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Tris(pentafluorphenyl)boron
Tris(pentafluorphenyl)boron
Tris(pentafluorophenyl)boron-3D-vdW.png
IUPAC name
Other names Perfluorotriphenylboron
Identifiers
CAS number 1109-15-5
Properties
Molecular formula C18F15B
Molar mass 511.98 g/mol
Appearance colorless solid
Melting point

126-131 °C

Solubility in water forms adduct
Structure
Molecular shape trigonal planar
Dipole moment 0 D
Hazards
R-phrases R36/37/38
S-phrases S26 S36
Related compounds
Related compounds Triphenylboron (C6H5)3B
BF3
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.

Contents

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:

Phosphino-borane Synthesis Welch 2006

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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