Phosphonium iodide

Phosphonium iodide is a chemical compound with the formula PH
4I. It is an example of a salt containing an unsubstituted phosphonium cation (PH+
4). Phosphonium iodide is commonly used as storage for phosphine[2] and as a reagent for substituting phosphorus into organic molecules.[3]

Phosphonium iodide
Ball-and-stick model of the phosphonium cation
Model of the iodide anion
Names
IUPAC name
Phosphanium iodide
Other names
Iodine phosphide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.978
EC Number
  • 235-189-0
UNII
Properties
PH
4I
Molar mass 161.910 g/mol
Boiling point 62 °C (144 °F; 335 K) Sublimes[1]
decomposes
Structure
Tetragonal (tI)
a = 6.34 Å, c = 4.62 Å
185.7 Å3
2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Preparation

Phosphonium iodide is prepared by mixing diphosphorus tetraiodide (P
2I
4) with elemental phosphorus and water at 80 °C and allowing the salt to sublime.[4]

Properties

Structure

Its crystal structure has a tetragonal tI space group, which is a distorted version of the NH4Cl crystal structure; the unit cell has approximate dimensions 634×634×462 pm.[5] The hydrogen bonding in the system causes the PH+
4 cations to orient such that the hydrogen atoms point toward the I−
 anions.[6]

Chemical

At 62 °C and atmospheric pressure, phosphonium iodide sublimates and dissociates reversibly into phosphine and hydrogen iodide (HI).[1] It oxidizes slowly in air to give iodine and phosphorus oxides; it is hygroscopic[4] and is hydrolyzed into phosphine and HI:[7]

PH
4I ⇌ PH
3 + HI

Phosphine gas may be devolved from phosphonium iodide by mixing an aqueous solution with potassium hydroxide:[8]

PH
4I + KOH → PH
3 + KI + H
2O

It reacts with elemental iodine and bromine in a nonpolar solution to give phosphorus halides; for example:

2PH
4I + 5I
2 → P
2I
4 + 8HI[4]

Phosphonium iodide is a powerful substitution reagent in organic chemistry; for example, it can convert a pyrilium into a phosphinine via substitution.[3] In 1951, Glenn Halstead Brown found that PH4I reacts with acetyl chloride to produce an unknown phosphine derivative, possibly CH
3C(=PH)PH
2·HI.[4]

References
  1. Smith, Alexander.; Calvert, Robert Peyton. (July 1914). "The Dissociation Pressures of Ammonium- and Tetramethylammonium Halides and of Phosphonium Iodide and Phosphorus Pentachloride". Journal of the American Chemical Society. 36 (7): 1363–1382. doi:10.1021/ja02184a003. Retrieved 6 October 2020.
  2. Morrow, B. A.; McFarlane, Richard A. (July 1986). "Trimethylgallium adsorbed on silica and its reaction with phosphine, arsine, and hydrogen chloride: an infrared and Raman study". The Journal of Physical Chemistry. 90 (14): 3192–3197. doi:10.1021/j100405a029. ISSN 0022-3654.
  3. Mei, Yanbo (2020). Complexes, Heterocycles, and Depolymerizable Polymers. Made from Building Blocks with Low-coordinated Phosphorus (Thesis). ETH Zurich. p. 18. doi:10.3929/ethz-b-000431853. hdl:20.500.11850/431853. Retrieved 6 October 2020.
  4. Brown, Glenn Halstead (1951). Reactions of phosphine and phosphonium iodide (PhD). Iowa State College. Retrieved 5 Oct 2020.
  5. Dickinson, Roscoe G. (July 1922). "The Crystal Structure of Phosphonium Iodide". Journal of the American Chemical Society. 44 (7): 1489–1497. doi:10.1021/ja01428a015.
  6. Sequeira, A.; Hamilton, Walter C. (September 1967). "Hydrogen Bonding in Phosphonium Iodide: A Neutron‐Diffraction Study". The Journal of Chemical Physics. 47 (5): 1818–1822. Bibcode:1967JChPh..47.1818S. doi:10.1063/1.1712171.
  7. Levchuk, Ievgen (2017). Design and optimization of luminescent semiconductor nanocrystals for optoelectronic applications (PDF) (faculty). University of Erlangen–Nuremberg. p. 140. Retrieved 6 Oct 2020.
  8. Osadchenko, Ivan M; Tomilov, Andrei P (30 June 1969). "Phosphorus Hydrides". Russian Chemical Reviews. 38 (6): 495–504. Bibcode:1969RuCRv..38..495O. doi:10.1070/RC1969v038n06ABEH001756.
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