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1. A Raman Spectroscopic and Computational Study of New Aromatic Pyrimidine-Based Halogen Bond Acceptors

   https://doi.org/10.3390/inorganics7100119  

   Hardin, April E.; Ellington, Thomas L.; Nguyen, Suong T.; Rheingold, Arnold L.; Tschumper, Gregory S.; Watkins, Davita L.; Hammer, Nathan I. (October 2019, Inorganics)

   Two new aromatic pyrimidine-based derivatives designed specifically for halogen bond directed self-assembly are investigated through a combination of high-resolution Raman spectroscopy, X-ray crystallography, and computational quantum chemistry. The vibrational frequencies of these new molecular building blocks, pyrimidine capped with furan (PrmF) and thiophene (PrmT), are compared to those previously assigned for pyrimidine (Prm). The modifications affect only a select few of the normal modes of Prm, most noticeably its signature ring breathing mode, ν1. Structural analyses afforded by X-ray crystallography, and computed interaction energies from density functional theory computations indicate that, although weak hydrogen bonding (C–H···O or C–H···N interactions) is present in these pyrimidine-based solid-state co-crystals, halogen bonding and π-stacking interactions play more dominant roles in driving their molecular-assembly. 

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2. 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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3. Ditopic halogen bonding with bipyrimidines and activated pyrimidines

   https://doi.org/10.1107/S2053229620005082  

   Nwachukwu, Chideraa I.; Patton, Leanna J.; Bowling, Nathan P.; Bosch, Eric (May 2020, Acta Crystallographica Section C Structural Chemistry)

   The potential of pyrimidines to serve as ditopic halogen-bond acceptors is explored. The halogen-bonded cocrystals formed from solutions of either 5,5′-bipyrimidine (C 8 H 6 N 4 ) or 1,2-bis(pyrimidin-5-yl)ethyne (C 10 H 6 N 4 ) and 2 molar equivalents of 1,3-diiodotetrafluorobenzene (C 6 F 4 I 2 ) have a 1:1 composition. Each pyrimidine moiety acts as a single halogen-bond acceptor and the bipyrimidines act as ditopic halogen-bond acceptors. In contrast, the activated pyrimidines 2- and 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine (C 14 H 13 N 3 ) are ditopic halogen-bond acceptors, and 1:1 halogen-bonded cocrystals are formed from 1:1 mixtures of each of the activated pyrimidines and either 1,2- or 1,3-diiodotetrafluorobenzene. A 1:1 cocrystal was also formed between 2-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4-diiodotetrafluorobenzene, while a 2:1 cocrystal was formed between 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4-diiodotetrafluorobenzene. 

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4. Probing non-covalent interactions driving molecular assembly in organo-electronic building blocks

   https://doi.org/10.1039/c9ce00219g  

   Johnson, Sarah N.; Ellington, Thomas L.; Ngo, Duong T.; Nevarez, Jorge L.; Sparks, Nicholas; Rheingold, Arnold L.; Watkins, Davita L.; Tschumper, Gregory S. (January 2019, CrystEngComm)

   Recent advancements in material science exploit non-covalent interactions, such as halogen bonding (XB) or π-stacking within solid-state molecular frameworks for application in organic electronic devices. Herein, we focus on these and other non-covalent interactions and the effect that furan and thiophene substituents play on the solid-state properties of co-crystals formed between pentafluoro(iodoethynyl)benzene ( F 5 BAI ; XB donor) and a pyridine disubstituted with either furans or thiophenes ( PyrFur 2 and PyrThio 2 ; XB acceptors). Spectroscopic and thermal analyses of 1 : 1 mixtures provide indirect evidence of XB interactions, whereas X-ray crystallography provides direct evidence that XB and π-stacking are present in both co-crystals. Density functional theory (DFT) computations provide insight into the relative electronic energetics of each pair-wise contact observed in the experimental F 5 BAI-PyrFur 2 and F 5 BAI-PyrThio 2 co-crystals. 

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5. Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions

   https://doi.org/10.3389/fchem.2021.620017  

   Jaynes, Tyler J.; Sharafi, Mona; Campbell, Joseph P.; Bocanegra, Jessica; McKay, Kyle T.; Little, Kassondra; Osadchey Brown, Reilly; Gray, Danielle L.; Woods, Toby J.; Li, Jianing; et al (April 2021, Frontiers in Chemistry)

   null (Ed.)

   This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (S N Ar) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1 H- 1 H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C–H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules. 

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