Stereoelectronic Effects of Substituent Groups in the Solid State

Aug 5, 2003 - School of Chemistry, University of Hyderabad, Hyderabad 500 046, India, ... effects of functional groups may be distinguished in the sol...
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CRYSTAL GROWTH & DESIGN

Stereoelectronic Effects of Substituent Groups in the Solid State. Crystal Chemistry of Some Cubanecarboxylic and Phenylpropiolic Acids

2003 VOL. 3, NO. 5 675-681

Dinabandhu Das,† Ram K. R. Jetti,‡ Roland Boese,*,‡ and Gautam R. Desiraju*,† School of Chemistry, University of Hyderabad, Hyderabad 500 046, India, and Institut fu¨ r Anorganische Chemie, Universita¨ t Duisburg-Essen, Standort Essen, Universita¨ tsstrasse 5-7, D-45177 Essen, Germany Received July 11, 2003

ABSTRACT: A series of 4-substituted cubanecarboxylic acids and phenylpropiolic acids have been studied with the aim of elucidating steric and electronic factors exerted by the 4-substituent in the formation of the dimer, or alternatively, the rare syn-anti catemer patterns in their respective crystal structures. It is shown that catemer formation depends critically on the ability of a proximal C-H group to form a supportive C-H‚‚‚O bond. In turn, this means that the C-H group must be sufficiently activated toward hydrogen bond formation. Such activation is inherent to the cubyl group but must be present additionally from a suitable electron withdrawing group in the phenylpropiolic acids. In any event, while C-H activation is necessary for catemer formation it is not sufficient. The substituent group that is present in the 4-position must also be sufficiently bulky so as to form a close packed array that is compatible with the catemer geometry. These trends are justified by the crystal structures of the 12 acids in the two families wherein the 4-substituent group is H, F, Cl, Br, I, and CH3. Our results indicate that electronic and steric effects of functional groups may be distinguished in the solid state, in that the formation of either a dimer or catemer may be rationalized on the basis of these effects. Introduction Complementary recognition between functional groups in molecules is the first step in crystallization. Because molecular recognition is complementary in nature,1 a functional group approach to crystal engineering is, at least currently, not possible,2 and prediction of crystal structures is correspondingly difficult.3 Current systematic synthetic strategies in purely organic systems attempt to tackle this issue by identifying systems wherein crystal construction may be taken up in a modular fashion and where interaction interference is minimal, so that the effect of any particular functionality is identifiable.4 Indeed, this is one of the wellaccepted goals of crystal engineering. Functional groups exert both steric and electronic effects on structure and reactivity, but even in solution, where the functional group approach is broadly applicable, separating out these two effects is difficult.5 In the solid state, this issue is more or less intractable because the effects of the functional groups are in themselves implicitly connected to the nature and positioning of the other functional groups in the molecule. In crystal engineering, steric and electronic effects are synonymous with geometrical and chemical effects on crystal packing.6 While the origin of all intermolecular interactions is electrostatic7 and there is no rigorous basis for such a distinction, it is still convenient to distinguish these components on the basis of distance dependence of the corresponding interactions.8,9 * Corresponding author. (G.R.D.) Fax: (+91) 40-2301-0567. Email: [email protected]. (R.B.) Fax: (+49) 201-183-2535. Email: [email protected]. † University of Hyderabad. ‡ Institut fu ¨ r Anorganische Chemie der Universita¨t DuisburgEssen.

Scheme 1

Such considerations have led to two schools of thought with respect to crystal packing analysis.10 In all cases except where the strongest and most directional of intermolecular interactions are implicated, the advocates of space filling argue that crystal structures are the result of close packing and shape matching (geometrical recognition).11,12 In contrast, one might also argue that specific structure directing interactions (chemical recognition) determine crystal packing in a decisive manner.13 Such a view does not negate the existence of geometrical factorssrather, it considers them as a sort of isotropic background that lacks structural and directional specificity.14 It is not possible to comment upon which of these two lines of argument is more appropriate on the basis of a single crystal structure analysis, or even with a just a few. Observed crystal structures are free energy minima, and most of the intermolecular separations will appear to be normal and satisfactory. To critically examine this question, one needs to look at an entire series of crystal structures so that chemical and geometrical effects of functional groups may be monitored one at a time, to the exclusion of other effects. However, such an analysis is possible in optimal cases and is reported in this paper,

10.1021/cg0341252 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/05/2003

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Table 1. Crystallographic Data for Acids 1b, 1f, 2a, 2b, 2e, and 2f 1b

1f

2a

2b

2e

2f

emp. formula formula wt. crystal system T (K)a space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) Z V (Å3) Dcalc (g/cm3) F (000) R1 wR2 GOF N-totalb N-indep.c N-obsd.d variables solvent

C9H7FO2 166.15 monoclinic 173(2) P21/c 9.529(2) 6.0556(15) 12.861(3) 90 105.810(4) 90 4 714.0(3) 1.546 344 0.0596 0.1475 1.074 8417 1752 1300 109 xylene

C10H10O2 162.18 orthorhombic 293(2) Pbcn 7.3167(12) 8.5089(12) 25.704(3) 90 90 90 8 1600.3(4) 1.346 688 0.0641 0.1687 1.099 5700 2013 1715 110 hexane

C9H6O2 146.14 monoclinic 203(2) P21/n 5.091(3) 14.988(9) 9.886(6) 90 90.180(11) 90 4 754.3(8) 1.287 304 0.0714 0.1863 1.037 9198 1872 1178 100 benzene

C9H5FO2 164.13 monoclinic 203(2) P21/c 3.849(8) 6.349(13) 31.47(7) 90 92.83(4) 90 4 768(3) 1.419 336 0.0585 0.1267 0.919 3286 1764 1028 117 acetonitrile

C10H8O2 160.16 orthorhombic 203(2) P212121 6.862(2) 12.442(4) 19.621(7) 90 90 90 8 1675.1(10) 1.270 672 0.0709 0.1635 1.069 10514 4129 3193 217 acetonitrile

Ck* e

0.73

0.70

0.67

0.69

C9H5IO2 272.03 monoclinic 183(2) P21 4.1162(14) 6.013(2) 35.398(12) 90 90.958(6) 90 4 876.1(5) 2.063 512 0.0570 0.1560 1.449 9460 4136 3861 219 xylene and acetonitrile 0.70

a

b

0.68

c

Temperature of data collection. N-total is the total number of reflections collected. N-indep. is the number of independent reflections. N-obsd. is the number of observed reflections based on the criteria I > 2σI. e Ck* is the packing fraction calculated from the program PLATON.

d

Table 2. Crystallographic Data from the Literature for Acids 1a, 1c, 1d, 1e, 2c, and 2d emp. formula formula wt. crystal system CSD refcode space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) Z V (Å3) Dcalc (g/cm3) R1 wR2

1a

1c

1d

1e

2c

2d

C9H8O2 148.2 monoclinic JUNQAK P21/c 8.599(2) 11.131(2) 14.588(4) 90 94.68(1) 90 8 1391.7(5) 1.41 0.039 0.054

C9H7ClO2 182.6 monoclinic JUNCUQ P21/c 7.175(4) 25.233(8) 8.394(4) 90 90.60(2) 90 8 1520(1) 1.60 0.045 0.057

C9H7BrO2 227.1 monoclinic JUNGII P21/c 8.306(3) 7.261 14.269(6) 90 113.13(2) 90 4 791.4(5) 1.91 0.032 0.043

C9H7IO2 274 monoclinic JUNKOS P21/c 8.304(4) 7.341(2) 14.701(7) 90 113.11(2) 90 4 824.2(7) 2.21 0.034 0.034

C9H5ClO2

C9H5BrO2 225.04 monoclinic JUKVIU P21/c 3.913(20) 6.141(2) 34.444(27) 90 91.79(5) 90 4 827.2(8) 1.81 0.030 0.030

which describes a series of crystal structures in two closely related structural families, the 4-substituted cubanecarboxylic acids and phenylpropiolic acids (Scheme 1). The main idea for the comparison of these two series was because they have an additional and critically placed C-H group (at the 3-position in the cubyl ring and at the ortho position in the phenyl ring in the two cases, respectively) and because this C-H group can act as a hydrogen bond donor to form a C-H‚‚‚O hydrogen bond. These acids can associate via O-H‚‚‚O (O‚‚‚O, 2.57-2.70 Å) hydrogen bonds to form, in principle, either zero-dimensional dimers or one-dimensional catemers. While the factors that lead to catemer or dimer formation in some benzoic acids have been discussed in terms of intermolecular interactions,15 little has been said in terms of steric and electronic functional group effects. The present paper attempts to explore this issue. Results and Discussion At the outset, it should be mentioned that no simple carboxylic acid (not containing other hydrogen bonding

triclinic SUHSET P1 h 6.120(4) 17.323(13) 3.944(2) 90.47(6) 92.70(5) 102.26(6) 2 408.1 2.063 0.065 0.071

groups), except oxalic acid16 and tetrolic acid,17 is known to form both the dimer (finite synthon) and the catemer (infinite synthon) structure; therefore, issues of polymorphism were not expected to pose problems in the present study. Table 1 shows the structural details of the acids for which crystal structures were determined or redetermined in this study.18 Table 2 gives the crystallographic details for the acids whose structures were determined earlier with the CSD refcodes. Figure 1 shows the crystal structures of 4-chlorocubanecarboxylic acid, 1c,19 and 4-chlorophenylpropiolic acid, 2c,20 studied previously by the Hyderabad group. The distinctive feature in both these structures is the very rare synanti O-H‚‚‚O (O‚‚‚O, 2.59-2.66 Å) catemer. In general, the catemer is less common than the dimer21 and among catemers the syn-anti version is very uncommon indeed, being restricted to the cubanecarboxylic acids and the phenylpropiolic acids. Acids such as 1c and 2c are therefore quite unique in that a very rare supramolecular synthon is repeated in molecules that seem to be,

Stereoelectronic Effects of Substituent Groups

Figure 1. Crystal structures of (a) 4-chlorocubanecarboxylic acid, 1c and (b) 4-chlorophenylpropiolic acid, 2c. In both cases, the syn-anti catemer and the type-I Cl‚‚‚Cl interactions are shown. Note the C-H‚‚‚O bonds from the proximal C-H groups.

Figure 2. Overlap diagram of the catemer structures of the chloro acids 1c and 2c. Note that the synthon comprising the O-H‚‚‚O syn-anti catemer and the supportive C-H‚‚‚O bond have a near exact match in the two cases. The H atoms of the carboxyl groups are disordered and were omitted for clarity.

upon casual inspection, different. It is this particular feature that lends itself to more detailed analysis in terms of functional groups. A closer inspection of the molecular structures of the two acids shows that all the atoms that constitute the catemer synthon are welloverlapped (Figure 2).22a In particular, the spatial orientation of the carboxyl group with respect to the C-H‚‚‚O forming C-H group (within the same molecule) is nearly the same.22b This is why both acids can assemble to give the syn-anti catemer synthon. In either catemer or dimer, the number of O-H‚‚‚O hydrogen bonds per carboxyl group is two, and so the

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factors that lead to discrimination between the two structure types must be at a secondary level. These factors have been discussed in supramolecular terms, that is, according to whether there exist or not other subsidiary interactions in either the catemer or the dimer case, which tips the structural balance.21 For the pair of acids mentioned above, one of the discriminating interactions is clearly the above-mentioned C-H‚‚‚O hydrogen bond formed by the 3-cubyl-H atom in acid 1c19 and by the ortho-H atom in acid 2c.20 The inference is that in the absence of a sufficiently activated C-H group, the catemer would not be formed.19,23 In the cubyl acids, the C-H activation by the cubyl skeleton is in itself sufficient to support the requisite C-H‚‚‚O interaction (1c, D, 3.63 Å, d, 2.53 Å, 152.1° and 2c, 3.47 Å, 2.53 Å, 143.7°).24 In the phenylpropiolic acids, further activation from the substituents may be necessary for the phenyl hydrogens are 105-106 times less acidic than cubyl hydrogens.19 In this light, one may conclude that the extra C-H activation by the Cl group in acid 2c is both necessary and sufficient to form the catemer. An additional chemical factor that should be considered for acid 2c is the putative C-H‚‚‚Cl interactions that are suggested by the geometry of the C-H groups as they straddle the Cl atoms related by a center of inversion. However, the relevant H‚‚‚Cl distances, d, are very long (3.11 and 3.29 Å). Further, recent work from several authors shows that the C-H‚‚‚Cl-C contact can hardly be considered a hydrogen bond. Only if the acceptor is activated via anion formation or metal coordination are the interactions of the hydrogen bond type.24 When these two factors are taken together, no further chemical effect of the Cl group that arises from a possible C-H‚‚‚Cl interaction needs to be considered. In addition to electronic factors that affect C-H activation, steric effects are also important in these crystal structures. This is revealed by the fact that the unsubstituted acids, 1a19 and 2a, adopt the dimer structure (Figures 3 and 4). Why does this happen? Analysis of the crystal structures of the chloro acids 1c and 2c shows that the catemer is geometrically compatible with the close packed arrangement of Cl groups in the type-I,25 or centrosymmetric, arrangement (Figure 1). The rigid catemer chains, when laid side by side, optimize type-I Cl‚‚‚Cl interactions (3.668 Å, θ1 ) θ2 ) 150.4° in 1c; 3.501 Å, θ1 ) θ2 ) 173.8° in 2c) to complete the layer structure. In contrast, both the steric bulk and the activating effect of the Cl group are absent in acids 1a and 2a, and the dimer structure results. Even if the activation inherent in the cubyl group was sufficient for C-H‚‚‚O bond formation in 1a, the H atom in the 4-position of the cubyl ring is too small (unlike the Cl group in acid 1c) to ensure close packing of the catemer chains. One may now hypothesize that catemer formation requires two structural features: a sufficiently activated C-H group andsequally importantsa ballast group at the other end of the molecule that can concomitantly stabilize the catemer through close packing. In other words, the Cl‚‚‚Cl contacts in acids 1c and 2c are not just compatible with the catemer, but they are actually required for its formation. The relative importance of the electronic effect of the 4-substituent upon catemer formation is therefore different in the two families of compounds studied here.

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Figure 3. Crystal structures of the cubane acids 1a-1f. The dimer and catemer synthons are shaded. The H atoms of the carboxyl groups were omitted for clarity.

In both cases, the electronic effect is transmitted through induction. However, the cubyl C-H groups in acids 1a-1f are already activated to an extent (pKa ∼38) that further activation or deactivation by substituents is not so critical. The role of the substituent group is mostly steric. For the phenylpropiolic acids 2a-2f, however, the baseline activation is lower (pKa ∼44), and so the electronic effect of the 4-substituent (activating or deactivating) is independently important. It is now easy to explain the formation of the catemer in the bromo acids, 1d19 and 2d26 (Figures 3 and 4). These acids are similar to the corresponding Cl acids 1c and 2c.27 The Br‚‚‚Br contacts play the same role as the Cl‚‚‚Cl contacts in terms of their space filling requirements or consequences. Since only the steric role of Br needs to be considered in the cubyl acid 1d, one may rationalize its catemer structure. In the phenylpropiolic acid 2d, the Br atom has a smaller activating effect than Cl in acid 2c, but this is, however, offset by its better ballast. The Br‚‚‚Br contact is also of typeII28 (Br‚‚‚Br, 3.933 Å, θ1 ) 167.4°, θ2 ) 88.5°) in this case, but this is unexceptional. The iodo acids 1e and 2e are very similar, and again the I‚‚‚I interaction in acid 2e is type-II28 (I‚‚‚I, 3.976 Å, θ1 ) 169.2°, θ2 ) 90.9°). The crystal structures of 2c-2e also illustrate that C-H‚‚‚X (X ) Cl, Br, I) interactions need not be considered seriously in this family. The mean H‚‚‚X distances (d) are, respectively, 3.15, 3.10, and 3.16 Å. The deviations from the respective van der Waals separations are 0.25, 0.05, and -0.01 Å. If these are electrostatic interactions and hydrogen bond-like (and

represent chemical perturbations of the system), the deviations from the van der Waals separations should increase rather than decrease in going from Cl to Br to I. We conclude that the C-H‚‚‚X distances are consequences rather than causes of any packing type. To summarize then, a combination of steric and electronic factors is sufficient for catemer formation for all six Cl, Br, and I substituted acids in this study. Our hypothesis above is tested and proved convincingly by the corresponding fluoro (1b and 2b) and methyl (1f and 2f) derivatives (Figures 3 and 4). The fluorocubane acid 1b forms a dimer, but the corresponding phenylpropiolic acid 2b is a catemer. The F group that behaves like an isostere to H (volumes: H, 14.7 Å3; F, 19.6 Å3)29 is too small to stabilize the catemer sterically; therefore, 1b is a dimer (like 1a). The small size of F is, however, offset by its markedly C-H activating role in 2b resulting in a catemer. Exactly the reverse situation prevails in the pair of methyl substituted acids, 1f and 2f. The CH3 group is an isostere to Cl (volumes: CH3, 23.5 Å3; Cl, 19.9 Å3)30; therefore, 1f forms a catemer. However, the deactivating effect of the CH3 group on C-H‚‚‚O hydrogen bridge formation more than offsets its steric role in acid 2f resulting in a dimer. The space group is noncentrosymmetric, and so the dimer is formed around a general position. Other acids studied by the Hyderabad group in the past confirm these trends. 4-(Methoxycarbonyl)-1-cubanecarboxylic acid forms a catemer because of the steric bulk and electron withdrawing character of the carbomethoxy group.19 4-Methoxy-, 3,4-dimethoxy-, and 3,4,5-tri-

Stereoelectronic Effects of Substituent Groups

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Figure 4. Crystal structures of the phenylpropiolic acids 2a-2f. The dimer and catemer synthons are shaded. The H atoms of the carboxyl groups were omitted for clarity.

methoxyphenylpropiolic acids form dimers because of the deactivating electronic effects of the substituent groups.31 Conclusions The series of 4-substituted cubanecarboxylic and phenylpropiolic acids constitute an excellent series of derivatives to probe and dissect out steric and electronic effects of substituent groups on solid state properties. In the cubane acids, the substituent groups mainly exert a steric effect, and the electronic effect is not critical. If the substituent is too small as in 1a and 1b, catemer formation is precluded. If the substituent is large enough (1c-1f), the catemer is formed. In the phenylpropiolic acids, both steric and electronic effects are very important. In 2c-2e, both effects are compatible with catemer formation, while in 2a both effects lead to the dimer. In contrast, electronic and steric effects predict opposite structural possibilities for 2b and 2f. Electronic effects dominate over steric effects in both cases leading to the catemer for 2b and to the dimer for 2f. While some progress has been made in this paper when it comes to understanding substituent effects, each system in and of itself should be considered primarily on its own. In very exceptional cases, such as described here, one can begin to compare related systems. In this particular case, the uncommon nature of the syn-anti catemer facilitates a detailed comparison between the

two families of acids. However, care must be taken in such an exercise. The present study also provides a clear answer to the somewhat fixed arguments that have questioned the crystal structure directing role of weak intermolecular interactions with the implication that it is not somehow possible to prove such a role.11 According to such arguments, the observed geometries in crystal structures could just as easily be rationalized with purely geometrical factors of shape, size, and close-packing. In the context of the present study, it would be maintained, for example, that the catemer arises merely from a suitable juxtaposition of the proximal C-H group and a neighboring O atom, questioning in effect the formation of a specific (and structure directing) C-H‚‚‚O hydrogen bridge. The results presented in this paper show that such a conclusion is not necessarily warranted. With the separation of steric and electronic effects of the substituent groups in the two structural families as reported here, it is possible to conclude that catemer formation can only follow from C-H‚‚‚O bonding, which in turn can only follow from C-H activation. Accordingly, the C-H‚‚‚O bridge may be described as a specific crystal structure directing interaction. The additional information obtained is that while C-H activation is a necessary condition for the formation of these hydrogen bridges, it is not a sufficient condition

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thereby identifying those situations wherein geometrical factors are important. Experimental Procedures In the early stages of this work, 1,4-bis(methoxycarbonyl)cubane, the starting material for 1b, was synthesized using the well-established Eaton and Cole32 procedure; later on, the commercial material (Aldrich) was used. All the compounds were characterized with NMR and IR spectra. 1H NMR and 13 C NMR were recorded at 200 and 50 MHz on a Bruker ACF instrument. IR spectra were recorded on a Jasco 5300 spectrophotometer. All the reactions of 1b were carried out using dry solvent. Commercially available acid 2a (Lancaster) was used directly for crystallization. The synthesis of acids 1b, 1f, 2b, 2e, and 2f is detailed here. 4-Fluorocubane-1-carboxylic acid (1b) was prepared by a literature procedure.33 IR (cm-1): 1689, 1628. 1H NMR (200 MHz, CDCl3): δ 4.85-4.45 (m, 6H); δ 8.25 (s, 1H). 13C NMR (50 MHz, CDCl3): δ 178.45, δ 102.68, δ 56.29, δ 54.09, δ 42.30. 4-Methylcubane-1-carboxylic acid (1f) was prepared by a literature procedure.34 IR (cm-1): 2976, 1674. 1H NMR (200 MHz, CDCl3): δ 4.15 (t, 3H), δ 3.65 (t, 3H), δ 1.28 (s, 3H). 4-Substituted phenylpropiolic acids (2b, 2e, 2f) were prepared by standard procedures from the corresponding cinnamic acids. The pure compounds were obtained by recrystallization from chloroform-acetonitrile (1:2) mixed solvent. The details of the pure acids are given below: (4-Fluorophenyl)propiolic Acid (2b). Yield 65%. IR (cm-1): 2210, 1726; 1H NMR (200 MHz, CDCl3): δ 8.95 (s, 1H), δ 7.66 (q, 2H), δ 7.16 (t, 2H). 13C NMR (50 MHz, CDCl3): δ 161.72, δ 158.10, δ 135.68, δ 130.71, δ 116.46, δ 87.98, δ 80.04. (4-Iodophenyl)propiolic Acid (2e). Yield 50%. IR (cm-1): 2222, 1684; 1H NMR (200 MHz, CDCl3): δ 7.78 (d, 2H), δ 7.34 (d, 2H). 13C NMR (50 MHz, DMSO-d6): δ 154.35, δ 138.13, δ 134.32, δ 118.69, δ 98.59, δ 83.76, δ 83.03. (4-Methylphenyl)propiolic Acid (2f). Yield 60%. IR (cm-1): 2197, 1676; 1H NMR (200 MHz, CDCl3): δ 9.35 (s, 1H), δ 7.54 (d, 2H), δ 7.23 (d, 2H), δ 2.40 (s, 3H); 13C NMR (50 MHz, CDCl3): δ 158.55, δ 141.96, δ 133.35, δ 129.51, δ 116.12, δ 89.73, δ 79.88, δ 21.79. X-ray Data Collection and Crystal Structure Determinations. X-ray data for 1f were collected on a Bruker P4 diffractometer, while those for 1b, 2a, 2b, 2e, and 2f were collected on a SMART diffractometer using MoKR radiation. The structure solution and refinement were carried out using SHELXL programs built in with the SHELXTL (Versions 6.10 and 6.12) package.35 The positions of the H atoms bound to phenyl and cubyl groups in 1b, 1f, 2a, 2b, 2e, and 2f were generated by a riding model on idealized geometries with Uiso(H) ) 1.2 Ueq(C), while the H atoms of the hydroxyl groups were located in difference Fourier maps, and these H atoms were also refined as riding, with Uiso(H) ) 1.5 Ueq(O). The hydrogen atoms of the hydroxyl groups in 1f, 2f, and 2b were disordered over two sites with occupancies of 0.5 each. In some cases, the U values seem to be a bit too large or too small, and this is because of poor crystal quality. The details of the X-ray data collection and structure solution and refinement are given in the Supporting Information.36

Acknowledgment. D.D. thanks the Council of Scientific and Industrial Research for a fellowship. G.R.D. is a recipient of the Alexander von Humboldt research award. R.K.R.J. and R.B. are grateful to the Deutsche Forschungsgemeinschaft for financial assistance. Supporting Information Available: Tables of crystal data, structure solution and refinement, atomic coordinates, bond lengths and angles, and anisotropic thermal parameters for all the acids reported (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

Das et al.

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Stereoelectronic Effects of Substituent Groups (12) Kitaigorodskii, A. I. Molecular Crystals and Molecules, Academic Press: New York, 1973. (13) Desiraju, G. R. Crystal Engineering. The Design of Organic Solids; Elsevier: Amsterdam, The Netherlands, 1989. (14) That some of these weak interactions have specific structural and functional roles even in complex biological systems is seen from two recent publications: (a) Brandl, M.; Weiss, M. S.; Jabs, A.; Su¨hnel, J.; Hilgenfeld, R. J. Mol. Biol. 2001, 307, 357-377. (b) Klaholz, B. P.; Moras, D. Structure 2002, 10, 1197-1204. (15) Moorthy, J. N.; Natarajan, R.; Mal, P.; Venugopalan, P. J. Am. Chem. Soc. 2002, 124, 6530-6531. (16) (a) Cox, E. G.; Dougill, M. W.; Jeffrey, G. A. J. Chem. Soc. 1952, 4854-4864. (b) Derissen, J. L.; Smit, P. H. Acta Crystallogr., Sect. B 1974, 30, 2240-2242. (c) Thalladi, V. R.; Nu¨sse, M.; Boese, R. J. Am. Chem. Soc. 2000, 122, 92279236. (17) Benghiat, V.; Leiserowitz, L. J. Chem. Soc., Perkin Trans. 2 1972, 1763-1768. (18) Rollett, J. S. Acta Crystallogr. 1955, 8, 487-494. It was noted by us that the space group of acid 2a is similar to the low-temperature structure in the original paper (photographic data). The monoclinic space group P21/n is now confirmed rather than Pnnm. (19) Kuduva, S. S.; Craig, D. C.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1999, 121, 1936-1944. (20) Desiraju, G. R.; Murty, B. N.; Kishan, K. V. R. Chem. Mater. 1990, 2, 447-449. (21) (a) Leiserowitz, L. Acta Crystallogr. 1976, B32, 775-802. (b) Berkovitch-Yellin, Z.; Leiserowitz, L. J. Am. Chem. Soc. 1982, 104, 4052-4064. (22) (a) This type of mapping of molecular structural features with respect to their supramolecular recognition properties is well-known in the drug design area. When it is carried out for a ligand, it constitutes the definition of a pharmacophore. (b) The rms deviation for the atoms in the carboxylic groups and the H atoms involved in C-H‚‚‚O hydrogen bridges in the two structures 1c and 2c when overlayed is 0.4248 Å. (23) Desiraju, G. R. Nature 2001, 412, 397-400. (24) (a) Aullo´n, G.; Bellamy, D.; Brammer, L.; Bruton, E.; Orpen, A. G. Chem. Commun. 1998, 653-654. (b) Aakero¨y, C. B.; Evans, T. A.; Seddon, K. R.; Pa´linko´, I. New J. Chem. 1999, 145-152. (c) Thallapally, P. K.; Nangia, A. CrystEngComm 2001, 27. (d) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1, 277-290.

Crystal Growth & Design, Vol. 3, No. 5, 2003 681 (25) Parthasarathi, R.; Desiraju, G. R. J. Am. Chem. Soc. 1989, 111, 8725-8726. In confirmation of the geometrical role of the Cl group in acid 1c, we note that type-I Cl‚‚‚Cl interactions are manifestations of pure close packing and that they do not arise from polarization. In contrast, the Cl‚‚‚Cl contacts of type-II are polarization induced and direction dependent. (26) Goud, B. S.; Desiraju, G. R. Acta Crystallogr., Sect. C 1993, 49, 292-294. (27) We restrict ourselves to a qualitative comparison of these crystal structures. A Kalman-type isostructurality parameter cannot be reliably calculated in these cases because there are too many differences in the hydrophobic regions of the structures. See (a) Kalman, A.; Parkanyi, L.; Argay, G. Acta Crystallogr., Sect. B 1993, 49, 1039-1049. (b) Fabian, L.; Kalman, A. Acta Crystallogr., Sect. B 1999, 55, 1099-1108. (28) The type-II geometry is very common for Br‚‚‚Br and I‚‚‚I interactions. (29) Nangia, A. New J. Chem. 2000, 24, 1049-1055. (30) (a) Desiraju, G. R.; Sarma, J. A. R. P. Indian Acad. Sci. (Chem. Sci.) 1986, 96, 599-605. (b) Thallpally, P. K.; Chakraborty, K.; Carrell, H. L.; Kotha, S.; Desiraju, G. R. Tetrahedron 2000, 56, 6721-6728. (c) Cso¨regh, I.; Weber, E.; Hens, T. Cryst. Eng. 2001, 4, 343-357. (31) Desiraju, G. R.; Kishan, K. V. R. J. Am. Chem. Soc. 1989, 111, 4838-4343. (32) (a) Eaton, P. E.; Cole, T. W. J. Am. Chem. Soc. 1964, 86, 962-964. (b) Eaton, P. E.; Cole, T. W. J. Am. Chem. Soc. 1964, 86, 3157-3158. (33) (a) Irngartinger, H.; Starck, S.; Gredel, F. Liebigs Ann. 1996, 311-315. (b) Della, E. W.; Head, N. J. J. Org. Chem. 1995, 60, 5303-5313. (34) Edward, J. T.; Farrell, P. G.; Langford, G. E. J. Am. Chem. Soc. 1976, 98, 3075-3085. (35) SHELXTL, Versions 6.10 and 6.12. (2000) Bruker AXS Inc.: Madison, WI. (36) In the crystal structure of acid 2e, the cell angle β is close to 90°, indicating an apparent orthorhombic symmetry. However, several attempts to solve the structure of this compound in the orthorhombic system failed. Further examination of the crystal structure revealed that there is no pseudosymmetry present and that the cell system is indeed monoclinic.

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