A second polymorph of bis(triphenyl-λ5-phosphanylidene)ammonium chloride–boric acid adduct

The title crystal structure is a new triclinic polymorph of [(Ph3P)2N]Cl·(B(OH)3) or C36H30NP2+·Cl−·BH3O3. The crystal structure of the orthorhombic polymorph was reported by [Andrews et al. (1983). Acta Cryst. C39, 880–882]. In the crystal, the [(Ph3P)2N]+ cations have no significant contacts to the chloride ions nor to the boric acid molecules. This is indicated by the P—N—P angle of 137.28 (8)°, which is in the expected range for a free [(Ph3P)2N]+ cation. The boric acid molecules form inversion dimers via pairs of O—H...O hydrogen bonds, and each boric acid molecule forms two additional O—H...Cl hydrogen bonds to one chloride anion. These entities fill channels, created by the [(Ph3P)2N]+ cations, along the c-axis direction

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