Vinko Nemec

We report a strategy for the synthesis of higher-order halogen-bonded cocrystals involving a metal complex as a multitopic acceptor, by utilizing an isothiocyanate ligand whose pendant sulfur atom enables the formation of multiple halogen bonds.

Discovery of a polymorph of tris( p‐ tolyl)antimony(III) reveals solid‐state supramolecular dimers connected through short, attractive Sb∙∙∙Sb contacts that, based on theoretical calculations and a systematic screen of the Cambridge Structural Database (CSD), could be an overlooked hallmark of the solid‐state and supramolecular chemistry of trivalent organoantimony(III) compounds. While the previously reported crystal structure of tris( p‐ tolyl)antimony(III) is known to exhibit phenyl embrace motifs, resembling the arsenic or phosphorus analogs, the new polymorph exhibits supramolecular dimers characterized by direct Sb∙∙∙Sb contacts shorter than the sum of the van der Waals radii, and stabilized by a significant dimer dissociation energy of ca. −26 kJ mol ⁻¹ established by periodic density‐functional calculations. Theoretical analysis indicates that the contacts are attractive and dispersion‐dominated, with an overview of the CSD showing that such direct Sb···Sb contacts are not unique to this polymorphic form of tris( p‐ tolyl)antimony(III) but appear to be a more general feature of trivalent organoantimony in the solid state. Specifically, the CSD shows that dimers based on short Sb···Sb contacts are found across a number of organoantimony structures, including some of the simplest representatives of the series, such as triphenyl‐ and trimethylantimony, for which the dimer interaction energy is herein calculated to be ca. −13 kJ mol ⁻¹ .

Discovery of a polymorph of tris( p‐ tolyl)antimony(III) reveals solid‐state supramolecular dimers connected through short, attractive Sb∙∙∙Sb contacts that, based on theoretical calculations and a systematic screen of the Cambridge Structural Database (CSD), could be an overlooked hallmark of the solid‐state and supramolecular chemistry of trivalent organoantimony(III) compounds. While the previously reported crystal structure of tris( p‐ tolyl)antimony(III) is known to exhibit phenyl embrace motifs, resembling the arsenic or phosphorus analogs, the new polymorph exhibits supramolecular dimers characterized by direct Sb∙∙∙Sb contacts shorter than the sum of the van der Waals radii, and stabilized by a significant dimer dissociation energy of ca. −26 kJ mol ⁻¹ established by periodic density‐functional calculations. Theoretical analysis indicates that the contacts are attractive and dispersion‐dominated, with an overview of the CSD showing that such direct Sb···Sb contacts are not unique to this polymorphic form of tris( p‐ tolyl)antimony(III) but appear to be a more general feature of trivalent organoantimony in the solid state. Specifically, the CSD shows that dimers based on short Sb···Sb contacts are found across a number of organoantimony structures, including some of the simplest representatives of the series, such as triphenyl‐ and trimethylantimony, for which the dimer interaction energy is herein calculated to be ca. −13 kJ mol ⁻¹ .

10 salts of selected mono- and dihalopyridines with benzenesulfonic and p -toluenesulfonic acids have been synthesised and structurally characterised in order to study halopyridinium cations as bifunctional donors.

In this work, we synthesized three novel 4-iodotetrafluorophenoxy-azobenzene ethers, which contain different substituents (X = −H, −Cl, −CN) on the opposite side of the molecule in relation to the perhalogenated moiety carrying the iodine atom. To explore the halogen bond donor potential of the prepared compounds, we performed cocrystal screening with a series of nitrogen-containing acceptors: 1,4-diazabicyclo[2.2.2]­octane, 4-dimethylaminopyridine, 2,2’-bipyridine, 4,4’-bipyridine, 4,4’-azopyridine, N,N’-bis­(pyridin-4-yl)­methylenehydrazine, 1,2-bis­(pyridin-4-yl)­ethane, and 1,2-bis­(pyridin-4-yl)­ethylene. These three azobenzenes were selected in order to investigate how bent molecules carrying a perhalogenated moiety would act as halogen bond donors, as well as how different substituents on a distant part of the molecule could affect the formation of cocrystals. Out of 24 combinations, only 8 experiments yielded cocrystals suitable for single-crystal X-ray diffraction with two out of three azobenzene derivatives (X = −Cl and −CN). Structural analysis revealed that in all obtained cocrystals, the robust interaction is the I···N halogen bond between the azobenzene iodine atom and the acceptor nitrogen atom. A majority of cocrystals feature two donor molecules per one acceptor molecule and display crystal packing based on discrete trimeric halogen-bonded complexes. Only in the case of the 4,4’-bipyridine cocrystal with a 1:1 stoichiometry is the crystal structure based on discrete halogen-bonded dimers. In order to investigate changes in the halogen bond donor ability of the azobenzene derivatives, we have calculated values of the molecular electrostatic potential (MEP) for the DFT-optimized molecular geometries. Calculations showed that the electrostatic potential on the iodine atom only slightly depends on the functional group located on the opposite side of the molecule, with relatively large MEP values (+135 kJ mol^–1 e^–1 on average).

In order to explore a strategy for synthesizing halogen‐bonded metal‐organic cocrystals by utilizing metal complexes whose pendant chloride group and the morpholinyl oxygen atom enables halogen bonding, we have synthesized four pentacoordinated Cu(II) and Zn(II) complexes of the MCl ₂ L general formula (L=imines prepared by the condensation reaction of 4‐aminoethylmorpholine with 2‐pyridinecarboxyaldehide or 2‐acetylpyridine). The prepared metal complexes were further cocrystallized with selected iodoperfluorinated benzenes. Out of 20 combinations, 14 experiments yielded crystals suitable for single‐crystal X‐ray diffraction. Structural analysis revealed that in 7 cocrystals halogen bonds are formed both with morpholinyl oxygen as well as with chloride atoms. In 6 cocrystals only I⋅⋅⋅Cl halogen bonds are present, while only one cocrystal exclusively featured I⋅⋅⋅O _morpholinyl halogen bonds. We observed 5 halogen bonding motifs to the MCl ₂ moiety, in which each chloride atom can be an acceptor of one halogen bond, two, or none at all. The most common motif in our work (6 cocrystals) is where one chlorine atom is an acceptor of one halogen bond, while the other chlorine atom does not participate in halogen bonding. The crystal packing in the prepared cocrystals is directed by halogen‐bonded architectures which are either zero‐, one‐ or two‐dimensional.

In order to study the competition between halogen bond acceptor sites of multifunctional N,O,X-based halogen bond acceptors (X = S, O, N or π aromatic ring) we have synthesized a family of oxazole derivatives and cocrystallized them with selected iodoperfluorinated benzenes as halogen bond donors. Out of 28 combinations, 19 experiments yielded crystals suitable for single-crystal X-ray diffraction. Structural analysis revealed that in all of the obtained cocrystals the most prominent supramolecular interaction is the I⋯N_oxazole halogen bond with relative shortening values of up to 18%, comparable to I⋯N_pyridine halogen bond shortening values. The acceptors are ditopic in 10 cocrystals and form additional I⋯N, I⋯O or I⋯π halogen bonds. A majority of cocrystals feature one donor molecule per one acceptor molecule. In order to rank the acceptor sites and establish how the electrostatic potential differences impact the supramolecular landscape around these molecules, the values of molecular electrostatic potentials (MEPs) were calculated on their optimized geometries. These calculations were in agreement with experimental observations, since the best (most negative MEP value) acceptor binding site in the series of used oxazoles is the oxazole nitrogen atom. Depending on the difference between MEP values of the oxazole oxygen atom and the peripheral functionalities, additional halogen bonds could potentially be formed with one of these acceptor sites, leading to the formation of two different types of halogen-bonded supramolecular chains.

In this work, we present a systematic study of the halogen bonding potential of different 2,2'-bipyridine derivatives in the synthesis of cocrystals by using selected perfluorinated iodobenzenes and N-haloimides as halogen bond donors. These halogen bond acceptor molecules were chosen to explore how different substituents on 2,2'-bipyridine affect halogen bond formation. Out of 24 combinations, we obtained only 8 cocrystals by using two methods, liquid-assisted grinding and crystallization from the solution. Of those 8 cocrystals, one has already been described in the literature. As expected, structural data revealed that 2,2'-bipyridine derivatives act as ditopic halogen bond acceptors in all structures. Dominant interactions in 7 of the cocrystals are I···N or Br···N halogen bonds, while in the one remaining cocrystal it is the I···C(π) halogen bond.

Periodic density-functional theory (DFT) calculations and dissolution calorimetry measurements were used to predict the thermodynamic stability and the likelihood of interconversion between a series of halogen-bonded cocrystals.