Diphosphorus tetraiodide

Diphosphorus tetraiodide is an orange crystalline solid with the formula P2I4. It has been used as a reducing agent in organic chemistry. It is a rare example of a compound with phosphorus in the +2 oxidation state, and can be classified as a subhalide of phosphorus. It is the most stable of the diphosphorus tetrahalides.[1]

Diphosphorus tetraiodide
Names
IUPAC name
Diphosphorus tetraiodide
Preferred IUPAC name
Tetraiododiphosphane
Other names
Phosphorus(II) iodide
Identifiers
3D model (JSmol)
ECHA InfoCard 100.033.301
EC Number
  • 236-646-7
Properties
P2I4
Molar mass 569.57 g/mol
Appearance Orange crystalline solid
Melting point 125.5 °C (257.9 °F; 398.6 K)
Boiling point Decomposes
Decomposes
Hazards
C
R-phrases (outdated) R14, R34, R37
Flash point Non-flammable
Related compounds
Other anions
 Diphosphorus tetrafluoride
Diphosphorus tetrachloride
Diphosphorus tetrabromide
Other cations
 diarsenic tetraiodide
Related Binary Phosphorus halides
 phosphorus triiodide
Related compounds
 diphosphane
diphosphines
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Synthesis and structure

Diphosphorus tetraiodide is easily generated by the disproportionation of phosphorus triiodide in dry ether:

2 PI3 → P2I4 + I2

It can also be obtained by treating phosphorus trichloride and potassium iodide in anhydrous conditions.[2]

The compound adopts a centrosymmetric structure with a P-P bond of 2.230 Å.[3]

Reactions

Inorganic chemistry

Diphosphorus tetraiodide reacts with bromine to form mixtures PI3−xBrx. With sulfur, it is oxidized to P2S2I4, retaining the P-P bond.[1] It reacts with elemental phosphorus and water to make phosphonium iodide, which is collected via sublimation at 80 °C.[4]

Organic chemistry

Diphosphorus tetraiodide is used in organic synthesis mainly as a deoxygenating agent.[5] It is used for deprotecting acetals and ketals to aldehydes and ketones, and for converting epoxides into alkenes and aldoximes into nitriles. It can also cyclize 2-aminoalcohols to aziridines[6] and to convert α,β-unsaturated carboxylic acids to α,β-unsaturated bromides.[7]

As foreshadowed by the work of Bertholet in 1855,[5] diphosphorus tetraiodide is used in the Kuhn–Winterstein reaction, the conversion of glycols to alkenes.[8]

References
  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. H. Suzuki; T. Fuchita; A. Iwasa; T. Mishina (December 1978). "Diphosphorus Tetraiodide as a Reagent for Converting Epoxides into Olefins, and Aldoximes into Nitriles under Mild Conditions". Synthesis. 1978 (12): 905–908. doi:10.1055/s-1978-24936.
  3. Z. Žák, M. Černík "Diphosphorus tetraiodide at 120 K" Acta Crystallographica Section C 1996, vol. C52, pp. 290-1. doi:10.1107/S0108270195012510
  4. Brown, Glenn Halstead (1951). Reactions of phosphine and phosphonium iodide (PhD). Iowa State College. Retrieved 5 Oct 2020.
  5. Alain Krief, Vikas N. Telvekar "Diphosphorus Tetraiodide" Encyclopedia for Reagents in Organic Synthesis 2009. doi:10.1002/047084289X.rd448.pub2
  6. H. Suzuki; H. Tani (1984). "A mild cyclization of 2-aminoalcohols to aziridines using diphosphorus tetraiodide". Chemistry Letters. 13 (12): 2129–2130. doi:10.1246/cl.1984.2129.
  7. Vikas N. Telvekar; Somsundaram N. Chettiar (June 2007). "A novel system for decarboxylative bromination". Tetrahedron Letters. 48 (26): 4529–4532. doi:10.1016/j.tetlet.2007.04.137.
  8. Richard Kuhn, Alfred Winterstein (1928). "Über konjugierte Doppelbindungen I. Synthese von Diphenyl-poly-enen". Helvetica Chimica Acta. 11 (1): 87–116. doi:10.1002/hlca.19280110107.
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