Supramolecular Synthesis by Cocrystallization of Oxalic and Fumaric

Supramolecular Synthesis by Cocrystallization of Oxalic and Fumaric Acid with Diazanaphthalenes Britta

Olenik,†

Tanja

Smolka,†

Roland

Boese,*,‡

and Reiner

Sustmann*,†

Institut fu¨ r Organische Chemie and Institut fu¨ r Anorganische Chemie, Universita¨ t Essen, 45117 Essen, Germany

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 2 183-188

Received January 6, 2003

ABSTRACT: 1:1 cocrystals of oxalic and fumaric acid with 1,4-diazanaphthalene (quinoxaline) and 2,3-diazanaphthalene (phthalazine) were prepared. 1,5-Diazanaphthalene (naphthyridine) formed a 1:1 cocrystal only with oxalic acid. X-ray structural analyses are reported. The influence of the position of the two nitrogen atoms in these diazanaphthalenes on the crystal packing is analyzed. The gross crystal structure is governed by strong OH‚‚‚N hydrogen bonds. The fine-tuning of the packing of the cocrystals of quinoxaline with oxalic and fumaric acid occurs via CH‚‚‚O hydrogen bonds. The stabilization by the CH‚‚‚O hydrogen bonds leads to a planar sheetlike structure in the cocrystal with fumaric acid, in contrast to the cocrystal with oxalic acid where the optimization of the CH‚‚‚O hydrogen bonds results in a nonplanar bent structure. The geometrical arrangement of the nitrogen atoms in 1and 5-positions in naphthyridine allows a planar sheetlike structure in the cocrystal with oxalic acid. In the case of phthalazine complete proton transfer from one carboxy group of oxalic and fumaric acid to a nitrogen atom of the heterocycle, generating ionic interactions is observed. The hydrogen atom of the second carboxy group of the acids is involved in a OH‚‚‚OdC hydrogen bond. Introduction The design of organic solids by crystal engineering is presently of high interest.1,2 Attempts are made to understand the rules that govern the construction of particular solid-state structures. Concepts that have been developed for covalent synthesis are extended to the area of noncovalent synthesis.3 Synthons can be defined that can be used as a guideline in the preparation of particular solid-state structures. The strength of noncovalent interactions ranges from weak, like van der Waals forces, CH‚‚‚X hydrogen bonds (X ) O, N, π),4,5 π‚‚‚π stacking, or CH‚‚‚π interactions,6 to strong forces such as O-H‚‚‚X hydrogen bonds (X ) O, N).7 Apart from the strong interactions that are often structure determining, it is the sum of a number of different weak forces which leads to the observed solidstate structure.8 This was demonstrated by Jones et al. with the molecular complexes of phenazine with various dicarboxylic acids.9 Other systems that utilize O-H‚‚‚ O and C-H‚‚‚O interactions with carboxylic acids10 or O-H‚‚‚N and C-H‚‚‚O in heterodimers11 face the same problem of the dependence on many cooperative factors which is the main problem in predicting crystal structures. In recent years, we have been interested in the formation of cocrystals of functional diols as hydrogenbond donors and imino nitrogen atom carrying molecules as hydrogen-bond acceptors, either in aliphatic bismines or in heterocyclic structures.12,13 In some cases, the construction of hydrogen bonded networks of such components has led to interesting macroscopic properties, like a thermally reversible photochromism.14-16 †

Institut fu¨r Organische Chemie. Institut fu¨r Anorganische Chemie. * To whom correspondence should be addressed. (R.B.) Fax: +49 201/183, E-mail: [email protected] and (R.S.) Fax: +49 201/ 1834259, E-mail: [email protected]. ‡

In this contribution, we use quinoxaline, phthalazine, and naphthyridine as diazanaphthalenes in the cocrystallization with fumaric and oxalic acid. The systematic study is directed toward the understanding of the influence of nitrogen atoms in different positions of the naphthalene skeleton on the observed crystal packing. Cocrystals could be obtained for all combinations of the synthons except for naphthyridine where only a cocrystal with oxalic and not with fumaric acid could be prepared. Cocrystals of these heterocycles with dicarboxylic acids were reported so far only for phthalazine, where Rogers et al. described cocrystals of phthalazine and phthalic acid in the ratio 2:1 and 2:3.17 Recently, we described cocrystals of azanaphthalenes and meso1,2-diphenyl-1,2-ethanediol.18 Results and Discussion24 Quinoxaline and Oxalic Acid. 1,4-Diazanaphthalene (quinoxaline) and oxalic acid were cocrystallized in a 1:1 ratio from an acetone solution. X-ray structural analysis (Table 1) reveals two molecules of quinoxaline and two molecules of oxalic acid in the asymmetric unit (space group P1 h ) (Figure 1a). A first inspection shows that the four molecules are held together by OH‚‚‚N hydrogen bonds with alternating molecules of oxalic acid and quinoxaline, forming four different hydrogen bonds to build infinite chains. The oxalic acid molecules are almost planar, but it is remarkable to see that the molecules in the chain are not arranged in a single plane. The mean planes of the oxalic acid molecules and quinoxalines are almost coplanar, but the mean planes of the heterocycle (I) and oxalic acid (II) as well as the mean plane of heterocycle (III) and oxalic acid (IV) are considerably tilted. This torsion brings O6 in a position to build a CH‚‚‚OdC bridge to the hydrogen atom at C6. This geometry allows further the formation of weak C-H‚‚‚OdC hydrogen bonds (hydrogen atom at C2 and

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Table 1. Data Collection and Refinement for Cocrystals of Diazanaphthalenes with Oxalic and Fumaric Acid formula

C8H6N2‚C2H2O4

C8H6N2‚C4H4O4

C8H6N2‚C2H2O4

C8H7N2‚C2HO4

C8H6N2‚C4H4O4

form wt (Da) density (g cm-3) T (K) cryst size (mm) cryst color crystal system space group a (pm) b (pm) c (pm) R (°) β (°) γ (°) V (106 pm3) Z 2θmax (°) refln collected refln indepen Rint refln obs (>4σF) param refin R1 (F) wR2 (F2, all data) residual electron density (106 e pm-3)

220.18 1.536 299(2) 0.18‚0.02‚0.01 blue triclinic P1 h 373.3(6) 1226.6(14) 2109(3) 87.59(3) 85.43(4) 81.90(5) 952(2) 4 50 8724 2867 0.0844 1431 289 0.0769 0.2189 0.298

246.22 1.450 293(2) 0.53‚0.46‚0.32 colorless triclinic P1 h 575.4(3) 884.2(3) 1193.1(5) 72.43(3) 82.26(4) 77.80(4) 564.1(4) 2 59.90 4175 3142 0.1416 1759 163 0.0736 0.2298 0.292

220.18 1.544 203(2) 0.21‚0.18‚0.07 pale yellow triclinic P1 h 371.0(6) 668.0(11) 974.0(16) 86.48(4) 79.37(6) 88.19(4) 236.7(7) 1 56.48 1441 885 0.0225 676 73 0.0763 0.1996 0.780

220.18 1.491 203(2) 0.12‚0.03‚0.02 pale yellow monoclinic P21/c 561.26(6) 1812.74(17) 991.98(10) 90 103.594(2) 90 980.98(17) 4 56.18 8384 2211 0.0515 1571 145 0.0601 0.1656 0.515

246.22 1.448 203(2) 0.18‚0.08‚0.06 colorless monoclinic P21/m 378.44(13) 4260.2(14) 1057.5(4) 90 96.547(7) 90 1693.9(10) 6 56.64 14587 4247 0.0300 1516 244 0.0846 0.2094 0.290

Figure 1. Cocrystal of 1,4-diazanaphthalene (quinoxaline) and oxalic acid, (a) thermal ellipsoid plot (50%) of the molecules in the asymmetric unit with numbering scheme, (b) molecular packing viewing down [100], (c) molecular packing viewing down [101], (d) schematic representation of a planarized chain, (e) schematic representation of a hypothetical linear and planar chain with synperiplanar oxalic acid and quinoxaline as in the cocrystal.

C15 of quinoxaline molecules), which interconnect the chains formed by the O-H‚‚‚N hydrogen bonds. Figure 1b displays two such chains viewing down the [100] vector. It is noteworthy that the chains are arranged antiparallel so that the nitrogen atoms of quinaxoline are on the inner side of the dimeric chain. This means that such dimer chains are positioned next to each other when constructing the cocrystal. The next dimer would appear above or below the chain shown in Figure 1b. The quinoxaline molecules in this new dimer fill the free space above or below the oxalic acid molecules of the original dimer leading to a close packing, which however is not as dense as within the dimers. The interplanar angle of the molecules in a single chain is especially visible in Figure 1c viewing down the [101] vector.

Neighboring V-shaped chains are alternately interlocked and linked by weak hydrogen bonds to the chains displayed below in Figure 1c. To gain more insight in the reason for the observed packing the question may be asked whether a fully planar arrangement as observed in other cases (see below) might be an option? Planarization in the actual disposition of oxalic acid and quinoxaline (Figure 1) does not seem to be feasible. If oxalic acid (II) is rotated about the central bond into the plane of quinoxaline (I), severe steric interaction between the hydrogen atom at C6 and O6 would be the consequence. The O‚‚‚H distance would be smaller than the van der Waals radii, unless oxalic acid (II) would be rotated about an axis perpendicular to the molecular plane which passes through the

Cocrystallization of Oxalic and Fumaric Acid

midpoint of the central bond C19-C20. This rotation could lead to C‚‚‚O distances expected for a CH‚‚‚O hydrogen bridge. It would however disturb the linearity of the individual chains (Figure 1d). On the other hand, a linear arrangement could be imagined as oxalic acid (II) and quinoxaline (III) demonstrate where besides the strong OH‚‚‚N hydrogen bond a CH‚‚‚O bridge exists between O7 and the hydrogen atom at C12. Distances and angles are such that no steric hindrance is established due to the planar disposition. This observation allows the construction of two planar and linear arrangements of the two molecules which are shown in Figure 1d,e. In both strimgs oxalic acid assumes an antiperiplanar conformation with respect to the carbonyl groups. In Figure 1d, the quinoxaline molecules are rotated alternatively by 180° about the axis connecting the two nitrogen atoms, similar to the cocrystal, in Figure 1e the nitrogen atoms of the quinoxaline molecules remain on the same same side of a string. Both arrangements have the disadvantage with respect to the cocrystal that there is no three-dimensional hydrogen bond network and that they are less compact, i.e., they constitute structures with less van der Waals contacts. The real arrangement has the advantage in having two hydrogen bridges within the chain and an additional one between different chains. The structure could be said to be optimized with respect to compactness and the maximum number of C-H‚‚‚O bridges. Quinoxaline and Fumaric Acid. The two carboxy groups in fumaric acid are separated by a CC double bond which can be considered a spacer between two carboxy groups that are directly linked in oxalic acid. The influence of this structural change on the crystal packing of a cocrystal with quinoxaline should produce insight in the factors which govern the placement of the molecules in the crystal lattice. The 1:1 triclinic cocrystal (space group P1 h ) was produced from an acetone solution. The asymmetric unit consists of one molecule of quinoxaline and one molecule of fumaric acid (Figure 2a). One carbonyl group of fumaric acid is positioned anti- and the other synperiplanar to the double bond. Fumaric acid is almost planar and is coplanar to quinoxaline. Due to the OH‚‚‚N hydrogen bridges, chains are formed that are different from the previous case. The quinoxaline molecules in the chain are identically oriented (Figure 2b) in contrast to the former cocrystal (Figure 1). An additional weak hydrogen bond is established between O2 and the hydrogen atom at C2, the geometrical conditions allowing planarity of the two molecules. Remember, that the nonplanar arrangement of oxalic acid and quinoxaline in the former case was due to the formation of a hydrogen bridge between the hydrogen atom at C8 (peri hydrogen) and OdC of the acid. This is avoided here allowing the complete planarity of the array. Again, C-H‚‚‚O interactions couple the chains. As in the case of oxalic acid, it is possible to identify dimeric chains in which the nitrogen atoms of quinoxaline point to the inside (the two chains in the middle of Figure 2b). Each pair of quinoxaline and fumaric acid in this dimeric arangement is held together by four CH‚‚‚O hydrogen bonds. The interaction to the next chain, above or below in the figure, is characterized by only two CH‚‚‚O interactions per dimer of quinoxaline and fumaric acid. In the case of the longer

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Figure 2. Cocrystal of 1,4-diazanaphthalene (quinoxaline) and fumaric acid, (a) thermal ellipsoid plot (50%) of the molecules in the asymmetric unit with numbering scheme, (b) molecular packing viewing onto the molecular planes, (c) molecular packing viewing along the OH‚‚‚N connected chains.

fumaric acid molecules, the benzene part of a quinoxaline molecule in a second chain fits perfectly between a unit of fumaric acid and two molecules of quinoxaline in the chain, producing a planar sheet. The arrangement of the planar chains is displayed in a view almost along the chains (Figure 2c). Two different C-H‚‚‚O interactions, arranged around an inversion center i1 couple the chains in one direction and one C-H‚‚‚O interaction exists at the other inversion center i2 at (benzene part of quinoxaline). Clearly, the CC double bond spacer of fumaric acid leads to separations of quinoxaline molecules in a single chain which produces an optimal space for the positioning of a quinoxaline molecule of another dimeric chain in between. This pattern also provides an explanation of the different asymmetric unit in the cocrystal of quinoxaline and oxalic acid. There nonplanar chains of alternating molecules of the components are formed. The nonplanar arrangement allows optimal CH‚‚‚O interactions between different chains which would not have been possible if the carbonyl groups were arranged as in the case of fumaric acid to form hydrogen bridges

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to the hydrogen atom at C2. The nonplanar arrangement leads to chains with smaller lateral extension allowing better interchain CH‚‚‚O bridges. A planar situation as in the case of fumaric acid would generate a less tight packing with a reduced number of CH‚‚‚O hydrogen bridges. Thus, the difference in crystal packing results from the fact to build as many CH‚‚‚O hydrogen bridges as possible. In the case of the cocrystal of quinoxaline and fumaric acid, planar sheets of molecules are superimposed and form a three-dimensional crystal lattice. The quinoxaline molecules of different layers are shifted offset face to face, probably generating additional stabilizing interactions between the sheets in view of the small separation of the different planes. There are no evident additional interactions between aromatic rings in different planes. Naphthyridine and Oxalic Acid. The two nitrogen atoms of naphthyridine are in 1- and 5-positions of the naphthalene skeleton, i.e., each six-membered ring of the bicycle is of pyridine type. The relative position of the nitrogen atoms leads to a hydrogen bonding motif which resembles the chains in the cocrystal of quinoxaline and fumaric acid and not the chains in the cocrystal with oxalic acid, with the essential difference that the position of the OH‚‚‚N bond connecting naphthyridine and fumaric acid alternates between the 1- and the 5-position and not between the 1- and the 4-position in quinoxaline. This disposition provides free space between two naphthyridine molecules in a chain to place a second chain so that oxalic acid can establish the maximum number of C-H‚‚‚O bridges to the neighboring naphthyridine molecule. Figure 3a represents the symmetry completed molecules, both being planar due to their respective centrosymmetry, both are almost coplanar to each other. The chains with the strong OH‚‚‚N hydrogen bridges are interconnected by CH‚‚‚O interactions in which all oxygen atoms of oxalic acid are involved shown in Figure 3b. The layers of the molecules, which are separated by 312 pm are shifted with respect to each other, so the carbonyl oxygens are situated almost above the C-atoms of the oxalic acid molecules, the equivalent atoms have distances according to the a-axis (371 pm) of the unit cell (Figure 3c). Phthalazine and Oxalic Acid. A 1:1 cocrystal was isolated from an acetone solution and characterized by X-ray analysis (Table 1). The asymmetric unit (space group of the monoclinic crystal P21/c) consists of one molecule of each component (Figure 4a). The major difference of this cocrystal to those of the previous diazanaphthalene molecules is a complete proton transfer from one carboxy group of oxalic acid to one of the nitrogen atoms of phthalazine. No significant electron density is found in the surrounding of O2. The hydrogen atom located at N2 could be clearly located and refined without constraints at a distance of 93 pm. Such a proton transfer is also observed in a 2:1 and a 2:3 cocrystal of phthalazine with phthalic acid and indicates a higher basicity of the nitrogen atoms of phthalazine as compared to those of the other heterocycles.17 It leads to salt formation, thus additional electrostatic interactions are generated. The N+-H‚‚‚O- hydrogen bridge is characterized by a relatively short O2‚‚‚N2 distance. A further but weaker and bifurcated interaction can be identified between H2 and O4. Oxalic acid is planar as

Olenik et al.

Figure 3. Cocrystal of 1,5-diazanaphthalene (naphthyridine) and oxalic acid, (a) thermal ellipsoid plot (50%) of the molecules in the asymmetric unit with numbering scheme, (b) molecular packing viewing down [100], (c) molecular packing viewing perpendicular to the molecular plane, showing the stacking of the molecules.

well as the heterocycle (max devation from mean plane 3.0 pm), but the two molecules of the asymmetric unit are not arranged in the same plane. The dihedral angle between the CC bond connecting the two carboxy groups of oxalic acid and the C1-N2 bond of phthalazine (C12C11‚‚‚N2-C1) is -36.8°. Intermolecular interactions can further be identified between C1-H1 and the oxygen atom O1 of oxalic acid as well as between a hydrogen atom (H8) in the benzene part of the heterocycle and a carboxy oxygen (O3) (Figure 4b). The geometric parameters, however, are such that only a weak stabilization should result. An analysis of the packing (Figure 4b) reveals that the oxalic acid molecules form linear chains (catamer motif) which are held together by strong O-H‚‚‚O hydrogen bridges. It is interesting to note that this hydrogen bond is established between a protonated and deprotonated carboxy group. The carboxylate group displays two almost identical C-O bond distances of 124 and 126 pm, respectively, in the carboxy group they are

Cocrystallization of Oxalic and Fumaric Acid

Figure 4. Cocrystal of 2,3-diazanaphthalene (phthalazine) and oxalic acid, (a) thermal ellipsoid plot (50%) of the molecules in the asymmetric unit with numbering scheme, (b) molecular packing viewing down [001], (c) molecular packing viewing along the molecular plane, showing the tilt and offset face-toface arrangement of the molecules.

120 for CdO and 130 pm for the C-O bond. The latter situation is very similar in the crystal structure of pure oxalic acid (CdO 121 pm, C-O 131 pm),19-22 whereas the egalization of the CO bonds in the former case is typical for carboxylate groups. The arrangement of the phthalazine molecules along the chain of oxalic acid shows that at one side the molecules are bonded by the NH‚‚‚O hydrogen bridges and at the other side by CH‚‚‚O interactions. The phthalazine molecules on each side form stacks with centroid-centroid distances of 561 pm, corresponding to the a-axis of the unit cell, and indicating the offset face to face arrangement which contributes to favorable interactions between the π systems of the heterocyclic ring systems (Figure 4c). The interactions take place between the heterocyclic and the benzene part of phthalazine molecules. In one stack, bonded by CH‚‚‚O, phthalazine is positioned to the oxalic acid parallel to the long axis, wheres in the other one, due to the NH‚‚‚O bridge this is parallel to the short axis. Although

Crystal Growth & Design, Vol. 3, No. 2, 2003 187

Figure 5. Cocrystal of 2,3-diazanaphthalene (phthalazine) and fumaric acid, (a) thermal ellipsoid plot of the molecules in the asymmetric unit completed by crystallographic mirror symmetry for one 2,3-diazanaphthalene and centrosymmetry for fumaric acid, with numbering scheme, (b) molecular packing viewing down [100], (c) molecular packing viewing along the chains, linked by OH‚‚‚N bridges and networked by CH‚‚‚O bridges.

phthalazine is a diazanaphthalene, an important distinction to the above-discussed cases is that the two nitrogen atoms of phthalazine are too close to be both hydrogen bonded to an acid molecule. This leads to the completely different pattern of the crystal packing. Phthalazine and Fumaric Acid. An acetone solution in the ratio 1:1 of the components led to the formation of 1:1 cocrystals which were analyzed by X-ray crystallography (Table 1). The asymmetric unit consists of one and a half molecule of each component (monoclinic space group P21/m). One phthalazine molecule (N20, C21-C24) is intercepted by the crystallographic mirror plane and one fumaric acid (O1, O2, C1, C2) has crystallographically centrosymmetry, thus both are completed by the respective symmetry operations (Figure 5a). Three crystallograpic independent strong hydrogen bonds can be identified, one between a protonated nitrogen atom of phthalazine and a deprotonated oxygen atom of fumaric acid, the second one between the same fumaric acid OH group and the Cs symmetric phthalazine and the third between the OH

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group of the Ci symmetric fumaric acid molecule and an oxygen atom of the deprotonated carboxy group of the acid molecule. Thus, the two molecules of each kind are diffferent in the unit cell, the phthalazine molecules are protonated and one of the fumaric acid molecules is deprotonated. The deprotonated acid is almost planar, and is coplanar with the protonated, again planar heterocycle (max deviation from mean plane 2.3 pm) and on the other side it is almost coplanar with the Cs symmetric heterocycle. These 1:2 complexes (one acid and two bases) are linked among each other via weak C-H‚‚‚O hydrogen bonds, with all these molecules lying almost in one plane. These units are further connected by the centrosymmetric acid molecules via the strong O-H‚‚‚ O and weaker C-H‚‚‚O bridges (Figure 5b), forming an interplanar angle of 50° with the planar units. The planar units form stacks along the a-axis corresponding to a separation of 378 pm (Figure 5c). Conclusion OH‚‚‚N and CH‚‚‚O hydrogen bonds determine the crystal packing in the 1:1 cocrystals of quinoxaline with oxalic acid and fumaric acid. The double bond in fumaric acid as spacer between two carboxy groups leads to a planar arrangement of quinoxaline and fumaric acid molecules. In contrast, the cocrystal of quinoxaline and oxalic acid is characterized by a nonlinear placement of the constituents. The position of the nitrogen atoms in naphthyridine as component of the cocrystal with oxalic acid permits a planar sheetlike structure as in the case of the cocrystal of quinoxaline and fumaric acid. The structures of the cocrystals of phthalazine with oxalic- and fumaric acid exhibit a proton transfer from one carboxy group to a nitrogen atom and a OH‚‚‚O hydrogen bond between two acid molecules leading to a backbone to which the nitrogen heterocycles are attached. The discussion has shown that the position of the two nitrogen atoms in these diazanaphthalenes influence strongly the crystal packing. Experimental Section The compounds were either commercially available or, as in the case of naphthyridine (5), were synthesized according to a literature procedure.23 Cocrystals were obtained by dissolving the components in a 1:1 ratio in the reported solvent, if not otherwise indicated, and letting the solvent slowly evaporate at ambient temperature. Cocrystal quinoxaline/ oxalic acid: acetone, mp: 175 °C. Cocrystal quinoxaline/ fumaric acid: acetone, mp: 230 °C. naphthyridine/oxalic acid: acetone, mp: 155 °C. Cocrystal phthalazine/oxalic acid: acetone, mp: 155 °C. Cocrystal phthalazine/fumaric acid: acetone, mp: 160 °C. The 1:1 composition of the cocrystal was also checked by solution 1H NMR spectra.

Olenik et al. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-190623 for 1‚3, 190624 for 2‚3, 190625 for 1‚4, 190626 for 1‚5, and 190627 for 2‚5. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: int. code + 44(1223)336-033; e-mail: [email protected]].

Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaaft and Sonderforschungs-bereich SFB 452. References (1) Desiraju, G. R. Crystal Engineering. The Design of Organic Solids; Elsevier: Amsterdam, 1989. (2) Desiraju, G. R. Comprehensive Supramolecular Chemistry, Vol. 6; MacNicol, D. D., Toda, F., Bishop, R., Eds.; Pergamon: Oxford, Chapter 1, pp 1-22, 1996. (3) Desiraju, G. R. Angew. Chem. 1995, 107, 2541-2558. (4) Calhorda, M. J. Chem. Commun. 2000, 801-809. (5) Braga, D.; Grepioni, F. New J. Chem. 1998, 1159-1161. (6) Nishio, M. The CH/π Interaction, 1st ed.; Wiley-VCH: New York, 1998. (7) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological Structures, 2nd ed.; Springer: Berlin, 1994. (8) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. (9) Batchelor, E.; Klinowski, J.; Jones, W. J. Mater. Chem. 2000, 10, 839-848. (10) Senthil, V. S.; Nangia, A.; Katz, A. K.; Carrell, H. L. Cryst. Growth Des. 2002, 2, 313-318. (11) Vishweshwar, P.; Nangia, A.; Lynch, V. M. J. Org. Chem. 2002, 67, 556-565. (12) Reyes-Arellano, A.; Boese, R.; Steller, I.; Sustmann, R. Struct. Chem. 1995, 6, 391-396. (13) Smolka, T.; Schaller, T.; Sustmann, R.; Bla¨ser, D.; Boese, R. J. Prakt. Chem. 2000, 342, 465-471. (14) Felderhoff, M.; Steller, I.; Reyes-Arellano, A.; Boese, R.; Sustmann, R. Adv. Mater. 1996, 8, 402-405. (15) Felderhoff, M.; Smolka, T.; Sustmann, R.; Steller, I.; Weiss, H.-C.; Boese, R. J. Prakt. Chem. 1999, 341, 639-648. (16) Smolka, T.; Sustmann, R.; Boese, R. J. Prakt. Chem. 1999, 341, 378-383. (17) Rogers, R. D.; Sharma, C. V. K.; Whitcomb, D. R. Cryst. Eng. 1998, 1, 255-262. (18) Olenik, B.; Boese, R.; Sustmann, R. Cryst. Growth Eng. 2003, 2, 175-182. (19) Derissen, J. L.; Smit, P. H. Acta Crystallogr. 1974, B30, 2240-2242. (20) Delaplane, R. G.; Ibers, J. A. Acta Crystallogr. 1969, B25, 2413-2437. (21) Sabine, T. M.; Cox, G. W.; Craven, B. M. Acta Crystallogr. 1969, B25, 2437-2441. (22) Coppens, P.; Sabine, T. M. Acta Crystallogr. 1969, B25, 2451-2460. (23) Rapoport, H.; Batcho, A. D. J. Org. Chem. 1963, 28, 17531759. (24) For all discussions, C-H and O-H distances have been normalized to their distances from neutron diffraction with 108 and 98.3 pm, respectively.

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