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 olefin 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 noncoordinating anions have been used to catalyze hydrogenation, hydrosilylation, oligomerization, and the living polymerization of olefins. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Noncoordinating anions are important components of many superacids, which result from the combination of Bronsted acids and Lewis acids.

Pre-"BARF" era

Prior to the 1990s, [BF₄]⁻, [PF₆]⁻, and [ClO₄]⁻ were considered weakly coordinating anions. These species are now known to bind well to electrophilic metal centers.^[2] 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 were considered to be low-coordinating with some cations.

Era of BARF

A revolution in this area occurred in the 1990s with the introduction of the [B[3,5-(CF₃)₂C₆H₃]₄]⁻ anion, commonly abbreviated as [BAr^F₄]⁻ and affectionately called "BARF."^[3] 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.^[4] 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 B[3,5-(CF₃)₂C₆H₃]₄⁻ were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion.^[5] Kobayashi's method of preparation has been superseded by a safer route.^[3]

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:^[6]

(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 CB₁₁H₁₂^-. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)₃Si][HCB₁₁Me₅Br₆] contains a noncoordinating anion derived from a carborane.^[7]

References

1. ^ I. Krossing and 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. ^ ^{a b} 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2600718. 
4. ^ 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. 
5. ^ H. Nishida, N. Takada, M. Yoshimura, T. Sonods, H. Kobayshi (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. 
6. ^ 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. 
7. ^ 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. doi:10.1126/science.1073540. PMID 12161650.