Proposed fluorination mechanism of CB(5)H(6)(-) and CB(9)H(10)(-) with HF. Evidence of kinetic control in the formation of 2-CB(5)H(5)F(-) and 6-CB(9)H(9)F(-)

Thiocyanation of closo-dodecaborate B(12)H(12)(2-). A novel synthetic route and theoretical elucidation of the reaction mechanism

Although vast experimental experience has been accumulated about the reactions of icosahedral B(12)H(12)(2-) borane cages, little is known about the mechanisms by which these reactions proceed. To address this issue, we have chosen the thiocyanation of B(12)H(12)(2-) and have studied this reaction using both experimental and theoretical methods. First, we present a novel and more convenient synthetic route using in situ generated thiocyanogen, (SCN)(2). The synthesized disubstituted product B(12)H(10)(SCN)(2)(2-) is exclusively the meta positional isomer, as confirmed by the X-ray crystallographic analysis. The quantum chemical calculations at the B3LYP/def2-TZVP//RI-PBE/def2-SVP level show that the free energy differences between the ortho, meta, and para disubstituted species are very small and as such cannot explain the observed positional preferences. The calculations of the kinetic aspects reveal that the reaction is best described as an electrophilic substitution. The calculated isomer preferences for the second SCN substituent are meta > para > ortho. The major outcome of this work is a clear and consistent picture of the electrophilic substitution reaction mechanism of the thiocyanation of B(12)H(12)(2-), thus contributing to our understanding of the general features of boron hydride reactivity.

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.

CB11Me11 Boronium Ylides: Carba-closo-dodecaboranes with a Naked Boron Vertex

In pentane solution, 2 equiv of the icosahedral CB(11)Me(12)(*) radical cleaves the Si-Si bond of hexaalkyldisilanes by boron to silicon methyl transfer with formation of 2 equiv of methyltrialkylsilanes. The loss of a methyl radical converts the CB(11)Me(12)(*) radical into an internally charge-compensated "boronium ylide" CB(11)Me(11) with a naked vertex, which can be formally viewed as a deprotonated hypercloso carborane. It has been isolated as an air-sensitive solid, stable only below approximately -60 degrees C. The naked vertex appears to be in position 12 since the material reacts instantaneously with alcohols and ethers to form the 12-alkoxy anions 12-CB(11)Me(11)OR. It reacts with many other nucleophiles to yield more complex mixtures containing similar products. DFT calculations for the four CB(11)Me(11) isomers give closed-shell ground-state electronic structures. For the isomer with naked vertex 12, a DFT computational search failed to reveal any skeletal dimers, apparently due to excessive methyl-methyl repulsions, and only a cyclic dimer bound through weak interactions of one of the 7-methyl hydrogen atoms on each cage with the empty exocyclic orbitals on B12 of the other cage was found. Natural hybrid orbital populations suggest that the three possible isomers of monomeric boronium ylides are close to true singlet ylides, with triplet states approximately 50 kcal/mol higher. The calculated electronic structure of the carbonium ylide is close to a singlet carbene, with a triplet state approximately 16 kcal/mol higher. An attempted preparation of Me(3)C(+)CB(11)Me(12)(-) yielded neopentane and products consistent with a sequential loss of methyl groups from the carborane cage with a transient formation of similar boronium ylides. Probable mechanisms of these methyl transfer reactions are considered, and the possibly quite general role of "ylide" structures in Lewis acid induced substitution reactions on the boron vertices of carboranes and boranes is noted.