Lithium tetrakis(pentafluorophenyl)borate explained

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 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.

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 (OEt₂) molecules are bound to the lithium cation, with Li-O bond lengths of approximately 1.95 Å. The [Li(OEt₂)₄]⁺ complex is tetrahedral.

Preparation

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:^[1]

(C₆F₅)₃B + Li(C₆F₅) → [Li(OEt₂)₃][B(C₆F₅)₄]

Since its discovery, many revised syntheses have been described.^[2]

Reactions

Lithium tetrakis(pentafluorophenyl)borate is primarily used to prepare cationic transition metal complexes:

LiB(C₆F₅)₄) + ML_nCl → LiCl + [ML_n]B(C₆F₅)₄

LiB(C₆F₅)₄ is converted to the trityl reagent [Ph₃C][B(C₆F₅)₄], which is useful activator of Lewis-acid catalysts.^{[3] [4]}

Safety

Lithium tetrakis(pentafluorophenyl)borate will deflagrate on melting (ca 265 C) giving thick black smoke, even under nitrogen.^[5] The mechanism is unknown. Metal tetrakis(pentafluorophenyl)borates of K and Na decompose vigorously as well.^{[6] [7]}

See also

• Tetraphenylborate

References

1. A.G. Massey, A.J. Park. "Perfluorophenyl derivatives of the elements : I. Tris(pentafluorophenyl)boron." Journal of Organometallic Chemistry, 1964, 2, 245-250.
2. Martin, Eddy, Hughes, David, and Lancaster, Simon. "The composition and structure of lithium tetrakis(pentafluorophenyl)borate diethyletherate." Inorganica Chimica Acta, 2010, 363, 275-278
3. Cordoneanu, Adina, Drewitt, Mark, Bavarian, Neda, Baird, Michael. "Synthesis and Characterization of Weakly Coordinating Anion Salts of a New, Stable Carbocationic Reagent, the Dibenzosuberenyl (Dibenzotropylium) Ion." New Journal of Chemistry, 2008, 32, 1890-1898.
4. The Reaction of Cp*₂ZrMe₂ with [CPh₃][B(C₆F₅)₄]: Triphenylethane Does Not form ηⁿ-Arene Complexes with [Cp*₂ZrMe]⁺. 2002. Journal of Organometallic Chemistry. 654. 221–223. 10.1016/S0022-328X(02)01319-0. Lancaster. Simon J.. Bochmann. Manfred. 1–2.
5. Martin . Eddy . Hughes . David L. . Lancaster . Simon J. . 2010-01-04 . The composition and structure of lithium tetrakis(pentafluorophenyl)borate diethyletherate . Inorganica Chimica Acta . en . 363 . 1 . 275–278 . 10.1016/j.ica.2009.09.013 . 0020-1693.
6. Stockmann . Tom J. . Ding . Zhifeng . 2012-10-25 . Tetraoctylphosphonium tetrakis(pentafluorophenyl)borate room temperature ionic liquid toward enhanced physicochemical properties for electrochemistry . The Journal of Physical Chemistry B . 116 . 42 . 12826–12834 . 10.1021/jp3081832 . 1520-5207 . 23009026.
7. Lee . Hyun-Ju . Baek . Jun-Won . Seo . Yeong-Hyun . Lee . Hong-Cheol . Jeong . Sun-Mi . Lee . Junseong . Lee . Chong-Gu . Lee . Bun-Yeoul . 2021-05-10 . Preparation of High-Purity Ammonium Tetrakis(pentafluorophenyl)borate for the Activation of Olefin Polymerization Catalysts . Molecules . 26 . 9 . 2827 . 10.3390/molecules26092827 . 1420-3049 . 8126221 . 34068755. free .