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1. Intermolecular binding preferences of haloethynyl halogen-bond donors as a function of molecular electrostatic potentials in a family of N -(pyridin-2-yl)amides

   https://doi.org/10.1039/d1ob01133b  

   Abeysekera, Amila M.; Averkiev, Boris B.; Le Magueres, Pierre; Aakeröy, Christer B. (January 2021, Organic & Biomolecular Chemistry)

   null (Ed.)

   In order to explore how σ-hole potentials, as evaluated by molecular electrostatic potential (MEP) calculations, affect the ability of halogen atoms to engage in structure-directing intermolecular interactions, we synthesized four series of ethynyl halogen-substituted amide containing pyridines (activated targets); ( N -(pyridin-2-yl)benzamides (Bz-act-X), N -(pyridin-2-yl)picolinamides (2act-X), N -(pyridin-2-yl)nicotinamides (3act-X) and N -(pyridin-2-yl) isonicotinamides (4act-X), where X = Cl/Br/I. The molecules are deliberately equipped with three distinctly different halogen-bond acceptor sites, π, N(pyr), and OC, to determine binding site preferences of different halogen-bond donors. Crystallographic data for ten (out of a possible twelve) new compounds were thus analyzed and compared with data for the corresponding unactivated species. The calculated MEPs of all the halogen atoms were higher in the activated targets in comparison to the unactivated targets and were in the order of iodine ≈ chloroethynyl < bromoethynyl < iodoethynyl. This increased positive σ-hole potential led to a subsequent increase in propensity for halogen-bond formation. Two of the four chloroethynyl structures showed halogen bonding, and all three of the structurally characterized bromoethynyl species engaged in halogen bonding. The analogous unactived species showed no halogen bonds. Each chloroethynyl donor selected a π-cloud as acceptor and the bromoethynyl halogen-bond donors opted for either π or N(pyr) sites, whereas all halogen bonds involving an iodoethynyl halogen-bond donor (including both polymorphs of Bz-act-I ) engaged exclusively with a N(pyr) acceptor site. 

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2. Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

   https://doi.org/10.3390/cryst10121075  

   Loy, Cody; Zeller, Matthias; Rosokha, Sergiy V. (December 2020, Crystals)

   null (Ed.)

   The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br…Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl− anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl−, with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br…Cl separations. DFT computations of an extended series of R–Br…Cl− complexes confirmed that the judicious choice of brominated electrophile allows varying halogen Br…Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding. 

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3. Differences in thermal expansion and motion ability for herringbone and face-to-face π-stacked solids

   https://doi.org/10.1107/S2052252521009593  

   Ding, Xiaodan; Zahid, Ethan; Unruh, Daniel K.; Hutchins, Kristin M. (January 2022, IUCrJ)

   A series of aromatic organic molecules functionalized with different halogen atoms (I/ Br), motion-capable groups (olefin, azo or imine) and molecular length were designed and synthesized. The molecules self-assemble in the solid state through halogen bonding and exhibit molecular packing sustained by either herringbone or face-to-face π-stacking, two common motifs in organic semiconductor molecules. Interestingly, dynamic pedal motion is only achieved in solids with herringbone packing. On average, solids with herringbone packing exhibit larger thermal expansion within the halogen-bonded sheets due to motion occurrence and molecular twisting, whereas molecules with face-to-face π-stacking do not undergo motion or twisting. Thermal expansion along the π-stacked direction is surprisingly similar, but slightly larger for the face-to-face π-stacked solids due to larger changes in π-stacking distances with temperature changes. The results speak to the importance of crystal packing and intermolecular interaction strength when designing aromatic-based solids for organic electronics applications. 

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4. Complementary, Cooperative Ditopic Halogen Bonding and Electron Donor-Acceptor π-π Complexation in the Formation of Cocrystals

   https://doi.org/10.3390/molecules27051527  

   Speetzen, Erin D.; Nwachukwu, Chideraa I.; Bowling, Nathan P.; Bosch, Eric (March 2022, Molecules)

   This study expands and combines concepts from two of our earlier studies. One study reported the complementary halogen bonding and π-π charge transfer complexation observed between isomeric electron rich 4-N,N-dimethylaminophenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the second study elaborated the ditopic halogen bonding of activated pyrimidines. Leveraging our understanding on the combination of these non-covalent interactions, we describe cocrystallization featuring ditopic halogen bonding and π-stacking. Specifically, red cocrystals are formed between the ditopic electron poor halogen bond donor 1-(3,5-dinitrophenylethynyl)-2,4,6-triflouro-3,5-diiodobenzene and each of electron rich pyrimidines 2- and 5-(4-N,N-dimethyl-aminophenylethynyl)pyrimidine. The X-ray single crystal structures of these cocrystals are described in terms of halogen bonding and electron donor-acceptor π-complexation. Computations confirm that the donor-acceptor π-stacking interactions are consistently stronger than the halogen bonding interactions and that there is cooperativity between π-stacking and halogen bonding in the crystals. 

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5. Critical comparison of RX⋯Y and RH⋯Y directionality in halogen and hydrogen bonds using modern computational chemistry methods

   https://doi.org/doi.org/10.1016/j.cplett.2020.137221  

   Riley, Kevin E. (April 2020, Chemical physics letters)

   Symmetry adapted perturbation theory anlysis and intermolecular overlap volume calculations are used to investigate the origins of halogen- and hydrogen bond directionality. The central finding is that exchange-repulsion is primarily responsible for destabilizing both types of interaction as the R-x⋯Y (x = H, Cl, Br) angle deviates from linearity. The particular shape of the electron density envelope on the halogen/hydrogen bond donor plays a large role in dictating the degree to which a complex is destabilized upon R-x⋯Y rotation, with halogen bonds exhibiting a roughly linear destabilization and hydrogen bonds exhibiting destabilization that is approximately quadratic. 

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