Tris(pentafluorophenyl)borane

Structure

The structure of tris(pentafluorophenyl)borane (BCF) was determined by gas electron diffraction. It has a propeller-like arrangement of its three pentafluorophenyl groups with a torsional angle of 40.6(3)° for the deviation of these groups from a hypothetically planar arrangement. Compared with a torsional angle of 56.8(4)° for tris(perfluoro-para-tolyl)borane, which is a stronger Lewis acid than BCF, this shows that there is some delocalization of electron density from the para-fluorine atoms to the boron atom that reduces its acidity.

Lewis acidity

The most noteworthy property of this molecule is its strong Lewis acidity. Its Lewis acid strength, as quantified by experimental equilibrium constants, is by 7 orders of magnitude higher than the one of structurally analogous triphenylborane. Experimental equilibrium measurements, its AN value (Gutmann-Beckett method) as well as quantum-chemical calculations all indicate that the Lewis acidity of B(C₆F₅)₃ is slightly lower than that of BF₃ and significantly reduced compared to BCl₃. B(C₆F₅)₃ forms a strong Lewis adduct with water, which was shown to be a strong Brønsted acid having an acidity comparable to hydrochloric acid (in acetonitrile). In consequence, even traces of moisture are able to deactivate B(C₆F₅)₃ and remaining catalytic activity might only be due to the Brønsted acidity of the water adduct.

Applications in catalysis

In one application (C₆F₅)₃B forms noncoordinating anions by removing anionic ligands from metal centers. Illustrative is a reaction that give rise to alkene polymerization catalysts where tris(pentafluorophenyl)boron is used as an activator or cocatalyst:

(C₆F₅)₃B + (C₅H₅)₂Zr(CH₃)₂ → [(C₅H₅)₂ZrCH₃]⁺[(C₆F₅)₃BCH₃]⁻

In this process, the strongly coordinating methyl group transfers to the boron to expose a reactive site on zirconium. The resulting cationic zirconocene species is stabilised by the non coordinating borane anion. The exposed site on the zirconium allows for coordination of alkenes, whereupon migratory insertion into the remaining carbon-methyl ligand gives rise to a propyl ligand this process continues resulting in the growth of a polymer chain. This reagent has led to the development of immobilised catalyst/activator species; where the catalyst/activator is immobilised on an inert inorganic support such as silica.

Tris(pentafluorophenyl)borane is also capable of abstracting hydride to give [(C₆F₅)₃BH]⁻, and it catalyzes hydrosilylation of aldehydes. Otherwise (C₆F₅)₃B binds to a wide range of Lewis bases, even weak ones. The compound is hygroscopic, forming the trihydrate [(C₆F₅)₃BOH₂](H₂O)₂, wherein one water in coordinated to boron and the other two waters are hydrogen-bonded to the coordinated water.

Related compounds are pentafluorophenylboron halides.

Frustrated Lewis pair

Tris(pentafluorophenyl)borane is a key reagent leading to the concept of frustrated Lewis pairs. The combination of BCF and bulky basic phosphines, such as tricyclohexylphosphine (PCy₃) cleaves H₂:

(C₆F₅)₃B + PCy₃ + H₂ → (C₆F₅)₃BH⁻ + HPCy₃⁺

Many related phosphines, boranes, and substrates participate in related reactions.

Other reactions

(C₆F₅)₃B was used to prepare a compound containing a Xe-C bond:

(C₆F₅)₃B + XeF₂ → [C₆F₅Xe]⁺[(C₆F₅)₂BF₂]⁻

Upon reaction with pentafluorophenyllithium, the salt of the noncoordinating anion lithium tetrakis(pentafluorophenyl)borate is formed.

(C₆F₅)₃B + C₆F₅Li → Li[(C₆F₅)₄B]

B(C₆F₅)₃ reacts with dimesitylphosphine to give the zwitterionic phosphonic-boronate (mes = C₆H₂Me₃):

(C₆F₅)₃B + mes₂PH → (C₆F₅)₂B(F)−C₆F₄−P(H)mes₂

This zwitterionic salt can be converted to a system that reversibly binds molecular H₂:

(C₆F₅)₂B(F)−C₆F₄−P(H)mes₂ + Me₂SiHCl → (C₆F₅)₂B(H)−C₆F₄−P(H)mes₂ + Me₂SiFCl

(C₆F₅)₂B(H)−C₆F₄−P(H)mes₂ → (C₆F₅)₂B−C₆F₄−Pmes₂ + H₂

Extra reading

• Lawson, James R.; Melen, Rebecca L. (3 February 2017). "Tris(pentafluorophenyl)borane and Beyond: Modern Advances in Borylation Chemistry". Inorganic Chemistry. 56 (15): 8627–8643. doi:10.1021/acs.inorgchem.6b02911. PMID 28157303.