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]^−	[Me2^iPrPhN]^+	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}·{(I^2− 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

References

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