Polyiodide

The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms.[1] The most common and simplest 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⁻.

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.[2]

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 are characterized by their highly complex and variable structures, and can be considered as associations of I₂, I⁻, and I⁻
₃ units. Discrete polyiodides are usually linear, reflecting the origin of the ion. 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. The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.[3]

Structure of higher polyiodides
Anion	Counter-cation	Structural description
[I₃]⁻	Cs⁺	linear
[I₄]²⁻	[Cu(NH₃)₄]²⁺	symmetric linear array of iodine atoms[4]
[I₅]⁻	[EtMe₃N]⁺	V-shaped with polymeric layers
[I₅]⁻	[EtMePh₂N]⁺	V-shaped with isolated [I₅]⁻ ions
[I₆]²⁻	[NH₃(CH₂)₈NH₃]²⁺	almost linear [[5]]
[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
[I₉]⁻	[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[6]
[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
[I₁₂]²⁻	[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)[7]
[I₁₆]²⁻	[Me₂Ph₂N]⁺	centrosymmetric arrangement of [I⁻ ₇·I₂·I⁻ ₇]
[I₁₆]²⁻	[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[8]
[I_∞]^δ−	Pyrroloperylene^+•	Infinite polyiodide homopolymer.[9]

Structures of some polyiodide ions.

References

Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 17: The group 17 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 547. ISBN 978-0-13-175553-6.
Brauer, G., ed. (1963). "Potassium triiodide". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 294.
King, R. Bruce (2005). "Chlorine, Bromine, Iodine, & Astatine: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry (2nd ed.). Wiley. p. 747. ISBN 9780470862100.
Svensson, Per H.; Kloo, Lars (2003). "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide–Iodine Systems". Chem. Rev. 103 (5): 1649–84. doi:10.1021/cr0204101. PMID 12744691.
Reiss, Guido J.; Van Megen, Martin (2013). "I₆²⁻ Anion Composed of Two Asymmetric Triiodide Moieties: A Competition between Halogen and Hydrogen Bond". Inorganics. 1 (1): 3–13. doi:10.3390/inorganics1010003.
Reiss, Guido J. (2019-06-26). "A cyclic I₁₀²⁻ anion in the layered crystal structure of theophyllinium pentaiodide, C₇H₉I₅N₄O₂". Zeitschrift für Kristallographie – New Crystal Structures. 234 (4): 737–739. doi:10.1515/ncrs-2019-0082. ISSN 2197-4578.
Reiss, Guido J.; Megen, Martin van (2012). "Two New Polyiodides in the 4,4′-Bipyridinium Diiodide/Iodine System". Zeitschrift für Naturforschung B. 67 (1): 5–10. doi:10.1515/znb-2012-0102. ISSN 1865-7117. S2CID 5857644.
Tebbe, Karl-Friedrich; Buchem, Rita (1997-06-16). "Das bisher iodreichste Polyiodid: Herstellung und Struktur von Fc₃I₂₉". Angewandte Chemie (in German). 109 (12): 1403–1405. doi:10.1002/ange.19971091233.
Madhu, Sheri; Evans, Hayden A.; Doan-Nguyen, Vicky V. T.; Labram, John G.; Wu, Guang; Chabinyc, Michael L.; Seshadri, Ram; Wudl, Fred (4 July 2016). "Infinite Polyiodide Chains in the Pyrroloperylene–Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes". Angewandte Chemie International Edition. 55 (28): 8032–8035. doi:10.1002/anie.201601585. PMID 27239781. S2CID 30407996.

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[InorgChem-1] Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 17: The group 17 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 547. ISBN 978-0-13-175553-6.

[2] Brauer, G., ed. (1963). "Potassium triiodide". Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 294.

[Encyclo-3] King, R. Bruce (2005). "Chlorine, Bromine, Iodine, & Astatine: Inorganic Chemistry". Encyclopedia of Inorganic Chemistry (2nd ed.). Wiley. p. 747. ISBN 9780470862100.

[4] Svensson, Per H.; Kloo, Lars (2003). "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide–Iodine Systems". Chem. Rev. 103 (5): 1649–84. doi:10.1021/cr0204101. PMID 12744691.

[5] Reiss, Guido J.; Van Megen, Martin (2013). "I₆²⁻ Anion Composed of Two Asymmetric Triiodide Moieties: A Competition between Halogen and Hydrogen Bond". Inorganics. 1 (1): 3–13. doi:10.3390/inorganics1010003.

[6] Reiss, Guido J. (2019-06-26). "A cyclic I₁₀²⁻ anion in the layered crystal structure of theophyllinium pentaiodide, C₇H₉I₅N₄O₂". Zeitschrift für Kristallographie – New Crystal Structures. 234 (4): 737–739. doi:10.1515/ncrs-2019-0082. ISSN 2197-4578.

[7] Reiss, Guido J.; Megen, Martin van (2012). "Two New Polyiodides in the 4,4′-Bipyridinium Diiodide/Iodine System". Zeitschrift für Naturforschung B. 67 (1): 5–10. doi:10.1515/znb-2012-0102. ISSN 1865-7117. S2CID 5857644.

[8] Tebbe, Karl-Friedrich; Buchem, Rita (1997-06-16). "Das bisher iodreichste Polyiodid: Herstellung und Struktur von Fc₃I₂₉". Angewandte Chemie (in German). 109 (12): 1403–1405. doi:10.1002/ange.19971091233.

[9] Madhu, Sheri; Evans, Hayden A.; Doan-Nguyen, Vicky V. T.; Labram, John G.; Wu, Guang; Chabinyc, Michael L.; Seshadri, Ram; Wudl, Fred (4 July 2016). "Infinite Polyiodide Chains in the Pyrroloperylene–Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes". Angewandte Chemie International Edition. 55 (28): 8032–8035. doi:10.1002/anie.201601585. PMID 27239781. S2CID 30407996.