This highlight presents an overview of the current advances in the preparation of halogen bonded metal–organic multi-component solids, including salts and cocrystals comprising neutral and ionic constituents.

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.

In order to study halogenide anions simultaneously acting as hydrogen bond and halogen bond acceptors, we have prepared and crystallised halogenide salts (chlorides, bromides and iodides) of the three isomers of iodopyridine, 2-iodopyridine (2-IPy), 3-iodopyridine (3-IPy) and 4-iodopyridine (4-IPy). In all nine structures the pyridinium cations and the halogenide anions are interconnected through C–I⋯X⁻ halogen and N–H⋯X⁻ hydrogen bonds (X = Cl, Br, I). Based on the trend observed in the relative shortening of the two interactions, both halogen and hydrogen bonds seem to be decreasing in strength with the size of the halogenide anion, although the decrease is apparently more pronounced for hydrogen bonds. To study this further, we have performed a CSD study of structures comprising a halogenide and both hydrogen and halogen bond donors, which has indicated a tendency of chloride towards hydrogen and of iodide towards halogen bonds, in accord with our experimental results. Additionally, we have obtained crystals of ((2-IPyH)₂ Cl I), the first example of a double salt comprising both hydrogen and halogen bond donors and two different halogenides. In this structure, the halogen bonds were formed exclusively with the iodide anion, while the hydrogen bonds were formed only with the chloride anion.

In order to examine the possibility of controlling formation of hydrogen bonded hexacyanoferrous networks through combination of hydrogen and halogen bonding, seven new crystalline compounds were obtained by reacting hexacyanoferrous acid (H₄[Fe(CN)₆]) with seven halogenopyridines. The (protonated) halogenopyridines were present in all seven obtained solids, and the extent in which they participate in halogen bonding varied with the halogen atom (Cl, Br and I) and the geometry of the halogenopyridines (o-, m- or p-). Iodopyridines have been found to always form halogen bonds with the cyano groups of the anions, while neither chloropyridine used in this study was found to act as a halogen bond donor. Of the prepared solids, four comprise hydrogen bonded hexacyanoferrate networks achieved by direct binding of H₂[Fe(CN)₆]²⁻ and H₃[Fe(CN)₆]⁻ anions through [Fe–CN⋯H⋯NC–Fe] hydrogen bonds into 2D and 3D networks respectively, two are formed from H₂[Fe(CN)₆]²⁻ anions bridged by water molecules, and one from [Fe(CN)₆]⁴⁻ anions bridged by H₃O⁺ cations. This variability in hydrogen bonding patterns can be brought into connection with different halogen bonding proclivities of the halopyridinium cations.

In order to study the proclivity of primary amine groups to act as halogen bond acceptors, three aromatic diamines (p-phenylenediamine (pphda), benzidine (bnzd) and o-tolidine (otol)) were cocrystallised with three perfluorinated iodobenzenes (1,4-tetrafluorodiiodobenzene (14tfib), 1,3-tetrafluorodiiodobenzene (13tfib) and 1,3,5-trifluorotriiodobenzene (135tfib)) as halogen bond donors. Five cocrystals were obtained: (pphda)(14tfib), (bnzd)(13tfib)₂, (bnzd)(135tfib)₄, (otol)(14tfib) and (otol)(135tfib)₂. In spite of the variability of both stoichiometries and structures of the cocrystals, in all the prepared cocrystals the amine groups form exclusively I···N halogen bonds, while the amine hydrogen atoms participate mostly in N–H⋯F contacts. The preference of the amine nitrogen atom toward the halogen bond, as opposed to the hydrogen bond (with amine as a donor), is rationalised by means of computed hydrogen and halogen bond energies, indicating that the halogen bond energy between a simple primary amine (methylamine) and a perfluorinated iodobenzene (pentafluoroiodobenze ne) is ca. 15 kJ mol⁻¹ higher than the energy of the (H)NH∙∙∙NH₂ hydrogen bond between two amine molecules.

We performed a structural study of N-alkylated halogenopyridinium cations to examine whether choice of the N-substituent has any considerable effect on the halogen bonding capability of the cations. For that purpose, we prepared a series of N-ethyl-3-halopyridinium iodides and compared them with their N-methyl-3-halopyridinium analogues. Structural analysis revealed that N-ethylated halogenopyridinium cations form slightly shorter C−X⋯I⁻ halogen bonds with iodide anion. We have also attempted synthesis of ditopic symmetric bis-(3-iodopyridinium) dications. Although successful in only one case, the syntheses have afforded two novel ditopic asymmetric monocations with an iodine atom bonded to the pyridine ring and another on the aliphatic N-substituent. Here, the C−I⋯I⁻ halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan.