Keggin-Type Anions as Halogen Bond Acceptors.

To study the potential of Keggin-type polyoxometalate anions to act as halogen bond acceptors, we have prepared a series of 10 halogen-bonded compounds starting from phosphomolybdic and phosphotungstic acid and halogenopyridinium cations as halogen (and hydrogen) bond donors. In all the structures, the cations and the anions were interconnected by halogen bonds, more often with terminal M=O oxygen atoms than with bridging oxygen atoms as acceptors. In four structures comprising protonated iodopyridinium cations capable of forming both hydrogen and halogen bonds with the anion, the halogen bond with the anion is apparently favored, whereas hydrogen bonds preferentially involve other acceptors present in the structure. In three obtained structures derived from phosphomolybdic acid, the corresponding oxoanion has been found in its reduced state [Mo12PO40]4-, which has also led to a decrease in halogen bond lengths as compared to the fully oxidated [Mo12PO40]3-. The electrostatic potential on the three types of anions involved in the study ([Mo12PO40]3-, [Mo12PO40]4-, and [W12PO40]3-) has been calculated for optimized geometries of the anions, and it has been shown that the terminal M=O oxygen atoms are the least negative sites of the anions, indicating that they act as halogen bond acceptors primarily due to their steric availability.

Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals.

Multicentre two-electron (mc/2e or 'pancake bonding') bonding between 7,7,8,8-tetra-cyano-quinodi-methane (TCNQ) radical anions was studied on its 14 novel salts with planar organic cations. The formal charges of the TCNQδ- moieties are -1/2 and -2/3, and they form mc/2e bonded dimers, trimers and tetramers which are further stacked into extended arrays. Multicentre bonding within these oligomers is characterized by short interplanar separations of 2.9-3.2 Å; distances between the oligomers are larger, typically >3.3 Å. The stacks are laterally connected by C-H⋯N hydrogen bonding, forming 2D arrays. The nature of mc/2e bonding is characterized by structural, magnetic and electrical data. The compounds are found to be semiconductors, and high conductivity [10-2 (Ω cm)-1] correlates with short interplanar distances between pancake-bonded oligomers.

Isostructural Halogen Exchange and Halogen Bonds: The Case of N-(4-Halogenobenzyl)-3-halogenopyridinium Halogenides.

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.

Conservation of the Hydrogen-Bonded Pyridone Homosynthon in Halogen-Bonded Cocrystals.

Seven cocrystals of pyridone and perfluorinated halocarbons have been prepared. In all cases pairs of pyridone molecules are connected into dimers by two N-H···O hydrogen bonds, forming the characteristic pyridone homosynthon of R2 2(8) topology. These dimers further act as acceptors of halogen bonds through the two pyridone oxygen atoms, forming two (in six cases) or three (in one case) halogen bonds with the donor molecules. The stoichiometry of the cocrystals obtained and the overall topology of the supramolecular architecture depend primarily on the topicity of the halogen bond donor, with the monotopic donor yielding a cocrystal of 1:1 stoichiometry comprising discrete supramolecular complexes, the ditopic donors cocrystals of 1:2 stoichiometry comprising chains, and the tritopic donor a cocrystal of 1:2 stoichiometry comprising hydrogen- and halogen-bonded layers. The results indicate that the pyridone homosynthon is a robust and reliable supramolecular synthon that is conserved in halogen-bonded cocrystals of pyridone.

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.