MATERIALS Today Perspectives of Crystal Engineering "Is Materials Science going soft?" photochemical reactions of cinnamic The most successful strategies of This question is raised by George acids? A broad and more meaningful crystal engineering are based on a Marsh in the first issue of Materials definition of crystal engineering was building block (or supramolecular Today and certainly deserves further provided by G. R. Desiraju in 1989 as synthon) approach, which simplifies consideration. Crystal Engineering, "the understanding of intermolecular the complex problem of structure one of these 'soft' areas in materials interactions in the context of crystal prediction into a simple problem of science, has recently been emerging packing and in the utilisation of such network architecture (like making as a major cross-disciplinary field of understanding in the design of new things out of LEGO ® blocks). For all basic and applied inquiry. solids with desired physical and practical purposes, the crystal chemical properties". 3 Crystal structures are assumed to be Let's examine the scope of this engineering is now a rapidly networks, where molecules, metals, exciting field as it applies to materials developing interdisciplinary field ions, etc., are considered as nodes and science. with a wide scope for basic research the intermolecular interactions or and promising industrial applications coordination bonds represent node Crystals are comprised of which may also drive the basic connections. 4 The design of one, two, molecules or ions, and the physical research effort. A few of the key or three-dimensional crystalline and chemical properties of the crystals research areas under the purview of network structures can thus be depend upon the geometrical crystal engineering include: achieved by choosing the desired arrangement of these internal building combination of nodes and connectors. blocks. To the materials scientist, • Hardness and Color of Solids For example, metal cations as the control of both the physical and • Separation Science nodes, coordinated to simple linear chemical properties of the materials • Catalysis bifunctional ligands as connectors would be a natural outcome of the • Optical Materials (NLO, will form a variety of 1D, 2D, and 3D ability to predict the crystal structure Octupolar, Switches) architectures depending upon the of a given compound. The ability to • Electronic Materials and Sensors metal coordination geometry and fine-tune such features as color, • Conducting and Magnetic metal to ligand ratio as shown by melting point, polarity, polymorphism, Materials Mike Zaworotko in reference 5 and or conductivity would offer unlimited • Nanotechnology redrawn in Figure 1. Interestingly, potential for materials modification. • Crystal Growth, Polymorphism many of these simple new • Liquid Crystals frameworks have no precedence in Unfortunately, it is not yet possible • Molecular Modeling naturally occurring solids and the to predict solid state structure merely • Drug Design bulk properties of these compounds from chemical composition. This was • Protein-Receptor Binding remains largely unexplored.5 the subject of John Maddox's, 1988 • Surfactants, Monolayers, and Nature editorial where he indicated Multilayers Two separate branches of crystal that "One of the continuing scandals in • Supramolecular Devices engineering independently deal with the physical sciences is that it remains • Solid State Chemical Reactivity design principles for organic solids in general impossible to predict the structure of even the simplest Metal crystalline solids from a knowledge of 0 their chemical composition". 1 While Bifunctional ligand m these statements may still be true in a - i i strict sense even today, continuing Linear Chain Ladder efforts of various research groups _ ! l around the globe to achieve the T:i:i objective of crystal structure :EL E]Z prediction have afforded significant Brick Square Grid Honeycomb progress toward this goal. The term crystal engineering was coined by G. M. J. Schmidt in the 1970s to address the problem of crystal structure prediction in the Figure 1. Control of metal ion coordination geometry and reaction context of organic solid state stoichiometry can lead to controllable ID, 2D, and 3D architectures. Page 27 NATEIiIA~ Today and inorganic solids (or coordination polymers). The organic solids are o o____._o .... o o____._o designed by considering molecular shape, symmetry, and intermolecular interactions such as O-H, N-H, and Figure 2. The 1:1 complex of 4-nitrobenzoic acid and 4-(N~V- C-H hydrogen bonding, or weaker dimethylamino)benzoic acid. The linear ribbon structure is stabilized by halogen...halogen, electrostatic, or centric O-H...O dimers and acentric C-H...O hydrogen bonded dimersJ ° van der Waals interactions. Inorganic solids are designed primarily by concerning the nature of weak one of nature's laws: "nature abhors a considering metal coordination intermolecular interactions such as C- vacuum". Despite these hurdles, quite geometry, chemical structure of the H---O and C-H...N interactions has a number of open porous networks organic ligand, metal to ligand ratio, been dispelled by crystallographic have been recently constructed. and the coordinating nature of the statistical analysis: it has been anions. 4,5 More recently, design o" o unequivocally proven that these strategies relying on a combination of interactions are directional and organic and inorganic synthons, electrostatic in nature like any other O~O" metal-organic solids, have become strong hydrogen bonds. 8,9 For increasingly prominent. Regardless example, the 1:1 molecular complex of the basic approach, in order to of 4-nitrobenzoic acid and 4-(N,N- design materials with reliable and dimethylamino)benzoic acid shows a 1 predictable crystalline networks, linear ribbon, that is stabilized by consideration of symmetry and robust acentric O-H-.-O dimers and C-H..-O and directional non-covalent hydrogen bonded dimers (Figure 2).1° interactions are paramount. Although The non-centrosymmetric nature of currently more difficult, it is also the C-H.-.O hydrogen bonded dimer possible to design solid state between the electron donating, and architecture with weak withdrawing functionalities serves as intermolecular interactions by careful a useful building block in efforts consideration of the interplay among toward the design of non- various intermolecular interactions. centrosymmetfic crystals with second CN CN 2 harmonic generation (SHG) Crystal engineering of organic properties. Very recently, C-H---O solids, especially with respect to NH2 hydrogen bonds have been used to understanding the nature of various construct trigonal acentric layered intermolecular interactions, has structures to generate a new class of profoundly influenced the concepts of SHG active octupolar solids, n supramolecular chemistry. It is now well accepted that the organic crystal The design of nanoporous solids a is the best example of a (both organic and inorganic) with supermolecule, and crystal controlled sizes and shapes and engineering has become an integral chemical environments using the The 1:3 complex of trimesic acid part of supramolecular chemistry, the principles of crystal engineering has and N,N-dicyclohexylamine forms an chemistry of the 21st century.4,6,7 generated enormous interest in recent open porous hexagonal network years because such designer solids (effective cavity size 12.7 ,~) stabilized The voluminous (and ever may be exploited for separations, by rigid hydrogen bonding between growing) crystallographic informa- shape selective catalysis, and trimesate anions, 1, and ammonium tion stored in the Cambridge optoelectronic applications.~2 In cations (Figure 3). 13 Interestingly, Structural Database (CSD) is the general, strategies for the synthesis of Yaghi and co-workers used the same primary source for an extremely porous solids rely upon propagation trimesate anions to generate porous reliable description of intermolecular of molecular symmetry into inorganic layered structures with interactions for the design of organic crystalline symmetry through rigid transition metals coordinated to solids. The statistical analysis of directional forces (hydrogen bonds carboxylates. The crystalline lattice of structural and chemical information and/or metal coordination bonds) these compounds is thermally stable provides accurate and unambiguous such that an open porous network up to 350°C, even after the removal of information on interrnolecular may result. The down side of this included aromatic guest molecules! TM interactions that no other strategy is that such open networks Moore and co-workers used a spectroscopic method (NMR, UV, IR) always tend to interpenetrate to obey somewhat similar strategy by could provide. The controversy Page 28 MATERIALS Today ~.o[~:: =4i'_°-.~ N N °° ~ .°~ ~.°° .~ ° .W• ..o• .W..o• . • •.'~ ~'.: - "--~ ~'.: N N :0.• •O: e ~.. :Ooe %0: ~ ~" . .~ ~" . --.*-- ,¢V•-.~*• °3 tie• .,Co %~ go...~_. ~.. :Q % ~.e • e% ooe'~ t-e • •o. -.~ ~,-.o._ .~- .o,,,~ ~, .o.• %=-.4"-' Figure 4. Angular bifunctional ligands, such as pyrimidine, and metal ions with a preference for linear coordination can be used to construct Figure 3. The 1:3 complex of square complexes. Here, the 1:1 Ag(I):pyrimidine complex forms trimesic acid and N,N- tetracationic supramolecular squares which face-to-face stack at the ligand dicyclohexylamine. Note the open, corners to form an open channel network in the solid state. 17 The