Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the weakly coordinating anion (B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>)<sup>âÂÂ</sup>. Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form in the catalyst composition for olefin polymerization reactions and in electrochemistry. It is a water-soluble compound. Its anion is closely related to the non-coordinating anion known as BARF. The tetrakis(pentafluorophenyl)borates have the advantage of operating on a one-to-one stoichiometric basis with Group IV transition metal polyolefin catalysts, unlike methylaluminoxane (MAO) which may be used in large excess.
The anion is tetrahedral with B-C bond lengths of approximately 1.65 Angstroms. The salt has only been obtained as the etherate, and the crystallography confirms that four ether (OEt<sub>2</sub>) molecules are bound to the lithium cation, with Li-O bond lengths of approximately 1.95 ÃÂ . The [Li(OEt<sub>2</sub>)<sub>4</sub>]<sup>+</sup> complex is tetrahedral.
The salt was first produced in studies on tris(pentafluorophenyl)boron, a well known Lewis acidic compound. Combining equimolar ether solutions of pentafluorophenyllithium and tris(pentafluorophenyl)boron gives the lithium salt of tetrakis(pentafluorophenyl)borate, which precipitates the etherate as a white solid:
Since its discovery, many revised syntheses have been described.
Lithium tetrakis(pentafluorophenyl)borate is primarily used to prepare cationic transition metal complexes:
LiB(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub> is converted to the trityl reagent [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>], which is useful activator of Lewis-acid catalysts.
Lithium tetrakis(pentafluorophenyl)borate will deflagrate on melting (ca. 265 ðC) giving thick black smoke, even under nitrogen. The mechanism is unknown. Metal tetrakis(pentafluorophenyl)borates of K and Na decompose vigorously as well.