BARF Acronym Chemistry — What Does BARF Stand For?

BARF (or more precisely BArF) stands for:

B = Borate (boron centre with four substituents) Ar = Aryl (aromatic ring) F = Fluorinated (trifluoromethyl groups on the aryl rings)

Full IUPAC name: Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate

Chemical formula: [B(C₆H₃(CF₃)₂)₄]⁻ Alternative notation: [B(ArF)₄]⁻ where ArF = 3,5-(CF₃)₂C₆H₃

Common salt forms: • NaBArF₄ — Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate • [H(OEt₂)₂][BArF₄] — Brookhart's acid (protonated diethyl ether with BArF counterion)

The BArF anion is classified as a weakly coordinating anion (WCA) — it interacts very weakly with cations, making it ideal for stabilising highly reactive cationic species in catalysis.

The BArF anion has a tetrahedral boron centre bonded to four identical fluorinated aryl groups:

Core structure: • Central atom: Boron (B) with a formal negative charge • Four substituents: 3,5-bis(trifluoromethyl)phenyl groups • Each phenyl ring has two –CF₃ groups at the 3 and 5 positions (meta positions) • Total of 8 CF₃ groups (2 per ring × 4 rings) • Total fluorine atoms: 24 (8 CF₃ × 3 F each)

Molecular formula: C₃₂H₁₂BF₂₄⁻ Molecular weight: ~863 g/mol

Key structural features:

1. Tetrahedral geometry around boron (sp³ hybridised)
2. Negative charge delocalised across the entire anion
3. Bulky — the large size prevents close approach to cations
4. Fluorine atoms withdraw electron density, making the anion very stable
5. No lone pairs available for coordination — non-nucleophilic

The combination of large size, charge delocalisation, and fluorination is what makes BArF an exceptionally weak coordinator.

BArF is one of the most important weakly coordinating anions (WCAs) in modern chemistry.

What is a weakly coordinating anion? A WCA is an anion that has very little interaction with the cation it is paired with. Unlike strongly coordinating anions (Cl⁻, Br⁻, CH₃COO⁻) that bind tightly to metal centres, WCAs leave the cation essentially 'free'.

Why weak coordination matters: • In catalysis, the active species is often a cation (e.g., [Pd(PPh₃)₂]²⁺) • If the counterion coordinates strongly, it blocks the active site • A WCA like BArF stays out of the way, leaving the metal centre open for substrates • This dramatically increases catalytic activity and selectivity

Ranking of anion coordinating ability (weak → strong): [B(ArF)₄]⁻ < [SbF₆]⁻ < [PF₆]⁻ < [BF₄]⁻ < [OTf]⁻ < [ClO₄]⁻ < [NO₃]⁻ < [Cl]⁻

BArF is among the weakest coordinating anions known — often THE weakest in practical use.

Advantages of BArF over other WCAs: • More stable than [SbF₆]⁻ (which can release toxic HF) • Less reactive than [PF₆]⁻ or [BF₄]⁻ (which can decompose) • Chemically inert under most reaction conditions • Soluble in organic solvents (unlike many inorganic anions)

BArF is used extensively in several areas of chemistry:

1. Homogeneous Catalysis: • Olefin polymerisation: BArF counterions activate metallocene catalysts for polyethylene and polypropylene production • C–H activation: Stabilises cationic Ir and Rh complexes used in C–H bond functionalisation • Hydrogenation: Ir-based asymmetric hydrogenation catalysts with BArF show higher activity than with PF₆⁻ or BF₄⁻ • Gold catalysis: Au(I) cationic catalysts with BArF are highly active for alkyne activation

2. Stabilising Reactive Cations: • Silylium ions (R₃Si⁺): Too reactive for most counterions, but stable with BArF • Carbocations: BArF stabilises reactive carbocations without quenching them • Low-valent metal centres: Highly electrophilic metal complexes need non-interfering counterions

3. Ionic Liquids: • BArF salts have low melting points and high thermal stability • Used as specialised ionic liquid components

4. Supramolecular Chemistry: • BArF is used as a counterion in self-assembled cages and molecular machines • Its size and inertness prevent interference with host-guest interactions

5. Electrochemistry: • Used as a supporting electrolyte in non-aqueous electrochemistry • Wide electrochemical window — resistant to oxidation and reduction

Comparison of BArF with other commonly used counterions in chemistry:

Anion | Formula | Coordinating Strength | Stability | Solubility in Organics BArF⁻ | [B(ArF)₄]⁻ | Very weak | Very high | Excellent SbF₆⁻ | [SbF₆]⁻ | Weak | Moderate (can release HF) | Good PF₆⁻ | [PF₆]⁻ | Moderate | Moderate (hydrolyses) | Good BF₄⁻ | [BF₄]⁻ | Moderate | Moderate (hydrolyses) | Good OTf⁻ | [CF₃SO₃]⁻ | Moderate–Strong | High | Good Cl⁻ | Cl⁻ | Strong | High | Low Br⁻ | Br⁻ | Strong | High | Low

Why chemists choose BArF: • Highest stability among common WCAs • No decomposition to release HF (unlike PF₆⁻ and BF₄⁻) • Excellent solubility in organic solvents like CH₂Cl₂, THF, toluene • Weakest coordination — gives maximum catalytic activity

Disadvantages of BArF: • Expensive (NaBArF₄ costs significantly more than NaPF₆ or NaBF₄) • High molecular weight (~863 g/mol) means large mass contribution • Synthesis requires fluorinated starting materials

The most common BArF salt, sodium BArF₄ (NaBArF₄), is prepared by a Grignard-type reaction:

Synthesis route:

Step 1: Prepare the Grignard reagent 3,5-(CF₃)₂C₆H₃Br + Mg → 3,5-(CF₃)₂C₆H₃MgBr (3,5-bis(trifluoromethyl)bromobenzene reacts with magnesium in THF)

Step 2: React with sodium tetrafluoroborate 4 ArFMgBr + NaBF₄ → NaBArF₄ + 4 MgBrF (4 equivalents of ArF Grignard react with NaBF₄)

Alternative: React ArFLi with BF₃·OEt₂ followed by NaCl metathesis.

Overall: 4 [3,5-(CF₃)₂C₆H₃]MgBr + NaBF₄ → Na[B(3,5-(CF₃)₂C₆H₃)₄] + 4 MgBrF

Purification: Recrystallisation from CH₂Cl₂/hexane or water/acetone mixtures.

Yield: Typically 60–80%

NaBArF₄ is a white crystalline solid that is air-stable and easy to handle.

Brookhart's acid [H(OEt₂)₂][BArF₄] is prepared by reacting NaBArF₄ with HCl in diethyl ether.