Anticooperativity of Multiple Halogen Bonds and Its Effect on Stoichiometry of Cocrystals of Perfluorinated Iodobenzenes

Perhalogenated Anilines as Bifunctional Donors of Hydrogen and Halogen Bonds in Cocrystals with Ditopic Nitrogen-Containing Acceptors

In this study, we examine the experimental and theoretical capabilities of two perhalogenated anilines, 2,3,5,6-tetrafluoro-4-bromoaniline (btfa) and 2,3,5,6-tetrafluoro-4-iodoaniline (itfa) as hydrogen and halogen bond donors. A series of 11 cocrystals derived from the two anilines and selected ditopic nitrogen-containing acceptors (4,4'-bipyridine, 1,2-bis(4-pyridyl)ethane, and 1,4-diazabicyclo[2.2.2]octane) in 1:1 and 2:1 stoichiometries were prepared by liquid-assisted grinding and crystallization from solution. Crystallographic analysis revealed bifunctional donor properties in both anilines. The dominant supramolecular interaction in four cocrystals of btfa is the N-H···Nacceptor hydrogen bond between btfa and acceptor molecules, while in the one remaining cocrystal, donor and acceptor molecules are connected via the N-H···Nacceptor hydrogen bond and the Br···Nacceptor halogen bond. In two cocrystals of itfa, the dominant supramolecular interaction is the I···Nacceptor halogen bond between itfa and acceptor molecules, while in the remaining four cocrystals, donor and acceptor molecules are additionally connected by the N-H···Nacceptor hydrogen bond. Periodic density-functional theory (DFT) calculations have been conducted to assess the formation energies of these cocrystals and the strengths of the established halogen and hydrogen bonds. Molecular DFT calculations on btfa and itfa indicate that the differences in electrostatic potential between the competing sites on the molecules are 261.6 and 157.0 kJ mol-1 e-1, respectively. The findings suggest that itfa, with a smaller electrostatic potential difference between donor sites, is more predisposed to act as a bifunctional donor.

Structural Landscape of α-Acetamidocinnamic Acid Cocrystals with Bipyridine-Based Coformers: Influence of Crystal Packing on Their Thermal and Photophysical Properties

Controlling the supramolecular synthon outcome in systems with different functionalities has been a key factor for the design of supramolecular materials, which also affected their physicochemical properties. In this contribution, we have analyzed the structural landscape of α-acetamidocinnamic acid (HACA) aiming to find its synthon outcome from the competitivity between its acidic and amidic groups. We prepared four multicomponent forms including one dihydrate (HACA·2H2O) and three cocrystals bearing different bipyridine coformers with formulas (HACA)2(1,2-bpe) (1), (HACA)2(4,4'-azpy) (2), and (HACA)2(4,4'-bipy)3 (3) (1,2-bpe = 1,2-bis(4-pyridyl)ethylene; 4,4'-azpy = 4,4'-azopyridine; 4,4'-bipy = 4,4'-bipyridine). First, we applied a virtual screening approach to assess the feasibility of cocrystal formation. Then, we synthesized the cocrystals, via liquid-assisted grinding (LAG) (1 and 2) or solvothermal (3) techniques, and single crystals of HACA, and their four multicomponent forms were obtained showing different synthons and crystal packings. Besides, a Cambridge Structural Database (CSD) search of the cocrystals presenting bipyridine-type coformers and molecules with acid and amide functionalities was performed, and the observed synthon occurrences as well as the possibility of synthon modification by tuning the H-donor/H-acceptor propensity of the acidic and amidic groups were shown. Finally, we measured their thermal and photophysical properties, which were correlated with their structural features.

Cocrystallization of Antifungal Compounds Mediated by Halogen Bonding

The application of halogen bonding in pharmaceutical chemistry remains a challenge. In this work, novel halogen-bonded cocrystals based on azole antifungal active pharmaceutical ingredients (APIs) and the ditopic molecule 1,4-diiodotetrafluorobenzene are reported. Their crystal structural features, spectroscopic properties, and thermal stability were studied. The components are bound through I···N from the triazole moieties present in all of the compounds. The molecular electrostatic potential (MEP) surfaces and quantum theory of atoms in molecules (QTAIM) calculations are used to rationalize the presence of hydrogen and halogen bonds in the resulting structures and their energetic analysis. The relative halogen bond ability of the different groups of voriconazole, fluconazole, and itraconazole was analyzed using MEP surfaces, demonstrating this approach to be an interesting tool to predict halogen-bonding preferences.

Anticooperativity and Competition in Some Cocrystals Featuring Iodine-Nitrogen Halogen Bonds

Phenomena such as anticooperativity and competition among non-covalent bond donors and acceptors are key considerations when exploring the polymorphic and stoichiomorphic landscapes of binary and higher-order cocrystalline architectures. We describe the preparation of four cocrystals of 1,3,5-trifluoro-2,4,6-triiodobenzene with N-heterocyclic compounds, namely acridine, 3-aminopyridine, 4-methylaminopyridine, and 1,2-di(4-pyridyl)ethane. The cocrystals, which are characterized by single-crystal and powder X-ray diffraction experiments, all show moderately strong and directional iodine⋅⋅⋅nitrogen halogen bonds with reduced distance parameters ranging from 0.79 to 0.92 and carbon-iodine⋅⋅⋅nitrogen bond angles ranging from 165.4(3) to 175.31(7)°. The cocrystal comprising 1,3,5-trifluoro-2,4,6-triiodobenzene and acridine provides a relatively rare example where all three halogen bond donor sites form halogen bonds with three acceptor molecules, overcoming an anticooperative effect. This effect manifests itself through the lengthening of non-halogen-bonded C-I bonds, weakening their potential to form halogen bonds. The effect is only observed once two halogen bonds have been formed to 1,3,5-trifluoro-2,4,6-triiodobenzene; one such bond does not appear to be adequate. Among the four cocrystals studied, competition between the pyridyl nitrogen atoms and the amine nitrogen atoms suggests that the former are the preferred halogen bond acceptors. Analysis by Hirshfeld fingerprint plots and ¹³ C and ¹⁹ F magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy provides additional insights into the prevalence of various short contacts in the crystal structures and into the spectral response to halogen-bond-induced cocrystallization.

Ability of Peripheral H Bonds to Strengthen a Halogen Bond

Quantum calculations study the manner in which the involvement of a halogen atom as a proton acceptor in one or more H bonds (HBs) affects the strength of the halogen bond (XB) it can form with a nucleophile aligned with the X σ-hole. A variety of Lewis acids wherein X = F, Cl, Br, and I are attached to a tetrel atom C or Ge engaged in a XB with nucleophile NH₃. One, two, and three HF molecules were positioned perpendicular to the XB axis so that they could form a HB to the X atom. Each such HB strengthened the XB by an increment of 1 kcal/mol or more that does not attenuate as each new HB is added, potentially increasing the interaction energy manyfold. Additionally, the presence of one or more HBs facilitates the formation of a XB by molecules which are reluctant to engage in such a bond in the absence of these auxiliary interactions. Even the F atom, which avoids such a XB, can be coaxed to participate in a XB of moderate strength by one or more of these external HBs.

The Halogen Bonding Proclivity of the sp3 Sulfur Atom as a Halogen Bond Acceptor in Cocrystals of Tetrahydro-4H-thiopyran-4-one and Its Derivatives

In this work, we present a systematic study of the capability of the sp³ hybridized sulfur atom for halogen bonding both in a small building block, tetrahydro-4H-thiopyran-4-one, and two larger ones derived from it, Schiff bases with a morpholine fragment on the other end of the molecule. These three building blocks were cocrystallized with six perhalogenated aromates: 1,4-diiodotetrafluorobenzene, 1,3,5-triiodotrifluorobenzene, 1,3-diiodotetrafluorobenzene, 1,2-diiodotetrafluorobenzene, iodopentafluorobenzene, and 1,4-dibromotetrafluorobenzene. Out of the 18 combinations, only 7 (39%) yielded cocrystals, although with a high occurrence of the targeted I···S halogen bonding motif in all cocrystals (71%), and in imine cocrystals the I···O_morpholine motif (100%) as well as, surprisingly, the I···N_imine motif (100%). The I···S halogen bonds presented in this work feature lower relative shortening values than those for other types of sulfur atoms; however, the sp³ sulfur atom could potentially be more specific an acceptor for halogen bonding.

Halogen Bond Motifs in Cocrystals of N,N,O and N,O,O Acceptors Derived from Diketones and Containing a Morpholine or Piperazine Moiety

In this study, we investigate the halogen bond acceptor potential of oxygen and nitrogen atoms of morpholine and piperazine fragments when they are peripherally located on N,O,O or N,N,O acceptor molecules. We synthesized four acceptor molecules derived from either acetylacetone or benzoylacetone and cocrystallized them with 1,4-diiodotetrafluorobenzene and 1,3,5-triiodotrifluorobenzene. This resulted in eight cocrystals featuring different topicities and geometric dispositions of donor atoms. In all cocrystals, halogen bonds are formed with either the morpholinyl oxygen atom or the terminal piperazine nitrogen atom. The I···O_morpholine halogen bonds feature lower relative shortening values than I···N_terminal, I···O_carbonyl, and I···N_proximal halogen bonds. The N and O halogen bond acceptor sites were evaluated through calculations of molecular electrostatic potential values.