Brookhart's acid

Brookhart's acid
2D drawing of the molecule
Identifiers
139362-04-2
ChemSpider 9301081
Jmol interactive 3D Image
PubChem 11125959
Properties
C40H33BF24O2
Molar mass 1012.46
Appearance White crystalline solid
Hazards
Main hazards Strong acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Brookhart's acid is the salt of the diethyl ether oxonium ion and tetrakis[3,5-bis(trifluoromethyl)phenyl)borate (BAr’4). It is a colorless solid, used as a strong acid. The compound was first reported by Volpe, Grant, and Brookhart in 1992.[1]

Preparation

This compound is prepared by treatment of NaBAr’4 in diethyl ether (Et2O) with hydrogen chloride:

NaBAr'4 + HCl + 2 Et2O → [H(OEt2)2]+BAr'4 + NaCl

NaBAr’4 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.[1]

Structure and properties

Crystal Structure of related acid
The crystal structure of the compound [H(OEt2)2][B(C6F5)4],[2] that is closely related to Brookhart's acid. The acidic proton, which resides between the ether oxygen centres, is not shown.

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.[1]

[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.[2]

Uses

Traditional weakly coordinating anions, such as perchlorate, tetrafluoroborate, and hexafluorophosphate will coordinate to very electrophilic cations, making these counterions unsuitable for some complexes.[3] The highly reactive species [Cp2Zr(CH3)]+, for example, has been reported to abstract F from PF6.[4] Starting in the 1980s, new types 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.[1] One common method of generating these cationic species is via protonolysis of a dialkyl complexes or an olefin complex. For example, an electrophilic palladium catalyst, [(2,2'-bipyridine)Pd(CH3)(CH3CN)][BAr'4], is prepared by protonating the dimethyl complex with Brookhart's acid. This electrophilic, cationic palladium species is used for the polymerization of olefins with carbon monoxide in aprotic solvents.[5]

References

  1. 1 2 3 4 Brookhart, M.; Grant, B.; Volpe, A. F. (1992). "[(3,5-(CF3)2C6H3)4B][H(OEt2)2]+: A convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes". Organometallics 11 (11): 3920. doi:10.1021/om00059a071.
  2. 1 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. Krossing, I.; Raabe, I. (2004). "Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition 43 (16): 2066–90. doi:10.1002/anie.200300620. PMID 15083452.
  4. Jordan, R. F.; Dasher, W. E.; Echols, S. F. (1986). "Reactive cationic dicyclopentadienyl zirconium(IV) complexes". Journal of the American Chemical Society 108 (7): 1718. doi:10.1021/ja00267a068.
  5. Brookhart, M.; Rix, F. C.; Desimone, J. M.; Barborak, J. C. (1992). "Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide". Journal of the American Chemical Society 114 (14): 5894. doi:10.1021/ja00040a082.
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