphs/ solvatomorphs) have distinct physicochemical and material properties which provides great opportunities to tune the structure-function relationships at the molecular level.3-6 The inclusion of guest solvent molecules in the crystal lattice of the host is quite common during the solution-mediated crystallization process.7-10The investigation of the phenomenon of desolvation from a crystal lattice is of topical interest in chemistry and material science.11-16In the area of pharmaceutical research, solvent removal from the crystalline state of the drug plays an important role resulting in the formation of new polymorphic forms which are not accessible by conventional crystallization techniques.17 Importantly, the phenomenon of desolvation is one of the potential strategy not only to form porous extended frameworks but also small molecular solids with promising gas adsorption abilities9, 13and photoemission properties. 18-20 The process of solvent elimination (desolvation) is systemspecific. In extended framework lattice, solid state dynamics is entirely controlled by strong coordination bonds and hence the evacuation of guest solvent molecules may occur from such a robust system without breaking the periodicity of the crystalline lattice, resulting in permanent porosity and makes them suitable candidates for single crystal device materials.9, 13 By contrast, organic molecular systems are stabilized by weak non-covalent interactions and nature tends to maximize the attractive interactions to minimize the

empty space to form densely packed crystalline lattice.21 In such a scenario, the solvent molecules can incorporate into the lattice through potential host-guest interactions which provides additional stability to the framework structure.22The loss of the guest molecule from such a molecular host in the absence of intrinsic porositydoes not allow quick solid state rearrangement to maintain the 3D periodicity and leads to disruption of single crystallinity23which make the event of desolvation involving SingleCrystal-to-Single-Crystal (SCSC) phase transition extremely rare.15, 24-27 Moreover, the structural transformation with the removal of guest molecules from crystal lattice often needs potential stimuli such as, elevated temperature, light or mechanical forces which often destroy the crystalline nature of the material with reduced properties. In this regard, spontaneous solvent elimination at room temperature (RT, 20-25 °C) from a nonporous organic host lattice with quick reorganization to another polymorphic form in SCSC fashion demands significant attention in both fundamental and application based research. During our investigation of the effect of fluorinated solvents on the crystallization outcomes of a newly synthesized fluorine containing compound, namelyN-(3-Ethynylphenyl)-4-fluorobenzamide (SYF33; Scheme 1),we observed that the crystals obtained from Scheme 1.Chemical Structure of the compound SYF33 with multiple solid forms

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portance. For this purpose, intermolecular interaction energies (IE) of important molecular pairs for all crystalline phases along with their total lattice energy were calculated using PIXEL. 28-29 List of all intermolecular interactions with their IE is summarized in Table S3.

Figure 2. Displacement ellipsoids (50% probability level) of solvated (left) and desolvated (right) phases. Dotted lines show intra- and intermolecular interactions. Figure 1. Images of very fast desolvation at RT (outside mother solvent) from Form FB followed by SCSC phase transition to Form III.

hexafluorobenzene (HFB) solvent undergoes very fast transition at RT outside the mother liquor. The event of phase transition proceeds rapidly as is reflected in the crystal turning opaque from transparent (Figure 1, and movie M1 in the Supporting Information). In order to capture this phenomenon we cautiously collected the Single Crystal X-ray Diffraction (SCXRD) data of the initial crystalline phase at very low temperature (100 K) which confirms the presence of guest HFB solvent molecule in the crystal lattice (Form FB). After that, the crystal settled down to RT to make the crystal fully opaque (within a minute) and again SCXRD data was collected. It is quite rare and interesting to observe that after the rapid phase transition at RT, the mono-crystallinity of the crystal is maintained through the complete elimination of solvent molecule in an SCSC manner resulting in the appearance of a new solvent-free polymorphic phase (Form III). The absence of the guest solvent molecule after desolvation in Form III phase was also confirmed by thermogravimetric analysis (Figure S2), and via an overlay of the experimental and simulated powder X-ray diffraction patterns (Figure S3). In addition to that, the crystallization screening in other organic solvents (Table S1) generates in addition, two new polymorphic forms (Form I and Form II) for the compound SYF33. It is noteworthy that the single crystal of Form III could not be obtained from any other solutionmediated crystallization techniques and the exposure of HFB solvent on Form III crystal (desolvated form) shows no reversible uptake of solvent molecule.Interestingly, dissolving the Form III crystals in HFB solvent by gentle heating results in “immediate recrystallization” of the solvated form (Form FB). All crystallographic and refinement details are tabulated in Table S2 and ORTEPs are given in Figure 2 and Figure S4. To rationalize the above unusual facile desolvation induced SCSC transition and the origin of occurrence of polymorphism in case of the compound SYF33, a detailed investigation into the intermolecular interactions which are present in the crystal packing is of extreme im-

The solvated phase (Form FB) of the compound crystallizes in monoclinic P21/n space group with 1:1 stoichiometric ratio for the host-HFB solvent (Figure 2). The supramolecular assembly is mainly guided by intermolecular strong N-H···O hydrogen bonding chains (motif I, IE -50.9 kJ/mol)operating in alternatedirections (see purple arrows in Fig. 3a), whichalso consists of stacking between fluoro-and ethynyl-phenyl rings (Figure S5). Such adjacent chains are held by weak C(sp)-H···Fhydrogen bonds (motif IX, IE -5.1 kJ/mol), formed between the highly acidic acetylenic hydrogen with the fluorine atom. The guest HFB solvent molecule is trapped in between such N-H···O chains (red circle in Figure 3a) in the crystal lattice through the presence of parallel-displaced stacking with fluorophenyl ring (motif III, IE -17.8 kJ/mol) and also with ethnylphenyl ring of the host (motif II, IE -19.0 kJ/mol) which bridges the two criss-cross N-H···O layers forming 3D crystal lattice (Figure 4). The driving force is the presence of donor...acceptor interactions between the electron deficient HFB and relatively electron rich fluoro-andethynylphenyl ring to stabilize the solvated structure of Form FB. It is also evident from the fact that the presence of both electrostatic (27-30%) along with dispersive contributions are significant for these stacking interactions (motif II and III, Table S3).Likely in Form FB, the robustness of most stabilized N-H···O chains (motif I, IE 51.2 kJ/mol) with C(sp)-H···F (motif VI, IE -7.0 kJ/mol) along with the C(sp2)-H···F (motif IV, IE -10.2 kJ/mol) interactions direct the supramolecular packing of molecules in Form III (Figure 3b). It is important to note that the IE of the most advantageous N-H···O hydrogen bonds in both the Form FB (-50.9 kJ/mol) and Form III (-51.2 kJ/mol) remains same which also operates in a similar fashion (purple arrows in Figure 3a-b). Moreover, the host-guest stacking interactions in Form FB is compensated by direct stacking of the fluorophenyl and ethynylphenyl rings forming a non-porous crystal structure in Form III (red circle in Figure 3a-b). This structural uniqueness leads to the preservation of the overall framework structure of Form III without losing monocrystallinity (SCSC transition) even after the abrupt removal of the solvent from the Form FB crystal lattice (Figure 4).

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In contrast, the Form I and Form II polymorphs are structurally very different (Figure 3c-d) from the above two forms (Form FB and Form III). The orientation of the two phenyl rings with respect to bridging N-H and C=O groups are alteredfrom that of Form FB and Form III as a consequence of conformational flexibilityat τ1, τ2, and τ3 in the molecule(Scheme 1, and see torsion angles in Table S4). The two symmetrically unique molecules (green and

Once the key structural features of all four crystalline phases have been quantitatively characterized, it is of extreme interest to understand theaffinity of the guest HFB solvent towards the host molecule via π···π stacking interactions in the Form FB(Figure 5a) as well as the sudden removal of the guest, resulting in the formation of Form IIIin terms of the quantitative complementarity in the molecular electrostatic potential (MESP). For this purpose the MESP was mapped for both the guest and the host molecules at the crystal geometry for theForm FB (Figure 5b-d). The center of the ring for HFBmoleculeexhibits

Figure 4. Preservation of similar supramolecular framework in Form IIIafter quick desolvation at RT from Form FB.

Figure 3. Supramolecular layer of molecules connected via strong N-H···O, weak C-H···F hydrogen bonds and stacking interactions in (a) Form FB (b) Form III (c) Form I and(d) Form II. Purple arrowsindicate the direction of N-H···O interactions along their interaction energies.

purple) in both Form I and Form IIpolymorphs rearrange to exhibit layered packing of the molecules (Figure 3c-d). In Form I, alternate layers of each symmetry unique molecules (Figure 3c) are stabilized by strong NH···O chains along with stacking interactions (motif I, IE 48.7 kJ/mol and motif II, IE -47.8 kJ/mol) and weak C(sp)H···F hydrogen bonds (motif XIII and XV). Each molecular layer in Form II (Figure 3d) contains both the symmetrically unique molecules by similar strong N-H···O chains (motif I, IE -52.1 kJ/mol and motif II, IE -45.6 kJ/mol) and connected by weak C(sp2)-H···F and C(sp2)H···π(triple bond) interactions (motif VII).

strong electropositive character (0.035 au) due to the presence of six highly electronegative fluorine atoms. Whereas, both the faces, correspond to the presence of the fluorophenyl and the ethynylphenyl rings for the host molecule, show nearly neutral [0.001 au (top view) and 0.0005 au (down view)] to strongly electronegative [-0.012 au (top view) and -0.007 au (down view)] regions as is shown in Figure 5b-d. The clear demarcation of the magnitude of the electrostatic potentials between the phenyl rings of the host-guest molecule facilitates the effective parallel-displaced type of stacking (Figure 5a). 31 But, it is noteworthy to mention that in spite of the presence of highly electropositive character onthe phenyl ring on the HFB molecule, it was found to stack preferably near the neutral-to-mild electropositive fluorophenyl ring (top view)in preference to the strongly electronegative ethynylphenyl ring in the crystal structure (asymmetric unit) of the Form FB(Figure 2, left). However the final crystal structure does get further stabilized via interaction of the HFB molecule with the more electronegative MESP of the ethynylphenyl (down view) on the other side. This preferential stacking between the solventfluorophenylring (motif III) of the host can also be elucidated using the topological treatment of the electron density by QTAIM. 32 The result shows themultiple presence of critical points (BCPs, RCP and CCP) in between the stacking of two rings due to the characteristic presence of closed-shell C···C as well as C···F and C···O contacts (Figure 6). The value of all topological parameters at the BCPs for such contacts is given in Table S5. The nature and strength of the host-guest complementarity was further visualized in real space through reduced density gradient (RDG) isosurfacessurrounding the NCI regions (Figure 7).33-34The fingerprint of RDG-NCI analysis is the sign of the second eigenvalue (λ2) of the Hessian matrixthat decides the characteristic ofthe

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Figure 5. (a) Host-guest stacking, and Electrostatic potential 30 (ESP) mapped over the Hirshfeld surfaces of (b) HFB solvent molecule and (c, d) host molecule of Form FB with top and down view. Colour range -0.02 au (red) through 0.00 (white) to +0.02 au (blue).

Figure 6. Molecular graph for the solvent and fluorophenyl ring interaction in Form FB.

interaction(λ2< 0 for attractive and λ2> 0 for repulsive). The colour of the RDG isosurface for stacking of solventfluorophenyl ring clearly indicates a combination of light blue (weakly destabilizing) and light green (weakly stabilizing) regions (Figure 7a). The plot of RDG versus electron density multiplied by the sign of the second Hessian eigenvalue [ρ*sign (λ2)] (Figure 7b) clearly depicts low gradient spike in the negative region which corresponds to the weakly attractive dispersion interaction due to the presence of interatomic BCPs.The spikes in the positive regions is indicative of steric clashes between the highly electron deficient phenyl ring of the solvent and the nearly neutral fluorophenyl ring of the host molecule. Thus, the affinity of HFB solvent towards the host in the crystalline phase Form FB depends on the net balance of both stabilizing and destabilizing stacking interactions which contribute to the overall metastability and facilitates the rapid elimination of the solvent from the crystal lattice at RT leading to the formation of the solvent free Form III phase. Furthermore, the stability of all crystalline phases was obtained from lattice energy calculations which clearly suggests that Form FB has reduced lattice energy (-84.2 kJ/mol) in comparison to the other three polymorphs (Table S6). This might be the driving force for the spontaneous phase transition at RT by desolvation of the metastableForm FB to the relatively stable Form III(lattice energy -127.4 kJ/mol). It is important to observe that although Form I and Form II have greater lattice energies (-130.9 kJ/mol and -128.0 kJ/mol respectively) than Form III, but Form FB transforms to Form III preferably afterdesolvation in accordance with the Ostwald’s

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rule of crystallization of polymorphs.The three stable forms were characterizedviadifferential scanning calorimetry (DSC) and hot stage microscopy experiments performed on crystals ofeach form (Figure 8 and S6). The DSC results reveal that the Form I crystals (red trace in Figure 8) melts directly with sharp endothermic peak at 119.4 °C. The crystals of both the polymorphs Form II and Form III show typical melting and immediate recrystallization endotherms (a) at 110.6 °C and 105.4 °C respectively (blue and green traces). It was expected that both the Form II and Form III after the melting in first endotherm (a) underwent quick recrystallization to most stable Form I phase followed by the subsequent melting of Form I phase in the second endotherms (b) at 118.4 °C and 115.4 °C respectively. Hence, they are monotropic polymorphs.5

Figure 7. (a)RDG isosurface (the isovalue is 0.6 coloured over the range of 0.02 < ρ*signλ2< 0.02 au.) and (b) Plot of RDG versus ρ*sign (λ2) for the solvent-flurophenyl ring stacking in Form FB.

Figure 8. DSC traces recorded at 2°C/ min for crystals of all three polymorphs (Form I, Form II and Form III).

Visual insights into all these thermal events were also obtained from hot stage microscopy (Figure S6). Thus, thermal experiments also establish the highest stability of Form I followed by Form II to Form III phase, which is consistent with the trends as obtained from calculated lattice energies. In summary, a unique desolvation with SCSC phase transition has been observed at ambient conditions from Form FB to Form III phase in the crystal landscape of N-(3-Ethynylphenyl)-4-fluorobenzamide which also consists of two other non-solvated polymorphic forms (Form Iand II). The structural study reveals that the oc-

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currence of desolvation involving SCSC transition undergoes by the preservation of the supramolecular structure with the subtle balance involving stacking interactions. This process involves the conservation of the crystal structure in a non-porous organic host. This can provide potential insights into the design and development of new molecular crystals in future based on the features of electrostatic complementarity that exists between the host-guest complexes.

ASSOCIATED CONTENT Supporting Information Details of synthesis, NMR, crystallizations, single crystal data collections, ORTEPs of polymorphs, hot stage microscopy images, PXRD patterns, FTIR, and theoretical calculations are provided in Supporting Information. movieM1 CCDC. 1558393-15558396

AUTHOR INFORMATION Corresponding Author

Email: [email protected]; Fax: +91-0755-6692392 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by funding from DSTSERB, INDIA. We acknowledge IISER Bhopal for research facilities and infrastructure. S. B. thanks IISER Bhopal for the research fellowship. S. B. is grateful to Mr. Kaushik Pal for helping us in imaging using microscope.

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Observation of Rapid Desolvation of Hexafluorobenzene Involving Single-Crystal-to-Single-Crystal Phase Transition in a Non-porous Organic Host SubhrajyotiBhandary,[a] and Deepak Chopra*[a]

Synopsis:A very fast and facile single-crystal-to-single-crystal phase transition was observed at room temperature which is driven by quick elimination of hexafluorobenzene in a non-porous organic host. Moreover, the host exists in two other solvent free polymorphs.

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