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

Abstract

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 (11)B NMR spectroscopy and to prepare stable [PNP][B2X7] salts failed.

Existence of Al2F7− in molten MF–AlF3 (M = K, Cs) systems as determined by Raman spectroscopy and structural simulation

“…− was also demonstrated in a study by Bertocco et al [42] . Al and B are the same main group elements, whereas F and Cl are halogens.…”

Systematics of BX3 and BX2+ Complexes (X = F, Cl, Br, I) with Neutral Diphosphine and Diarsine Ligands

“…The [B 2 F 7 ] − ion is known to be unstable in solution (exchanging with [BF 4 ] − and F – ), and the NMR data do not resolve the question of its presence in the complex. The IR spectrum of the solid shows bands at 1240 and 830 cm –1 which may be due to [B 2 F 7 ] − . The diarsine complexes are the first examples of boronium salts with arsenic donor ligands, but are markedly less stable than the phosphine analogues.…”

Bis(triphenylphosphine)iminium tetrafluoroborate chloroform monosolvate

“…To the best of our knowlege, only four reports of [PPN]BF 4 structures are known: solvent-free [PPN]BF 4 (Bertocco et al, 2016;Denny & Darensbourg, 2016;Folda et al, 2015) (Liau et al, 2002).…”