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Crystal Structure-Mechanical Property Correlations in N-(3-ethynylphenyl)-3-fluorobenzamide Polymorphs Subhrajyoti Bhandary, Kiran S.R.N. Mangalampalli, Upadrasta Ramamurty, and Deepak Chopra Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01432 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Crystal Growth & Design

Crystal Structure-Mechanical Property Correlations in N-(3ethynylphenyl)-3-fluorobenzamide Polymorphs Subhrajyoti Bhandary,aKiran S. R. N. Mangalampalli,bUpadrasta Ramamurty,cand Deepak Chopra*a 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. b SRM Research Institute, Department of Physics and Nanotechnology, SRM University, Kattankulathur-603203, Chennai, Tamilnadu (India). c Department of Materials Engineering, Indian Institute of Science Bangalore 560012 (India). ABSTRACT:During the solution-mediated crystallization ofN-(3-ethynylphenyl)-3-fluorobenzamide, small variation in the process conditions can lead to two new polymorphic forms in addition to the three previously reported forms. Structural features of the two new forms and mechanical properties of the three stable polymorphs, amongst the five forms, have been investigated using instrumented nanoindentation. The results show that among the three stable forms (Form I, Form II and Form III) of the compound, the Form II crystal exhibits lowest hardness (H), and elastic modulus (E), while these values are nearly similar for Form I and Form III crystals. Interestingly, the direct correlation of mechanical properties with the density of crystals was found for three polymorphs, but their melting points do not follow the similar trends. The quantitative analysis of structural features with the inputs from energy frameworks suggest that the anisotropy in mechanical properties of the three polymorphs originate from the different orientations of strong to moderate N-H···O hydrogen bonds and weak to strong π···π stacking interactions, which mainly stabilize the crystal packing of the three polymorphs.

The implications of crystal polymorphism on the variations in physical and chemical properties of a compound is well documented in the literature.1-3 Although there are many compounds reported in the literature which exist in numerous solid state forms. There exists 16 cases in which the compounds give rise to more than five polymorphic forms, whose structures have been determined correctly for all the isolated polymorphic forms.4-5A careful analysis of such polymorphic landscape of a compound could provide insights into the dynamic aspects of crystallization process and also in their structure-activity relations at the molecular level.6-7In addition, it is possible to explore the mechanical properties of polymorphs, with a view of not only studying structureproperty relations within the context of crystal engineering and solid-state material chemistry, but also exploit it for identifying the most suitable form for pharmaceutical manufacturing.8-12Moreover, mechanical characterization of different solid-state systems offers a unique way to screen the nature and energetics of intermolecular interactions that stabilize the supramolecular structure,13in addition to the observed phase transitions and features of molecular migration.14The study of mechanical behaviour of molecular crystals by experiments on nanoindentation 15 has already emerged as an important technique to interconnect the molecular level events with various microscopic/ macroscopic phenomena,16-17 but the quantitative understanding of mechanical responses for crystal polymorphs still needs further attention. 9, 10, 12, 18-20 In this regard, during our ongoing investigation to understand the mechanical properties of crystal polymorphs by nanoindentation,10 we observed the concomitant appearance of two polymorphic forms of the compound, N-(3ethynylphenyl)-3- fluorobenzamide (2; Scheme 1).21 Although our target was to regenerate the previously reported polymorphic form (Form II) for the mechanical study, crystallization of the compound2 in toluene solvent at room temperature(24-28 °C) surprisingly generates single crystals of two different morphologies (rod and long-plates; Figure 1). A

preliminary examination of the unit cell followed by single crystal X-ray diffraction (SCXRD) data collection on the obtained crystals for the two distinct morphologies revealed that the compound 2crystallizes in two new polymorphic forms (FormsIII and IID; CCDC. 1576168-1576169; Table S1) in the solvent toluene,in addition to three known and established forms (FormsI, II and ID;CCDC. 1555052,1555055 and 1555053respectively). It is important to mention that the Form II polymorphic phase crystallizes exclusively in the same solvent at low temperature (4-5 °C).21 However, Form IIIcan be crystallizedin dichloromethane-hexane solvent mixture exclusively, but the Form IIDwas not obtainedfurther. Even after extensive crystallization attempts in several conditions using various solvents (Table S2) this form could not be isolated and hence it can be regarded as a “disappearing polymorph” or metastable form. The crystallographic parameters of the two new polymorphs (FormsIII and IID) are summarized in Table S1 and ORTEPs are given in Figure S1, Supporting Information. The newly obtained polymorphic forms were also characterized by powder X-ray diffraction technique and the phase stability was determined by differential scanning calorimetry measurements (Figures S2-S4). The mechanical response of different polymorphs of the compound 2,in terms ofH and E were evaluated22bynanoindentation (TI 950, Hysitron Inc, Minneapolis, USA) whichwas carried outon the major faces of single crystalsof FormsI, II, and IIIusing a three-sided pyramidal diamond indenter.The indentation could not be performed on FormsID Scheme 1.Chemical Structure of the compound (2)and existing pentamorphic phases.

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variation), the variations in E values are quite substantial. In particular, Form II's stiffness is less than half of that measured for Form I. (Table 1). It is generally believed that higher values of ρ, which largely depends on close packing, and E, which depends on packing as well as the number and strength of intermolecular interactions, lead to a greater melting point (Tm) of solid. However, it was found to be not true in the current context. 25-26

Figure 1. Concomitant occurrence of Form III (plate) and Form IID (rod) crystals after crystallization of compound2from toluene solvent at room temperature (24-28 °C).

andIIDdue to the unavailability ofsuitably large single crystals for reliable data acquisition. Representativeload, P, vs. depth of penetration, h, curves obtained from the major faces of different polymorphs are shown in Figure 2a. All the P-h curves of three polymorphs show significant residual penetration depths upon complete unloading indicating that the crystals absorbed majority of the imposed indentation strain through plastic (or permanent) deformation. In case of Form II, the indentation depth at the peak load (~ 950 nm) was the highest, whereas FormsI and Form III, exhibit much smaller but similar, peak penetration depths (~750 nm) at the same peak load. This result implies that Form II crystals are less resistant to plastic deformation than FormsI and III. In addition, the occurrence of pop-ins or displacement bursts in the loading segments of the P-h curves at different loads on all the three polymorphic forms (see arrows in loading segments of Figure 2a) was noted. Their occurrence indicatesthat the plastic flow behavior is heterogeneous or irregular in nature which can be attributed to the elastic compression of the layers followed by a sudden slip when reached to a critical load.23Scanned 3D image of the residual indent on the major face of Form IIIare displayed in Figures 2b and 2c. They indicate tomaterial pileup at one of the corners, which is due to the incompressible (or volume conserving) nature of plastic deformation in it. The indents made on FormsI and Form II crystals are devoid of such features (Figure S5). The values of H and E, extracted using the standard Oliver-Pharr method from the P-h responses, obtained for the three polymorphs are listed in Table 1. The average values of both H and Eof Form III are greater than those obtained for Form I, which, in turn, are higher than the respective values of Form II.Thus, Form III is relatively harder and stiffer polymorph amongst the three polymorphs indented. Moreover, the hierarchy in the mechanical response of the three crystal polymorphs examined is commensurate with the order in which their densities (ρ)are organized, as can be seen from Table 1. However, no such correlation can be found between the mechanical properties and the melting point (Tm) of solid. While the range of H values measured in the three polymorhs is relatively small (0.30 to 0.35 GPa, i.e, less than 20%

Figure 2. (a) Representative Load (P)-displacement (h) curves obtained using Berkovich indenter of radius ~ 75 nm on the major faces of Form I, Form II and Form III single crystals. (b) Plan view and (c) 3D AFM image of the residual indent of the Form III crystal.

Table 1. Hardness (H), Elastic Modulus (E), Density (ρ) and Melting point (Tm) of three Polymorphs. indented face

H (GPa)

E (GPa)

ρ (gcm3 )

Tm(°C)24

Form I

(110)

0.33± 0.09

10.66± 0.69

1.371

102.8

Form II

(100)

0.30± 0.02

4.91±0 .14

1.353

90.4

Form III

(200)

0.35± 0.01

12.89± 0.16

1.388

92.4

The mechanical properties of the three polymorphs can be rationalized in terms of their crystal structures and packingarrangement of molecules. The SCXRD results establishthattwo new polymorphic forms of the compound crystallize in monoclinic C2/c(Form III) and orthorhombic Pca21(Form IID) space groups with one molecule in the asymmetric unit (Zʹ) for each form (Table S1). As mentioned earlier21, the formation of the layered supramolecular structure was common toall the three polymorphs (FormsI,II, and ID). In Form IID, where thefluorine atomexhibits positionaldisorder, Figure S1, the molecules are packed in zigzag layers via strong N-H···O hydrogen bonding chains and two adjacent layers are linked through C(sp)-H···π (ring) interactions (Figure S6). The arrangement of molecules in Form III polymorph is similar to that in Form I(Figure 3). The layered structure of both the polymorphs are guided by strong N-H···O hydrogen bonding chains and non-directional π···π stacking interactions, which are also present in the same direction (Figures 3a and 3c). Two such adjacent chains are connected via weak C(sp)H···F hydrogen bond. Also, several weak C(sp2)-H···O, C(sp/sp2)-H···π (ring/triple bond) and C(sp2)-H···F interactions were found to stabilize the crystal packing of both the Forms I and III. Because of the layered nature of the packing which exists in the solid state, the large anisotropy in their mechanical properties are expected for both the polymorphs.


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Crystal Growth & Design But, both the Form I and Form III exhibit high H and E values as observed from nanoindentation experiments on the major crystal face (Table 1 and Figure S7). This is due to the fact that although the Form I has slip planes, but they are not effective for sliding of molecular layers along the indentation direction (Figure S8).

Figure 3. Similar layered crystal packing of (a-b) Form I and (cd) Form III. Red shades (b and d) indicate the indentation planes for Form I (110) and Form III (200). Dark red dotted lines in (d) show possible slip planes hindered by C-H···O and C-H···C dimers of Form III. Yellow arrows indicate indentation directions.

Upon indentation on the major crystal face (110) of the Form I, the indenter tip experiences strong N-H···O along with weak C(sp2)-H···O, C(sp2)-H···π (triple bond) and C(sp)-H···F hydrogen bonds(Figure 3b) which makes the crystal hard to deform, both elastically as well as plastically. Although the structure of FormIII is closely related to Form I, a clear difference in the intermolecular interactions exists across the indentation face (200) (Figure 3d), which leads to slightly greater H and E values in the former.By the simple visualization of the Form III structure it can be assumed that the same consists of slip planes (red dotted lines in Figure 3d), but they are inter-linked through numerous directional interactions such as C(sp2)-H···O dimer, C(sp2)-H···π (ring) dimer and C(sp)-H···F hydrogen bonds, which are collectively strong.So, these are not favourable for mechanical sliding or displacement of molecular layers during indentation on the (200) face of Form III (Figure 3d) leading to slightly higher resistance to deformation than Form I. However, on loading, plastic flow can be caused by the breaking of such hydrogen bonding dimers and C(sp)-H···F interactions, leading to popins (see blue arrow in Figure 2a) and material pile-up for Form IIIcrystal (Figure 2c-d). In contrast, the packing of molecules in Form IIis mainly constructed by strong N-H···O hydrogen bonding chains which are stacked by π···π interactions to form zigzag layers in nearly perpendicular direction to each other (Figure 4a). This closely perpendicular arrangement of strong N-H···O and π···π stacking interaction is a unique feature in the crystal packing of Form II. The neighboring molecules in two criss-cross hydrogen bonding chains are further connected via C(sp)-H···F hydrogen bond along a-direction leading to the formation of molecular layers (between blue shades,Figure 4b) perpendicular to the indentation face (100). Two such adjacent layers are connected with weak displaced π···π stacking and C(sp2)-H···π (ring/triple bond) hydrogen bonds along bdirection (Figure 4b). When the major (100) face of Form II crystal is indented, these π···π stacking arrays can be compressed as a result of the very low E value of the same. The movement of the molecular layers can also be possible by

disrupting of the weak C(sp2)-H···π (ring/triple bond) hydrogen bond (see pop-in for red P-h curve) in Form II, which is the reason for the lowest H value of Form II polymorph than the other two stable polymorphs (Forms I and II) examined.The probable mechanism of molecular layer displacement is nearly similar (Figure S9) to the previously reported nanoindentation study for the curcumin polymorphs.27 To further quantify and rationalize the observed mechanical responses of the three polymorphs, their interaction topologies have been mappedin terms of energy frameworks using CrystalExplorer17 28. This method recently got considerable research attentionto correlate mechanical properties with a graphical representation of interaction hierarchy for molecular crystals.13, 29The framework of energy is based on the strength and nature of pairwise interaction energy (IE) which is proportional to the thickness of cylinders joining the molecules. From the energy frameworks of FormsI(Figure S10) andIII(Figure 5a), it is clear that the higher diameter tubes (red arrow) originate from both the electrostatic and dispersive dominated interactions along the b-axis. This is on account of the inter-layers that comprise of both the specific N-H···O hydrogen bonding as well as non-directional π···π stacking interactions with IE of -63.1 kJ/mol (Form I) and 49.5 kJ/mol (Form III). Despite the presence of possible slip planes in the structures of Form I and Form III, easy molecular layer sliding in the later is restricted by the collectively strong several weak hydrogen bonding interactions such as, CH···O (-27.8 kJ/mol), C-H··· π (-27.8 kJ/mol) and the displaced stacking (-32.7 kJ/mol), which is evident from the comparable sizes of the framework along the c-axis in between two adjacent layers (Figure 5a). For this reason, the anisotropy in interaction topology of Form III is not prominent along the indentation direction and hence the observed stiff nature. Similarly, the movement of molecular layers in Form I is strongly resisted mainly by N-H···O hydrogen bond which exhibits higher IE (-63.1 kJ/mol) in comparison to the other two polymorphs.In case of Form II polymorph, the energy framework exhibits moderately strong N-H···O hydrogen bonding chains (-45.8 kJ/mol) which are strongly stacked to each other by π···π interactions (totaling of -67.2 kJ/mol) along with few C-H···π interactions (totaling of -22.7 kJ/mol) in a nearly perpendicular manner (see blue topology and red arrows in Fig. 5b). This orthogonal arrangement of the N-H···O and π···π interactions with their significant difference of IE generate anisotropy in interaction topology for Form II.



Figure 4. (a) Anisotropy in crystal packing in different directionscomprisingof strong N-H···O and weak π···π stacking interactions, and (b) red and blue shades show the indented facewith possible slidingof layers respectively in Form II polymorph. Yellow arrow indicates the indentation direction.

Figure 5. Energy frameworks for crystal structures of (a) Form III and (b) Form II polymorphs representing the total interaction energy (blue), electrostatic (red) and dispersive (green) components.

Moreover, the anisotropy is also evident from separate frameworks (red and green in Figure 5b)indicativeof directional electrostatic N-H···O hydrogen bonding chains andstrong dispersive dominated stacking layers, resulting in the lowest H and E values of Form II polymorph in contrast to others. In summary, two new polymorphic forms of N-(3ethynylphenyl)-3- fluorobenzamide have been reported in addition to the three previously known forms. The existence of this pentamorphic forms of anorganic compound is rare and noteworthy for understanding the landscape of experimentally determined crystal structures. In addition, the concomitant appearance of the new polymorphsis important forunderstandingthe subtle interplay of thermodynamic and kinetic factors which play an important role during the process of crystallization leading toenergetically closely related different solid-state forms. The mechanical characterization of three stable polymorphic forms (Form I, Form II and Form III) by nanoindentation experiment reveal that inspite of having layered kind of structures in all the polymorphic forms, the single crystals of Form II displayed the lowest values of hardness and elastic modulus, while Form III exhibitsthe

hardest and stiffest nature. The structural insightsusingthe interaction topology from energy frameworks demonstrate that the different arrangement of the most stabilized electrostatic N-H···O hydrogen bonds and dispersive dominated π···π interactions are responsible for the observed anisotropy in interaction topology for Form II. Contrary to this, the coexistence of such interactions in similar direction leads to seemingly isotropic packing for Form I and Form III polymorphs. Finally, this study delineates the molecular level understanding of crystal polymorphism in the context of fundamental as well as of practical applications.

ASSOCIATED CONTENT Supporting Information Details of Crystallization, experimental procedures, description of single crystal data collection and structure solution, ORTEPs, PXRD patterns, DSC of polymorphs, experimental face indexing, BFDH morphologies, energy frameworks and interaction energy calculations are provided in Supporting Information.


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Crystal Growth & Design Accession Codes CCDC. 1576168-1576169 contain the supplementary crystallographic data for this paper.

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 DST-SERB, INDIA. D.C. and S.B. acknowledge IISER Bhopal for research facilities and infrastructure. S. B. thanks, IISER Bhopal for the research fellowship. S. B. is also grateful to Dr. Sajesh P Thomas for needful suggestions regarding energy framework calculations.M.S.R.N.K thanks, DST-SERB, INDIA for an Early Career Research Award (File No: ECR/2016/000827).

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For Table of Contents Use Only

Crystal Structure-Mechanical Property Correlations in N-(3ethynylphenyl)-3-fluorobenzamide Polymorphs Subhrajyoti Bhandary,a Kiran S. R. N. Mangalampalli,bUpadrasta Ramamurty,c and Deepak Chopra*a

Synopsis:A rare case of crystal polymorphism was experienced, in which an organic compound exists in pentamorphic phases. All three stable phases show mechanical anisotropy upon nanoindentation.


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