Pressure-Induced Formation of Molecular B₂X₄(μ-X)₂ (X = Cl, Br, I) Species

Abstract

Boron(III) halides (BX3, where X = F, Cl, Br, I) at ambient pressure conditions exist as strictly monomeric, trigonal-planar molecules. Using correlated ab initio calculations, the three heavier halides (X = Cl, Br, I) are shown to possess B2X4(μ-X)2 local minima, isostructural with the diborane molecule. The calculated dissociation barrier of the B2I4(μ-I)2 species [≈14 kJ/mol with CCSD(T)/cc-pVTZ] may be high enough to allow cryogenic isolation. The remaining dimer structures are more labile, with dissociation barriers of less than 6 kJ/mol. All three dimer species may be stabilized by application of external pressure. Periodic density functional theory calculations predict a new dimer-based P1̅ solid, which becomes more stable than the P63/m monomer-derived solids at 5 (X = I) to 15 (X = Cl) GPa. Metadynamics simulations suggest that B2X4(μ-X)2-based solids are the kinetically preferred product of pressurization of the P63/m solid.

BH3 under Pressure: Leaving the Molecular Diborane Motif

“…But we did look at the infinite chain, the limit of a large ring. In this, we were encouraged by our finding of such a geometry being important in a study of B 2 I 6 under pressure . We began with a simple chain with linear B–H–B linkages, shown at the top in Figure .…”

Boron‐Boron One‐Electron Sigma Bonds versus B‐X‐B Bridged Structures

“…The neutral dimer di‐μ‐hydroxytetrahydroxy‐diborane B 2 (OH) 4 (μ‐OH) 2 may also present the same structure, this time with the hydroxyl groups acting as bridges . Halogen atoms are similarly able to connect two borane monomers . It is also well‐known that when an additional electron is added to planar BX 3 compounds, which have a formal sp 2 hybridization, this new electron occupies a previously empty p boron orbital, changing the hybridization of the boron atom to nearly sp 3 , with a pyramidal geometry.…”

Boron, aluminium, gallium, indium and thallium

“…The results of these studies suggest that at elevated pressures the three heavier halides (X = Cl, Br, I) possess local energy minima that represent a B 2 X 4 (m-X) 2 structure that are isostructural with the diborane molecule. 216 The syntheses, X-ray crystal structures, and electrochemistry of BH 3 and BF 3 adducts of 1 0 ,2 0 ,3 0 ,4 0 ,5 0 -pentamethylazaferrocene have been reported. 217 The diarylamido/bis(phosphine) PNP pincer ligand (2-i Pr 2 P-4-MeC 6 H 3 ) 2 N has been evaluated as a scaffold for supporting the {BF 2 } fragment.…”