Over the past several decades, halogen bonding has become a standard ‘tool’ in supramolecular chemistry and particularly in crystal engineering. Traditionally, the study of halogen bonding has primarily been focused...

Structures of iodopyridinium halogenides have demonstrated why iodide, the weakest halogen bond acceptor among the halogenides, preferentially forms halogen bonds.

We demonstrate a design for halogen-bonded metal−organic cocrystals involving coordinatively unsaturated square-planar Cu(II) and Ni(II) centers, by utilizing a Schiff base ligand whose pendant acetyl group enables halogen bonding. The robustness of this design is evident by the assembly of a large family of eight cocrystals based on zero-, one-, and twodimensional halogen bonded architectures involving mono-or ditopic halogen bond donors based on iodine or bromine.

Six N -(4-halogenobenzyl)-3-halogenopyridinium cations were prepared by reacting meta -halogenopyridines (Cl, Br, and I) with (4-halogenobenzyl) bromides (Br and I) and were isolated as bromide salts, which were further used to obtain iodides and chlorides. Sixteen compounds (out of 18 possible cation/anion combinations) were obtained; two crystallized as hydrates and 14 as solvent free salts, 11 of which belonged to one isostructural series and 3 to another. All crystal structures comprise halogen-bonded chains, with the anion as an acceptor of two halogen bonds, with the pyridine and the benzyl halogen substituents of two neighboring cations. The halogen bonds with the pyridine halogen show a linear correlation between the relative halogen bond length and angle, which primarily depend on the donor halogen. The parameters of the other halogen bonds vary with all three halogens, indicating that the former halogen bond is the dominant interaction. This is also in accord with the calculated electrostatic potential in the σ-holes of the halogens and the thermal properties of the solids. The second isostructural group comprises combinations of the best halogen bond donors and acceptors, and features a more favorable halogen bond geometry of the dominant halogen bond, reaffirming its significance as the main factor in determining the structure.

Novel halogen bonded cocrystals of aromatic aldehydes have been synthesized. We present the halogen bond acceptor potential of the aldehyde group oxygen atom in competition with the hydroxy, methoxy and pyridine groups.