3D model (JSmol)
|Molar mass||511.98 g/mol|
|Melting point||126 to 131 °C (259 to 268 °F; 399 to 404 K)|
|S-phrases (outdated)||S26 S36|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Tris(pentafluorophenyl)borane, sometimes referred to as "BCF", is the chemical compound (C6F5)3B. It is a white, volatile solid. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC3 core is planar. It has been described as the “ideal Lewis acid” because of its versatility and the relative inertness of the B-C bonds. Related fluoro-substituted boron compounds, such as those containing B-CF3 groups, decompose with formation of B-F bonds.
- 3 C6F5MgBr + BCl3 → (C6F5)3B + 3MgBrCl
Originally the synthesis employed C6F5Li, but this reagent can detonate with elimination of LiF.
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 torisonal angle of 40.6(3)° for the devation of these groups from a hypothetically planar arrangement.Compared with a torisonal angle of 56.8(4)° for tris(perfluoro-para-tolyl)borane (BTolF), which is a stronger Lewis acid than BCF, this shows that there is some delocaization of electron density from the para-fluorine atoms to the boron atom that reduces its acidity.
The most noteworthy property of this molecule is its strong Lewis acidity. Its acid strength, as determined by the Gutmann-Beckett method and the Childs method, is comparable to BF3 but weaker than BCl3. This property indicates that the electronegativity of the C6F5 group and a halide are similar.
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.
(C6F5)3B was used to prepare a compound containing a Xe-C bond:
- (C6F5)3B + XeF2 → [C6F5Xe]+[(C6F5)2BF2]−
- (C6F5)3B + C6F5Li → Li[(C6F5)4B]
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