Darstellung und spektroskopische Charakterisierung der Monofluorohydro-closo-Borate [B6H5F]2? und [B12H11F]2?

Improvement in hydrogen binding ability of closo-dicarboranes via functionalization and designing of extended frameworks

Neutral closo-dicarboboranes are reported to have very low H₂ binding ability. Herein, we report an improvement in H₂ binding energy (E_b) of C₂B₄H₆ by substituting H atoms with different functional groups like X = F, Cl, Br, and XY = BO, CN and NC via quantum-chemical density functional theory based computations. In going from B₆H₆^2- to C₂B₄H₆, the E_b value is reduced from 14.6 kJ mol^-1 to 2.7 kJ mol^-1. C₂B₄X₆ and C₂B₄(XY)₆ systems, which can bind a total of eight H₂ molecules, with one H₂ molecule occupying at each B-B-C face, possess an E_b value per H₂ in the range of 4.5 kJ mol^-1 for X = F, 3.9 kJ mol^-1 for X = Cl, 5.9 kJ mol^-1 for X = Br, 6.8 kJ mol^-1 for XY = BO, 5.8 kJ mol^-1 for XY = CN and 5.2 kJ mol^-1 for XY = NC. The improvement in E_b value is found to be the highest in case of C₂B₄(BO)₆, which has the ability to bind 6.6 gravimetric wt% of H₂. The situation can be made more favorable by applying an external electric field. Energy decomposition analysis reveals that although the dispersion interaction (ca. 55-65%) has significant role in binding H₂ with such types of molecules, contribution from electrostatic and orbital interaction is also considerable. Further, we modeled an extended system by linking C₂B₄(BO)_n through 'C ≡ C' units for H₂ storage purpose. The energy difference between the highest occupied and the lowest unoccupied molecular orbitals gradually lessens with the increase in molecular length. Therefore, it can be tuned gradually by controlling the chain length, which may further open up their potency in the field of electronics. Graphical abstract C₂B₄X₆ (X = F, Cl, Br) and C₂B₄(XY)₆ (XY = BO, CN, NC) show enhanced H₂ binding ability from C₂B₄H₆. Further, 1D, 2D and 3-D frameworks can be built by joining C₂B₄(BO)_n units via 'C ≡ C' linkage.

B12Fn0/- (n = 1-6) series: when do boron double chain nanoribbons become global minima?

We present an extensive density-functional and wave function theory study of partially fluorinated B₁₂F_n^0/- (n = 1-6) series, which show that the global minima of B₁₂F_n^0/- (n = 2-6) are characterized to encompass a central boron double chain (BDC) nanoribbon and form stable BF₂ groups at the corresponding BDC corner when n ≥ 3, but the B₁₂F^0/- system maintains the structural feature of the well-known quasi-planar C_3v B₁₂. When we put the spotlight on B₁₂F₆^0/- species, our single-point CCSD(T) results unveil that albeit with the 3D icosahedral isomers not being their global minima, C₂ B₁₂F₆ (6.1, ¹A) and C₁ B₁₂F₆^- (12.1, ²A) as typical low-lying isomers are 0.60 and 1.95 eV more stable than their 2D planar counterparts D_3h B₁₂F₆ (6.7, ¹A') and C_2v B₁₂F₆^- (12.7, ²A₂), respectively, alike to B₁₂H₆^0/- species in our previous work. Detailed bonding analyses suggest that B₁₂F_n^0/- (n = 2-5) possess ribbon aromaticity with σ plus π double conjugation along the BDC nanoribbon on account of their total number of σ and π delocalized electrons conforming the common electron configuration (π²⁽ⁿ⁺¹⁾σ²ⁿ). Furthermore, the simulated PES spectra of the global minima of B₁₂F_n^- (n = 1-6) monoanions may facilitate their experimental characterization in the foreseeable future. Our work provides new examples for ribbon aromaticity and powerful support for the F/H/Au/BO analogy.

exo-Substituent effects in halogenated icosahedral (B12H122–) and octahedral (B6H62–) closo-borane skeletons: chemical reactivity studied by experimental and quantum chemical methods

The exo-substituent effects in halogenated icosahedral B12H122– (B12) and octahedral B6H62– (B6) closo-borane skeletons were studied both experimentally and theoretically. Firstly, the equilibrium geometries of exo-substituted B12 and B6 clusters were obtained using quantum chemical calculations at the MP2/def2-SVP level. A comparison with the available X-ray crystallographic data revealed a very good agreement between the theoretical and experimental values. Secondly, other descriptors of the molecular structure of these borane compounds – 11B NMR chemical shifts – were experimentally determined and compared with the calculated values obtained by the ab initio/GIAO approach at the MP2/def2-TZVP level. It was shown that the calculated data reproduced the experiment very closely. Thirdly, we investigated experimentally the halogenation reactions of B12 and attempted to explain the observed ratios between the two obtained disubstituted products (meta/ortho ~ 4:1) by calculating their thermodynamic stabilities using the DFT/B3LYP method. These calculations showed the enhanced stability of the meta disubstituted B12 but did not explain why the para product had not been observed in the experiment. We thus turned our attention to the kinetic aspects of exo-substitution reactions by exploring the possible reaction pathways and transition states. In spite of the complexity of the plausible reaction mechanisms, reasonable agreement was obtained between the calculated activation barriers and the experimental observations concerning the halogenation reactions of the B6 and B12 molecules. It also allowed to exclude from considerations certain reaction pathways leading to the mono- and dihalogenated B12 and B6 species.

Unexpected formation of new fluoroboranes from the reaction of NMe4B3H8 with BF3 and MeC identical with CH: exo-2-FB4H9 and trans-MeCH=CHBF2

The new fluoroboranes exo-2-FB4H9 1 and trans-MeCH=CHBF2 2 have been obtained unexpectedly and in good yield from the reaction of tetramethylammonium octahydrotriborate (NMe4B3H8) with boron trifluoride and propyne (MeC identical to CH).

Chemistry of closo-Dodecaborate Anion [B12H12]2-: A Review

Synthesis and chemical properties of the closo-dodecaborate anion [B12H12]2- and its derivatives are reviewed. Attention is also paid to potential applications of the closo-dodecaborate derivatives with emphasis on medical applications. A review with 325 references.