Brookhart's acid

Preparation

This compound is prepared by treatment of a diethyl ether (Et2O) solution of NaBArF4 (BArF4- = B(C6H3(CF3)2)4-) with hydrogen chloride:

: NaBArF4 + HCl + 2 Et2O → [H(OEt2)2]+[BArF4]- + NaCl

[H(OEt2)2][BArF4] is soluble in diethyl ether, whereas sodium chloride is not. Precipitation of sodium chloride thus drives the formation of the oxonium acid compound, which is isolable as a solid.

Structure and properties

The acid crystallizes as a white, hygroscopic crystalline solid. NMR and elemental analysis showed that the crystal contains two equivalents of diethyl ether. In solution, the compound slowly degrades to m-C6H3(CF3)2 and BAr′3.

[H(OEt2)2][B(C6F5)4] is a related compound with a slightly different weakly coordinating anion; it was first reported in 2000. An X-ray crystal structure of that compound was obtained, showing the acidic proton coordinated by both ethereal oxygen centers, although the crystal was not good enough to determine whether the proton is located symmetrically or unsymmetrically between the two.

Uses

Traditional weakly coordinating anions, such as perchlorate, tetrafluoroborate, and hexafluorophosphate, will nonetheless coordinate to very electrophilic cations, making these counterions unsuitable for some complexes. The highly reactive species [Cp2Zr(CH3)]+, for example, has been reported to abstract F− from PF6. Starting in the 1980s, new types of weakly coordinating anions began to be developed. BAr′4 anions are used as counterions for highly electrophilic, cationic transition metal species, as they are very weakly coordinating and unreactive towards electrophilic attack.

Potential application

Polyketones, thermoplastic polymers, are formed by the copolymerisation of carbon monoxide and one or more alkenes (typically ethylene with propylene). The process utilises a palladium(II) catalyst with a bidentate ligand like 2,2′-bipyridine or 1,10-phenanthroline (phen) with a non-coordinating BARF counterion, such as [(phen)Pd(CH3)(CO)]BArF4. The preparation of the catalyst involves the reaction of a dimethyl palladium complex with Brookhart's acid in acetonitrile with loss of methane and the catalytic species is formed by uptake of carbon monoxide to displace acetonitrile.

:[(Et2O)2H]BArF4 + [(phen)Pd(CH3)2] + MeCN → [(phen)Pd(CH3)(MeCN)]BArF4 + 2 Et2O + CH4

:[(phen)Pd(CH3)(MeCN)]BArF4 + CO → [(phen)Pd(CH3)(CO)]BArF4 + MeCN

The mechanism involves migratory insertion Use of monodentate phosphine ligands also leads to undesirable side-products but bidentate phosphine ligands like 1,3-bis(diphenylphosphino)propane have been used industrially.

References

References

1. (1992). "[(3,5-(CF₃)₂C₆H₃)₄B]⁻[H(OEt₂)₂]⁺: A convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes". Organometallics.
2. Jutzi, P.; Müller, C.; Stammler, A.; Stammler, H. G. (2000). "Synthesis, Crystal Structure, and Application of the Oxonium Acid [H(OEt₂)₂]⁺B(C₆F₅)₄]⁻". Organometallics vol. 19, p. 1442. {{doi. 10.1021/om990612w
3. (2004). "Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition.
4. (1986). "Reactive cationic dicyclopentadienyl zirconium(IV) complexes". Journal of the American Chemical Society.
5. (1992). "Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide". Journal of the American Chemical Society.
6. (2001). "Encyclopedia Of Polymer Science and Technology".
7. (2002). "Alternating copolymerization of carbon monoxide and olefins by single-site metal catalysis". [[Coord. Chem. Rev.]].
8. (1992). "Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide". [[J. Am. Chem. Soc.]].
9. (1996). "Mechanistic studies of the palladium(II)-catalyzed copolymerization of ethylene with carbon monoxide". [[J. Am. Chem. Soc.]].
10. (1996). "Palladium-catalyzed alternating copolymerization of alkenes and carbon monoxide". [[Chem. Rev.]].
11. (2000). "Kinetic studies of migratory insertion reactions at the (1,3-bis(diphenylphosphino)propane)Pd(II) center and their relationship to the alternating copolymerization of ethylene and carbon monoxide". [[J. Am. Chem. Soc.]].