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 atom; the core is
planar
Planar is an adjective meaning "relating to a plane (geometry)".
Planar may also refer to:
Science and technology
* Planar (computer graphics), computer graphics pixel information from several bitplanes
* Planar (transmission line technologies), ...
. It has been described as the “ideal
Lewis acid” 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 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
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 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, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
(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
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
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 is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
, whereupon migratory insertion into the remaining carbon-methyl
ligand
In coordination chemistry, a ligand is an ion or molecule ( 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 elec ...
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 , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
.
is also capable of abstracting hydride to give , and it catalyzes
hydrosilylation Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 2009 ...
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 pair
A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit ...
s. 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
Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the weakly coordinating anion (B(C6F5)4)−. Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form ...
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