Molecules to Crystals, Crystals to Molecules ... and Back Again

Molecules to Crystals, Crystals to Molecules ... and Back Again? Michael J. Zaworotko* Department of Chemistry, UniVersity of South Florida, CHE205, 4202 East Fowler AVenue, Tampa, Florida, 33620

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 4-9

ReceiVed NoVember 30, 2006

ABSTRACT: Self-assembly of more than one molecular component, modular self-assembly, is a particularly attractive approach to synthesizing new crystalline compounds, because it can be accomplished in one-pot reactions with existing molecular components and it allows for facile fine-tuning of structural and functional features. The challenges and opportunities that face crystal engineering are illustrated by concentrating on the structural diversity that has been exhibited in two classes of compounds that exemplify these opportunities: metal-organic nets and molecular co-crystals. 1. Molecules to Crystals One of the continuing scandals in the physical sciences is that it remains in general impossible to predict the structure of eVen the simplest crystalline solids from a knowledge of their chemical composition.10 My interest and involvement in crystal engineering arose because of a “eureka moment” that occurred in 1991 when I read James D. Wuest’s paper titled “Use of Hydrogen Bonds to Control Molecular Aggregation. Self-Assembly of ThreeDimensional Networks with Large Chambers”.1 This paper, the beautiful structure it described, and the references cited therein caused me to reassess my research program, which was then focused on the design and synthesis of novel ionic liquids.2 This reassessment caused an overnight redirection of my research program toward the then-emerging field of crystal engineering. Such a change in direction might appear counterintuitive until one realizes that, from a supramolecular perspective, crystalline networks are effectively the antithesis of ionic liquids. It now seems clear that 1988-1991 were the watershed years in enabling the dream3,4 of crystal engineering to accelerate its journey toward fruition. During these years, the independent efforts of Desiraju,5 Ermer,6 Etter,7 Fujita,8 Robson,9 Wuest,1 and others set the stage for what has transpired over the subsequent 15 years. Indeed, it is fair to assert that crystal design from first principles was about to cease being the oxymoron suggested by John Maddox almost as soon as he made his controversial assertion in 1988.10 In 1991, little could I, even in my most optimistic mode, have envisaged how the concept of crystal engineering would expand and advance so dramatically and afford novel materials that could have great practical or commercial value such as metal-organic frameworks or MOFs,11 pharmaceutical co-crystals,12 and appropriately, given Schmidt’s focus upon solid-state synthesis, chemically reactive crystals.13 In this contribution, I highlight my two “pet projects” in crystal engineering, coordination polymers and co-crystals, and emphasize how they are inextricably intertwined with each other because of their reliance on geometric principles and modular approaches to self-assembly. Coordination Polymers. Figure 1 illustrates by one simple measure how research activity in coordination polymers has grown exponentially since 1990. The first generation of coordination polymers is exemplified by compounds in which transition metal moieties serve as nodes that are connected by * E-mail: [email protected].

Michael John Zaworotko was born in South Wales in August 1956 and attended Bedwellty Grammar School and Imperial College, where he received a B.Sc. (Hons) and ARCS in 1977. His doctoral research at the University of Alabama focused on organoaluminum chemistry under the supervision of Prof. Jerry L. Atwood and the Ph.D. was granted in 1982. This was followed by postdoctoral research in structural and organometallic chemistry with Prof. S. R. Stobart at University of Victoria, Canada. He joined the faculty at Saint Mary’s University, Halifax, Canada, in 1985, where he remained until 1998. His interest in crystal engineering was developed in 1991−1992 during a sabbatical leave at the Frank J. Seiler Research Laboratory at the United States Air Force Academy in Colorado Springs, CO. He moved to the University of Winnipeg, Canada, in 1998, where he served as Dean of Arts and Science and to the University of South Florida, Tampa, FL, in September 1999, where he currently serves as Chairperson of the Department of Chemistry. His research interests continue to focus on two classes of modular solids that can be designed using supramolecular concepts: coordination polymers and co-crystals. Particular emphasis is now being placed on the following: expanding even further into the nanoscale domain by concentrating on coordination polymer networks sustained by synthetic supermolecules, and improving the performance of active pharmaceutical ingredients by exploring their ability to exist as novel co-crystal forms.

multifunctional ligands such as 4,4′-bipyridine (bipy). Bipy and other simple ligands, such as carboxylates, have served effectively as nanometer-scale spacers.14 Indeed, bipy was affectionately called the “carbon-carbon bond of crystal engineering” by one of my former students, Len MacGillivray. In the mid1990s, we and others were able to exploit bipy’s ability to propagate the inherent coordination geometries exhibited by metal centers in order to generate prototypal examples of octahedral,15 diamondoid,16 square grid,17 ladder,18 and bilayer19 nets (Figure 2). The implied modularity of such nets facilitated their use as blueprints, and the schematic format used in Figure 2 emphasizes both the simplicity and the generality of the modular approach. Numerous examples of such structures have since been reported from octahedral (octahedral, square grid, ladder, bilayer), tetrahedral (diamondoid), or square planar (square grid) metal moieties and a broader range of spacer ligands. Figure 2 also implies the open nature of these structures, which has

10.1021/cg0680172 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006

Perspective

Figure 1. Occurrence of the term “coordination polymers” in titles and abstracts of papers published between 1990 and 2005 (SciFinder search 11/2006).

Figure 2. Schematic representations of some of the nets exhibited by “first-generation” coordination polymers, i.e., nets sustained by transition metal moieties (nodes) and the appropriate stoichiometric ratio of linear bifunctional ligands such as 4,4′-bipyridine.

subsequently been demonstrated in spectacular fashion through the permanent porosity exhibited by “second generation” MOFs that are based on secondary building units (SBUs).11,20 Co-crystals. There are those who might see crystal engineering as being two-faceted with different philosophies and goals, i.e., coordination polymers (inorganic chemistry) and molecular organic crystals (organic chemistry). However, in my opinion,21 there is a continuum that facilitates a synergy that cuts across such chemical silos. Indeed, the result of our very first crystal engineering experiment22 helps to illustrate such a perspective, because it involved the generation of a hydrogen-bonded diamondoid network from an organometallic building block, the tetramanganese cubane, [Mn(CO)3(µ3-OH)]4. The organometallic moiety is able to express its tetrahedral symmetry via hydrogen bonding to 2 equiv of 1,2-ethylenediamine (Figure 3). This compound was prototypal for a range of co-crystals that illustrated the modularity of co-crystals.23 The term “co-crystal” as we employ it herein was first coined in the context of complexes between nucleic bases24 and subsequently popularized by Etter.7 However, the definition of co-crystal remains a matter of debate.25,26 In the Zaworotko research group, we have been using the following operating definition: A co-crystal is a multiple component crystal in which all components are solid under ambient conditions when in their

Crystal Growth & Design, Vol. 7, No. 1, 2007 5

pure form. These components consist of a target molecule or ion and a molecular co-crystal former(s); when in a co-crystal, they coexist at the molecular level within a single crystal.12 That all components are solid under ambient conditions has important practical considerations, because synthesis of co-crystals can be achieved via the solid-state.27 Furthermore, chemists can execute a certain degree of control over the composition of a co-crystal, because they can invoke molecular recognition, especially hydrogen bonding, during the selection of co-crystal formers.12,28,29 These features distinguish co-crystals from another broad group of multiple component compounds, solvates, and it should be noted that most molecular compounds are amenable to formation of such co-crystals, as they exist as solids under ambient conditions.30 If one uses the criterion that all components of a co-crystal must be solid under ambient condition, then co-crystals represent a long-known class of compounds; a prototypal example is quinhydrone, which was reported at least as early as 1844 by Wo¨hler31 and was studied during the 1890s.32 Co-crystals were subsequently described as organic molecular compounds.33 However, structural information on co-crystals was largely absent until the 1960s, when the term complexes was coined, primarily in the context of molecular recognition between nucleic bases.34 Co-crystals are relatively lightly studied, as less than 0.5% of molecular solids that have been structurally characterized would be regarded as being co-crystals. However, as Figure 4 reveals, there is growing interest in the subject. Thus, it would be fair to state that co-crystals represent a long-known but little-studied class of compounds. However, there is every reason to assume that co-crystals will play an ever increasing role in pharmaceutical science and elsewhere: • That all components of a co-crystal are solid under ambient conditions has important practical considerations allowing for co-crystals to be prepared without the use of solvent.28 • There are many crystal forms possible for molecules that can form co-crystals, because there are many co-crystal formers possible for a given molecule. • Molecules are not modified from a covalent perspective when they are present in a co-crystal, spawning interest in an increasingly important subset of co-crystals, pharmaceutical cocrystals, i.e. co-crystals in which the target molecule or ion is an active pharmaceutical ingredient, API.12 It is anticipated that pharmaceutical co-crystals will afford a diverse range of new forms of APIs with improved physical properties such as solubility, stability, hygroscopicity, and dissolution rate.35 2. Crystals to Molecules The number, type, sophistication, and beauty of synthetic supramolecular entities are limited only by the boundless imagination of chemists and the practical considerations of the currently aVailable techniques for the separation, analysis and proper characterization of the resulting product.36 Nanoscale Polyhedra. The geometric and self-assembly principles that have been so successfully applied to building coordination polymer nets and co-crystals from first principles have also been exploited to generate aesthetically pleasing molecules or ions that are unprecedented in terms of their scale, at least from the perspective of a chemist. Particularly striking examples of novel high-molecular-weight compounds are exemplified by spheroid architectures that are based on regular (Platonic, Figure 2a37) and semiregular (Archimedean, Figure 2b38) solids; there are now numerous examples of such

6 Crystal Growth & Design, Vol. 7, No. 1, 2007

Perspective

Figure 3. Adamantoid portions of the hydrogen-bonded diamondoid networks formed between an organometallic cubane [Mn(CO)3(µ3-OH)]4 and 1,2-ethylenediamine (left) and 4,4′-bipyridine (right).

Figure 5. Early examples of Platonic (a, tetrahedron sustained by Mgmalonate moieties) and Archimedean (b, snub cube sustained by hydrogen bonds between 6 resorcinarenes) nanoscale polyhedral structures Figure 4. Occurrence of the term “co-crystal” in titles and abstracts of papers published between 1990 and 2005 (SciFinder search 11/2006).

structures.39 There also exists a class of polyhedron that is closely related to Platonic and Archimedean solids, but differ as they are built by the direct linking of the vertices of polygons rather than the edges. Such structures can be termed partially open uniform polyhedra or faceted polyhedra,40 because they necessarily contain both open and closed faces. The first molecular examples of such polyhedra were based on the dicopper tetracarboxylate secondary building units (SBUs) linked by angular 1,3-benzenedicarboxylate (bdc) anions and were reported in 2001.41,42 Such structures, which we have termed “nanoballs”,41 can be viewed as if they are based on the small rhombihexahedron (Figure 6) and possess the following attractive features: they are facile to prepare from ubiquitous, inexpensive, and air/water stable molecular squares such as dimetal tetracarboxylate SBUs; they require a ligand that connects the vertices of the molecular squares at 120°, an angle subtended by 1,3-functionalized aromatics such as bdc; they are inherently modular in nature; they contain multiple sites for decoration; and even with bdc, they contain nanoscale cavities and channels. Subsequent studies have demonstrated that the prototypal nanoball is indeed modular, because its exterior surface can be substituted and cavities are sufficiently large enough to bind aromatic guest molecules. We have demonstrated that hydroxylated nanoballs form polymer-stable composites with hydroxylated biocompatible polymers,43 and others have shown that nanoballs can self-assemble on graphite surfaces.44

Figure 6. Small rhombihexahedron (left) and its prototypal molecular equivalent (right).

Co-crystal-Controlled Solid-State Synthesis. An attractive feature of co-crystals is that they can be prepared without solvent. Indeed, the prototypal co-crystal quinhydrone was first prepared in such a manner,31 which begs the question, can we use co-crystals to avoid use of solvent and, in principle at least, achieve 100% yield of reaction product with no waste? Whereas solid-state organic synthesis represents a well-established area of research,45 co-crystal-controlled solid-state synthesis is currently limited to photodimerizations or photopolymerizations13 and nucleophilic substitution.46 In short, just as co-crystals are long-known but little-explored, the same can be said about cocrystal-controlled solid-state synthesis. In the case of the photodimerization and photopolymerization, one co-crystal

Perspective

Crystal Growth & Design, Vol. 7, No. 1, 2007 7

Figure 7. The conversion via heating of a yellow mixture of amine and anhydride substrates to a purple co-crystal (130 °C) which in turn converts to the corresponding condensation product at 160 °C.

former typically serves to align or “template” the reactant, which is the other co-crystal former. In the case of the latter, both co-crystal formers are reactants, although there are examples in which the reactive moieties are in the same molecule and therefore generate polymeric structures.47 We are currently investigating whether co-crystals might offer even broader potential in the context of green chemistry, especially for condensation reactions. For example, imides have been long studied for their ability to form via solution methods.48 Imides are also under the spotlight for their biological activity: Thalidomide is once again being exploited therapeutically,49 and Virstatin shows great promise as an antiviral agent.50 Imides are routinely formed via solution methods, but they have also been afforded in the solid state by utilizing microwave irradiation.51 We have recently demonstrated that it is possible to expand the range of synthesis methods for imides by exploiting co-crystals. Co-crystals sustained by the amine-acid anhydride supramolecular heterosynthon have been prepared via grinding, solvent drop grinding, or if a co-crystal former is low melting, melting/cooling. As revealed by Figure 7, pale yellow substrates convert to a deeply colored co-crystal, which in turn converts to the orange condensation product simply by heating. Such an approach, which is inherently modular, is well-suited to combinatorial strategies for the synthesis of wide ranges of condensation products, many of which do not yet exist according to SciFinder. It is also conducive to the preparation of compounds that tend to exhibit low solubility such as polyacids. 3. ... And Back Again? What would the properties of materials be if we could really arrange the atoms the way we want them? ... I can hardly doubt that when we haVe some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can haVe, and of different things that we can do. Richard P. Feynman, Dec 29, 1959 So what lies ahead of us? Crystal engineering is anything but a mature paradigm. Indeed, Maddox’s comment about crystal structure prediction remains largely true today despite considerable effort and progress.52 However, as the result of several decades of focused research, crystal engineering has to a certain extent addressed Feynman’s vision, especially in the context of metal-organic (coordination polymer) and organic network structures. Although the term crystal engineering was introduced in 19553 and enabled in 1971,4 it seems evident that solid-state chemists are only now finally positioned to address Feynman’s vision in a more comprehensive manner so as to develop materials that are perhaps revolutionary and fine-tunable rather than evolutionary.

Figure 8. The bcc network formed by nanoballs with sulfonated exteriors. Cover art Cryst. Growth Des., Vol. 4.

Figure 9. Crystal packing of 1D chains of self-assembled nanoballs (left) might be expected based upon comparison with wine bottles in the world’s largest collection of wine (Chisinau, Moldova, right).

One such situation is exemplified by the possibility of using nanoscale polyhedral molecules as nanoscale SBUs, nSBUs, for the generation of nets in which the basic building block is a nanostructure rather than a metal ion or SBU. Indeed, the metal-organic nanoballs described herein appear to be ideally suited for exploitation as nSBUs: they are predisposed toward further decoration at either the axial metal sites or the bdc ligand; they possess Oh point group symmetry, which means that they could in principle sustain high-symmetry infinite networks. Sulfonated anionic nanoballs, {[Cu2(5-SO3-bdc)2(4-methoxypyridine)0.50(MeOH)x (H2O)1.50-x]12}24-, contain 24 sulfonate moieties that are exposed at the exterior of the nanoball. They are therefore predisposed for coordination to Cu(II), and in the presence of excess copper (II) nitrate, 16 sulfonate moieties bond to 16 [Cu(methoxypyridine)4]2+ cations that facilitate crosslinking via axial coordination to a sulfonate moiety that lies on an adjacent nanoball. In total, each nanoball cross-links to eight other nanoballs to generate an extended bcc network (Figure 8).53 The symmetry of nSBUs can be further reduced to as low as 1D when methoxylated nanoballs are crystallized and selfassemble in a head-to-tail fashion.54 Figure 9 illustrates how the overall crystal packing can be rationalized on the basis of an analogy with the macro world. Indeed, it is not hard to see other examples of blueprints from the world around us that have been successfully reproduced at the molecular level. Buckminsterfullerene54 and more exotic examples of 2D tiling patterns55 immediately come to mind as such examples, as do mineralomimetic architectures.56

8 Crystal Growth & Design, Vol. 7, No. 1, 2007

Co-crystals also represent a significant opportunity in terms of fundamental knowledge. As noted by Lehn in 1995 “Molecular chemistry, thus, has established its power over the covalent bond. The time has come to do the same for noncovalent intermolecular forces.”57 Co-crystals are particularly well-suited to address the hierarchy of noncovalent bonding for the following reasons: a detailed understanding of the supramolecular chemistry of the functional groups present in a given molecule is a prerequisite for designing a co-crystal and selection of appropriate co-crystal formers; the very existence of cocrystals is dependent on noncovalent bonding between co-crystal formers. Unfortunately, when multiple functional groups are present in a molecule, the Cambridge Structural Database58 rarely contains enough information to address the hierarchy of the possible supramolecular synthons. Furthermore, the role of solvent in the nucleation of crystals and co-crystals remains poorly understood, yet solvent can be critical in obtaining a particular co-crystal. Our research program is addressing the hierarchy of the supramolecular synthons that can occur for a range of common functional groups such as carboxylic acids, amides, and alcohols, with emphasis on supramolecular heterosynthons, i.e., noncovalent bonding between different but complementary functional groups.59 It is becoming evident that supramolecular heterosynthons are key to implementing a broadly based design strategy for co-crystals in which a target molecule forms co-crystals with a series of co-crystal formers that are carefully selected for their ability to form supramolecular heterosynthons with the target molecule. However, such noncovalent interactions between molecules are not just important in the context of crystalline solids. They are also critically important in gaining a better understanding and control over solution- (e.g., drug-protein interactions, crystal nucleation), surface- (e.g., sensors), and gas-phase (e.g., air pollution) phenomena that are of profound importance. To summarize, coordination polymers and co-crystals represent archetypal compounds that have played a significant role in the evolution of crystal engineering, but there is every reason to believe that they will continue to offer scientific challenges of the highest order, e.g., crystal structure prediction and related issues such as polymorphism, and opportunities, e.g., is there any longer an upper limit of scale for synthetic chemists? To paraphrase the oft-cited quote of McCrone,60 it now seems fair to assert that the number of forms known for certain classes of compounds is indeed proportional to the time and energy spent in research on that compound. References (1) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc. 1991, 113, 46964698. (2) Wilkes, J. S.; Zaworotko, M. J. Chem. Commun. 1992, 965-967. (3) Pepinsky, R. Phys. ReV. 1955, 100 (3), 971. (4) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647-678. (5) Desiraju, G. R. Crystal Engineering: the Design of Organic Solids; Elsevier: Amsterdam, 1989. (6) Ermer, O. J. Am. Chem. Soc. 1988, 110, 3747-3754. (7) Etter, M. C. J. Phys. Chem. 1991, 95, 4601-4610. (8) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem. Soc. 1990, 112, 56455647. (9) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 15461554. (10) Maddox, J. Nature 1988, 335, 201. (11) Eddaoudi, M.; Moler, D. B.; Li, H. L.; Chen, B. L.; Reineke, T. M.; O’Keefe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319-330. (12) Almarsson, O ¨ .; Zaworotko, M. J. Chem. Commun. 2004, 1889-1896. (13) (a) Gao, X. C.; Friscic, T.; MacGillivray, L. R. Angew. Chem., Int. Ed. 2004, 43, 232-236. (b) Tanaka, K.; Toda, F. Chem. ReV. 2000, 100, 1025. (c) Fowler, F. W.; Lauher, J. W. J. Phys. Org. Chem. 2000, 13, 850.

Perspective (14) (a) Biradha, K.; Sarkar, M.; Rajput, L. Chem. Commun. 2006, 41694179. (b) Janiak, C. Dalton Trans. 2003, 14, 2781-2804. (c) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Withersby, M. A.; Schroder, M. Coord. Chem. ReV. 1999, 183, 117-138. (d) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. 1998, 37, 1460-1494. (e) Batten, S. R.; Hoskins, B. F.; Moubaraki, B.; Murray, K. S.; Robson, R. Dalton Trans. 1999, 2977. (f) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511-522. (g) Evans, O. R.; Ngo, H. L.; Lin, W. J. Am. Chem. Soc. 2001, 123, 10395. (h) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762-1765. (i) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982. (j) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (15) Subramanian, S.; Zaworotko, M. J. Angew. Chem., Int. Ed. 1995, 34, 2127. (16) (a) MacGillivray, L. R.; Subramanian, S.; Zaworotko, M. J. Chem. Commun. 1994, 1325. (b) Zaworotko, M. J. Chem. Soc. ReV. 1994, 23, 283. (17) (a) Gable, R. W.; Hoskins, B. F.; Robson, R. Chem. Commun. 1990, 1677. (b) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151. (18) Hennigar, T. L.; MacQuarrie, D. C.; Losier, P.; Rogers, R. D.; Zaworotko, M. J. Angew. Chem. Intl. Ed. 1997, 36, 972-973. (19) (a) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Angew. Chem., Int. Ed. 1997, 36, 1725. (b) Power, K. N.; Hennigar, T. L.; Zaworotko, M. J. New J. Chem. 1998, 22, 177. (20) (a) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. (b) Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble, S.; Margiolaki, I. Science 2005, 309, 2040-2042. (c) Liu, Y. L.; Kravtsov, V. C.; Larsen, R.; Eddaoudi, M. Chem. Commun. 2006, 1488-1490. (21) Moulton, B.; Zaworotko, M. J. Chem. ReV. 2001, 101, 1629-1658. (22) Copp, S. B.; Subramanian, S.; Zaworotko, M. J. J. Am. Chem. Soc. 1992, 114, 8719-8720. (23) Copp, S. B.; Holman, K. T.; Sangster, J. O. S.; Subramanian, S.; Zaworotko, M. J. J. Chem. Soc. Dalton Trans. 1995, 22332243. (24) Schmidt, J.; Snipes, W. Int. J. Radiat. Biol. 1967, 13, 101-109. (25) Desiraju, G. R. Cryst. Eng. Commun. 2003, 5, 466-467. (26) Dunitz, J. D. Cryst. Eng. Commun. 2003, 5, 506. (27) Shan, N.; Toda, F.; Jones, W. Chem. Commun. 2002, 2372. (28) Aakero¨y, C. B.; Salmon, D. J. Cryst. Eng. Commun. 2005, 7, 439448. (29) Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J. J. Pharm. Sci. 2006, 95, 499. (30) Ulrich, J. Cryst. Growth Des. 2004, 4, 879-890. (31) Wo¨hler, F. Annalen 1844, 51, 153. (32) Ling, A. R.; Baker, J. K. J. Chem. Soc. Trans. 1893, 63, 13141327. (33) Anderson, J. S. Nature 1937, 140, 583-584. (34) Hoogsteen, K. Acta Crystallogr. 1963, 16, 907-916. (35) (a) Li, Z. J.; Abramov, Y.; Bordner, J.; Leonard, J.; Medek, A.; Trask, A. V. J. Am. Chem. Soc. 2006, 128, 8199-8210. (b) Remenar, J. F.; Morissette, S. L.; Peterson, M. L.; Moulton, B.; MacPhee, J. M.; Guzma´n, H. R.; Almarsson, O ¨ . J. Am. Chem. Soc. 2003, 25, 8456. (c) Childs, S. L.; Chyall, L. J.; Dunlap, J. T.; Smolenskaya, V. N.; Stahly, B. C.; Stahly, G. P. J. Am. Chem. Soc. 2004, 126, 13335. (d) Nehm, S. J.; Rodriguez-Spong, B.; Rodrigues-Hornedo, N. Cryst. Growth Des. 2006, 6, 592-600. (36) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502-518. (37) Saalfrank, R. W.; Stark, A.; Peters, K.; von Schnering, H. G. Angew. Chem., Int. Ed. 1988, 27, 851. (38) MacGillivray, L. R.; Atwood, J. L. Nature 1997, 389, 469-472. (39) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853-907. (b) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509-518. (c) Caulder, D. L.; Raymond, K. N. Acc. Chem. Res. 1999, 32, 975982. (d) MacGillivray, L. R.; Atwood, J. L. Angew. Chem., Int. Ed. 1999, 38, 1018-1033. (e) Hof, F.; Craig, S. L.; Nuckolls, C.; Rebek, J., Jr. Angew. Chem., Int. Ed. 2002, 41, 1488-1508. (40) Holden, A. Shapes, Space, and Symmetry; Columbia University Press: New York, 1971; p 94. (41) Moulton, B.; Lu, J.; Mondal, A.; Zaworotko, M. J. Chem. Commun. 2001, 863-864. (42) Eddaoudi, M.; Kim, J.; Wachter, J. B.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 4368-4369.

Perspective (43) (a) Abourahma, H.; Coleman, A. W.; Moulton, B.; Rather, B.; Shahgaldian, P.; Zaworotko, M. J. Chem. Commun. 2001, 23802381. (b) Mohomed, K.; Abourahma, H.; Zaworotko, M. J.; Harmon, J. P. Chem. Commun. 2005, 3277-3279. (44) Furukawa, H.; Kim, J.; Plass, K. E.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 8398-8399. (45) (a) Tanaka, K. SolVent-Free Organic Synthesis; Wiley: New York, 2003. (b) Kaupp, G. Top. Curr. Chem. 2005, 254, 95-183. (46) Etter, M. C.; Frankenbach, G. M.; Bernstein, J. Tetrahedron Lett. 1989, 30, 3617. (47) Foxman, B. M.; Sandor, R. B. Tetrahedron 2000, 56, 6805. (48) (a) Solomons, T. W. G.; Fryhle, C. B. Organic Chemistry, 8th ed.; John Wiley & Sons, Inc.: New York, 2004. (b) Bessemov, M. I.; Koton, M. M.; Kudryavtsev, V. V.; Laius, L. A. PolyimidesThermally Stable Polymers; Plenum: New York, 1987. (49) Corral, L. G.; Kaplan, G. Ann. Rheum. Dis. 1999, 58, 107-113. (50) Hung, D. T.; Shakhnovich, E. A.; Pierson, E.; Mekalanos, J. J. Science 2005, 310, 670-674. (51) (a) Vidal, T.; Petit, A.; Loupy, A.; Gedye, R. Tetrahedron 2000, 56, 5473-5478. (b) Martelanc, M.; Kranjc, K.; Polanc, S.; Kocevar, M. Green Chem. 2005, 7, 737-741.

Crystal Growth & Design, Vol. 7, No. 1, 2007 9 (52) (a) Gavezzotti, A. Acc. Chem. Res. 1994, 27, 309-314. (b) Dunitz, J. D. Chem. Commun. 2003, 545-548. (53) McManus, G. J.; Wang, Z.; Zaworotko, M. J. Cryst. Growth Des. 2004, 4, 11-13. (54) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (55) Moulton, B.; Lu, J. J.; Zaworotko, M. J. J. Am. Chem. Soc. 2001, 123, 9224-9225. (56) Wells, A. F. Structural Inorganic Chemistry; Oxford University Press: Oxford, U.K., 1975. (b) Wells, A. F. Three-Dimensional Nets and Polyhedra; Wiley: New York, 1977. (57) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995; p 2. (58) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380-388. (59) Fleischman, S.; Morales, L.; Moulton, B.; Rodrı´guez-Hornedo, N; Bailey Walsh, R.; Zaworotko, M. J. Chem. Commun. 2003, 186-187. (60) McCrone, W. C. In Physics and Chemistry of the Organic Solid State; Fox, D., Labes, M. M., Weissberger, A., Eds.; Interscience: NewYork, 1965; Vol II, pp 725-767.

CG0680172