Exploring the Competition between Halogen Bonding and CH Hydrogen Bonding in Bromoarenes Using an Aryldiyne Template

The interplay between pnicogen bonds and halogen bonds in nanostructured 4-XPhNH2---PH2FO---BrF (X = H, Me, NH2, N(Me)2, OH, OMe, F, Cl, Br, CHO, CN and NO2) complexes have been investigated using M06-2X/aug-cc-pVDZ quantum chemical calculations. The attraction energy of P---N and O---Br in the nanostructured 4-XPhH2N---PH2FO and PH2FO---BrF dimer systems increased because of the introduction of a third molecule and hence a positive cooperativity is observed during formation of the pnicogen bond and halogen bond in trimer complexes. The effect of substituent exchange on cooperativity between pnicogen bonds and halogen bonds was studied and the results cleared that the cooperativity between pnicogen and halogen bonds increases with an increase in the electron donor capability of substituents. In addition, the obtained values for cooperativity, showed a good correlation with Hammett’s substituent constants. Both the pnicogen and halogen bonds distances in the trimer complexes are dependent on the power of the P---N and O---Br bonds respectively and are shortened compared to the corresponding dimer complex systems. The shortening of both the pnicogen and halogen bond distances in the trimer complexes increases as the electron withdrawing nature of the substituent increases. Natural bond orbital theory is used to investigate how charge redistribution during molecular interactions, leads to the positive cooperative effects. Good correlations between amounts of charge transfer and pnicogen/halogen bond distance variations are obtained. Finally, molecular electrostatic potential and electron density difference maps were used for to increase the depth of understanding. The positive interplay between pnicogen and halogen bonds within the studied complexes is consistent with the results of both the molecular electrostatic potential and electron density difference maps.

A number of prototypical weak electron donor–electron acceptor complexes are investigated by the Symmetry Adapted Perturbation Theory, some of which belong to novel classes of weak bonds such as halogen and chalcogen bonds. Also included are complexes involving strong Lewis acids such as BeO and AuF. The common view in the literature is to associate these novel bonds with a variety of “holes”, σ, π, δ, or positive areas in their electrostatic potential maps. The presumption is that these positive areas of the electrostatic potential are indicative of the electrostatic nature of these noncovalent bonds. The electrostatic view extends to the explanations of the directionality of approaches between the subsystems forming these bonds. This work demonstrates that one common feature of these electrostatic potential “holes” is the local depletion of electron density of which the best detector is the first-order Pauli repulsion. The minimization of this repulsion determines the bond directionality and its relative angular rigidity. In relatively strong complexes of BeO with rare gases, where BeO shows a clear cavity in electron density—an ultimate “σ hole”—the electrostatic effect does not control the bending potential—the exchange repulsion does. In halogen bonds, the halogen atom is nonspherical, displaying an axial “σ hole” in its electrostatic potential. However, in no examined case, from rare gas acting as an electron donor to a polar donor to an anionic donor, is the electrostatic energy responsible for the directionality of the halogen bond. In fact, it is not even maximized in the direction of the σ hole in N2-ClF and NH3-ClF. Yet, in all the cases, the exchange repulsion is minimized in the direction of the σ hole. The minimized exchange repulsion associated with the subtle and less subtle depletions of the electron density occur on the nodal planes or on the intersections thereof in the highest occupied molecular orbitals of Lewis acids, provided that the systems are closed-shell. The role of nodal planes in covalent and coordinate covalent bonds is well recognized. This work points to their similarly equal importance in certain types of donor–acceptor noncovalent interactions.

Summary This review covers advances in anion complexation in 2013, 2014, and 2015. The review focuses on the applications of anion receptor chemistry, including sensing, self-assembly, extraction, transport, catalysis, and fundamental advances in the area.

A “nucleophilic” iodine in a halogen-bonded iodonium complex manifests an unprecedented I+···Ag+ interaction

The capacity of SX2 (X = F, Cl, and Br) to engage in different kinds of noncovalent bonds was investigated by ab initio calculations. SCl2 (SBr2) has two σ-holes upon extension of Cl (Br)-S bonds, and two σ-holes upon extension of S-Cl (Br) bonds. SF2 contains only two σ-holes upon extension of the F-S bond. Consequently, SCl2 and SBr2 form chalcogen and halogen bonds with the electron donor H2CO while SF2 forms only a chalcogen bond, i.e., no F···O halogen bond was found in the SF2:H2CO complex. The S···O chalcogen bond between SF2 and H2CO is the strongest, while the strongest halogen bond is Br···O between SBr2 and H2CO. The nature of these two types of noncovalent interaction was probed by a variety of methods, including molecular electrostatic potentials, QTAIM, energy decomposition, and electron density shift maps. Termolecular complexes X2S···H2CO···SX'2 (X = F, Cl, Br, and X' = Cl, Br) were constructed to study the interplay between chalcogen bonds and halogen bonds. All these complexes contained S···O and Cl (Br)···O bonds, with longer intermolecular distances, smaller values of electron density, and more positive three-body interaction energies, indicating negative cooperativity between the chalcogen bond and the halogen bond. In addition, for all complexes studied, interactions involving chalcogen bonds were more favorable than those involving halogen bonds. Graphical Abstract Molecular electrostatic potential and contour map of the Laplacian of the electron density in Cl2S···H2CO···SCl2 complex.

In the present study we have characterized the halogen bonding in selected molecules H3N–ICF3 (1-NH3), (PH3)2C–ICF3 (1-CPH3), C3H7Br–(IN2H2C3)2C6H4 (2-Br), H2–(IN2H2C3)2C6H4 (2-H2) and Cl–(IC6F5)2C7H10N2O5 (3-Cl), containing from one halogen bond (1-NH3, 1-CPH3) up to four connections in 3-Cl (the two Cl–HN and two Cl–I), based on recently proposed ETS-NOCV analysis. It was found based on the NOCV-deformation density components that the halogen bonding C–X…B (X-halogen atom, B-Lewis base), contains a large degree of covalent contribution (the charge transfer to X…B inter-atomic region) supported further by the electron donation from base atom B to the empty σ*(C–X) orbital. Such charge transfers can be of similar importance compared to the electrostatic stabilization. Further, the covalent part of halogen bonding is due to the presence of σ-hole at outer part of halogen atom (X). ETS-NOCV approach allowed to visualize formation of the σ-hole at iodine atom of CF3I molecule. It has also been demonstrated that strongly electrophilic halogen bond donor, [C6H4(C3H2N2I)2][OTf]2, can activate chemically inert isopropyl bromide (2-Br) moiety via formation of Br–I bonding and bind the hydrogen molecule (2-H2). Finally, ETS-NOCV analysis performed for 3-Cl leads to the conclusion that, in terms of the orbital-interaction component, the strength of halogen (Cl–I) bond is roughly three times more important than the hydrogen bonding (Cl–HN).FigureETS-NOCV reprezentation of σ-hole at iodine together with the molecular electrostatic potential picture

Halogen and hydrogen bonds [1] are most often associated with the structure of molecular crystals. Even weak specific interactions, such as halogen···halogen and CH···halogen contacts, can compete between themselves and with Kitaigorodski's close packing rule. The competition between halogen···halogen and CH···halogen interactions has been studied at high pressure for the series of six dihalomethanes CH2XY (X,Y = Cl, Br, I). They crystallize in several structural types of space groups Pbcn, C2/c, Pnma, Pna21or Fmm2. In all these compounds and in their polymorphs the halogen···halogen and CH···halogen interactions persist despite considerable structural differences. The group of monohalomethanes (CH3X, X = Cl, Br, I) are the simplest organic polar compounds and ideal models for studying halogen···halogen and CH···halogen interactions. For these simplest haloalkanes, the halogen···halogen competition with CH···halogen bonds, scaled in the function of electrostatic potential in the Cl, Br, I series, is affected by pressure. Phase α-CH3Br, isostructural with CH3I (orthorhombic space group Pnma) and dominated by halogen···halogen bonds, is destabilized by pressure. At 1.5 GPa the ambient-pressure α-CH3Br phase transforms into phase β-CH3Br governed by CH···halogen interactions. Phase β of CH3Br is isostructural with CH3Cl, orthorhombic space group Cmc21[2,3]. The CH3Br molecules are more evenly accommodated in space group Cmc21and CH···halogen interactions are favoured by the close-packing effect.

The influence of the halogen substituent on crystal packing and redox properties is investigated in a series of heteroleptic complexes [Fe(qsal-X)(dipic)]MeOH (qsal-X = 4-halogen-2-[(8-quinolylimino)methyl]phenolate; dipic = 2,6-pyridinedicarboxylate; X = F 1, Cl 2, Br 3 and I 4).Compounds 1 and 2 exhibit triclinic symmetry (P1̅), whereas 3 and 4 crystallise in monoclinic P21/n. The crystal packing shows self-sorting of the ligands with - interactions between the qsal-X ligands and overlap of the dipic ligands to form a 1D chain, that is supported by C-H···O interactions. In 1 and 2, the cross-section of the 1D chain is square, while for 3 and 4, it is rectangular. In the former, the dipic ligands interact through C=O··· interactions, while - interactions are found in 3 and 4. Neighbouring chains are connected via - interactions involving the quinoline rings, but their relative position is driven by the preference of 1 and 2, for C-H···X interactions, whereas 3 and 4 form O···X halogen bonds. The nature and topology of the electron density of these interactions have been investigated using molecular electrostatic potential (MEP) mapping, quantum theory of atoms in molecules (QTAIM) and ‘non-covalent interactions’ (NCI) analysis. UV-Visible experiments show MLCT bands associated with the qsal-X ligands, confirming the structure is stable in solution. Electrochemical studies reveal slight tuning of the Fe3+/Fe2+ redox couple showing a linear relationship between E° and the Hammett parameter σp.

The electron-electron relaxation and correlation-driven charge migration process, which features pure electronic aspect of ultrafast charge migration phenomenon, occurs on a very short timescale in ionized molecules and molecular clusters, prior to the onset of nuclear motion. In this article, we have presented nature of ultrafast pure electronic charge migration dynamics through Cl…..N, Cl…..O, Br…..N, and Br…..O halogen bonds, explored using density functional theory. We have explored the role of donor, acceptor, electron correlation, vibration and rotation in charge migration dynamics through these halogen bonds. For this work, we have selected ClF, Cl2, ClOH, ClCN, BrF, BrCl, BrOH, and BrCN molecules paired with either NH3 or H2O. We have found that the timescale for pure electron-electron relaxation and correlation-driven charge migration through the Cl…..N, Br…..N, Cl…..O, and Br…..O halogen bonds falls in the range of 300–600 attosecond. The primary driving force behind the attosecond charge migration through the Cl…..N, Br…..N, Cl…..O, and Br…..O halogen bonds is the energy difference (ΔE) between two stationary cationic orbitals (LUMO- β and HOMO- β), which together represents the initial hole density immediately following vertical ionization. We have also predicted that the strength of electron correlation has significant effect on the charge migration timescale in Cl…..N, Br…..N, Cl…..O, and Br…..O halogen bonded clusters. Vibration and rotation are also found to exhibit profound effect on attosecond charge migration dynamics through halogen bonds. The attosecond charge migration dynamics through Cl…..N, Cl…..O, Br…..N, and Br…..O halogen bonds depends on strength of electron correlation, donor and acceptor, the energy difference (ΔE) between two stationary cationic orbitals (LUMO-β and HOMO-β) involved in electronic superposition, vibration and rotation.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate hydrogen bonding, halogen bonding, and pnicogen bonding involving tetrahedral P4 and the FH, ClH, and FCl molecules. P4 has three unique interaction sites: at a vertex (designated the P1 atom); at an edge (the P2-P3 bond); and at the P2-P3-P4 face. The uniqueness of molecular P4 is its ability to act as an electron donor and an electron acceptor at the same site, except for the P2-P3 bond, which is only an electron donor. FCl and FH form five different complexes with P4, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P4 and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P4 molecule acting as the base are greater than the binding energies when P4 is the acid. Thus, P4 is a better electron donor than an electron acceptor. Charge-transfer interactions and EOM-CCSD spin-spin coupling constants across hydrogen, halogen, and pnicogen bonds are reported for all of the P4 complexes. Relative to 1J(Pi-Pj) in molecular P4, 1J(P1-P2) coupling constants decrease in absolute value and 1J(P2-P3) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a PF halogen bond. Absolute values of 1J(P1-P2) increase and those of 1J(P2-P3) decrease in hydrogen-bonded complexes and complexes with PCl halogen bonds. 1J(P1-P2) and 1J(P2-P3) exhibit a single linear correlation with the corresponding Pi-Pj distances.

O-H...X and O-H...O H-bonds as well as C-X...X dihalogen and C-X...O halogen bonds have been investigated in halomethanol dimers (bromomethanol dimer, iodomethanol dimer, difluorobromomethanol…bromomethanol complex and difluoroiodomethanol…iodomethanol complex). Structures of all complexes were optimized at the counterpoise-corrected MP2/cc-pVTZ level and single-point energies were calculated at the CCSD(T)/aug-cc-pVTZ level. Energy decomposition for the bromomethanol dimer complex was performed using the DFT-SAPT method based on the aug-cc-pVTZ basis set. OH...O and OH...X H-bonds are systematically the strongest in all complexes investigated, with the former being the strongest bond. Halogen and dihalogen bonds, being of comparable strength, are weaker than both H-bonds but are still significant. The strongest bonds were found in the difluoroiodomethanol…iodomethanol complex, where the O-H...O H-bond exceeds 7 kcal mol(-1), and the halogen and dihalogen bonds exceed 2.5 and 2.3 kcal mol(-1), respectively. Electrostatic energy is dominant for H-bonded structures, in halogen bonded structures electrostatic and dispersion energies are comparable, and, finally, for dihalogen structures the dispersion energy is clearly dominant.

To explore the nature of unconventional halogen bonds, the halogen bonds between a series of halides FXOn (X = Cl, Br; n = 0-3) and CH3CN have been studied at M05-2X/6-311++G(d,p), MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels. Our calculations reveal that the electrostatic potentials of these hyper-valent halogen atoms are greatly different to those of monovalent halogen atoms and, accordingly, the strength and interaction modes of these halogen bonds show different characteristics. For complexes of monovalent and heptavalent halides, only one interaction mode was found, but for those of tervalent and pentavalent halides, three kinds of stable configurations with different interaction modes were recognized. Configuration I is a linear structure formed by a single halogen bond, while configurations II and III are cyclic structures formed by the cooperation of a halogen bond and a hydrogen bond. From an energy point of view, monovalent halides form the strongest halogen bonds, and heptavalent halides feature the weakest halogen bonds. For tervalent and pentavalent halides, the interaction energies of the three configurations are similar. In the two cyclic configurations of complexes formed by tervalent and pentavalent halides, due to the competition of the halogen bond with the hydrogen bond, the halogen bonds are weaker than that of configuration I. NBO analysis indicates that the three configurations of complexes display different donor-acceptor orbital interactions. AIM and LMO-EDA analysis reveal that the electrostatic interaction is the dominant driving force for the formation of the complexes, and all the halogen and hydrogen bonds in these complexes are closed-shell interactions.

The complexes formed as a result of the interactions between cyanophosphine (CP, H2PCN) and hypohalous acid molecules (HOX, X = F, Cl, Br, and I) were studied by employing ab initio computations conducted at the MP2/aug-cc-pVTZ level. Three types of complexes were acquired (I, II, and III) as a result of the (O∙∙∙P) pnicogen bond, the (N∙∙∙H) hydrogen bond, and the (N∙∙∙X) halogen bond interaction, respectively. The results of harmonic vibrational frequency calculations with no imaginary frequencies confirmed the structures as minima. In addition, given the interaction energy of the complexes, hydrogen bond complexes of structure II have the highest stability compared to other structures. In all studied complexes, the strength of the interactions depended on the electronegativity of the halogen atoms. The characteristics and nature of the whole three types of complexes were examined and evaluated with natural bond orbital (NBO), atom in molecules (AIM), molecular electrostatic potential (MEP) maps, non-covalent interaction (NCI) index, and electron density difference (EDD) analyses. The optimization of all complexes and corresponding monomers was conducted through the ab initio method, employing the MP2 level along with the aug/cc-pVTZ basis set for all atoms, except for the iodine (I) atom, for which the aug-cc-pVTZ (PP) basis set was employed. Subsequent frequency calculations were executed to ascertain the minimum energy state of the complexes at the MP2 level and the aug/cc-pVTZ basis set, utilizing Gaussian09 software. The MEP maps of the monomers were generated using the analysis-surface suite (WFA-SAS) software package. To probe the orbital interactions within the studied complexes, NBO analysis was performed employing NBO software. The assessment of bond nature, topological features, and electron density values at critical points for the studied complexes was undertaken using AIMAll software. The NCI index was derived utilizing Multiwfn software, and its three-dimensional representation was rendered using VMD software.

N-heterocyclic carbenes (NHCs) are important ligands in organometallic catalytic reactions. This study focuses on the halogen bonds with NHCs as the electron donors. The results show that NHCs are better electron donors in halogen bonds. Our interest is how to make a halogen bond having a partially covalent property, which depends on the strength of halogen bonding. The covalent property of halogen bonding is related to the nature of the halogen donor. Iodine is favourable to form a halogen bond with covalent property than chlorine and bromine. The covalent property of halogen bond is greatly affected by substituents. Strong electron-donating groups in NHCs could reinforce the covalent attribute of bromine bonding, whereas strong electron-withdrawing groups in NHCs make iodine bonding lose the covalent nature. The covalent property of halogen bond is further heightened through the cooperative effect between the carbene–halogen bond and another interaction. This covalent property results in a much short binding distance and a prominent bond elongation. The covalent property of halogen bond has been analysed with the energy density, electron local function, and electron density shifts.

To study the potential of Keggin-type polyoxometalateanions toact as halogen bond acceptors, we have prepared a series of 10 halogen-bondedcompounds starting from phosphomolybdic and phosphotungstic acid andhalogenopyridinium cations as halogen (and hydrogen) bond donors.In all the structures, the cations and the anions were interconnectedby halogen bonds, more often with terminal M=O oxygen atomsthan with bridging oxygen atoms as acceptors. In four structures comprisingprotonated iodopyridinium cations capable of forming both hydrogenand halogen bonds with the anion, the halogen bond with the anionis apparently favored, whereas hydrogen bonds preferentially involveother acceptors present in the structure. In three obtained structuresderived from phosphomolybdic acid, the corresponding oxoanion hasbeen found in its reduced state [Mo12PO40]4–, which has also led to a decrease in halogen bondlengths as compared to the fully oxidated [Mo12PO40]3–. The electrostatic potential on the three typesof anions involved in the study ([Mo12PO40]3–, [Mo12PO40]4–, and [W12PO40]3–) has beencalculated for optimized geometries of the anions, and it has beenshown that the terminal M=O oxygen atoms are the least negativesites of the anions, indicating that they act as halogen bond acceptorsprimarily due to their steric availability.