Non-coordinating anion

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Lewis acids
.

Pre-"BARF" era

Before the 1990s, tetrafluoroborate (BF−
4), hexafluorophosphate (PF−
6), and perchlorate (ClO−
4) were considered weakly coordinating anions. These species are now known to bind to strongly electrophilic metal centers.[2][3] Tetrafluoroborate and hexafluorophosphate anions are coordinating toward highly electrophilic metal ions, such as cations containing Zr(IV) centers, which can abstract fluoride from these anions. Other anions, such as triflates are considered to be low-coordinating with some cations.

Era of BARF

Structure of the weakly coordinating anion [Al(OC(CF3)3)4]−,[4] illustrating its high symmetry. Color code: green = F, red = O.

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".[5] This anion is far less coordinating than tetrafluoroborate, hexafluorophosphate, and perchlorate, and consequently has enabled the study of still more electrophilic cations.[6] 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

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 For that reason, it is sometimes referred to as Kobayashi's anion.[7] Kobayashi's method of preparation has been superseded by a safer route.[5]

Brookhart's Acid

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:[9]

(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

mesityl)3Si][HCB11Me5Br6] contains a non-coordinating anion derived from a carborane.[10]

References

  1. PMID 15083452
    .
  2. doi:10.1021/ic00313a034
    .
  3. doi:10.1039/J19710002279
    .
  4. PMID 18049748
    .
  5. ^
    PMID 19079785
    .
  6. doi:10.1021/om00059a071
    .
  7. doi:10.1246/bcsj.57.2600
    .
  8. doi:10.1021/om990612w
    .
  9. PMID 15909033
    .
  10. S2CID 9072540
    .
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