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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) ismore »present in these pyrimidine-based solid-state co-crystals, halogen bonding and π-stacking interactions play more dominant roles in driving their molecular-assembly.« less
2. 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)

   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 andmore »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.« less
3. 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-stackingmore »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.« less
4. 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)

   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 withmore »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.« less
5. Increasing the structural boundary of quasiracemate formation: 4-substituted naphthylamides

   https://doi.org/10.1039/D0CE01331E  

   Craddock, Drew E. ; Parks, McKenzie J. ; Taylor, Lauren A. ; Wagner, Benjamin L. ; Ruf, Michael ; Wheeler, Kraig A. ( January 2021 , CrystEngComm)

   Quasiracemates – materials consisting of pairs of near enantiomers – form crystalline motifs that mimic the inversion relationships observed for their racemic counterparts. Recent investigations from our group explored a family of chiral ( N -benzoyl)methylbenzylamines to understand the structural boundary of cocrystallization. This investigation extends these earlier studies to include naphthylamide quasiracemates, where the molecular framework is ∼20% larger by volume than the previous diarylamides. A family of naphthylamides was prepared where the pendant functional group differs incrementally in size ( i.e. , H to C 6 H 5 ) to give 55 possible unique pairs of racemic andmore »quasiracemic combinations. Data collected from these materials using X-ray crystallography, thermal analysis methods and lattice energy calculations offer important insight into how a spatially larger naphthylamide molecular framework promotes greater structural variance of substituents during the pairwise assembly of quasienantiomers.« less