Proline and Induced Conformational Polymorphism - American

Cocrystal of 1,1-Dicyano-2-(4-hydroxyphenyl)-ethene with L-Proline and Induced Conformational Polymorphism of 1,1-Dicyano-2-(4-hydroxy3-methoxyphenyl)-ethene

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 3 383-391

Tatiana V. Timofeeva,*,† Genevieve H. Kuhn,† Volodymyr V. Nesterov,† Vladimir N. Nesterov,† Donald O. Frazier,§ Benjamin G. Penn,§ and Mikhail Yu. Antipin†,‡ Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701, USA, Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, B-334, Moscow, Russia, and NASA G.C. Marshall Space Flight Center, Huntsville, Alabama 35812, USA Received September 20, 2002

ABSTRACT: Crystallization of 1,1-dicyano-2-(4-hydroxyphenyl)-ethene (I) and 1,1-dicyano-2-(4-hydroxy-3-methoxyphenyl)-ethene (II) with L-proline (III) and L-tartaric acid (IV) was carried out. Compound I formed cocrystals with L-proline I‚III. However, cocrystallization of II was unsuccessful; instead in the presence of III or IV formation of two conformational polymorphs IIa and IIb was observed. In cocrystals I‚III, hydrogen bonded chains of proline molecules with molecules of I attached to them were found. Similar synthons were found in the literature in other proline-containing crystals, and we believe that it is possible to use these acentric supramolecular associates in crystal engineering of nonlinear optical materials. Possible reasons for the formation of two conformational polymorphs of compound II are discussed based on the results of X-ray analysis, molecular and crystal structure energy calculations, and thermal behavior of compound II. 1. Introduction Many potential nonlinear optical (NLO) compounds form centrosymmetric crystals and so do not manifest second harmonic generation in the crystalline state. To overcome this problem, a cocrystallization strategy (crystal engineering approach) was proposed.1 In many cases, cocrystallization of the polarizable molecules with chiral molecules, which are connected with hydrogen bonds, or form organic salts, or both, aims to build acentric crystalline structures.2,3 In our previous papers,4-9 we presented structural investigations of a large series of dicyanovinylaromatic NLO compounds, many of which manifest strong second harmonic generation in solution but do not form acentric crystals. In the present investigation, we applied the cocrystallization technique to several representative dicyanovinylaromatics bearing hydroxy or amino groups that potentially can form hydrogen bonds. Results are presented for two representative of this group: 1,1-dicyano-2-(4hydroxyphenyl)-ethene (I) and 1,1-dicyano-2-(4-hydroxy3-methoxyphenyl)-ethene (II). We crystallized I and II in the presence of L-proline (III) and with L-tartaric acid (IV). We choose these chiral agents for cocrystallization since we found in the literature data on success with these agents, for instance, cocrystallization of 2-amino-5-nitropyridine with L-tartaric acid.2,10 Indeed, for compound I we obtained an acentric cocrystal with III. Results of cocrystallization of compound II with these two agents are signifi* To whom correspondence should be addressed. Phone: (505) 454 3362. Fax (505) 454 3103. E-mail: [email protected]. † New Mexico Highlands University. ‡ Russian Academy of Sciences. § NASA G.C. Marshall Space Flight Center.

cantly more unexpected and in our opinion more interesting. In the case of compound II, cocrystals were not found but instead two conformational polymorphs of II were revealed by X-ray single-crystal analysis: monoclinic phases IIa and IIb. In this paper, we present results of structural investigation of four crystals: compound I, cocrystal of I and III in the ratio 1:1, and conformational polymophs IIa and IIb. To investigate conformational polymorphism in more detail, we carried out ab initio quantum calculations of the energy of the two conformers of molecule II, and crystal energy of the two polymorphs of compound II. 2. Experimental Section and Calculation Details Synthesis and Crystal Growth. 1,1-Dicyano-2-(4-hydroxyphenyl)-ethene (I) and 1,1-dicyano-2-(4-hydroxy-3-methoxyphenyl)-ethene (II) were prepared from 4-hydroxy-benzaldehyde and 4-hydroxy-3-methoxybenzaldehyde, respectively, with malononitrile in the presence of catalytic amounts of morpholine in ethanol using the standard procedure we described before.8,11-13 The yield and 1H NMR spectra are the same as we presented in our previous paper.8 To cocrystallize compound I, we used the following procedure. Equimolar amounts of I and III (0.5 mmol) in a mixture of 20 mL of acetonitrile and 10 mL of ethanol were boiled for

10.1021/cg0200513 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003

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Table 1. Crystallographic Data, Details of Data Collection and Refinement formula formula weight T (K) crystal system space group unit cell dimensions a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z goodness of fit on F2 final R indices

I

I‚III

IIa

IIb

C10H6N2O 170.17 295 monoclinic P21/c 9.271(2)

C15H15N3O3 285.30 110 monoclinic P21 14.755(2)

C11H8N2O2 200.19 295 monoclinic P21/n 4.836(1)

C11H8N2O2 200.19 295 monoclinic P21/c 9.199(2)

3.811(8) 23.632(5) 90 100.15(3) 90 821.9(3) 4 0.984

5.283(1) 18.856(2) 90 111.45(1) 90 1368.1(3) 4 1.493

13.042(3) 32.438(7) 90 92.94(3) 90 2043.2(7) 8 0.908

13.980(3) 16.125(3) 90 100.09(3) 90 2041.6(7) 8 1.140

0.0467

0.0529

0.0672

0.0624

two minutes. The solution was covered with filter paper and left for slow evaporation of solvent. In two weeks, yellow crystals of complex I‚III were obtained. A similar procedure to obtain cocrystals of I and IV gave no complex but made it possible to crystallize individual compound I suitable for X-ray analysis. Compound I crystallizes as yellow needles, mp 188189 °C, cocrystals I‚III are yellow prisms, mp 170-171 °C. Melting points for all compounds were measured on a heating stage (HS1) with temperature controller (STC200B) from INSTEC mounted on a Zeiss polarizing microscope. We grew single crystals of pure compound II by slow evaporation from ethanol and acetonitrile solutions. We were unable to obtain from these solutions crystals suitable for X-ray analysis. On the contrary, when we crystallized II with III or IV (equimolar amounts) we obtained by slow evaporation from ethanol in both cases crystals suitable for X-ray analysis. The nature of these crystals was revealed only after their X-ray analyses had been completed. Among crystals obtained by crystallization of II and III in ethanol, we found the first modification of compound II (IIa), and among crystals obtained by crystallization of II in the presence of IV we found the second polymorph of II (IIb). Single crystals of both polymorphs are yellow plates with mp 141.5-142.0 and 145.5146.0 °C for IIa and IIb, respectively. X-ray Analysis. Crystals of I, I‚III, IIa, and IIb were studied by single crystal X-ray diffraction. Some details of data collection and structure refinement are given in Table 1. All crystal structures were solved by direct methods and refined by full-matrix least squares in anisotropic approximation for non-hydrogen atoms. In all structures the hydrogen atoms of hydroxy groups we localized on difference Fourier maps and refined isotropically. All other hydrogen atoms were placed geometrically and refined using the riding model. All calculations were carried out using the program SHELXL97.14 Obtained information including atomic coordinates and their isotropic equivalent displacement parameters, bonds lengths, bond and torsion angles for the four crystals studied are presented in CIF form as Supporting Information. Selected bond lengths and bond angles in structures I and I‚III are presented in Table 2 and in polymorphs IIa and IIb in Table 3. Cocrystal I‚III contains two molecules of I and two molecules of III in the symmetrically independent part of the unit cell (molecules IA, IB, IIIA, and IIIB). The general view of molecule I from crystal I, and symmetrically independent complexes I(A)‚III(B) and I(B)‚III(A) from cocrystal I‚III with atomic numbering schemes are presented in Figure 1. For cocrystal I‚III molecular pairs that are connected with hydrogen bonds are shown. Crystals of IIa and IIb are also built of two systems of symmetrically independent molecules (A and B). Coordinates of the molecules B in structures IIa and IIb are denoted with primes. Since geometry characteristics of symmetrically independent molecules in structures IIa and IIb are very

Table 2. Bond Lengths (Å) and Angles (°) in Molecules I (Crystal I) and Molecules IA and IB (Crystal I‚III) According to X-ray Data I

I‚III-A

I‚III-B

O(1)-C(4) N(1)-C(9) N(2)-C(10) C(1)-C(6) C(1)-C(2) C(1)-C(7) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(7)-C(8) C(8)-C(9) C(8)-C(10)

bond/angle

1.352(3) 1.136(3) 1.140(3) 1.402(3) 1.404(3) 1.440(3) 1.378(3) 1.386(4) 1.390(3) 1.373(3) 1.356(3) 1.431(3) 1.440(3)

1.339(2) 1.138(2) 1.141(2) 1.407(2) 1.410(2) 1.437(2) 1.373(2) 1.397(2) 1.408(2) 1.369(2) 1.364(2) 1.428(2) 1.434(2)

1.338(2) 1.147(2) 1.155(2) 1.405(2) 1.410(2) 1.436(2) 1.373(2) 1.396(2) 1.408(2) 1.373(2) 1.364(2) 1.431(2) 1.425(2)

C(6)-C(1)-C(2) C(6)-C(1)-C(7) C(2)-C(1)-C(7) C(3)-C(2)-C(1) C(2)-C(3)-C(4) O(1)-C(4)-C(5) O(1)-C(4)-C(3) C(5)-C(4)-C(3) C(6)-C(5)-C(4) C(5)-C(6)-C(1) C(8)-C(7)-C(1) C(7)-C(8)-C(9) C(7)-C(8)-C(10) C(9)-C(8)-C(10) N(1)-C(9)-C(8) N(2)-C(10)-C(8)

117.5(2) 125.8(2) 116.6(2) 121.4(2) 119.7(2) 122.6(2) 117.3(2) 120.0(2) 120.0(2) 121.3(2) 131.8(2) 125.1(2) 118.5(2) 116.4(2) 179.3(3) 176.9(3)

118.4(1) 124.8(1) 116.8(1) 121.1(2) 120.1(1) 118.0(1) 122.7(1) 119.3(1) 120.6(2) 120.6(1) 131.2(2) 126.6(2) 118.5(2) 114.9(1) 176.9(2) 179.6(2)

118.1(1) 125.0(1) 116.9(1) 121.5(2) 119.8(1) 118.1(1) 122.5(1) 119.4(1) 120.5(2) 120.7(1) 131.0(2) 126.9(2) 118.6(2) 114.5(1) 176.8(2) 179.4(2)

similar, in Figure 2 we present only molecules A from the crystal structures IIa and IIb. Quantum Calculations of the Isomers of Compound II and Crystal Energy Calculations of Polymorphs of II. To compare relative energies of the isomers IIa and IIb, we optimized their geometry and calculated their energies using ab initio quantum approximation (GAUSSIAN program15) on RHF/6-31G** and B3LYP/6-31G** levels of theory. The experimental (X-ray) and calculated bond lengths and bond angles after optimization on the B3LYP/6-31G** level are listed in Table 3. We evaluated crystal energy of the two polymorphs using the commercial software Cerius2 from Accelrys, with a Dreiding16 force field.

3. Results and Discussion Molecular and Crystal Structures of Compound I and Complex I‚III. Molecules of I in crystals of I and I‚III have standard geometry parameters. In both crystals, bond lengths distribution in the benzene ring of molecule I shows slight quinoid character (see Table 2). In the crystal I‚III, where molecules I are connected by hydrogen bonds to L-proline, quinoid character is somewhat more pronounced than in the crystal I, which indicates that hydrogen bonding of I with L-proline is slightly stronger than between two molecules of I. It is worthwhile to mention that the orientation of the hydroxy group in the molecules of pure compound I and in its complex with III is different (Figure 1) due to formation of different systems of intermolecular hydrogen bonds. Molecules of I form a centrosymmetric crystal structure, while cocrystallysation of I with chiral agent III leads to formation of an acentryc crystal I‚III (Table 1). It is very likely that in both structures I and I‚III molecular packing is determined by formation of intermolecular hydrogen bonds, since the energetic impact of these bonds is significantly higher than the impact

1,1-Dicyano-2-(4-hydroxyphenyl)-ethene with L-Proline

Crystal Growth & Design, Vol. 3, No. 3, 2003 385

Figure 1. Molecular structure and crystallographic numbering scheme for molecule I and two symmetrically independent hydrogen bonded associates for cocrystal I‚III. Table 3. Bond Lengths (Å) and Angles (°) in Molecules IIa (A and B) and IIb (A and B) According to X-ray Data and Obtained by ab Initio Calculations (B3LYP/6-31G**) IIa-A

IIa-B

IIb-A

IIb-B

O(1)-C(4) O(2)-C(3) O(2)-C(11) N(1)-C(9) N(2)-C(10) C(1)-C(6) C(1)-C(2) C(1)-C(7) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(7)-C(8) C(8)-C(9) C(8)-C(10)

bond/angle

1.336(6) 1.364(6) 1.417(6) 1.156(6) 1.129(6) 1.395(7) 1.408(6) 1.424(7) 1.357(6) 1.415(7) 1.365(7) 1.353(7) 1.354(7) 1.413(8) 1.436(7)

1.352(5) 1.363(5) 1.417(6) 1.141(7) 1.135(6) 1.382(7) 1.407(6) 1.438(6) 1.370(7) 1.385(7) 1.375(7) 1.374(7) 1.343(6) 1.424(8) 1.429(7)

1.348 1.367 1.428 1.165 1.164 1.393 1.419 1.443 1.381 1.418 1.393 1.389 1.372 1.430 1.433

1.357(4) 1.365(4) 1.423(4) 1.149(4) 1.138(4) 1.384(4) 1.398(4) 1.448(4) 1.384(4) 1.380(4) 1.384(4) 1.381(4) 1.352(4) 1.419(5) 1.438(4)

1.352(4) 1.363(3) 1.404(4) 1.150(4) 1.142(4) 1.384(4) 1.405(4) 1.442(4) 1.363(4) 1.395(4) 1.388(4) 1.372(4) 1.350(4) 1.429(5) 1.430(5)

1.349 1.370 1.432 1.164 1.164 1.411 1.420 1.445 1.382 1.415 1.395 1.389 1.371 1.430 1.433

C(3)-O(2)-C(11) C(6)-C(1)-C(2) C(6)-C(1)-C(7) C(2)-C(1)-C(7) C(3)-C(2)-C(1) C(2)-C(3)-O(2) C(2)-C(3)-C(4) O(2)-C(3)-C(4) O(1)-C(4)-C(5) O(1)-C(4)-C(3) C(5)-C(4)-C(3) C(6)-C(5)-C(4) C(5)-C(6)-C(1) C(8)-C(7)-C(1) C(7)-C(8)-C(9) C(7)-C(8)-C(10) C(9)-C(8)-C(10) N(1)-C(9)-C(8) N(2)-C(10)-C(8)

116.9(4) 116.9(5) 118.5(5) 124.7(5) 121.1(5) 126.3(5) 120.0(5) 113.7(4) 120.0(5) 120.9(5) 119.1(5) 120.5(5) 122.4(5) 131.7(5) 124.6(4) 120.5(4) 114.9(5) 179.8(6) 178.7(6)

118.7(4) 118.3(5) 117.7(5) 123.9(5) 119.6(5) 125.3(5) 121.1(5) 113.6(4) 119.3(5) 121.0(5) 119.7(5) 119.6(5) 121.7(5) 132.2(5) 125.6(5) 120.2(5) 114.3(5) 177.6(7) 178.7(7)

118.3 118.5 117.0 124.5 120.0 126.2 120.7 113.1 120.4 119.9 119.7 119.6 121.5 131.7 124.9 119.3 115.7 179.9 179.6

117.0(2) 118.6(3) 125.8(3) 115.5(3) 121.1(3) 125.7(3) 119.3(3) 114.9(3) 117.8(3) 122.1(3) 120.1(3) 120.5(3) 120.4(3) 130.6(3) 124.7(3) 119.3(3) 116.0(3) 179.3(4) 178.9(4)

117.9(3) 118.3(3) 125.4(3) 116.3(3) 120.9(3) 127.3(3) 120.1(3) 112.7(3) 119.3(3) 121.2(3) 120.1(3) 119.5(3) 121.1(3) 131.5(3) 125.7(3) 119.9(3) 114.3(3) 179.2(4) 178.5(4)

118.4 118.5 125.1 116.4 120.7 126.7 120.0 113.4 120.1 120.2 119.7 120.4 120.7 131.9 125.3 119.0 115.7 179.2 179.4

of other intermolecular nonbonded interactions. In crystal I molecules form infinite antiparallel chains (symmetry 21) along axis b (Figure 3). Relative orienta-

IIa DFT

IIb DFT

tion of the hydrogen bonded chains in part can be related to intermolecular contacts of the acidic (positively charged) hydrogen atom at C(7) in the CdC bond

386


Figure 2. Molecular structure and crystallographic numbering scheme for polymorphs IIa and IIb. In both cases, only symmetrically independent molecules A are presented.

with the negatively charged N atom of the cyano group (see Figures 1 and 3, and Table 4) [C(7)-H(7)‚‚‚N(2)]. Such contacts are relatively common in the structures of other dicyanovinylbenzene derivatives that we studied earlier.4-7 Cocrystals I‚III are built of hydrogen-bonded chains of L-proline molecules III, with molecules of I attached to these chains. Projection of crystal structure I‚III along these chains (axis b) is shown in Figure 4. Each L-proline molecule is involved in formation of three hydrogen bonds. Two hydrogen bonds between the N-H groups of the protonated nitrogen atoms of this molecule and the deprotonated oxygen atoms of the carboxyl groups connect L-proline molecules in translational chains (Figure 5a,b). Only one of the H atoms at a protonated nitrogen atom is involved in hydrogen bond formation.

Figure 3. Projection of crystal structure of compound I.

Timofeeva et al.

Molecules of I are attached to this chain by a hydrogen bond that is formed between the hydroxy group of molecule I and the second oxygen atom of the carboxyl group of an L-proline molecule (Figure 1). Geometry parameters of hydrogen bonds in crystals I and I‚III are listed in Table 4. Projection of crystal structure I‚III perpendicular to the above-mentioned chains is shown in Figure 4. To understand better the hydrogen bonding system in I‚ III, partial projections that include chains formed by symmetrically independent L-proline molecules A and B, surrounded by molecules I, are presented in Figure 5a,b, respectively. From partial projections of structure I‚III along axis c (Figure 5a,b), it is obvious that symmetrically independent molecules of III (A and B) form similar molecular chains, but the mode of attachment of molecules I (A and B) is different (Figure 1). Chains formed by L-proline molecules are parallel (Figure 5a,b) and that corresponds to the asymmetric nature of cocrystal I‚III. Analyzing data in the Cambridge Structural Database,17 we found that proline and several prolinecontaining cocrystals (QIJYIR, L-prolinium hydrogen tartrate, PROLIN, L-proline, RUWGEV, L-proline monohydrate, POKHAY10, 4-(2,4,6-tri-isopropyl-benzoyl)benzoic acid (S)-(-)-proline, DLPROM01, DL-proline monohydrate) form translational chains of proline molecules with parameters close to those observed in I‚III (b ) 5.283 Å). Parameters (Å) along the proline molecular chains in these structures, presented with their refcodes, are

QIJYIR18 PROLIN19 RUWGEW20 a ) 5.007 c ) 5.200 c ) 5.136 DLPROM0121 POKHAY1022 a ) 5.274 a/2 ) 5.825 In the latter structure, there are two symmetrically



Table 4. Selected Characteristics of Intermolecular Hydrogen Bonds and Short Intermolecular Contacts in Structures I, I‚III, IIa, and IIb Structure I H-bond

symmetry

O/C‚‚‚N

O/C-H

H‚‚‚N

angle O/C-H‚‚‚N (°)

O(1)-H(1O)‚‚‚N(1) C(7)-H(7)‚‚‚N(2)

1-x, 0.5+y, 0.5-z -x, 1-y, -z

2.895(3) 3.474(4)

0.84(4) 0.92(4)

2.07(4) 2.60(4)

171(2) 158(2)

Structure I‚III H-bond

independent molecules

symmetry

O/N‚‚‚O

O/N-H

H‚‚‚O

angle O/N-H‚‚‚O (°)

O(1)-H(1)‚‚‚O(2B) N(1B)-H(NC)‚‚‚O(1B) O(1′)-H(1′)‚‚‚O(2A) N(1A)-H(NA)‚‚‚O(1A)

IA-IIIB IIIB-IIIB IB-IIIA IIIA-IIIA

x, y, z x, y-1, z x, y, z x, y-1, z

2.619(2) 2.780(2) 2.604(2) 2.805(2)

0.92(2) 0.95(2) 0.91(2) 0.79(2)

1.71(2) 1.86(2) 1.71(2) 2.04(2)

170(1) 161(1) 170(1) 163(1)

Structure IIa H-bond


symmetry

O‚‚‚N/O

O-H

H‚‚‚N/O

angle O-H‚‚‚N/O (°)

O(1)-H(1)‚‚‚N(1) O(1′)-H(1′)‚‚‚N(2) C(7′)-H(7′a)‚‚‚N(2′)

2.5-x, 0.5+y, 0.5-z x+1, y, z -x-1, -y, -z

2.906(6) 2.852(6) 3.504(6)

0.88(2) 1.03(2) 0.93

2.08(2) 2.01(2) 2.59(2)

157(2) 137(2) 169

O(1)-H(1)‚‚‚O(2) O(1′)-H(1′)‚‚‚O(2′)

A-A B-A B-B intramolecular A B

2.652(6) 2.629(6)

0.88(2) 1.03(2)

2.28(2) 2.00(2)

105(2) 104(2)

H-bond


symmetry

O‚‚‚N

O-H

H‚‚‚N

angle O-H‚‚‚N (°)

O(1)-H(1)‚‚‚N(2) O(1′)-H(1′)‚‚‚N(2′)

A-A B-B intramolecular A B

x+1, 0.5-y, 0.5+z x+1, 0.5-y, 0.5+z

2.861(4) 2.860(4)

1.05(2) 0.89(2)

1.98(2) 2.03(2)

140(2) 154(2)

2.678(4) 2.624(4)

1.05(2) 0.89(2)

2.16(2) 2.19(2)

108(2) 109(2)

Structure IIb

O(1)-H(1)‚‚‚O(2) O(1′)-H(1′)‚‚‚O(2′)

independent molecules forming chains along axis a. The chain found in structure POKHAY10 formed by (S)(-)-proline and 4-(2,4,6-tri-isopropylbenzoil)benzoic acid is shown in Figure 6. This and similar chains (onedimensional hydrogen-bonded associates) can be considered to be synthons which are potentially useful for crystal engineering and for crystal structure prediction and modeling.23 We believe that it will be possible to find other examples of cocrystals in which chromophore molecules are attached to the chains of proline molecules via hydrogen bonds, and these chains will be packed in an acentric manner. They might give us analogues of

Figure 4. Projection of structure of cocrystal I‚III along axis b. Relative positions of hydrogen bonded chains formed by molecules I and III are presented.

polymer structures with NLO pendants, and the favorable characteristic of such structures, when compared with unpoled polymers, will be their acentric packing mode. Molecular Structure of the Conformers IIa and IIb and Crystal Structure of Polymorphs IIa and IIb. Since we were able to obtain cocrystals of compound I with one of the chiral reagents, we presumed that we would observe cocrystallization with hydrogen bond formation using chiral reagents for the similar molecules of II. However, X-ray analysis revealed that we have not obtained cocrystals but rather two polymorphs (IIa and IIb) of compound II. An interesting peculiarity of these polymorphs is that both crystals consist of two systems of symmetrically independent molecules (A and B). The other peculiarity of these polymorphs is that they are built of the different conformers (Figure 2). So in this case, we can describe polymorphism of compound II as induced conformational polymorphism caused most probably by the presence of chiral reagents in solution. The molecular geometry of the conformers studied (Figure 2) is quite common for this class of compounds (see ref 7). Inside every polymorph, molecules A and B, which are not related by symmetry operations, are characterized by almost identical geometry parameters. Molecular geometry of the two conformers is also in a good agreement with the results of DFT quantum calculations (Table 3). The only significant difference between conformers is the relative orientation of the dicyanovinyl group around the C(1)-C(7) bond and corresponding differences in bond angles at the C(7) atom (Figure 2, Table 3). According to the X-ray data both conformers are almost planar; mean-square deviations of non-hydrogen atoms from the molecular plane in structure IIa are equal to 0.091 (A) and 0.078 Å (B),

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Figure 5. Relative orientation of molecular chains formed by symmetrically independent proline molecules A (a) and B (b) in structure of cocrystal I‚III. Projection along axis c demonstrates that these chains are parallel.

and in the structure IIb are equal to 0.073 (A) and 0.049 Å (B). According to quantum calculations, both conformers IIa and IIb are also planar. The planar conformation of these molecules is stabilized by intramolecular hydrogen bonds O(1)-H(1)‚‚‚O(2) between hydroxy and methoxy groups (see Figure 2 and Table 4). According to our quantum calculations, the total energy for the conformer IIa is slightly lower than for conformer IIb. The energy difference is equal to 1.63 kcal/mol on the RHF/6-31G** level of theory, and to 1.70 kcal/mol on the B3LYP/6-31G** level or 1.62 and 1.55 kcal/mol, respectively, if we consider zero point energy (ZPE) correction. This result definitely shows that conformer IIa is more thermodynamically stable than conformer IIb. On the other hand, kinetic factors during chemical reaction can significantly shift the balance between the reaction products. It is possible to conclude that kinetic factors play an important role in this case since two conformers of the same compound characterized by different energy have been detected experimentally. Although crystal structures IIa and IIb belong to the same space group (Table 1 gives two different settings of this group) with two molecules in the symmetrically independent part of the unit cell (Z ) 8), molecular packing modes in these crystals are quite different. Figure 7a,b clearly shows a difference between the two structures. If structure IIb can be described as a layered one, the packing mode in the structure IIa belongs to a more complicated type. The difference is most probably due to the different conformations of the molecules in

the two modifications, which leads to distinctions in the systems of hydrogen bonds and to distinctions in the other intermolecular contacts important for structure building. In crystal structure IIa hydrogen bonds of the O-H‚‚‚NtC type, that were found between A molecules, and between A and B molecules (Figure 6a, Table 4), are forming a 3D network. Molecules B are also involved in centrosymmetrical “dimers” connected by intermolecular contacts of the CtN‚‚‚H-C type (C(7′)H(7′a)‚‚‚N(2′), Table 4). The same type of contact was found in structure I and in similar structures that we studied previously. 4-7 Molecules in crystal IIb form layers built of symmetrically independent molecules A or B (Figure 7b). These layers are normal to the [102h ] direction. Layered type structures with molecular planes coplanar within molecular layers (sheets) are quite common for related crystalline cyanovinyl compounds. 4-6,9 In some cases, molecular packings inside such layers are almost identical in different polymorphs, but the sequence of their superposition is different. This phenomenon was described in our recent work9 and characterized as organic polytypism. In structure IIb, molecules inside both layers (A and B) are connected with very similar hydrogen bonds of the O-H‚‚‚NtC type (Figure 8a,b, Table 4). The topology of the layers formed by molecules A and B is the same (Figure 8a,b), and the corresponding



Figure 7. Crystal structures of polymorphs IIa (a) and IIb (b).

Figure 6. Chain of hydrogen-bonded proline molecules A and B with attached molecules of 4-(2,4,6-tri-isopropylbenzoil)benzoic acid found in cocrystal POKHAY1022.

intermolecular hydrogen bonds with cyano groups in A and B layers are almost identical (Table 4). It is interesting to mention that crystal structure IIb contains pseudo glide plane a. The coordinate indexes of pseudo glide plane a [-0.057 0.986 -0.155] are close to crystallographic glide plane c with indexes [0 1 0]. The shift along the a axis is equal to 4.475 Å which is close to 1/2 of parameter a (4.599 Å). The hypothetical superposition of both pseudo and crystallographic glide plains would lead to the transformation of the latter into glide plane n and appearance of a pseudo group of the second type described recently.24 This means that the pseudo group possesses the same class of symmetry, but half-fold cell volume with decreased parameter c: [P21/ c, Z′ ) ) 2] w [P21/n, Z′ ) 1 + c/2]. Actually, we can see in this structure a situation similar to what was found in organic polytypes.9 In polytypes, an “error” in

layer superposition leads to several polymorphs; in the case of structure IIb a small “error” in superposition of two almost identical layers leads to the existence of two symmetrically independent layers and to doubling of one of the cell parameters. Calculations of crystal packing energy (Cerius2) gave us -20.26 kcal mol-1 for polymorph IIa and -20.64 kcal mol-1 for polymorph IIb. If we summarize the energy difference between conformers obtained from quantum DFT calculations with ZPE correction and crystalline energy, the difference between total energies of the crystalline polymorphs will be even more pronounced, and is in fact reversed (-21.81 and -20.64 kcal mol-1 for IIa and IIb, respectively). This result gives us reason to suppose that crystallization of polymorph IIa under equal conditions will be preferable, mostly because of the lower conformational energy of conformer IIa. On the other hand, according to our data, the volumes of the unit cells of both modifications measured at room temperature are very close 2043.2(7) IIa and 2041.6(7) Å3 IIb (difference ∼0.08%), which gives no grounds to speculate on their relative stability.

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Figure 8. Fragments of molecular sheets in structure IIb formed by symmetrically independent molecules A (a) and B (b).

Measuring the melting points of the compounds, we found that polymorph IIa melts at a lower temperature (∼142 °C) than polymorph IIb (∼146 °C). It is remarkable however that after cooling and solidification both samples melt a second time at the same temperature ∼142 °C that corresponds to polymorph IIa. This gives us reason to suggest that in a melt molecules of II adopt low energy conformation IIa, and crystallize in the form of the corresponding conformational polymorph. We should mention that according to our quantum calculations (HF/6-31G**) the barrier to interconversion of conformers IIa and IIb is not too high and is equal to 6.1 kcal/mol. In fact, since we are dealing in our case with conformers (conformations corresponding to two energy minima), standard criteria for the most stable polymorphs, built of molecules with identical conformations, will not be completely applicable. To support any particular mechanism of induced conformational polymorphism we need to perform modeling of corresponding molecular complexes in solution, and we plan to do so in the nearest future. At present, we believe that a mixture of a solvent with any additional low molecular compound can behave as a different solvent and can lead to obtaining of different polymorph modifications.

Acknowledgment. We gratefully acknowledge financial support of this research by NASA grant NAG81708, and NASA cooperative agreement NCC8-195-A. We are also grateful to Dr. L. N. Kuleshova for help with the description of pseudosymmetry in crystal IIb and to Dr. M. Minton for manuscript preparation. Supporting Information Available: X-ray crystallographic information files (CIF) for crystals I, I‚III, IIa, and IIb. These materials are available free of charge via Internet http://pubs.acs.org.

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Proline and Induced Conformational Polymorphism - American

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