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

The program Mercury, developed at the Cambridge Crystallographic Data Centre, was originally designed primarily as a crystal structure visualization tool. Over the years the fields and scientific communities of chemical crystallography and crystal engineering have developed to require more advanced structural analysis software. Mercury has evolved alongside these scientific communities and is now a powerful analysis, design and prediction platform which goes a lot further than simple structure visualization.

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Mercury 4.0: from visualization to analysis, design and prediction

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Halogen Bonding in Supramolecular Chemistry.

Metal–organic frameworks (MOFs), also known as coordination polymers (CPs), are crystalline materials constructed from metal ions or clusters bridged by organic ligands to form one-, two-, or three-dimensional infinite networks. In contrast with the prolific production of MOFs based on rigid ligands (RL-MOFs), the design, syntheses and applications of MOFs based on flexible ligands (FL-MOFs) are somewhat overlooked. Although sacrificing a measure of control, the use of flexible ligands may provide unique opportunities to obtain novel crystalline framework materials exhibiting desirable attributes. In this review, emphasis has been placed on the design and the structural diversity of FL-MOFs. Homochiral FL-MOFs and dynamic frameworks induced by flexible ligands are also briefly outlined. An overview is also shown for the applications of FL-MOFs as platforms for gas adsorption, heterogeneous catalysis, proton conduction etc.

Metal–organic frameworks based on flexible ligands (FL-MOFs): structures and applications

Mechanochemical solvent-free reactions by milling, grinding or other types of mechanical action have emerged as a viable alternative to solution chemistry. Mechanochemistry offers not only a possibility to eliminate the need for bulk solvent use, and reduce the generation of waste, but it also unlocks the door to a different reaction environment in which synthetic strategies, reactions and molecules previously not accessible in solution, can be achieved. This Minireview examines the potential of mechanochemistry in chemical and materials synthesis, by providing a cross-section of the recent developments in using ball milling for the formation of molecules and materials based on covalent and coordination bonds.

Mechanochemistry for Synthesis

This study reveals the self-assembly patterns of six-coordinate complexes of the tetra(4-pyridyl)- and tetra(3-pyriyl)-tin-porphyrin moieties (SnT4PyP and SnT3PyP, respectively) with multidentate carboxylic acids as axial ligands. Detailed structural characterization of the supramolecular organization in the resulting ordered solids by X-ray diffraction is reported. Crystals of the five new Sn(acid)2-TPyP complexes consist of multiporphyrin polymeric chains and networks that are sustained by extensive hydrogen bonding, involving the functional substituents on the axial ligands as proton donors and the peripheral N-sites of the porphyrin as proton acceptors. The use of different ligands leads to different connectivity features of the supramolecular assemblies that form. Structures with the 5-hydroxy-isophthalic acid and trimesic acid ligands (1 and 2) reveal the formation of one-dimensional hydrogen-bonded chains only, as solvation effects prevent interporphyrin interaction in other directions. Reaction of...

Crystal Engineering of Molecular Networks: Tailoring Hydrogen-Bonding Self-Assembly of Tin-Tetrapyridylporphyrins with Multidentate Carboxylic Acids As Axial Ligands

As part of our extensive investigations of hybrid organic–inorganic coordination frameworks, we report herein on the construction and characterization of new polymeric assemblies obtained by reacting the title tricarboxylic ligand with various transition metal ions. While polymeric structures of binary metal–ligand composition and three-dimensional connectivity scheme could be obtained with the f-shell lanthanoid ions, formulation of crystalline supramolecular assemblies with d-shell transition metals required incorporation of a third bipyridyl component in the reaction mixture. The latter present materials of ternary metal–ligand–bipyridyl content and layered structures composed of two-dimensional coordination polymers. The detailed coordination patterns of varying geometric features have been revealed by single-crystal X-ray diffraction. It has been also shown that selected polymeric solids act as heterogeneous catalysts in acetylation reactions of alcohols. The two-dimensional polymeric assemblies with...

Coordination Polymers of 5-(2-Amino/Acetamido-4-carboxyphenoxy)-benzene-1,3-dioic Acids with Transition Metal Ions: Synthesis, Structure, and Catalytic Activity

This study targets the construction of porphyrin assemblies directed by halogen bonds, by utilizing a series of purposely synthesized Sn(axial ligand)2–(5,10,15,20-tetraarylporphyrin) [Sn(L)2-TArP] complexes as building units. The porphyrin moiety and the axial ligands in these compounds contain different combinations of complimentary molecular recognition functions. The former bears p-iodophenyl, p-bromophenyl, 4′-pyridyl, or 3′-pyridyl substituents at the meso positions of the porphyrin ring. The latter comprises either a carboxylate or hydroxy anchor for attachment to the porphyrin-inserted tin ion and a pyridyl-, benzotriazole-, or halophenyl-type aromatic residue as the potential binding site. The various complexes were structurally analyzed by single-crystal X-ray diffraction, accompanied by computational modeling evaluations. Halogen-bonding interactions between the lateral aryl substituents of one unit of the porphyrin complex and the axial ligands of neighboring moieties was successfully expressed in several of the resulting samples. Their occurrence is affected by structural (for example, specific geometry of the six-coordinate complexes) and electronic effects (for example, charge densities and electrostatic potentials). The shortest intermolecular I⋅⋅⋅N halogen-bonding distance of 2.991 A was observed between iodophenyl (porphyrin) and benzotriazole (axial ligand) moieties. Manifestation of halogen bonds in these relatively bulky compounds without further activation of the halophenyl donor groups by electron-withdrawing substituents is particularly remarkable.

Exploring supramolecular self-assembly of tetraarylporphyrins by halogen bonding: crystal engineering with diversely functionalized six-coordinate tin(L)2-porphyrin tectons.

We present a rapid and readily scalable methodology for the mechanosynthesis of diverse metal–organic frameworks (MOFs) in the absence of milling media typically required for other types of mechanochemical syntheses. We demonstrate the use of liquid-assisted resonant acoustic mixing (LA-RAM) methodology for the synthesis of three- and two-dimensional MOFs based on Zn(II), Co(II) and Cu(II), including a mixed ligand system. Importantly, the LA-RAM approach also allowed the synthesis of the ZIF-L framework that has never been previously obtained in a mechanochemical environment, as well as its Co(II) analogue. Straightforward scale-up from milligrams to at least 25 grams is demonstrated using the metastable framework ZIF-L as the model.

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Hatem M. Titi

Papers

Mechanochemistry for Synthesis

Crystal Engineering of Molecular Networks: Tailoring Hydrogen-Bonding Self-Assembly of Tin-Tetrapyridylporphyrins with Multidentate Carboxylic Acids As Axial Ligands

Coordination Polymers of 5-(2-Amino/Acetamido-4-carboxyphenoxy)-benzene-1,3-dioic Acids with Transition Metal Ions: Synthesis, Structure, and Catalytic Activity

Exploring supramolecular self-assembly of tetraarylporphyrins by halogen bonding: crystal engineering with diversely functionalized six-coordinate tin(L)2-porphyrin tectons.

Simple, scalable mechanosynthesis of metal–organic frameworks using liquid-assisted resonant acoustic mixing (LA-RAM)