Non-coordinating anion

Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion.[1] Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes with an unsaturated coordination sphere. These special anions are essential components of homogeneous alkene polymerisation catalysts, where the active catalyst is a coordinatively unsaturated, cationic transition metal complex. For example, they are employed as counterions for the 14 valence electron cations [(C5H5)2ZrR]+ (R = methyl or a growing polyethylene chain). Complexes derived from non-coordinating anions have been used to catalyze hydrogenation, hydrosilylation, oligomerization, and the living polymerization of alkenes. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Non-coordinating anions are important components of many superacids, which result from the combination of Brønsted acids and 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 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.[7] Kobayashi's method of preparation has been superseded by a safer route.[5]

Crystal Structure of related acid
The crystal structure of the compound [H(Et2O)2][B(C6F5)4],[8] which is closely related to 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 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.[10]

References
  1. I. Krossing & I. Raabe (2004). "Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition. 43 (16): 2066–2090. doi:10.1002/anie.200300620. PMID 15083452.
  2. Honeychuck, R. V.; Hersh, W. H. (1989). "Coordination of "Noncoordinating" Anions: Synthesis, Characterization, and X-ray Crystal Structures of Fluorine-Bridged [SbF6]−, [BF4]−, and [PF6]− Adducts of [R3P(CO)3(NO)W]+. An Unconventional Order of Anion Donor Strength". Inorganic Chemistry. 28 (14): 2869–2886. doi:10.1021/ic00313a034.
  3. Mayfield, H. G.; Bull, W. E. (1971). "Co-ordinating Tendencies of the Hexafluorophosphate Ion". J. Chem. Soc. A (14): 2279–2281. doi:10.1039/J19710002279.
  4. Santiso-Quiñones, Gustavo; Reisinger, Andreas; Slattery, John; Krossing, Ingo (2007). "Homoleptic Cu–phosphorus and Cu–ethene complexes". Chemical Communications (47): 5046–5048. doi:10.1039/b710899k. PMID 18049748.
  5. N. A. Yakelis; R. G. Bergman (2005). "Sodium Tetrakis(3,5-trifluoromethyl)phenylborate (NaBArF24): Safe Preparation, Standardized Purification, and Analysis of Hydration". Organometallics. 24 (14): 3579–3581. doi:10.1021/om0501428. PMC 2600718. PMID 19079785.
  6. M. Brookhart; B. Grant; A. F. Volpe, Jr. (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): 3920–3922. doi:10.1021/om00059a071.
  7. H. Nishida; N. Takada; M. Yoshimura; T. Sonods; H. Kobayashi (1984). "Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. Highly lipophilic stable anionic agent for solvent-extraction of cations". Bulletin of the Chemical Society of Japan. 57 (9): 2600. doi:10.1246/bcsj.57.2600.
  8. 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. 19 (7): 1442. doi:10.1021/om990612w.
  9. G.Erker (2005). "Tris(pentafluorophenyl)borane: a Special Boron Lewis Acid for Special Reactions". Dalton Transactions (11): 1883–1890. doi:10.1039/b503688g. PMID 15909033.
  10. Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin, L.; Lambert, J. B. (2002). "Crystallographic Evidence for a Free Silylium Ion". Science. 297 (5582): 825–827. Bibcode:2002Sci...297..825K. doi:10.1126/science.1073540. PMID 12161650. S2CID 9072540.
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