Polyiodide

The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms.^[1][2] The most common member is the triiodide ion, I⁻
₃. Other known larger polyiodides include [I₄]²⁻, [I₅]⁻, [I₆]²⁻, [I₇]⁻, [I₈]²⁻, [I₉]⁻, [I₁₀]²⁻, [I₁₀]⁴⁻, [I₁₁]³⁻, [I₁₂]²⁻, [I₁₃]³⁻, [I₁₄]^4-, [I₁₆]²⁻, [I₂₂]⁴⁻, [I₂₆]³⁻, [I₂₆]⁴⁻, [I₂₈]⁴⁻ and [I₂₉]³⁻. All these can be considered as formed from the interaction of the I^–, I₂, and I⁻
₃ building blocks.

Preparation

The polyiodides can be made by addition of stoichiometric amounts of I₂ to solutions containing I⁻ and I⁻
₃, with the presence of large countercations to stabilize them. For example, KI₃·H₂O can be crystallized from a saturated solution of KI when a stoichiometric amount of I₂ is added and cooled.^[3]

Structure

The 14-membered ring array of iodine atoms in [([16]aneS₄)PdIPd([16]aneS₄)][I₁₁

]

The primitive cubic lattice of iodide ions bridge by I₂ molecules, present in [Cp*₂Fe]₄[I₂₆

]

Polyiodides adopt diverse structures. Most can be considered as associations of I₂, I⁻, and I⁻
₃ units. Discrete polyiodides are usually linear. The more complex two- or three-dimensional network structures of chains and cages are formed as the ions interact with each other, with their shapes depending on their associated cations quite strongly, a phenomenon named dimensional caging.^[4][5] The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.^[6]

Structure of higher polyiodides

Anion	Counter-cation	Structural description
[I2]−	Na(C3H6O)+ 3	linear[7][8]
[I3]−	Cs+, (C4H9)4N+	linear
[I4]2−	[Cu(NH3)4]2+	symmetric linear array of iodine atoms[9]
[I5]−	[EtMe3N]+	V-shaped with polymeric layers
	[EtMePh2N]+	V-shaped with isolated [I5]− ions
[I6]2−	[NH3(CH2)8NH3]2+	almost linear [[10]]
[I7]−	[Ag(18aneS6)]+	an anionic network derived from a primitive rhombohedral lattice of iodide ions bridged by I2 molecules
[I8]2−	[Ni(phen)3]2+	regular anionic shapes, can be described as [I− 3·I2·I− 8] or [I− 3·I− 5]
[I9]−	[Me2iPrPhN]+	14-membered ring tied by two I2 bridges to give 10-membered rings
	[Me4N]+	non-octahedral, but a twisted "h"-like arrangement of I− 3 and I2 units
[I10]2−	[Cd(12-crown-4)2]2+; Theophyllinium	twisted ring configuration with two I− 3 units linked by two I2 molecules[11]
[I11]3−	[(16aneS4)PdIPd(16aneS4)]3+	14-membered ring (9.66 × 12.64 Å) around the complex cation, with the rings interlink further to give an infinite 2D sheet
[I12]2−	[Ag2(15aneS5)2]2+	extended 3D spiral superstructure supported by Ag–I bonds and weak I···S interactions
	[Cu(Dafone)3]2+	planar configuration
[I13]3−	[Me2Ph2N]+	consists of zigzag chains of I− and I2
[I14]4−	4,4′-bipyridinium	double hook (I− 3·I2·I−·I2·I−·I2·I− 3)[12]
[I16]2−	[Me2Ph2N]+	centrosymmetric arrangement of [I− 7·I2·I− 7]
	[iPrMe2PhN]+	the anion forms 14-membered rings catenated by I2 molecules, which further link into layers with 10- and 14-membered rings
[I22]4−	[MePh3P]+	two L-shaped [I5]− units linked by an I2 molecule and completed by two end-on [I5]− groups
[I26]3−	[Me3S]+	consists of [I5]− and [I7]− ions with intercalated I2 molecules
[I26]4−	Cp*2Fe+	an anionic network derived from a primitive cubic lattice built from I− ions, with I2 bridges on all edges and systematically removing 1⁄12 of the I2 molecules
[I29]3−	Cp2Fe+	an anionic 3D network with a cage-like structure of [{(I− 5)1⁄2·I2}·{(I2− 12)1⁄2·I2}·I2], with [Cp2Fe]+ ions interacting with the anion in the cavities[13]
[I∞]δ−	Pyrroloperylene+•	Infinite polyiodide homopolymer.[14]

Structures of some polyiodide ions.

Reactivity

Polyiodide compounds are generally sensitive to light.

Triiodide, I⁻
₃, undergoes unimolecular photodissociation.^[15][16] Polyiodide has been used to improve the scalability in the synthesis of halide perovskite photovoltaic materials.^[17]

Conductivity

Solid state compounds containing linear-chain polyiodide ions exhibit enhanced conductivity^[18][19] than their simple iodide counterparts. The conductivity can be drastically modified by external pressure, which changes the interatomic distances between iodine moieties and the charge distribution.^[20]

See also

• Triiodide
• Polyhalogen ions
• Iodine–starch test
• Dye-sensitized solar cell
• Halogen bond
• Catenation
• Inorganic polymer