BN Bond Length Research Articles

Synthesis and structural analysis of conjugated benzoxazaborine derivatives

Benzoxazaborine derivatives were synthesized by the dehydration condensation reaction of 2-aminobenzyl alcohols with arylboronic acids. The insensitivity of the benzoxazaborines to hydrolysis allowed these compounds to be isolated by silica gel column chromatography. The single crystal X-ray structural analysis demonstrated the highly planar structure of the benzoxazaborine unit, and the BN bond length indicated an extended π-conjugated system.

Probing Dative and Dihydrogen Bonding in Ammonia Borane with Electronic Structure Computations and Raman under Nitrogen Spectroscopy.

Although ammonia borane is isoelectronic with ethane and they have similar structures, BH3NH3 exhibits rather atypical bonding compared to that in CH3CH3. The central bond in ammonia borane is actually a coordinate covalent or dative bond rather than the conventional covalent C-C bond in ethane where each atom donates one electron. In addition, strong intermolecular dihydrogen bonds can form between two or more ammonia borane molecules compared to the relatively weak dispersion forces between ethane molecules. As a result, ammonia borane's physical properties are very sensitive to the environment. For example, gas-phase and solid-state ammonia borane have very different BN bond lengths and BN stretching frequencies, which led to much debate in the literature. It has been demonstrated that the use of cluster models based on experimental crystal structures led to better agreement between theory and experiment. Here, we employ a variety of cluster models to track how the interaction energies, bond lengths, and vibrational normal modes evolve with the size and structural characteristics of the clusters. The M06-2X/6-311++G(2df,2pd) level of theory was selected for this analysis on the basis of favorable comparison with CCSD(T)/aug-cc-pVTZ data for the ammonia borane monomer and dimer. Fourteen unique fully optimized molecular cluster geometries, (BH3NH3)n≤12, and nine crystal models, (BH3NH3)n≤19, were used to elucidate how the local environment impacts ammonia borane's physical properties. Computational results for the BN stretching frequencies are also compared directly to the Raman spectrum of solid ammonia borane at 77 K using Raman under liquid nitrogen spectroscopy (RUNS). A strong linear correlation was found to exist between the BN bond length and stretching frequency, from an isolated monomer to the most distorted BH3NH3 unit in a cluster or crystal structure model. Excellent agreement was seen between the frequencies computed for the largest crystal model and the RUNS experimental spectra (typically within a few wavenumbers).

Ground state structure of BNB using fixed-node diffusion Monte Carlo

BNB is a challenging example of artifactual symmetry-breaking effects due to its susceptibility to a pseudo second-order Jahn–Teller interaction, which results in a structure with unequal BN bondlengths. The fixed-node diffusion Monte Carlo method is employed to calculate the potential curves along the symmetric and asymmetric stretching coordinates. With a multi-determinant wavefunction, the symmetric and asymmetric structures are found degenerate within statistical errors, with the asymmetric configuration lower in energy. The energy difference between the two structures is smaller than the multi-reference coupled cluster result obtained with four determinants, and supports previous conclusions that BNB has a floppy quasi-symmetric ground-state.

On the nature of the B⋯N interaction and the conformational flexibility of arylboronic azaesters

It is shown that the structural and physicochemical properties of arylboronic azaesters result from the properties of the key B-N bond. This bond is different in arylboronic azaesters from typical single/double/triple B-N bonds present in other boron-nitrogen compounds. By studying a model example, 6-tert-butyl-2-(3',5'-difluorophenyl)-(N-B)-1,3,6,2-dioxazaborocane, it is proved that the molecule adopts the closed form in the solid state with the BN bond length equal to 1.7646(3) Å. This represents the longest B-N bond length for an arylboronic ester reported in literature. According to the results of experimental charge density studies and QTAIM analysis, the BN bond/contact shows good separation of the respective atomic basins and significant flexibility. The source function and NBO analyses indicate an evident involvement of the oxygen atoms in the creation of the B-N bonding. According to our DFT calculations the isolated model molecule exists in the open conformer. The constrained energy scans reveal an E(d(B-N)) potential energy surface which depends strongly on the nature of the substituents at the nitrogen and aromatic carbon atoms. It appears that an appropriate substitution can produce either conformer. A dipole moment analysis provides a good insight into the reactivity properties of the compound in solution. The opening of the azaester cage is associated with a breaking of the B-N bond, thus decreasing the magnitude of the dipole moment. The existence of the open conformer is confirmed by multi-temperature NMR studies. When decreasing the temperature of the measurements the (1)H and (11)B NMR spectra show features corresponding to the open form of the model arylboronic azaester.

The Molecular Structure of 1,3,5,7- Tetra(tert-butyl)-2,4,6,8-tetraazidoborazocine [1

The tetra(tert-butyl)-tetraazido-borazocine, 1, has the expected tub shape as determined for (SCNB=NtBu)4. However, it crystallizes in the triclinic system and shows no symmetry element in contrast to the isothiocyanato derivate which has C2 symmetry. There are two kinds of BN bonds which alter from long to short (average 1.491 and 1.388 A° ). The BN bond lengths to the azido groups are, on average, 1.486 °A. All B and N atoms of the ring reside in a trigonal planar environment. The B-N-N bond angles are close to 120◦ showing that the central nitrogen atom can be considered as sp2-hybridized. The N-N-N bond angles are on average 173.6◦. The two sets of the opposite B2N2 planes include an angle of 114.8◦. The atoms of the azido groups are almost coplanar with the respective NBN planes. This together with the short BN(N2) bond lengths indicates that these azido groups are π-bonded to their corresponding boron atom.

Unusual substituent effects on the bonding of iminoboranes

Substituent effects on iminoboranes XBNH, HBNX and XBNX (X = H, CH3, NH2, OH, F) have been analyzed in the framework of the NBO, AIM and ELF approaches, using B3LYP/6-311++G(d,p) optimized geometries and electron densities. Boron-substituted derivatives, XBNH, are more stable than the corresponding nitrogen-substituted isomers HBNX, with the energy difference increasing as the electron withdrawing character of the substituent increases. The BN linkage is not much affected by N-substitution, but it is significantly altered when the substituent is attached to the boron atom in both XBNH and XBNX series of compounds. Moreover, substituent effects on the structures of iminoboranes are opposite those observed for the corresponding isoelectronic acetylene derivatives. The ELF analysis indicates that electron-withdrawing substituents enhance the localization of electrons in a torus around the CC or the BN axis. As a result, although electron density is depleted at the bcp, the bond does not necessarily become weaker, since density increases around the periphery, a phenomenon named the "hole" effect. The dissimilarities between acetylene and iminoborane derivatives are primarily a consequence of the significant distortion of this torus in the latter, due to the large difference between the electronegativities of B and N, which leads to a large contribution of the X-B=:N-Y resonance structure in some cases. The "hole" effect is reflected in a reasonable correlation between the Laplacian of the electron density at the bcp and the BN bond length.

On symmetry breaking in BNB: real or artifactual?

The ground state of the linear BNB molecule has been examined with multireference-based ab initio methods coupled with quantitative basis sets. Previous computational studies on BNB, even those using highly correlated single reference-based methods, e.g., the CCSD(T) and BDT methods, suggested that the two BN bond lengths were unequal. In this paper, the BN(X 3Pi) + B(2Pu) potential energy curve is constructed using a state-averaged multireference-based correlated method (SA-CASSCF+PT2). The four lowest states of BN were included in the state averaging procedure. These calculations reveal no symmetry breaking along the antisymmetric stretching mode of the molecule.

Tris(triphenylphosphaniminato)boran, B(NPPh3)3

Tris(triphenylphosphoraneiminato)borane, B(NPPh3)3 B(NPPh3)3 has been prepared by the reaction of BF3 · OEt2 with LiNPPh3 in toluene/tetrahydrofuran. Moisture sensitive single crystals of B(NPPh3)3 · 0.5 C7H8 were obtained and characterized by NMR and IR spectroscopy as well as by a crystal structure determination. Space group P21/n, Z = 4, lattice dimensions at –70 °C; a = 2147.8(3), b = 978.5(2), c = 2423.8(2) pm, β = 114.11(1)°, R1 = 0.070. The compound forms monomeric molecules with a planar BN3 skeleton and BN bond lengths of 144.7 pm on average.

The molecular and solid‐state structure of borazine has been determined at 115 and 160 K by single‐crystal X‐ray crystallography. The molecular structure has C2 symmetry with small deviations from the expected D3h symmetry. The mean value of the BN bond length is 1.429(1) A and the interbond angles of the six‐membered ring are 117.1(1)° at the boron atoms and 122.9(1)°

Borazin im festen Zustand: Verdient es, als “anorganisches Benzol” bezeichnet zu werden?

Darstellung und struktur monomerer N-borylierter Galliumamide

The reaction of N-lithiated aminoboranes of type R 2BN( tBu)Li (R  Me, Mes) with Diorganylgallium chlorides R′ 2GaCl(R′  Me, Et, CH 2C(Ph)Me 2) yields the N-borylated gallium amides R′ 2GaN( tBu)BR 2. A single crystal X ray structure determination did show the molecule Mes 2BN( tBu)GaEt 2 to be monomeric with trigonal planar coordinated gallium, nitrogen and boron atoms. The BN bond length is short (1.384(6) Å), the GaN bond length is rather long (1.937(3) Å), although the nitrogen is non bridging. The described compounds are the first well characterized examples of monomeric gallium monoamides, on the other hand they represent a new class of compounds having two different elements of the III main group bound to one nitrogen atom.

Microwave spectra and structure of hydrogen cyanide-boron trifluoride: an almost weakly bound complex

Rotational spectra of the complex HCN-BF 3 have been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectra have been observed for the 11 B and 10 B species with both HC 14 N and HC 15 N, and the hyperfine structure has been analyzed. The complex has the expected C 3υ structure, with the nitrogen end of the HCN toward the boron, but the observed BN bond length of 2.473(29) A is notably shorter than that in the related weakly bound systems N≡N-BF 3 and N≡C-C≡N-BF 3 . The out-of-plane distortion of the BF 3 cannot be determined accurately but is probably less than 3 o

Calculation of NMR Chemical Shifts — The Third Dimension of Quantum Chemistry

AbstractNMR chemical shift calculations provide the basis for an intensive collaboration between quantum chemists and experimentalists. Calculated shift data can be used to describe the magnetic properties of a molecule, to identify unknown compounds by comparison of experimental and theoretical shift values, to determine equilibrium geometries, to investigate conformational changes, to elucidate the mechanism of molecular rearrangements, to determine solvent effects on NMR data, to identify complexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to describe chemical bonding. This is demonstrated by three examples, namely the determination of the equilibrium structure of the homotropylium cation, the description of BH3NH3 in solution or condensed phases, and the investigation of stannyl cation complexes in solution. IGLO calculations of 13C, 11B, 15N, and 119Sn chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium cation possesses an equilibrium 1,7 distance of 2 Å that is indicative of strong through‐space interactions and, as a consequence, homoaromatic character. (2) In solution, the charge transfer from NH3 to BH3 is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental δ(11B) and δ(15N) values can be reproduced when the geometry effect and the direct solvent effect are included in the shift calculations. (3) Stannyl cations form strongly‐bounded coordination complexes with solvent molecules (binding energy: ≥ 50 kcal/mol) that make the cation properties, in particular δ(119Sn) values, similar to those of covalently‐bounded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive solvent variations.

Solvation Effects on Geometry and Chemical Shifts. An Ab Initio/IGLO Reconciliation of Apparent Experimental Inconsistencies on H3B · NH3

To what extent can the structural parameters of a molecule be altered by the surrounding medium? According to model calculations in a polar solvent continuum, the BN bond length in H3B. NH3 in aqueous solution is indicated to be shortened by ca. 0.1 Å over the value in the gas phase (1.672 Å, microwave), i.e. to nearly the BN separation in the solidstate (1.564 Å, X‐ray). Similar calculations with the solvent hexane gave a value of 1.62 Å.

Beiträge zur chemie des bors: CLXXV. Über den einfluss sterischer faktoren auf die konformation von diborylaminen

The aminoborane Ph 2BN(CMe 3SiMe 3 possesses a relatively long BN bond (1.433 Å) and a nonplanar C 2BNCSi (τ(C 2B/NCSi) 45°) skeleton as shown by X-ray structure analysis. In addition, the structure of five diborylamines have been determined by X-ray methods. The compounds (mes 2B) 2NH and (mes 2BNH) 3B belong to diborylamines with a coplanar conformation for the C 2BNHBC 2 (or C 2BNHBN 2) units, while the diborylamines (Ph 2B) 2)NH and (Ph 2B) 2NMe adopt a twist conformation. Finally, an orthogonal conformation was detected for t-Bu 2BNMe-9-borafluorene. The T-Bu 2B group stands almost perpendicular on the BNC-9-borafluorene plane. Consequently, two distinctly different BN bond lengths (1.482 and 1.392 Å) are observed. The influence of steric effects on the conformation and bonding parameters is discussed.

Beiträge zur Chemie des Bors, 163 [1]. Die Molekülstruktur zweier Silaborazine / Contributions to the Chemistry of Boron, 163 [1]. The Molecular Structure of Two Silaborazines

Abstract The crystal and molecular structure of Ph2Si(NMeBMe)2NMe (1) and MeB(NMeSiPh2 )2NMe (2) have been determined by X-ray structure analyses. Both compounds possess almost planar heterocycles. The average BN bond lengths in both compounds are practically identical (1.44 Å), but the two nonequivalent SiN bonds in 2 are significantly different (1.719, 1.732 Å). NMR data ( 1H, 11B, 13C, 27Si) are consistent with planar silaborazine rings.

Beiträge zur Chemie des Bors, 137 [1]. Über eine einfache Synthese eines Amino-imino-borans und seiner Addukte mit Bortrichlorid und Bortribromid / Contribution to the Chemistry of Boron, 137 [1]. A Convenient Synthesis of an Amino-Imino-Borane and its Adducts with Boron Trichloride and Boron Tribromide

The amino-imino-borane (1) has been synthesized in high yield by a three step reaction. Pure 1 dimerizes fairly rapidly but can be stored as a monomer in solution for several weeks. It adds BCl3 and BBr3 with haloboration. The resulting diborylamines, 6 and 7, form a four membered cyclic internal coordination compound. The structure of the corresponding bromo derivative has been determined by X-ray crystallography. Its B2N2- ring is planar and contains four significantly different BN bonds. One of these is the shortest BN bond length yet observed for tricoordinated boron and nitrogen atoms (134 pm). Considering bond lengths the adduct can be looked upon as a coordination compound of an aminoborane with an iminoborane.

Beiträge zur Chemie des Bors, 133 [1] Molekül-und Kristallstruktur von Tris(methylanilido)boran / Contributions to the Chemistry of Boron, 133 [1] Molecular and Crystal Structure of Tris(methylanilido) Borane

Abstract The title compound crystallizes in the rhombohedral space group R3, Z = 1. The boron atom occupies a special position on the threefold axis therefore imposing C3 symmetry to the molecule. The C2BN plane is twisted relative to the BN3 plane by 37.1°, and the BN bond length of 144.8(3) pm is on the upper limit of BN bond lengths found for other triaminoborane systems.

Contributions to the Chemistry of Boron, LXXV The Crystal and Molecular Structure of a Tetraazadiborine: 3,6-Bis(dimethylamino)-1,2,4,5-tetramethyl-1,2,4,5,3,6-tetraazadiborine

The title compound crystallizes in the monoclinic system with dimensions of the unit cell a = 14.09(2), b = 8.60(1), c = 11.34(1) Å and β = 101.7°. The space group was determined to be A 2/a with four molecules in the unit cell. The structure was solved by direct methods, and all hydrogen atoms were located. Full matrix refinement led to R = 0.059. The B2N4 six membered ring is nonplanar as expected for an 8π electron heterocycle. The molecule possesses C2 symmetry but the ring itself is near the D2 symmetry as derived from earlier CNDO/S-calculations. The BN bond lengths are 1.43 A, the NN distances 1.43 Å and the average CN distances 1.44 A. The boron atoms and four of the six nitrogen atoms are trigonal-planarly coordinated while the remaining two N-atoms (N1,N2) deviate little, but significantly, from sp2 hybridisation. The structure of the molecule can be interpreted as the best compromise between ΒΝ-π-bonding, nonbonding repulsion between adjacent CH3 groups, and the interaction between neighbouring ‘free pairs of electrons’ of the ring N-N-units.

Rotational barrier and structure of aminoborane determined by AB initio calculations

Ab initio calculations have been used to determine the structural parameters of both the planar and the orthogonal form of H 2BNH 2. The BN bond lengths were found to be 1.378 Å and 1.469 Å, respectively. The arrangement about nitrogen is pyramidal in the orthogonal form. The best estimate of the barrier to internal rotation is 33.3 kcal/mole.

Self-consistent molecular orbital calculations on π-electron systems. Part 3.—The σ-electrons in borazine

The valency electron self-consistent molecular orbital method CNDO/2 is used to obtain the minimum energy as a function of the BN, BH, and NH bond lengths in borazine. The charge distribution of the σ- and π-electrons differs from the values given in part 1. Reasonable values for the bond lengths, quadrupole coupling constants and ionization potentials are obtained; but the force constants seem too high. The highest filled orbital is of σ-type.