Theoretical and Synthetic Study on the Existence, Structures, and Bonding of the Halide-Bridged [B₂X₇]⁻ (X = F, Cl, Br, I) Anions

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 <i>C</i>₂ 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