View source for Persistent carbene

{{Short description|Type of carbene demonstrating particular stability}}
[[File:SIMes.png|thumb|300px|[[SIMes|1,3-Dimesityl-imidazol-4,5-dihydro-2-ylidene]], a representative persistent carbene]]
A '''persistent carbene''' (also known as '''stable carbene''') is an [[organic molecule]] whose natural [[resonance structure]] has a carbon atom with [[octet rule|incomplete octet]] (a [[carbene]]), but does not exhibit the tremendous instability typically associated with such moieties.  The best-known examples and by far largest subgroup are the '''''N''-heterocyclic carbenes''' (NHC)<ref name="Hopkinson-2014">{{cite journal |author1=Hopkinson, M. N. |author2=Richter, C. |author3=Schedler, M. |author4=Glorius, F. |year=2014 |title=An Overview of ''N''-Heterocyclic Carbenes |journal=[[Nature (journal)|Nature]] |volume=510 |issue=7506 |pages=485–496 |bibcode=2014Natur.510..485H |doi=10.1038/nature13384 |pmid=24965649 |s2cid=672379}}</ref> (sometimes called '''Arduengo carbenes'''), in which nitrogen atoms flank the formal carbene.

Modern theoretical analysis suggests that the term "persistent carbene" is in fact a [[misnomer]].  Persistent carbenes do not in fact have a carbene electronic structure in their [[ground state]], but instead an [[ylide]] stabilized by [[Aromaticity|aromatic resonance]] or [[steric shielding]].  Acid catalyzes the carbene-like dimerization that some persistent carbenes undergo over the course of days.

Persistent carbenes in general, and Arduengo carbenes in particular, are popular [[ligand]]s in [[organometallic chemistry]].

==History==

===Early evidence===
In 1957, [[Ronald Breslow]] proposed that a relatively stable [[nucleophilic]] carbene, a [[thiazol-2-ylidene]] derivative of [[vitamin B1|vitamin B<sub>1</sub>]] (thiamine), was the catalyst involved in the [[benzoin condensation]] that yields [[furoin]] from [[furfural]].<ref name=Breslow2>{{cite journal |title = Mechanism of Thiamine Action: Participation of a Thiazolium Zwitterion |author = [[Ronald Breslow]] |journal=[[Chem. Ind.]] |volume = 26 |pages = 893 |year = 1957}}</ref><ref>{{cite journal | author = Ronald Breslow| year = 1958 | title = On the Mechanism of Thiamine Action. IV.1 Evidence from Studies on Model Systems | journal =[[J. Am. Chem. Soc.]] | volume = 80 | issue = 14| pages = 3719–3726 | doi = 10.1021/ja01547a064}}</ref> In this cycle, the vitamin's [[thiazolium]] ring exchanges a hydrogen atom (attached to carbon 2 of the ring) for a furfural residue. In [[heavy water|deuterated water]], the C2-[[proton]] was found to rapidly exchange for a [[deuteron]] in a statistical [[chemical equilibrium|equilibrium]]:<ref name=Breslow3>{{cite journal |title = Rapid Deuterium Exchange in Thiazolium Salts |author = R. Breslow
 |journal =[[J. Am. Chem. Soc.]] |volume = 79 |issue = 7|pages = 1762–1763 |year = 1957 |doi = 10.1021/ja01564a064}}</ref>
[[File:Breslow-carbene.png|center|600px|Deuterium exchange of the C2-proton of thiazolium salt.|frameless]]

This exchange was proposed to proceed via intermediacy of a thiazol-2-ylidene. In 2012 the isolation of the so-called ''Breslow intermediate'' was reported.<ref>{{cite journal |author1=Berkessel A. |author2=Elfert S. |author3=Yatham V. R. |author4=Neudörfl J.-M. |author5=Schlörer N. E. |author6=Teles J. H. | year = 2012 | title = Umpolung by ''N''-Heterocyclic Carbenes: Generation and Reactivity of the Elusive 2,2-Diamino Enols (Breslow Intermediates)| journal =[[Angew. Chem. Int. Ed.]] | volume = 51| issue = 49| pages = 12370–12374| doi = 10.1002/anie.201205878 |pmid=23081675|doi-access=free }}</ref><ref>[http://cen.acs.org/articles/90/i44/Chemists-Approach-Elusive-Breslow-Intermediate.html Chemists Approach Elusive Breslow Intermediate Carmen Drahl]</ref>

In 1960, [[Hans-Werner Wanzlick]] and coworkers conjectured that carbenes derived from [[dihydroimidazol-2-ylidene]] were produced by [[vacuum pyrolysis]] of the corresponding 2-trichloromethyl [[dihydroimidazole]] compounds with the loss of [[chloroform]].<ref name=Wanzlick1>{{cite journal|title = Ein neuer Zugang zur Carben-Chemie |trans-title=A new way into carbene chemistry |author1=Hans-Werner Wanzlick |author2=E. Schikora |journal=[[Angew. Chem.]] |volume = 72 |issue = 14 |pages = 494|year = 1960 |doi = 10.1002/ange.19600721409|bibcode=1960AngCh..72..494W }}</ref><ref name=Wanzlick2>{{cite journal |title = Ein nucleophiles Carben |trans-title=A nucleophilic carbene |author1=H. W. Wanzlick |author2=E. Schikora |journal =[[Chem. Ber.]] |volume = 94 |issue = 9 |pages = 2389–2393 |year = 1960 |doi = 10.1002/cber.19610940905}}</ref>
<ref name=Wanzlick3>{{cite journal|author = H. W. Wanzlick|journal = [[Angew. Chem. Int. Ed.]]|year = 1962|volume = 1|pages = 75–80|doi = 10.1002/anie.196200751|title = Aspects of Nucleophilic Carbene Chemistry|issue = 2}}</ref> They conjectured that the carbene existed in equilibrium with its [[Dimer (chemistry)|dimer]], a [[tetraaminoethylene]] derivative, the so-called [[Wanzlick equilibrium]]. This conjecture was challenged by [[David M. Lemal|Lemal]] and coworkers in 1964, who presented evidence that the dimer did not dissociate;<ref name=Lemal1>{{cite journal|author1=D. M. Lemal |author2=R. A. Lovald |author3=K. I. Kawano |title = Tetraaminoethylenes. The Question of Dissociation| journal=[[J. Am. Chem. Soc.]]| year=1964 |volume=86 |pages=2518–2519 |doi=10.1021/ja01066a044|issue = 12}}</ref> and by Winberg in 1965.<ref name=Winberg1>{{cite journal|author1=H. E. Winberg |author2=J. E. Carnahan |author3=D. D. Coffman |author4=M. Brown |title = Tetraaminoethylenes |journal=[[J. Am. Chem. Soc.]] |year=1965 |volume=87 |pages=2055–2056 |doi=10.1021/ja01087a040 |issue=9}}</ref> However, subsequent experiments by Denk, Herrmann and others have confirmed this equilibrium, albeit in specific circumstances.<ref name=denk99>{{cite journal |author1=Denk M. K. |author2=Hatano K. |author3=Ma M. | year = 1999 | title = Nucleophilic Carbenes and the Wanzlick Equilibrium A Reinvestigation | journal =[[Tetrahedron Lett.]] | volume = 40 | issue = 11| pages = 2057–2060 | doi = 10.1016/S0040-4039(99)00164-1}}</ref><ref>{{cite journal |author1=Böhm Volker P. W. |author2=Herrmann Wolfgang A. | year = 2000 | title = The Wanzlick Equilibrium | journal =[[Angew. Chem. Int. Ed.]] | volume = 39 | issue = 22| pages = 4036–4038 | doi = 10.1002/1521-3773(20001117)39:22<4036::AID-ANIE4036>3.0.CO;2-L|pmid=11093196 }}</ref>

===Isolation===
In 1970, Wanzlick's group generated imidazol-2-ylidene carbenes by the deprotonation of an [[imidazolium]] salt.<ref name="Wanzlick-1970">{{cite journal |author1=H. W. Wanzlick |author2=H. J. Schonherr |year=1970 |title=Chemie nucleophiler Carbene, XVIII, 1) 1.3.4.5-Tetraphenyl-imidazoliumperchlorat |trans-title=Chemistry of nucleophilic carbenes, XVIII. 1) 1,3,4,5-Tetraphenylimidazolium perchlorate |journal=[[Liebigs Ann. Chem.]] |volume=731 |pages=176–179 |doi=10.1002/jlac.19707310121}}</ref> Wanzlick as well as [[Roald Hoffmann]],<ref name=Wanzlick3 /><ref>{{cite journal|author1=R. Gleiter |author2=R. Hoffmann |title = Stabilizing a singlet methylene |journal=[[J. Am. Chem. Soc.]] |year=1968 |volume=90| pages=5457–5460 |doi=10.1021/ja01022a023 |issue=20}}</ref> proposed that these imidazole-based carbenes should be more stable than their 4,5-dihydro analogues, due to Hückel-type [[aromaticity]]. Wanzlick did not however isolate imidazol-2-ylidenes, but instead their [[Transition metal carbene complex|coordination compound]]s with [[mercury (element)|mercury]] and [[isothiocyanate]]:<ref name="Wanzlick-1970" />

[[File:Wanzlick-1970.png|center|600px|Preparation and trapping of an imidazol-2-ylidene.|frameless]]

In 1988, [[Guy Bertrand (chemist)|Guy Bertrand]] and others isolated a phosphinocarbene. These species can be represented as either a λ<sup>3</sup>-phosphinocarbene or λ<sup>5</sup>-[[phosphaacetylene]]:<ref name=Bertrand4>{{cite journal|title = Analogous α,α{{prime}}-bis-carbenoid, triply bonded species: synthesis of a stable λ<sup>3</sup>-phosphino carbene-λ<sup>3</sup>-phosphaacetylene |author1=A. Igau |author2=H. Grutzmacher |author3=A. Baceiredo |author4=G. Bertrand |journal=[[J. Am. Chem. Soc.]] |volume=110 |pages=6463–6466 |year=1988 |issue=19 |doi=10.1021/ja00227a028}}</ref><ref name=Bertrand5>{{cite journal |title = λ<sup>3</sup>-Phosphinocarbenes λ<sup>5</sup>-phosphaacetylenes |author1=G. Bertrand |author2=R. Reed |journal=[[Coord. Chem. Rev.]] |volume=137 |pages=323–355 |year=1994 |doi=10.1016/0010-8545(94)03005-B}}</ref>
[[File:Bertrand1.png|center|600px|Alkyne and carbene resonances structures of Bertrand's carbene|frameless]]

These compounds were called "push-pull carbenes" in reference to the contrasting electron affinities of the phosphorus and silicon atoms, and exhibited both carbenic and [[alkyne|alkynic]] reactivity; their electronic structure was (and would remain!) unclear. 
In 2000, Bertrand would obtain additional carbenes of the phosphanyl type, including (phosphanyl)(trifluoromethyl)carbene, stable in solution at -30&nbsp;°C.<ref>{{cite journal |author1=Christophe Buron |author2=Heinz Gornitzka |author3=Vadim Romanenko |author4=Guy Bertrand |year=2000 |title=Stable Versions of Transient Push-Pull Carbenes: Extending Lifetimes from Nanoseconds to Weeks |journal=[[Science (journal)|Science]] |volume=288 |issue=5467 |pages=834–836 |bibcode=2000Sci...288..834B |doi=10.1126/science.288.5467.834 |pmid=10796999}}</ref> 

In 1991, Arduengo and coworkers obtained the first crystalline diaminocarbene by [[deprotonation]] of an imidazolium cation:<ref name="Arduengo1">{{cite journal |author1=Arduengo, A.J. |author2=Harlow, R.L. |author3=Kline, M. |title=A stable crystalline carbene |year=1991 |journal=[[J. Am. Chem. Soc.]] |volume=113 |issue=1|pages=361–363 |doi=10.1021/ja00001a054}}</ref>

[[File:Arduengo2.png|center|600px|Preparation of ''N'',''{{prime|N}}''-diadamantyl-imidazol-2-ylidene|frameless]]

This carbene, heralding a large family of carbenes with the imidazol-2-ylidene core, is indefinitely stable at room temperature in the absence of oxygen and moisture, and melts at 240–241&nbsp;°C without decomposition.  

The first air-stable Arduengo carbene, a chlorinated member of the imidazol-2-ylidene family, was obtained in 1997.<ref name="Arduengo5" />

===New examples and new theory===
[[file:AllylMO.png|left|120px|thumb|[[Molecular orbitals|MO's]] of the  allylic system.]]
In the modern understanding, the superficially unoccupied ''p''-orbital on a stable carbene is not, in fact, fully empty.  Instead, the carbene Lewis structures are in [[resonance (chemistry)|resonance]] with [[dative bond]]s toward adjacent lone-pair or [[pi-bond|π bond]] orbitals.<ref>{{Cite web |last=Rzepa |first=Henry |author-link=Henry Rzepa |date=11 Sep 2016 |title=What's in a name? Carbenes: a reality check. |url=https://www.ch.imperial.ac.uk/rzepa/blog/?p=16758 |access-date=15 Feb 2024 |website=Chemistry with a Twist}}</ref>  

That persistent carbenes have ylidic character is hardly obvious, and indeed was initially contradicted.  
The [[X-ray diffraction|X-ray]] structure of ''N'',''{{prime|N}}''-diadamantyl-imidazol-2-ylidene revealed longer N–C [[bond length]]s in the ring of the carbene than in the parent imidazolium compound, suggesting very little [[double bond]] character to these bonds.<ref>{{cite journal |last1=Arduengo |first1=Anthony J. |last2=Harlow |first2=Richard L. |last3=Kline |first3=Michael |date=January 1991 |title=A stable crystalline carbene |journal=[[J. Am. Chem. Soc.]] |volume=113 |issue=1 |pages=361–363 |doi=10.1021/ja00001a054}}</ref>  Hence early workers attributed the stability of Arduengo carbenes to the bulky ''N''-[[adamantyl]] substituents, which [[steric hindrance|prevent reaction]] with other molecules.{{Cn|date=June 2025}} 

However, replacement of the ''N''-adamantyl groups with [[methyl]] groups also affords 1,3,4,5-tetramethylimidazol-2‑ylidene (Me<sub>4</sub>ImC:), a [[thermodynamic reaction control|thermodynamically stable]] unhindered NHC 
([http://ajarduengo.net/Jmol/htmlfiles/0161Viewer.html 3D]):<ref name="Arduengo2" />

[[File:Tetramethyl imidazol-2-ylidene.png|120px|1,3,4,5-tetramethyl&shy;imidazol-2-ylidene, a relatively unhindered carbene. |center|frameless]]
[[File:Bis(diisopropylamino)carbene.png|left|thumb|120px|Bis(diisopropylamino) carbene, the first acyclic stable carbene.]]
In 1995, Arduengo's group obtained a carbene derivative of [[dihydroimidazol-2-ylidene]], proving that stability did not arise from the [[aromatic (chemistry)|aromaticity]] of the conjugated [[imidazole]] backbone.<ref name="Arduengo3">{{cite journal |author1=J. Arduengo |author2=J. R. Goerlich |author3=W. J. Marshall |year=1995 |title=A stable diaminocarbene |journal=[[J. Am. Chem. Soc.]] |volume=117 |issue=44 |pages=11027–11028 |doi=10.1021/ja00149a034}}</ref>  The following year, the first acyclic persistent carbene demonstrated that stability did not require even cyclicity.<ref name="Alder1">{{cite journal |author1=R. W. Alder |author2=P. R. Allen |author3=M. Murray |author4=A. G. Orpen |year=1996 |title=Bis(diisopropylamino)carbene |journal=[[Angew. Chem. Int. Ed.]] |volume=35 |issue=10 |pages=1121–1123 |doi=10.1002/anie.199611211}}</ref>  

Unhindered derivatives of the hydrogenated<ref name="Denk1">{{cite journal |author1=M. K. Denk |author2=A. Thadani |author3=K. Hatano |author4=A. J. Lough |year=1997 |title=Steric Stabilization of Nucleophilic Carbenes |journal=[[Angew. Chem. Int. Ed.]] |volume=36 |issue=23 |pages=2607–2609 |doi=10.1002/anie.199726071}}</ref><ref name="AlderEtDAC">{{cite journal |last1=Alder |first1=RW |last2=Chaker |first2=L |last3=Paolini |first3=FP |year=2004 |title=Bis(diethylamino)carbene and the mechanism of dimerisation for simple diaminocarbenes |journal=Chemical Communications |issue=19 |pages=2172–2173 |doi=10.1039/b409112d |pmid=15467857}}</ref> and acyclic<ref name="AlderEtDAC" /><ref name="Alder2">{{cite journal |author1=R. W. Alder |author2=M. E. Blake |year=1997 |title=Bis(''N''-piperidyl)carbene and its slow dimerisation to tetrakis(''N''-piperidyl)ethene |journal=[[Chem. Commun.]] |issue=16 |pages=1513–1514 |doi=10.1039/a703610h}}</ref><ref name="Alder3">{{cite journal |author1=R. W. Alder |author2=M. E. Blake |author3=J. M. Oliva |year=1999 |title=Diaminocarbenes; Calculation of Barriers to Rotation about C<sub>carbene</sub>–N Bonds, Barriers to Dimerization, Proton Affinities, and <sup>13</sup>C NMR Shifts |journal=[[J. Phys. Chem. A]] |volume=103 |issue=50 |pages=11200–11211 |bibcode=1999JPCA..10311200A |doi=10.1021/jp9934228}}</ref> carbenes dimerize over time, but proved key to resolving the electronic structure.  
Acyclic carbenes are flexible and bonds to the carbenic atom admit rotation. But bond rotation in the compound appeared [[bond rotation barrier|hindered]], suggesting that they did indeed have a [[double bond]] character.<ref name="Alder1" /> 

Subsequent research has focused on expanding the array of [[Heteroatom|heteroatoms]] stabilizing the ylide.  
Most persistent carbenes are stabilized by two flanking nitrogen centers.  The outliers include an aminothiocarbene and an aminooxycarbene ([http://ajarduengo.net/Jmol/htmlfiles/0165Viewer.html 3D])...<ref name="Arduengo4">{{cite journal |author=A. J. Arduengo, J. R. Goerlich and W. J. Marshall |year=1997 |title=A Stable Thiazol-2-ylidene and Its Dimer |journal=[[Liebigs Ann. Chem.]] |volume=1997 |issue=2 |pages=365–374 |doi=10.1002/jlac.199719970213}}</ref><ref name="Alder4">{{cite journal |author1=R. W. Alder |author2=C. P. Butts |author3=A. G. Orpen |year=1998 |title=Stable Aminooxy- and Aminothiocarbenes |journal=[[J. Am. Chem. Soc.]] |volume=120 |issue=44 |pages=11526–11527 |doi=10.1021/ja9819312}}</ref> [[File:Alder2.png|center|600px|Stable carbenes with oxygen or sulfur atoms bound to the carbenic atom |frameless]]...and room-temperature-stable bis(diisopropylamino)cyclopropenylidene, in which the amines are connected through [[vinylogy]].<ref name="bertrand2006">{{cite journal |last1=Lavallo |first1=Vincent |last2=Canac |first2=Yves |last3=Donnadieu |first3=Bruno |last4=Schoeller |first4=Wolfgang W. |last5=Bertrand |first5=Guy |year=2006 |title=Cyclopropenylidenes: From Interstellar Space to an Isolated Derivative in the Laboratory |journal=[[Science (journal)|Science]] |volume=312 |issue=5774 |pages=722–724 |bibcode=2006Sci...312..722L |doi=10.1126/science.1126675 |pmc=2427275 |pmid=16614171}}</ref>  In 2000, Bertrand obtained a moderately stable (amino)(aryl)carbene with only one heteroatom adjacent to the carbenic atom.<ref>{{cite journal |last1=Solé |first1=Stéphane |last2=Gornitzka |first2=Heinz |last3=Schoeller |first3=Wolfgang W. |last4=Bourissou |first4=Didier |last5=Bertrand |first5=Guy |year=2001 |title=(Amino)(Aryl)Carbenes: Stable Singlet Carbenes Featuring a Spectator Substituent |journal=[[Science (journal)|Science]] |volume=292 |issue=5523 |pages=1901–1903 |bibcode=2001Sci...292.1901S |doi=10.1126/science.292.5523.1901 |pmid=11397943}}</ref><ref>{{cite journal |author1=Lai Chun-Liang |author2=Guo Wen-Hsin |author3=Lee Ming-Tsung |author4=Hu Ching-Han |year=2005 |title=Ligand properties of ''N''-heterocyclic and Bertrand carbenes: A density functional study |journal=[[J. Organomet. Chem.]] |volume=690 |issue=24–25 |pages=5867–5875 |doi=10.1016/j.jorganchem.2005.07.058}}</ref>
<!-- TO DO (?): 
* Triplet carbenes 
* Wanzlick equilibrium mechanism elucidation
* Mesoionic carbenes -->

==Classes of stable carbenes==
Stable carbenes rely on adjacent heteroatoms to stabilize the "carbenic" carbon.  Stable carbenes can be usefully categorized by the number of such atoms that are nitrogen.  

Carbenes with sulfur, oxygen, or other [[chalcogen]]s at ''both'' [[Locant|α]] locations are expected to dissociate into an [[alkyne]] (R<sup>1</sup>C≡CR<sup>2</sup>) and a carbon [[chalcogenide|dichalcogenide]] (X<sup>1</sup>=C=X<sup>2</sup>).  Evidence for the reverse process exists: [[carbon disulfide]] (CS<sub>2</sub>) reacts with electron-deficient [[acetylene]] derivatives to conjecturally give transient [[1,3-dithiolium]] carbenes (i.e. where X<sup>1</sup> = X<sup>2</sup> = S), which then dimerise to [[tetrathiafulvene]] derivatives.<ref name="Haztzler1970">{{cite journal |author=H. D. Haztzler |year=1970 |title=Nucleophilic 1,3-dithiolium carbenes |journal=[[J. Am. Chem. Soc.]] |volume=92 |issue=5 |pages=1412–1413 |doi=10.1021/ja00708a058}}</ref><ref name="Haztzler1972">{{cite journal |author=H. D. Hartzler |year=1972 |title=1,3-Dithiolium carbenes from acetylenes and carbon disulfide |journal=[[J. Am. Chem. Soc.]] |volume=95 |issue=13 |pages=4379–4387 |doi=10.1021/ja00794a039}}</ref>

=== Diaminocarbenes ===
A wide variety of [[Azomethine ylide|bisazomethine ylides]] are known, both cyclic<ref name="Arduengo3" /><ref name="Denk1" /><ref name="Alder5">{{cite journal |author1=R. W. Alder |author2=M. E. Blake |author3=C. Bortolotti |author4=S. Buffali |author5=C. P. Butts |author6=E. Lineham |author7=J. M. Oliva |author8=A. G. Orpen |author9=M. J. Quayle |year=1999 |title=Complexation of stable carbenes with alkali metals |journal=[[Chem. Commun.]] |issue=3 |pages=241–242 |doi=10.1039/a808951e}}</ref> and acylic:<ref name="Alder1" /><ref name="Alder2" /><ref name="Alder3" />
{{vanchor|2NSatCNMRTables|text=[[File:diaminocarbenes1.png|center|thumb|600px|Some saturated metastable diaminocarbenes]]}}
The most useful such carbenes are [[Aromatic compound|aromatic]], for otherwise the [[Wanzlick equilibrium]] favors dimerization.<ref name="Denk1" /><ref name="Alder2" />  
Typically, they are derived from [[imidazole]] or [[triazole]] rings.  However, one stable ''N''-heterocyclic carbene derives from [[borazine]]:<ref name="Prasang-2005">{{cite journal |last1=Präsang |first1=C |last2=Donnadieu |first2=B |last3=Bertrand |first3=G |year=2005 |title=Stable Planar Six-π-Electron Six-Membered ''N''-Heterocyclic Carbenes with Tunable Electronic Properties |journal=[[J. Am. Chem. Soc.]] |volume=127 |issue=29 |pages=10182–10183 |doi=10.1021/ja052987g |pmc=2440681 |pmid=16028925}}</ref>[[File:Prasang-carbene.png|center|800px|frameless]]

==== Imidazol-2-ylidenes ====
Imidazol-2-ylidenes are known with [[alkyl]], [[aryl]],<ref name="Arduengo2">{{cite journal|author1=A. J. Arduengo |author2=H. V. R. Dias |author3=R. L. Harlow |author4=M. Kline |title = Electronic stabilization of nucleophilic carbenes|journal=[[J. Am. Chem. Soc.]]|year = 1992|volume = 114|pages = 5530–5534|doi = 10.1021/ja00040a007|issue = 14}}</ref> alkyloxy, alkylamino, alkylphosphino and even [[Chirality (chemistry)|chiral]] substituents on the nitrogen atoms.<ref name="Herman1">{{cite journal |author1=W. A. Herrmann |author2=C. Kocher |author3=L. J. Goossen |author4=G. R. J. Artus |year=1996 |title=Heterocyclic Carbenes: A High-Yielding Synthesis of Novel, Functionalized ''N''-Heterocyclic Carbenes in Liquid Ammonia |journal=[[Chem. Eur. J.]] |volume=2 |issue=12 |pages=1627–1636 |doi=10.1002/chem.19960021222}}</ref>  

1,3-Dimesityl-4,5-dichloroimidazol-2-ylidene, the first air-stable carbene, bears two [[chlorine]] atoms on the "backbone" ([http://ajarduengo.net/Jmol/htmlfiles/0173Viewer.html 3D]):<ref name="Arduengo5">{{cite journal|author1=A. J. Arduengo |author2=F. Davidson |author3=H. V. R. Dias |author4=J. R. Goerlich |author5=D. Khasnis |author6=W. J. Marshall |author7=T. K. Prakasha |title = An Air Stable Carbene and Mixed Carbene "Dimers"|journal=[[J. Am. Chem. Soc.]]|year = 1997|issue = 52|pages = 12742–12749|doi = 10.1021/ja973241o|volume = 119}}
</ref> 

[[File:1,3-di-1-(2,4,6-mesityl)-4,5-dichloroimidazol-2-ylidene.png|upright=0.7|alt=1,3-Dimesityl-4,5-dichloroimidazol-2-ylidene|center|frameless]]
The chlorines likely [[Electron-withdrawing group|reduce]] the [[electron density]] on the carbenic/ylidic carbon via [[inductive effect|induction]] through the σ system.  

Because imidazolylidenes are stable against dimerization, molecules can contain multiple imidazol-2-ylidene groups:<ref name="Herman2">{{cite journal|author1=W. A. Herrmann |author2=M. Elison |author3=J. Fischer |author4=C. Kocher |author5=G. R. J. Artus |journal =[[Chem. Eur. J.]] |year = 1996|volume = 2|pages = 772–780|doi = 10.1002/chem.19960020708|title = ''N''-Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis|issue = 7}}</ref><ref name="Dias1">{{cite journal|author1=H. V. R. Dias |author2=W. C. Jin |journal = [[Tetrahedron Lett.]]|year = 1994|volume = 35|pages = 1365–1366|doi = 10.1016/S0040-4039(00)76219-8|title = A stable tridentate carbene ligand|issue = 9}}</ref>
<gallery mode="packed">
File:Poly-imidazolylidenes1.png
File:Poly-imidazolylidenes2.png
</gallery>

==== Triazol-5-ylidenes ====
In principle, [[triazol-5-ylidene]]s occur in two isomeric families, the [[1,2,3-Triazole|1,2,3-triazol]]-5-ylidenes and [[1,2,4-Triazole|1,2,4-triazol]]-5-ylidenes:

[[File:Triazol-5-ylidenes.png|400px|center|Triazol-5-ylidene isomers.|frameless]]

Few such carbenes have been reported, but a triphenyl molecule is commercially available:{{Cn|date=June 2025}}
[[File:Triphenyltriazolylidene.png|alt=1,3,4-triphenyl-1,2,4-triazol-5-ylidene.|center|frameless|250x250px|Commercial 1,2,4-triazol-5-ylidene.]]

===Monoaminocarbenes===
The non-nitrogen atom adjacent to the carbene may be [[carbon]] (the [[Cyclic alkyl amino carbenes|cyclic monoamino carbenes]]),{{Cn|date=June 2025}} 
oxygen,<ref name="Alder4" /> sulfur,<ref name="Arduengo4" /><ref name="Alder4" /> or [[phosphorus]]:<ref name="Bertrand4" /><ref name="Bertrand5" />

[[File:Heteroaminocarbenes1.png|center|thumb|upright=2.5|Synthesised heteroamino carbenes (top and bottom right) and Bertrand's carbenes (bottom left)]]

Since [[oxygen]] and sulfur are [[divalent]], [[steric]] protection of the carbenic centre is particularly limited. 

A claimed [[isothiazole]] carbene ('''2b''')<ref name="wolf2007">{{cite journal |last1=Wolf |first1=J |last2=Böhlmann |first2=W |last3=Findeisen |first3=M |last4=Gelbrich |first4=T |last5=Hofmann |first5=HJ |last6=Schulze |first6=B |year=2007 |title=Synthesis of stable isothiazole carbenes |journal=[[Angew. Chem. Int. Ed.]] |volume=46 |issue=17 |pages=3118–3121 |doi=10.1002/anie.200604305 |pmid=17372997}}</ref> is not stable, rearranging instead to a β{{Nbh}}thiolactam:<ref name="DeHope-2006">{{cite journal |last1=DeHope |first1=A |last2=Lavallo |first2=V |last3=Donnadieu |first3=B |last4=Schoeller |first4=WW |last5=Bertrand |first5=G |year=2007 |title=Recently reported crystalline isothiazole carbenes: Myth or reality |journal=[[Angew. Chem. Int. Ed.]] |volume=46 |issue=36 |pages=6922–6925 |doi=10.1002/anie.200702272 |pmid=17661300}}</ref><ref>{{cite journal |author1=Wolf Janine |author2=Böhlmann Winfried |author3=Findeisen Matthias |author4=Gelbrich Thomas |author5=Hofmann Hans-Jorg |author6=Schulze Borbel |year=2007 |title=Reply to "Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality" |journal=[[Angew. Chem. Int. Ed.]] |volume=46 |issue=36 |page=6926 |doi=10.1002/anie.200702746}}</ref> 

[[File:Isothiazole-carbene-DeHope-2006.png|frameless|upright=2.5|center]]

===Cyclopropenylidenes===
Another family of carbenes is based on a [[cyclopropenylidene]] core, a three-carbon ring with a double bond between the two atoms adjacent to the carbenic one. This family is exemplified by [[bis(diisopropylamino)cyclopropenylidene]].<ref name="bertrand2006" />

===Bertrand's carbenes===
In Bertrand's persistent carbenes, the unsaturated carbon is bonded to a [[phosphorus]] and a [[silicon]].<ref>{{cite journal|author1=G. Bertrand |author2=A. Igau |author3=A. Baceiredo |author4=G. Trinquier |journal =[[Angew. Chem. Int. Ed.]]|year = 1989|volume = 28|pages = 621–622|doi = 10.1002/anie.198906211|title = [Bis(diisopropylamino)phosphino]trimethylsilylcarbene: A Stable Nucleophilic Carbene|issue=5}}
</ref> However, these compounds exhibit some alkynic properties and may instead be a hypervalent [[phosphaalkyne]].  The exact nature of these red oils remained unclear {{As of|2006|lc=y}}.<ref name="Bertrand5" />
[[File:Bertrand1.png|center|thumb|600px|Alkyne and carbene resonances structures of Bertrand's carbene]]

===Triplet state carbenes===
Persistent carbenes tend to exist in the [[singlet state|singlet]], dimerizing when forced into triplet states.  Nevertheless, [[Hideo Tomioka]] and associates used [[Delocalized electron|electron delocalization]] to produce a comparatively stable triplet carbene ([[bis(9-anthryl)carbene]]) in 2001.  It has an unusually long [[half-life]] of 19 minutes.<ref>{{cite journal |last1=Tomioka |first1=H |last2=Iwamoto |first2=E |last3=Itakura |first3=H |last4=Hirai |first4=K |year=2001 |title=Generation and characterization of a fairly stable triplet carbene |journal=[[Nature (journal)|Nature]] |volume=412 |issue=6847 |pages=626–628 |bibcode=2001Natur.412..626T |doi=10.1038/35088038 |pmid=11493917 |s2cid=4373216 |ref={{harvid|Tomioka|2001}}}}</ref><ref>{{cite journal|journal = [[Chemical & Engineering News]]|date = 2001-08-13|volume = 79|issue = 33|pages = 11|url=http://pubs.acs.org/cen/topstory/7933/7933notw5.html|title = Triplet Carbene has Long Life|author1=Michael Freemantle|doi=10.1021/cen-v079n033.p011a|url-access = subscription}}</ref>

[[File:Tomioka Carbene 2001.svg|center|thumb|650px|Delocalization in a stable triplet carbene reported by {{Harvtxt|Tomioka|2001}}.  
Note that the molecule is not planar; each arene system forms a plane [[perpendicular]] to the other]]

In 2006 a triplet carbene was reported by the same group with a [[half-life]] of 40 minutes. This carbene is prepared by a [[photochemistry|photochemical]] [[chemical decomposition|decomposition]] of a [[diazomethane]] precursor by 300&nbsp;[[nanometer|nm]] light in benzene with expulsion of [[nitrogen]] gas.<ref>{{cite journal |last1=Itoh |first1=T |last2=Nakata |first2=Y |last3=Hirai |first3=K |last4=Tomioka |first4=H |year=2006 |title=Triplet Diphenylcarbenes Protected by Trifluoromethyl and Bromine Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature |journal=[[J. Am. Chem. Soc.]] |volume=128 |issue=3 |pages=957–967 |doi=10.1021/ja056575j |pmid=16417387 |ref={{harvid|Itoh|2006}}}}</ref>

[[File:Persistent triplet carbene.png|center|thumb|600px|A persistent triplet carbene (right), synthesized by {{Harvtxt|Itoh|2006}}.  
Note that the molecule is neither bent at the central carbon nor planar; that carbon is [[Sp hybrid orbital|''sp'' hybridized]] and each arene system forms a plane [[perpendicular]] to the other]]

Exposure to oxygen (a triplet diradical) converts this carbene to the corresponding [[benzophenone]]. A diphenylmethane compound{{Which|date=June 2025}} is formed when it is trapped by [[cyclohexa-1,4-diene]].{{Cn|date=June 2025}} 

As with the other carbenes, this species contains large bulky substituents, namely [[bromine]] and the trifluoromethyl groups on the phenyl rings, that shield the carbene and prevent or slow down the process of dimerization to a 1,1,2,2-tetra(phenyl)alkene. Based on [[Computational chemistry|computer simulations]], the [[bond length|distance]] of the divalent carbon atom to its neighbors is claimed to be 138 [[picometer]]s with a [[bond angle]] of 158.8°. The planes of the phenyl groups are almost at right angles to each other (the [[dihedral angle]] being 85.7°).{{OR?|date=June 2025}}

=== Mesoionic carbenes ===
[[Mesoionic carbene]]s (MICs) are similar to ''N''-heterocyclic carbenes (NHCs), except that canonical resonance structures with the carbene depicted cannot be drawn without adding additional charges. Mesoionic carbenes are also referred to as abnormal ''N''-heterocyclic carbenes (aNHC) or remote ''N''-heterocyclic carbenes (rNHC).

==Chemical properties==
Enders ''et al.''<ref name="Enders-1995">{{cite journal |author1=D. Enders |author2=K. Breuer |author3=G. Raabe |author4=J. Runsink |author5=J. H. Teles |author6=J. P. Melder |author7=K. Ebel |author8=S. Brode |year=1995 |title=Preparation, Structure, and Reactivity of 1,3,4-Triphenyl-4,5-dihydro-1''H''-1,2,4-triazol-5-ylidene, a New Stable Carbene |journal=[[Angew. Chem. Int. Ed.]] |volume=34 |issue=9 |pages=1021–1023 |doi=10.1002/anie.199510211}}</ref><ref name="Enders-2019">{{cite journal |author1=Enders, D. |author2=Breuer, K. |author3=Runsink, J. |author4=Teles, J.H. |year=1996 |journal=[[Liebigs Ann. Chem.]] |pages=2019–2028|title=Chemical Reactions of the Stable Carbene 1,3,4-Triphenyl-4,5-dihydro-1''H''-1,2,4-triazol-5-ylidene|volume=1996 |issue=12 |doi=10.1002/jlac.199619961212}}</ref><ref name=Enders-1997>
{{cite journal |author1=Enders, D. |author2=Breuer, K. |author3=Teles, J.H. |author4=Ebel, K. |year=1997 |journal=[[J. Prakt. Chem.]] |pages=397–399 |title=1,3,4-Triphenyl-4,5-dihydro-1''H''-1,2,4-triazol-5-ylidene – applications of a stable carbene in synthesis and catalysis |doi=10.1002/prac.19973390170 |volume=339}}</ref>
have performed a range of organic reactions involving a model triazol-5-ylidene:
[[File:triazol5ylidene reactions.png|center|thumb|600px|
{|class=wikitable
|+Reactions of triazol-5-ylidene<ref name=Enders-1997/>
!a
|3,6-diphenyl-1,2,4,5-[[tetrazine]], toluene
|92%
|style="border-top: 0px; border-bottom: 0px"|
!e
|2 equiv., PhNCO, toluene, reflux
|92%
|-
!b
|RXH, RT
|95–97%
|style="border-top: 0px; border-bottom: 0px"|
!f
|CS<sub>2</sub>, toluene, or PhNCS, THF, RT
|71–90%
|-
!c
|O<sub>2</sub>, S<sub>8</sub>, or Se, toluene, reflux
|54–68%
|style="border-top: 0px; border-bottom: 0px"|
!g
|Maleimide, THF, RT
|47–84%
|-
!d
|R<sub>1</sub>CH=CHR<sup>2</sup>, THF, RT
|25–68%
|style="border-top: 0px; border-bottom: 0px"|
!h
|Dimethylacetylene dicarboxylate, THF, reflux
|21%
|}]]
The unprotonated molecule performed [[nucleophilic]] addition ('''e''' and '''f'''), possibly [[Conjugate addition|in conjugate]] ('''d''', '''g''' and '''h''').  As a base, it [[deprotonation|abstracts labile proton]]s easily; the resulting cation can easily add a nucleophile (a net [[insertion reaction]]; '''b''').  Chalcogens [[addition reaction|add]] at the carbene to recover the (thio)[[Ureas|urea]] ('''c''') and activated [[Diene|dienes]] add the carbene in [4+1] cycloadditions ('''a'''). 

===Basicity and nucleophilicity===

The imidazol-2-ylidenes are strong bases, having conjugate [[pKa|p''K''<sub>a</sub>]]&nbsp;≈&nbsp;24 in [[dimethyl sulfoxide]] (DMSO):<ref name=Alder6/>

[[File:imidazol2ylidene pka.png|center|600px|Measurement of the pKa value for the conjugate acid of an imidazol-2-ylidene|frameless]]

Conjugate p''K''<sub>a</sub> values for several NHC families have been examined in aqueous solution. pK<sub>a</sub> values of triazolium ions lie in the range 16.5–17.8,<ref>{{cite journal |author=Massey Richard S |year=2012 |title=Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pKaValues for Twenty Triazolium Salts in Aqueous Solution |url=http://dro.dur.ac.uk/17557/1/17557.pdf |journal=[[J. Am. Chem. Soc.]] |volume=134 |issue=50 |pages=20421–20432 |doi=10.1021/ja308420c |pmid=23173841}}</ref> around 3 p''K''<sub>a</sub> units more acidic than related imidazolium ions.<ref>{{Cite journal |last1=Higgins |first1=Eleanor M. |last2=Sherwood |first2=Jennifer A. |last3=Lindsay |first3=Anita G. |last4=Armstrong |first4=James |last5=Massey |first5=Richard S. |last6=Alder |first6=Roger W. |last7=O'Donoghue |first7=Annmarie C. |year=2011 |title=p''K''<sub>a</sub>s of the conjugate acids of ''N''-heterocyclic carbenes in water |url=https://semanticscholar.org/paper/666ab73922ee8874fa58dfa9f87544420f4f40bb |journal=[[Chem. Commun.]] |volume=47 |issue=5 |pages=1559–1561 |doi=10.1039/C0CC03367G |pmid=21116519 |s2cid=205757477}}</ref>  
Contrariwise, diaminocarbenes will deprotonate DMSO solvent, with the resulting anion reacting with the resulting amidinium salt:

[[File:AlderTIPcarbeneDMSO.png|center|thumb|600px|Using [[D6-DMSO]] as an NMR solvent can have unexpected results.]]

The molecules are likely also reasonably [[nucleophilic]].  Reaction of imidazol-2-ylidenes with [[1-bromohexane]] gave 90% of the 2-substituted adduct, with only 10% of the corresponding [[alkene]].

Stable carbenes derived from [[thiazole]] underlie the action of [[thiamine]] in biological systems, and its [[Biomimetics|biomimetic]] descendant, the [[Stetter reaction]].<ref>{{cite journal |author=R. Breslow |year=1957 |title=Rapid Deuterium Exchange in Thiazolium Salts |journal=[[J. Am. Chem. Soc.]] |volume=79 |issue=7 |pages=1762–1763 |doi=10.1021/ja01564a064}}</ref>  

===Dimerisation===
At one time, stable carbenes were thought to reversibly [[Dimer (chemistry)|dimer]]ise through the so-called [[Wanzlick equilibrium]].<ref name="AlderEtDAC" /> 
The uncatalyzed reaction is typically quite slow, presumably in part because direct, planar dimerization ('''A''') requires first crossing the high [[Singlet state|singlet]]-[[Triplet state|triplet]] barrier.  In the preferred pathway ('''B'''), the empty carbon [[p-orbital|''p'' orbital]] attacks a nearby carbene [[lone pair]]:<ref name="Alderreview">{{cite journal |last1=Alder |first1=Roger W. |last2=Blake |first2=Michael E. |last3=Chaker |first3=Leila |last4=Harvey |first4=Jeremy N. |last5=Paolini |first5=François |last6=Schütz |first6=Jan |year=2004 |title=When and How Do Diaminocarbenes Dimerize? |journal=[[Angew. Chem. Int. Ed.]] |volume=43 |issue=44 |pages=5896–5911 |doi=10.1002/anie.200400654 |pmid=15457494 |doi-access=free}}</ref> 

[[File:Routes of carbene dimerisation.png|center|600px|frameless]]
Protons, which create [[formamidinium]] salts, catalyze the reaction,<ref name="AlderEtDAC" /> as do other [[Lewis acids and bases|Lewis acids]].<ref name="Alderreview" />   

However, imidazol-2-ylidenes and triazol-5-ylidenes are thermodynamically stable and do not dimerise even under relatively forcing conditions.  They have been stored in [[Solution (chemistry)|solution]] in the absence of water and air for years. This is presumably due to the [[aromatic]] nature of these carbenes, which is lost upon dimerisation.<ref name="Denk1" /><ref name="Alder2" /> 

Chen and Taton demonstrated that a sufficiently short tether (i.e., propylene, but not butylene) could force aromatic stable carbenes to dimerize:<ref name="Chen">{{cite journal |author1=T. A. Taton |author2=P. Chen |year=1996 |title=A Stable Tetraazafulvalene |journal=[[Angew. Chem. Int. Ed.]] |volume=35 |issue=9 |pages=1011–1013 |doi=10.1002/anie.199610111}}</ref>

[[File:Chen dimer.png|center|300px|Dimerisation of tethered diimidazol-2-ylidenes|frameless]]

If a dicarbene, the carbenic [[lone pair]]s would be forced into close proximity. To avoid [[electrostatic]] repulsion between the lone pairs, the orbitals hybridize into bonds.

===Metal complexes===
{{Main|Transition metal NHC complex}}
Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) coordinate to a plethora of elements: from [[main group element]]s, [[transition metal]]s and [[actinides]] to even [[alkali metals]] and [[lanthanides]]. A [[periodic table]] of elements gives some idea of the complexes which have been prepared.  
{{Periodic table (persistent carbene)}}
In many cases, the complexes have been identified by single crystal [[X-ray crystallography]].<ref name="Alder5" /><ref name="Herrmann3">{{cite journal|title = ''N''-Heterocyclic Carbenes|journal =[[Angew. Chem. Int. Ed.]]|volume = 36|issue = 20|year = 1997|pages = 2162–2187|author1=[[Wolfgang A. Herrmann]] |author2=Christian Köcher |doi = 10.1002/anie.199721621|s2cid =97336589}}</ref><ref name="Boche1">{{cite journal|title = Crystal Structure of the Dimeric (4-''tert''-Butylthiazolato)(glyme)lithium: Carbene Character of a Formyl Anion Equivalent|journal =[[Angew. Chem. Int. Ed.]]|volume = 34|issue = 4|year = 1995|pages = 487–489|author1=Gernot Boche |author2=Christof Hilf |author3=Klaus Harms |author4=Michael Marsch |author5=John C. W. Lohrenz |doi =10.1002/anie.199504871}}</ref>
Stable carbenes are roughly [[Isolobal principle|isolobal]] with [[organophosphine]]s.  The carbenic [[lone pair]] is a good σ donor, and the adjacent, stabilizing heteroatoms enrich the [[Conjugated system|π system]] with such electrons as to inhibit [[Pi backbonding|π backbonding]].  Enders<ref name="Enders-ChemBer-1996">{{cite journal|author1=D. Enders |author2=H. Gielen |author3=G. Raabe |author4=J. Runsink |author5=J. H. Teles |journal = [[Chem. Ber.]]|year = 1996|volume = 129|pages = 1483–1488|doi = 10.1002/cber.19961291213|title = Synthesis and Stereochemistry of the First Chiral (Imidazolinylidene)- and (Triazolinylidene)palladium(II) Complexes|issue = 12}}
</ref> and Hermann<ref name="Herrmann3" /><ref name="Herrman4">
 {{cite journal|title = Metal Complexes of ''N''-Heterocyclic Carbenes – A New Structural Principle for Catalysts in Homogeneous Catalysis|journal =[[Angew. Chem. Int. Ed.]]|issue = 21|year = 1995|pages = 2371–2374|author1=[[Wolfgang A. Herrmann]] |author2=Martina Elison |author3=Jakob Fischer |author4=Christian Köcher |author5=Georg R. J. Artus |doi = 10.1002/anie.199523711|volume = 34}}
</ref><ref name="Herrman5">
 {{cite journal|title = Chiral Heterocylic Carbenes in Asymmetric Homogeneous Catalysis|journal =[[Angew. Chem. Int. Ed.]]|volume = 35|issue = 23–24|year = 1996|pages = 2805–2807|author1=[[Wolfgang A. Herrmann]] |author2=Lukas J. Goossen |author3=Christian Köcher |author4=Georg R. J. Artus |doi = 10.1002/anie.199628051}}
</ref> have shown [[ligands|ligand]] rough equivalence between stable carbenes and organophosphines in several [[catalytic cycles]]: the carbenes do not activate the metal near so much, but the resulting complexes are far more robust. Grubbs has reported replacing a phosphine ligand (PCy<sub>3</sub>) with an imidazol-2-ylidene in the [[olefin metathesis]] catalyst RuCl<sub>2</sub>(PCy<sub>3</sub>)<sub>2</sub>CHPh, and noted increased ring closing metathesis as well as exhibiting "a remarkable air and water stability".<ref name="Grubbs1">
 {{cite journal|author1=M. Scholl |author2=T. M. Trnka |author3=J. P. Morgan |author4=R. H. Grubbs |journal = [[Tetrahedron Lett.]]|year = 1999|volume = 40|pages = 2247–2250|doi = 10.1016/S0040-4039(99)00217-8|title = Increased ring closing metathesis activity of ruthenium-based olefin metathesis catalysts coordinated with imidazolin-2-ylidene ligands|issue = 12}}
</ref> 

Molecules containing two and three carbene moieties have been prepared as potential [[bidentate]] and [[tridentate]] carbene ligands.<ref name="Herman2" /><ref name="Dias1" />

==Physical properties==

Those carbenes that have been isolated to date tend to be colorless solids with low melting points. These carbenes tend to sublime at low temperatures under high vacuum.{{Cn|date=June 2025}}

X-ray structures of imidazolic carbenes show N–C–N bond angles of 103–110°, but typically 104°.<ref name="HZGBS">{{cite journal |display-authors=3 |vauthors=Haque RA, Zulikha HZ, Ghdhayeb MZ, Budagumpi S, Salman AW |date=January 2012 |title=Cationic Nitrile-Functionalized Ag(I)- and Hg(II)-N-Heterocyclic Carbene Complexes of CCC, CNC, and NCN Pincer-Type Carbene Ligands: Synthesis, Crystal Structures, and Characterization |journal=Heteroatom Chemistry |publisher=[[Wiley (publisher)|Wiley]] |volume=23 |issue=5 |pages=486–497 |doi=10.1002/hc.21041 |doi-access=free}}</ref><ref name="HCBAAM">{{cite journal |display-authors=3 |vauthors=Haque RA, Choo SY, Budagumpi S, Abdullah AA, Ahamed MK, Majid AA |date=July 2015 |title=Synthesis, crystal structures, characterization and biological studies of nitrile-functionalized silver(I) N-heterocyclic carbene complexes |journal=[[Inorganica Chimica Acta]] |volume=433 |pages=35–44 |doi=10.1016/j.ica.2015.04.023 |doi-access=free}}</ref><ref name=HGBSAM>{{cite journal |display-authors=3 |vauthors=Haque RA, Ghdhayeb MZ, Budagumpi S, Salman AW, Ahamed MK, Majid AA |date=January 2013 |title=Non-symmetrically substituted N-heterocyclic carbene–Ag(I) complexes of benzimidazol-2-ylidenes: Synthesis, crystal structures, anticancer activity and transmetallation studies |journal=[[Inorganica Chimica Acta]] |volume=394 |pages=519–525 |doi=10.1016/j.ica.2012.09.013 |doi-access=free}}</ref><ref name="LZWGW">{{cite journal |display-authors=3 |vauthors=Liu QX, Zhao XJ, Wu XM, Guo JH, Wang XG |date=December 2007 |title=New mercury(II) and silver(I) complexes containing NHC metallacrown ethers with the π–π stacking interactions |journal=[[Journal of Organometallic Chemistry]] |publisher=[[Elsevier]] |volume=692 |issue=25 |pages=5671–5679 |doi=10.1016/j.jorganchem.2007.09.027 |doi-access=free}}</ref>  
Nonaromatic carbenes typically exhibit larger angles: dihydroimidazole-2-ylidene shows a N–C–N bond angle of about 106°, whilst the angle of an acyclic carbene{{Which|date=June 2025}} is 121°.  Contrariwise, monoamino carbenes X-ray structures have shown N–C–X bond angles of around 104° and 109° respectively.{{Cn|date=June 2025}}

===NMR===
One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the <sup>13</sup>C-[[NMR]] spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the <sup>13</sup>C-[[NMR]] spectrum. For example, bis(isopropyl)imidazolidinylidene exhibits a peak at 238&nbsp;ppm:{{Cn|date=June 2025}}
[[File:Carbene 13C NMR.png|Carbene peak in <sup>13</sup>C NMR|center|frameless|upright=4]]
Imidazole-based carbenes generally have diagnostic <sup>13</sup>C&nbsp;NMR chemical shift values between 210 and 230&nbsp;ppm for the carbenic carbon:<ref>{{cite journal |last1=Tapu |first1=Daniela |last2=Dixon |first2=David A. |last3=Roe |first3=Christopher |date=12 August 2009 |title=13C NMR Spectroscopy of "Arduengo-type" Carbenes and Their Derivatives |journal=[[Chem. Rev.]] |volume=109 |issue=8 |pages=3385–3407 |doi=10.1021/cr800521g |pmid=19281270}}</ref>
[[File:2N 13C-NMR tables.png|center|frameless|Stable imidazol-2-ylidenes|upright=1.3]]
[[Triazole]]-based carbenes have shifts between 210 and 220&nbsp;ppm, while nonaromatic diaminocarbenes have shifts between 230 and 270&nbsp;ppm (see [[#2NSatCNMRTables|diagram]]).  
Acyclic, monoamino carbenes have shifts between 250 and 300&nbsp;ppm for the carbenic carbon, further downfield than any other table carbene.{{cn|date=February 2024}}  

Upon coordination to metal centers, the <sup>13</sup>C carbene resonance usually shifts highfield, depending on the Lewis acidity of the complex fragment. Based on this observation, Huynh ''et al.'' developed a new methodology to determine ligand donor strengths by <sup>13</sup>C NMR analysis of ''trans''-palladium(II)-carbene complexes. The use of a <sup>13</sup>C-labeled N-heterocyclic carbene ligand also allows for the study of mixed carbene-phosphine complexes, which undergo ''trans''-''cis''-isomerization due to the [[trans effect]].<ref>
 {{cite journal|doi = 10.1021/om900667d|journal = [[Organometallics]]|title = 13C NMR Spectroscopic Determination of Ligand Donor Strengths Using N-Heterocyclic Carbene Complexes of Palladium(II)|year = 2009|author = Han Vinh Huynh | volume = 28|issue = 18|pages = 5395–5404 |display-authors = 1 |last2 = Han |first2 = Yuan |last3 = Jothibasu |first3 = Ramasamy |last4 = Yang |first4 = Jie An|doi-access = free }}
</ref>

==Applications==
[[File:Grubbs'-2G-3D-balls.png|right|thumb|150px|A second generation Grubbs' catalyst.]]
NHCs are widely-used [[ligand|ancillary ligands]] in [[organometallic]] chemistry. One practical application is the [[ruthenium]]-based [[Grubbs' catalyst]], for [[olefin metathesis]]; and [[NHC-Palladium Complexes|various palladium complexes]] for cross-coupling reactions.<ref>S. P. Nolan [editor] (2006). [https://books.google.com/books?id=9JOG1g9iFrwC N-Heterocyclic carbenes in synthesis], Wiley-VCH {{ISBN|3-527-31400-8}}</ref><ref>F. Glorius [editor] (2007) [https://books.google.com/books?id=Fv2orfXKVnoC&pg=PP1 N-Heterocyclic carbenes in transition metal catalysis], Springer {{ISBN|3-540-36929-5}}
</ref><ref>{{Cite journal|last1=Díez-González|first1=Silvia|last2=Marion|first2=Nicolas|last3=Nolan|first3=Steven P.|date=2009-08-12|title=N-Heterocyclic Carbenes in Late Transition Metal Catalysis|journal=[[Chem. Rev.]]|volume=109|issue=8|pages=3612–3676|doi=10.1021/cr900074m|pmid=19588961|s2cid=206902952|issn=0009-2665}}</ref> 

Ag(I)-NHC complexes have been widely tested for their biological applications.<ref>{{cite journal |author1=Garrison Jered C. |author2=Youngs Wiley J. | year = 2005 | title = Ag(I) N-Heterocyclic Carbene Complexes: Synthesis, Structure, and Application | journal = Chem. Rev. | volume = 105 | issue = 11| pages = 3978–4008 | doi = 10.1021/cr050004s |pmid=16277368 |s2cid=43090499 }}</ref>

==Preparation methods==
NHCs are often strongly [[Base (chemistry)|basic]] (the [[pKa]] value of the [[conjugate acid]] of an imidazol-2-ylidene was measured at ca. 24)<ref name=Alder6>
 {{cite journal|author1=R. W. Alder |author2=P. R. Allen |author3=S. J. Williams |journal=[[Chem. Commun.]]|year = 1995|pages = 1267|doi = 10.1039/c39950001267|title = Stable carbenes as strong bases|issue = 12}}
</ref> and react with [[oxygen]]. Their synthesis, then must be [[air-free technique|performed free of air]] and compounds of even moderate [[acidity]].  
Conversely, provided rigorously dry, relatively non-acidic and air-free materials are used, stable carbenes are reasonably robust to handling ''per se''. 
[[File:Kcarbene.png|thumb|right|300px|X-ray structure of a [[potassium bis(trimethylsilyl)amide|KHMDS]]-diaminocarbene complex, formed when excess KHMDS was used to deprotonate the parent [[formamidinium]] salt.{{Cn|date=June 2025}} ]]
The simplest syntheses deprotonate a parent salt, 
but the byproducts can be difficult to separate out, 
because NHCs coordinate strongly to even alkali metal cations.  
Potassium and sodium salts tend to precipitate from solution and can be removed, 
but lithium ions are especially problematic, requiring [[cryptand]]s or [[crown ether]]s.  

Alternate techniques have been developed to avoid such purification difficulties. 

===Deprotonation===
[[Deprotonation]] of carbene precursor salts with strong bases reliably produces almost all stable carbenes:

[[File:deprotonation1.png|center|thumb|600px|Deprotonation of precursor salts to give stable carbenes.]]

Imidazol-2-ylidenes and dihydroimidazol-2-ylidenes, such as [[IMes]], have been prepared by the deprotonation of the respective [[imidazolium]] and [[imidazoline|imidazolinium]] salts. Acyclic carbenes<ref name=Alder1/><ref name=Alder2/> and tetrahydropyrimidinyl-based carbenes<ref name=Alder5/> were prepared by deprotonation using strong homogeneous bases.

However, the reaction depends on the correct choice of base.  
Although imidazolium salt precursors are stable to [[nucleophilic]] addition, other non-aromatic salts (i.e. [[formamidinium]] salts) are not.<ref name="Alderformamidinium">{{cite journal |author1=Roger W. Alder |author2=Michael E. Blake |author3=Simone Bufali |author4=Craig P. Butts |author5=A. Guy Orpen |author6=Jan Schütz |author7=Stuart J. Williams |year=2001 |title=Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes |journal=[[J. Chem. Soc., Perkin Trans. 1]] |issue=14 |pages=1586–1593 |doi=10.1039/b104110j}}
</ref>  In these cases, strong unhindered nucleophiles are avoided whether they are generated in ''situ'' or are present as an impurity in other reagents (such as LiOH in BuLi).  

[[Alkyllithiums]] are unreliable bases for the reaction,<ref name="Arduengo1" /> because they are too nucleophilic and often act as [[Transfer hydrogenation|hydridic reductants]]:

[[File:Basebutyllithium.png|center|600px|Reduction of [[formamidinium]] salts with tert-butyllithium|frameless]]


In principle, [[Sodium hydride|sodium]] or [[potassium hydride]]<ref name="Arduengo3" /><ref name="Arduengo4" /> would be the ideal base for deprotonating these precursor salts, but in practice the [[Salt (chemistry)|salt]] dissolves too slowly for effective reaction.  [[Dimethyl sulfoxide|DMSO]] or [[Tert-Butanol|''t''-BuOH]] catalyze the reaction through the soluble tert-butoxide or [[dimsyl sodium|dimsyl anion]] bases,<ref name="Arduengo1" /><ref name="Arduengo2" /> but those compounds are too nucleophilic for non-aromatic carbenes.  
Deprotonation with [[sodium]] or [[potassium]] hydride in a mixture of liquid [[ammonia]]/[[tetrahydrofuran|THF]] at −40&nbsp;°C has been reported<ref name="Herman1" /> for imidazole-based carbenes, and Arduengo and coworkers<ref name="Arduengo4" /> managed to prepare a dihydroimidazol-2-ylidene using NaH. However, this method has not been applied to the preparation of diaminocarbenes. 

In some cases, [[potassium tert-butoxide]] can be employed directly.<ref name="Arduengo2" />

Lithium amides like the [[Lithium diisopropylamide|diisopropylamide (LDA)]] and [[lithium tetramethylpiperidide|tetramethylpiperidide (LiTMP)]]<ref name="Alder1" /><ref name="Alder2" /> generally work well for the deprotonation of all types of salts, providing that not too much [[lithium hydroxide|LiOH]] impurity is present. Metal [[hexamethyldisilazides]]<ref name="Alder5" /> deprotonate almost all salts cleanly, except for unhindered formamidinium salts, where this base can act as a nucleophile to give a triaminomethane adduct.

=== Dechalcogenation and dechlorination ===
For carbenes stable at elevated temperatures, a rare approach [[desulfurisation|desulfurizes]] [[thiourea]]s in [[THF]] with molten [[potassium]]:<ref name="Denk1" /><ref>{{cite journal|author1=N. Kuhn |author2=T. Kratz |journal = [[Synthesis (journal)|Synthesis]]|year = 1993|pages = 561–562|doi = 10.1055/s-1993-25902|title = Synthesis of Imidazol-2-ylidenes by Reduction of Imidazole-2(3''H'')-thiones|volume = 1993|issue = 6}}</ref> 
[[File:NHC desulfurization.png|center|frameless|500x500px]]
A contributing factor to the reaction's success is that the [[potassium sulfide]] byproduct is insoluble in the solvent.{{Cn|date=June 2025}} 

A single example of [[deoxygenation|deoxygenating]] a [[urea]] with a [[fluorene]] derived carbene to give the tetramethyldiaminocarbene and fluorenone has also been reported:<ref>{{cite journal|title = Carbene-to-Carbene Oxygen Atom Transfer|author1=D. Kovacs |author2=M. S. Lee |author3=D. Olson |author4=J. E. Jackson |journal = [[J. Am. Chem. Soc.]]|year = 1996|volume = 118|pages = 8144–8145|doi = 10.1021/ja961324j|issue = 34}}</ref>
[[File:Deoxygenation.png|center|frameless|500x500px]]
[[Bis(trimethylsilyl)mercury]] (CH<sub>3</sub>)<sub>3</sub>Si-Hg-Si(CH<sub>3</sub>)<sub>3</sub> reacts with chloro-[[iminium]] and chloro-[[amidinium]] salts to give a metal-free carbene and elemental [[Mercury (element)|mercury]].<ref>{{cite journal |author1=Michael Otto |author2=Salvador Conejero |author3=Yves Canac |author4=Vadim D. Romanenko |author5=Valentyn Rudzevitch |author6=Guy Bertrand |year=2004 |title=Mono- and Diaminocarbenes from Chloroiminium and -amidinium Salts: Synthesis of Metal-Free Bis(dimethylamino)carbene |journal=[[J. Am. Chem. Soc.]] |volume=126 |issue=4 |pages=1016–1017 |doi=10.1021/ja0393325 |pmid=14746458}}</ref> For example:
:(CH<sub>3</sub>)<sub>3</sub>Si−Hg−Si(CH<sub>3</sub>)<sub>3</sub> + R<sub>2</sub>N=C(Cl)−{{chem|NR|2|+}}Cl<sup>−</sup> → R<sub>2</sub>N−C−NR<sub>2</sub> + Hg<sub>{{abbr|(l)|liquid}}</sub> + 2(CH<sub>3</sub>)<sub>3</sub>SiCl

=== Vacuum pyrolysis ===
Vacuum pyrolysis, with the removal of neutral volatile byproducts i.e. methanol or chloroform, has been used to prepare dihydroimidazole and triazole based carbenes. Historically the removal of chloroform by [[vacuum pyrolysis]] of adducts '''A''' was used by Wanzlick<ref name="Wanzlick2" /> in his early attempts to prepare dihydroimidazol-2-ylidenes but this method is not widely used. The Enders laboratory has used vacuum pyrolysis of adduct '''B''' to generate a triazol-5-ylidene:<ref name="Enders-1995" />

[[File:Vacuum pyrolysis-NHC-preparation.png|center|thumb|400px|Preparation of carbenes via vacuum pyrolysis.]]

===Purification===
A stable carbene prepared from potassium hydride can be filtered through a dry celite pad to remove excess KH (and resulting salts) from the reaction. On a relatively small scale, a suspension containing a stable carbene in solution can be allowed to settle and the supernatant solution pushed through a dried membrane [[syringe filter]]. 

[[Recrystallization (chemistry)|Recrystallisation]] of stable carbenes is difficult, because stable carbenes are readily soluble in non-polar solvents, and polar solvents are insuitably acidic. 

Air-free [[sublimation (chemistry)|sublimation]] purifies effectively, even giving [[Monocrystal|monocrystals]] suitable for X-ray analysis. However, strong complexation to metal ions like [[lithium]] will in most cases prevent sublimation.  Also, the process must be performed at high vacuum, as persistent carbenes decompose above 60&nbsp;°C.  
[[File:Air-free sublimation.png|thumb|600x600px|Apparatus:<dl>
<dt>a</dt><dd>Rubber cone (typically used to form a vacuum seal in a Büchner flask filtration) which is selected so as to fit snugly around the neck of the Schlenk tube</dd>
<dt>b</dt><dd>Schlenk tube</dd>
<dt>c</dt><dd>Gas/vacuum inlet</dd>
<dt>d</dt><dd>Teflon tap (or stopcock)</dd>
<dt>e</dt><dd>Syringe</dd>
</dl>
Method (steps '''4''' and '''5''' can be repeated as required; steps '''6''' and '''7''' are not essential):<dl>
<dt>1</dt><dd>Impure solid to sublime (brown) is placed in a Schlenk tube, avoiding contaminating the sides of the tube (e.g. by careful evaporation from a solution containing the brown solid).</dd>
<dt>2</dt><dd>Rubber cone (black) is pushed near top of the Schlenk tube (forming a tight seal around flask) and filled with a coolant such as dry ice/acetone (blue/white). The bottom of the Schlenk tube is heated (red shading) under vacuum (blue arrow), so that the impure solid (brown) sublimes as a pure solid (purple) at the cooled neck area (blue shading).</dd>
<dt>3</dt><dd>The flask is held under an atmosphere of an inert gas via Schlenk tube side arm, until step '''7'''.  The cooling-cone (black) is removed, leaving the concentrated impurity as a residue (dark brown) in the bottom of the flask, and the purified sublimed solid (purple) at the neck.</dd>
<dt>4</dt><dd>Solvent (blue) is inserted via syringe to dissolve the residue (green/brown), taking care to avoid washing off the sublimed solid (purple).</dd>
<dt>5</dt><dd>Residue solution is then removed by syringe (green/brown).</dd>
<dt>6</dt><dd>Purified sublimed solid (purple) is washed off the neck of the flask with fresh solvent (blue) via a syringe.</dd>
<dt>7</dt><dd>Solvent is removed under vacuum to give the purified sublimed solid (dark purple).</dd>
</dl>|center]]

==References==
{{Reflist|30em}}

==Further reading==
Reviews on persistent carbenes:
* {{cite journal|author1=Hopkinson, M. N. |author2=Richter, C. |author3=Schedler, M. |author4=Glorius, F. |title=An Overview of N-Heterocyclic Carbenes|journal=[[Nature (journal)|Nature]]|year=2014|volume=510|issue=7506 |pages=485–496|doi=10.1038/nature13384 |pmid=24965649|bibcode=2014Natur.510..485H|s2cid=672379 }}.
* [https://books.google.com/books?id=ElV85eHbubQC&dq=Alder+Bertrand&pg=PA153 Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, (Chapter 4, Hideo Tomioka (triplet state); Chapter 5 (singlet state), Roger W. Alder) - ed. Guy Bertrand]
* [https://books.google.com/books?id=wp0C10qO0A8C&dq=moss+jones+platz+Bertrand&pg=PA329 Reactive Intermediate Chemistry By Robert A. Moss, Matthew Platz, Maitland Jones (Chapter 8, Stable Singlet Carbenes, Guy Bertrand)]
* R. W. Alder, in 'Diaminocarbenes: exploring structure and reactivity', ed. G. Bertrand, New York, 2002
* {{cite journal|author=M. Regitz|title=Stable Carbenes—Illusion or Reality?|journal=[[Angew. Chem. Int. Ed.]]|year=1996|volume=30|issue=6|pages=674–676|doi=10.1002/anie.199106741}}

For a review on the physico-chemical properties (electronics, sterics, ...) of N-heterocyclic carbenes:
* {{cite journal |author1=T. Dröge |author2=F. Glorius |title=The Measure of All Rings - N-Heterocyclic Carbenes |journal=[[Angew. Chem. Int. Ed.]] |year=2010 |volume=49 |pages=6940–6952 |doi=10.1002/anie.201001865 |issue=39 |pmid=20715233 }}

[[Category:Functional groups]]
[[Category:Carbenes]]
[[Category:Organometallic chemistry]]
[[Category:Organic compounds]]