Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

We have performed a database survey and a structural and computational study of the potential and the limitations of halogenopyridinium cations as halogen bond donors. The database survey demonstrated that adding a positive charge on a halogenopyridine ring increases the probability that the halogen atom will participate in a halogen bond, although for chloropyridines it remains below 60%. Crystal structures of both protonated and N-methylated monohalogenated pyridinium cations revealed that the iodo- and bromopyridinium cations always form halogen-bonding contacts with the iodide anions shorter than the sum of the vdW radii, while chloropyridinium cations mostly participate in longer contacts or fail to form halogen bonds. Although a DFT study of the electrostatic potential has shown that both protonation and N-methylation of halogenopyridines leads to a considerable increase in the ESP of the halogen σ-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

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.

Exploring the Halogen-Bonded Cocrystallization Potential of a Metal-Organic Unit Derived from Copper(ii) Chloride and 4-Aminoacetophenone

In this work, we describe a novel halogen-bonded metal-organic cocrystal involving a square-planar Cu(ii) complex and 1,4-diiodotetrafluorobenzene (14tfib) by utilizing an amine ligand whose pendant acetyl group enables halogen bonding. The cocrystal was prepared by both mechanochemical synthesis (liquid-assisted grinding) and the conventional solution-based method. Crystal structure determination by single crystal X-ray diffraction revealed that the dominant supramolecular interactions are the I···O halogen bond between 14tfib and CuCl₂(aap)₂ building blocks, and the N–H···Cl hydrogen bonds between CuCl₂(aap)₂ molecules. The combination of halogen and hydrogen bonding leads to the formation of a 2D network. Overall, this work showcases an example of the possibility for extending the complexity of metal-organic crystal structures by using halogen bonding in a way that does not affect other hydrogen bonding synthons.

Halogen bonded metal bis(dithiolene) 2D frameworks

Short and directional I⋯S halogen bonding interactions in iodinated bis(dithiolene) complexes lead to the crystallization of 2D or 3D anionic frameworks.

C—I...NC halogen bonding in two polymorphs of the mixed-valence 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), with segregatedversusalternated stacks

Oxidation of diiodoethylenedithiotetrathiafulvalene (EDT-TTF-I2), C8H4I2S6, with the strong oxidizer tetrafluorotetracyanoquinodimethane (TCNQF4), C12F4N4, affords, depending on the crystallization solvent, two polymorphs of the 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), represented asD2A. In both salts, the TCNQF4is reduced to the radical anion state, and is associated through short C—I...NC halogen bonds to two EDT-TTF-I2molecules. The two polymorphs differ in the solid-state association of these trimericD–A–Dmotifs. In polymorph (I) the trimeric motif is located on an inversion centre, and hence both EDT-TTF-I2molecules have +0.5 charge. Together with segregation of the TTF and TCNQ derivatives into stacks, this leads to a charge-transfer salt with high conductivity. In polymorph (II) two crystallographically independent EDT-TTF-I2molecules bear different charges, close to 0 and +1, as deduced from an established correlation between intramolecular bond lengths and charge. Overlap interactions between the halogen-bondedD0–A^{{-}{\bullet}}–D^{{+}{\bullet}} motifs give rise, in a perpendicular direction, to diamagneticA22−andD0–D22+–D0entities, where the radical species are paired into the bonding combination of respectively the acceptor LUMOs and donor HOMOs. The strikingly different solid-state organization of the halogen-bondedD–A–Dmotifs provides an illustrative example of two modes of face-to-face interaction between π-type radicals, into either delocalized, uniform chains with partial charge transfer and conducting behaviour, or localized association of radicals into face-to-faceA22−andD22+dyads.