Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds

Influence of Substituents in the Benzene Ring on the Halogen Bond of Iodobenzene with Ammonia

The effects on the CI··N halogen bond between iodobenzene and NH3 of placing various substituents on the phenyl ring are monitored by quantum calculations. Substituents R = N(CH3)2, NH2, CH3, OCH3, COCH3, Cl, F, COH, CN, and NO2 were each placed ortho, meta, and para to the I. The depth of the σ-hole on I is deepened as R became more electron-withdrawing which is reflected in a strengthening of the halogen bond, which varied between 3.3 and 5.5 kcal/mol. In most cases, the ortho placement yields the largest perturbation, followed by meta and then para, but this trend is not universal. Parallel to these substituent effects is a progressive lengthening of the covalent C-I bond. Formation of the halogen bond reduces the NMR chemical shielding of all three nuclei directly involved in the C-I··N interaction. The deshielding of the electron donor N is most closely correlated with the strength of the bond, as is the coupling constant between I and N, so both have potential use as spectroscopic measures of halogen bond strength.

Promotion of TH3 (T = Si and Ge) group transfer within a tetrel bond by a cation-π interaction

The possibility of the transfer of the TH₃ group across a tetrel bond is considered by ab initio calculations. The TB is constructed by pairing PhTH₃ (Ph = phenyl; T = Si and Ge) with bases NH₃, NHCH₂, and the C₃N₂H₄ carbene. The TH₃ moves toward the base but only by a small amount in these dimers. However, when a Be²⁺ or Mg²⁺ dication is placed above the phenyl ring, the tetrel bond strength is greatly magnified reaching up to nearly 100 kcal mol^-1. This dication also induces a much higher degree of transfer which can be best categorized as half-transfer for the two N-bases and a near complete transfer for the carbene.

Search for an exothermic halogen bond between anions

A halogen-bonded complex containing a pair of anions can be made more stable than the isolated anions if the Lewis acid is a long carbon chain, fully substituted by CN groups, with an I atom on one end and a COO− group on the other, with Cl− as base.

1,2-Dihydro-1,3,2-diazaborinine tautomer as an electron-pair donor in hydrogen-bonded complexes

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate 1,2-dihydro-1,3,2-diazaborinine:HX complexes for HX = H+, HF, HCl, H2O, HCN, NH3, HCP, and HCCH. Most complexes are stabilized by linear, traditional hydrogen bonds except for those with H2O and NH3, which have bridging structures and nonlinear hydrogen bonds. H-atom transfer from N to B can occur in complexes with HF and HCl, with formation of a traditional F–H···N bond and a proton-shared Cl···H···N bond. The binding energies of the uncharged complexes range from 25 to 88 kJ mol−1. Spin-spin coupling constants have been used to characterize these hydrogen-bonded complexes.

Probing Halogen Bonds by Scalar Couplings

As halogen bonding is a weak, transient interaction, its description in solution is challenging. We demonstrate that scalar coupling constants (J) are modulated by halogen bonding. The binding-induced magnitude change of one-bond couplings, even up to five bonds from the interaction site, correlates to the interaction strength. We demonstrate this using the NMR data of 42 halogen-bonded complexes in dichloromethane solution and by quantum chemical calculations. Our observation puts scalar couplings into the toolbox of methods for characterization of halogen bond complexes in solution and paves the way for their applicability for other types of weak σ-hole interactions.

Diboron Bonds Between BX3 (X=H, F, CH3 ) and BYZ2 (Y=H, F; Z=CO, N2 , CNH)

The ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations. Due to electron donation from its substituents, the trivalent B atom of BYZ₂ (Z=CO, N₂ , and CNH; Y=H and F) has the ability to in turn donate charge to the B of a BX₃ molecule (X=H, F, and CH₃ ), thus forming a B⋅⋅⋅B diboron bond. These bonds are of two different strengths and character. BH(CO)₂ and BH(CNH)₂ , and their fluorosubstituted analogues BF(CO)₂ and BF(CNH)₂ , engage in a typical noncovalent bond with B(CH₃ )₃ and BF₃ , with interaction energies in the 3-8 kcal/mol range. Certain other combinations result in a much stronger diboron bond, in the 26-44 kcal/mol range, and with a high degree of covalent character. Bonds of this type occur when BH₃ is added to BH(CO)₂ , BH(CNH)₂ , BH(N₂ )₂ , and BF(CO)₂ , or in the complexes of BH(N₂ )₂ with B(CH₃ )₃ and BF₃ . The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components, complemented by smaller polarization energy, while polarization is primarily responsible for the stronger ones.

Calculated coupling constants 1 J(X-Y) and 1 K(X-Y), and fundamental relationships among the reduced coupling constants for molecules Hm X-YHn , with X, Y ═ 1 H, 7 Li, 9 Be, 11 B, 13 C, 15 N, 17 O, 19 F, 31 P, 33 S, and 35 Cl

Equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to determine coupling constants ¹ J(X-Y) for 65 molecules H_m X-YH_n , with X,Y ═ ¹ H, ⁷ Li, ⁹ Be, ¹¹ B, ¹³ C, ¹⁵ N, ¹⁷ O, ¹⁹ F, ³¹ P, ³³ S, and ³⁵ Cl. The computed ¹ J(X-Y) values are in good agreement with available experimental data. The reduced coupling constants ¹ K(X-Y) have been derived from ¹ J(X-Y) by removing the dependence on the magnetogyric ratios of X and Y. Patterns are found for the reduced coupling constants on a ¹ K(X-Y) surface that are related to the positions of X and Y in the periodic table.

Hydrogen vs. Halogen Bonds in 1-Halo-Closo-Carboranes

A theoretical study of the hydrogen bond (HB) and halogen bond (XB) complexes between 1-halo-closo-carboranes and hydrogen cyanide (NCH) as HB and XB probe has been carried out at the MP2 computational level. The energy results show that the HB complexes are more stable than the XBs for the same system, with the exception of the isoenergetic iodine derivatives. The analysis of the electron density with the quantum theory of atoms in molecules (QTAIM) shows the presence of a unique intermolecular bond critical point with the typical features of weak noncovalent interactions (small values of the electron density and positive Laplacian and total energy density). The natural energy decomposition analysis (NEDA) of the complexes shows that the HB and XB complexes are dominated by the charge-transfer and polarization terms, respectively. The work has been complemented with a search in the CSD database of analogous complexes and the comparison of the results, with those of the 1-halobenzene:NCH complexes showing smaller binding energies and larger intermolecular distances as compared to the 1-halo-closo-carboranes:NCH complexes.

Anion Influence on the Packing of 1,3-Bis(4-Ethynyl-3-Iodopyridinium)-Benzene Halogen Bond Receptors

Rigid and directional arylethynyl scaffolds have been widely successful across diverse areas of chemistry. Utilizing this platform, we present three new structures of a dicationic 1,3-bis(4-ethynyl-3-iodopyridinium)-benzene halogen bonding receptor with tetrafluoroborate, nitrate, and hydrogen sulfate. Structural analysis focuses on receptor conformation, anion shape, solvation, and long range packing of these systems. Coupled with our previously reported structures, we conclude that anions can be classified as building units within this family of halogen bonding receptors. Two kinds of antiparallel dimers are observed for these dicationic receptors. An off-centered species is most frequent, present among geometrically diverse anions, and assorted receptor conformations. In contrast, the centered antiparallel dimers are observed with receptors adopting a bidentate conformation in the solid-state. While anions support the solid-state formation of dimers, the molecular geometry and characteristics (planarity, rigidity, and directionality) of arylethynyl systems increases the likelihood of dimer formation by limiting efficient packing arrangements. The significantly larger cation may have considerable influence on the solid-state packing, as similar cationic arylethynyl systems also display these dimers, suggesting.

A Continuum from Halogen Bonds to Covalent Bonds: Where Do λ3 Iodanes Fit?

The intrinsic bonding nature of λ 3 -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set ( ω B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in λ 3 -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF 2 − and the covalent FI molecule. The equatorial bonds in λ 3 -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of λ 3 -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties.

Dual function of the boron center of BH(CO)2/BH(N2)2 in halogen- and triel-bonded complexes with hypervalent halogens

The complexes between BH(CO)₂/BH(N₂)₂ and XF₃/XF₅ are stabilized by a halogen bond and a triel bond. The MEP analyses of BH(CO)₂/BH(N₂)₂ indicate that there are both a region with negative MEPs on the B atom in the vertical direction of the molecular plane and a σ-hole at the B-H bond end. Therefore, the boron atom in BH(CO)₂/BH(N₂)₂ plays a dual role of a Lewis base and an acid in the halogen bond and triel bond, respectively. The halogen and triel bonds are stronger in order of IF₃< BrF₃< ClF₃, IF₅< BrF₅< ClF₅, and BH(CO)₂< BH(N₂)₂ in most complexes. These complexes have large stability since the interaction energy varies from -5 to -115 kcal/mol. The halogen bond belongs to a covalent interaction or a partially covalent interaction in most complexes. The subsystems in these complexes have prominent deformation, accompanied with big charge transfer and large polarization.

Fostering the Basic Instinct of Boron in Boron-Beryllium Interactions

A set of complexes L₂HB···BeX₂ (L = CNH, CO, CS, N₂, NH₃, NCCH₃, PH₃, PF₃, PMe₃, OH₂; X = H, F) containing a boron-beryllium bond is described at the M06-2X/6-311+G(3df,2pd)//M062-2X/6-31+G(d) level of theory. In this quite unusual bond, boron acts as a Lewis base and beryllium as a Lewis acid, reaching binding energies up to -283.3 kJ/mol ((H₂O)₂HB···BeF₂). The stabilization of these complexes is possible thanks to the σ-donor role of the L ligands in the L₂HB···BeX₂ structures and the powerful acceptor nature of beryllium. According to the topology of the density, these B-Be interactions present positive laplacian values and negative energy densities, covering different degrees of electron sharing. ELF calculations allowed measuring the population in the interboundary B-Be region, which varies between 0.20 and 2.05 electrons upon switching from the weakest ((CS)₂HB···BeH₂) to the strongest complex ((H₂O)₂HB···BeF₂). These B-Be interactions can be considered as beryllium bonds in most cases.

Sigma-hole carbon-bonding interactions in carbon-carbon double bonds: an unnoticed contact

In this manuscript, we combine high-level ab initio calculations on some small complexes and a CSD survey to analyze the existence of unprecedented noncovalent carbon bonds in X₂C[double bond, length as m-dash]CH₂Y systems (Y = electron-rich atom or group). The methylene group is usually seen as a weak hydrogen bond donor when interacting with an electron-rich atom. However, we demonstrate that when the electron-rich atom is located equidistant from the two H atoms and along the C[double bond, length as m-dash]C bond a σ-hole noncovalent carbon-bonding interaction is established, instead of a bifurcated hydrogen bond, as derived from Atoms-in-Molecules (AIM) and Natural bond orbital (NBO) analyses. The physical nature of the interaction has been analyzed using the Symmetry Adapted Perturbation Theory (SAPT) method. The results indicate that electrostatics is very important followed by either the induction or dispersion terms in anionic and neutral complexes, respectively. In addition the CSD analysis reveals the existence of such interactions, giving reliability to our calculations, which are much more numerous for neutral than for anionic Y systems.

Hydrogen-Bonding Acceptor Character of Be3, the Beryllium Three-Membered Ring

The ability of Be₃ as a hydrogen bond acceptor has been explored by studying the potential complexes between this molecule and a set of hydrogen bond donors (HF, HCl, HNC, HCN, H₂O, and HCCH). The electronic structure calculations for these complexes were carried out at the MP2 and CCSD(T) computational levels together with an extensive NBO, ELF, AIM, and electrostatic potential characterization of the isolated Be₃ system. In all the complexes, the Be-Be σ bond acts as electron donor, with binding energies between 19 and 6 kJ mol^-1. A comparison with the analogous cyclopropane:HX complexes shows similar binding energies and contributions of the DFT-SAPT energetic terms. A blue-shift of the harmonic frequencies of Be₃ is observed upon complexation.

Halogen Bonding Involving CO and CS with Carbon as the Electron Donor

MP2/aug'-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH₂. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC:ClNC and SC:ClCl surfaces. SC:ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC:ClY complexes. The largest binding energies are found for the ion-pair SC-Cl⁺:-Y complexes. The transition structures which connect the complex and the ion pair on SC:ClNC and SC:ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC:ClY and SC:ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY. This secondary interaction assumes increased importance in the SC:ClNH₂ complex, and is a factor leading to its unusual structure. C-O and C-S stretching frequencies and 13C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC:ClY complexes and SC-Cl⁺:-Y ion pairs. Spin-spin coupling constants 1xJ(C-Cl) for OC:ClY complexes increase with decreasing distance. As a function of the C-Cl distance, 1xJ(C-Cl) and ¹J(C-Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.

Comparison of Various Means of Evaluating Molecular Electrostatic Potentials for Noncovalent Interactions

The various heterodimers formed by a series of Lewis acids with NH₃ as Lewis base are identified. Lewis acids include those that can form chalcogen (HSF and HSBr), pnicogen (H₂ PF and H₂ PBr), and tetrel (H₃ SiF and H₃ SiBr) bonds, as well as H-bonds and halogen bonds. The molecular electrostatic potential (MEP) of each Lewis acid is considered in a number of ways. Pictorial versions show broad regions of positive and negative MEP, on surfaces that vary with respect to either the value of the chosen isopotential, or their distance from the nuclei. Specific points are identified where the MEP reaches a maximum on a particular isodensity surface (V_s,max ). The locations and values of V_s,max were evaluated on different isodensity surfaces, and compared to the stabilities of the various equilibrium geometries. As the chosen isodensity is decreased, and the MEP maxima drift away from the molecule, some points maintain their angular positions with respect to the molecule, whereas others undergo a reorientation. The lowering isodensity also causes some of the maxima to disappear. In general, there is a fairly good correlation between the energetic ordering of the equilibrium structures and the values of V_s,max . A number of possible Lewis acid sites on the heteroaromatic imidazole ring were also considered and presents some cautions about application of V_s,max as the principal criterion for predicting equilibrium geometries. © 2017 Wiley Periodicals, Inc.

Borylenes: An Emerging Class of Compounds

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H₂ , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.