Era of BARF
A revolution in this area occurred in the 1990s with the introduction of the 4]− anion, commonly abbreviated as − and affectionately called "BARF." This anion is far less coordinating than the tetrafluoroborate BF4−, hexafluorophosphate PF6−, and perchlorate ClO4−, and consequently has enabled the study of still more electrophilic cations. Related tetrahedral anions include tetrakis(pentafluorophenyl)borate, B(C6F5)4- and Al(OC(CF3)3)4-.
In the bulky borates and aluminates, the negative charge is symmetrically distributed over many electronegative atoms. Related anions are derived from tris(pentafluorophenyl)boron B(C6F5)3. Another advantage of these anions is that their salts are more soluble in non-polar organic solvents such as dichloromethane, toluene, and, in some cases, even alkanes. Polar solvents, such as acetonitrile, THF, and water tend to bind to electrophilic centers, in which cases, the use of a noncoordinating anion is pointless.
Salts of the anion B4− were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion. Kobayashi's method of preparation has been superseded by a safer route.
The neutral molecules that represent the parents to the non-coordinating anions are strong Lewis acids, e.g. boron trifluoride, BF3 and phosphorus pentafluoride, PF5. A notable Lewis acid of this genre is tris(pentafluorophenyl)boron, B(C6F5)3, which abstracts alkyl ligands:
- (C5H5)2Zr(CH3)2 + B(C6F5)3 → +-
Read more about this topic: Non-coordinating Anion
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