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1. 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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2. Crystal structure and photoreactivity of a halogen-bonded cocrystal based upon 1,2-diiodoperchlorobenzene and 1,2-bis(pyridin-4-yl)ethylene

   https://doi.org/10.1107/S2053229620006233  

   Bosch, Eric ; Battle, Jessica D. ; Groeneman, Ryan H. ( June 2020 , Acta Crystallographica Section C Structural Chemistry)

   null (Ed.)

   The formation of a photoreactive cocrystal based upon 1,2-diiodoperchlorobenzene ( 1,2-C 6 I 2 Cl 4 ) and trans -1,2-bis(pyridin-4-yl)ethylene ( BPE ) has been achieved. The resulting cocrystal, 2( 1,2-C 6 I 2 Cl 4 )·( BPE ) or C 6 Cl 4 I 2 ·0.5C 12 H 10 N 2 , comprises planar sheets of the components held together by the combination of I...N halogen bonds and halogen–halogen contacts. Notably, the 1,2-C 6 I 2 Cl 4 molecules π-stack in a homogeneous and face-to-face orientation that results in an infinite column of the halogen-bond donor. As a consequence of this stacking arrangement and I...N halogen bonds, molecules of BPE also stack in this type of pattern. In particular, neighbouring ethylene groups in BPE are found to be parallel and within the accepted distance for a photoreaction. Upon exposure to ultraviolet light, the cocrystal undergoes a solid-state [2 + 2] cycloaddition reaction that produces rctt -tetrakis(pyridin-4-yl)cyclobutane ( TPCB ) with an overall yield of 89%. A solvent-free approach utilizing dry vortex grinding of the components also resulted in a photoreactive material with a similar yield. 

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3. Halogen-bonded co-crystal containing 1,3-diiodoperchlorobenzene and the photoproduct rtct -tetrakis(pyridin-4-yl)cyclobutane resulting in a zigzag topology

   https://doi.org/10.1107/S2056989023001408  

   Bosch, Eric ; Unruh, Daniel K. ; Santana, Carlos L. ; Groeneman, Ryan H. ( March 2023 , Acta Crystallographica Section E Crystallographic Communications)

   The formation and crystal structure of a zigzag network held together by I...N halogen bonds is reported. In particular, the halogen-bond donor is 1,3-diiodoperchlorobenzene ( C 6 I 2 Cl 4 ) while the acceptor is the photoproduct rtct -tetrakis(pyridin-4-yl)cyclobutane ( TPCB ). Curiously, within the resulting co-crystal ( C 6 I 2 Cl 4 )·( TPCB ), the photoproduct accepts only two halogen bonds between neighbouring 4-pyridyl rings and as a result behaves as a bent two-connected node rather than the expected four-connected centre. In addition, the photoproduct, TPCB , is also found to engage in C—H...N hydrogen bonds, forming an extended zigzag chain. 

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4. Characterizing the interplay of Pauli repulsion, electrostatics, dispersion and charge transfer in halogen bonding with energy decomposition analysis

   https://doi.org/10.1039/c7cp06959f  

   Thirman, Jonathan ; Engelage, Elric ; Huber, Stefan M. ; Head-Gordon, Martin ( January 2018 , Physical Chemistry Chemical Physics)

   The halogen bond is a class of non-covalent interaction that has attracted considerable attention recently. A widespread theory for describing them is the σ-hole concept, which predicts that the strength of the interaction is proportional to the size of the σ-hole, a region of positive electrostatic potential opposite a σ bond. Previous work shows that in the case of CX 3 I, with X equal to F, Cl, Br, and I, the σ-hole trend is exactly opposite to the trend in binding energy with common electron pair donors. Using energy decomposition analysis (EDA) applied to a potential energy scan as well as the recent adiabatic EDA technique, we show that the observed trend is a result of charge transfer. Therefore a picture of the halogen bond that excludes charge transfer cannot be complete, and permanent and induced electrostatics do not always provide the dominant stabilizing contributions to halogen bonds. Overall, three universally attractive factors, polarization, dispersion and charge transfer, together with permanent electrostatics, which is usually attractive, drive halogen bonding, against Pauli repulsion. 

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5. 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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