Tris(pentafluorophenyl)borane, sometimes referred to as "BCF", is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. It is a white, volatile solid. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

atom; the core is planar. It has been described as the “ideal

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

” because of its high thermal stability and the relative inertness of the B-C bonds. Related fluoro-substituted boron compounds, such as those containing groups, decompose with formation of B-F bonds. Tris(pentafluorophenyl)borane is thermally stable at temperatures well over 200 °C, resistant to oxygen, and water-tolerant.

Preparation

Tris(pentafluorophenyl)borane is prepared using a

Grignard reagent Grignard reagents or Grignard compounds are chemical compounds with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromi ...

derived from bromopentafluorobenzene: : The synthesis originally employed , but this reagent can detonate with elimination of .

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 , 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

ity. 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 is slightly lower than that of and significantly reduced compared to . forms a strong Lewis adduct with water, which was shown to be a strong Brønsted acid having an acidity comparable to

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

(in acetonitrile). In consequence, even traces of moisture are able to deactivate and remaining catalytic activity might only be due to the Brønsted acidity of the water adduct.

Applications in catalysis

In one application 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: : 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 In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

, whereupon migratory insertion into the remaining carbon-methyl

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

. is also capable of abstracting hydride to give , and it catalyzes hydrosilylation of aldehydes. Otherwise binds to a wide range of Lewis bases, even weak ones. The compound is hygroscopic, forming the trihydrate , wherein one water in coordinated to boron and the other two waters are hydrogen-bonded to the coordinated water. Related compounds are 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 Tricyclohexylphosphine is the tertiary phosphine with the formula P( C6H11)3. Commonly used as a ligand in organometallic chemistry, it is often abbreviated to PCy3, where Cy stands for cyclohexyl. It is characterized by both high basicity (p''K ...

() cleaves :Stephan, D. W., ""Frustrated Lewis Pairs": A New Strategy to Small Molecule Activation and Hydrogenation Catalysis", Dalton Trans. 2009, 3129. : Many related phosphines, boranes, and substrates participate in related reactions.

Other reactions

was used to prepare a compound containing a Xe-C bond: : Upon reaction with , the salt of the noncoordinating anion lithium tetrakis(pentafluorophenyl)borate is formed. : reacts with to give the zwitterionic phosphonic-boronate (mes = ): : This zwitterionic salt can be converted to a system that reversibly binds molecular : : :

References

Extra reading

*{{open access Organoboranes Pentafluorophenyl compounds Perfluorinated compounds Acid catalysts