Tris(pentafluorophenyl)boron

Tris(pentafluorophenyl)boron is the chemical compound (C₆F₅)₃B. 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.

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Preparation

(C₆F₅)₃B is prepared using a Grignard reagent:

3C₆F₅MgBr + BCl₃ → (C₆F₅)₃B + 3MgBrCl

Originally the synthesis employed C₆F₅Li, but this reagent can detonate with elimination of LiF.^[1]

Lewis acidity

The most noteworthy property of this molecule is its strong Lewis acidity: stronger than BF₃ but weaker than BCl₃. This property indicates that the electronegativity of the C₆F₅ group and a halide are similar. An most important application of (C₆F₅)₃B is that it forms noncoordinating anions by removing anionic ligands from metal centers.^[2] Illustrative is a reaction that give rise to alkene polymerization catalyst:

(C₆F₅)₃B + (C₅H₅)₂Zr(CH₃)₂ → [(C₅H₅)₂ZrCH₃⁺][ C₆F₅)₃BCH₃⁻]

In this process, the strongly coordinating methyl group transfers to the boron to expose a reactive site on zirconium. Alkenes can bind to this site, whereupon they couple to the remaining methyl ligand to give a propyl ligand, thereby starting the growth of a chain of polyethylene.

(C₆F₅)₃B is also capable of abstracting hydride to give [(C₆F₅)₃BH]⁻, and it catalyzes hydrosilylation of aldehydes. Otherwise (C₆F₅)₃B binds to a wide range of Lewis bases, even weak ones.^[3] The compound is hygroscopic, forming the trihydrate [(C₆F₅)₃BOH₂](H₂O)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. ^[4]

Other reactions

(C₆F₅)₃B was used to prepare a compound containing a Xe-C bond:

(C₆F₅)₃B + XeF₂ → [C₆F₅Xe⁺][(C₆F₅)₂BF₂⁻]

One study ^[5] reported a nucleophilic aromatic substitution on one of the pentafluorinephenyl rings by dimesityl phosphane:

The bulky mesityl groups prevent the phosphorus atom from coordinating directly to boron and instead the ring is attacked. When the fluorine atom on boron is replaced by hydrogen with dimethylchlorosilylhydride, the resulting phosphazenium borate is capable of reversible hydrogen storage ^[6]

References

1. ^ Piers, W. E.; Chivers, T. “Pentafluorophenylboranes: from Obscurity to Applications” Chemical Society Reviews volume 26, pages 345-354 (1997)
2. ^ Fuhrmann, H.; Brenner, S.; Arndt, P.; Kempe, R. “Octahedral Group 4 Metal Complexes That Contain Amine, Amido, and Aminopyridinato Ligands: Synthesis, Structure, and Application in α-Olefin Oligo- and Polymerization” Inorganic Chemistry 1996, 35, 6742-6745.
3. ^ Erker, G. "Tris(pentafluorophenyl)borane: A Special Boron Lewis Acid for Special Reactions" Dalton Transactions (2005), 1883-1890.
4. ^ Chivers, T. “Pentafluorophenylboron halides: 40 years later” Journal of Fluorine Chemistry (2002), volume 115, page 1-8.
5. ^ Reversible, Metal-Free Hydrogen Activation Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan Science (journal) 17 November 2006: Vol. 314. no. 5802, pp. 1124 - 1126 doi:10.1126/science.1134230
6. ^ See also hydrogen storage

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