Non-coordinating anion
Anion which interacts weakly with cations
- 2 min read
- 3 chapters
Related on Lably
Through Electrophile, Coordination complex and Transition metal
Pre-"BARF" era
Before the 1990s, tetrafluoroborate, hexafluorophosphate, and perchlorate were considered weakly coordinating anions. Usually, only by exclusion of conventional solvents were transition metal perchlorate complexes found to exist, for example. It is now appreciated that BF−4, PF−6, and ClO−4 bind to electrophilic metal centers of the type use in some catalytic such as cationic Zr(IV) centers. Other anions, such as triflates coordinate to metal cations. On that same theme, tetraphenylborate forms pi-complexes with utilizing its electron-rich phenyl groups as ligands.
Sign in to keep reading
2 more chapters are free with a Lably account.
Era of BARF
A revolution in this area occurred in the 1990s with the introduction of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ion, B[3,5-(CF
3)
2C
6H
3]−
4, commonly abbreviated as B(ArF)4− and colloquially called "BARF". This anion is far less coordinating than tetrafluoroborate, hexafluorophosphate, and perchlorate, and consequently has enabled the study of still more electrophilic cations. Related tetrahedral anions include tetrakis(pentafluorophenyl)borate B(C
6F
5)−
4, and Al[OC(CF
3)
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 non-coordinating anion is pointless.
Salts of the anion B[3,5-(CF
3)
2C
6H
3]−
4 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 Kobayashi's method of preparation has been superseded by a safer route.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)borane, B(C6F5)3, which abstracts alkyl ligands:
- (C5H5)2Zr(CH3)2 + B(C6F5)3 → [(C5H5)2Zr(CH3)]+[(CH3)B(C6F5)3]−
Other types of non-coordinating anions
Another large class of non-coordinating anions are derived from carborane anion CB
11H−
12. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)3Si][HCB11Me5Br6] contains a non-coordinating anion derived from a carborane.