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1. Role of secondary interactions in a series of 2:1 halogen-bonded cocrystals formed between 4-(dimethylamino)pyridine and ditopic halogen-bond donors

   https://doi.org/10.1107/S205322962400771X  

   Bosch, Eric; Bowling, Nathan P (September 2024, Acta Crystallographica Section C Structural Chemistry)

   The structures of a series of 2:1 cocrystals formed between 4-(dimethylamino)pyridine and each of 1,2,4,5-tetrachloro-3,6-diiodobenzene, 2C₇H₁₀N₂·C₆Cl₄I₂, 1,2,4,5-tetrabromo-3,6-diiodobenzene, 2C₇H₁₀N₂·C₆Br₄I₂, 1-bromo-4-iodo-2,3,5,6-tetrafluorobenzene, 2C₇H₁₀N₂·C₆BrF₄I, and 1,2-dibromo-4,5-difluoro-3,6-diiodobenzene, 2C₇H₁₀N₂·C₆Br₂F₂I₂, are reported. In all five structures, the core halogen-bonded 2:1 trimolecular units have geometrically similar parameters, with the central halogen-bond donor flanked by two pyridine halogen-bond acceptors twisted with respect to the central halogen-bond donor at angles ranging from 76 to 86°. The I...N halogen-bond separations are all short, ranging from 73.3 to 76.7% of the sum of the van der Waals radii, while the C—I...N bond angles are essentially linear. The Br...N halogen-bond separation in the cocrystal formed with 1-bromo-4-iodo-2,3,5,6-tetrafluorobenzene is 80.4% of the sum of the van der Waals radii. Subtle differences in the crystal packings are attributed to the role of secondary C—H...π and weak π-type interactions with chloro and bromo substituents. The cocrystals 2C₇H₁₀N₂·C₆Cl₄I₂and 2C₇H₁₀N₂·C₆Br₄I₂are isomorphous. 

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2. Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals

   https://doi.org/10.3390/molecules25040907  

   Quentin, Jay; C. Swenson, Dale; R. MacGillivray, Leonard (February 2020, Molecules)

   null (Ed.)

   The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2′-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe), the donor molecules support the C=C bonds of 2,2′-bpe to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of 2,2′-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2′-tpcb). In each case, the reactivity occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2′-bpe molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of 2,2′-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2′-tpcb) and (1,4-di-I-tFb)·(2,2′-tpcb). 

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3. Partial transfer of bridging atom in halogen-bonded complexes

   https://doi.org/10.1016/j.comptc.2021.113398  

   Lu, Jia; Scheiner, Steve (October 2021, Computational and Theoretical Chemistry)
4. Achieving a series of solid-state [2 + 2] cycloaddition reactions involving 1,2-bis(2-pyridyl)ethylene within halogen-bonded co-crystals

   https://doi.org/10.1039/D4CE00097H  

   Andren, Max; Bosch, Eric; Krueger, Herman R; Groeneman, Ryan H (March 2024, CrystEngComm)

   The formation of a halogen-bonded co-crystal based upon 1,2-bis(2-pyridyl)ethylene along with iodoperchlorobenzene is reported. The co-crystal undergoes a nearly quantitative [2 + 2] cycloaddition reaction in the organic solid state. 

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5. Crystal structure of a (carboxymethyl)triethylazanium bromide–2-(triethylazaniumyl)acetate (1/1) hydrogen-bonded dimer

   https://doi.org/10.1107/S2056989023006850  

   Carlson, Faith M; Staples, Richard J; Biros, Shannon M (September 2023, Acta Crystallographica Section E Crystallographic Communications)

   The title compound, C₈H₁₈NO₂⁺·Br⁻·C₈H₁₇NO₂, crystallizes as the bromide salt of a 50:50 mixture of (triethylazaniumyl)carboxylic acid and the zwitterionic (triethylazaniumyl)carboxylate. The two organic entities are linked by a half-occupied bridging carboxylic acid hydrogen atom that is hydrogen-bonded to the carboxylate group of the second molecule. The tetralkylammonium group adopts a nearly perfect tetrahedral shape around the nitrogen atom with bond lengths that agree with known values. The carboxylic acid/carboxylate group is orientedantito one of the ethyl groups on the ammonium group, and the carbonyl oxygen atom is engaged in intramolecular C—H...O hydrogen bonds. 

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