Bridge Orientation as a Selector of Intermolecular Interactions in a

Nov 8, 2008 - Series of Crystalline Isomeric Benzylideneanilines. William H. Ojala,* Kendra M. Lystad, Tera L. Deal, Jessica E. Engebretson, Jill M. S...
0 downloads 0 Views 1MB Size
Download PDF
Bridge Orientation as a Selector of Intermolecular Interactions in a Series of Crystalline Isomeric Benzylideneanilines William H. Ojala,* Kendra M. Lystad, Tera L. Deal, Jessica E. Engebretson, Jill M. Spude, Barjeta Balidemaj, and Charles R. Ojala

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 964–970

Department of Chemistry, UniVersity of St. Thomas, St. Paul, Minnesota 55105-1079, and Department of Chemistry, Normandale Community College, Bloomington, Minnesota 55431 ReceiVed July 10, 2008; ReVised Manuscript ReceiVed NoVember 8, 2008

ABSTRACT: Comparison of the crystal structures of compounds we have designated “bridge-flipped isomers,” which on the molecular level differ only in the orientation of a bridge of atoms connecting two larger parts of the molecule, offers a useful context for the examination and evaluation of intermolecular interactions and their robustness as supramolecular synthons. Intermolecular interactions in the crystal structures of five pairs of bridge-flipped isomeric benzylideneanilines are examined here, including interactions between nitrile groups and ring hydrogen atoms, between halogen atoms, and between nitrile groups and halogen atoms. Of these, only the halogen-nitrile interaction in which the halogen is iodine is present in both bridge-flipped isomers of the pair, although its influence on the molecular packing arrangement is insufficient to compel the two isomers to assume identical packing arrangements. Pairwise analysis of crystal structures of bridge-flipped isomers thus provides a perspective beyond that provided by the isolated analysis of the structures of the individual compounds. Introduction To design and prepare molecular crystals having desired and predictable structures requires an understanding of the intermolecular interactions that link molecules together in the solid state. Generally it is through observing how these interactions are manifested in multiple known crystal structures that sufficient insight is gained to allow at least some predictability at the design stage of how they may be manifested in, and used to prepare, new molecular solids. These interactions range from the relatively strong (classical hydrogen bonds) to the relatively weak (weak hydrogen bonds, van der Waals interactions, Lewis acid-Lewis base interactions). Certain of these, demonstrated to be robust by virtue of their occurrence in a variety of crystal structures, have become recognized in recent years as supramolecular synthons. Sufficient knowledge of and experience with both strong and weak intermolecular interactions have made possible the preparation of a variety of useful solid materials by design in recent years through the application of the supramolecular synthon concept. We are conducting a study of intermolecular interactions in crystals of isomeric organic compounds we have designated “bridge-flipped isomers.”1 Molecules of these compounds differ from each other only in the orientation of a chain or bridge of atoms connecting two major parts of the molecule. Pairs of bridge-flipped isomers are found among the phenylhydrazones (Ar-NH-N)CH-Ar′ vs Ar-CH)N-NH-Ar′ where Ar ) aryl) and the benzylideneanilines (Ar-CH)N-Ar′ vs ArN)CH-Ar′ where Ar ) aryl). Examples related by dual iminogroup reversals are found among the azines and the Schiff-base derivatives of glyoxal (Ar-CH)N-N)CH-Ar′ vs ArN)CH-CH)N-Ar′ where Ar ) aryl). We are interested in identifying pairs of bridge-flipped isomers in which the different isomers assume the same molecular packing arrangement (producing isostructural crystals) because their anticipated mutual solid-state solubility might make them especially suitable for the preparation of new materials through the formation of solid solutions. Thus far our investigation has identified some * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (651)-962-5585. Fax: (651)-962-5209.

isostructural pairs, but it has also demonstrated that in general isostructuralism between bridge-flipped isomers is rare. Differences between the isomers with respect to their molecular conformations and solid-state intermolecular interactions tend to differentiate their solid-state molecular packing arrangements. On the other hand, we note that certain solid-state intermolecular interactions such as hydrogen bonding operating in both isomers of a bridge-flipped pair might link molecules of the different isomers into similar packing motifs in their respective crystals. Although this by itself would not guarantee isostructuralism, as similar motifs in different isomers can be packed in different ways, it might increase the likelihood that isostructural crystals of the two different isomers could be obtained (e.g., by seeding experiments). Among these interactions, the intermolecular Lewis acidLewis base contact that can occur between halogen atoms and nitrile nitrogen atoms in the solid state has been a major focus of our work. An extensive literature exists concerning close halogen-nitrogen contacts in which the nitrogen atom has been part of a variety of functional groups,2-4 and the interaction between halogen atoms and nitrile nitrogen atoms in particular has been closely examined by previous investigators of a variety of relevant crystal structures.5-7 For our part, we previously determined and described the molecular and crystal structures of the 4-X-4′-C≡N-substituted benzylideneanilines in which X ) Cl, Br, and I.8 In accord with the increasing Lewis acidity (polarizability) of these halogens in the order Cl < Br < I, we found that contacts sufficiently robust to organize the molecules in this series in an ordered packing arrangement (as opposed to an arrangement with end-for-end molecular disorder, a common occurrence among the benzylideneanilines) occur only for the two iodo-substituted compounds. Moreover, although the close C≡N · · · I contacts found in each of those structures organize the molecules of the two different isomers into similar chains, the three-dimensional packing of the chains is different in the two isomers; the isomers are not isostructural. Although this previous study failed to demonstrate that halogen-nitrile interactions can compel different bridge-flipped isomers to assume identical solid-state packing arrangements, it did provide a unique context for evaluating the role of

10.1021/cg8007443 CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008

Crystalline Isomeric Benzylideneanilines Scheme 1. Isomeric “Bridge-Flipped” Benzylideneanilines

halogen-nitrile interactions in molecular packing, allowing a comparison of C≡N · · · X interactions (in this case, where X ) Cl, Br, or I) in molecules structurally related in a specific way. More generally, we propose here that comparing the crystal structures of bridge-flipped isomers offers a useful evaluation of the robustness of particular intermolecular interactions and of their significance as supramolecular synthons by virtue of whether or not, or under what conditions, these interactions are of sufficient strength and directionality either to compel two bridge-flipped isomers to assume identical packing arrangements or to compel them to assume different ones. In accord with using bridge-flipped isomers as an especially useful context for comparing solid-state intermolecular interactions, we present here the packing arrangements we have found to be assumed by five pairs of bridge-flipped isomeric benzylideneanilines. These include four pairs of halogen/nitrile substituted benzylideneanilines, all bearing the 2-C≡N-4′-X substitution pattern, where X ) F, Cl, Br, and I (Compounds 1a-4a and 1b-4b, Scheme 1). For comparison to the halogenated structures, we also discuss here the structures of the two corresponding 2-C≡N-4′-CH3 isomers (Compounds 5a and 5b, Scheme 1). Although several examples of isostructuralism occur among these 10 compounds, all occur between compounds in which one halogen replaces a different halogen on a given benzylideneaniline framework; no isostructural pairs of bridge-flipped isomers occur among the five pairs we describe in this paper. With respect to the halogen · · · nitrile interaction, halogen atoms are found to be in intermolecular solid-state contact with nitrile nitrogen atoms in only one pair of isomers in this group, the iodo pair 4a and 4b. Although this interaction occurs in both iodo-nitrile isomers, it fails to compel these isomers to form isostructural crystals. Of direct relevance to the supramolecular synthon concept is our finding that the differences in packing arrangement between isomers in all five pairs can be related to the occurrence of different supramolecular synthons in the solid state depending on the orientation of the -CH)N- linkage. In this respect, this molecular bridge can be viewed as a solidstate switch; its position determines which synthons are turned on and which are turned off. Experimental Section All 10 compounds were prepared by the standard method, condensation of a benzaldehyde derivative with an aniline derivative by heating in ethanol solution. Melting points were determined on a Fisher-Johns melting point apparatus and are uncorrected. Crystals suitable for singlecrystal X-ray diffraction studies were obtained either directly from the cooled reaction mixture or by slow evaporation of an ethanol solution. Compound 1a (X ) F) was obtained as yellow needles, mp 120-122 °C; 1b (X ) F) as colorless plates, mp 104-108 °C; 2a (X ) Cl) as yellow needles, mp 106-108 °C; 2b (X ) Cl) as yellow plates, mp 116-118 °C; 3a (X ) Br) as yellow needles, mp 112-115 °C; 3b (X ) Br) as yellow needles, mp 96-98 °C; 4a (X ) I) as yellow needles, mp 148-151 °C; 4b (X ) I) as colorless plates, mp 93-96 °C; 5a (X ) CH3) as colorless needles, mp 95-97 °C; and 5b (X ) CH3) as

Crystal Growth & Design, Vol. 9, No. 2, 2009 965 colorless needles, mp 72-76 °C. Data collection was performed on a Bruker (for structures 1a, 4a, and 3b) or Siemens (for structures 2a, 3a, 5a, 1b, 2b, 4b, and 5b) platform CCD diffractometer using Mo KR radiation (λ ) 0.71073 Å) at 173 K using the SMART9 software package. Data reduction was accomplished using SAINT-Plus;10 absorption corrections were applied using SADABS.11,12 Structure solution was accomplished using SHELXS13 from the SHELXTL14 software package except for structures 3a, 4a, and 5b, for which SIR9715 was used. Refinement was performed using SHELXL-97.16 Hydrogen atoms were placed in calculated positions and refined as riding [C-H ) 0.95 Å and Uiso(H) ) 1.2Ueq(C) for aryl and bridge H atoms; C-H ) 0.98 Å and Uiso(H) ) 1.5Ueq(C) for methyl H atoms]. The presence of residual electron density between the methyl hydrogen atoms of 5a and 5b resulted in the use of a disordered model for all of the methyl groups in both structures; for each methyl group two orientations related by a 60° rotation about the C-C bond were included in the model, with each hydrogen atom assigned half-occupancy. PLATON17 was used for structure verification and analysis. MERCURY18 was used for preparation of packing diagrams. Crystal data, data collection parameters, and refinement parameters for all 10 structures are presented in Table 1. No unusual geometric features or anomalous crystallographic features were observed among the 10 structures with the exception of the disorder in the positions of the methyl hydrogens in 5a and 5b, which was accounted for in the final structural model.

Results and Discussion A summary of crystallographic parameters and additional details of the structure determination and refinement are given in Table 1. The overall conformations found for the 10 compounds are essentially those shown in Scheme 1; in the cyanobenzylidene series 1a-5a the C≡N bond points in the same general direction as the bridge C-H bond, while in the cyanoaniline series 1b-5b the C≡N bond points in the direction opposite that of the bridge C-H bond. As noted previously, we found that none of the bridge-flipped isomeric pairs (1a vs 1b, 2a vs 2b, etc.) assumed identical packing arrangements when we crystallized them, although it is important to note that we did not conduct crystallization studies extensive enough to rule out the existence of additional polymorphs that might have led to isostructural bridge-flipped pairs for any of these 10 compounds. Within the cyanobenzylidene series 1a through 5a we obtained four different packing arrangements: one assumed by 1a (X ) F), one assumed by both 2a (X ) Cl) and 3a (X ) Br), one assumed by 4a (X ) I), and one assumed by 5a (X ) CH3). Only one of these packing arrangements, the one assumed by 5a, had more than one molecule in the asymmetric unit (Z′ ) 4 for 5a). Within the cyanoaniline series 1b through 5b we obtained three packing arrangements: one assumed by the 1b (X ) F)-2b (X ) Cl)-3b (X ) Br) series, one assumed by 4b (X ) I), and one assumed by 5b (X ) CH3), all of which had more than one molecule in the asymmetric unit (Z′ ) 2). Only one of these 10 compounds (5b) assumed a non-centrosymmetric packing arrangement. Although attempts to explain the observed solid-state packing arrangements of molecules simply on the basis of functional group interactions must be considered highly speculative at best, the presence of particular functional groups on these 10 molecules does suggest that certain intermolecular interactions reasonably could be expected to play a significant role in determining the three-dimensional packing pattern. Some possibilities for compounds 1a-5a and 1b-5b include halogen · · · halogen interactions, halogen · · · nitrogen interactions (involving either the bridge N or the nitrile N, or both), π-π stacking interactions, and dipolar nitrile · · · nitrile interactions. In particular, the occurrence of centrosymmetric dipolar interactions between nitrile groups in crystal structures and their influence

966 Crystal Growth & Design, Vol. 9, No. 2, 2009

Ojala et al.

Table 1. Crystallographic Data for Compounds 1a-5a and 1b-5b compound

1a

2a

3a

4a

5a

formula mw crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalc (g cm-3) Rint µ (mm-1) R1[I > 2σ(I)], wR2 S refl. meas., ind., obs. parameters

C14H9FN2 224.23 monoclinic P21/n 3.9182(8) 11.901(2) 23.687(5) 90 92.089(4) 90 1103.9(4) 4 1.349 0.048 0.09 0.048, 0.121 1.03 6927, 2510, 1541 154

C14H9ClN2 240.68 monoclinic P21/n 7.057(3) 3.8807(15) 42.011(15) 90 90.606(6) 90 1150.5(7) 4 1.390 0.031 0.31 0.041, 0.093 1.07 7854, 2518, 1963 154

C14H9BrN2 285.14 monoclinic P21/n 7.1031(7) 3.9075(4) 42.454(4) 90 90.857(2) 90 1178.2(2) 4 1.607 0.036 3.47 0.038, 0.077 1.19 12624, 2682, 2285 154

C14H9IN2 332.13 triclinic P1j 4.1333(4) 11.5489(12) 13.6092(14) 70.583(1) 88.690(2) 83.109(2) 608.16(11) 2 1.814 0.022 2.61 0.019, 0.049 1.06 7134, 2740, 2621 154

C15H12N2 220.27 triclinic P1j 7.0689(11) 15.980(3) 22.180(4) 75.792(3) 83.549(3) 88.317(3) 2413.4(7) 8 1.212 0.034 0.07 0.051, 0.129 1.03 25575, 9763, 6155 613

compound

1b

2b

3b

4b

5b

formula mw crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalc (g cm-3) Rint µ (mm-1) R1[I > 2σ(I)], wR2 S refl. meas., ind., obs. parameters

C14H9FN2 224.23 monoclinic P21/c 12.7404(10) 7.1801(5) 24.0894(18) 90 90.724(1) 90 2203.5(3) 8 1.352 0.027 0.09 0.037, 0.100 1.05 25041, 5056, 4187 307

C14H9ClN2 240.68 monoclinic P21/c 13.4815(11) 7.3059(6) 23.461(2) 90 90.224(1) 90 2310.8(3) 8 1.384 0.037 0.31 0.047, 0.099 1.08 25955, 5298, 4198 307

C14H9BrN2 285.14 monoclinic P21/c 13.7628(13) 7.4094(7) 23.245(2) 90 90.104(2) 90 2370.4(4) 8 1.598 0.039 3.44 0.027, 0.070 1.02 26816, 5406, 4499 307

C14H9IN2 332.13 monoclinic P21/c 9.9873(7) 14.2680(10) 18.0622(12) 90 94.243(1) 90 2566.8(3) 8 1.719 0.035 2.47 0.025, 0.051 1.02 29633, 5872, 4808 307

C15H12N2 220.27 monoclinic P21 11.5779(12) 7.3556(7) 13.7938(14) 90 91.074(2) 90 1174.5(2) 4 1.246 0.043 0.08 0.042, 0.096 1.05 13909, 2900, 2406 308

Scheme 2. Centrosymmetric R22(10) Nitrile N: Ring H Synthon-“Motif 1”

on molecular packing has been the focus of recent investigation.19 For nitrile groups engaged in this kind of interaction that are located on aromatic rings, the presence of a nearby ring hydrogen atom makes possible a centrosymmetric R22(10) interaction between nitrile nitrogen atoms and ring hydrogen atoms that links molecules into dimers (Scheme 2), a longrecognized supramolecular synthon20 that we refer to herein as “Motif 1.” At the same time, as we examine these 10 crystal structures for these and other intermolecular contacts that might be significant in determining the isostructuralism or nonisostructuralism of isomeric pairs, we are aware of the influence of the molecular conformation. A conformational feature specific to benzylideneanilines and potentially structure-differentiating between bridge-flipped isomeric ones is the steric interaction between the bridge hydrogen atom and an aniline ring hydrogen atom ortho to the bridge that can force the aniline ring farther out of coplanarity with the bridge atoms than the benzylidene ring (Scheme 3). Pairs of bridge-flipped isomeric benzylideneanilines exhibiting this difference in conformation, especially if it is severe, are not likely to assume isostructural packing arrangements. On the other hand, several benzylideneaniline

Scheme 3. Difference in Conformation Between Bridge-Flipped Isomeric Benzylideneanilines Due to H-H Steric Interaction

crystal structures in which the molecule is nearly planar have been reported in the literature,21-24 which suggests that this conformational difference between bridge-flipped isomers need not exist between every such pair of benzylideneanilines generally. It does exist between all five of ours. Molecules of all 10 compounds deviate significantly from planarity, and the members of each pair differ significantly in conformation (Table 2). The angle between the plane of the six-carbon benzylidene ring and the plane defined by the C-CdN-C bridge atoms is generally smaller than 10 degrees (regardless of whether or not the benzylidene ring is the one substituted in the ortho position), while the six-carbon aniline rings are twisted out of the plane of the bridge atoms by 30-40°, aniline rings substituted with the ortho-nitrile group showing the greatest displacements. Given a difference in conformation this severe between the bridge-flipped isomeric pairs in our series, it is on the one hand not surprising that they do not assume isostructural packing arrangements, and this conformational difference might appear

Crystalline Isomeric Benzylideneanilines

Crystal Growth & Design, Vol. 9, No. 2, 2009 967

Table 2. Torsional Angles (deg) between Six-Membered Rings and Bridgea benzylidene ring

aniline ring

Cyanobenzylidenes 1a 2a 3a 4a 5a

(X (X (X (X (X

) ) ) ) )

F) Cl) Br) I) CH3)

8.06(21) 8.14(20) 6.93(31) 3.15(16) 16.34(09) 7.84(11) 6.57(11) 2.76(05)

30.89(12) 33.44(11) 33.72(17) 34.38(12) 28.83(11) 32.85(09) 41.70(08) 28.96(09)

Cyanoanilines 1b (X ) F) 2b (X ) Cl) 3b (X ) Br) 4b (X ) I) 5b (X ) CH3)

3.66(02) 6.31(16) 4.72(04) 6.61(25) 4.76(03) 6.58(27) 4.99(24) 13.81(27) 5.04(06) 4.89(34)

40.38(08) 39.23(10) 39.62(12) 40.24(17) 38.33(13) 40.87(18) 46.02(17) 41.88(18) 37.18(17) 40.14(18)

a Defined as the C)N group and the ring carbon atoms to which it is attached.

to render superfluous any discussion of intermolecular interactions as potentially structure-differentiating. On the other hand, it is not clear to us whether conformation determines packing or packing determines conformation for these compounds. We have not performed computational studies that would allow us to make that judgment. The flexibility in conformation that benzylideneanilines exhibit across a variety of crystal structures25 suggests that the conformation can be strongly affected by the packing arrangement, especially by certain specific intermolecular contacts,21 so analysis and comparison of these structures in terms of intermolecular interactions appear reasonable to us. A comparison of molecular packing motifs found in the fluoro-substituted isomeric pair 1a and 1b (X ) F in both structures) is shown in Figures 1 and 2. (Dashed lines in all figures indicate contact distances equal to or less than the sum of the van der Waals radii.) In 1a is found Motif 1, the R22(10) nitrile N: ring H synthon also shown in Scheme 2. In 1a it is strictly centrosymmetric, the interacting molecules being related by a crystallographic inversion center. The nitrile nitrogen atom participates in two intermolecular interactions shorter than the sum of the van der Waals radii; one is part of this centrosymmetric interaction, and the other is a contact to a ring hydrogen atom ortho to the fluorine atom. In fact, in all 10 structures the nitrile group makes not a single, linear -C≡N · · · H-C or -C≡N · · · X-C close intermolecular contact but at least two intermolecular contacts that are nonlinear in the -C≡N · · · H or -C≡N · · · X moiety. Close molecular stacking in 1a (X ) F) is reflected in one short unit cell axis (∼4 Å or shorter; 3.9182(8) Å in 1a), a feature also found in the chloro, bromo, and iodo analogues 2a, 3a, and 4a but not in any of the cyanoanilines 1b-5b. In contrast, the Motif 1 synthon is absent from the packing arrangement of 1b (X ) F) and from the chloro and bromo analogues 2b and 3b, with which 1b is isostructural. Contacts between nitrile groups and ring hydrogen atoms found in 1b-3b are not of the centrosymmetric type shown in Scheme 2 but are of the non-centrosymmetric type shown in Scheme 4, an R22(16) pattern here designated “Motif 2,” in which a ring hydrogen atom meta to the nitrile group participates in the interaction rather than a ring hydrogen atom ortho to the nitrile group (as in Scheme 2). This interaction

Figure 1. Molecular packing in 1a (X ) F). Selected close intermolecular approaches are indicated by dashed lines. Motif 1 is visible halfway along the c-axis (right-hand side of unit cell-compare Scheme 2).

Figure 2. Molecular packing in 1b (X ) F), 2b (X ) Cl), and 3b (X ) Br) (shown for 1b). Selected close intermolecular approaches are indicated by dashed lines. Motif 2 is shown (compare Scheme 4).

links together the two symmetry-independent molecules in the asymmetric unit of 1b, and it is maintained in both 2b and 3b. It is not a common motif among crystal structures; a search of the Cambridge Structural Database26 (Version 5.29 through the January 2008 update) for this motif (with contact distances equal to or less than the sum of the van der Waals radii of 1.2 Å for H and 1.55 Å for N) produced no similar examples involving aromatic rings, although examples could be found in which the functional groups were linked by more flexible hydrocarbon chains. It is not obvious to us why a simple reversal of the bridge orientation should cause such a fundamental difference in packing preference and solid-state synthon type as is shown between 1a on the one hand and 1b-3b on the other, although

968 Crystal Growth & Design, Vol. 9, No. 2, 2009

Ojala et al.

Scheme 4. Non-Centrosymmetric R22(16) Nitrile N: Ring H Synthon-“Motif 2”

Scheme 5. Centrosymmetric R22(10) Nitrile N: Ring H Synthon-“Motif 3”

bridge reversal can be expected to change the charge distribution throughout the molecule and thereby the favored intermolecular contacts as well. Although there is a close approach between the bridge hydrogen atom and a nitrile group in 1b, this contact is not maintained in any of the remaining structures in either series; thus, neither it nor any other intermolecular interaction directly involving the bridge adequately explains the reluctance of the cyanobenzylidene series 1a-5a to assume the same packing arrangement as their bridge-flipped isomers in the cyanoaniline series 1b-5b. In going from the fluoro-substituted isomeric pair 1a:1b to the chloro- and bromo-subtituted isomeric pairs 2a:2b and 3a: 3b, the increased size of the halogen atoms leads to new kinds of intermolecular contacts in the cyanobenzylidene isomers 2a and 3a but leaves the cyanoaniline isomers 2b and 3b essentially unchanged. The molecular packing observed in 2a (X ) Cl) is shown in Figure 3; compound 3a (X ) Br) is isostructural with 2a. As in 1a (X ) F), the Motif 1 synthon is present in 2a and 3a, and as in 1a it is strictly centrosymmetric, the interacting molecules being related by crystallographic inversion centers. Whereas in 1a the closest intermolecular contact involving the fluorine atom is to a ring hydrogen atom, in 2a and 3a this halogen-hydrogen interaction is replaced by a halogen-halogen

interaction (Cl · · · Cl in 2a, Br · · · Br in 3a). This may reflect increased significance of the X · · · X interaction for the larger and more polarizable halogens in these structures, although whether these are genuinely attractive interactions, particularly for chlorine, has been questioned in the past.27 (In the bridgeflipped isomers 1b-3b, halogen-halogen contacts are not found.) The ring hydrogen atom in intermolecular contact with the fluorine atom in 1a (the ring H meta to the nitrile group) is in contact in 2a (X ) Cl) and 3a (X ) Br) not with a halogen atom but with the nitrile group of a neighboring molecule, defining a new R22(10) motif, Motif 3 (Scheme 5). This motif occurs about inversion centers in 2a and 3a and links molecules into ribbons (Figure 3). Unlike Motif 2, Motif 3 occurs often in crystal structures. A search through the Cambridge Structural Database (Version 5.29 through the January 2008 update) for the fragment shown in Scheme 5 (with contact distances equal to or less than the sum of the van der Waals radii of 1.2 Å for H and 1.55 Å for N) produced 34 hits, including acetonitrile solvates. It is thus not surprising that it occurs among at least some of our structures, but it is rather surprising that it occurs among certain of our cyanobenzylidenes but not at all among our cyanoanilines. In both the cyanobenzylidene series and the cyanoaniline series, a unique molecular packing arrangement occurs when the halogen atom is iodine. Although the overall molecular packing arrangements of cyanobenzylidene 4a and cyanonaniline 4b differ from each other, halogen-nitrile contacts, which are absent from 1a-3a and 1b-3b, are present in both 4a (X ) I) and 4b (X ) I). Views of the packing are shown in Figure 4 for 4a and Figure 5 for 4b; relevant distances and angles are given in Table 3. The occurrence of halogen-nitrile contacts in the packing of the iodinated compounds 4a and 4b and their absence from the packing of the fluorinated, chlorinated, and brominated analogues are consistent with the greater Lewis acidity (polarizability) of the iodine atom compared to that of these other halogens. Although iodine-nitrile contacts link molecules into dimeric pairs in both 4a and 4b, the interactions are not identical, and 4a and 4b are not isostructural. The interaction in 4a is strictly centrosymmetric, the molecules in the pair being related to each other by a crystallographic inversion center. The interaction in 4b is non-centrosymmetric; the interaction is between the two independent molecules of the asymmetric unit. In both cases the nitrogen-iodine distances (Table 3) are within the range previously described as sufficiently short to be significant in organizing molecules into structure-defining motifs, although the distances in 4a are toward the longer end of that range. The interactions are not entirely linear but instead are bent toward a trigonal geometry at the nitrogen atom while tending toward linearity at the iodine atom, a geometry observed also in previously determined iodine-nitrile structures. In 4a the Motif 1 synthon links the dimers into ribbons of molecules, but in 4b the contact that links dimers into ribbons is a two-pronged one, a nitrile-hydrogen contact and an interaction

Figure 3. Molecular packing in 2a (X ) Cl) and 3a (X ) Br). Motifs 1 and 3 are shown (compare Schemes 2 and 5), as are halogen-halogen contacts.

Crystalline Isomeric Benzylideneanilines

Crystal Growth & Design, Vol. 9, No. 2, 2009 969

Figure 4. Molecular packing in 4a (X ) I). Motif 1 and centrosymmetric iodine-nitrile interactions define the packing arrangement.

Figure 5. Molecular packing in 4b (X ) I). In contrast to 4a (X ) I), in 4b the iodine-nitrile contacts are non-centrosymmetric. Table 3. Distances (Å) and Angles (deg) in Intermolecular Iodine-Nitrile Contacts in 4a and 4ba 4a N· · ·I C≡N · · · I N · · · I-C a

3.486(2) 129.45(17) 165.91(7)

4b 3.163(3),3.219(2) 125.9(2),127.16(19) 165.54(8),170.28(8)

Contact radii: N)1.55 Å; I ) 1.98 Å.28

between a ring hydrogen atom and the benzylidene pi cloud of a neighboring molecule. As noted previously, we have not examined

Figure 6. Molecular packing in 5a (X ) CH3). One orientation of the disordered methyl group is shown. Molecules within each layer are interlinked by Motifs 1 and 3 as are molecules in 2a (X ) Cl) and 3a (X ) Br) (compare Figure 3).

by computation the role of charge distribution within the molecule as a potential influence on the types of intermolecular interactions preferred by a given benzylideneaniline. Reversal of the bridge orientation would be expected to directly influence the charge distribution, perhaps changing a preference for a C-H · · · N≡C interaction to a preference for a C-H · · · pi interaction. Pairs of bridge-flipped isomers such as the benzylideneanilines described here may be of particular interest to workers in the area of the computational chemistry of the organic solid state. If the influence of the halogen atom on these packing arrangements were limited to space-filling considerations, replacing the chlorine or bromine atom by the roughly similar in size methyl group might well leave the molecular packing unchanged. Obtaining a new packing arrangement would suggest an electronic role, as well as a steric role, for the halogen. We find that 5a (X ) CH3) and 5b (X ) CH3) assume unique packing arrangements (indicating that the effect of the halogens is not solely space-filling) that nonetheless share some of the motifs found in other structures discussed previously. A view of the packing arrangement assumed by 5a is given in Figure 6. This complicated structure (four molecules in the asymmetric unit) can be viewed simply as a superposition of molecular layers within which the molecules are interlinked by the same synthons, Motif 1 and Motif 3, that interlink the molecules of 2a (X ) Cl) and 3a (X ) Br), with the difference that in 5a the Motif 1 interaction is not located about a crystallographic inversion center and is thus not strictly centrosymmetric. As a member of the cyanoaniline series, 5b might be expected to show Motif 2; this motif does reappear (Figure 7), but not completely. The replacement of the halogen atom by the methyl group is accompanied by the loss of the contact between the hydrogen atom ortho to that position and the nitrile group of the neighboring molecule (the lower dashed line in Scheme 4). Conclusion The analysis of the molecular packing motifs observed in these 10 structures is given a unique context by the pairwise comparison

970 Crystal Growth & Design, Vol. 9, No. 2, 2009

Ojala et al. 1a-5a and 1b-5b. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 7. Molecular packing in 5b (X ) CH3). One orientation of the disordered methyl group is shown. The major motif is a distorted or incomplete version of Motif 2.

of bridge-flipped isomers. We find that certain motifs, such as Motifs 1 and 3, are present in only the cyanobenzylidene structures, while Motif 2 is present in only the cyanoaniline structures. One motif in particular, the iodine-nitrile contact, is sufficiently robust that it is found in both bridge-flipped isomers, although its occurrence alone is insufficient to compel the two isomers to assume identical packing arrangements. Although few close intermolecular approaches involving the bridge atoms were found in these structures, the bridge orientation and the molecular charge distribution that results from it can be expected to influence significantly which motifs (supramolecular synthons) will be present in a given crystal structure; thus, the bridge orientation can be regarded as the switch that switches certain motifs on and switches others off. We are continuing to examine the crystal structures of other bridge-flipped isomeric pairs of organic molecules from this perspective. Acknowledgment. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. The authors express their thanks to Dr. Victor G. Young, Jr. (Director), Benjamin E. Kucera, and William W. Brennessel of the X-ray Crystallographic Laboratory of the Department of Chemistry, University of Minnesota, Minneapolis, MN, for their assistance. We are grateful to Dr. William B. Gleason of the Department of Laboratory Medicine and Pathology of the University of Minnesota, Minneapolis, MN and Dr. Doyle Britton of the Department of Chemistry of the University of Minnesota, Minneapolis, MN for helpful discussions. Supporting Information Available: Ellipsoid plots and X-ray crystallographic information files (CIF) are available for compounds

(1) (a) Ojala, W. H.; Smieja, J. M.; Spude, J. M.; Arola, T. M.; Kuspa, M. K.; Herrera, N.; Ojala, C. R. Acta Crystallogr., Sect. B: Struct. Sci. 2007, 63, 485–496. (b) Ojala, W. H.; Arola, T. M.; Herrera, N.; Balidemaj, B.; Ojala, C. R. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2007, 63, o207-o211. (2) (a) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386–395. (b) Crihfield, A.; Hartwell, J.; Phelps, D.; Walsh, R. B.; Harris, J. L.; Payne, J. F.; Pennington, W. T.; Hanks, T. W. Cryst. Growth Des. 2003, 3, 313–320. (c) Walsh, R. B.; Padgett, C. W.; Metrangolo, P.; Resnati, G.; Hanks, T. W.; Pennington, W. T. Cryst. Growth Des. 2001, 1, 165–175. (3) Cho, H. M.; Moore, J. S.; Wilson, S. R. Acta Crystallogr., Sect. E: Struct. Rep. Online 2005, 61, o3773-o3774. (4) Ahrens, B.; Jones, P. G. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1999, 55, 1308–1310. (5) Merz, K. Cryst. Growth Des. 2006, 6, 1615–1619. (6) (a) Britton, D. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, 60, o184-o186. (b) Britton, D. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 553–563. (c) Britton, D.; Gleason, W. B. Acta Crystallogr., Sect. E: Struct. Rep. Online 2002, 58, o1375-o1377. (d) Britton, D. Acta Crystallogr., Sect. E: Struct. Rep. Online 2001, 57, o702-o704. (7) (a) Reddy, D. S.; Ovchinnikov, Y. E.; Shishkin, O. V.; Struchkov, Y. T.; Desiraju, G. R. J. Am. Chem. Soc. 1996, 118, 4085–4089. (b) Desiraju, G. R.; Harlow, R. L. J. Am. Chem. Soc. 1989, 111, 6757– 6764. (c) Desiraju, G. R. In Crystal Engineering: The Design of Organic Solids; Elsevier, New York, 1989; Chapter 6, pp 197-198. (8) (a) Ojala, C. R.; Ojala, W. H.; Gleason, W. B.; Britton, D. J. Chem. Crystallogr. 2001, 31, 377–386. (b) Ojala, C. R.; Ojala, W. H.; Gleason, W. B.; Britton, D. J. Chem. Crystallogr. 1999, 29, 27–32. (9) SMART, Version 5.054; Bruker AXS Inc.: Madison, WI, 2001. (10) SAINT-Plus, Version 6.45; Bruker AXS Inc.: Madison, WI, 2003. (11) SADABS, Version 2.03; Bruker AXS Inc.: Madison, WI, 2000. (12) Blessing, R. H. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 33–38. (13) Sheldrick, G. M. SHELXS-97: Program for the Solution of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (14) SHELXTL, Version 6.10; Bruker AXS Inc.: Madison, WI, 2000. (15) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (16) Sheldrick, G. M.; Schneider, T. R. SHELXL-97: Program for the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (17) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13. (18) 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. (19) Wood, P. A.; Borwick, S. J.; Watkin, D. J.; Motherwell, W. D. S.; Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2008, 64, 393– 396. (20) Desiraju, G. R.; Sharma, C. V. K. Crystal Engineering and Molecular Recognition-Twin Facets of Supramolecular Chemistry. In The Crystal as a Supramolecular Entity: PerspectiVes in Supramolecular Chemistry; Desiraju, G. R., Ed.; John Wiley and Sons: Chichester, 1996; Vol. 2, p. 48. (21) Navon, O.; Bernstein, J. Struct. Chem. 1997, 8, 3–11. (22) Clegg, W.; Elsegood, M. R. J.; Heath, S. L.; Houlton, A.; Shipman, M. A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, 52, 2548–2552. (23) Ahmet, M. T.; Silver, J.; Houlton, A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, 50, 1814–1818. (24) Nakai, H.; Shiro, M.; Ezumi, K.; Sakata, S.; Kubota, T. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, 32, 1827– 1833. (25) Nakai, H.; Ezumi, K.; Shiro, M. Acta Crystallogr., Sect. B: Struct. Sci. 1981, 37, 193–197. (26) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380– 388. (27) Price, S. L.; Stone, A. J.; Lucas, J.; Rowland, R. S.; Thornley, A. E. J. Am. Chem. Soc. 1994, 116, 4910–4918. (28) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.

CG8007443