Van der Waals Complexes between Unsaturated Hydrocarbons and Boron Trifluoride: An Infrared and ab Initio Study of Ethene·BF3 and Propene·BF3

Strong Triel Bonds with Be as Electron Donor

A systematic theoretical study was conducted on the triel bonds (TrBs) within the TrX3···Be(CO)3 complexes (Tr = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I). The interaction energies of these systems range between 4 and 38 kcal/mol. The TrB weakens as X becomes more electronegative in the B and Al systems, while the opposite pattern of stronger bonds is observed in the In and Tl analogues. The dominant component of the TrB is polarization energy, which arises from charge transfer from Be(CO)3 to TrX3. The source of the density is a confluence of CO π-bonding orbitals at the Be center that resembles a Be lone pair, and which makes the molecular electrostatic potential above the Be somewhat negative. This π-lump is paired with the highly positive π-hole above the Tr, and a large amount of charge is transferred to the empty pz orbital of Tr. These factors, when considered in conjunction with large AIM measures, confer on this TrB a certain degree of covalency.

Triel Bonds between BH3/C5H4BX and M(MDA)2 (X = H, CN, F, CH3, NH2; M = Ni, Pd, Pt, MDA = Enolated Malondialdehyde) and Group 10 Transition Metal Electron Donors

A systematic theoretical study was conducted on the triel bonds (TrB) within the BH3∙∙∙M(MDA)2 and C5H4BX∙∙∙M(MDA)2 (M = Ni, Pd, Pt, X = H, CN, F, CH3, NH2, MDA = enolated malondialdehyde) complexes, with BH3 and C5H4BX acting as the electron acceptors and the square-coordinated M(MDA)2 acting as the electron donor. The interaction energies of these systems range between -4.71 and -33.18 kcal/mol. The larger the transition metal center M, the greater the enhancement of the TrB, with σ-hole TrBs found to be stronger than π-hole TrBs. In the σ-hole TrB complex, an electron-withdrawing substituent on the C opposite to the B atom enhances the TrB, while an electron-donating substituent has little effect on the strength of TrB in the Pd and Pt complexes but enhances the TrB in the Ni-containing complexes. The van der Waals interaction plays an important role in stabilizing these binary systems, and its contribution diminishes with increasing M size. The orbital effect within these systems is largely due to charge transfer from the dz2 orbital of M into the empty pz orbital of B.

Skeletal rearrangement of a boron-containing annulenic molecule into a macrocycle bridged by an electronically stabilized boron cation

An annulenic molecule containing a three-coordinate chloroborane moiety, which exhibits a borane-olefin proximity effect, undergoes a skeletal rearrangement upon chloride abstraction, to generate a three-dimensional macrocyclic molecule featuring a borocenium (η5-cyclopentadienyl-B+-R) structure.

Bringing out the potential of organoboron compounds by designing the chemical bonds and spaces around boron

Since the structures, reactivity and properties of organoboron compounds stem from the electron deficiency and low electronegativity of boron, the design of the chemical bonds attached to boron as well...

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

Study of the influence of intermolecular interaction on classical and reverse substituent effects in para-substituted phenylboranes

The substituent effect and the reverse substituent effect in para-substituted phenylboranes and the influence of the intermolecular interaction of H⋯B type with either silane or methylsilane on the latter of these effects are extensively studied.

Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

The BCl₃ promoted aminoboration of alkenes and alkynes was investigated both computationally and experimentally, leading to the discovery of a metal-free hydroamination of alkynes.

Hydride-Triel Bonds

In this article, we present the results of our comprehensive studies of 72 dimers of the R3XXH⋯YR3Y type (X = Si, Ge; Y = B, Al, Ga; R^X  = H, Cl, Me; R^Y  = H, F, Cl, Me) and featuring hydride-triel bonds (i.e., charge-inverted hydrogen bonds). Influence of X and Y atoms as well as R^X and R^Y substituents on various properties of these dimers is investigated in detail. In particular the strength of the H⋯Y hydride-triel bonds is paid a close attention and it is shown that hydride-triel bonds can be strong enough to considerably determine structure and properties of molecular systems. In addition, properties of the investigated dimers are largely governed by the charge transfer from the Lewis base to the Lewis acid, which is particularly important if more bulky and polarizable R^Y and Y atoms are present in the YR3Y molecule. Several excellent linear (R² close to 1) and exponential correlations between pairs of diverse parameters are presented. Few instances are discussed where somewhat unexpected bond paths exist between two atoms featuring partial negative charges (e.g., between hydride hydrogen and halogen and between lateral sides of two halogens) showing that in some cases a bond path prefers to link two closely spaced electron-rich atoms instead of two atoms that are expected to form a bond. © 2018 Wiley Periodicals, Inc.

Revealing the physical nature and the strength of charge-inverted hydrogen bonds by SAPT(DFT), MP2, SCS-MP2, MP2C, and CCSD(T) methods

The physical nature of charge-inverted hydrogen bonds in H₃ XH ⋯YH₃ (X = Si, Ge; Y = Al, Ga) dimer systems is studied by means of the SAPT(DFT)-based decomposition of interaction energies and supermolecular interaction energies based on MP2, SCS-MP2, MP2C, and CCSD(T) methods utilizing dimer-centered aug-cc-pCVnZ (n = D, T, Q) basis sets as well as an extrapolation to the complete basis set limit. It is revealed that charge-inverted hydrogen bonds are inductive in nature, although dispersion is also important. Computed interaction energies form the following relation: EintSAPT<EintSCS-MP2≤EintMP2C<EintMP2≈EintCCSD(T). It is confirmed that the aug-cc-pCVDZ basis set performs poorly and that very accurate values of interaction and dispersion energies require basis sets of at least quadrupole-ζ quality. Considerably large binding energies suggest potential usefulness of charge-inverted hydrogen bonds as an important structural motif in molecular binding. Terminology applying to σ- and π-hole interactions as well as to triel and tetrel bonds is discussed. According to this new terminology the charge-inverted hydrogen bond would become the first described case of a hydride-triel bond. © 2017 Wiley Periodicals, Inc.

Efficient Synthesis of Dimeric Oxazoles, Piperidines and Tetrahydroisoquinolines from N-Substituted 2-Oxazolones

A mild and practical method for the construction of heterocycles from N-substituted 2-oxazolones through cascade, BF3 ⋅Et2 O/H2 O-catalyzed reactions involving iminium ion generation and trapping by external or internal olefinic and aryl moieties is described. Mechanistic and computational studies revealed the strong protic acid HBF4 as the initiating catalyst for these cascade reactions. Providing access to novel molecular diversity, these processes may facilitate chemical biology studies, drug discovery efforts and natural products synthesis.

Reversible [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine with ethylene

Under ambient conditions, a [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine 1 with ethylene afforded a bicyclo[2.2.2] derivative 2, which was structurally characterized.

Under ambient conditions, a [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine 1 with ethylene afforded a bicyclo[2.2.2] derivative 2, which was structurally characterized. The cyclization process was found to be reversible, and thus retro-[4 + 2] cycloaddition reproduced 1 quantitatively, concomitant with the release of ethylene. Compound 1 reacted regio-selectively and stereo-selectively with styrene derivatives and norbornene, respectively, and these processes were found to be reversible too. Computational studies determined the reaction pathways which were consistent with the regio-selectivity observed in the reaction of styrene, and the reaction was suggested to be essentially concerted but highly asynchronous.

Triel Bonds, π-Hole-π-Electrons Interactions in Complexes of Boron and Aluminium Trihalides and Trihydrides with Acetylene and Ethylene

MP2/aug-cc-pVTZ calculations were performed on complexes of aluminium and boron trihydrides and trihalides with acetylene and ethylene. These complexes are linked through triel bonds where the triel center (B or Al) is characterized by the Lewis acid properties through its π-hole region while π-electrons of C₂H₂ or C₂H₄ molecule play the role of the Lewis base. Some of these interactions possess characteristics of covalent bonds, i.e., the Al-π-electrons links as well as the interaction in the BH₃-C₂H₂ complex. The triel-π-electrons interactions are classified sometimes as the 3c-2e bonds. In the case of boron trihydrides, these interactions are often the preliminary stages of the hydroboration reaction. The Quantum Theory of “Atoms in Molecules” as well as the Natural Bond Orbitals approach are applied here to characterize the π-hole-π-electrons interactions.

Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Recently, main group reagents have attracted a lot of attention in bond-forming reactions in organic synthesis. This article highlights the use of pentafluorophenyl substituted boron reagents in their reactions with C=C and C≡C π-bonds for the synthesis of heterocyclic and aromatic compounds. These cyclisation reactions fall into four general classes although there is some overlap between classes and often combinations of these different types of reactivity are observed in the formation of the final heterocyclic product: (i) 1,2- (and 1,4-) additions of nucleophile and Lewis-acidic boron centre, (ii) 1,1-carboboration, (iii) carbocation rearrangements and (iv) cycloaddition chemistry. In addition, the prospect of using such boron reagents catalytically in the synthesis of aromatic compounds such as oxazoles and dibenzopentalene derivatives is emphasised.

Frustrated Lewis pairs: a new strategy to small molecule activation and hydrogenation catalysis

The combination of Lewis acids and bases that are sterically precluded from forming Lewis acid-base adducts, termed Frustrated Lewis pairs provide a unique route to the activation of small molecules and applications in catalysis.

Gas Sensing Mechanism in Chemiresistive Cobalt and Metal-Free Phthalocyanine Thin Films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.

An ab initio and cryospectroscopic study of the hydrogen chloride and boron trifluoride complexes of cyclopropene

Mid-infrared spectra of cyclopropene (c-C3H4) mixtures with HCl or BF3, dissolved in liquefied argon and in liquefied nitrogen have been examined. Evidence was found for the formation of two different isomers of the 1:1 complexes: a pi-type complex in which the interaction occurs with the pi-bond of cyclopropene, and a sigma-type complex in which the electron acceptors interact with one of the CC single bonds. At lower temperatures, indications for the formation of a 1:2 chain-type complex were found. Using spectra recorded between 90 and 124 K in liquid argon, the complexation enthalpies for the pi- and sigma-complexes with HCl were determined to be -8.8(3) and -7.9(3) kJ mol-1, respectively. For the pi-complex with BF3 a value of -7.4(3)kJ mol-1 was found. Structural and spectral information on the 1:1 complexes was obtained from B3LYP/6-311++G(d,p) and MP2(FC)/6-31+G(d) calculations. Using Free Energy Perturbation Monte Carlo simulations to calculate the solvent influences, and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the solution enthalpies were transformed into complexation energies with values of -15.5(6) and -13.8(8) kJ mol-1 for the pi- and sigma-isomers of c-C3H4.HCl, respectively, and of -12.7(8) kJ mol-1 for the pi-type BF3 complex.

A cryosolution infrared and ab initio study of the van der Waals complexes of cyclopentene with hydrogen chloride and boron trifluoride

The formation of weak molecular complexes of cyclopentene with HCl and BF3, dissolved in liquid argon is investigated using infrared spectroscopy. Evidence is found for the formation of 1:1 complexes in which the Lewis acid under study binds to the CC double bond. At higher concentrations of HCl, weak absorption bands due to 1:2 species are also observed. From spectra recorded at different temperatures between 92 and 127 K, the complexation enthalpies for CP.HCl and CP.BF3 are determined to be -9.5(3) and -16.1(9) kJ mol-1, while for CP.(HCl)2 a value of -17.0(6) kJ mol-1 is obtained. For the 1:1 and 1:2 complexes, structural and spectral information is obtained from ab initio calculations at the MP2/6-31+G(d) level. Using free energy perturbation Monte Carlo simulations to calculate the solvation enthalpies and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the complexation energies for CP.HCl and CP.(HCl)2 are estimated from the experimental complexation enthalpies to be -17.4(14) and -34.0(20) kJ mol-1, while the value for CP.BF3 was derived to be -23.4(22) kJ mol-1. The experimental complexation energies are compared with the theoretical values derived from the MP2/6-31+G(d) potential energy surfaces and with single point energies calculated at the MP2/6-311++G(3df,2pd) level.