Pentacyanocyclopentadiene
{{Chembox
| ImageFile = Pentacyanocyclopentadien.png
| ImageSize = 150px
| ImageAlt = Skeletal formula of pentacyanocyclopentadiene
| ImageFile1 = Pentacyanocyclopentadiene-3D-spacefill.png
| ImageSize1 = 160
| ImageAlt1 = Space-filling model of the pentacyanocyclopentadiene molecule
| PIN = Cyclopenta-1,3-diene-1,2,3,4,5-pentacarbonitrile
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|Section1={{Chembox Identifiers
| CASNo = 69239-40-3
| PubChem = 22597206
| ChemSpiderID = 11484097
| SMILES = C(#N)C1C(=C(C(=C1C#N)C#N)C#N)C#N
| StdInChI = 1S/C10HN5/c11-1-6-7(2-12)9(4-14)10(5-15)8(6)3-13/h6H
| StdInChIKey = SZRONZXSOSCLOK-UHFFFAOYSA-N
}}
|Section2={{Chembox Properties
| C=10 | H=1 | N=5
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|Section3={{Chembox Hazards
| MainHazards =
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Pentacyanocyclopentadiene is a derivative of cyclopentadiene with five cyano groups with the molecular formula C5H(CN)5. The corresponding anion, pentacyanocyclopentadienide, is a ligand with the molecular formula {{chem|C|5|(CN)|5|−}}. In contrast to other anions based on a C5 ring unit it binds to metals through the pendant cyano groups rather than the C5 ring. The anion was first synthesised by Webster in the 1960s{{cite journal | last1 = Webster | first1 = O. W. | year = 1966 | title = Diazotetracyanocyclopentadiene | journal = Journal of the American Chemical Society | volume = 88 | issue = 17| pages = 4055–4060 | doi = 10.1021/ja00969a029 }} and its conjugate acid much later on.{{cite journal|last1=Reed|first1=C.|journal=Chem. Commun.|date=2004|page=706|doi= 10.1039/b316122f|title=Exploration of the pentacyano-cyclo-pentadienide ion, {{chem|C|5|(CN)|5|−}}, as a weakly coordinating anion and potential superacid conjugate base |url=https://escholarship.org/content/qt9gr3s0g8/qt9gr3s0g8.pdf?t=o9pcf8}} More recently Wright has discovered its extensive coordination chemistry.{{cite journal|last1=Wright|first1=D. S.|journal=Chem. Commun.|date=2011|volume=47|issue=36|page=10007|doi = 10.1039/c1cc13021h|pmid=21833428|title=Transition metal complexes of the pentacyanocyclopentadienide anion}}{{cite journal|last1=Wright|first1=D. S.|journal=Dalton Trans.|date=2012|volume=41|issue=19|page=5919|doi=10.1039/c2dt30274h|pmid=22473357|title=Group 11 complexes containing the [C5(CN)5]− ligand; 'coordination-analogues' of molecular organometallic systems}} By virtue of a combination of mesomeric and aromatic stabilization of its anion, pentacyanocyclopentadiene is a superacid, with an estimated aqueous pKa of −11.{{Cite book|title=Organic Chemistry|last=Jonathan|first=Clayden|date=2012|publisher=Oxford University Press|last2=Greeves|first2=Nick|last3=Warren|first3=Stuart G.|isbn=9780199270293|edition= 2nd|location=Oxford|oclc=761379371}} The free acid was prepared by Reed in 2004 and was assigned a polymeric structure with protons that bridge planar C5(CN)5 units.{{Cite journal|last=Richardson|first=Christopher|last2=Reed|first2=Christopher A.|date=2004|title=Exploration of the pentacyano-cyclo-pentadienide ion, {{chem|C|5|(CN)|5|−}}, as a weakly coordinating anion and potential superacid conjugate base. Silylation and protonation|url=http://xlink.rsc.org/?DOI=B316122F|journal=Chem. Commun.|language=en|issue=6|pages=706–707|doi=10.1039/B316122F|issn=1359-7345}}
Synthesis
Pentacyanocyclopentadiene is synthesised by coupling carbon disulfide and sodium cyanide in dimethylformamide before oxidation using ammonium persulfate and final purification generates the ammonium pentacyanocyclopentadiene salt. Further reaction with sodium hydride generates NaC5(CN)5 which is a starting point for its coordination chemistry with transition metals.
Reactions and coordination chemistry
Coupling of sodium pentacyanocyclopentadiene (NaC5(CN)5) with transition metal halide salts generates metal complexes containing the {{chem|C|5|(CN)|5|−}} anion.
Because the anion binds to metals through the cyanide group it can act as a pentagonal node, unlike most cyclopentadienyl complexes that bind to the face of the cyclopentadienyl ring. Thus it can form fullerene-like structures with large voids containing solvent.{{cite journal|last1=Bacsa|first1=J.|journal=Angew. Chem. Int. Ed.|date=2011|volume=50|issue=36|pages=8279–8282|doi=10.1002/anie.201102783|title=Assembly of the First Fullerene-Type Metal-Organic Frameworks Using a Planar Five-Fold Coordination Node}}{{cite journal|last1=Less|first1=R. J.|journal=Eur. J. Inorg. Chem.|date=2013|volume=2013|issue=7|pages=1161–1169|doi=10.1002/ejic.201201342|title=Solvent Direction of Molecular Architectures in Group 1 Metal Pentacyanocyclopentadienides}} This has important implications for gas storage and separation.