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

Ball-and-stick model of the diphosphorus tetraiodide molecule
Names
IUPAC name Diphosphorus tetraiodide
Preferred IUPAC name Tetraiododiphosphane
Other names Phosphorus(II) iodide
Identifiers
CAS Number	13455-00-0 Y
3D model (JSmol)	Interactive image
ChemSpider	75322
ECHA InfoCard	100.033.301
EC Number	236-646-7
PubChem CID	83484
CompTox Dashboard (EPA)	DTXSID6065474
InChI InChI=1S/I4P2/c1-5(2)6(3)4 Key: YXXQTQYRRHHWFL-UHFFFAOYSA-N
SMILES P(P(I)I)(I)I
Properties
Chemical formula	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
Solubility in water	Decomposes
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501
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). Y verify (what is YN ?) 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]

Another synthesis route involves combining phosphonium iodide with iodine in a solution of carbon disulfide. An advantage of this route is that the resulting product is virtually free of impurities.^[3]

2PH₄I + 5I₂ → P₂I₄ + 8HI

The compound adopts a centrosymmetric structure with a P-P bond of 2.230 Å.^[4]

Reactions

Inorganic chemistry

Diphosphorus tetraiodide reacts with bromine to form mixtures PI_3−nBr_n. 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.^[3]

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, diphosphorus tetraiodide can convert glycols to trans alkenes.^[5][8] This reaction is known as the Kuhn–Winterstein reaction, after the chemists who applied it to the production of polyene chromophores.^[5][9]