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1. Spectroscopic Measurement of a Halogen Bond Energy

   https://doi.org/10.1002/anie.201906279  

   Zhang, Xinxing; Liu, Gaoxiang; Ciborowski, Sandra; Wang, Wei; Gong, Chu; Yao, Yifan; Bowen, Kit (July 2019, Angewandte Chemie International Edition)

   Abstract Halogen bonding (XB) has emerged as an important bonding motif in supramolecules and biological systems. Although regarded as a strong noncovalent interaction, benchmark measurements of the halogen bond energy are scarce. Here, a combined anion photoelectron spectroscopy and density functional theory (DFT) study of XB in solvated Br⁻anions is reported. The XB strength between the positively‐charged σ‐hole on the Br atom of the bromotrichloromethane (CCl₃Br) molecule and the Br⁻anion was found to be 0.63 eV (14.5 kcal mol⁻¹). In the neutral complexes, Br(CCl₃Br)_1,2, the attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl₃Br, which is a second type of halogen bonding, was estimated to have interaction strengths of 0.15 eV (3.5 kcal mol⁻¹) and 0.12 eV (2.8 kcal mol⁻¹). 

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2. From weak to strong interactions: structural and electron topology analysis of the continuum from the supramolecular chalcogen bonding to covalent bonds

   https://doi.org/10.1039/d1cp05441d  

   Miller, Daniel K.; Chernyshov, Ivan Yu.; Torubaev, Yury V.; Rosokha, Sergiy V. (April 2022, Physical Chemistry Chemical Physics)

   The relationship between covalent and supramolecular bonding, and the criteria of the assignments of different interactions were explored via the review of selenium and tellurium containing structures in the Cambridge Structural Database and their computational analysis using Quantum Theory of Atoms in Molecules (QTAIM). This combined study revealed continuums of the interatomic Se⋯Br and Te⋯I distances, d Ch⋯X , in the series of associations from the sums of the van der Waals radii of these atoms ( r Ch + r X ) to their covalent bond lengths. The electron densities, ρ ( r ), at Bond Critical Points (BCPs) along the chalcogen bond paths increased gradually from about 0.01 a.u. common for the non-covalent interactions to about 0.1 a.u. typical for the covalent bonds. The log  ρ ( r ) values fell on the same linear trend line when plotted against normalized interatomic distances, R XY = d Ch⋯X /( r Ch + r X ). The transition from the positive to negative values of the energy densities, H ( r ), at the BCPs (related to a changeover of essentially non-covalent into partially covalent interactions) were observed at R XY ≈ 0.80. Synchronous changes of bonding characteristics with R XY (similar to that found earlier in the halogen-bonded systems) designated normalized interatomic separation as a critical factor determining the nature of these bondings. The uninterrupted continuums of Te⋯I and Se⋯Br bond lengths and BCPs’ characteristics signified an intrinsic link between limiting types of bonding involving chalcogen atoms and between covalent and supramolecular bonding in general. 

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3. Halogen Bonding versus Nucleophilic Substitution in the Co-Crystallization of Halomethanes and Amines

   https://doi.org/10.3390/cryst14020124  

   Grounds, Olivia; Zeller, Matthias; Rosokha, Sergiy V (February 2024, Crystals)

   Haloalkanes and amines are common halogen-bond (XB) donors and acceptors as well as typical reagents in nucleophilic substitution reactions. Thus, crystal engineering using these molecules requires an understanding of the interchange between these processes. Indeed, we previously reported that the interaction of quinuclidine (QN) with CHI3 in acetonitrile yielded co-crystals showing a XB network of these two constituents. In the current work, the interactions of QN with C2H5I or 1,4-diazabicyclo[2.2.2]octane (DABCO) with CH2I2 led to nucleophilic substitution producing I− anions and quaternary ammonium (QN-CH2CH3 or DABCO-CH2I+) cations. Moreover, the reaction of QN with CHI3 in dichloromethane afforded co-crystals containing XB networks of CHI3 with either Cl− or I− anions and QN-CH2Cl+ counter-ions. A similar reaction in acetone produced XB networks comprising CHI3, I− and QN-CH2COCH3+. These distinctions were rationalized through a computational analysis of XB complexes and the transition-state energies for the nucleophilic substitution. It indicated that the outcome of the reactions was determined mostly by the relative energies of the products. The co-crystals obtained in this work showed bonding between the cationic (DABCO-CH2I+, QN-CH2Cl+) or neutral (CHI3) XB donors and the anionic (I−, Cl−) or neutral (CHI3) acceptors. Their analysis showed comparable electron and energy densities at the XB bond critical points and similar XB energies regardless of the charges of the interacting species. 

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4. “Anti-electrostatic” halogen bonding in solution

   https://doi.org/10.1039/d1sc01863a  

   Loy, Cody; Holthoff, Jana M.; Weiss, Robert; Huber, Stefan M.; Rosokha, Sergiy V. (June 2021, Chemical Science)

   null (Ed.)

   Halogen-bonded (XB) complexes between halide anions and a cyclopropenylium-based anionic XB donor were characterized in solution for the first time. Spontaneous formation of such complexes confirms that halogen bonding is sufficiently strong to overcome electrostatic repulsion between two anions. The formation constants of such “anti-electrostatic” associations are comparable to those formed by halides with neutral halogenated electrophiles. However, while the latter usually show charge-transfer absorption bands, the UV-Vis spectra of the anion–anion complexes examined herein are determined by the electronic excitations within the XB donor. The identification of XB anion–anion complexes substantially extends the range of the feasible XB systems, and it provides vital information for the discussion of the nature of this interaction. 

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5. Halogen Bonding and Rearrangements in Complexes of N-Chlorosuccinimide with Halides

   https://doi.org/10.3390/molecules30030639  

   Hardin, Maison; Zeller, Matthias; Rosokha, Sergiy V (February 2025, Molecules)

   The role of halogen bonding (HaB) in the reactions of N-chlorosuccinimide (SimCl), a versatile reagent in organic synthesis, was investigated through experimental and computational analyses of its interactions with halides. The reactions of SimCl with Br− or I− resulted in the crystallization of HaB complexes of chloride with N-iodosuccinimide (SimI) or N-bromosuccinimide (SimBr). Computational analysis revealed that halogen rearrangements, which occurred even at −73 °C, were facilitated by halogen bonding. The dissociation of SimCl∙Y− (Y = I or Br) complexes into a Sim− + ClY pair (followed by the rotation and re-binding of the interhalogen molecules) bypassed the formation of the high-energy Sim− + Cl+ pair and drastically (about tenfold) reduced the dissociation energy of the N–Cl bond. Furthermore, while the dissociation energy of individual SimCl is higher (and its HaB is weaker) compared to that of SimI or SimBr, the dissociation of the N-Cl bond in SimCl∙Y− requires less energy than in the complexes of SimBr or SimI. The facile cleavage of such bonds in HaB complexes explains the high reactivity of SimCl and its effectiveness as a halogenating agent. 

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