Supramolecular Interactions and Smart Materials: C–X…X′–M Halogen Bonds and Gas Sorption in Molecular Solids

Abstract

The ultimate goal of a solid-state chemist is the rational design and synthesis of crystalline materials with the presence of specific and tunable chemical or physical properties, the so-called ''smart materials.'' Porosity, magnetism, chirality, conductivity, superconductivity, spin-transition, optical, and photophysical properties can be exploited for a wide variety of applications including gas storage, separations, electronic, catalysis, nonlinear optics, or drug delivery, and are therefore the objectives of intense research activity. The occurrence of these properties in crystalline materials 1) results from the combination of two aspects: (i) the molecular units (building blocks) that compose the crystal and (ii) the arrangement of these building blocks in the crystal. Controlling the former is relatively simple, and essentially it is only limited to the imagination and creativity of the researcher, since molecular chemistry has, over about two centuries, developed a wide range of very powerful procedures for creating even more sophisticated molecules from atoms linked by covalent bonds. On the contrary, organizing the molecules, or building blocks, in a predetermined manner is difficult to accomplish given that the structure of crystalline solids results from a delicate balance between all the intermolecular interactions present in the crystal, maximizing the attractive ones and minimizing the repulsive ones while, although not always, adopting the densest packing [1]. When a single strong interaction dominates in the system other weak intermolecular interactions normally play a secondary role in the crystal packing, providing support to the stronger interactions. In this situation, control over the molecular packing can successfully be achieved [2], although the presence of numerous weak interactions can in some cases overwhelm much stronger interactions as a result of their abundance [3]. Further difficulties can be encountered if the building blocks are 1) It should of course be acknowledged that crystalline products might not always represent the most desirable form of a material with respect to a given application.

Competing hydrogen-bond and halogen-bond donors in crystal engineering

“…Recently, halogen bonds 6 which can play important roles in areas such as biochemistry 7 , medicinal chemistry 8 , and material science, 9 have found uses in crystal engineering because these interactions have properties that parallel those of hydrogen 25 bonds in terms of directionality and strength. 10,11 Typical hydrogen-bond strength ranges from approximately 4-60 kJ/mol 12 while halogen bonds range from 5-180 kJ/mol (the strong interaction I 2 I -in I 3 -is the extreme).…”

“…Many hydrogen-bond based synthons 1 such as acid … pyridine, 2 acid … amide, 3 phenol … pyridine, 4 oxime … N(heterocycle) 5 have been explored extensively as robust and reliable tools for crystal engineering and supramolecular synthesis. Recently, halogen bonds, 6 which play important roles in areas such as biochemistry, 7 medicinal chemistry, 8 and material science, 9 have found uses in crystal engineering because these interactions have properties that parallel those of hydrogen bonds in terms of directionality and strength. 10,11 Typical hydrogenbond strengths range from approximately 4-60 kJ mol 21 , 12 while halogen bonds range from 5-180 kJ mol 21 (the strong interaction I 2 … I 2 in I 3 2 is the extreme).…”

Extending the halogen-bonded supramolecular synthon concept to 1,3,4-oxadiazole derivatives

“…The overall goal of crystal engineering is the design and control of molecular packing in crystalline materials with desired properties. 1 This approach has attracted attention in different areas such as biochemistry, 2 medicinal chemistry, 3 and materials chemistry 4 among others. The precise understanding of intermolecular recognition between molecular building tectons has transformed this field into a multidisciplinary tool with impact in the development of organic devices such as organic conductors 5 and magnets, 6 chemical separation agents, 7 NLO-active materials, 8 liquid crystals 9 and porous organic solids.…”

“…The first is relatively simple to control, whereas the second is more difficult given that the crystal packing results from a delicate balance of all the intermolecular synthon interactions present in the crystal while normally adopting the densest accessible packing. 4 The concept of the supramolecular synthon was introduced into molecular crystal chemistry in 1995 (ref. 11) and involves the defined structural kernel of a crystal structure, which encapsulates the essence of a crystal in terms of molecular recognition.…”

“…Thus, they are ideally positioned to be involved in intermolecular interactions and can play, therefore, a fundamental role in the aggregation of molecules into solids. 4 In recent years, halogen bonding has grown from a scientific curiosity to one of the most interesting non-covalent interactions for constructing supramolecular assemblies. 14 Halogen bonding (XB) interactions span over a very wide range, from the weak Cl⋯Cl interaction in chlorocarbons to the strong I − ⋯I 2 interaction in I 3…”