Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the
weakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
(B(C
6F
5)
4)
−. 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
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
. 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
Methylaluminoxane, commonly called MAO, is a mixture of organoaluminium compounds with the approximate formula (Al(CH3)O)''n''. It is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene ...
(MAO) which may be used in large excess.
Structure and properties
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
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
(OEt
2) molecules are bound to the lithium cation, with Li-O bond lengths of approximately 1.95 Å. The
i(OEt2)4sup>+ complex is tetrahedral.
Preparation
The salt was first produced in studies on
tris(pentafluorophenyl)boron, a well known
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 sp ...
ic 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:
:(C
6F
5)
3B + Li(C
6F
5) →
i(OEt2)3B(C
6F
5)
4]
Since its discovery, many revised syntheses have been described.
Reactions
Lithium tetrakis(pentafluorophenyl)borate is primarily used to prepare cationic
transition metal complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es:
:LiB(C
6F
5)
4) + ML
nCl → LiCl +
Ln(C
6F
5)
4
LiB(C
6F
5)
4 is converted to the trityl reagent
h3CB(C
6F
5)
4], which is useful activator of Lewis-acid catalysts.
Safety
Lithium tetrakis(pentafluorophenyl)borate will deflagrate on melting (ca 265 C) giving thick black smoke, even under nitrogen. The mechanism is unknown. Metal tetrakis(pentafluorophenyl)borates of K and Na decompose vigorously as well. (unpublished data)
See also
*
Tetraphenylborate
Tetraphenylborate (IUPAC name: Tetraphenylboranuide) is an organoboron anion consisting of a central boron atom with four phenyl groups. Salts of tetraphenylborate uncouple oxidative phosphorylation
Oxidative phosphorylation (UK , US ) or e ...
References
{{Lithium compounds
Lithium salts
Organoboron compounds
Pentafluorophenyl compounds