Collect. Czech. Chem. Commun.
2009, 74, 1-27
https://doi.org/10.1135/cccc2008189
Published online 2009-01-13 10:11:56
exo-Substituent effects in halogenated icosahedral (B12H122–) and octahedral (B6H62–) closo-borane skeletons: chemical reactivity studied by experimental and quantum chemical methods
Martin Lepšíka, Martin Srneca, Drahomír Hnykb, Bohumír Grünerb, Jaromír Plešekb, Zdeněk Havlasa and Lubomír Rulíšeka,*
a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Gilead Sciences Research Center & IOCB, Flemingovo nám. 2, 166 10 Prague 6, Czech Republic
b Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Husinec-Řež 1001, 250 68 Řež, Czech Republic
Abstract
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.
Keywords: Boranes; Boron clusters; Halogenations; DFT; Ab initio calculations; Reaction mechanism; Substituent effects.
References: 77 live references.
