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The C2-symmetric structures of [B2X7]− (X = F, Cl, Br, I) were optimized. The bonding in [B2F7]− is mainly governed by electrostatics, while in [B2I7]− the charge is almost equally distributed over all atoms and additional weak iodine···iodine interactions are observed as shown by an atoms in molecules analysis. On the basis of thermodynamic estimations salts of these anions may be stabilized by large cations in the solid state, even though experimental attempts have yet failed.
While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B2X7]− anions (X = F, Cl, Br, I), of which only [B2F7]− has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B2X7]− anions is presented. The structures of [B2X7]− anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of C2 symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B2F7]− the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B2I7]− and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B2X7]− anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B2X7]− anions from [BX4]− and BX3 is favored in accord with the high Lewis acidity of the BX3 molecules. In solution and in the solid state only [B2F7]− is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B2X7]− (X = Cl, Br, I) anions in solution by 11B NMR spectroscopy and to prepare stable [PNP][B2X7] salts failed.
|حجم فایل||844 B|