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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. 

The present study evaluates the potential combination of charge-transfer electron-donor–acceptor π–π complexation and C—H hydrogen bonding to form colored cocrystals. The crystal structures of the red 1:1 cocrystals formed from the isomeric pyridines 4- and 3-{2-[4-(dimethylamino)phenyl]ethynyl}pyridine with 1-[2-(3,5-dinitrophenyl)ethynyl]-2,3,5,6-tetrafluorobenzene, both C 14 H 4 F 4 N 2 O 4 ·C 15 H 14 N 2 , are reported. Intermolecular interaction energy calculations confirm that π-stacking interactions dominate the intermolecular interactions within each crystal structure. The close contacts revealed by Hirshfeld surface calculations are predominantly C—H interactions with N, O, and F atoms. 

1,3-Diiodo-5-nitrobenzene, C₆H₃I₂NO₂, and 1,3-dibromo-5-nitrobenzene, C₆H₃Br₂NO₂, crystallize in the centrosymmetric space groupP2₁/m, and are isostructural with 1,3-dichloro-5-nitrobenzene, C₆H₃Cl₂NO₂, that has been redetermined at 100 K for consistency. While the three-dimensional packing in all three structures is similar, the size of the halogen atom affects the nonbonded close contacts observed between molecules. Thus, the structure of 1,3-diiodo-5-nitrobenzene features a close Type 1 I...I contact, the structure of 1,3-dibromo-5-nitrobenzene features a self-complementary nitro-O...Br close contact, while the structure of 1,3-dichloro-5-nitrobenzene also has a self-complementary nitro-O...Cl interaction, as well as a bifurcated C—H...O(nitro) close contact. Notably, the major energetically attractive intermolecular interaction between adjacent molecules in each of the three structures corresponds to a π-stacked interaction. The self-complementary halogen...O(nitro) and C—H...O(nitro) interactions correspond to significant cohesive attraction between molecules in each structure, while the Type 1 halogen–halogen contact is weakly cohesive. 

The rapid evaporation of 1:1 solutions of diethynylpyridines and N -halosuccinimides, that react together to form haloalkynes, led to the isolation of unreacted 1:1 cocrystals of the two components. The 1:1 cocrystal formed between 2,6-diethynylpyridine and N -iodosuccinimide (C 4 H 4 INO 2 ·C 9 H 5 N) contains an N -iodosuccinimide–pyridine I...N halogen bond and two terminal alkyne–succinimide carbonyl C—H...O hydrogen bonds. The three-dimensional extended structure features interwoven double-stranded supramolecular polymers that are interconnected through halogen bonds. The cocrystal formed between 3,5-diethynylpyridine and N -iodosuccinimide (C 4 H 4 INO 2 ·C 9 H 5 N) also features an I...N halogen bond and two C—H...O hydrogen bonds. However, the components form essentially planar double-stranded one-dimensional zigzag supramolecular polymers. The cocrystal formed between 3,5-diethynylpyridine and N -bromosuccinimide (C 4 H 4 BrNO 2 ·C 9 H 5 N) is isomorphous to the cocrystal formed between 3,5-diethynylpyridine and N -iodosuccinimide, with a Br...N halogen bond instead of an I...N halogen bond. 

The treatment of 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine with a threefold excess of 1,2,3,5-tetrafluoro-4,6-diiodobenzene in dichloromethane solution led to the formation of the unexpected 1:2 title co-crystal, C 14 H 13 N 3 ·2CF 4 I 2 . In the extended structure, two unique C—I...N halogen bonds from one of the 1,2,3,5-tetrafluoro-4,6-diiodobenzene molecules to the pyrimidine N atoms of the 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine molecule generate [110] chains and layers of these chains are π-stacked along the a- axis direction. The second 1,2,3,5-tetrafluoro-4,6-diiodobenzene molecule resides in channels formed parallel to the a -axis direction between stacks of 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine molecules and interacts with them via C—I...π(alkyne) contacts. 

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. 

We report the design, synthesis, and crystal structure of a conjugated aryleneethynyl molecule, 2-(2-{4,5-dimethoxy-2-[2-(2,3,4-trifluorophenyl)ethynyl]phenyl}ethynyl)-6-[2-(pyridin-2-yl)ethynyl]pyridine, C 30 H 17 F 3 N 2 O 2 , that adopts a planar rhombus conformation in the solid state. The molecule crystallizes in the space group P -1, with Z = 2, and features two intramolecular sp 2 -C—H...N hydrogen bonds that co-operatively hold the arylethynyl molecule in a rhombus conformation. The H atoms are activated towards hydrogen bonding since they are situated on a trifluorophenyl ring and the H...N distances are 2.470 (16) and 2.646 (16) Å, with C—H...N angles of 161.7 (2) and 164.7 (2)°, respectively. Molecular electrostatic potential calculations support the formation of C—H...N hydrogen bonds to the trifluorophenyl moiety. Hirshfeld surface analysis identifies a self-complementary C—H...O dimeric interaction between adjacent 1,2-dimethoxybenzene segments that is shown to be common in structures containing that moiety.