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Competition between Polar and Centrosymmetric Packings in Molecular Crystals: Analysis of Actual and Virtual Structures Roberto Centore,*,† Sandra Fusco,† Fabio Capone,‡ and Mauro Causà‡ †

Department of Chemical Sciences, University of Naples ”Federico II”, Via Cintia, I-80126 Naples, Italy Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, Piazzale Tecchio, I-80125 Naples, Italy

‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Imines obtained by condensation of 4-hydroxybenzohydrazide with aliphatic ketones are a rare example of a class of compounds showing a remarkable tendency to crystallize in acentric polar space groups (Pna21 or Cc). In fact, all of the (seven) compounds studied up to now show at least one polar polymorph. In some cases, polymorphism was detected, and a nonpolar centrosymmetric phase was also identified (P21/c or P21/n space group). With the aim to disclose the conditions that can favor the formation of acentric structures in molecular crystals, we report, in this paper, a theoretical analysis (ab initio density functional theory with periodic boundary) of the lattice energy and density of all the packing modes observed in the whole set of imines. The computational analysis has been performed by optimizing each compound in its own experimental packings (actual crystal structures) and also in the packings of the other compounds of the class (virtual structures). The experimental crystallographic data and the theoretical analysis suggest that two conformers, basically differing for the orientation of the phenolic H atom in the plane of the phenyl ring, compete, in solution, for the formation of polar or centrosymmetric packings. The transitions between polar and centrosymmetric polymorphs are of diffusive type, and single crystals are not preserved, while the transitions between different polar polymorphs can be of singlecrystal-to-single-crystal type.

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with aliphatic ketones, Chart 1,9,10 a class of compounds with a persistent tendency to form acentric polar crystal structures (Pna21 or Cc space groups).11−16 That tendency was checked against change of the ketone, spanning from acyclic (acetone, methylethylketone) to cyclic ketones (cyclobutanone, cyclohexanone);9 it was confirmed in different polymorphs of the same compound (out of the four polymorphs identified up to now for imine 1, three are polar, with the transitions between the polar phases being of singlecrystal-to-single-crystal type);9,17 finally, the tendency was confirmed in the case of the racemic mixture of a chiral imine.10 In the latter case (rac-3-methylcyclopentanone as the ketone in imine 5), two different crystal phases were observed: a polar phase (Pna21) and a centrosymmetric phase (P21/n), with the second being thermodynamically stable at room temperature. In the present paper we report the solid state analysis of the new compound 4, in which cyclopentanone was used as the ketone reagent, and a comparative theoretical analysis (ab initio density functional theory (DFT) with periodic boundary) of

INTRODUCTION In a polar crystal, there is a direction that is not transformed in the opposite direction by any symmetry operation of the crystal class. That direction is called the polar axis of the crystal.1 There is a general interest in polar crystals, because some physical properties of materials, represented by odd rank tensors and highly desired for advanced applications, such as pyroelectricity, piezoelectricity, ferroelectricity, second harmonic generation, electrooptic effect, are only allowed or they are strongly enhanced in polar space groups.1 The center of symmetry transforms each direction in the opposite one, so that centrosymmetric crystals are not polar. Only 10, out of the 21 classes lacking the center of symmetry, are polar. As a matter of fact, polar symmetry is rare. It is well-known that a large fraction of organic nonchiral compounds crystallize in centrosymmetric space groups, mainly P21/c and P1̅,2−4 and also in crystals of enantiomerically pure chiral compounds, the most frequent space group is P212121 that is acentric but not polar.1−4 Actually, the bias for centrosymmetric over acentric crystals is a problem lying at the very heart of crystallography, addressed since the beginning of the discipline. It is still a long debated, challenging problem of the structural science.5−8 In view of all this, it is noteworthy that we have found in the imines obtained by condensation of 4-hydroxybenzohydrazide © XXXX American Chemical Society

Received: January 12, 2016 Revised: February 29, 2016

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DOI: 10.1021/acs.cgd.6b00054 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Chart 1. Chemical Formulae of the Studied Compounds and the Space Groups of the Observed Crystal Phasesa

Table 1. Crystal and Refinement Data for the Two Polymorphs of 4 4ort emp. formula M system space group a/Å b/Å c/Å β/° V (Å3) Z, T/K ρcalc/g·cm−3 reflns collected unique reflns (Rint) R1 [I > 2σ(I)] wR2 [all data] max. peak/hole (e·Å−3)

a

packing was performed using the program Mercury,23 which was also used for the calculation of the powder diffraction patterns. Hirshfeld surface analysis24 was performed using the program CrystalExplorer.25 Computational Details. Ab initio calculations with periodic boundary conditions were performed using the Hybrid Density Functional method: the Becke’s B3LYP functional was applied.26 We have used a 6-31G(dp) Gaussian basis set,27 which gives reasonable results for organic molecular crystals.28,29 The dispersion forces were treated by the Grimme’s method,30,31 adopting the parameters optimized for molecular crystals.28,29 The treatment of the dispersion forces using a damped Lennard-Jones potential as proposed by Grimme30,31 is absolutely mandatory in molecular crystals for getting realistic optimized geometries. The lattice energy (Ulat) was calculated as the difference between the DFT molar energy of the optimized crystal and the DFT molar energy of the free molecule. The crystal structures were optimized with respect to both the atomic fractional coordinates and the lattice parameters, keeping the experimental space group symmetry. The generation of the starting virtual structures to be optimized has been performed as already described32 and is detailed in the Supporting Information (henceforth SI). All calculations were performed using the CRYSTAL program.33

In the case of 5, the racemic mixture is considered.

the lattice energy and density of all the different modes of packing observed for the whole set of imines studied up to now. The aim is to find a rationale for the observed tendency to form non-centrosymmetric structures.

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4mon

C12H14N2O2 218.25 orthorhombic monoclinic Pna21 P21/c 9.9150(6) 7.825(3) 9.717(1) 13.922(5) 11.2870(8) 10.140(4) 90 98.27(2) 1087.44(15) 1093.2(7) 4, 173 4, 173 1.333 1.326 7232 7396 2288 (0.0288) 2489 (0.0664) 0.0281 0.0574 0.0735 0.1541 0.151/−0.146 0.261/−0.278

EXPERIMENTAL SECTION

Materials and Methods. Differential scanning calorimetric (DSC) analysis was performed using a PerkinElmer Pyris instrument, under flowing nitrogen, at 10 K/min scanning rate. Temperature controlled optical microscopy was performed with a Zeiss Axioskop polarizing microscope equipped with a Mettler FP90 heating stage. 1H NMR spectra were recorded with a Varian spectrometer operating at 200 MHz. All chemicals were obtained commercially and used as received, except for 4-hydroxybenzohydrazide, which was prepared as described in the literature.18 Synthesis of 4. The compound was obtained by refluxing, in absolute ethanol, 4-hydroxybenzohydrazide and cyclopentanone (in stoichiometric ratio 1:1.30 by mol) for 1 h (20 mL of ethanol were used for 0.5 g of 4-hydroxybenzohydrazide). After that period, about half of the solvent was removed by gentle boiling, and, on cooling to room temperature, the product was obtained as a white crystalline solid, which was recovered by filtration and dried in oven at 100 °C for 2 h. Single crystals for X-ray analysis were grown from ethanol solution by evaporation. 4. N′-Cyclopentylidene-4-hydroxybenzohydrazide. C12H14N2O2. Yield 88%; mp 268 °C (from EtOH, dec). δH (200 MHz; DMSOd6, 25 °C) 1.73 (4 H, m), 2.39 (4 H, m), 6.80 (2 H, d, 3J = 8.2 Hz), 7.69 (2 H, d, 3J = 8.8 Hz), 10.01 (2 H, s, broad) ppm. X-ray Analysis. All data for crystal structure determinations were measured on a Bruker-Nonius KappaCCD diffractometer equipped with Oxford Cryostream 700 apparatus, using graphite monochromated MoKa radiation (0.71073 Å). Reduction of data and semiempirical absorption correction were done using SADABS program.19 The structures were solved by direct methods (SIR97 program20) and refined by the full-matrix least-squares method on F2 using SHELXL-97 program21 with the aid of the program WinGX.22 H atoms bonded to C were generated stereochemically and refined by the riding model; those bonded to O and N were found in difference Fourier maps and their coordinates were refined. To all H atoms, Uiso equal to 1.2 times Ueq of the carrier atom was given. Crystal and refinement data are summarized in Table 1. The analysis of the crystal

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RESULTS AND DISCUSSION Polymorphism of Imine 4. Crystallization of 4 produces samples of different morphologies, and this can suggest solid state polymorphism.34,35 As it will be clear later, the two morphologies do correspond to two different polymorphs: a polar orthorhombic modification (Pna21, Figure 1a) and a centrosymmetric monoclinic modification (P21/c, Figure 1b). The polymorphism was confirmed by DSC analysis and optical observations. In the DSC heating curve of a sample only containing prisms like that of Figure 1a, melting at 268 °C is the only transition recorded (see SI). When a lozenge-shaped crystal as that of Figure 1b is heated under the polarizing microscope, an irreversible solid−solid transition is observed at 175 °C, and the solid phase obtained melts at 268 °C. So, for 4, we deduce that the polar orthorhombic phase is thermodynamically stable in the whole investigated temperature range up to melting, while the monoclinic centrosymmetric phase is metastable. Moreover, the solid state transition from the monoclinic to the orthorhombic phase is not topotactic, and single crystals are not preserved during the transition (see SI for a detailed analysis; a movie of the transition in avi format is also available). This suggests that the transition is of diffusive type B



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and imino N acceptor are wrapped around the polar c axis. In terms of symmetry, the two orthogonal chains, shown in Figure 3, are generated by a glide plane and by a 21 axis parallel to the glide plane. This fixes the crystal class as mm2, noncentrosymmetric and polar (see SI for a more detailed discussion based on topological and symmetry considerations). The H bonding patterns of 4mon are similar to 4ort, but the H-bonded chains are no longer orthogonal to each other. Amide chains parallel to c are formed by H bonding between the N−H donor and the carbonyl oxygen acceptor of glide related molecules. Chains of H bonded molecules parallel to (a + c/2) are formed by H bonding between O−H donors and imino N acceptors of glide related molecules. The two patterns are shown in Figure 4. In this case, the glide plane and the 21 axis are perpendicular, and the crystal class is 2/m centrosymmetric. The packing of 4mon is completely analogous to the monoclinic P21/c structure of 1.17 The two crystal structures can be considered isomorphous. So, notwithstanding the coincidence of the H bonding synthons in the two structures and the formation of chains between the same set of donor and acceptor groups, the crystal packing is basically different in the two cases. The packing diagrams reported in Figure 5, in particular, clearly evidence the polarity of the packing of 4ort, and the centrosymmetric, non polar packing of 4mon. Theoretical Calculations, Actual and Virtual Structures. From the present and previous works,9,10,17 altogether four different packings have been identified for the compounds of Chart 1: two polar packings (Pna21 and Cc space groups) and two centrosymmetric packings (P21/c and P21/n space groups). Following a recently proposed approach,32 we have optimized, by ab initio DFT calculations with periodic boundary, each imine in its own experimental packings (actual crystal structures) and also in the different crystal packings of the other imines (as an example, for 4, also in the packing Cc of imine 3 and P21/n of racemic imine 5). We will call virtual crystal structures these latter, since they have not (yet) been found experimentally.32 In this way, we have access to calculated thermodynamic properties (lattice energy and density) of each imine in all the crystal packings observed experimentally for the whole set of compounds.38 Imines 2 and 5 have not been considered in this computational study because their crystal structures show some disorder.9 The results are shown in Tables 2−5. A survey of Tables 2−5 reveals that the polar orthorhombic structure has always a low lattice energy in all the studied compounds, within 0.6 kcal/mol of the absolute minimum, and its lattice energy monotonically decreases with increasing the size of the R group (Chart 1). The polar monoclinic packing (Cc) has instead a high lattice energy in all the imines (≥2.4 kcal/mol of the absolute minimum) but 3, in which its lattice energy is the lowest (actual structure). The trend of the lattice density with the size of the R group is remarkable. If we consider the centrosymmetric P21/c packing, the lattice density monotonically decreases by increasing the size of R, so that the maximum density is for 1 and the minimum for 6. In the case of the polar packing Pna21, and of the centrosymmetric packing P21/n, the trend is exactly reversed, as the density substantially increases by increasing the size of R. In the case of the polar packing Cc the trend is not monotonic, and ρlat has the maximum just for 3. So, it seems that in the centrosymmetric packings it is possible to optimize the lattice density both for R small and large. The analysis of Tables 2−5 also shows that, in

Figure 1. (a) Prismatic crystal of the orthorhombic modification of 4; (b) lozenge shaped crystal of the monoclinic modification of 4. In both cases the scale bar is 200 μm.

(nucleation and growth process) with relevant differences in the crystal packing of the two phases. Discussion of the Crystal Structures of 4. The X-ray molecular structures of the orthorhombic and monoclinic modifications of 4 (henceforth 4ort and 4mon respectively) are shown in Figure 2.

Figure 2. (a) Ortep diagram of 4ort. (b) Ortep diagram of 4mon. In both cases, thermal ellipsoids are drawn at 30% probability level.

The two molecules basically differ for the orientation of the phenolic hydrogen in the plane of the phenyl ring: in Figure 2a it is placed on the same side of the lone pair on N2, while in Figure 2b it is on the opposite side. It is important to note that the orientation shown in Figure 2a is common to all polar polymorphs (space group Pna21) of the imines of Chart 1, while the orientation of Figure 2b is found in all the centrosymmetric polymorphs (P21/c or P21/n) and in the polar polymorph Cc. So, it is as if the two conformations fitted different packings. Of course, the sole observation of the conformational polymorphism of 4 implies that both the conformers are present in solution.36 The packing of 4ort is completely analogous to the imines of Chart 1 with the same space group, described in refs 9 and 10. Amide chains running along a are formed by H-bonding between the N−H donor and the carbonyl acceptor.37 The chains formed through H-bonding between the O−H donor C



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Figure 3. Different H-bonded chains in the crystal structure of 4ort. Structural data of the H-bonds: 37N1−H···O2i: 0.81(4), 2.31(4), 3.122(4) Å, 178(4)°, i = 1/2 + x, 1.5 − y, z; O1−H···N2ii: 0.97(5), 1.86(5), 2.827(4) Å, 169(4)°, ii = −x, 2 − y, −1/2 + z.

Figure 4. Different H-bonded chains in the crystal structure of 4mon. Structural data of the H-bonds: 371−H···O2i: 0.83(3), 2.29(3), 3.115(3) Å, 172(3)°, i = x, 1/2 − y, −1/2 + z; O1−H···N2ii: 1.00(3), 1.82(3), 2.781(3) Å, 159(2)°, ii = −1 + x, 1/2 − y, −1/2 + z.

Table 3. Calculated Lattice Energy (kcal/mol) and Lattice Density (g/cm3) for Actual and Virtual Crystal Structures of 3 at 0 Ka Ulat ρlat a

3 (P21/c)

3 (Pna21)

3 (Cc)

3 (P21/n)

−53.9 1.405

−55.00 1.383

−55.2 1.482

−50.3 1.383

Entries for the actual structures are given in bold.

Table 4. Calculated Lattice Energy (kcal/mol) and Lattice Density (g/cm3) for Actual and Virtual Crystal Structures of 4 at 0 Ka Figure 5. (a) Partial crystal packing of 4ort viewed down a; (b) partial crystal packing of 4mon viewed down c.

Ulat ρlat


1 (P21/c)

1 (Pna21)

1 (Cc)

1 (P21/n)

−50.9 1.431

−50.9 1.246

−47.9 1.371

−51.4 1.375

a

4 (P21/c)

4 (Pna21)

4 (Cc)

4 (P21/n)

−55.8 1.399

−56.9 1.414

−54.5 1.452

−56.0 1.389


many cases the highest density is shown by the packing Cc that has a high lattice energy. Concerning the polymorphism of 1 and 4, a relevant difference comes out from the inspection of Tables 2 and 4. In fact, in 4 the two actual structures have lattice energy and density similar to each other (ΔUlat = 1.1 kcal/mol, Δρlat = 0.015 g/cm3). In the case of 1, on the contrary, the minimum of Ulat is reached in two different packings (both actual structures)


general, the packing with the lowest lattice energy also exhibits a high density; the reverse, however, is not true because in D



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aliphatic ketones has been further confirmed experimentally and is quite remarkable if compared with other literature examples.12−16 In fact, all the seven imines investigated up to now have a polar crystal structure, either as the thermodynamic stable phase or as a metastable phase. This confirms that transverse molecules9,40 are real candidates to yield acentric polar crystals in high score. A general look at the experimental and theoretical results reported in this paper would also suggest the provocative observation that, in the case study, the bias for acentric over centric structures (or the reversed bias) is only apparent and strongly dependent on the identification of the polymorphs. After all, several imines of Chart 1 show, at the present level of investigation, both an acentric and a centric polymorph (all in space groups that, according to Kitaigorodskii, allow the close packing) and, in some cases in which only one has been found, there are reasons to believe that also the other could be obtained.41 In many cases, one of the polymorphs was identified by chance, or after several trials and in a scarcely reproducible way or it was long sought, and eventually found, because it was supposed to exist.42


6 (P21/c)

6 (Pna21)

6 (Cc)

6 (P21/n)

−50.3 1.314

−58.6 1.408

−50.7 1.426

−54.1 1.405


having quite different lattice density (Δρlat = 0.18 g/cm3). That “softness” of the othorhombic packing of 1 can be related with its rich polymorphism and single-crystal-to-single-crystal transitions.9,17 The pattern emerging from the analysis of the actual and virtual crystal structures is intriguing and shows apparent contradictions that can be summarized as follows: (1) In some cases, two crystal structures with slightly differing thermodynamic parameters are not both observed experimentally, but only one.39 This is the case, for instance, of the two centrosymmetric structures of 4 (ΔUlat ≅ 0.2 kcal/ mol), but also of the two centrosymmetric structures of 1 (ΔUlat ≅ 0.6 kcal/mol); (2) In other cases, two crystal structures with slightly differing lattice energy are both observed experimentally notwithstanding a relevant difference in lattice density. This is the case, for instance, of the polar and centrosymmetric structures of 1 (ΔUlat ≅ 0 kcal/mol, (Δρlat≅ 0.2 g/cm3); (3) In other cases, two crystal structures with thermodynamic parameters differing by a greater extent are both observed experimentally. This is the case of the Pna21 and P21/c structures of 4 (ΔUlat ≅ 1.1 kcal/mol). In the search of a possible rationalization of these results, we note that case 1 corresponds to crystal structures that are both centrosymmetric and in which the molecular conformation is the same. Cases 2 and 3 corresponds to crystal structures profoundly different in their symmetry (i.e., polar versus centrosymmetric) and molecular conformation (see Figure 2). So, the reason could be related with the balance between kinetic and thermodynamic factors. In the first stages of nucleation, before critical clusters are formed, with reference to 1, the activation energy for the interconversion is low, and therefore the structure with lower (free) energy is selected.34,35 With reference to 2 and 3, the profound difference in symmetry and molecular conformation results in a higher activation energy for the interconversion and thereby both crystal structures can develop independently. So, the presence in solution of both conformers is a prerequisite for polymorphism, because the two conformers fit different sets of packings (polar or centrosymmetric). The actual observation of it relies on the relative (free) energies of the two sets of packings. As a final remark, we note that data of Tables 2−5 provide a confirmation, on a completely different class of compounds and on a set of space groups including acentric ones, of the hypothesis of virtual isomorphism that we have recently proposed (see also the SI): in a class of similar compounds, all the different modes of packing observed experimentally for single members of the class correspond to minima of the lattice energy and to acceptable lattice densities for every member of the class.32

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00054. Analysis of the polymorphism of 4. Symmetry and topological analysis of H bonding patterns in relation with the different conformation of the imine molecules. Analysis of Hirshfeld fingerprint plots of 4ort and 4mon. Discussion of the optimized actual and virtual structures. 1 H NMR spectrum of 4 (PDF) W Web-Enhanced Feature *

Movie of the phase transition from the monoclinic to the orthorhombic phase of 4, recorded on a single crystal. Accession Codes

CCDC 1417087−1417088 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to the CRdC NTAP of Regione Campania (Italy) for the X-ray facility. R.C. thanks the COST Association for support and critical discussions within the COST Action CM-1402- Crystallize.

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REFERENCES

(1) Hahn, Th.; Klapper, H. Point Groups and Crystal Classes. In International Tables for Crystallography, Vol. A, Hahn, Th., Ed.; D. Reidel Publishing Company: Dordrecht (Holland), 1983; pp 746− 785. (2) Mighell, A. D.; Himes, V. L.; Rodgers, J. R. Acta Crystallogr., Sect. A: Found. Crystallogr. 1983, 39, 737−740.

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CONCLUSIONS The tendency to form acentric polar crystal structures in imines obtained by condensation of 4-hydroxy-benzohydrazide with E



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(3) Padmaja, N.; Ramakumar, S.; Viswamitra, M. A. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 725−730. (4) The most recent figures released by the Cambridge Structural Database, on February 16, 2015 (www.ccdc.cam.ac.uk/ SupportandResources/Resources/pages/Resources.aspx), are 78.1% of structures in centrosymmetric space groups (58.9% in P21/c and P1̅), 21.9% in non-centrosymmetric space groups and 16.5% in chiral space groups. (5) The fundamental work on this problem was delivered by Kitaigorodskii who, in his book (Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultant Bureau: New York, NY, 1961) ranked the space groups in terms of suitability for the close packing of molecules of arbitrary shape. His study ruled out many of the 230 space groups as unfit for the close packing. It resulted, in particular, that the two most frequent space groups, P21/c and P1̅ (both centrosymmetric), are particularly suited for close packing of molecules. However, according to his analysis, other, far less frequent, non-centrosymmetric (and polar) space groups, are also suited for close packing (e.g., P21, Pna21, Pca21). More recent contributions on this topic are given in refs 6−8. (6) Brock, C. P.; Dunitz, J. D. Chem. Mater. 1994, 6, 1118−1127. (7) Dunitz, J. D.; Filippini, G.; Gavezzotti, A. Helv. Chim. Acta 2000, 83, 2317−2335 (“The centre of symmetry is the best packing operator for molecules with awkward shape”).. (8) Kelley, S. P.; Fábián, L.; Brock, C. P. Acta Crystallogr., Sect. B: Struct. Sci. 2011, 67, 79−93. (9) Centore, R.; Jazbinsek, M.; Tuzi, A.; Roviello, A.; Capobianco, A.; Peluso, A. CrystEngComm 2012, 14, 2645−2653. (10) Centore, R.; Fusco, S.; Jazbinsek, M.; Capobianco, A.; Peluso, A. CrystEngComm 2013, 15, 3318−3325. (11) Non trivial examples of classes of compounds with tendency to form acentric crystals are rare. References 12−16 contain a list with pertinent citations. (12) Nitroanilines: Panunto, T. W.; Urbànczyk-Lipkowska, Z.; Johnson, R.; Etter, M. C. J. Am. Chem. Soc. 1987, 109, 7786−7797. (13) Ortho-substituted benzoic acids: Frankenbach, G. M.; Etter, M. C. Chem. Mater. 1992, 4, 272−278. (14) Hydrazone derivatives: Serbutoviez, C.; Bosshard, C.; Knöpfle, G.; Wyss, P.; Prêtre, P.; Günter, P.; Schenk, K.; Solari, E.; Chapuis, G. Chem. Mater. 1995, 7, 1198−1206. (15) Phenolic polyene compounds: Kwon, O.-P.; Jazbinsek, M.; Yun, H.; Seo, J.-I.; Seo, J.-Y.; Kwon, S.-J.; Lee, Y. S.; Günter, P. CrystEngComm 2009, 11, 1541−1544. (16) N-substituted stilbazolium compounds: Kim, P. J.; Jeong, J.-H.; Jazbinsek, M.; Kwon, S.-J.; Yun, H.; Kim, J.-T.; Lee, Y. S.; Baek, I.-H.; Rotermund, F.; Günter, P.; Kwon, O.-P. CrystEngComm 2011, 13, 444−451. (17) Sahoo, S. C.; Panda, M. K.; Nath, N. K.; Naumov, P. J. Am. Chem. Soc. 2013, 135, 12241−12251. (18) Centore, R.; Carella, A.; Tuzi, A.; Capobianco, A.; Peluso, A. CrystEngComm 2010, 12, 1186−1193. (19) SADABS, Bruker-Nonius: Delft, The Netherlands, 2002. (20) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (22) Farrugia, L. J. J. Appl. Crystallogr. 2012, 45, 849−854. (23) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466−470. (24) Spackman, M. A.; Jayatilaka, D. CrystEngComm 2009, 11, 19− 32. (25) CrystalExplorer (Version 3.1); Wolff, S. K.; Grimwood, D. J.; McKinnon, J. J.; Turner, M. J.; Jayatilaka, D.; Spackman, M. A. University of Western Australia: Perth, 2012. (26) Becke, A. D. J. Chem. Phys. 1996, 104, 1040−1046. (27) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654−3665.

(28) Civalleri, B.; Zicovich-Wilson, C. M.; Valenzano, L.; Ugliengo, P. CrystEngComm 2008, 10, 405−410. (29) Centore, R.; Causa, M.; Fusco, S.; Carella, A. Cryst. Growth Des. 2013, 13, 3255−3260. (30) Grimme, S. J. Comput. Chem. 2004, 25, 1463−1473. (31) Grimme, S. J. Comput. Chem. 2006, 27, 1787−1799. (32) Centore, R.; Causà, M.; Cerciello, F.; Capone, F.; Fusco, S. CrystEngComm 2014, 16, 9168−9175. (33) Dovesi, R.; Orlando, R.; Erba, A.; Zicovich-Wilson, C. M.; Civalleri, B.; Casassa, S.; Maschio, L.; Ferrabone, M.; De La Pierre, M.; D’Arco, P.; Noël, Y.; Causà, M.; Rérat, M.; Kirtman, B. Int. J. Quantum Chem. 2014, 114, 1287−1317. (34) Bernstein, J. Polymorphism in Molecular Crystals; Clarendon Press: Oxford, 2002. (35) Bernstein, J. Cryst. Growth Des. 2011, 11, 632−650. (36) It is worthy of great reflection that a structural feature very minimal in relation to the single molecule, as it is the orientation of the phenolic H atom in the plane of the phenyl ring, has far reaching consequences in terms of molecular aggregation (polar versus centrosymmetric packings) and thereby of physicochemical properties of the final materials. (37) The geometric parameters of the hydrogen bond D−H···A are given, here and throughout the paper, in the following order: D−H (Å), H···A (Å), D···A (Å), D−H···A (deg), symmetry code of the acceptor atom. (38) Actually, in the case of imine 1, there are three different polymorphs having the space group Pna21 and very similar packing: they are named I, II, and III after ref 9. Among those three, the polymorph stable at room temperature is III and it corresponds to the Pna21 structures observed in the other imines of Chart 1. The transitions I−II and II−III are all single-crystal-to-single-crystal, and the I−II is also thermosalient, ref 9. Polymorphs I and II have not been considered in the present theoretical analysis. (39) The caveat to this statement is that, in principle, we can never be sure that all the crystal polymorphs of a given compound have been identified. (40) In ref 9 a transverse molecule was defined as a molecule in which the ground state dipole moment is transverse to the head-to-tail molecular vector. (41) This is the case of imine 2 for which the P21/c polymorph, although not yet discovered, is very likely to exist. There remains the case of imine 3. However, this is a particular case, because the molecular conformation in the known polar packing Cc (see ref 9) is the same as found in the centrosymmetric (P21/c and P21/n) packings of the other imines, so, in this case, based on the data of Table 3, the expected conformational polymorph should be that in the polar orthorhombic space group Pna21. (42) The chronological development of the research on the imines of Chart 1 may be instructive in this regard. We started with the experimental observation that all the studied imines only formed polar structures (ref 9 for imines 1, 2, 3 and 6). After one year, Prof. Naumov (in Abu Dhabi) identified, by chance, the centrosymmetric polymorph of imine 1 (ref 17), but he admitted (personal communication to R.C.) that the reproducibility in the preparation of that polymorph was very poor. Then, we passed to imine 5, for which we isolated the expected polar polymorph at first (ref 10). In order to collect better X-ray data, we recrystallized imine 5, but surprisingly and frustratingly, we were no longer able to get the polar polymorph, but only the (new) centrosymmetric P21/n one. In the case of 4 (this work), we first identified the centrosymmetric polymorph, and after, driven by the belief that a polar polymorph should exist, we found, by recrystallization, the orthorhombic one. Subsequent trials to get again the centrosymmetric monoclinic phase were (of course!) all pointless. So, it seems that imines 1, 4, and 5 provide new examples of Ostwald’s rule of stages and of the mysterious phenomenon of disappearing polymorphs (refs 34 and 35).

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