Apersistent carbene (also known asstable carbene) is anorganic molecule whose naturalresonance structure has a carbon atom withincomplete octet (acarbene), but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are theN-heterocyclic carbenes (NHC)^[1] (sometimes calledArduengo carbenes), in which nitrogen atoms flank the formal carbene.

Modern theoretical analysis suggests that the term "persistent carbene" is in fact amisnomer. Persistent carbenes do not in fact have a carbene electronic structure in theirground state, but instead anylide stabilized byaromatic resonance orsteric 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 popularligands inorganometallic chemistry.

In 1957,Ronald Breslow proposed that a relatively stablenucleophilic carbene, athiazol-2-ylidene derivative ofvitamin B₁ (thiamine), was the catalyst involved in thebenzoin condensation that yieldsfuroin fromfurfural.^[2][3] In this cycle, the vitamin'sthiazolium ring exchanges a hydrogen atom (attached to carbon 2 of the ring) for a furfural residue. Indeuterated water, the C2-proton was found to rapidly exchange for adeuteron in a statisticalequilibrium:^[4]

This exchange was proposed to proceed via intermediacy of a thiazol-2-ylidene. In 2012 the isolation of the so-calledBreslow intermediate was reported.^[5][6]

In 1960,Hans-Werner Wanzlick and coworkers conjectured that carbenes derived fromdihydroimidazol-2-ylidene were produced byvacuum pyrolysis of the corresponding 2-trichloromethyldihydroimidazole compounds with the loss ofchloroform.^[7][8][9] They conjectured that the carbene existed in equilibrium with itsdimer, atetraaminoethylene derivative, the so-calledWanzlick equilibrium. This conjecture was challenged byLemal and coworkers in 1964, who presented evidence that the dimer did not dissociate;^[10] and by Winberg in 1965.^[11] However, subsequent experiments by Denk, Herrmann and others have confirmed this equilibrium, albeit in specific circumstances.^[12][13]

In 1970, Wanzlick's group generated imidazol-2-ylidene carbenes by the deprotonation of animidazolium salt.^[14] Wanzlick as well asRoald Hoffmann,^[9][15] proposed that these imidazole-based carbenes should be more stable than their 4,5-dihydro analogues, due to Hückel-typearomaticity. Wanzlick did not however isolate imidazol-2-ylidenes, but instead theircoordination compounds withmercury andisothiocyanate:^[14]

In 1988,Guy Bertrand and others isolated a phosphinocarbene. These species can be represented as either a λ³-phosphinocarbene or λ⁵-phosphaacetylene:^[16][17]

These compounds were called "push-pull carbenes" in reference to the contrasting electron affinities of the phosphorus and silicon atoms, and exhibited both carbenic andalkynic 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 °C.^[18]

In 1991, Arduengo and coworkers obtained the first crystalline diaminocarbene bydeprotonation of an imidazolium cation:^[19]

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 °C without decomposition.

The first air-stable Arduengo carbene, a chlorinated member of the imidazol-2-ylidene family, was obtained in 1997.^[20]

In the modern understanding, the superficially unoccupiedp-orbital on a stable carbene is not, in fact, fully empty. Instead, the carbene Lewis structures are inresonance withdative bonds toward adjacent lone-pair orπ bond orbitals.^[21]

That persistent carbenes have ylidic character is hardly obvious, and indeed was initially contradicted. TheX-ray structure ofN,N′-diadamantyl-imidazol-2-ylidene revealed longer N–Cbond lengths in the ring of the carbene than in the parent imidazolium compound, suggesting very littledouble bond character to these bonds.^[22] Hence early workers attributed the stability of Arduengo carbenes to the bulkyN-adamantyl substituents, whichprevent reaction with other molecules.^{[citation needed]}

However, replacement of theN-adamantyl groups withmethyl groups also affords 1,3,4,5-tetramethylimidazol-2‑ylidene (Me₄ImC:), athermodynamically stable unhindered NHC (3D):^[23]

In 1995, Arduengo's group obtained a carbene derivative ofdihydroimidazol-2-ylidene, proving that stability did not arise from thearomaticity of the conjugatedimidazole backbone.^[24] The following year, the first acyclic persistent carbene demonstrated that stability did not require even cyclicity.^[25]

Unhindered derivatives of the hydrogenated^[26][27] and acyclic^[27][28][29] 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 appearedhindered, suggesting that they did indeed have adouble bond character.^[25]

Subsequent research has focused on expanding the array ofheteroatoms stabilizing the ylide.

Most persistent carbenes are stabilized by two flanking nitrogen centers. The outliers include an aminothiocarbene and an aminooxycarbene (3D)...^[30][31]

...and room-temperature-stable bis(diisopropylamino)cyclopropenylidene, in which the amines are connected throughvinylogy.^[32] In 2000, Bertrand obtained a moderately stable (amino)(aryl)carbene with only one heteroatom adjacent to the carbenic atom.^[33][34]

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 otherchalcogens atbothα locations are expected to dissociate into analkyne (R¹C≡CR²) and a carbondichalcogenide (X¹=C=X²). Evidence for the reverse process exists:carbon disulfide (CS₂) reacts with electron-deficientacetylene derivatives to conjecturally give transient1,3-dithiolium carbenes (i.e. where X¹ = X² = S), which then dimerise totetrathiafulvene derivatives.^[35][36]

A wide variety ofbisazomethine ylides are known, both cyclic^[24][26][37] and acylic:^[25][28][29]

The most useful such carbenes arearomatic, for otherwise theWanzlick equilibrium favors dimerization.^[26][28]

Typically, they are derived fromimidazole ortriazole rings. However, one stableN-heterocyclic carbene derives fromborazine:^[38]

Imidazol-2-ylidenes are known withalkyl,aryl,^[23] alkyloxy, alkylamino, alkylphosphino and evenchiral substituents on the nitrogen atoms.^[39]

1,3-Dimesityl-4,5-dichloroimidazol-2-ylidene, the first air-stable carbene, bears twochlorine atoms on the "backbone" (3D):^[20]

The chlorines likelyreduce theelectron density on the carbenic/ylidic carbon viainduction through the σ system.

Because imidazolylidenes are stable against dimerization, molecules can contain multiple imidazol-2-ylidene groups:^[40][41]

In principle,triazol-5-ylidenes occur in two isomeric families, the1,2,3-triazol-5-ylidenes and1,2,4-triazol-5-ylidenes:

Few such carbenes have been reported, but a triphenyl molecule is commercially available:^{[citation needed]}

The non-nitrogen atom adjacent to the carbene may becarbon (thecyclic monoamino carbenes),^{[citation needed]} oxygen,^[31] sulfur,^[30][31] orphosphorus:^[16][17]

Sinceoxygen and sulfur aredivalent,steric protection of the carbenic centre is particularly limited.

A claimedisothiazole carbene (2b)^[42] is not stable, rearranging instead to a β‑thiolactam:^[43][44]

Another family of carbenes is based on acyclopropenylidene core, a three-carbon ring with a double bond between the two atoms adjacent to the carbenic one. This family is exemplified bybis(diisopropylamino)cyclopropenylidene.^[32]

In Bertrand's persistent carbenes, the unsaturated carbon is bonded to aphosphorus and asilicon.^[45] However, these compounds exhibit some alkynic properties and may instead be a hypervalentphosphaalkyne. The exact nature of these red oils remained unclear as of 2006^[update].^[17]

Persistent carbenes tend to exist in thesinglet, dimerizing when forced into triplet states. Nevertheless,Hideo Tomioka and associates usedelectron delocalization to produce a comparatively stable triplet carbene (bis(9-anthryl)carbene) in 2001. It has an unusually longhalf-life of 19 minutes.^[46][47]

In 2006 a triplet carbene was reported by the same group with ahalf-life of 40 minutes. This carbene is prepared by aphotochemicaldecomposition of adiazomethane precursor by 300 nm light in benzene with expulsion ofnitrogen gas.^[48]

Exposure to oxygen (a triplet diradical) converts this carbene to the correspondingbenzophenone. A diphenylmethane compound^[which?] is formed when it is trapped bycyclohexa-1,4-diene.^{[citation needed]}

As with the other carbenes, this species contains large bulky substituents, namelybromine 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 oncomputer simulations, thedistance of the divalent carbon atom to its neighbors is claimed to be 138picometers with abond angle of 158.8°. The planes of the phenyl groups are almost at right angles to each other (thedihedral angle being 85.7°).^{[original research?]}

Mesoionic carbenes (MICs) are similar toN-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 abnormalN-heterocyclic carbenes (aNHC) or remoteN-heterocyclic carbenes (rNHC).

Enderset al.^[49][50][51]have performed a range of organic reactions involving a model triazol-5-ylidene:

The unprotonated molecule performednucleophilic addition (e andf), possiblyin conjugate (d,g andh). As a base, itabstracts labile protons easily; the resulting cation can easily add a nucleophile (a netinsertion reaction;b). Chalcogensadd at the carbene to recover the (thio)urea (c) and activateddienes add the carbene in [4+1] cycloadditions (a).

The imidazol-2-ylidenes are strong bases, having conjugatepK_a ≈ 24 indimethyl sulfoxide (DMSO):^[52]

Conjugate pK_a values for several NHC families have been examined in aqueous solution. pK_a values of triazolium ions lie in the range 16.5–17.8,^[53] around 3 pK_a units more acidic than related imidazolium ions.^[54] Contrariwise, diaminocarbenes will deprotonate DMSO solvent, with the resulting anion reacting with the resulting amidinium salt:

The molecules are likely also reasonablynucleophilic. Reaction of imidazol-2-ylidenes with1-bromohexane gave 90% of the 2-substituted adduct, with only 10% of the correspondingalkene.

Stable carbenes derived fromthiazole underlie the action ofthiamine in biological systems, and itsbiomimetic descendant, theStetter reaction.^[55]

At one time, stable carbenes were thought to reversiblydimerise through the so-calledWanzlick equilibrium.^[27] The uncatalyzed reaction is typically quite slow, presumably in part because direct, planar dimerization (A) requires first crossing the highsinglet-triplet barrier. In the preferred pathway (B), the empty carbonp orbital attacks a nearby carbenelone pair:^[56]

Protons, which createformamidinium salts, catalyze the reaction,^[27] as do otherLewis acids.^[56]

However, imidazol-2-ylidenes and triazol-5-ylidenes are thermodynamically stable and do not dimerise even under relatively forcing conditions. They have been stored insolution in the absence of water and air for years. This is presumably due to thearomatic nature of these carbenes, which is lost upon dimerisation.^[26][28]

Chen and Taton demonstrated that a sufficiently short tether (i.e., propylene, but not butylene) could force aromatic stable carbenes to dimerize:^[57]

If a dicarbene, the carbeniclone pairs would be forced into close proximity. To avoidelectrostatic repulsion between the lone pairs, the orbitals hybridize into bonds.

Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) coordinate to a plethora of elements: frommain group elements,transition metals andactinides to evenalkali metals andlanthanides. Aperiodic table of elements gives some idea of the complexes which have been prepared.

v t e Periodic table: Persistent carbene
Group →	1	2		3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	1 H																		2 He
2	3 Li	4 Be												5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg												13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca		21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr		39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba		71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra		103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
				57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb
				89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No

Legend

  Carbene complex with element known

  No carbene complex with element known

In many cases, the complexes have been identified by single crystalX-ray crystallography.^[37][58][59]Stable carbenes are roughlyisolobal withorganophosphines. The carbeniclone pair is a good σ donor, and the adjacent, stabilizing heteroatoms enrich theπ system with such electrons as to inhibitπ backbonding. Enders^[60] and Hermann^[58][61][62] have shownligand rough equivalence between stable carbenes and organophosphines in severalcatalytic 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 (PCy₃) with an imidazol-2-ylidene in theolefin metathesis catalyst RuCl₂(PCy₃)₂CHPh, and noted increased ring closing metathesis as well as exhibiting "a remarkable air and water stability".^[63]

Molecules containing two and three carbene moieties have been prepared as potentialbidentate andtridentate carbene ligands.^[40][41]

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.^{[citation needed]}

X-ray structures of imidazolic carbenes show N–C–N bond angles of 103–110°, but typically 104°.^{[64][65][66][67]} 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?] is 121°. Contrariwise, monoamino carbenes X-ray structures have shown N–C–X bond angles of around 104° and 109° respectively.^{[citation needed]}

One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the¹³C-NMR spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the¹³C-NMR spectrum. For example, bis(isopropyl)imidazolidinylidene exhibits a peak at 238 ppm:^{[citation needed]}

Imidazole-based carbenes generally have diagnostic¹³C NMR chemical shift values between 210 and 230 ppm for the carbenic carbon:^[68]

Triazole-based carbenes have shifts between 210 and 220 ppm, while nonaromatic diaminocarbenes have shifts between 230 and 270 ppm (seediagram). Acyclic, monoamino carbenes have shifts between 250 and 300 ppm for the carbenic carbon, further downfield than any other table carbene.^{[citation needed]}

Upon coordination to metal centers, the¹³C carbene resonance usually shifts highfield, depending on the Lewis acidity of the complex fragment. Based on this observation, Huynhet al. developed a new methodology to determine ligand donor strengths by¹³C NMR analysis oftrans-palladium(II)-carbene complexes. The use of a¹³C-labeled N-heterocyclic carbene ligand also allows for the study of mixed carbene-phosphine complexes, which undergotrans-cis-isomerization due to thetrans effect.^[69]

In academia, NHCs are widely-usedancillary ligands. They are components of theruthenium-basedGrubbs' catalyst forolefin metathesis, which have been intensively investigated.NHC-Palladium Complexes catalyze cross-coupling reactions.^[70][71][72]

Ag(I)-NHC complexes have been widely tested for their biological applications.^[73]

NHCs are often stronglybasic (thepKa value of theconjugate acid of an imidazol-2-ylidene was measured at ca. 24)^[52] and react withoxygen. Their synthesis, then must beperformed free of air and compounds of even moderateacidity. Conversely, provided rigorously dry, relatively non-acidic and air-free materials are used, stable carbenes are reasonably robust to handlingper se.

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, requiringcryptands orcrown ethers.

Alternate techniques have been developed to avoid such purification difficulties.

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

Imidazol-2-ylidenes and dihydroimidazol-2-ylidenes, such asIMes, have been prepared by the deprotonation of the respectiveimidazolium andimidazolinium salts. Acyclic carbenes^[25][28] and tetrahydropyrimidinyl-based carbenes^[37] were prepared by deprotonation using strong homogeneous bases.

However, the reaction depends on the correct choice of base. Although imidazolium salt precursors are stable tonucleophilic addition, other non-aromatic salts (i.e.formamidinium salts) are not.^[74] In these cases, strong unhindered nucleophiles are avoided whether they are generated insitu or are present as an impurity in other reagents (such as LiOH in BuLi).

Alkyllithiums are unreliable bases for the reaction,^[19] because they are too nucleophilic and often act ashydridic reductants:

In principle,sodium orpotassium hydride^[24][30] would be the ideal base for deprotonating these precursor salts, but in practice thesalt dissolves too slowly for effective reaction.DMSO ort-BuOH catalyze the reaction through the soluble tert-butoxide ordimsyl anion bases,^[19][23] but those compounds are too nucleophilic for non-aromatic carbenes. Deprotonation withsodium orpotassium hydride in a mixture of liquidammonia/THF at −40 °C has been reported^[39] for imidazole-based carbenes, and Arduengo and coworkers^[30] 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.^[23]

Lithium amides like thediisopropylamide (LDA) andtetramethylpiperidide (LiTMP)^[25][28] generally work well for the deprotonation of all types of salts, providing that not too muchLiOH impurity is present. Metalhexamethyldisilazides^[37] deprotonate almost all salts cleanly, except for unhindered formamidinium salts, where this base can act as a nucleophile to give a triaminomethane adduct.

For carbenes stable at elevated temperatures, a rare approachdesulfurizesthioureas inTHF with moltenpotassium:^[26][75]

A contributing factor to the reaction's success is that thepotassium sulfide byproduct is insoluble in the solvent.^{[citation needed]}

A single example ofdeoxygenating aurea with afluorene derived carbene to give the tetramethyldiaminocarbene and fluorenone has also been reported:^[76]

Bis(trimethylsilyl)mercury (CH₃)₃Si-Hg-Si(CH₃)₃ reacts with chloro-iminium and chloro-amidinium salts to give a metal-free carbene and elementalmercury.^[77] For example:

(CH₃)₃Si−Hg−Si(CH₃)₃ + R₂N=C(Cl)−NR⁺
₂Cl⁻ → R₂N−C−NR₂ + Hg_(l) + 2(CH₃)₃SiCl

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 byvacuum pyrolysis of adductsA was used by Wanzlick^[8] in his early attempts to prepare dihydroimidazol-2-ylidenes but this method is not widely used. The Enders laboratory has used vacuum pyrolysis of adductB to generate a triazol-5-ylidene:^[49]

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 membranesyringe filter.

Recrystallisation of stable carbenes is difficult, because stable carbenes are readily soluble in non-polar solvents, and polar solvents are insuitably acidic.

Air-freesublimation purifies effectively, even givingmonocrystals suitable for X-ray analysis. However, strong complexation to metal ions likelithium will in most cases prevent sublimation. Also, the process must be performed at high vacuum, as persistent carbenes decompose above 60 °C.

Reviews on persistent carbenes:

• Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. (2014). "An Overview of N-Heterocyclic Carbenes".Nature.510 (7506):485–496.Bibcode:2014Natur.510..485H.doi:10.1038/nature13384.PMID 24965649.S2CID 672379..
• Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, (Chapter 4, Hideo Tomioka (triplet state); Chapter 5 (singlet state), Roger W. Alder) - ed. Guy Bertrand
• 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
• M. Regitz (1996). "Stable Carbenes—Illusion or Reality?".Angew. Chem. Int. Ed.30 (6):674–676.doi:10.1002/anie.199106741.

For a review on the physico-chemical properties (electronics, sterics, ...) of N-heterocyclic carbenes:

• T. Dröge; F. Glorius (2010). "The Measure of All Rings - N-Heterocyclic Carbenes".Angew. Chem. Int. Ed.49 (39):6940–6952.Bibcode:2010ACIE...49.6940D.doi:10.1002/anie.201001865.PMID 20715233.

• Functional groups
• Carbenes
• Organometallic chemistry
• Organic compounds

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