Trisborane, sometimes referred to as "BCF", is the chemical compound3B. 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.
Preparation
Trisborane is prepared using a Grignard reagent derived from bromopentafluorobenzene: Originally the synthesis employed C6F5Li, but this reagent can detonate with elimination of LiF.
Structure
The structure of trisborane was determined by gas electron diffraction. It has a propeller-like arrangement of its three pentafluorophenyl groups with a torsional angle of 40.6° for the deviation of these groups from a hypothetically planar arrangement. Compared with a torsional angle of 56.8° for trisborane, 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.
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. Owing to the Lewis acidity of the boron center, this compound is quite sensitive to the presence of water and is irreversibly deactivated by it.
Applications in catalysis
In one application 3B forms noncoordinating anions by removing anionic ligands from metal centers. Illustrative is a reaction that give rise to alkene polymerization catalysts where trisboron 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, 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. Trisborane is also capable of abstracting hydride to give −, and it catalyzes hydrosilylation of aldehydes. Otherwise 3B binds to a wide range of Lewis bases, even weak ones. The compound is hygroscopic, forming the trihydrate 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.
Trisborane is a key reagent leading to the concept of frustrated Lewis pairs. The combination of BCF and bulky basic phosphines, such as tricyclohexylphosphine cleaves H2: Many related phosphines, boranes, and substrates participate in related reactions.
Other reactions
3B was used to prepare a compound containing a Xe-C bond: Upon reaction with pentafluorophenyllithium, the salt of the noncoordinating anion lithium tetrakisborate is formed.
