Non-coordinating anion

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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.

Contents

Pre-"BARF" era

Before the 1990s, tetrafluoroborate, hexafluorophosphate, and perchlorate were considered weakly coordinating anions. 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 strongly electrophilic metal centers of the type use in some catalytic reactions. [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] , illustrating its high symmetry. Color code: green = F, red = O, blue = Al. Al(ORf)4.png
Structure of the weakly coordinating anion [Al(OC(CF3)3)4] , illustrating its high symmetry. Color code: green = F, red = O, blue = Al.

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.

NonCoordAnion2011.png

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.[ citation needed ] 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]

The crystal structure of the compound [H(Et2O)2][B(C6F5)4], . H atoms are omitted from the image. Color code: red = O, yellow = F, gray = C. BArF acid crystal structure.png
The crystal structure of the compound [H(Et2O)2][B(C6F5)4], . H atoms are omitted from the image. Color code: red = O, yellow = F, gray = C.

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]

Related Research Articles

<span class="mw-page-title-main">Counterion</span> Ion which negates another oppositely-charged ion in an ionic molecule

In chemistry, a counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium ion is the counterion for the chloride ion and vice versa.

<span class="mw-page-title-main">Silver perchlorate</span> Chemical compound

Silver perchlorate is the chemical compound with the formula AgClO4. This white solid forms a monohydrate and is mildly deliquescent. It is a useful source of the Ag+ ion, although the presence of perchlorate presents risks. It is used as a catalyst in organic chemistry.

A salt metathesis reaction is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding affiliations. This reaction is represented by the general scheme:

<span class="mw-page-title-main">Tetrafluoroborate</span> Anion

Tetrafluoroborate is the anion BF−
4. This tetrahedral species is isoelectronic with tetrafluoroberyllate (BeF2−
4), tetrafluoromethane (CF4), and tetrafluoroammonium (NF+
4) and is valence isoelectronic with many stable and important species including the perchlorate anion, ClO−
4, which is used in similar ways in the laboratory. It arises by the reaction of fluoride salts with the Lewis acid BF3, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

<span class="mw-page-title-main">Tris(pentafluorophenyl)borane</span> Chemical compound

Tris(pentafluorophenyl)borane, sometimes referred to as "BCF", is the chemical compound (C6F5)3B. It is a white, volatile solid. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC3 core is planar. It has been described as the “ideal Lewis acid” because of its high thermal stability and the relative inertness of the B-C bonds. Related fluoro-substituted boron compounds, such as those containing B−CF3 groups, decompose with formation of B-F bonds. Tris(pentafluorophenyl)borane is thermally stable at temperatures well over 200 °C, resistant to oxygen, and water-tolerant.

Organofluorine chemistry describes the chemistry of organofluorine compounds, organic compounds that contain a carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil and water repellents to pharmaceuticals, refrigerants, and reagents in catalysis. In addition to these applications, some organofluorine compounds are pollutants because of their contributions to ozone depletion, global warming, bioaccumulation, and toxicity. The area of organofluorine chemistry often requires special techniques associated with the handling of fluorinating agents.

<span class="mw-page-title-main">Hexafluorophosphate</span> Anion with the chemical formula PF6–

Hexafluorophosphate is an anion with chemical formula of [PF6]−. It is an octahedral species that imparts no color to its salts. [PF6]− is isoelectronic with sulfur hexafluoride, SF6, and the hexafluorosilicate dianion, [SiF6]2−, and hexafluoroantimonate [SbF6]−. In this anion, phosphorus has a valence of 5. Being poorly nucleophilic, hexafluorophosphate is classified as a non-coordinating anion.

<span class="mw-page-title-main">Silver hexafluorophosphate</span> Chemical compound

Silver hexafluorophosphate, sometimes referred to "silver PF-6," is an inorganic compound with the chemical formula AgPF6.

<span class="mw-page-title-main">Sodium tetraphenylborate</span> Chemical compound

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and caesium ions, and some organic nitrogen compounds.

<span class="mw-page-title-main">Tetrakis(acetonitrile)copper(I) hexafluorophosphate</span> Chemical compound

Tetrakis(acetonitrile)copper(I) hexafluorophosphate is a salt with the formula [Cu(CH3CN)4]PF6. It is a colourless solid that is used in the synthesis of other copper complexes. The cation [Cu(CH3CN)4]+ is a well-known example of a transition metal nitrile complex.

<span class="mw-page-title-main">Ferrocenium tetrafluoroborate</span> Chemical compound

Ferrocenium tetrafluoroborate is an organometallic compound with the formula [Fe(C5H5)2]BF4. This salt is composed of the cation [Fe(C5H5)2]+ and the tetrafluoroborate anion (BF−
4). The related hexafluorophosphate is also a popular reagent with similar properties. The ferrocenium cation is often abbreviated Fc+ or Cp2Fe+. The salt is deep blue in color and paramagnetic. Ferrocenium salts are sometimes used as one-electron oxidizing agents, and the reduced product, ferrocene, is inert and readily separated from ionic products. The ferrocene–ferrocenium couple is often used as a reference in electrochemistry. The standard potential of ferrocene-ferrocenium is dependent on specific electrochemical conditions.

<span class="mw-page-title-main">Lithium tetrakis(pentafluorophenyl)borate</span> Chemical compound

Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the weakly coordinating anion (B(C6F5)4)−. Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form in the catalyst composition for olefin polymerization reactions and in electrochemistry. It is a water-soluble compound. Its anion is closely related to the non-coordinating anion known as BARF. The tetrakis(pentafluorophenyl)borates have the advantage of operating on a one-to-one stoichiometric basis with Group IV transition metal polyolefin catalysts, unlike methylaluminoxane (MAO) which may be used in large excess.

<span class="mw-page-title-main">Ferrocenium hexafluorophosphate</span> Chemical compound

Ferrocenium hexafluorophosphate is an organometallic compound with the formula [Fe(C5H5)2]PF6. This salt is composed of the cation [Fe(C5H5)2]+ and the hexafluorophosphate anion (PF−
6). The related tetrafluoroborate is also a popular reagent with similar properties. The ferrocenium cation is often abbreviated Fc+ or Cp2Fe+. The salt is deep blue in color and paramagnetic.

<span class="mw-page-title-main">Rhodocene</span> Organometallic chemical compound

Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.

<span class="mw-page-title-main">Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate</span> Chemical compound

Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate is an anion with chemical formula [{3,5-(CF3)2C6H3}4B]−, which is commonly abbreviated as [BArF4]−, indicating the presence of fluorinated aryl (ArF) groups. It is sometimes referred to as Kobayashi's anion in honour of Hiroshi Kobayashi who led the team that first synthesised it. More commonly it is affectionately nicknamed "BARF." The BARF ion is also abbreviated BArF24−, to distinguish it from the closely related BArF−
20, [(C6F5)4B]−. However, for a small group of chemists, the anion is abbreviated as TFPB otherwise, short for Tetrakis[3,5-bis(triFluoromethyl)Phenyl]Borate.

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl dimer</span> Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

<span class="mw-page-title-main">Boranylium ions</span>

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands:

<span class="mw-page-title-main">Brookhart's acid</span> Chemical compound

Brookhart's acid is the salt of the diethyl ether oxonium ion and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr′4). It is a colorless solid, used as a strong acid. The compound was first reported by Volpe, Grant, and Brookhart in 1992.

In chemistry, the Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide is used as a probe molecule and systems are evaluated by 31P-NMR spectroscopy. In 1975, Viktor Gutmann used 31P-NMR spectroscopy to parameterize Lewis acidity of solvents by acceptor numbers (AN). In 1996, Michael A. Beckett recognised its more generally utility and adapted the procedure so that it could be easily applied to molecular species, when dissolved in weakly Lewis acidic solvents. The term Gutmann–Beckett method was first used in chemical literature in 2007.

<span class="mw-page-title-main">Transition metal nitrile complexes</span> Class of coordination compounds containing nitrile ligands (coordinating via N)

Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile.

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. 1 2 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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