Chemistry for Everyone
Designing Advanced Materials As Simple As Assembling Lego® Blocks!† C. V. Krishnamohan Sharma‡ Department of Chemistry, Texas A&M University, College Station, TX 77843-4355;
[email protected] Did you ever wonder whether one day the creativity you showed as a child in making architectures out of Lego blocks would come in handy for designing advanced materials? This may sound rather unreal at first, but as we shall see later the success of recent rational designing strategies for the synthesis of advanced materials will prove that careful self-assembly of molecular building blocks holds the key for a new generation of materials (1). Our quest for novel materials with useful properties Figure 1. A schematic representation of seven Lego blocks (six symmetric and one awkwardly shaped) has led to the rapid development and their possible network structures. Note that it is difficult to form well-defined patterns using of new interdisciplinary fields; awkwardly shaped building blocks. crystal engineering and combinatorial chemistry are conspicuous among them (2, 3). The basic approaches of these two robust metal–metal bonds and metal–ligand coordination fields for materials synthesis are quite different: while crystal bonds (7, 8). The weak intermolecular interactions may also engineering studies are based on structural information, comcontribute to the stabilization of inorganic complexes if organic binatorial chemistry automates the synthesis and screening moieties are a significant part of the structure. Therefore, of properties for a large group of compounds in order to idenunderstanding the nature of intermolecular forces and coortify the targeted materials at a faster pace. This article predination bonds constitutes a primary step in the process of sents an overview of crystal engineering strategies and their controlling the organization of molecules or ions in the solid role in the design of novel materials. state. It is well known that chemical and physical properties Molecules in the crystalline state are related through of materials are governed by the geometrical positions of certain symmetric operations (translation, rotation, glidemolecules and ions in the solid state. Therefore, it is possible plane, etc.) to give highly ordered periodic structures while to manipulate the properties of materials predictably by balancing the intermolecular interactions both attractive and controlling the arrangement of molecules in the solid state. repulsive in a chemically meaningful way. For all practical So now, the obvious question would be “how do we organize purposes crystal structures can be viewed as networks where molecules predictably in the solid state?” The answer is nonmolecules and ions can be considered as nodes and the intrivial and rather difficult. In fact, crystal engineering emerged termolecular interactions or coordinate bonds between the as an independent discipline in the process of answering molecules as node connections. The major advantage of dethis very question! fining crystal structures as networks is that we can simplify the complex crystalline structural features into easily identifiable network topologies based on chemical and structural Defining Crystal Structures as Networks information of the molecular building blocks (4–8). ThereThe molecules and ions in organic crystals attract each fore, the design of crystal structures is no longer a complex other through numerous medium-range isotropic interactions task, but a mere topological organization of molecules. (e.g., C…C, H…H, C…H), long-range electrostatic interactions (e.g., O–H…O᎑, N–H+…O᎑, O–H…O, N–H…O), Lego Blocks—Network Topology and weakly directional hydrogen bonds (e.g., C–H…O, O–H…π) (4–6 ). On the other hand, metals and ligands in Let us ignore molecules, intermolecular interactions, and inorganic compounds are primarily interconnected through coordinate bonds and just consider a group of seven blocks provided in Figure 1 (a cube, ⴚ-, L-, T-, Y-, and ⴙ- shaped † This article is dedicated to Prof. Abraham Clearfield on his units, and an awkwardly shaped object). Assume that these 72nd birthday. are our Lego blocks; you can interlink all symmetric building ‡ Current address: Eastman Kodak Research Laboratories, Mail blocks through their edges and the awkwardly shaped buildBox 02002, 1669 Lake Avenue, Rochester, NY 14650. ing block has no well-defined directional preference. JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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Chemistry for Everyone
Now think of permutations and combinations to make networks using this set of building blocks and imagine the possibilities of networking. We can construct four kinds of architectures using these building blocks: zero-, one-, two-, and three-dimensional structures. A selected few of the several possible architectures one can make from these building blocks are shown in Figure 1. For example, a discrete square network can be made by combining four ⴚ and cubic building blocks or by assembling four L and cubic building blocks (1a, 1b). Infinite one-dimensional chains can be obtained by linking ⴚ building blocks or by combining ⴚ and cubic building blocks (2a, 2b). We can interlink T blocks in such a way that they can form either a one-dimensional ladderlike structure or a two-dimensional structure like a brick wall (3a, 3b). An infinite honeycomb network can be constructed by assembling complementary Y blocks or by combining Y and ⴚ or cubic building blocks (4a, 4b). The two-dimensional square networks may be formed by linking complementary ⴙ building blocks or by combining ⴚ and cubic blocks (5a, 5b). A variety of three-dimensional structures can be constructed by exploiting six-face connectivity of the cubes, but this is not depicted. It is clear that putting together awkwardly shaped building blocks into predictable arrays is no easy task because these building blocks do not form well-defined patterns and the number of possible geometrical arrangements is very great. This simple exercise of assembling Lego blocks is a prelude to the discussion on assembling molecular building blocks in the solid state. Molecular Building Blocks—Crystalline Networks So far we have discussed that (i) intermolecular forces and coordinate M–N bonds cement the molecules or ions together in the solid state and (ii) certain symmetrical objects form predictable patterns. If we combine these two aspects judiciously (i.e., by understanding the intriguing correlations between molecular shape, symmetry, and intermolecular forces), there emerges a rational strategy for the design of new materials. The ingenuity of chemists plays a major role in identifying the complementary molecular building blocks for a proposed purpose. In general, one needs to target certain zero-, one-, two-, or three-dimensional networks and then work backward to determine the suitable molecular building blocks. In this section we examine the type of molecular building blocks needed to achieve the networks identified in Figure 1.
Figure 2. Two types of molecular squares. (a) Metals as corners and ligands as spacers [Pd(en)(4,4′-bipy)][NO3]2. (b) Ligands as corners and metals as spacers, [Ag(pyrimidine)][NO3].
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Synthesis. The synthesis of predictable inorganic or organic solids generally requires complexation of divergent multifunctional ligands with metal salts or with appropriate complementary organic molecular building blocks. Some of the popular symmetrical molecular building blocks used for crystal engineering purposes are pyrazine (1), pyrimidine (2), 4,4′bipyridine (3), 1,4-dicyanobenzene (4), 1,4-benzenedicarboxylic acid (5), 1,3-benzenedicarboxylic acid (6), 1,3,5-triazine (7), 1,3,5-tricyanobenzene (8), 1,3,5-benzenetricarboxylic acid (9), 1,3,5,7-adamantanetetracarboxylic acid (10), hexamethylenetetramine (11), and tetrapyridyl porphyrin (12) (Scheme I). CN
N
COOH
N
N N
N
N HOOC
1
2
N
CN
COOH
3
4
5 COOH N HOOC
NC
CN
8
HOOC
N
7
6
COOH
CN
COOH
N
COOH
COOH HOOC
9
N
11
10
N
NH
N N
N
N
N N
HN
N
12
Scheme I
A typical laboratory experiment involves the crystallization of molecular complexes or metal complexes (depending upon the type of network one would like to target) using different types of crystallization techniques (9). In general, dissolution of the molecular building blocks in common organic solvents and slow evaporation of the solvent(s) yields desirable single crystals for X-ray diffraction experiments. However, in several cases direct addition of organic ligands to a metal salt leads to the immediate precipitation of insoluble metal complexes that are difficult to crystallize. Two possible ways to circumvent this problem are slowing down the reaction rate using layering techniques and using high-pressure, high-temperature conditions (hydrothermal synthesis) to partially dissolve the inorganic complexes (9, 10). X-ray diffraction studies on single crystals give fractional coordinates of the molecule(s) in the solid state and crystallographic visualization software is used to critically analyze the molecular networks (11). Discrete Macrocycles. Molecular squares can be formed using metal corners and linear bifunctional ligand spacer units. Pd(NO3)2 chelated with ethylenediamine readily reacts with 4,4′-bipyridine to yield a molecular square (Fig. 2a) (12). Note that chelation of metal coordination sites is necessary to force the formation of a discrete macrocycle rather than a possible infinite network. A molecular square can also be obtained using metals as spacers and ligands as corners. The tetranuclear molecular square of [Ag(pyrimidine)][(NO3)] is one such example (Fig. 2b) (13). Linear Chains. Self-recognition of carboxylic acids through hydrogen-bonded dimers in the crystal structure of terephthalic acid (1,4-benzenedicarboxylic acid) leads to an infinite one-
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a
b Figure 3. One-dimensional linear chain structures. (a) Hydrogenbonded self-assembly of terephthalic acid. (b) Coordination polymer of [Ag(pyrazine)][(NO3)]. Anions are not shown.
a
b
Figure 4. Molecular ladder and brick structures obtained by Tshaped building blocks in 1:1.5 metal complexes. (a) [Co(4,4′bipy)1.5][NO3]. (b) [Cu(pyrazine)1.5][NO3]. Anions are not shown.
a
dimensional chain structure (Fig. 3a) (5). The 1:1 coordination complexes of linear bifunctional ligands with a transition metal also form linear chain structures such as the onedimensional polymeric structure of [Ag(pyrazine)][NO3] (7). Molecular Ladders and Bricks. Adoption of three-coordinate T-shaped geometry by certain transition metals in their coordination complexes with bifunctional ligands may lead to molecular ladder and brick wall structures. The coordination polymers [Co(NO3)2(4,4′-bipy)1.5] and [Cu(pyrazine)1.5][SiF6] are such examples (Figs. 4a and 4b) (7). Hexagonal Networks. The trigonally disposed carboxylic acid groups of trimesic acid (1,3,5-benzenetricarboxylic acid) predictably self-assemble through hydrogen-bonded dimers into a honeycomb grid (Fig. 5a) (14). The coordination complex of Ag(CF3SO3) with 1,3,5-tricyanobenzene forms a two-dimensional hexagonal grid as shown in Figure 5b (15). Square Networks. The metallated tetrakis(4-carboxyphenyl)porphyrin with its fourfold molecular symmetry selfassembles through hydrogen bonding of the carboxylic acid groups to form a two-dimensional square network (Fig. 6a) (16 ). The 2:1 complexes of linear bifunctional ligands with transition metal complexes can form square networks in suitable circumstances, for example [Cd(dipy)2(NO3)2] as shown in Figure 6b (17 ). Crystalline Network Determining Factors. One may ask the following questions with regard to crystal structure prediction: Apart from molecular symmetry and directional interactions, what other factors control the formation of crystalline networks? What determines whether a ladder structure is formed rather than a brick wall structure? Do all 2:1 metal complexes with bifunctional ligands form square networks? The answers to these questions are not straightforward. Controlling factors are close-packing requirements; solvent(s) of crystallization; experimental conditions (temperature, pH, impurities, type of crystallization technique employed); nature of the metal cation (coordination geometry), counteranion (coordinating ability, size and shape), and ligand (conformational flexibility, shape and positioning of functional groups); and presence or absence of a template (a template is a guest molecule that can potentially be included in the crystalline lattice of the host framework, see Fig. 8) (5, 7, 18). Small variations in any of these factors may lead to significant changes in the observed structural patterns.
b
a
Figure 5. Honeycomb networks obtained through self-assembly of trigonal building blocks. (a) Trimesic acid forms hexagonal network through hydrogen bonds. (b) Ag(I) cations function as spacer units between 1,3,5-tricyanobenzene in [Ag(TCB)][CF3SO3] to form a hexagonal coordination network. Anions are not shown.
b
Figure 6. (a) Metallated tetrakis(4-carboxyphenyl)porphyrin forms a square-network through hydrogen-bonded carboxylic acid dimers. (b) The coordination complex of [Cd (4,4′-bipy)2][NO3]2 leads to a square-network. Anions are not shown.
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Three-Dimensional Solids. Three-dimensional structures can also be constructed using the above strategies. For example, how do we approach the problem of constructing threedimensional diamondlike networks using molecular building blocks (Fig. 7a)? First, we need to identify the geometry of a basic repeating unit(s) in the network. In this case, the repeating unit is a tetrahedral (T d) symmetric building block. We can achieve this symmetry by considering the three geometrically distinct objects shown in Figure 7b. If we assemble these objects together along the edges of Td symmetry we will end up with a diamondoid network. Appropriate molecular building blocks for the generation of three-dimensional diamond structures are given in Figure 7c (19). It is very difficult to predict the solid-state architectures of molecules with irregular shape, conformational flexibility, and no strong directional forces in molecular building block approach (20). The majority of organic molecules fall under this category! Nevertheless, the building-block methodology has had a significant role in predicting the crystal structure of many industrially important organic molecules (e.g., pigments, pharmaceuticals) in conjunction with molecular modeling techniques (21).
a
b
Figure 7. Retrosynthesis of diamondoid network leads to three distinct building blocks with tetrahedral symmetry. Arrow signs indicate the direction of propagation to obtain a diamondoid network. Notice that molecular building blocks should have strong directional forces along Td symmetry.
Architecture to Functionality Identification of crystalline networks is a way of codifying crystal structures with an ultimate objective of controlling functional properties. For the precise control of functional properties one needs to control the entire three-dimensional structure of the crystals. However, certain chemical and physical properties can be predicted simply on the basis of our limited knowledge of crystalline architectures. Inorganic Macrocyclic Hosts. Synthesis of inorganic analogues of organic macrocycles (e.g., crown ethers, cyclophanes, and calixeranes) with predetermined shapes is of great importance in developing a new family of inorganic host materials having conformational rigidity for binding neutral, chiral, and ionic guests. The macrocycles shown in Figure 1 function as good host or ion-exchange materials because of their cavity size, shape, and ionic nature. Porous Solids and Ion Exchange Materials. In recent years, there has been a great deal of interest in the synthesis of porous solids (artificial zeolites) because such materials will have tremendous applications in the areas of catalysis, separation science, and electronics (7, 15, 22–24). Ladder, brick, hexagonal, and square networks shown in Figures 2, 4, 5, and 6 are open porous and have different cavity shapes and sizes. In principle all these open networks could be used as synthetic analogues of natural zeolites. Unfortunately, these open networks may not be stable because of the amount of unfilled space: nature abhors a vacuum. When we try to construct an open network using crystal engineering strategies, nature will outsmart us through the interweaving of identical open networks (Fig. 8) (25, 26 ). But this problem can be solved using template-directed synthesis, where the holes in the networks are occupied by guest molecules and the regular nature of the network is retained (15, 26 ). Depending upon the stability of the open network, we can either exchange or remove the guest molecules (Fig. 8). Two-dimensional layered structures will also intercalate guest molecules between layers as in natural clays (22, 27 ). 620
c
Guest exchange Template directed synthesis–cavities filled by guest
(if the network is robust, guest molecules can be burned out to form porous solids)
yes
Fill it somehow! Nature abhors a vacuum
no
No open network will be formed
yes Targeted network with big open cavities
Nature's way of avoiding voids–cavities filled by interweaving
Figure 8. A schematic representation for the generation of porous networks. The targeted open networks undergo interpenetration if the open cavities are not filled by guest molecules. Also, note that the guest molecules in open networks can be readily exchanged or removed depending upon the nature and stability of the network.
Molecular Metals and Magnets. Recently, crystal engineering principles are increasingly used to design novel moleculebased metals and magnets (28–31). Here we briefly discuss how rational designing strategies are crucial in attaining these important functional properties. The charge-transfer complexes of organic donors (D) and acceptors (A) commonly result in a mixed stack (…DADADADAD…) to optimize the electrostatic interactions of the partially negative and positive charges of the molecular fragments. However, the donor–acceptor complex of tetrathiafulvalene (TTF, D) and tetracyanoquinodimethane (TCNQ, A) forms segregated stacks (…DDDDD…AAAAA…) and
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exhibits conductivity (28). This seemingly unlikely organization of all negatively charged ions into one set of stacks and positively charged ions into another is a prerequisite for high electrical conductivity (29). A critical analysis of this structure and other structures of this family suggests that the weak intermolecular interactions such as S…S, S…N, C–H…S, and C–H…N are important in controlling the structural features of these complexes. Therefore, several design strategies aimed at synthesizing organic conducting or superconducting materials strive to make segregated stacks by optimizing various types of intermolecular interactions (2). Molecule-based magnets offer several advantages over traditional atom-based systems with regard to size and synthetic versatility (30, 31). They represent the ultimate in miniaturization for the microelectronics industry and allow the maximum computer memory to be packed into a limited space (32). The basic difference between traditional magnets and these molecule-based magnets is that the active spin sites in the traditional magnets are located in d or f orbitals, whereas in the molecular magnets the active spin sites are located in σ and π molecular orbitals made up of s or p atomic orbitals. It has been realized that the molecule-based magnetic properties can be directly correlated to the nature of crystalline architectures. Therefore, one can fine-tune molecular magnetism by controlling the orientation and interacting distance between the spin sites. For example, the molecular complex [Fe(Cp*2)][TCNQ], DA, has been isolated as three polymorphs, each with different magnetic properties (30). The purple-platelet phase (α form, monoclinic) has an array of isolated DAAD units; each repeat unit has one independent spin and is paramagnetic. The green triclinic β form and the purple parallelepiped γ form (monoclinic) contain a …DADADA…type one-dimensional chain with two spins per repeat unit and exhibit interesting metamagnetic and ferromagnetic properties, respectively. The magnetic properties of these three polymorphs can be directly correlated to the coupling distances between adjacent FeIII sites and [TCNE]ⴢ᎑ moieties in the solid state. Optical Materials. Non-centrosymmetric or chiral solids exhibit very interesting functional properties such as optical properties, pyroelectricity, piezoelectricity, and triboluminescence (33, 34 ). The potential applications of optical materials in efficient frequency-conversion and high-speed light modulation stimulated the efforts to produce such materials. The design of non-centrosymmetric crystals from achiral molecular building blocks is challenging because close-packing principles predict that structures with inversion centers pack more efficiently than those without (20). The non-centrosymmetric solids were successfully designed in a few cases using the building block approach, where the crystalline networks were built by complementary acentric hydrogen bonding motifs. For example, the 1:1 molecular complex of 4-aminobenzoic acid and 3,5-dinitrobenzoic acid forms non-centrosymmetric crystals, as the networks are stabilized by hydrogen bonding motifs between hetero carboxylic acids and nitro-amine functional groups (35). The major problem in the design of optical materials is that one not only has to achieve the noncentrosymmetry but should also be able to control phasematchability, hyperpolarizability, thermal stability, and color transparency for realizing optical materials with practical applications (34 ).
The properties highlighted here are just a few of those one can target with the crystalline network approach (36 ). Crystal engineering studies exploit the voluminous crystallographic information stored in the Cambridge Structural Database (CSD) for a reliable description of intermolecular interactions and coordinate bonds in the building block approach (37). Limitations of Current Strategies Although current crystal engineering strategies provide a powerful means to attack the complex problem of crystal structure prediction, they have some disadvantages. First, the empirical rules formulated on structural motifs of different intermolecular interactions or coordination networks cannot be generalized and several deviations from these rules are possible. Second, only symmetrical molecular building blocks with functional groups capable of forming robust intermolecular interactions or coordinate bonds are currently considered for designing purposes. Third, to date there are no proper theoretical models or force fields for reliably predicting the crystal structures of even simple organic molecules. Fourth, polymorphism is an intriguing aspect of crystal engineering and the prediction of polymorphs using the building block approach is difficult. Summary and Outlook The importance of the molecular building block approach for the discovery of novel materials can be gauged by the huge collection of research articles in contemporary literature and the release of three new journals on this subject (Crystal Engineering, Crystal Engineering Communications, and Crystal Growth and Design). The molecular-building-block approach is quite successful in synthesizing porous solids, clay-like materials, and ion exchange materials for separations and catalysis, but application of these strategies to the design of novel optical, electronic, and molecular magnetic materials is still in the developmental stage. The next five to ten years will witness great progress in the development of new functional materials (38, 39). The teaching community is encouraged to introduce crystal engineering in the undergraduate curriculum as part of the structural chemistry course. The laboratory course for this subject involves only the synthesis of organic or inorganic crystals by mixing appropriate molecular building blocks (you can challenge students to come up with a variety of molecular building blocks for a given architecture). With modern X-ray crystallographic instruments (charge-coupled device, CCD, area detectors) and advanced computational tools, crystal structure determination has become a fairly routine exercise, like recording an IR or NMR spectrum. Literature Cited 1. Mallouk, T. E.; Lee, H. J. Chem. Educ. 1990, 67, 829–834. 2. Desiraju, G. R. Crystal Engineering: The Design of Organic Solids; Elsevier: Amsterdam, 1989. 3. Dagani, R. Chem. Eng. News 1999, 77 (Mar 8) 51–60. 4. Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311– 2327.
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Chemistry for Everyone 5. Desiraju, G. R.; Sharma, C. V. K. In The Crystal as a Supramolecular Entity; Desiraju, G. R. Ed.; Perspectives in Supramolecular Chemistry, Vol. 2; Wiley: New York, 1996; pp 31–61. 6. Aakeroy, C. B. Acta Crystallogr. 1997, B53, 569–586. 7. Zaworotko, M. J. In Crystal Engineering: The Design and Application of Functional Solids; Seddon, K. R.; Zaworotko, M., Eds.; NATO ASI Series C, Mathematical and Physical Sciences, Vol. 539; Kluwer: Dordrecht, Netherlands, 1998. 8. Robson, R. In Comprehensive Supramolecular Chemistry; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vögtle, F; Lehn, J.-M., Eds.; Pergamon: Oxford, 1996; Vol. 6, pp 733–755. 9. Jones, P. Chem. Br. 1981, 17, 222–225. 10. Laudise, R. A. Chem. Eng. News 1987, 65 (39), 30. 11. A variety of shareware and commercial crystallographic software is available for solving and visualizing crystal structures. For more information on this subject visit the Web site maintained by the International Union of Crystallography, http:// www.iucr.org (accessed Dec 2000). 12. Fujita, M.; Sasaki, O.; Mitsuhashi, T.; Fujita, T.; Yazaki, J.; Yamaguchi, K.; Ogura, K. Chem. Commun. 1996, 1535–1536. 13. Sharma C. V. K.; Griffin, S. T.; Rogers, R. D. Chem. Commun. 1998, 215–216. 14. Herbstein, F. H. In Comprehensive Supramolecular Chemistry; Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vogtle, F.; Lehn, J.-M., Eds.; Pergamon: Oxford, 1996; Vol. 6, pp 61–83. 15. Gardener, G. B.; Venkataraman, D.; Moore, J. S.; Lee, S. Nature 1995, 374, 792–795. 16. Kumar, R. K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541–552. 17. Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151–1152. 18. Sharma, C. V. K.; Rogers, R. D. Cryst. Eng. 1998, 1, 19–38. 19. Zaworotko, M. J. Chem. Soc. Rev. 1994, 283–288. 20. Kitaigorodoskii, A. I. Molecular Crystals and Molecules; Academic: New York, 1973. The close-packing principle formulated by A. I. Kitaigorodskii is the cornerstone in understanding the packing of irregularly shaped organic molecular crystals. The gross structural features of molecular crystals can be well understood by using this theory. 21. For a good overview of the latest developments in crystal engineering, see Crystal Engineering: From Molecules and Crystals to
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