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 Kobayashi's method of preparation has been superseded by a safer route.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
Famous quotes containing the words era of and/or era:
“The lover of nature is he whose inward and outward senses are still truly adjusted to each other; who has retained the spirit of infancy even into the era of manhood.”
—Ralph Waldo Emerson (1803–1882)
“Erasmus was the light of his century; others were its strength: he lighted the way; others knew how to walk on it while he himself remained in the shadow as the source of light always does. But he who points the way into a new era is no less worthy of veneration than he who is the first to enter it; those who work invisibly have also accomplished a feat.”
—Stefan Zweig (18811942)