Recent Developments in Crystal Engineering - ACS Publications

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Recent Developments in Crystal Engineering Kumar Biradha,*,† Cheng-Yong Su,*,‡ and Jagadese J. Vittal*,#,§ †

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China # Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore § Department of Chemistry, Gyeongsang National University, Jinju 660-701, S. Korea Downloaded via OPEN UNIV OF HONG KONG on January 29, 2019 at 09:30:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: In recent years, Asian countries, especially China and India, are making significant progress in the field of crystal engineering. The recent surge of publications in this area from these countries in high impact journals is a tangible measure of this activity. An important milestone in this direction was the China-India-Singapore Symposium on Crystal Engineering recently held at the National University of Singapore. This symposium brought researchers working in this field to meet, discuss, present, and exchange their research work and has generated momentum to further accelerate the growth of this field. This perspective highlights the recent advances discussed by the researchers at this symposium in the fast growing field of crystal engineering.

’ INTRODUCTION Crystal engineering (CE) is an emerging area of research encompassing various domains of chemistry, physics, biology, materials science, engineering, and pharmaceuticals. The field has evolved from designing structures to designing properties. Even at this advanced stage with a plethora of successful design strategies and properties that can be engineered, several fundamental questions remain unanswered. For example, even the definition of the fundamental concept of hydrogen bonding continues to evolve and the importance of weaker interactions continues to be challenged.1 Crystal structure prediction given the chemical component(s) is not yet routine despite major advances in recent years. The “crystallization reaction” is subject to phenomena such as polymorphism, hydration, solvation, and co-crystal and can even be difficult to reproduce. The supramolecular synthon approach is now widely accepted in organic crystal engineering for designing new solids and analyzing existing structures. However, the introduction of the term synthon promotes more questions such as the robustness of a synthon, interference of a functional group in robust synthons, synthomorphism, and synthon mimicry.2 Crystal engineering with metal and organic building blocks deals with relatively strong coordination bonds in addition to all other weak interactions and is called inorganic crystal engineering (ICE).3 It can involve engineering discrete species [e.g., metal-organic polygons/polyhedra (MOPs) or metal-organic r 2011 American Chemical Society

containers/cages (MOCs)] to polymeric crystalline materials [e.g., coordination polymers (CPs) or metal-organic frameworks (MOFs)].4 While the field flourishes, several terminologies emerged in the field which are mostly originated based either on the nature of the structure or on the property of the materials. In order to avoid confusion, it is necessary to establish unanimous terminologies in this field to gradually unify conflicting terms which are often used inappropriately in the current literature. Discussion at this symposium demonstrated that advances in synthetic strategy and design methodology have undoubtedly motivated in-depth studies in functionality of crystalline solids, evoking convergence of two inherent aspects of CE, namely, structural engineering based on the building blocks (BBs) and functional engineering for targeting collective crystal properties. One of the approaches is to utilize large functionalized BBs as opposed to metal ions. As depicted in Figure 1, MOPs and MOCs of well-defined shapes and sizes are now recognized as supramolecular building blocks (SBBs)4a in the area of ICE.4c Similar to metal ions, these giant BBs also possess predesigned extension sites that serve as architectural blueprints, and the following strategies of crystal engineering have been developed: (a) direct linking of BBs with organic “struts” using coordination bonds; (b) 0D f 3D, 1D f 3D, and 2D f 3D dimensional Received: September 20, 2010 Revised: January 25, 2011 Published: February 24, 2011 875

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increase via weak noncovalent interactions such as hydrogen bonds, and (c) 1D f 3D and 2D f 3D dimensional increase via interpenetration or catenation.5 Recently, the first China-India-Singapore Symposium on Crystal Engineering held at the National University of Singapore provided an excellent forum and opportunities for the researchers from these three countries to showcase their research and exchange ideas. The focus of this perspective is to review the invited talks which were presented at this symposium by the top researchers from these countries working in the field of crystal engineering, crystal growth, and supramolecular chemistry. A wide range of topics was discussed that includes new directions in organic crystal engineering, fundamental theoretical aspects of supramolecular interactions, crystallization of biomaterials, polymorphism, co-crystals, design of organic crystals, covalent organic frameworks, porous organic frameworks, oxo clusters, rotaxanes, coordination polymers, supramolecular gels, solid-state dynamics and structural transformations, mechanical properties of pharmaceutics, sorption, optical and magnetic properties to applications in graphene and plastic solar cells.

packing and hence the crystal structures and bulk properties. Hence, an understanding of these weak interactions is important in CE. In this context, computational methods are an important tool in the study of intermolecular interactions. They complement experimental methods such as crystallography to advance our knowledge in molecular packing. Wong’s laboratory has been interested in theoretical investigations of the various supramolecular interactions that are present in simple organic systems.6 Wong presented the cooperative behavior of CH 3 3 3 π interactions in several hydrocarbon-benzene complexes using highlevel ab initio calculations at the CCSD(T)/aug-cc-pVTZ//MP2/ aug(d,p)-6-311G(d,p) level for small saturated hydrocarbons.6c He concluded his talk by asserting that because the non-negligible interaction energy of the multiple CH 3 3 3 π interactions and the fact that cycloalkyl, long-chain alkyl, and aromatic functional groups are almost ubiquitous in organic compounds and biomolecules, the CH 3 3 3 π interaction is even more important than one may have anticipated to understanding conformational behavior of organic molecules, molecular recognition, crystal engineering, protein structures, and hydrophobic effect. Traditionally, interaction mimicry was considered for estimating the relative importance of one type of hydrogen bond (weak) when compared to the other type of hydrogen bond (strong).7 Desiraju and co-workers highlighted the legitimacy of the CH 3 3 3 O hydrogen bond in mimicking strong N-H 3 3 3 O hydrogen bonds in the co-crystals of 5,5-diethylbarbituric acid (barbital) with urea and acetamide (Figure 2). The analyses of single crystal X-ray structures at variable temperatures revealed that the C-H 3 3 3 O bond in the barbital-acetamide co-crystal plays a similar chemical and structural role to the N-H 3 3 3 O bond in the barbital-urea co-crystal. More interestingly studies on the formation of ternary co-crystals of barbitalacetamide-urea highlighted the relative importance of both these interactions.8a The exploration of robustness of a particular interaction or a synthon is a recurring theme in the crystal engineering of organic molecules. In this regard, Desiraju’s laboratory has shown that the homologous nature of C-H 3 3 3 N interactions leads to the formation of similar 1D chains in the crystal structures of HCN, cyanoacetylene, 4-ethynylcyanobenzene, and 4-cyano-4-ethynylbiphenyl. Alternatively, these four crystal structures can be considered as structural homologues.8b The genuineness of C-H 3 3 3 F-C interactions and how their directional nature is similar to those of well-established hydrogen bonds were also addressed by Desiraju by studying the crystal

’ WEAK INTERACTIONS: C-H 3 3 3 O, C-H 3 3 3 F, AND HALOGEN 3 3 3 HALOGEN INTERACTIONS Although small in energy, weak interactions outnumber covalent bonds in crystals and they often govern molecular

Figure 1. Demonstration of design and synthetic strategies in ICE by means of different BBs and dimensional increasing fashions.

Figure 2. Schematic representations of barbital interactions with (a) urea and (b) acetamide; the motifs observed in the crystals structures of co-crystals of barbital with (c) urea, (d) acetamide, and (e) acetamide and urea. 876

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Figure 3. (a) Space filling mode of alloxan molecule; similar molecular chains observed in the crystal structures of (b) fluorobenzene and (c) alloxan.

Figure 4. Crystal packing of 4-fluoro-N-(4-fluorophenyl)benzamide. Note the N-H 3 3 3 O, C-H 3 3 3 O, C-H 3 3 3 F, and F 3 3 3 F interactions between the molecules.

The gas sorption properties of MOF materials have been extensively explored, while organic materials are not as widely studied.10 Biradha discussed the use of halogen 3 3 3 halogen interactions in combination with N-H 3 3 3 O hydrogen bonds to design organic porous networks.11a,b These solids are composed of 3-fold symmetrical molecules such as tris(4-halophenyl)benzene-1,3,5-tricarboxamide which form 3-fold triple helical columns via N-H 3 3 3 O hydrogen bonds. The halogen3 3 3 halogen interactions assemble these columns into 3D-networks such that continuous channels exist which are occupied by THF molecules (Figure 5). Interestingly iodo, bromo, and chloro derivatives exhibited isostructurality and gas absorption properties, while the fluoro derivative generates close packed crystal lattice via C-H 3 3 3 F interactions.

structures of various fluoro-substituted benzenes.8c Fluorobenzene was found to exhibit interesting supramolecular similarities with a complex molecule such as alloxan, which does not exhibit strong N-H 3 3 3 O hydrogen bonds in its crystal structure despite having several N-H and CdO groups. The tetragonal P41212 packing of fluorobenzene and alloxan defines a structure type which is governed by two interactions formed from intersecting chains of molecules (Figure 3). One of these interactions is a hydrogen bond that involves the 1,3 H-atoms. The other could be a dipolar interaction such as CdO 3 3 3 CdO (in alloxan) or weak hydrogen bonds such as C-H 3 3 3 π (fluorobenzene). In contrast, the molecule 1,2,3,5-tetrafluorobenzene, which has a shape and size similar to those that adopt the tetragonal fluorobenzene structure, does not adopt a tetragonal structure because of the chemical nonviability of the CF 3 3 3 π interactions that would be needed to mediate in the adoption of this structure. Chopra’s group approached the problem of establishing the authenticity of interactions containing organic fluorine by studying a series of isomeric fluorinated benzanilides (Figure 4). These structures were found to exhibit several robust synthons involving fluorine atoms. It was also found that even in the presence of strong N-H 3 3 3 O hydrogen bonds weak yet directional CH 3 3 3 F interactions cooperatively steer the packing of molecules in the crystal lattice.9

’ STRONG HYDROGEN BONDS: CO-CRYSTALS, HOST-GUEST COMPLEXES, POLYMORPHISM, AND SOLID-STATE REACTIONS Biradha also highlighted the functional group approach in crystal engineering. This centers on how the molecules containing multiple functional groups deviate from forming well-explored robust synthons. A simple solvent such as phenol can profoundly influence crystallization of trimesic acid (H3TMA) and pyromellitic acid (H4PMA).11c The crystallization of H3TMA in the presence of phenolic solvents was found to distort the well-known hexagonal network to a brick-wall network (Figure 6). Further, the addition of phenol to the crystallization reaction between H3TMA and bispyridine promoted cocrystal formation by preventing salt formation.11d,e Biradha also touched upon the solid state [2 þ 2] reactivity of molecules containing olefins which are unreactive in the solid state when pure but become reactive in their co-crystals or complexes due to the template effects of phluroglucinol or Ag(I) 3 3 3 Ag(I) interactions.11f,g In these studies, a double [2 þ 2] cycloaddition reaction of bis(4-pyridyl)-1,5-pentadiene was shown to occur via stepwise mechanism. In other words, the double bonds of the diene do not react simultaneously but sequentially (Figure 7). This mechanism was shown to be a single-crystal to single-crystal (SCSC) transformation by 1H NMR and UV-spectroscopy.11f 877



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Figure 5. Triple helical N-H 3 3 3 O hydrogen bonded columns (a) top view, (b) side view, and (c) assembling of columns via halogen 3 3 3 halogen interactions to form a porous network, and the channels were filled by disordered THF molecules.

Figure 6. (Left) Brick wall network of H3TMA via modified synthon of carboxylic acids; (right) square grid network of H4PMA with the inclusion of disordered phenol.

ethenzamide does not form polymorphs; indeed, it is very difficult to crystallize at all. However, the co-crystallization of ethenzamide with various carboxylic acids readily gave a family of co-crystals and their polymorphs. Saccharin, 3,5-dinitrobenzoic acid, and ethylmalonic acid form dimorphic co-crystals, while cocrystallization with another analgesic drug, gentisic acid, yields trimorphic co-crystals (Figure 10).14 All these co-crystalline solid forms were characterized by various analytical techniques including single-crystal X-ray diffraction. Analyses of the crystal structures revealed that supramolecular synthons in the polymorphic structures are similar, and the cause of polymorphism in these co-crystals is from minor conformational differences in the co-crystal constituents. The results presented here contradict previous claim that the co-crystallization may reduce the probability of exhibiting polymorphism.14d Although the example of carbamazepine and the statistics in CSD are in support of such claims, our view is that the multicomponent systems have more or equal probability to form polymorphs than single-component systems. This association is based on the fact that the multicomponent systems have more conformational freedom, association modes, and supramolecular synthons compared to singlecomponent systems. Norfloxacin is known to exhibit polymorphism and a varied degree of bioavailability from its anhydrate to hydrates.15a Sarma and co-workers who have studied the anomalous behavior of norfloxacin using differential scanning calorimetry (DSC), differential thermogravimetric (DTG), powder X-ray diffraction (PXRD), and single crystal X-ray diffraction (XRD) techniques. They found a new salt of norfloxacin with ethylenediaminetetraacetic acid (EDTA) that exhibits ∼100% more bioavailability than the anhydrous form of the API.15b

Figure 7. Illustration for stepwise [2 þ 2] dimerization of pentadiene3-one via hydrogen bonding template (blue clips).

Hydrothermal techniques have been routinely employed for the synthesis of MOFs and hybrid materials. Du’s group found that hydrothermal synthesis can be used to generate co-crystals which cannot be obtained by conventional crystallization procedures.12a Further, their results also indicate that polymorphic forms of co-crystals can be obtained via hydrothermal synthesis at different temperatures, in particular when the basic building blocks contain multiple binding sites (Figure 8).12b Baruah presented crystal engineering aspects of various hostguest complexes in which the host contains multiple carboxylic acids groups attached to naphthalene or phthalimide units.13 It was also demonstrated that the co-crystals of a dicarboxylic acid with pyridine exhibit two polymorphic forms as shown in Figure 9. Polymorphism in co-crystals is increasingly gaining interest because of the overwhelming interest in pharmaceutical cocrystals. Aitipamula reported polymorphic co-crystals of an analgesic drug called ethenzamide. It is interesting to note that 878



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Figure 8. Illustration of two different hydrogen bonding patterns in the polymorphic co-crystals.

Figure 9. Illustration for polymorphic forms of pyridine co-crystal of diacid.

’ CRYSTAL ENGINEERING STUDIES WITH NUCLEOBASES Supramolecular interactions involving nucleobases and their derivatives are of importance due to their relevance to biology. Swamy’s group studied interactions between such nucleobases by appending them with organophosphonate substrates.16 Their studies indicate that the phosphoryl (PdO) group, despite being a strong hydrogen-bond acceptor, does not engage itself in hydrogen bonding with the nucleobase residues in nonsolvated structures and hence does not disturb the interaction between the nucleobase moieties. Interaction of metal ions with nucleobases has implications in the understanding of nucleic acid structure and catalysis, medicinal action of metal-organic complexes at the nucleic acid level, and for the construction of coordination architectures. Verma’s research is focused toward developing catalytic nucleobase polymers, construction of metal-nucleobase frameworks to study coordination, and surface patterning.16 He described structures of silver-adenine(purine) coordination motifs containing N-coordinated silver-adenine duplexes and metallamacrocyclic tetramers17a and hexamers.17b The emergence of various metallacycles and evolution of complexities in a silver-adenine framework based on defined substitution patterns in the nucleobase skeleton was discussed in his presentation.

class of foldamers exhibits selective cation-binding, as revealed by the differential binding affinities toward alkaline metal cations (e.g., Liþ, Naþ, Kþ, Rbþ, and Csþ). Metal-directed self-assembly has been widely used to construct metal-organic structures. Yip’s laboratory has been interested in investigating how supramolecular interactions control the topology of coordination polymeric structures.19 He showed how the anion and the solvent used for crystallization influences Ag(PAnP)X (PAnP, a diphosphine containing a π-conjugated anthracene backbone; X = ClO4, OTf, PF6, BF4) which exists as a helical polymer or discrete metallacycles. The solution dynamics of the complexes show equilibrium between the metallacycles and the helix. The binuclear Au(I) complex [Au2(PAnP)Cl2] exhibits a U-shaped molecular conformation. Interestingly, reaction of the gold complex with substoichiometric amounts of AgSbF6 does not lead to precipitation of AgCl. Instead, a coordination polymer {[Au2(μ-PAnP)(Cl)2]2Ag}n[SbF6]n resulted which is sustained by AgI-AuI attractions. In another study,19c a series of binuclear Pt(II) and Au(I) anthracenyldiacetylide and tetracenyldiacetylide complexes were used as building blocks for coordination polymers and supramolecular gels. It was demonstrated that (Me3PAu)2-tetracenyldiacetylide can self-assemble into a honeycomb-like structure via d10-d10 aurophilic interactions. Wu’s laboratory is interested in the design of novel supramolecular architectures and their materials applications. These include template-directed synthesis of various architectures such as rotaxanes, catenanes, molecular necklaces, and supramolecular dendrimers, etc. In his talk, Wu discussed various methodologies to synthesize new supramolecular architectures including the following: (1) efficient synthesis of separable pseudo[n]rotaxanes by a “threading-followed-by precipitation” approach;20a (2) synthesis of a hetero[4]rotaxane by a “threading-stoppering-followed-by-clipping” approach;20b (3) synthesis of linear and rectangular rotaxanes by a “template-directed regioselective clipping” approach.20c

’ DISCRETE ENTITIES Shape-persistent aromatic foldamers with rigid, noncollapsible backbones have attracted a great deal of attention. Zeng discussed such shape-persistency by biasing the conformational preferences of aromatic oligoamides using inward intramolecular hydrogen bonds along the aromatic backbones as presented in Figure 11.18 They have also explored the tunability of macrocyclic inner cavities (e.g., effective cavity size, steric crowdedness, hydrophobicity, and cation-binding selectivity) in circularly folded aromatic pentamers. Interestingly, it was found that this 879



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Figure 10. Illustration for hydrogen bonding patterns in various co-crystal polymorphs of ethenzamide.

Figure 11. (a) Chemical structure of aromatic pentamers 1 and 2; top view of X-ray crystal structure of the complexes (b) Kþ@anionic 1 and (c) Csþ@anionic 2. Both anionic 1 and 2 were generated by deprotonating their hydroxyl groups using tetrabutylammonium hydroxide. Methoxy methyl groups in (b) and (c) that stay below the pentamer plane were removed for clarity.

Figure 12. (a) Schematic representative of high-nuclearity Ag(I) alkynyl clusters. X stands for a templating anion, and Y is a counterion. (b) Giant silver cluster containing Mo6O228- cores and Ag60 shell. Peripheral alkynyl ligands have been omitted for clarity.

’ ROLE OF CLUSTERS IN CRYSTAL ENGINEERING ICE by creating cluster building blocks (CBBs) is a fast moving topic. Yang and co-workers21 have developed some synthetic strategies to make oxo clusters containing transition metals (TM), lanthanides, and main group elements, which were in turn used to construct porous cluster-organic frameworks. Their strategies include structure-directing of the lacunary sites of polyoxometalate (POM) units, the synergistic directing between two or more lacunary fragments, the cooperation of

the structure-directing of the lacunary sites of POM units and the directing assembly of the organic ligands, as well as combination between the structure-directing and the decomposition of lacunary POM fragment. Strategies such as ligand inducement, synergistic coordination between two types of ligands, and combination of self-polymerization and ligands inducement have been utilized to synthesize high-nuclearity Ln-cluster organic frameworks, Ln-TM-cluster organic frameworks, and Ln-organogermanate cluster frameworks, respectively. Further, chiral transfer from the guest to the host and the synergistic 880



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Figure 13. Illustrations for (left) pyrazine complexes and (right) pyrazole complexes of POMs.

strategy between the chiral groups and the acentric borate clusters were applied to make the nonlinear optical materials such as metal borates, germanates, and borogermanates constructed from B-O, Ge-O, and B-Ge-O cluster units. Long presented recent work from his laboratory on the assembly of high-nuclearity 4f and 3d-4f metal clusters based on metalloligand and anion-template approaches, as well as their effort in the assembly of metal-organic framework on the basis of high nuclear 3d-4f metal clusters.22 Wang and co-workers23 are working on anion-templated synthesis of silver alkynyl clusters in terms of CBB control in crystal engineering. A facile template approach has been developed for the synthesis of high-nuclearity silver(I) alkynyl clusters with a general structure shown in Figure 12a, which afforded a variety of silver clusters isolated by using spherical chlorides,23a triangle carbonates,23b and tetrahedral chromates as templating agents.23c Furthermore, POMs of good affinity toward silver ions have been incorporated into silver alkynyl systems, leading to the formation of a nanosized peanut-like cluster (Figure 12b).23c The core-shell structure consists of Mo6O228- cores and a Ag60 shell. Electrochemical studies indicate that there is electronic communication between the core and the shell; that is, the silver shell works like an electronic relay, while POM cores function as a redox buffer to stabilize the cluster. Polyoxomolybdates (POM) are versatile inorganic materials built of nanosize CBBs of oxomolybdate units containing a few molybdenum atoms to several hundred. Ramanan discussed the supramolecular reactivity of metal pyrazole and metal pyrazine complexes (metal = Co, Ni, Cu, and Zn) with chromium-based Anderson-Evans clusters (Figure 13) and evaluated the structures from a crystal engineering perspective.24 The studies with pyrazine revealed that the ratio of metal and pyrazine influences supramolecular aggregation and hence the nucleation of Anderson-Evans cluster based solids with the incorporation of a metal pyrazine complex or a metal pyrazine coordination polymer. In the case of pyrazole complexes, all three complexes contain a unique oxalate bridged copper pyrazole moiety linking Anderson-Evans cluster into 1D chains.

Figure 14. Classification of porous crystals: Type-I porous crystals collapse on guest removal, Type-II porous crystals show permanent porosity, Type-III porous crystals are flexible and dynamic, while Type-IV porous crystals can sustain postsynthetic functionalization (modifiable positions: 1, metal/cluster sites; 2, organic sites, and 3, vacant space).

Porous crystals receive great attention due to their relevance to gas storage/separation and energy.10a An important trend is to explore framework flexibility and dynamic behaviors of porous materials.10b More interestingly, postsynthetic modification of the porous crystals retaining the framework structures is a newly emerging method in ICE.10c Tremendous progress is pushing the frontiers of the burgeoning field of porous crystal engineering (PCE), which spans many areas such as covalent-organic frameworks (COFs),10d metal-organic frameworks (MOFs), or porous coordination polymers (PCPs),10a hybrid porous solids (containing both organic and inorganic moieties linked by strong bonds),10b hydrogen-bonded porous crystals, etc. Studies in PCE are now expanding to other functionalities such as reactivity, catalysis, chirality, flexibility, sensing, etc., that exploit pores to interact with external stimuli and uptakes. In this context, Su proposed a new type of porous crystals, shown in Figure 14, as an extension to Kitagawa’s classification.10a Type-I porous crystals have nonpermanent porosity because of inseparable host-guest dependence, typical of charged frameworks with pores filled by counterions. On the other hand, type-II porous crystals possess stable and robust porosity toward guest removal, as exemplified by zeolite-like MOFs and COFs. Whereas type-III porous crystals display flexible and dynamic framework responsive to external stimuli,

’ POROSITY, FLEXIBILITY, DYNAMICS, AND FUNCTIONALITY Despite the progress made in crystal structure design, the challenge of functional engineering with desired physical and chemical properties for potential applications is still in its infancy.10 These issues were addressed in this symposium. 881



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Du and co-workers28 reported a 3D Cu(II) coordination framework which displays dual character by removing the included water clusters under different conditions (under kinetic and thermodynamic control), affording two anhydrous polymorphs and revealing both rigid and flexible features in a microporous system. In addition, coordinated water induced structural transformation of a Cu2þ coordination framework which exhibited reactivity in a [Cu2(OOCR)4] paddle-wheel unit. Destruction and reconstruction of the robust [Cu2(OOCR)4] structural motif can be finely controlled through addition and removal of aqua ligands around the metal centers in solid state, showing a drastic structural transformation between two distinct 2D and 3D coordination networks. Topological analyses simplifies complicated frameworks for better understanding and provides a blueprint for targeting particular packing arrangements and their associated properties. In this regard, a highly connected MOF of uninodal ncb net (9connected) was obtained by Chen’s group,29a and a uninodal bct (10-connected) net was observed by Bu and co-workers.29b Su and co-workers30 have succeeded in synthesizing of a series of intriguing Borromean networks from C3-symmetric tripodal ligands. A promising development in topological chemistry is associating topology with functionality such as chirality,31 porosity,30d and magnetism. Chen and co-workers32 demonstrated that an iron(III) acetate hybrid material with a star topology, an alternative to the spin-frustrated Kagome lattice, can also exhibit spin frustration and long-range magnetic order simultaneously. Moreover, interwoven MOFs are expected to display unusual gas sorption and selectivity.33 Nevertheless, topological treatment of the complicated frameworks should be always justified by proper selection of the molecular BBs which embed the assembly information into the nodes and connectivities. Oversimplification may result in loss of essential geometrical instructions of BBs, while deficient simplification may create “new” topology which is meaningless for crystal design and prediction. Banerjee34 highlighted his group’s recent work on developing methodologies for enhanced H2 storage and CO2 capture in MOFs by introducing open metal sites and functionalizing organic linkers. His group has been engaged in the design and synthesis of fluorinated MOFs (F-MOFs). These materials containing porous surfaces with exposed fluorine atoms show interesting H2-storage properties. He also highlighted the crystal engineering aspects of porous lightweight and environmentally friendly Mg- and Ca-based MOFs for selective CO2 capture. In the last part of his talk, Banerjee discussed the selective gas sorption studies of POM-based ionic crystals (Figure 15). This appears to be the first report on selective hydrogen sorption in POM based ionic crystals, and such selective H2 storage capacity could be efficiently utilized for H2/N2 separation. Qiu35 described his group’s recent work on design and synthesis of porous organic frameworks (POFs) including MOFs, COFs, and conjugated microporous polymers (CMPs) with well-defined and predictable networks. With the help of developed theoretical predictions, his team synthesized a porous aromatic framework, PAF-1 as shown in Figure 16, with unprecedented surface area (BET surface area, 5640 m2 g-1; Langmuir surface area, 7100 m2 g-1 by N2 sorption). The N2 sorption measurement is commensurate with the uptakes observed for gases such as H2 (7.0 wt % excess at 77 K/48 bar) and CO2 (1.3 g g-1 at 40 bar, 298 K). These impressive sorption characteristics are combined with exceptional thermal and

Type-IV porous crystals can be defined as postsynthetically modifiable porous solids which can maintain the framework integrity upon structural modifications but do not necessarily remain as single crystals. The emergence of Type-IV porous crystals is leading CE to a new stage of postsynthetic functionalization and means that CE may no longer require that every solid be predesigned. Functional engineering of porous crystals in this manner has several advantages: a variety of weak interactions besides coordination bonds are involved, which are the origins of crystal softness and dynamics; organic, inorganic, and coordination crystals encompass a wider scope to exploit and realize solidstate functionality. Chen’s laboratory25 is working on CE of functional CPs, especially microporous and dynamic CPs. Recently they obtained a multifunctional porous cuprous triazolate framework, known as MAF-2, which shows exceptional framework flexibility and sorption properties, for example, temperature-controllable sorption behavior, superior organic solvent/water and benzene/ cyclohexane selectivity, and multimode structural transformations.25a Later, they found that this dynamic porous crystal can be used for the storage and purification of acetylene (C2H2) over CO2 due to large C2H2 uptake (70 cm3 g-1) and high C2H2/ CO2 uptake ratio (3.7) at 298 K, 1 atm. The estimated storage capacity of C2H2 is such that one volume of MAF-2 could deliver 20 vol of C2H2, that is, 40 times higher than that of a gas cylinder working between practical limits of 1.0-1.5 atm.25b Furthermore, they reported a flexible 3D porous magnet [KCo7(OH)3(ip)6(H2O)4] 3 12H2O (ip = isophthalic acid) based on interesting trigonal-prismatic heptanuclear Co(II) clusters. Reversible SCSC transformation on the desorption/adsorption of guest molecules was observed to accompany reversible magnetic property changes. All aqua ligands bonded the Co2þ and Kþ centers were removed to offer two kinds of coordinatively unsaturated metal centers (UMCs). This represents a rare case type of postmodified porous CPs, leading to a heterometallic UMCfunctionalized pore surface having a sensing effect for coordinative, paramagnetic, small molecules such as O2 and NO.25c Su’s group26 is interested in solid-state dynamics of porous crystals. He described a series of dynamic behaviors effected by ligand exchange, guest exchange, desolvation, and framework flexibility. Such solid-state dynamics is usually triggered by external stimuli such as temperature and gas/guest uptake via solid-gas or solid-solution reactions.27 A special case26a is observed for a doubly interpenetrated Cd2þ grid network which exhibits complicated dynamics, involving ligand-exchange in the coordination sphere, guest-exchange in the cavity, Cd-O bonds cleavage/formation and CF3SO3- anion shifting. Robustness and dynamics were observed in a series of thermally stable porous hydrogen-bonded lanthanide crystals.26b Different hydrogen bonding motifs afforded two polymorphs which display distinct porous structures and gas adsorption behavior. Dynamic coordination change and framework shrinkage/expansion was observed in a 3D framework [Co2(PPCA)2(H2O)(V4O12)0.5] which contains both coordinated and lattice water molecules. Dehydration of these water molecules triggered reversible framework and coordination dynamics. Such a mechanical and vaporchromic response could lead to potential sensing materials.26c It is noteworthy that postsynthetic modification of porous crystals can be carried out through multiple approaches as summarized in Figure 14c: at the metal center via creating UMCs and ligandexchanges; at organic spacers by chemical reaction; at vacant space via guest-exchange or doping. 882



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Figure 15. POM-based ionic solids exhibiting selective H2 adsorption properties over N2.

doped in the CNTs. Further studies show that some simple metal salts such as cobalt and nickel acetates can also be used to prepare CNTs. Chirality is a important topic in CE due to its relevance to catalysis and recognition. Sun and co-workers31a constructed two homochiral enantiomers based on mixed ligands (1,3,5-tris(1imidazolyl)benzene and 1,4-benzenedicarboxylate) by decorating two interpenetrating (10,3)-b frameworks with homochiral left- or right-handed helical chains. The ligands display distortion and the complex shows ferroelectric and second harmonic generation (SHG) properties. Bu and co-workers31b synthesized a porous guest-free MOF, [Zn(dtp)] (dtp =2,3-di-1H-tetrazol5-ylpyrazine), which possesses a unique zeolite-like, etd-(8,3) topological framework with chiral open channels. This compound exhibits high thermal stability, permanent porosity, and gas-adsorption selectivity toward O2 and CO2 over N2. Molecular magnetism has stimulated considerable interest because of potential applications in information storage and quantum computing. By introduction of a organic carboxylate into a azido system, Bu and co-workers37 obtained a 3D azidocopper-isonicotinate complex which is a ferromagnet with a critical temperature of 6.5 K. This compound represents the first ferromagnet azido-carboxylate system. They further introduced a series of acid and trizaole ligands to generate more ferromagnets and antiferromagnets by fine-tuning the magnetic properties in a more controllable manner. They also constructed molecule magnetic compounds from other bridging ligands. For example, a 3D Cu2þ complex has been constructed from 5-(pyrimidin2-yl)tetrazolate and VO3- anions. Magnetic studies indicate that this compound is a ferrimagnet with a critical temperature of 10 K, representing rare homospin heterometallic ferrimagnets. Tong gave an overview on the coordination chemistry of five cyclohexane polycarboxylic acids, that is, 1,2- and 1,4-cyclohexane dicarboxylic acid, 1,2,4-cyclohexanetricarboxylic acid, 1,2,4,5-cyclohexane tetracarboxylic acid, and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid,38 and explored their utilization in materials science, especially as magnetic materials. Dastidar stressed the importance of the design aspects of supramolecular gels which are highly relevant to materials research due to their potential application as structure-directing agents, cosmetics, conservation of arts, sensors, electroptics/ photonics, catalysis, drug delivery, and biomedical applications.39 Two different crystal engineering approaches were discussed to establish a working hypothesis that addresses which target gelator molecule/system may be designed. It has been demonstrated

Figure 16. Upper: structure model of diamond and PAF-1. Lower: experimental and simulated N2 sorption isotherms of PAF-1 at 77 K (left) and high-pressure hydrogen sorption isotherms at 77 K (right).

hydrothermal stability. In fact, PAF-1 is thermally stable up to 520 K and its sorption properties were unaffected by boiling in water for 7 days. The success of PAF-1 shows that simple organic reactions such as the Yamamoto type Ullmann reaction can afford high surface area and excellent physicochemical features.

’ MAGNETISM, ORGANO GELS, MECHANICAL PROPERTIES, AND DEVICES Bai’s group36 has developed a novel route to carbon nanotube (CNTs) synthesis by the pyrolysis of a MOF [Ni3(btc)2 3 12H2O] (btc = benzene-1,3,5-tricarboxylate) in one end closed quartz tube (Figure 17) and the other end is open to air. Openended N atom doped CNTs with high specific surface area have also been prepared by direct pyrolysis of [Co(HTTG)(H2O)2]n (TTG = N,N0 ,N00 -1,3,5-triazine-2,4,6-triyltris-glycine). Tube diameter, length, and N concentration, as well as the proportion of two types of N doping of the products, can be controlled by adjusting the reaction temperature and time. Single-component CO2, CH4 adsorption experiments and ideal adsorbed solution theory (IAST) multicomponent adsorption simulations indicate that they are promising candidates for CO2/CH4 separation. The selectivity was found to increase with increasing the content of N 883



PERSPECTIVE

Figure 17. Schematic view of a synthetic route to CNTs by solid state pyrolysis of MOFs.

Figure 18. Various supramolecular synthons used in designing supramolecular gelators.

Figure 20. DPP-based conjugated polymer.

Examples of crystals with parallel layered structure having strong intralayer and weak interlayer interactions were shown to be of shearing type (Figure 19).41 It was also demonstrated that in bending type crystals the strength of intermolecular interactions in orthogonal directions will be significantly different, while comparable intermolecular interactions in three orthogonal directions exist in brittle crystals. The structure-mechanical property relationship in some polymorphic forms of a compound that exhibits distinct mechanical properties was also presented to emphasize the need for a structure-based approach to API formulation in the pharmaceutical industry. New initiatives for conversion of solar light into electricity with greater efficiency are needed to meet global demand for clean energy. In recent years, organic photovoltaics (OPV) represent an important component, and a low cost, scalable approach to producing renewable energy. Patil presented some background on organic photovoltaic devices. He discussed conjugated polymers based on diketopyrrolopyrrole (DPP) which has been recently developed in his laboratory as a promising candidate for use in optoelectronic applications, particularly in field-effect transistors (FETs) and organic photovoltaic cells (Figure 20).42 Graphene is a hot next-generation material for nanoelectronic devices.43 It consists of a single atomic sheet of conjugated sp2 carbon atoms with finite size unlike graphite. Loh provided an overview of graphene materials in device applications and highlighted the significance of improving the solubility of graphene in

Figure 19. Trimorphs of 6-chloro-2,4-dinitroaniline show distinct mechanical properties.

that while a 1-D hydrogen bonding network that forms supramolecular synthons is crucial for organo-gelation, those materials that are gels as lattice inclusion complexes are a good target for serving as gelators. Various organic salt based supramolecular synthons were exploited to generate a large number of organogelators, whereas mixed ligand based coordination polymers capable of occluding a large number of solvent molecules were shown to form metallogels with aqueous solvents (Figure 18). Functional materials that consist of interconnecting networks have become increasingly important in science and technology. The special crystal network structure of amino acids in spider silk leads to tensile strength several magnitudes higher than single chains of amino acids. Liu’s interests vary from the control of single crystals, such as size and shape of single crystals, to the engineering of crystal network.40 Liu presented the latest developments in the kinetics of fiber network formation, the correlation between the structures of macromolecular functional materials and their properties, including the engineering of nano phase and ultrafunctional materials. The importance of establishing structure--mechanical property correlation in materials science and engineering for successful design of materials was emphasized by Malla Reddy using some molecular crystals that exhibit shearing, bending, and brittle mechanical behavior under external mechanical stress. 884



PERSPECTIVE

order to increase its processability by incorporating functional groups which can promote different types of supramolecular interactions. These functional groups can further broaden the properties of the graphene through the formation of donoracceptor complex with graphene, which affords tuneability in electrical conductivity, optical and photovoltaic properties. Realizing the importance of graphene as a promising and versatile material for the future, this year’s Nobel prize in physics was awarded to Andre Geim and Konstantin Novoselov who made pioneering contributions in this area.

(2) (a) Desiraju, G. R. Angew. Chem. Int. 1995, 34, 2311. (b) Karfunkel, H.; Wilts, H.; Hao, Z.; Iqbal, A.; Mizuguchi, J.; Wu, Z. Acta Crystallogr. 1999, B55, 1075. (c) Desiraju, G. R. Cryst. Growth Des. 2008, 8, 5. (d) Biradha, K.; Rajput, L. Crystal Engineering with Molecules Containing Amide and Pyridine Functionalities, in Organic Crystal Engineering: Frontiers in Crystal Engineering; Tiekink, E. R.T.; Vittal, J. J.; Zaworotko, M. J., Eds.; John Wiley Publishers: New York, 2010. (3) (a) Biradha, K.; Ramanan, A.; Vittal, J. J. Cryst. Growth Des. 2009, 9, 2969. (b) Braga, D. J. Chem. Soc., Dalton Trans. 2000, 3705. (c) Chen, C.-L.; Kang, B.-S; Su, C.-Y. Aust. J. Chem. 2006, 59, 3. (d) Biradha, K.; Sarkar, M.; Rajput, L. Chem. Commun. 2006, 4169. (4) (a) Perry, J. J., IV; Perman, J. A.; Zaworotko, M. J. Chem. Soc. Rev. 2009, 38, 1400. (b) Zhang, Q.; Zhang, J. Y.; Yu, Q.-Y.; Pan, M.; Su, C.-Y. Cryst. Growth Des. 2010, 10, 4076. (c) Zhang, X.-L.; Guo, C.-P.; Yang, Q.-Y.; Lu, T.-B.; Tong, Y.-X.; Su, C.-Y. Chem. Mater. 2007, 19, 4630. (5) (a) Li, X.-P.; Zhang, J.-Y.; Pan, M.; Zheng, S.-R.; Liu, Y.; Su, C.-Y. Inorg. Chem. 2007, 46, 4617. (b) Li, X.-P.; Pan, M.; Zheng, S.-R.; Liu, Y.-R.; He, Q.-T.; Kang, B.-S.; Su, C.-Y. Cryst. Growth Des. 2007, 7, 2481. (c) Yang, Q.-Y.; Zheng, S.-R.; Yang, R.; Pan, M.; Cao, R.; Su, C.-Y. CrystEngComm 2009, 11, 680. (d) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; zur Loye, H.-C. Chem. Commun. 2004, 2158. (6) (a) Ran, J.; Wong, M. W. Aust. J. Chem. 2009, 62, 1062. (b) Wong, M. W.; Steudel, Y.; Steudel, R. Chem.—Eur. J. 2007, 13, 502. (c) Ran, J.; Wong, M. W. J. Phys. Chem. A 2006, 110, 9702. (7) (a) Desiraju, G. R. Chem. Commun. 2005, 2995. (b) Anthony, A.; Jaskolski, M.; Nangia, A.; Desiraju, G. R. Chem. Commun. 1998, 2537. (8) (a) Thakur, T. S.; Azim, Y.; Srinu, T.; Desiraju, G. R. Curr. Sci. 2010, 98, 25. (b) Mahapatra, S.; Azim, Y.; Desiraju, G. R. J. Mol. Struct. 2010, 976, 200. (c) Thakur, T. S.; Kirchner, M. T.; Bl€aser, D.; Boese, R.; Desiraju, G. R. CrystEngComm 2010, 12, 2079. (d) Ganguly, P.; Desiraju, G. R. CrystEngComm 2010, 12, 817–833. (9) Chopra, D.; Guru Row, T. N. CrystEngComm. 2008, 10, 54. (10) (a) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem., Int. Ed. 2004, 43, 2334. (b) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. (c) Wang, Z.; Cohen, S. M. Chem. Soc. Rev. 2009, 38, 1315. (d) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Côte, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268. (11) (a) Rajput, L.; Chernyshev, V. V.; Biradha, K. Chem. Commun. 2010, 46, 6530. (b) Rajput, L.; Biradha, K. Cryst. Growth Des. 2009, 9, 40. (c) Rajput, L; Jana, N; Biradha, K Cryst. Growth. Des. 2010, 10, 4565. (d) Santra, R.; Ghosh, N.; Biradha, K. New J. Chem. 2008, 1673. (e) Santra, R.; Biradha, K. Cryst. Growth Des. 2009, 9, 4969. (f) Santra, R.; Biradha, K. CrystEngComm 2008, 10, 1524. (g) Santra, R.; Biradha, K. Cryst. Growth Des. 2010, 10, 3315. (12) (a) Du, M.; Zhang, Z.-H.; Wang, X.-G.; Wu, H.-F.; Wang, Q. Cryst. Growth Des. 2006, 6, 1867. (b) Du, M.; Jiang, X.-J.; Tan, X.; Zhang, Z.-H.; Cai, H. CrystEngComm 2009, 11, 454. (13) (a) Singh, D.; Baruah, J. B. CrystEngComm 2009, 11, 2688. (b) Singh, W. M.; Baruah, J. B. Dalton Trans. 2009, 2352. (c) Mondal, P.; Karmakar, A.; Singh, W. M.; Baruah, J. B. CrystEngComm 2008, 10, 1550. (14) (a) Aitipamula, S.; Chow, P. S.; Tan, R. B. H. CrystEngComm 2009, 11, 889. (b) Aitipamula, S.; Chow, P. S.; Tan, R. B. H. Cryst. Growth Des. 2010, 10, 2229. (c) Aitipamula, S.; Chow, P. S.; Tan, R. B. H. € Zaworotko, M. J. CrystEngComm 2009, 11, 1823. (d) Almarsson, O.; Chem. Commun. 2004, 1889. (15) (a) Puigjaner, C.; Barbas, R.; Portell, A.; Font-Bardia, M.; Alcobe, X.; Prohens, R. Cryst. Growth Des. 2010, 10, 2948. (b) Sarma, J. A. R. P.; Nittala, R. V.; Panpalia, G. M. Unpublished results. (16) Swamy, K. C. K.; Balaraman, E.; Kumar, N. S. Tetrahedron 2006, 62, 10152. (17) (a) Purohit, C. S.; Verma, S. J. Am. Chem. Soc. 2006, 128, 400. (b) Kumar, J.; Verma, S. Inorg. Chem. 2009, 48, 6350. (c) Verma, S.; Mishra, A. K.; Kumar, J. Acc. Chem. Res. 2010, 46, 79. (d) Mishra, A. K.; Verma, S. Inorg. Chem. 2010, 49, 3691. (e) Kumar, J.; Verma, S. Chem. Commun. 2008, 2526. (f) Purohit, C. S.; Verma, S. J. Am. Chem. Soc. 2007, 129, 3488. (18) (a) Qin, B.; Chen, X. Y.; Fang, X.; Shu, Y. Y.; Yip, Y. K.; Yan, Y.; Pan, S. Y.; Ong, W. Q.; Ren, C. L.; Su, H. B.; Zeng, H. Q. Org. Lett. 2008,

’ CONCLUSIONS The work reviewed here describes presentations during this trilateral symposium and is by no means a comprehensive coverage of research done either by these speakers or all aspects of CE research that is going on in these countries. It may be obvious from this Perspective that the major research activity in India is in the area of OCE while research activities in ICE especially PCPs are dominated by China. This Perspective also highlights the complementary nature between India and China and stresses the need for collaborative efforts between the countries in the future. This marks the beginning of the paradigm shift in CE research in this region and a synergistic outcome to the beginning of scientific interactions among the Asian giants may be felt in the near future. Increase in funding opportunities for scientific research will further fuel the growth of CE research in this region. Because of interdisciplinary collaborations and technological advancements in instrumentations and computational power, CE is expected to progress toward device applications mainly addressing environmental and health issues as well as tackling future energy storage problems. Hence, life scientists, biomedical researchers, engineers, and technologists are expected to plunge into CE in the near future. ’ AUTHOR INFORMATION Corresponding Author

*(K.B.) Phone: 91-3222-283346. Fax: 91-3222-255303. E-mail: [email protected]; (C.Y.S.) Phone: þ86-20-84115178. Fax: þ86-20-84115178. E-mail: [email protected]. (J.J.V.) Fax: þ65-6779-1691. Tel: þ65-6516-2975. E-mail: [email protected].

’ ACKNOWLEDGMENT J.J.V. would like to thank the Department of Chemistry, Faculty of Science and India Research Initiatives at the National University of Singapore for their generous support to conduct this trilateral symposium. We would like to thank Prof. P. A. W. Dean, University of Western Ontario, and Prof. M. J. Zaworotko, University of South Florida, for going through the manuscript. K. B., J.J.V. and C.Y.S. thank DST of India, NNSF of China and World Class University (WCU) project through the Grant No. R32-2008-000-2003-0 (S. Korea) for the financial support respectively. ’ REFERENCES (1) (a) Desiraju, G. R. Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1998; (b) For a description of a very recent recommendation to IUPAC on a modern definition of the term “hydrogen bond”, see http://ipc.iisc. ernet.in/∼arunan/iupac/. 885


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PERSPECTIVE

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’ NOTE ADDED AFTER ASAP PUBLICATION This manuscript was originally published on February 24, 2011 with an error to reference 21. The corrected version was reposted on March 8, 2011.

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