11B NMR study of the tautomeric transformations of the 7,8-dicarba-nido-und ecaborane(11) adducts with pyridine derivatives

The tautomeric transformations of o-carborane(12) 7,8-C₂B₉H₁₁-Py(X), where Py(X) = 4-picoline (4-CH₃-C₅H₄N), 3-picoline (3-CH₃-C₅H₄N 4-stilbazole (4-C₆H_5-C₂H₂-C₅H₄N, or 3-bromopyridine (3-Br-C₅H₄N), and the iodo derivative 7,8-C₂B₉H₁₀I C₅H₅N were studied by¹¹B NMR spectroscopy in solution. The tautomeric transformations found are caused by migration of the bridging hydrogen cluster C₂B₉H₁₁ which is present in the structures. In the¹¹B NMR spectra, the boron signal is observed in the high-field region of the spectrum if the nearest two boron atoms are additionally bound by the bridging hydrogen. This enables the recording of the dynamic structural transformations of the adducts. It was found that the tautomeric equilibrium depends on the nature of substituents introduced in the heterocyclic ligand and the carborane cluster. Substituents with greater electron-accepting ability induce a shift of equilibrium toward the tautomer in which the bridging hydrogen is farther from the boron atom bonded to the substituent.     

Thermal conversions of chitosanium dodecahydro-<Emphasis Type="Italic">closo</Emphasis>-dodecaborate

The thermal behavior of chitosanium dodecahydro-closo-dodecaborate, (C₆O₄H₉NH₃)₂B₁₂H₁₂, was studied by thermal analysis, X-ray diffraction, and IR and X-ray photoelectron spectroscopy. As this compound is heated at a rate above 10–20 K/min, it ignites at a temperature of about 300°C. As the compound is heated to 1000°C at a rate below 10 K/min in an inert atmosphere, it yields a mixture of carbon and amorphous boron and/or boron carbides. The presence of a small amount of boron oxide in the product is explained by the formation of a partially oxidized hydroborate anion at the early stages of (C₆O₄H₉NH₃)₂B₁₂H₁₂ decomposition via the interaction between oxygen of the chitosanium cation and the B₁₂H₁₂²⁻ anion. Heating the initial compound in air at a rate below 10 K/min yields carbon and boron oxide as the main products. Molten boron oxide protects boron and/or boron carbides and boron nitride forming in small amounts in the particle bulk from oxidation.     

Coordination compounds of electron-deficient boron cluster anions BnH n2? (n = 6, 10, 12)

This survey concerns the coordination ability of B _n H _n ²⁻ (n = 6, 10, 12) boron cluster anions and their derivatives in complex formation. Boron cluster anions form four types of compounds: salts of organic cations and alkali-metal cations, including Cat₂B _n H _n , where specific interactions can be observed between a cation Cat and a boron cluster anion; salts of protonated anions CatB₆H₇ and CatB₁₀H₁₁, analogues of Cat[MB _n H _n ] complexes, where an extra hydrogen atom appears bound with the BBB face of a boron polyhedron and performs as a hard acceptor; metal complexes with outer-sphere boron cluster anions where specific ligand-ligand interactions may be observed between a boron cluster anion and an inner-sphere ligand; and true metal complexes with boron cluster anions that enter the inner coordination sphere. The last case characterizes closo-hydroborate anions as polydentate ligands whose denticity can vary widely under the effect of substituents or other ligands in the complex.     

Theoretical study of molecular hydrogen elimination from the undecahydrodecaborate monoanion [B10H11]−. Exopolyhedral substitution intermediates: [B10H9]− monoanion and neutral [B10H10] cluster

The elementary reaction of molecular hydrogen elimination from the [B₁₀H₁₁]^? anion, which is presumably the rate-limiting stage of acid-catalyzed reactions of substitution of exopolyhedral H atoms in the [B₁₀H₁₀]^2? decahydro-closo-decaborate anion, has been calculated by the density functional theory method (in the B3LYP/6-311++G** approximation). Specific transition states of H₂ elimination in which vacancies form near the boron atoms have been localized. It has been demonstrated that regioselectivity of substitution reactions can be related to the significant difference between the activation barriers for the pathways of H₂ elimination from boron atoms with different coordination numbers (CN 6 and 5). The electron density of the [B₁₀H₉]^? anion that forms after hydrogen molecule elimination has a characteristic shape of the lowest unoccupied molecular orbital for the interaction with nucleophilic reagents; in acid-catalyzed reactions, different anions, for example, a carboxylic acid residue, can act as such. The direct reaction of the [B₁₀H₉]^? intermediate with nucleophilic anions is hindered by the Coulomb charge repulsion. To overcome this hindrance, the possibility of [B₁₀H₉]^? protonation to form the neutral [B₁₀H₁₀] system has been considered. It has been shown that the proton affinity of the [B₁₀H₉]^? anion is ～280–290 kcal/mol. For the [B₁₀H₁₀] cluster, the lowest-lying and low-lying isomers have been considered. For all the systems under consideration, the electronic chemical potential and Pearson hardness have been evaluated.     

π-Aromatic B<Subscript>16</Subscript>H<Subscript>6</Subscript>: A Neutral Boron Hydride Analogue of Naphthalene

Systematic ab inito calculations are carried out in this work to investigate the geometrical and electronic structures of B₁₆H₆ neutral and its anion B₁₆H₆ ⁻. The quasi-planar C_2v B₁₆H₆(¹A₁) with 10 delocalized π-electrons proves to be the neutral boron hydride analogue of naphthalene (D_2h C₁₀H₈). This π-aromatic neutral may be obtained from the experimentally known π-antiaromatic C_2h B₁₆ (Sergeeva et al., J. Am. Chem. Soc. 130:7244, 2008) upon hydrogenation at the six corner positions and is expected to be undistinguishable from a perfect planar D_2h B₁₆H₆ in experiments. Detailed adaptive natural density partitioning (AdNDP) analyses clearly reveal the bonding pattern of B₁₆H₆ and the calculated nucleus independent chemical shifts (NICS) strongly support its global π-aromaticity. C_2v B₁₆H₆ ⁻(²B₂) anion with one extra electron appears to have a similar quasi-planar structure with even a less severe out-of-plane distortion. Ultraviolet (UV) absorption spectrum of B₁₆H₆ and photoelectron spectroscopy (PES) spectrum of B₁₆H₆ ⁻ are simulated to facilitate their spectroscopic characterizations.     

Derivatives of the closo-dodecaborate anion and their application in medicine

The paper presents a comparative analysis of the possibilities and characteristic features of the application of various polyhedral boron compounds, viz., the closo-decaborate anion [B₁₀H₁₀]^2–, the closo-dodecaborate anion [B₁₂H₁₂]^2–, the carba-closo-dodecaborate anion [CB₁₁H₁₂]^–, carboranes C₂B₁₀H₁₂, and the bis(dicarbollide) complexes [M(C₂B₉H₁₁)₂]^– (M = Fe, Co, or Ni), in boron neutron capture therapy (BNCT) for cancer. The requirements on compounds used in BNCT are formulated and the advantages of the application of the closo-dodecaborate anion are considered. The data on the synthesis of various derivatives of the closo-dodecaborate anion, which either already found use in BNCT or are most promising in this field, are summarized. The possibilities of the application of agents derived from the closo-dodecaborate anion in medical diagnostics are discussed.     

Analysis of Why Boron Avoids sp2 Hybridization and Classical Structures in the BnHn+2 Series

We performed global minimum searches for the B_nH_n+2 (n=2‐5) series and found that classical structures composed of 2c–2e B? H and B? B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol^?1 in B₂H₄ to 62.3 kcal mol^?1 in B₅H₇. We believe this occurs because boron atoms in the studied molecules are trying to avoid sp² hybridization and trigonal structure at the boron atoms, as in that case one 2p‐AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p‐AOs and avoiding having one 2p‐AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron–hydride clusters as well as for pure boron clusters and boron compounds in general.     

Darstellung, 11B-, 13C-, 1H-NMR- und Schwingungsspektren von Monoethoxyhydro-closo-Dodecaborat(2–) sowie Kristallstruktur von [(C5H5N)2CH2][B12H11(OC2H5)]

Preparation, ¹¹B, ¹³C, ¹H NMR and Vibrational Spectra of Monoethoxyhydro-closo-dodecaborate(2–), and the Crystal Structure of [(C₅H₅N)₂CH₂][B₁₂H₁₁(OC₂H₅)] By treatment of Na₂[B₁₂H₁₂] with dry HF in ethanol Na₂[B₁₂H₁₁(OC₂H₅)] is formed which has been separated by ion exchange chromatography on diethylaminoethyl(DEAE) cellulose from the starting compound and by-products. The X-ray structure determination of [(C₅H₅N)₂CH₂][B₁₂H₁₁(OC₂H₅)] (monoclinic, space group P2₁/m, a = 9.1906(3), b = 12.6612(8), c = 9.3640(12) Å, β = 112.947(6)°, Z = 2) reveals the complete ordering of the anion sublattice. The ¹¹B nmr spectrum exhibits the characteristic feature (1:5:5:1) of a mono substituted B₁₂ cage with a strong down-field shift of ipso-B at +6.5 ppm. In the ¹³C nmr spectrum a triplet at 67.9 ppm of the methylene group and a quartet at 19.5 ppm of the methyl group is observed. Correspondingly, the ¹H nmr spectrum shows two multiplets at 3.7 and 1.3 as expected for an ethoxy substituent, and a multiplet at 2.1 ppm due to the protons of the boron cluster. The i.r. and Raman spectra exhibit strong CH stretching vibrations between 2 963 and 2 863 cm^?1, and in the i.r. spectrum the CO and BO stretching frequencies of the B? O? C bridge are observed at 1 175 and 1 140 cm^?1.     

Laser-Assisted Chemical Vapor Deposition of Boron

Ar⁺ laser-induced chemical vapor deposition (LCVD) of B from gaseous mixtures of BCl₃ and H₂ has been investigated for temperatures between 1000 K and 2250 K and for partial pressure within the ranges 25 mbar ≤ p(BCI₃) ≤ 800 mbar and 10 mbar ≤p(H₂) ≤ 400 mbar. For the lowest temperatures, deposition is controlled by the chemical kinetics which, with p(BCI₃) = p(H₂) = 100 mbar, is characterized by an apparent chemical activation energy of about 26.5 kcal/mol. In this regime, the deposited boron is amorphous. At high temperatures, deposition becomes limited by gas-phase transport and polycrystalline boron with βrhombohedral structure is obtained.     

Designed Boron-Rich Polymeric Nanoparticles Based on Nano-ion Pairing for Boron Delivery

Boron-rich particles with the boron fraction ca.10–20 wt % of controllable shape and size that can be easily prepared via simple ion co-assembly are promising material for tumor treatment by boron neutron capture therapy. Electroneutral, dynamic core-shell polymeric nanoparticles were prepared by co-assembly of cationic PEO-block-PGEA diblock copolymer with sodium closo-dodecaborate, Na₂[B₁₂H₁₂]. This is the first example of polymer nanoparticles based on [B₁₂H₁₂]²⁻nano-ion pairing. The high [B₁₂H₁₂]²⁻ loading is proven by calorimetry at physiological salt concentration. As a result of rational design, rod-, worm- and sphere-like particles were produced and further tested using human glioblastoma and cervical carcinoma cell lines. Rod-like particles yielded the highest internalization capability in all tested cell lines.     

A zwitterion produced by a strong intramolecular N→B interaction in 1‐hydroxy‐2‐(pyridin‐2‐ylcarbonyl)benzo[d][1,2,3]diazaborinine

2‐Acylated 2,3,1‐benzodiazaborines can display unusual structures and reactivities. The crystal structure analysis of the boron heterocycle obtained by condensing 2‐formylphenylboronic acid and picolinohydrazide reveals it to be an N→B‐chelated zwitterionic tetracycle (systematic name: 1‐hydroxy‐11‐oxo‐9,10,17λ⁵‐triaza‐1λ⁴‐boratetracyclo[8.7.0.0^2,7.0^12,17]heptadeca‐3,5,7,12,14,16‐hexaen‐17‐ylium‐1‐uide), C₁₃H₁₀BN₃O₂, produced by the intramolecular addition of the Lewis basic picolinoyl N atom of 1‐hydroxy‐2‐(pyridin‐2‐ylcarbonyl)benzo[d][1,2,3]diazaborinine to the boron heterocycle B atom acting as a Lewis acid. Neither of the other two pyridinylcarbonyl isomers (viz. nicotinoyl and isonicotinoyl) are able to adopt such a structure for geometric reasons. A favored yet reversible chelation equilibrium provides an explanation for the slow D₂O exchange observed for the OH resonance in the ¹H NMR spectrum, as well as for its unusual upfield chemical shift. Deuterium exchange may take place solely in the minor open (unchelated) species present in solution.     

Thermoelectric properties of photo- and thermal CVD boron and boron phosphide films

The electrical properties of n-BP films newly prepared by thermal CVD in the B₂H₆-PH₃-H₂ system were improved by a deuterium lamp excitation. High-temperature electrical conductivity and thermoelectric power of amorphous boron and polycrystalline boron phosphide films grown on silica glass were measured to evaluate the thermoelectric figure-of-merit (Z). In particular, the Z-value for photo-thermal BP films was higher (10⁻⁴/K) than that of boron films, indicating that they are promising for high-temperature thermoelectric materials.     

Simulation of the Solid State Vibrational Spectra of Aminodichloroborane and Ammonia Boron Trichloride

This is the second part of a quantum chemical investigation on the reaction between boron trichloride and ammonia. In part I [9] we focused on the energetic course of successive chlorine substitutions which are relevant for the chemical vapor deposition of BN. In this work we analyze in detail the vibrational spectra of reaction products accessible at room temperature. Regarding an experimental IR spectrum of the condensation product of the reaction between BCl₃ and NH₃ [10], there are more signals than expected for monomeric aminodichloroborane. Since this molecule shows the tendency to oligomerize, we have studied whether the presence of aminodichloroborane dimers or trimers or an impurity of the ammonia boron trichloride complex – an intermediate from which aminodichloroborane is formed – can explain the shape of the measured spectrum. To this end we have calculated the vibrational frequencies of monomeric, dimeric, and trimeric BCl₂NH₂, H₃N · BCl₃, and several van der Waals complexes at the level of a Møller‐Plesset second order perturbation theory. For the verification of the methodology, the vibrational frequencies of the dimethylaminodichloroborane molecule have been determined, in good accord with experimental gas phase spectra. Also the solid state spectrum of H₃N · BCl₃ is well reproduced by the computed vibrational spectrum of the monomeric adduct and a hydrogen‐bonded aggregate. Our studies concerning the spectrum of the condensation product of the reaction of boron trichloride with ammonia indicate that the substance investigated by Kwon and McGee had contained, besides monomeric aminodichloroborane, also its trimer and ammonia boron trichloride.     

Thermal and thermomechanical properties of trialkylammonium dodecahydro-<Emphasis Type="Italic">closo</Emphasis>-dodecaborates (R<Subscript>3</Subscript>NH)<Subscript>2</Subscript>[B<Subscript>12</Subscript>H<Subscript>12</Subscript>] (R = Et, Вu)

This work is devoted to specific features of the thermal and thermomechanical properties of trialkylammonium salts of the [B₁₂H₁₂]^2– anion (R₃NH)₂ [B₁₂H₁₂], where R = Et, Вu, and is of fundamental importance for the development of composite materials with an increased boron content. The results are given in comparison with similar data for (Et₃NH)₂[B₁₀H₁₀].     

The undecahydrodecaborate anion B<Subscript>10</Subscript>H<Stack><Subscript>11</Subscript><Superscript>−</Superscript></Stack> as the starting reagent in exopolyhedral substitution and complexation: Theoretical and experimental prerequisites

Using the electron density functional theory (B3LYP approximation) with the 6-31G* basis set, the potential energy surface of the undecahydrodecaborate anion B₁₀H₁₁⁻ was calculated and the activation energies and the activation barriers for the elementary reactions of proton H* migration around the boron polyhedron were estimated. Analysis of the calculation results in comparison with the experimental data accumulated recently implies that the salts of the B₁₀H₁₁⁻ anion represent a new type of starting compounds for exopolyhedral substitution and complexation involving decaborate anions. Of particular interest is the targeted preparation of isomers of metal complexes containing a decaborate anion depending on the use of B₁₀H₁₀²⁻ or B₁₀H₁₁^¨- as the starting reagent. Certain trends in the reactivity of B₁₀H₁₀⁻ and B₁₀H₁₁⁻ anions can be explained in terms of the simple analysis of Mulliken charge distribution on atoms.     

Fast and Long-Lasting Iron(III) Reduction by Boron Toward Green and Accelerated Fenton Chemistry

Generation of hydroxyl radicals in the Fenton system (Fe^II/H₂O₂) is seriously limited by the sluggish kinetics of Fe^III reduction and fast Fe^III precipitation. Here, boron crystals (C-Boron) remarkably accelerate the Fe^III/Fe^II circulation in Fenton-like systems (C-Boron/Fe^III/H₂O₂) to produce a myriad of hydroxyl radicals with excellent efficiencies in oxidative degradation of various pollutants. The surface B−B bonds and interfacial suboxide boron in the surface B₁₂ icosahedra are the active sites to donate electrons to promote fast Fe^III reduction to Fe^II and further enhance hydroxyl radical production via Fenton chemistry. The C-Boron/Fe^III/H₂O₂ system outperforms the benchmark Fenton (Fe^II/H₂O₂) and Fe^III-based sulfate radical systems. The reactivity and stability of crystalline boron is much higher than the popular molecular reducing agents, nanocarbons, and other metal/metal-free nanomaterials.     

Synthesis of a Tyr-octreotate conjugated closo-carborane [HC2B10H10]: a potential compound for boron neutron capture therapy

A novel Tyr³-octreotate conjugated closo-carborane as a potential compound for boron neutron capture therapy was obtained via Fmoc solid phase peptide synthesis. The boron cluster [C₂B₁₀H₁₁] was introduced through the reaction of 6,9-bis(acetonitrile)decaborane and 5-hexynoic acid yielding a new closo-carborane conjugated carboxylic acid which was coupled subsequently with solid phase conjugated Tyr³-octreotate. The final boron-containing peptide was purified by preparative reverse phase HPLC and structural identity was proved applying MALDI-TOF mass spectrometry.     

Organic ions in the gas phase—XXI. Competitive primary loss of C2H4 and CO from 1-tetralone under electron impact

Competing primary reactions by which 1-tetralone loses C₂H₄ and CO give rise to a doublet at mass 118 in the high-resolution mass spectrum corresponding to 96·4% C₈H₆O⁺ and 3·6% C₉H₁₀⁺, respectively. Study of the remainder of the spectrum however, suggests that these intensities constitute a poor measure of the relative importance of the two reaction paths. The C₉H₁₀⁺ ion evidently degrades more readily than C₈H₆O⁺, since an estimated 26% of the ions arising in these paths involve primary loss of CO. This estimate is supported by voltage dependence of the intensity distribution between the members of the doublet. As ionizing voltage is decreased, the intensity distribution is constant to about 20 volts, but between 20 and 10 volts the value for C₉H₁₀⁺ rises smoothly from about 3.5% to about 25%.     

Specific interactions in metal salts and complexes with cluster boron anions BnH n2? (n = 6, 10, 12)

Specific interactions that appear in metal salts and complexes with cluster boron anions B _n H _n ²⁻ (n = 6, 10, 12) have been discussed. These interactions, as well as chemical bonds, involve vertices, edges, or faces of boron polyhedra. Specific interactions have a considerable effect on the structure of compounds, making a significant contribution to the formation of the unit cell and forming supramolecular assemblies. Compounds containing B _n H _n ²⁻ cluster anions shed new light onto the nature of specific interactions owing to their many-center character and great variety.     

Visible‐Light‐Promoted Photocatalytic B−C Coupling via a Boron‐Centered Carboranyl Radical: Facile Synthesis of B(3)‐Arylated o‐Carboranes

A visible‐light‐mediated in situ generation of a boron‐centered carboranyl radical (o‐C₂B₁₀H₁₁ ^. ) has been described. With eosin Y as a photoredox catalyst, 3‐diazonium‐o‐carborane tetrafluoroborate [3‐N₂‐o‐C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron‐centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal‐free alternative for the synthesis of 3‐(hetero)arylated‐o‐carboranes.