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1. The HOX⋯SO ₂ (X=F, Cl, Br, I) Binary Complexes: Implications for Atmospheric Chemistry

   https://doi.org/10.1002/cphc.202000746  

   Kitzmiller, Nathaniel_L; Wolf, Mark_E; Turney, Justin_M; Schaefer, III, Henry_F (December 2020, ChemPhysChem)

   Abstract Sulfur dioxide and hypohalous acids (HOX, X=F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. We present the first theoretical examination of the HOX⋯SO₂binary complexes and the associated trends due to halogen substitution. Reliable geometries were optimized at the CCSD(T)/aug‐cc‐pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug‐cc‐pV(D+d)Z level of theory with the exception of the Br and I atoms which were modeled with an aug‐cc‐pwCVDZ‐PP pseudopotential. 27 HOX⋯SO₂complexes were characterized and the focal point method was employed to produce CCSDT(Q)/CBS interaction energies. Natural Bond Orbital analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each principle interaction. The interaction energies of all HOX⋯SO₂complexes in this study ranged from 1.35 to 3.81 kcal mol⁻¹. The single‐interaction hydrogen bonded complexes spanned a range of 2.62 to 3.07 kcal mol⁻¹, while the single‐interaction halogen bonded complexes were far more sensitive to halogen substitution ranging from 1.35 to 3.06 kcal mol⁻¹, indicating that the two types of interactions are extremely competitive for heavier halogens. Our results provide insight into the interactions between HOX and SO₂which may guide further research of related systems. 

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2. Weak σ‐Hole Triel Bond between C ₅ H ₅ Tr (Tr=B, Al, Ga) and Haloethyne: Substituent and Cooperativity Effects

   https://doi.org/10.1002/cphc.202000955  

   Yang, Qingqing; Zhou, Bohua; Li, Qingzhong; Scheiner, Steve (February 2021, ChemPhysChem)

   Abstract The replacement of a CH group of benzene by a triel (Tr) atom places a positive region of electrostatic potential near the Tr atom in the plane of the aromatic ring. This σ‐hole can interact with an X lone pair of XCCH (X=F, Cl, Br, and I) to form a triel bond (TrB). The interaction energy between C₅H₅Tr and FCCH lies in the range between 2.2 and 4.4 kcal/mol, in the order Tr=B+cation above the ring pulls density toward itself and thus magnifies the Tr σ‐hole. The TrB to the XCCH nucleophile is thereby magnified as is the strength of the TrB. This positive cooperativity is particularly large for Tr=B. 

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3. A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H ₂ O (X = F, Cl, Br)

   https://doi.org/10.1039/C9CP00422J  

   Wolf, Mark E.; Zhang, Boyi; Turney, Justin M.; Schaefer, Henry F. (March 2019, Physical Chemistry Chemical Physics)

   Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol −1 , −5.56 kcal mol −1 , and −4.97 kcal mol −1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol −1 and −3.03 kcal mol −1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size. 

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4. Halogen Bonding in the Complexes of Brominated Electrophiles with Chloride Anions: From a Weak Supramolecular Interaction to a Covalent Br–Cl Bond

   https://doi.org/10.3390/cryst10121075  

   Loy, Cody; Zeller, Matthias; Rosokha, Sergiy V. (December 2020, Crystals)

   null (Ed.)

   The wide-range variation of the strength of halogen bonds (XB) not only facilitates a variety of applications of this interaction, but it also allows examining the relation (and interconversion) between supramolecular and covalent bonding. Herein, the Br…Cl halogen bonding in a series of complexes of bromosubstituted electrophiles (R-Br) with chloride anions were examined via X-ray crystallographic and computational methods. Six co-crystals showing such bonding were prepared by evaporation of solutions of R-Br and tetra-n-propylammonium chloride or using Cl− anions released in the nucleophilic reaction of 1,4-diazabicyclo[2.2.2]octane with dichloromethane in the presence of R-Br. The co-crystal comprised networks formed by 3:3 or 2:2 halogen bonding between R-Br and Cl−, with the XB lengths varying from 3.0 Å to 3.25 Å. Analysis of the crystallographic database revealed examples of associations with substantially longer and shorter Br…Cl separations. DFT computations of an extended series of R–Br…Cl− complexes confirmed that the judicious choice of brominated electrophile allows varying halogen Br…Cl bond strength and length gradually from the values common for the weak intermolecular complexes to that approaching a fully developed covalent bond. This continuity of halogen bond strength in the experimental (solid-state) and calculated associations indicates a fundamental link between the covalent and supramolecular bonding. 

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5. Di(indenyl)beryllium

   https://doi.org/10.1002/anie.202107980  

   Koby, Ross_F; Schley, Nathan_D; Hanusa, Timothy_P (August 2021, Angewandte Chemie International Edition)

   Abstract The bonding in beryllocene, [BeCp₂], took decades to establish, owing to its unexpected mixed hapticity structure (i.e., [Be(η⁵‐Cp)(η¹‐Cp)]). Beryllium complexes containing the indenyl ligand, which is a close relative of the cyclopentadienyl anion, but which is also known to exhibit its own bonding peculiarities (e.g., facile η⁵⇄ η³shifts), have remained unknown. Standard metathetical approaches to their synthesis (e.g., with K[Ind′] + BeX₂in an ether solvent) give rise to intractable oils from which nothing identifiable can be isolated. In contrast, mechanochemical preparation, involving the solvent‐free grinding of BeBr₂and potassium indenides, leads to the production of discrete (indenyl)beryllium complexes, including [Be(C₉H₇)₂] (1) and [Be{1,3‐(SiMe₃)₂C₉H₅}Br] (2). The former displays η⁵/η¹‐coordinated ligands in the solid state, but DFT calculations indicate that an η⁵/η⁵‐conformation is less than 5 kcal mol⁻¹higher in energy. 

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