J. Appl. Cryst.の発刊に際して

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

How do molecules aggregate in solution, and how do these aggregates consolidate themselves in crystals? What is the relationship between the structure of a molecule and the structure of the crystal it forms? Why do some molecules adopt more than one crystal structure? Why do some crystal structures contain solvent? How does one design a crystal structure with a specified topology of molecules, or a specified coordination of molecules and/or ions, or with a specified property? What are the relationships between crystal structures and properties for molecular crystals? These are some of the questions that are being addressed today by the crystal engineering community, a group that draws from the larger communities of organic, inorganic, and physical chemists, crystallographers, and solid state scientists. This Perspective provides a brief historical introduction to crystal engineering itself and an assessment of the importance and utility of the supramolecular synthon, which is one of the most important concepts in the practical use and implementation of crystal design. It also provides a look to the future from the viewpoint of the author, and indicates some directions in which this field might be moving.

Crystal engineering: from molecule to crystal.

Halogen Bonding in Supramolecular Chemistry.

CONSPECTUS: The halogen bond is an attractive interaction in which an electrophilic halogen atom approaches a negatively polarized species. Short halogen atom contacts in crystals have been known for around 50 years. Such contacts are found in two varieties: type I, which is symmetrical, and type II, which is bent. Both are influenced by geometric and chemical considerations. Our research group has been using halogen atom interactions as design elements in crystal engineering, for nearly 30 years. These interactions include halogen center dot center dot center dot halogen interactions (X center dot center dot center dot X) and halogen center dot center dot center dot heteroatom interactions (X center dot center dot center dot B). Many X center dot center dot center dot X and almost all X center dot center dot center dot B contacts can be classified as halogen bonds. In this Account, we illustrate examples of crystal engineering where one can build up from previous knowledge with a focus that is provided by the modern definition of the halogen bond. We also comment on the similarities and differences between halogen bonds and hydrogen bonds. These interactions are similar because the protagonist atoms halogen and hydrogen are both electrophilic in nature. The interactions are distinctive because the size of a halogen atom is of consequence when compared with the atomic sizes of, for example, C, N, and O, unlike that of a hydrogen atom. Conclusions may be drawn pertaining to the nature of X center dot center dot center dot X interactions from the Cambridge Structural Database (CSD). There is a clear geometric and chemical distinction between type I and type II, with only type II being halogen bonds. Cl/Br isostructurality is explained based on a geometric model. In parallel, experimental studies on 3,4-dichlorophenol and its congeners shed light on the nature of halogen center dot center dot center dot halogen interactions and reveal the chemical difference between Cl and Br. Variable temperature studies also show differences between type I and type II contacts. In terms of crystal design, halogen bonds offer a unique opportunity in the strength, atom size and interaction gradation; this may be used in the design of ternary cocrystals. Structural modularity in which an entire crystal structure is defined as a combination of modules is rationalized on the basis of the intermediate strength of a halogen bond. The specific directionality of the halogen bond makes it a good tool to achieve orthogonality in molecular crystals. Mechanical properties can be tuned systematically by varying these orthogonally oriented halogen center dot center dot center dot halogen interactions. In a further development, halogen bonds are shown to play a systematic role in organization of LSAMs (long range synthon aufbau module), which are bigger structural units containing multiple synthons. With a formal definition in place, this may be the right time to look at differences between halogen bonds and hydrogen bonds and exploit them in more subtle ways in crystal engineering.

Halogen Bonds in Crystal Engineering: Like Hydrogen Bonds yet Different

The updated concepts on the nature of π–π interactions and their use in various fields ranging from crystal engineering to materials science to biochemistry are discussed. This is the opening paper...

The Nature and Applications of π–π Interactions: A Perspective

Molecular tapes mediated via strong O–H⋯N hydrogen bonds and weak C–I⋯O interactions are present in complexes of 4-nitrobenzoic acid·4-iodopyridine and 3,5-dinitrobenzoic acid·4-iodopyridine. Crystal structure of 4-nitrobenzamide·4-iodobenzamide has amide dimer tape and iodo⋯nitro interaction in orthogonal directions. There is good structural insulation in the hydrogen bonding and halogen bonding domains in these crystal structures. However, the less polarizable bromine and chlorine atoms show cross-pairing interactions that are less predictable. These preliminary results show that both carboxylic acid⋯pyridine and iodo⋯nitro heterosynthons may be simultaneously exploited for crystal design.

Crystal engineering with hydrogen bonds and halogen bonds

Halogen atoms Cl, Br, and I steer crystallization of 2-halo-3-hydroxypyridine 1, pyridine-N-oxides 2, and 2-halo-3-aminopyridine 3 in noncentrosymmetric space groups efficient for nonlinear optical materials. Whereas the strong hydrogen-bond chains are aligned antiparallel, the L- or V-geometry of weak interhalogen interaction steers polar alignment in the solid state, even for high dipole moment (μ) halogenated N-oxides 2 (μ > 5.0 D). Six out of seven structures evaluated crystallize in polar/chiral space groups due to interhalogen interactions compared to the one in six probability of noncentrosymmetric packing for achiral molecules.

Using halogen···halogen interactions to direct noncentrosymmetric crystal packing in dipolar organic molecules

The hexagonal layer structure of host atoms in chloro, bromo, and iodo derivatives of 2,4,6-tris(4-halophenoxy)-1,3,5-triazine, X-POT, is stabilized by a cyclic and cooperative halogen trimer (X···X) synthon. The X···X distance is ∼3.5 A in isostructural channel inclusion adducts of Cl-POT and Br-POT, whereas I···I is ∼3.8 A in cage structures of I-POT with aromatic and hydrophobic guests. X-ray crystal structures of I-POT with mesitylene, collidine, tribromomesitylene, triiodomesitylene, hexachlorobenzene, hexafluorobenzene, 1-methylnaphthalene, CH2Cl2, CH2Br2, and CH2I2 guests in R3 space group are reported. Host molecules are fully ordered in these isostructural clathrates, whereas guest atoms are disordered except for C6Cl6 and C6F6. The guest molecule resides in a penta-decker sandwich surrounded by double layers of iodo trimer and triazine ring. Supramolecular isomerism from channel to cage framework and the persistent crystallization of trigonal X-POT molecules in high-symmetry host networks (spac...

Halogen Trimer-Mediated Hexagonal Host Framework of 2,4,6-Tris(4-halophenoxy)-1,3,5-triazine. Supramolecular Isomerism from Hexagonal Channel (X = Cl, Br) to Cage Structure (X = I)

Binoy K. Saha

Papers

The Nature and Applications of π–π Interactions: A Perspective

Crystal engineering with hydrogen bonds and halogen bonds

Using halogen···halogen interactions to direct noncentrosymmetric crystal packing in dipolar organic molecules

Halogen Trimer-Mediated Hexagonal Host Framework of 2,4,6-Tris(4-halophenoxy)-1,3,5-triazine. Supramolecular Isomerism from Hexagonal Channel (X = Cl, Br) to Cage Structure (X = I)

Helical water chains in aquapores of organic hexahost: remarkable halogen-substitution effect on the handedness of water helix