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1. Illuminating the Performance of Electron Withdrawing Groups in Halogen Bonding

   https://doi.org/10.1002/cphc.202400607  

   Devore, Daniel_P; Ellington, Thomas_L; Shuford, Kevin_L (November 2024, ChemPhysChem)

   Abstract Throughout the halogen bonding literature, electron withdrawing groups are relied upon heavily for tuning the interaction strength between the halogen bond donor and acceptor; however, the interplay of electronic effects associated with various substituents is less of a focus. This work utilizes computational techniques to study the degree ofσ‐ andπ‐electron donating/accepting character of electron withdrawing groups in a prescribed set of halo‐alkyne, halo‐benzene, and halo‐ethynyl benzene halogen bond donors. We examine how these factors affect theσ‐hole magnitude of the donors as well as the binding strength of the corresponding complexes with an ammonia acceptor. Statistical analyses aid the interpretation of how these substituents influence the properties of the halogen bond donors and complexes, and show that the electron withdrawing groups that are bothσ‐ andπ‐electron accepting form the strongest halogen bond complexes. 

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2. 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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3. Shedding Light on the Vibrational Signatures in Halogen‐Bonded Graphitic Carbon Nitride Building Blocks

   https://doi.org/10.1002/cphc.202200812  

   Ellington, Thomas_L; Devore, Daniel_P; Uvin_G_De_Alwis, W_M; French, Kirk_A; Shuford, Kevin_L (January 2023, ChemPhysChem)

   Abstract The relative contributions of halogen and hydrogen bonding to the interaction between graphitic carbon nitride monomers and halogen bond (XB) donors containing C−X and C≡C bonds were evaluated using computational vibrational spectroscopy. Conventional probes into select vibrational stretching frequencies can often lead to disconnected results. To elucidate this behavior, local mode analyses were performed on the XB donors and complexes identified previously at the M06‐2X/aVDZ‐PP level of theory. Due to coupling between low and high energy C−X vibrations, the C≡C stretch is deemed a better candidate when analyzing XB complex properties or detecting XB formation. The local force constants support this conclusion, as the C≡C values correlate much better with theσ‐hole magnitude than their C−X counterparts. The intermolecular local stretching force constants were also assessed, and it was found that attractive forces other than halogen bonding play a supporting role in complex formation. 

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4. The relationship between the crystal habit and the energy framework pattern: a case study involving halogen bonding on the edge of a covalent bond

   https://doi.org/10.1039/d3ce00316g  

   Torubaev, Yury V.; Howe, Devin; Leitus, Gregory; Rosokha, Sergiy V. (June 2023, CrystEngComm)

   The relationship between solid-state supramolecular interactions and crystal habits is highlighted based on experimental and computational analysis of the crystal structure of strong halogen-bonded (HaB) associations between iodine-containing dihalogens (ICl, IBr) with 1,4-diazabicyclo[2,2,2]octane (DABCO) as well as with substituted pyridines and phenazine. The pattern of the energy frameworks and the interplay of the attractive and repulsive interactions in the solid-state associations involving these HaB donors and acceptors directly correlated with their crystal habits. This correlation suggests that analysis of the energy framework serves as a useful tool (complementary to the earlier developed methods) to rationalize and predict the crystal habit. The X-ray structural analysis also revealed that the I⋯N distances in the complexes were in the 2.24–2.54 Å range, i.e. they were much closer to the I⋯N covalent bond length than to the van der Waals separation. The computational analysis of the nature of halogen bonding in these complexes showed delocalization of their molecular orbitals' between donor and acceptors resulting in a substantial charge transfer from the nucleophiles to dihalogens and elongation of the I⋯X bond. As a result, both I⋯N and I⋯X bonds in the strongest complexes ( e.g. , ICl with DABCO or 4-dimethylaminopyridine) are characterized by the comparable Mayer bonds orders of about 0.6, along with the electron and energy densities at their bond critical points of about 0.1 a.u. and −0.02 a.u., respectively. These data as well as the density overlap regions indicator (DORI) point to the covalency of the I⋯N bonding and suggest that the interaction within the IX complexes can be described as (unsymmetrical) hypervalent 3c/4e N⋯I⋯X bonding akin to that in trihalide or halonium ions. 

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5. 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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