Establishing a Hierarchy of Halogen Bonding by Engineering Crystals

Article pubs.acs.org/crystal

Establishing a Hierarchy of Halogen Bonding by Engineering Crystals without Disorder Christer B. Aakeröy,* Prashant D. Chopade, and John Desper Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States S Supporting Information *

ABSTRACT: It has been shown, using a foundation of new structural data, that the relative strength and capability of iodo- and bromo-based molecules to act as halogen-bond donors in a competitive supramolecular arena accurately reflect a ranking of halogen-bond donors based upon electrostatic molecular potentials. Furthermore, to obtain the critical structural information, a protocol (comprising a lowering of molecular symmetry and the addition of strong and directional hydrogen bonds) for engineering crystals without positional disorder was successfully developed.

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An improved insight into how these interactions may compete with each other is important for the design and evaluation of new catalysts, improved molecular receptors, as well as in drug design.6 The primary goal of this study is to establish if a series of XB donors can be ranked according to expected strength and structural influence. This, in turn, will allow us to forge supramolecular synthetic strategies that incorporate a range of different intermolecular interactions that can operate in parallel with little or no “synthon crossover”.7 Halogen bonding is primarily electrostatic in nature, and therefore, the electrostatic charges on XB donors and acceptors should govern the outcome of a supramolecular reaction.8 To test this hypothesis, we need a series of ditopic molecules that carry two different XB donor groups that can be ranked according to their relative halogen-bonding ability. Experimental data confirm the theoretical predictions that the strength of the XB donor increases in the order F < Cl < Br < I.9 Moreover, the strength of the XB donor increases as the electron-withdrawing nature of the moiety bound to a given halogen atom increases.10 In this study, we set out to investigate binding abilities of ditopic XB donors by co-crystallizing them with (a) symmetric ditopic XB acceptors, (b) dissymmetric ditopic XB acceptors, and (c) single-point XB acceptors (Scheme 1). Against this background, and complemented with an analysis of relevant crystallographic data,11 we prepared suitable cocrystals of 1-bromo-4-iodo-2,3,5,6-tetrafluorobenzene 1 and its extended analogue 2 as ditopic XB donors equipped with an “activated” iodo substituent (the best XB donor) and a bromo

ntermolecular forces are principally responsible for most physical properties expressed by molecular solids and liquids. For example, the anomalous behavior of water is a consequence of hydrogen bonding, the low aqueous solubility of many pharmaceutical compounds can be traced to incompatible solute−solvent interactions, and every act of molecular recognition is determined by a balance between different intermolecular interactions. As a result, terms such as ion-dipole interactions, van der Waals forces, and hydrogen bonding are essential to our everyday chemical language. Intermolecular interactions are often classified according to their strengths and geometric requirements, and this is often driven by a need to arrange them within a strength-based hierarchy. For example, the relative importance of different hydrogen bonds has been systematically explored using both co-crystallizations, and a variety of empirically derived theoretical models.1 An important take-home message from many of these studies is that, in a system with multiple potential synthons,2 the best hydrogen-bond acceptor frequently seeks out the best hydrogen-bond donor, while the second-best donor binds to the second-best acceptor,3 a behavior that can be rationalized in the context of a primarily electrostatic view of the relative strengths of hydrogen-bond donor/acceptors across a wide range of chemical functional groups.1c,d Halogen bonds (XBs) have features that parallel many of those of hydrogen bonds in terms of strength and directionality.4 In halogen bonding, halogen atoms act as electrophiles and XB can play a vital role in fundamental crystal engineering as well as in applied materials science, including liquid crystals and magnetic materials, chiral discrimination, ion pair recognition, anion receptor, supramolecular polymer formation, porous material design, and chemical separation.5 © XXXX American Chemical Society

Received: July 1, 2013 Revised: July 25, 2013

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dx.doi.org/10.1021/cg400988m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Postulated Outcomes of Supramolecular Reactions (Where X1 = Best XB Donor, X2 = Second-Best XB Donor, A1 = Best XB Acceptor, and A2 = Second-Best XB Acceptor)

substituent (the second-best XB donor) (Scheme 2). Molecular electrostatic potential calculations were carried out on 1 and 2 Scheme 2. Ditopic Halogen-Bond Donors with Electrostatic Charges on Activated Halogens, Calculated Using the DFT/ B3LYP Level of Theory with the 6-311+G** Basis Set Using Spartan ’0812

Figure 2. MEP surfaces of (a) tetramethylpyrazine mono-N-oxide B and (b) 4,4′-bipyridine mono N-oxide C.

Figure 3. 1D network formed in 1·B. Structure shows disordered 1 and B in a space-filling model.

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using the DFT/B3LYP level of theory with a 6-311+G** basis set. The values confirm that the iodine atom should be considered as a better donor than the bromine atom against a backdrop of an electrostatic description of halogen bonds (Scheme 2).13

possible to establish any binding preferences between iodine and bromine. We also varied the stoichiometries of XB acceptors and donors (0.5:1, 1:1, and 1:2) in parallel solution experiments. For 0.5:1 and 1:1 acceptor/donor ratios, we obtained 1:1 co-crystals. However, for 1:2 acceptor/donor stoichiometry, donors and acceptors recrystallized separately. Positional disorder is common in solids containing atoms of similar sizes.14 The disorder between fluorine and hydrogen atoms has been studied in depth by Guru Row and coworkers.15 Their study shows that of all reported structures containing fluorine at ortho or meta to hydrogen atom, around 18% are disordered. Chlorine atoms also exhibit positional disorder with nitro groups16 and methyl groups.17 Bromine atoms, apart from exhibiting positional disorder with methyl and nitro groups, also show positional disorder with methoxy groups.18 In this context, we analyzed crystal structures of ortho and para phenyl-substituted pairs of heavy halogen atoms in the CSD (Cambridge Structural Database, http://www.ccdc.cam. ac.uk). For symmetric molecules (17 structures), disorder is found between iodo−bromo, iodo−chloro, and bromo−chloro atoms, in all but two structures (Table S1, Supporting Information). Moreover, para iodo/bromo positional disorder is observed in the co-crystal of N,N-dimethylpyridin-4-amine with 1-bromo-4-iodobenzene, which is in agreement with our work.19 The CSD survey indicates that disorder is a commonly occurring issue, which can hamper or prevent the analysis of intermolecular interactions. To overcome the crystallographic disorder problem, we hypothesized that, if we lowered the symmetry of the XB acceptor (tetramethylpyrazine, A), we might be able to avoid the positional disorder. We, therefore, prepared tetramethylpyrazine mono-N-oxide B and 4,4′bipyridine mono N-oxide C equipped with two acceptor sites of different strengths.20 The molecular electrostatic potential

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RESULTS AND DISCUSSION Co-crystallizations with Symmetric XB Acceptor Molecules. In our initial co-crystallizations, we combined 1

Figure 1. 1D halogen-bonded network formed in co-crystals of (a) 1·A and (b) 2·A. Both structures display I/Br disorder.

and 2 with a symmetric ditopic XB acceptor, tetramethylpyrazine A. The co-crystals 1·A and 2·A are formed via N···XI/Br halogen bonding (Figure 1). Unfortunately, the crystal structures of both 1 and 2·A show disordered halogen atoms at the para position. The occupancy factors for iodine/bromine are 0.50/0.50, and due to this positional disorder, it is not B



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Figure 4. 1D halogen-bonded network formed in 1·C. Structure shows disordered 1.

Figure 5. Ditopic dissymmetric XB acceptor D with MEP charges (left) and halogen-bonded network formed in 2·D via C−XI/Br···N interaction (right).

Figure 8. 2D sheet in the crystal structure of 8·G generated via a combination of N···I, N···Br, and CO···H−N interactions.

Figure 6. 1:1 co-crystal formed by combination of 2 with (a) 4-(Npyrrolidinyl)pyridine E and (b) 4-(N,N-dimethylamino)pyridine F.

Figure 9. Zig-zag network formed via N···I and N−H···N interactions in the structure of 8·H.

preferentially with the oxygen atoms of N-oxides (best acceptor) and the bromine atom should bind to the nitrogen atom (second-best acceptor). However, the crystal structure of 1·B shows, once again, I/Br disorder (50:50). To make matters worse, the N−O and O groups of B are disordered (50:50) (Figure 3). Similar disorder has previously been observed in cocrystals based on N−H/S (imino/thio) and N−H/O (imino/ oxo) acceptor groups.21 The second structure that we obtained with a dissymmetric acceptor resulted from the combination of 1 and 4,4′-bipyridine mono N-oxide C, and again, the disorder persists (Figure 4). The dissymmetry of the XB acceptor molecules was seemingly not sufficient to resolve the problem of positional disorder. Therefore, we decided to abandon the linear shape of the acceptor and instead synthesized a “bent” molecule equipped with two different XB acceptors. A ditopic acceptor containing benzimidazole and pyridine moieties (bridged by a methylene group to impart the desired shape) was thought to be suitable for the purpose of establishing intermolecular

Figure 7. (a) The molecular structure of 7. (b) Ladder formation via CO···H−N interactions in the crystal structure of 7. (c) 8 with MEP charges on iodo and bromo XB donor.

(MEP) charges on oxygen atoms are −173 and −184 kJ/mol for B and C, respectively, which are higher than the corresponding charges on the nitrogen atoms, −150 and −173 kJ/mol, respectively (Figure 2). Consequently, if the disorder were absent in the co-crystals of B and C with ditopic XB donor molecules, and if our hypothesis is correct, then the iodine atom should interact C



Article

Figure 10. 2D sheet in the crystal structure of 8·I formed via N···I, Br···Br, and CO···H−N bonds.

on the iodine atom is higher (182 kJ/mol) than that on the bromine atom (136 kJ/mol), making the former the better donor (Figure 7). The first crystal structure from a combination of 8 and 1,2bis(4-pyridyl)ethylene G shows the presence of the desired CO···H−N interaction, which results in a stable infinite chain. More importantly, we were finally able to engineer a structure without positional disorder for the I/Br atoms (Figure 8).23 Both bromo and iodo XB donors are halogen-bonded to nitrogen atoms of G; the C−X···N distance for X = I is 2.73 Å, which is a shorter distance compared to that for X = Br (2.75 Å). The former represents a 23% contraction and the latter a 20% contraction of the combined van der Waals radii, respectively, which is in line with reported data.24 In our next attempt, we combined 8 with 1,2-bis(4pyridyl)ethane25 H. The crystal structure of 8·H again reveals that there is no I/Br disorder (Figure 9), and the desired I··· N(pyridyl) halogen bond is present. However, the amide ladder (CO···H−N) is disrupted as an N−H···N (pyridyl from bipyethane) interaction is now instrumental in the selfassembly.26 Another salient feature of this structure is that the activated bromine atoms are not taking part in any structure-directing interactions (despite the fact that a carbonyl oxygen atom is available27).28 The preference for an I···N interaction over a Br···XB acceptor interaction supports the hypothesis that halogen bonding is governed by electrostatic charges on the XB donors. The last co-crystal was obtained from a combination of 8 and 4,4′-bipyridine I. The crystal structure of 8·I shows that a 2:1 co-crystal is formed and driven by N···I halogen bonds, which produces trimers that are subsequently linked into 1D chains via amide CO···H−N hydrogen bonds (Figure 10). The weaker XB donor, the bromine atom, is unable to compete with an iodine atom for an N(pyridyl) acceptor site and instead participates in a Br···Br Type I halogen··halogen contact.29 Again, the best donor (iodo) seeks out the best acceptor (py). Moreover, in the absence of a second acceptor site, the bromine atom is essentially left with a close-packing interaction involving an adjacent bromine atom.

ranking, due to the presence of two acceptor sites with different strengths (Nbenzimidazole = −206 kJ/mol and Npyridine = −181 kJ/ mol) (Figure 5). However, yet again, the resulting co-crystal 2· D displays positional disorder (Figure 5). Next, to further limit the chances of disorder, we moved from ditopic to monotopic XB acceptor molecules (e.g., substituted pyridines). Our choice of XB acceptor was based on the wellknown XB forming ability of 4-(N-alkylamino)pyridines.10 We chose 4-(N-pyrrolidinyl)pyridine E and 4-(N,N-dimethylamino)pyridine F as the new XB acceptor. We obtained two crystal structures (Figure 6), and in both of them, 1:1 cocrystals are formed. The disorder 2·E is I/Br 85:15/15:85. However, the 2·F co-crystal shows I/Br 81:19 disorder, with the iodine atom halogen bonding to the pyridyl nitrogen, and the bromine atom is structurally inactive. Although the distribution of disorder in the structure of 2·F favors the hypothesis that iodine, and not bromine, will prevail in a competitive co-crystallization, we still needed more conclusive evidence. Co-crystallizations of Dissymmetric XB Donor Molecules. Our strategies for reducing the symmetry of the XB acceptor molecules were, up to this point, ineffective for solving the disorder problem. Therefore, we synthesized four new lesssymmetric, ester-bridged, XB donor molecules 3−6 bearing at least two XB donor groups of varying electrostatic charges (Supporting Information, Figure S2). However, all attempts to grow suitable single crystals from co-crystallization reactions of 3−6 with a wide range of XB acceptors only yielded oils. To overcome problems with soft/oily products, we decided to strengthen intermolecular interactions, thereby enhancing lattice energies with the hope of facilitating crystal growth. Therefore, we synthesized XB donor molecules with an amide linker 7 using an acid chloride and amine coupling reaction (Figure 8). Amides are known to form amide···amide selfcomplementary hydrogen bonds, and this would also ensure that we have no interference from the N−H donor competing with XB donors for the acceptor site.22 The crystal structure of 7 by itself shows that CO···H−N hydrogen bonds create chains with no disorder in I and Br atoms. The strategy of lowering the symmetry seems to work in this case for resolving positional disorder. However, we were unable to obtain single crystals from co-crystallization reactions of 7 with any XB acceptors. Therefore, we synthesized another amide-based XB donor 8, bearing both activated halogen atoms (I and Br), which should aid in forming halogen bonds. We coupled 4-bromotetrafluoroaniline with 4-iodotetrafluorocarboxylic acid chloride to obtain 8. Because the NH group is directly connected to the phenyl ring bearing the bromo donor, it should lower the charge (due to electron-donating NH lone pair) and the presence of CO on the phenyl ring of the iodine donor will enhance the electrostatic charge on the iodine atom. DFT/B3LYP calculations on 8 confirm that the charge

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CONCLUSIONS To obtain structural data that unambiguously could test our hypothesis regarding halogen-bond preferences, we had to solve a problem with positional disorder by engineering crystal structures that would be less likely to be afflicted with disorder. After several attempts involving lowering the molecular symmetry of XB donors and acceptors as well as introducing strong and directional hydrogen bonds to support crystallization, we were eventually able to obtain three nondisordered crystal structures of co-crystals comprising dissymmetric ditopic halogen-bond donors with a variety of ditopic XB acceptors. In two of these cases, the best XB donor (using an electrostatically D



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Zimmermann, M. O.; Lange, A.; Joerger, A. C.; Boeckler, F. M. J. Med. Chem. 2013, 56, 1363. (7) (a) George, S.; Nangia, A.; Lam, C.-K.; Mak, T. C. W.; Nicoud, J.F. Chem. Commun. 2004, 1202. (b) Saha, B. K.; Nangia, A.; Jaskólski, M. CrystEngComm. 2005, 7, 355. (c) Reddy, L. S.; Chandran, S. K.; George, S.; Babu, N. J.; Nangia, A. Cryst. Growth. Des. 2007, 12, 2675. (d) Aakeröy, C. B.; Desper, J.; Helfrich, B. A.; Metrangolo, P.; Pilati, T.; Resnati, G.; Stevenazzi, A. Chem. Commun. 2007, 4236. (e) Aakeröy, C. B.; Schultheiss, N.; Rajbanshi, A.; Desper, J.; Moore, C. Cryst. Growth Des. 2009, 9, 432. (f) Aakeröy, C. B.; Chopade, P. D.; Ganser, C.; Desper, J. Chem. Commun. 2011, 47, 4688. (g) Aakeröy, C. B.; Chopade, P. D.; Desper, J. Cryst. Growth Des. 2011, 11, 5333. (8) Clark, T. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2013, 3, 13. (9) Legon, A. C. Angew. Chem., Int. Ed. 1999, 38, 2686. (10) Präsang, C.; Whitwood, A. C.; Bruce, D. W. Cryst. Growth Des. 2009, 9, 5319. (11) A CSD analysis shows 62 co-crystals containing halogen bonds between 1,4-diiodotetrafluorobenzene and pyridine derivatives, and 13 co-crystals for 1,4-dibromotetrafluorobenzene and pyridine derivatives. (12) (a) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol. Model. 2007, 13, 291. (b) Sarwar, M. G.; Dragisic, B.; Salsberg, L. J.; Gouliaras, C.; Taylor, M. S. J. Am. Chem. Soc. 2010, 132, 1646. (13) MEP surface was constructed using Spartan ’08 (Wavefunction, Inc., Irvine, CA). All the molecules in this paper are optimized using the DFT/B3LYP level of theory with a 6-311+G** basis set, with the maxima and minima in the electrostatic potential surface (0.002 e au−1 isosurface) determined using a positive point charge in the vacuum as a probe. The charges bear the units of potential as they are calculated based on the maxima and minima of a calculated molecular electrostatic potential surface, which represents the points of highest and lowest charge on the molecule. (14) (a) Olejniczak, A.; Katrusiak, A.; Vij, A. CrystEngComm 2009, 11, 1073. (b) Olejniczak, A.; Katrusiak, A.; Vij, A. CrystEngComm 2009, 11, 1240. (15) Nayak, S. K.; Kishore Reddy, M.; Chopra, D.; Guru Row, T. N. CrystEngComm 2012, 14, 200. (16) CSD REFCODES: JOCMIY, RAZZOI, and XIKKOS. (17) CSD REFCODES: IHATIU, LATWIN, and NULVOG. (18) CSD REFCODE: EBOBEC. (19) Roper, L. C.; Präsang, C.; Whitwood, A. C.; Bruce, D. W. CrystEngComm 2010, 12, 3382. (20) Co-crystals of N-oxides with carboxylic acids, phenols, and amides are prevalent. Moreover, a halogen-bonded co-crystal of an Noxide derivative with DITFB has been reported by Resnati and coworkers; see: Messina, M. T.; Metrangolo, P.; Panzeri, W.; Pilati, T.; Resnati, G. Tetrahedron 2001, 57, 8543. (21) For examples of disorder in 1,4-thiomorpholine and 1,4thioxane, see: (a) Cincic, D.; Friscic, T.; Jones, W. J. Am. Chem. Soc. 2008, 130, 7524. (b) Cincic, D.; Friscic, T.; Jones, W. Chem.Eur. J. 2008, 14, 747. (22) (a) Aakeröy, C. B.; Scott, B. M. T.; Desper, J. New J. Chem. 2007, 31, 2044. (b) Lauher, J. W.; Fowler, F. W.; Goroff, N. S. Acc. Chem. Res. 2008, 41, 1215. (c) Aakeröy, C. B.; Forbes, S.; Desper, J. J. Am. Chem. Soc. 2009, 131, 17048. (23) The halogen-bonded co-crystals of bipyethylene with DITFB and DBTFB are reported (CSD codes: QIHCAL and IKUHUR, respectively). The C−I···N distance in QIHCAL is 2.811 Å, and the angle is 179.29°. The C−Br···N distance in IKUHUR is 2.814 Å, and the angle is 179.11°. (24) CSD survey for C−I···N short contact shows an average distance of 2.90 Å and an angle of 171.95° as compared to C−Br···N (3.12 Å distance and 167.79° angle). (25) The halogen-bonded co-crystals of bipyethane with DITFB and DBTFB are reported (CSD codes: MEKWOO and QIHDAM, respectively). The C−I···N distance in MEKWOO is 2.811Å, and the angle is 175.84°. The C−Br···N distance in QIHDAM is 3.025 Å, and the angle is 172.26°.

based ranking) binds to the best XB acceptor, whereas the second-best XB donor is structurally inactive in the selfassembly process. In the third structures, both donor atoms interacted with separate N(pyridyl) moieties. This hierarchical halogen-bonding approach may provide useful guidelines in preparation of difficult to synthesize multimeric solid-state materials. Furthermore, to obtain precise structural information, we avoided positional disorder using a rational molecularstructure-based approach, which, in turn, may be applicable to numerous current challenges in applied solid-state and materials science.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, spectral data, crystallographic data, CCDC 923640−923650 (CIF), and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from NSF (CHE0957607) and to Victor Day (University of Kansas) for X-ray single-crystal data collection on 1-B.

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Establishing a Hierarchy of Halogen Bonding by Engineering Crystals

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