Phosphonium

Types of phosphonium cations

Summarize

Perspective

Protonated phosphines

The parent phosphonium is PH⁺
₄ as found in the iodide salt, phosphonium iodide. Salts of the parent PH⁺
₄ are rarely encountered, but this ion is an intermediate in the preparation of the industrially useful tetrakis(hydroxymethyl)phosphonium chloride:

PH₃ + HCl + 4 CH₂O → P(CH
₂OH)⁺
₄Cl⁻

Many organophosphonium salts are produced by protonation of primary, secondary, and tertiary phosphines:

PR₃ + H⁺ → HPR⁺
₃

The basicity of phosphines follows the usual trends, with R = alkyl being more basic than R = aryl.^[2]

Tetraorganophosphonium cations

The most common phosphonium compounds have four organic substituents attached to phosphorus. The quaternary phosphonium cations include tetraphenylphosphonium, (C₆H₅)₄P⁺ and tetramethylphosphonium P(CH
₃)⁺
₄.

Thumb — Tetramethylphosphonium bromide^[3]

Quaternary phosphonium cations (PR⁺
₄) are produced by alkylation of organophosphines.^[3] For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide:

PPh₃ + CH₃Br → [CH₃PPh₃]⁺Br⁻

The methyl group in such phosphonium salts is mildly acidic, with a pK_a estimated to be near 15:^[5]

[CH₃PPh₃]⁺ + base → CH₂=PPh₃ + [Hbase]⁺

This deprotonation reaction gives Wittig reagents.^[6]

Phosphorus pentachloride and related compounds

Solid phosphorus pentachloride is an ionic compound, formulated [PCl₄]⁺[PCl₆]⁻ (tetrachlorophosphonium hexachlorophosphate(V)), that is, a salt containing the tetrachlorophosphonium cation.^[7]^[8] Dilute solutions dissociate according to the following equilibrium:

PCl₅ ⇌ PCl⁺
₄ + Cl⁻

Triphenylphosphine dichloride (Ph₃PCl₂) exists both as the pentacoordinate phosphorane and as the chlorotriphenylphosphonium chloride, depending on the medium.^[9] The situation is similar to that of PCl₅. It is an ionic compound (PPh₃Cl)⁺Cl⁻ in polar solutions and a molecular species with trigonal bipyramidal molecular geometry in apolar solution.^[10]

Alkoxyphosphonium salts: Arbuzov reaction

The Michaelis–Arbuzov reaction is the chemical reaction of a trivalent phosphorus ester with an alkyl halide to form a pentavalent phosphorus species and another alkyl halide. Commonly, the phosphorus substrate is a phosphite ester (P(OR)₃) and the alkylating agent is an alkyl iodide.^[11]

Uses

Summarize

Perspective

Textile finishes

Tetrakis(hydroxymethyl)phosphonium chloride has industrial importance in the production of crease-resistant and flame-retardant finishes on cotton textiles and other cellulosic fabrics.^[12]^[13] A flame-retardant finish can be prepared from THPC by the Proban Process,^[14] in which THPC is treated with urea. The urea condenses with the hydroxymethyl groups on THPC. The phosphonium structure is converted to phosphine oxide as the result of this reaction.^[15]

Phase-transfer catalysts and precipitating agents

Organic phosphonium cations are lipophilic and can be useful in phase transfer catalysis, much like quaternary ammonium salts. Salts or inorganic anions and tetraphenylphosphonium (PPh⁺
₄) are soluble in polar organic solvents. One example is the perrhenate (PPh₄[ReO₄]).^[16]

Reagents for organic synthesis

Wittig reagents are used in organic synthesis. They are derived from phosphonium salts. A strong base such as butyllithium or sodium amide is required for the deprotonation:

[Ph₃P⁺CH₂R]X⁻ + C₄H₉Li → Ph₃P=CHR + LiX + C₄H₁₀

One of the simplest ylides is methylenetriphenylphosphorane (Ph₃P=CH₂).^[6]

The compounds Ph₃PX₂ (X = Cl, Br) are used in the Kirsanov reaction.^[17] The Kinnear–Perren reaction is used to prepare alkylphosphonyl dichlorides (RP(O)Cl₂) and esters (RP(O)(OR′)₂). A key intermediate are alkyltrichlorophosphonium salts, obtained by the alkylation of phosphorus trichloride:^[18]

RCl + PCl₃ + AlCl₃ → [RPCl₃]⁺AlCl⁻
₄

Ammonia production for "green hydrogen"

The main industrial procedure for the production of ammonia today is the thermal Haber-Bosch process, which generally uses fossil gas as a source of hydrogen, which is then combined with nitrogen to produce ammonia. In 2021, Professor Doug MacFarlane and collaborators Alexandr Simonov and Bryan Suryanto of Monash University devised a method of producing green ammonia that has the potential to make Haber-Bosch plants obsolete.^[19] Their process is similar to the electrolysis approach for producing hydrogen. While working with local company Verdant, which wanted to make bleach from saltwater by electrolysis, Suryanto discovered that a tetraalkyl phosphonium salt allowed the efficient production of ammonia at room temperature.^[20]