[[File:Imidazol-2-ylidene.jpg|thumb|A stable carbene: isolated 1,3-dimesitylimidazol-2-ylidene in a [[Schlenk flask]] ([[stir bar]] also present).]] A '''persistent carbene''' (also known as '''stable carbene''' or '''Arduengo carbene''') is a type of [[carbene]] demonstrating particular stability. The best-known examples are [[diaminocarbene]]s with the general formula (R2N)2C''':''', where the 'R's are various [[functional group]]s. The groups can be bridged so that the carbon with unfilled orbitals is part of an [[heterocyclic compound|heterocycle]], such as [[imidazole]] or [[triazole]]. Carbenes have long been known as very reactive and short lived [[molecules]] that could not be isolated, and were usually studied by observing the reactions they undergo. Stable carbenes had been proposed to exist by [[Ronald Breslow|R. Breslow]] in 1957, {{cite journal |title = Mechanism of Thiamine Action: Participation of a Thiazolium Zwitterion |author = [[Ronald Breslow]] |journal = Chemistry and Industry |volume = 26 |pages = 893 |year = 1957}} {{cite journal |title = Rapid Deuterium Exchange in Thiazolium Salts |author = R. Breslow |journal = [[Journal of the American Chemical Society]] |volume = 79 |issue = 7 |pages = 1762–1763 |year = 1957 |doi = 10.1021/ja01564a064}} and the first examples of stable carbenes coordinated to metal atoms were synthesized by [[Hans-Werner Wanzlick|H.-W. Wanzlick]] and collaborators {{cite journal |title = Ein neuer Zugang zur Carben-Chemie |author = Hans-Werner Wanzlick and E. Schikora |journal = [[Angewandte Chemie]] |volume = 72 |issue = 14 |pages = 494 |year = 1960 |doi = 10.1002/ange.19600721409}} {{cite journal |title = Ein nucleophiles Carben |author = H. W. Wanzlick and E. Schikora |journal = [[Chemische Berichte]] |volume = 94 |issue = 9 |pages = 2389–2393 |year = 1960 |doi = 10.1002/cber.19610940905}} . The isolation of a stable liquid dicarbene was reported in 1989 by [[Guy Bertrand (chemist)|G. Bertrand]] and others. {{cite journal |title = Analogous α,α'-bis-carbenoid, triply bonded species: synthesis of a stable λ3-phosphino carbene-λ3-phosphaacetylene |author = A. Igau, H. Grutzmacher, A. Baceiredo, G. Bertrand |journal = [[J. Am. Chem. Soc.]] |volume = 110 |pages = 6463–6466 |year = 1988 |doi = 10.1021/ja00227a028}} {{cite journal |title = λ3-Phosphinocarbenes λ5-phosphaacetylenes |author = G. Bertrand, R. Reed |journal =Coordination Chemistry Reviews |volume = 137 |pages = 323–355 |year = 1994 |doi = 10.1016/0010-8545(94)03005-B}} In 1991, the group of [[Anthony Joseph Arduengo III|A. Arduengo]] reported the synthesis of a stable, isolated, crystalline carbene. {{cite journal |author = A. J. Arduengo, R. L. Harlow and M. Kline |title = A stable crystalline carbene |year = 1991 |journal = [[J. Am. Chem. Soc.]] |volume = 113 |issue = 1 |pages = 361–363 |doi = 10.1021/ja00001a054}} Persistent carbenes are still fairly reactive substances, and many will undergo [[dimerisation]], sometimes reversibly. Persistent carbenes can exist in the [[singlet state]] or the [[triplet state]], with the singlet state carbenes being more stable. The relative stability of these compounds is only partly due to [[steric hindrance]] by bulky groups. Some singlet carbenes are [[thermodynamics|thermodynamically stable]] in the absence of moisture and (in most cases) [[oxygen]], and can be isolated and indefinitely stored. Others are not thermodynamically stable and will dimerise slowly over days. The less stable triplet state carbenes have [[half-lives]] measured in seconds, and therefore can be observed but not stored. ==History== ===Conjectures=== In 1957, Breslow proposed that a relatively stable [[nucleophilic]] carbene, a [[thiazol-2-ylidene]] derivative, was involved in the [[catalytic cycle]] of [[vitamin B1]] (thiamine) that yields [[furoin]] from [[furfural]]. In this cycle, the vitamin's [[thiazolium]] ring exchanges an hydrogen atom (attached to carbon 2 of the ring) for a furfural residue. Through a [[deuterium]] exchange experiment, Breslow demonstrated that under standard reaction conditions (in [[heavy water|deuterated water]]) the C2-[[proton]] was rapidly exchanged for a [[deuteron]] in a statistical [[chemical equilibrium|equilibrium]]. [[Image:breslow2.png|center|thumb|600px|Deuterium exchange of the C2-proton of thiazolium salt]] This confirmed that the C2-proton was readily removed, and Breslow claimed that the exchange occurred through the generation of a stable thiazol-2-ylidene intermediate. This was the first example of a stable carbene being implicated in a [[reaction mechanism]]. In 1960, H.-P. Wanzlick and co-workers 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]]. {{cite journal|author = H. W. Wanzlick|journal = [[Angew. Chem., Int. Ed. Engl.]]|year = 1962|volume = 1|pages = 75|doi = 10.1002/anie.196200751|title = Aspects of Nucleophilic Carbene Chemistry}} They conjectured that the carbene existed in an unfavourable equilibrium with its corresponding [[dimer]] (a [[tetraaminoethylene]] derivative), in the so-called [[Wanzlick equilibrium]]. This conjecture was chellenged by Lemal and co-workers in 1964, who presented evidence that the dimer did not dissociate; {{cite journal|author = D. M. Lemal, R. A. Lovald, and K. I. Kawano|title = Tetraaminoethylenes. The Question of Dissociation|journal = [[J. Am. Chem. Soc.]]|year = 1964|volume = 86|pages = 2518|doi = 10.1021/ja01066a044}} and also by Winberg in 1965. {{cite journal|author = H. E. Winberg, J. E. Carnahan, D. D. Coffman, and M. Brown|title = Tetraaminoethylenes|journal = [[J. Am. Chem. Soc.]]|year = 1965|volume = 87|pages = 2055|doi = 10.1021/ja01087a040}} However, subsequent experiments by [[Michael K. Denk|M. Denk]], [[W. A. Herrmann]] and others have confirmed the reality of the equilibrium, in specific circumstances. Denk, M. K., Hatano. K., Ma, M. (1999), ''Nucleophilic Carbenes and the Wanzlick Equilibrium A Reinvestigation''. Tetrahedron Letters, volume 40, 2057-2060. {{DOI|doi:10.1016/S0040-4039(99)00164-1}} Volker P. W. Böhm, Wolfgang A. Herrmann (2000), ''The Wanzlick Equilibrium'' [[Angewandte Chemie]] volume 39, issue 22, pages 4036-4038 {{DOI|10.1002/1521-3773(20001117)39:22<4036::AID-ANIE4036>3.0.CO;2-L}} ===First synthesis=== In 1970, Wanzlick's group prepared the first imidazol-2-ylidene carbene, by the deprotonation of an [[imidazolium]] salt. {{cite journal|author = H. W. Wanzlick and H. J. Schonherr|journal = [[Liebigs Ann. Chem.]]|year = 1970|volume = 731|pages = 176|doi = 10.1002/jlac.19707310121|title = Chemie nucleophiler Carbene, XVIII1) 1.3.4.5-Tetraphenyl-imidazoliumperchlorat}} Wanzlick, as well as Hoffmann, {{cite journal|author = R. Gleiter and R. Hoffmann|title = Stabilizing a singlet methylene|journal = [[J. Am. Chem. Soc.]]|year = 1968|volume = 90|pages = 5457|doi = 10.1021/ja01022a023}} believed that these imidazole-based carbenes, with a [[Hückel's rule|4n+2]] π-electron ring system, should be more stable than the 4,5-dihydro analogues, due to Hückel-type [[aromaticity]]. The carbenes were not isolated, but obtained as [[coordination compound]]s with [[mercury (element)|mercury]] and [[isothiocyanate]]: [[Image:Wanzlick2.png|center|thumb|600px|Preparation and trapping of an imidazol-2-ylidene]] In 1988, G. Bertrand and others isolated a red oil, the molecular structure of which can be represented as either a λ3-[[phosphinocarbene]] or λ5-[[phosphaacetylene]]: [[Image:Bertrand1.png|center|thumb|600px|Alkyne and carbene resonances structures of Bertrand’s carbene.]] These molecules, nicknamed "push-pull carbenes" in reference to the contrasting electron affinities of the phosphorus and silicon atoms, exhibit both carbenic and [[alkyne|alkynic]] reactivity. An X-ray structure of this molecule has not been obtained and at the time of publication some doubt remained as to their exact carbenic nature. In 1991, a stable, isolated, and crystalline dicarbene was finally obtained by A. Arduengo and co-workers, by [[deprotonation]] of an imidazolium chloride with a strong base: [[Image:Arduengo2.png|center|thumb|600px|Preparation of N,N'-diadamantyl-imidazol-2-ylidene]] This carbene, the forerunner of a large family of carbenes with the imidazol-2-ylidene core, was found to be indefinitely stable at room temperature (in the absence of oxygen and moisture), and melted at 240–241 °C without decomposition. Another interesting chemical property of this molecule was a characteristic resonance in the 13C [[NMR]] spectrum at 211 ppm for the carbenic atom. The [[X-ray diffraction|X-ray]] structure {{cite journal|url=http://home.att.net/~ajarduengo/RotateStructures/0153Viewer.html|title=1,3-diadamantylimidazol-2-ylidene|author=A. J. Arduengo ''et al.''|journal=Am. Chem. Soc.|year=1991|volume=113|page=361}} revealed longer N–C [[bond length]]s in the ring of the carbene than in the parent imidazolium compound, indicating that there was very little [[double bond]] character to these bonds. The first air-stable carbene, a chlorinated member of the imidazol-2-ylidene family, was obtained in 1997. In 2000, Bertrand obtained additional carbenes of the phosphanyl type, including (phosphanyl)(trifluoromethyl)carbene, stable in solution at – 30° Christophe Buron, Heinz Gornitzka, Vadim Romanenko, Guy Bertrand (2000), ''Stable Versions of Transient Push-Pull Carbenes: Extending Lifetimes from Nanoseconds to Weeks.'' Science, Vol. 288. no. 5467, pp. 834 - 836. {{doi|10.1126/science.288.5467.834}} and a moderately stable (amino)(aryl)carbene with only one heteroatom adjacent to the carbenic atom. Stéphane Solé, Heinz Gornitzka, Wolfgang W. Schoeller, Didier Bourissou, Guy Bertrand (2001): ''(Amino)(Aryl)Carbenes: Stable Singlet Carbenes Featuring a Spectator Substituent''. Science, Vol. 292. no. 5523, pp. 1901 - 1903. {{doi|10.1126/science.292.5523.1901}} Chun-Liang Lai, Wen-Hsin Guo, Ming-Tsung Lee and Ching-Han Hu (2005), ''Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study'' Journal of Organometallic Chemistry, Volume 690, Issues 24-25, Pages 5867-5875. {{doi|10.1016/j.jorganchem.2005.07.058}} ===Understanding their stability=== [[Image:Arduengo3.svg|right|thumb|160px|Dimethyl imidazol-2-ylidene, a stable dicarbene with little steric hindrance.]] Initially many researchers believed that the unique stability of this carbene was due to the bulky N-[[adamantyl]] substituents, which prevented the carbene from dimerising due to [[steric hindrance]]. However, Arduengo's group later obtained an imidazol-2-ylidene in which the bulky N-adamantyl groups were replaced with smaller [[methyl]] groups. This showed that [[steric hindrance]] was not the predominant stabilising factor, and that imidazole-2-ylidenes were [[thermodynamic reaction control|thermodynamically stable]]. [[Image:Arduengo4.svg|left|thumb|160px|1,3-Dimesityl-imidazol-4,5-dihydro-2-ylidene, a stable carbene without the aromatic imidazolium ring.
([http://home.att.net/~ajarduengo/RotateStructures/0159Viewer.html external viewer])]] It had been also conjectured that the double bond between carbons 4 and 5 of the imidazolium ring backbone, which gave [[aromaticity|aromatic]] character to that system, was important for the carbene's stability. This conjecture was disproved in 1995 by Arduengo's group, who obtained a derivative [[dihydroimidazol-2-ylidene]] without that double bond. The thermodynamical stability in this compound, and the role of steric protection in preventing dimerisation, has been a topic of some dispute.. [[Image:Alder1.svg|right|thumb|160px|Bis(diisopropylamino) carbene, the first acyclic stable carbene.]] In 1996, Alder and others synthesized the first acyclic persistent carbene, thus showing that a cyclic backbone was not necessary for their stability. Unlike previous examples, this molecule allowed rotation around the bonds of the carbenic atom. By measuring the [[bond rotation barrier|barrier to rotation]] of these bonds, the extent of their [[double bond]] character could be measured, and the [[ylide|ylidic]] nature of this carbene could be determined. Like the cyclic diaminocarbenes, unhindered variants tend to dimerise. Until 1997, all stable carbenes known had two nitrogen atoms bound to the carbenic atom. This pattern was broken in 1997–1998 with the sythesis of a thiazol-2-ylidene derivative by Arduengo's group and an aminothiocarbene and an aminooxycarbene by Alder's group,. In these stable compounds, the carbenic atom lies between a nitrogen atom and either a [[sulfur]] or oxygen atom: [[Image:Alder2.png|center|thumb|600px|Stable carbenes with oxygen or sulfur atoms bound to the carbenic atom. ([http://home.att.net/~ajarduengo/RotateStructures/0165Viewer.html External viewer])]] However, these carbenes are not thermodynamically stable as decomposition and dimerisation have been observed for unhindered examples. A more radical development was the synthesis in 2006 of [[bis(diisopropylamino)cyclopropenylidene]] by Bertrand's group. In this compound, stable at room temperature, the carbene atom is connected to two carbon atoms, in a three-member ring that retains the aromaticity and geometry of the [[cyclopropenylidene]] ring. This example demonstrated that the presence of heteroatoms next to the carbene is not necessary for stability, either. ==Classes of stable carbenes== The following are examples of the classes of stable carbenes isolated to date: ===Imidazol-2-ylidenes=== The first stable carbenes to be isolated were based on an [[imidazole]] ring, with the hydrogen in carbon 2 of the ring (between the two nitrogen atoms) removed, and other hydrogens replaced by various groups. These [[imidazol-2-ylidene]]s are still the most stable and the most well studied and understood family of persistent carbenes. A considerable range of imidazol-2-ylidenes have been synthesised, including those in which the 1,3-positions have been functionalised with [[alkyl]], [[aryl]], {{cite journal|author = A. J. Arduengo, H. V. R. Dias, R. L. Harlow, and M. Kline|title = Electronic stabilization of nucleophilic carbenes|journal = [[J. Am. Chem. Soc.]]|year = 1992|volume = 114|pages = 5530|doi = 10.1021/ja00040a007}} alkyloxy, alkylamino, alkylphosphino {{cite journal|author = W. A. Herrmann, C. Kocher, L. J. Goossen, and G. R. J. Artus|journal = [[Chem. Eur. J.]]|year = 1996|volume = 2|pages = 1627|doi = 10.1002/chem.19960021222|title = Heterocyclic Carbenes: A High-Yielding Synthesis of Novel, Functionalized N-Heterocyclic Carbenes in Liquid Ammonia}} and even [[Chirality (chemistry)|chiral]] substituents: [[Image:Imidazol2ylidenes1.png|center|thumb|600px|Stable imidazol-2-ylidenes.]] [[Image:Arduengo5.svg|160px|right|thumb|1,3-Dimesityl-4,5-dichloroimidazol-2-ylidene, the first air-stable carbene.
([http://home.att.net/~ajarduengo/RotateStructures/0173Viewer.html external viewer])]] In particular, substitution of two [[chlorine]] atoms for the two hydrogens at ring positions 4 and 5 yielded the first air-stable carbene. {{cite journal|author = A. J. Arduengo, F. Davidson, H. V. R. Dias, J. R. Goerlich, D. Khasnis, W. J. Marshall, T. K. Prakasha|title = An Air Stable Carbene and Mixed Carbene "Dimers"|journal = [[J. Am. Chem. Soc.]]|year = 1997|issue = 119|pages = 12742|doi = 10.1021/ja973241o|volume = 119}} Its extra stability probably results from the [[electron-withdrawing group|electron-withdrawing]] effect of the [[chlorine]] atoms, which must reduce the [[electron density]] on the carbon atom bearing the [[lone pair]], via [[inductive effect|induction]] through the sigma-backbone. Molecules containing two and even three imidazol-2-ylidene groups have also been synthesised. {{cite journal|author = W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, and G. R. J. Artus|journal = [[Chem. Eur. J.]]|year = 1996|volume = 2|pages = 772|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}} {{cite journal|author = H. V. R. Dias and W. C. Jin|journal = [[Tetrahedron Lett.]]|year = 1994|volume = 35|pages = 1365|doi = 10.1016/S0040-4039(00)76219-8|title = A stable tridentate carbene ligand}} Imidazole-based carbenes are thermodynamically stable and generally have diagnostic 13C [[NMR]] chemical shift values between 210–230 ppm for the carbenic carbon. Typically, X-ray structures of these molecules show N-C-N bond angles of 101–102°. ===Triazol-5-ylidenes=== Another family of persistent carbenes are based on the [[triazole]] ring, with the unfilled orbitals in carbon 5 of this ring. The [[triazol-5-ylidene]]s pictured below were first prepared by Enders and co-workers {{cite journal|author = D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J. P. Melder, K. Ebel, and S. Brode|journal = [[Angew. Chem., Int. Ed. Engl.]]|year = 1995|volume = 34|pages = 1021|doi = 10.1002/anie.199510211|title = Preparation, Structure, and Reactivity of 1,3,4-Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene, a New Stable Carbene}} by [[vacuum pyrolysis]] through loss of methanol from 2-methoxytriazoles. Only a limited range of these molecules have been reported, with the triphenyl substituted molecule being commercially available. [[Image:Triazol5ylidenes.png|thumb|600px|center|Triazol-5-ylidenes]] [[Triazole]]-based carbenes are thermodynamically stable and have diagnostic 13C NMR chemical shift values between 210–220 ppm for the carbenic carbon. The X-ray structure of the triphenyl substituted carbene above shows an N-C-N bond angle of ca. 101°. The 5-methoxytriazole precursor to this carbene was made by the treatment of a triazolium salt with sodium methoxide, which attacks as a [[nucleophile]]. This may indicate that these carbenes are less aromatic than imidazol-2-ylidenes, as the imidazolium precursors do not react with nucleophiles due to the resultant loss of [[aromaticity]]. ===Other diaminocarbenes=== The two families above can be seen as special cases of a broader class of compounds which have a carbenic atom bridging two nitrogen atoms. A range of such [[diaminocarbenes]] have been prepared principally by [[Roger Alder]]'s research group. In some of these compounds, the N-C-N unit is a member of a 5 or 6 membered non-aromatic ring, {{cite journal|author = J. Arduengo, J. R. Goerlich, and W. J. Marshall|title = A stable diaminocarbene|journal = [[J. Am. Chem. Soc.]]|year = 1995|volume = 117|pages = 11027|doi = 10.1021/ja00149a034}} {{cite journal|author = M. K. Denk, A. Thadani, K. Hatano, and A. J. Lough|journal = [[Angew. Chem., Int. Ed. Engl.]]|year = 1997|volume = 36|pages = 2607|doi = 10.1002/anie.199726071|title = Steric Stabilization of Nucleophilic Carbenes}} {{cite journal|author = R. W. Alder, M. E. Blake, C. Bortolotti, S. Buffali, C. P. Butts, E. Lineham, J. M. Oliva, A. G. Orpen, and M. J. Quayle|journal = [[Chem. Commun.]]|year = 1999|pages = 241|doi = 10.1039/a808951e|title = Complexation of stable carbenes with alkali metals}} including a bicyclic example. In other examples, the adjacent nitrogens are connected only through the carbenic atom, and may or may not be part of separate rings. {{cite journal|author = R. W. Alder, P. R. Allen, M. Murray, and A. G. Orpen|journal = [[Angew. Chem., Int. Ed. Engl.]]|year = 1996|volume = 35|pages = 1121|doi = 10.1002/anie.199611211|title = Bis(diisopropylamino)carbene}} {{cite journal|author = R. W. Alder and M. E. Blake|journal = [[Chem. Commun.]]|year = 1997|pages = 1513|doi = 10.1039/a703610h|title = Bis(N-piperidyl)carbene and its slow dimerisation to tetrakis(N-piperidyl)ethene}} {{cite journal|author = R. W. Alder, M. E. Blake, and J. M. Oliva|journal = [[J. Phys. Chem.]] a|title = Diaminocarbenes; Calculation of Barriers to Rotation about Ccarbene-N Bonds, Barriers to Dimerization, Proton Affinities, and 13C NMR Shifts|year = 1999|volume = 103|pages = 11200|doi = 10.1021/jp9934228}} [[Image:diaminocarbenes1.png|center|thumb|600px|Synthesised cyclic and acyclic diaminocarbenes]] Unlike the aromatic imidazol-2-ylidenes or triazol-5-ylidenes, these carbenes appear not to be thermodynamically stable, as shown by the dimerisation of some unhindered cyclic and acyclic examples. However more recent work by Alder suggests that these carbenes dimerise via acid catalysed dimerisation (as in the [[Wanzlick equilibrium]]). Diaminocarbenes have diagnostic 13C NMR chemical shift values between 230–270 ppm for the carbenic atom. The X-ray structure of dihydroimidazole-2-ylidene shows a N-C-N bond angle of ca. 106°, whilst the angle of the acyclic carbene is 121°, both greater than those seen for imidazol-2-ylidenes. ===Heteroamino carbenes=== There exist several variants of the stable carbenes above where one of the nitrogen atoms adjacent to the carbene center (the ''α'' nitrogens) has been replaced by an alternative heteroatom, such as oxygen, sulfur, or [[phosphorus]]. {{cite journal|author = A. J. Arduengo, J. R. Goerlich and W. J. Marshall|title = A Stable Thiazol-2-ylidene and Its Dimer|year = 1997|journal = [[Liebigs Annalen]]|volume = 1997|issue = 2|pages = 365–374|doi = 10.1002/jlac.199719970213}} {{cite journal|author = R. W. Alder, C. P. Butts, and A. G. Orpen|journal = [[J. Am. Chem. Soc.]]|year = 1998|volume = 120|pages = 11526|title = Stable Aminooxy- and Aminothiocarbenes|doi = 10.1021/ja9819312}} : [[Image:Heteroaminocarbenes1.png|center|thumb|600px|Synthesised heteroamino carbenes (top and bottom right) and Bertrand's carbenes (bottom left).]] In particular, the formal substitution of sulfur for one of the nitrogens in imidazole would yield the aromatic heterocyclic compound [[thiazole]]. A thiazole based carbene (analogous to the carbene postulated by Breslow) {{cite journal|author = R. Breslow|journal = [[J. Am. Chem. Soc.]]|year = 1957|volume = 79|pages = 1762|title = Rapid Deuterium Exchange in Thiazolium Salts|doi = 10.1021/ja01564a064}} has been prepared and characterised by X-ray crystallography. Other non-aromatic aminocarbenes with O, S and P atoms adjacent (i.e. alpha) to the carbene centre have been prepared, e.g. [[thioiminium|thio-]] and [[oxyiminium|oxy-iminium]] based carbenes have been characterised by X-ray crystallography. Since [[oxygen]] and sulfur are [[divalent]], [[steric]] protection of the carbenic centre is limited especially when the N-C-X unit is part of a ring. These acyclic carbenes have diagnostic 13C NMR chemical shift values between 250–300 ppm for the carbenic carbon, further downfield than any other types of stable carbene. X-ray structures have show N-C-X bond angles of ca. 104 ° and 109 ° respectively. ===Non-amino carbenes=== Carbenes that formally derive from imidazole-2-ylidenes by substitution of sulfur, oxygen, or other [[chalcogen]]s for ''both'' ''α''-nitrogens are expected to be unstable, as they have the potential to dissociate into an [[alkyne]] (R1C≡CR2) and a carbon [[chalcogenide|dichalcogenide]] (X1=C=X2).{{citation needed|date=December 2009}} [[Image:DecompositionHeteroaminocarbenes.png|center|thumb|600px|A possible decomposition pathway for aromatic N-C-X (X = O, S) substituted carbenes]] On the other hand, the reaction of [[carbon disulfide]] CS2 with electron deficient [[acetylene]] derivatives gives transient [[1,3-dithiolium]] carbenes (i.e. where X1 = X2 = S) which then dimerise. Thus it is possible that the reverse of this process might be occurring in similar carbenes. {{cite journal|author = H. D. Haztzler|title = Nucleophilic 1,3-dithiolium carbenes|journal = [[J. Am. Chem. Soc.]]|year = 1970|volume = 92|pages = 1412|doi = 10.1021/ja00708a058}} {{cite journal|author = H. D. Hartzler|title = 1,3-Dithiolium carbenes from acetylenes and carbon disulfide|journal = [[J. Am. Chem. Soc.]]|year = 1972|volume = 95|pages = 4379|doi = 10.1021/ja00794a039}} ===Bertrand's carbenes=== In Bertrand's persistent carbenes, the unfilled carbon is bonded to a [[phosphorus]] and a [[silicon]]. {{cite journal|author = G. Bertrand, A. Igau, A. Baceiredo, and G. Trinquier|journal = [[Angew. Chem. Int. Ed. Engl.]]|year = 1989|volume = 28|pages = 621|doi = 10.1002/anie.198906211|title = [Bis(diisopropylamino)phosphino]trimethylsilylcarbene: A Stable Nucleophilic Carbene}} However, these compounds seem to exhibit some alkynic properties, and when published the exact carbenic nature of these red oils was in debate. ===Other nucleophilic carbenes=== One stable N-heterocyclic carbene{{cite journal|title = Stable Planar Six--Electron Six-Membered N-Heterocyclic Carbenes with Tunable Electronic Properties|author = Carsten Präsang, Bruno Donnadieu, and Guy Bertrand|journal = [[J. Am. Chem. Soc.]]|year = 2005|volume = 127|issue = 29|pages = 10182–10183|doi = 10.1021/ja052987g|pmid = 16028925|last1 = Präsang|first1 = C|last2 = Donnadieu|first2 = B|last3 = Bertrand|first3 = G}} has a structure analogous to [[borazine]] with one [[boron]] atom replaced by methylene. This results in a planar 6 electron compound. [[Image:Borazine carbene.gif|center|thumb|600px|In the second step of this reaction sequence the proton is abstracted by [[Lithium tetramethylpiperidide|LiTMP]], two [[cyclohexane|cyclohexyl]] groups shield the carbene.]] ===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]] Vincent Lavallo, Yves Canac, Bruno Donnadieu, Wolfgang W. Schoeller, Guy Bertrand (2006), ''Cyclopropenylidenes: From Interstellar Space to an Isolated Derivative in the Laboratory ''. Science, volume 312, issue 5774, pp. 722–724. {{doi|0.1126/science.1126675}}. ===Triplet state carbenes=== In 2001, [[Hideo Tomioka]] and his associates were able to produce a comparatively stable triplet carbene ([[bis(9-anthryl)carbene]], with a half-life of 19 minutes), by taking advantage of [[resonance (chemistry)|resonance]]. {{cite journal|journal = [[Nature (journal)|Nature]]|volume = 412|pages = 626|year = 2001|doi = 10.1038/35088038|author = Hideo Tomioka, Eri Iwamoto, Hidetaka Itakura and Katsuyuki Hirai|title = Generation and characterization of a fairly stable triplet carbene|pmid = 11493917|last1 = Tomioka|first1 = H|last2 = Iwamoto|first2 = E|last3 = Itakura|first3 = H|last4 = Hirai|first4 = K|issue = 6847}} {{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}} In 2006 the same group reported a triplet carbene with a [[half-life]] of 40 minutes. {{cite journal|title = Triplet Diphenylcarbenes Protected by [[Trifluoromethyl]] and [[Bromine]] Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature|author = Tetsuji Itoh, Yoshimaru Nakata, Katsuyuki Hirai, Hideo Tomioka|journal = [[J. Am. Chem. Soc.]]|year = 2006|volume = 128|issue = 3|pages = 957–967|doi = 10.1021/ja056575j|pmid = 16417387|last1 = Itoh|first1 = T|last2 = Nakata|first2 = Y|last3 = Hirai|first3 = K|last4 = Tomioka|first4 = H}} This carbene is prepared by a [[photochemistry|photochemical]] [[chemical decomposition|decomposition]] of a [[diazomethane]] with expulsion of [[nitrogen]] gas at a [[wavelength]] of 300 [[nanometer]]s in benzene. [[Image:Persistent triplet carbene.png|center|thumb|600px|A persistent triplet carbene (right), synthesised by Itoh (2006]).]] Exposure to oxygen (diradical) converts this carbene to the corresponding [[benzophenone]] and the diphenylmethane compound is formed when it is trapped by [[1,4-cyclohexadiene]]. As with the other carbenes this species contains large bulky substituents, namely [[bromine]] and the trifluoromethyl groups, that shield the carbene and prevent or slow down the process of dimerisation to a 1,1,2,2-tetra(phenyl)alkene. Based on [[in silico|computer simulations]], the [[bond length|distance]] of the divalent carbon atom to its neighbours 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°). ==Chemical properties== ===Basicity and nucleophilicity=== The nucleophilicity and basicity of imidazol-2-ylidenes have been studied by Alder ''et al.'' who revealed that these molecules are strong bases, having a [[pKa]] of ca. 24 for the conjugate acid in [[Dimethyl sulfoxide]] (DMSO): [[Image:imidazol2ylidene pka.png|center|thumb|600px|Measurement of the pKa value for the conjugate acid of an imidazol-2-ylidene.]] However, further work by Alder has shown that diaminocarbenes will deprotonate the DMSO solvent, with the resulting anion reacting with the resulting amidinium salt. [[Image:AlderTIPcarbeneDMSO.png|center|thumb|600px|Using [[D6-DMSO]] as an NMR solvent can have unexpected results.]] Reaction of imidazol-2-ylidenes with [[1-bromohexane]] gave 90% of the 2-substituted adduct, with only 10% of the corresponding [[alkene]], indicating that these molecules are also reasonably [[nucleophilic]]. ===Dimerisation=== At one time, stable carbenes were thought to reversibly [[dimer]]ise through the so-called [[Wanzlick equilibrium]]. However, imidazol-2-ylidenes and triazol-5-ylidenes are thermodynamically stable and do not dimerise, and have been stored in [[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. In fact imidazol-2-ylidenes are so thermodynamically stable that only in highly constrained conditions are these carbenes forced to dimerise. Chen and Taton {{cite journal|author = T. A. Taton and P. Chen|journal = [[Angew. Chem., Int. Ed. Engl.]]|year = 1996|volume = 35|pages = 1011|doi = 10.1002/anie.199610111|title = A Stable Tetraazafulvalene}} made a doubly-tethered diimidazol-2-ylidene by [[deprotonating]] the respective diimidazolium salt. Only the deprotonation of the doubly-tethered diimidazolium salt with the shorter [[methylene]] (-CH2-) linkage resulted in the dicarbene dimer: [[Image:Chen dimer.png|center|thumb|300px|Dimerisation of tethered diimidazol-2-ylidenes.]] If this dimer existed as a dicarbene, the electron [[lone pair]]s on the carbenic carbon would be forced into close proximity. Presumably the resulting repulsive [[electrostatic]] interactions would have a significant destabilising effect. To avoid this electronic interaction, the [[carbene]] units dimerise. On the other hand, heteroamino carbenes (''e.g.'' R2N-C:-OR or R2N-C:-SR) and non-aromatic carbenes such as diaminocarbenes (''e.g.'' R2N-C:-NR2) have been shown to dimerise, R. W. Alder, L. Chaker, J. N. Harvey, F. P. V. Paolini, and J. Schütz (2004), ''When and How Do Diaminocarbenes Dimerize?'' Angewandte Chemie Int. Ed. Engl., volume = 43, issue = 44, pages 5896–5911 {{doi|10.1002/anie.200400654}} {{pmid|15457494}} albeit quite slowly. This has been presumed to be due to the high barrier to [[singlet state]] dimerisation: [[Image:diaminocarbene dimer.png|center|thumb|600px|"Least motion" (path A - not allowed) and "non-least motion" (path B) routes of carbene dimerisation.]] However, more recent work by Alder Roger W. Alder, Leila Chaker and François P. V. Paolini (2004), ''Bis(diethylamino)carbene and the mechanism of dimerisation for simple diaminocarbenes''. Chemical Communications, issue 19, pages {{doi|10.1039/b409112d}}{{pmid|15467857}} suggests that diaminocarbenes do not truly dimerise, but rather form the dimer by reaction via [[formamidinium]] salts, a protonated precursor species Accordingly, this reaction can be acid catalysed. This reaction occurs because unlike imidazolium based carbenes, there is no loss of aromaticity in protonation of the carbene. Unlike the dimerisation of [[triplet state]] carbenes, these [[singlet state]] carbenes do not approach head to head ("least motion"), but rather the carbene [[lone pair]] attacks the empty carbon [[p-orbital]] ("non-least motion"). Carbene dimerisation can also be acid or metal catalysed, and so care must be taken when determining if the carbene is undergoing true dimerisation. ===Reactivity=== The chemistry of stable carbenes has not been fully explored. However, Enders ''et al.'' D. Enders, K. Breuer, J. Runsink, and J. H. Teles (1996), Liebigs Ann. Chem., page 2019. D. Enders, K. Breuer, J. H. Teles, and K. Ebel (1997), ''Journal Fur Praktische Chemie-Chemiker-Zeitung'' volume 339, page 397. have performed a range of organic reactions involving a triazol-5-ylidene. These reactions are outlined below and may be considered as a model for other carbenes. [[Image:triazol5ylidene reactions.png|center|thumb|600px|Reactions of triazol-5-ylidene.]] [[Image:triazol5ylidene reactions text.png|center|600px|]] These carbenes tend to behave in a [[nucleophilic]] fashion ('''e''' and '''f'''), performing [[insertion reaction]]s ('''b'''), [[addition reaction]]s ('''c'''), [2+1] [[cycloaddition]]s ('''d''', '''g''' and '''h'''), [4+1] [[cycloaddition]]s ('''a''') as well as simple [[deprotonation]]s. The [[insertion reaction]]s ('''b''') probably proceed via [[deprotonation]], resulting in the generation of a nucleophile (-XR) which can attack the generated salt giving the impression of a H-X insertion. Care must be taken to check that a stable carbene is truly stable. The discovery of a stable [[isothiazole]] carbene ('''2''') from an isothiazolium perchlorate ('''1''') by one research group Janine Wolf, Winfried Böhlmann, Matthias Findeisen, Thomas Gelbrich, Hans-Jorg Hofmann, and Borbel Schulze (2007), ''Synthesis of Stable Isothiazole Carbenes''. Angewandte Chemie Int. Ed., volume 46, issue 17, pages 3118–3121 {{doi|10.1002/anie.200604305}} {{pmid|17372997}} was questioned by another group Alan DeHope, Vincent Lavallo, Bruno Donnadieu, Wolfgang W. Schoeller, and Guy Bertrand (2007), ''Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality''. Angewandte Chemie Int. Ed., volume 46, issue 36, pages 6922–6925, {{doi|10.1002/anie.200702272}}{{pmid|17661300}} who were only able to isolate ''2-imino-2H-thiete'' ('''4'''). The intermediate '''3''' was proposed through a [[rearrangement reaction]]. This carbene is no longer considered stable. Janine Wolf, Winfried Böhlmann, Matthias Findeisen, Thomas Gelbrich, Hans-Jorg Hofmann, and Borbel Schulze (2007), ''Reply to "Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality"''. Angewandte Chemie Int. Ed., volume 46, page 6926 {{doi|10.1002/anie.200702746}} [[Image:IsothiazoleCarbene.png|400px|center|Isothiazole carbene DeHope 2007]] ===Carbene complexation=== Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) have been shown to co-ordinate to a plethora of elements, from [[alkali metals]], [[main group element]]s, [[transition metal]]s and even [[lanthanides]] and [[actinides]]. A [[periodic table]] of elements gives some idea of the complexes which have been prepared, and in many cases these have been identified by single crystal [[X-ray crystallography]]. {{cite journal|title = N-Heterocyclic Carbenes|journal = [[Angew. Chem., Int. Ed. Engl.]]|volume = 36|issue = 20|year = 1997|pages = 2162–2187|author = Wolfgang A. Herrmann, Christian Köcher|doi = 10.1002/anie.199721621}} {{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. Engl.]]|volume = 34|issue = 4|year = 1995|pages = 487–489|author = Gernot Boche, Christof Hilf, Klaus Harms, Michael Marsch, John C. W. Lohrenz|doi = 10.1002/anie.199504871}}
{| border="0" cellpadding="0" cellspacing="1" style="width:80%; {{{1|}}}" |- ! width="1.0%"|[[Periodic table group|Group]] → ! width="5.5%"|[[Alkali metal|1]] ! width="5.5%"|[[Alkaline earth metal|2]] ! width="5.5%"|[[Group 3 element|3]] ! width="5.5%"|[[Group 4 element|4]] ! width="5.5%"|[[Group 5 element|5]] ! width="5.5%"|[[Group 6 element|6]] ! width="5.5%"|[[Group 7 element|7]] ! width="5.5%"|[[Group 8 element|8]] ! width="5.5%"|[[Group 9 element|9]] ! width="5.5%"|[[Group 10 element|10]] ! width="5.5%"|[[Group 11 element|11]] ! width="5.5%"|[[Group 12 element|12]] ! width="5.5%"|[[Boron group|13]] ! width="5.5%"|[[Carbon group|14]] ! width="5.5%"|[[Nitrogen group|15]] ! width="5.5%"|[[Chalcogen|16]] ! width="5.5%"|[[Halogen|17]] ! width="5.5%"|[[Noble gas|18]] |- ! ↓ [[Periodic table period|Period]] | colspan="19"|
|- ! [[Period 1 element|1]] | {{element cell| 1|hydrogen |H |1.00794(7) |Gas |Nonmetals|Primordial}} | colspan="16"|
| {{element cell| 2|helium |He|4.002602(2) |Gas |Poor metals|Primordial}} |- ! [[Period 2 element|2]] | {{element cell| 3|lithium |Li|6.941(2) |Solid|Nonmetals|Primordial}} | {{element cell| 4|beryllium |Be|9.012182(3) |Solid|Nonmetals|Primordial}} | colspan="10"|
| {{element cell| 5|boron |B |10.811(7) |Solid|Nonmetals|Primordial}} | {{element cell| 6|carbon |C |12.0107(8) |Solid|Nonmetals|Primordial}} | {{element cell| 7|nitrogen |N |14.00674(7) |Gas |Nonmetals|Primordial}} | {{element cell| 8|oxygen |O |15.9994(3) |Gas |Poor metals|Primordial}} | {{element cell| 9|fluorine |F |18.9984032(5) |Gas |Poor metals|Primordial}} | {{element cell|10|neon |Ne|20.1797(6) |Gas |Poor metals|Primordial}} |- ! [[Period 3 element|3]] | {{element cell|11|sodium |Na|22.98976928(2)|Solid|Nonmetals|Primordial}} | {{element cell|12|magnesium |Mg|24.3050(6) |Solid|Nonmetals|Primordial}} | colspan="10"|
| {{element cell|13|aluminium |Al|26.9815386(8) |Solid|Nonmetals|Primordial}} | {{element cell|14|silicon |Si|28.0855(3) |Solid|Nonmetals|Primordial}} | {{element cell|15|phosphorus|P |30.973762(2) |Solid|Nonmetals|Primordial}} | {{element cell|16|sulfur |S |32.066(6) |Solid|Nonmetals|Primordial}} | {{element cell|17|chlorine |Cl|35.4527(9) |Gas |Poor metals|Primordial}} | {{element cell|18|argon |Ar|39.948(1) |Gas |Poor metals|Primordial}} |- ! [[Period 4 element|4]] | {{element cell|19|potassium |K |39.0983(1) |Solid|Nonmetals|Primordial}} | {{element cell|20|calcium |Ca|40.078(4) |Solid|Nonmetals|Primordial}} | {{element cell|21|scandium |Sc|44.955912(6) |Solid|Poor metals|Primordial}} | {{element cell|22|titanium |Ti|47.867(1) |Solid|Nonmetals|Primordial}} | {{element cell|23|vanadium |V |50.9415(1) |Solid|Nonmetals|Primordial}} | {{element cell|24|chromium |Cr|51.9961(6) |Solid|Nonmetals|Primordial}} | {{element cell|25|manganese |Mn|54.938045(5) |Solid|Nonmetals|Primordial}} | {{element cell|26|iron |Fe|55.845(2) |Solid|Nonmetals|Primordial}} | {{element cell|27|cobalt |Co|58.933195(5) |Solid|Nonmetals|Primordial}} | {{element cell|28|nickel |Ni|58.6934(2) |Solid|Nonmetals|Primordial}} | {{element cell|29|copper |Cu|63.546(3) |Solid|Nonmetals|Primordial}} | {{element cell|30|zinc |Zn|65.39(2) |Solid|Nonmetals|Primordial}} | {{element cell|31|gallium |Ga|69.723(1) |Solid|Nonmetals|Primordial}} | {{element cell|32|germanium |Ge|72.61(2) |Solid|Nonmetals|Primordial}} | {{element cell|33|arsenic |As|74.92160(2) |Solid|Poor metals|Primordial}} | {{element cell|34|selenium |Se|78.96(3) |Solid|Nonmetals|Primordial}} | {{element cell|35|bromine |Br|79.904(1) |Liquid|Poor metals|Primordial}} | {{element cell|36|krypton |Kr|83.80(1) |Gas |Poor metals|Primordial}} |- ! [[Period 5 element|5]] | {{element cell|37|rubidium |Rb|85.4678(3) |Solid|Poor metals|Primordial}} | {{element cell|38|strontium |Sr|87.62(1) |Solid|Nonmetals|Primordial}} | {{element cell|39|yttrium |Y |88.90585(2) |Solid|Nonmetals|Primordial}} | {{element cell|40|zirconium |Zr|91.224(2) |Solid|Nonmetals|Primordial}} | {{element cell|41|niobium |Nb|92.90638(2) |Solid|Nonmetals|Primordial}} | {{element cell|42|molybdenum|Mo|95.94(1) |Solid|Nonmetals|Primordial}} | {{element cell|43|technetium|Tc|[97.9072] |Solid|Poor metals|Synthetic}} | {{element cell|44|ruthenium |Ru|101.07(2) |Solid|Nonmetals|Primordial}} | {{element cell|45|rhodium |Rh|102.90550(2) |Solid|Nonmetals|Primordial}} | {{element cell|46|palladium |Pd|106.42(1) |Solid|Nonmetals|Primordial}} | {{element cell|47|silver |Ag|107.8682(2) |Solid|Nonmetals|Primordial}} | {{element cell|48|cadmium |Cd|112.411(8) |Solid|Poor metals|Primordial}} | {{element cell|49|indium |In|114.818(3) |Solid|Poor metals|Primordial}} | {{element cell|50|tin |Sn|118.710(7) |Solid|Nonmetals|Primordial}} | {{element cell|51|antimony |Sb|121.760(1) |Solid|Poor metals|Primordial}} | {{element cell|52|tellurium |Te|127.60(3) |Solid|Nonmetals|Primordial}} | {{element cell|53|iodine |I |126.90447(3) |Solid|Nonmetals|Primordial}} | {{element cell|54|xenon |Xe|131.29(2) |Gas |Poor metals|Primordial}} |- ! [[Period 6 element|6]] | {{element cell|55|caesium |Cs|132.9054519(2)|Solid|Poor metals|Primordial}} | {{element cell|56|barium |Ba|137.327(7) |Solid|Nonmetals|Primordial}} | {{element cell|*|lanthanoid |Lanthanoids||Solid|Poor metals|Undiscovered}} | {{element cell|72|hafnium |Hf|178.49(2) |Solid|Nonmetals|Primordial}} | {{element cell|73|tantalum |Ta|180.94788(2) |Solid|Nonmetals|Primordial}} | {{element cell|74|tungsten |W |183.84(1) |Solid|Nonmetals|Primordial}} | {{element cell|75|rhenium |Re|186.207(1) |Solid|Nonmetals|Primordial}} | {{element cell|76|osmium |Os|190.23(3) |Solid|Nonmetals|Primordial}} | {{element cell|77|iridium |Ir|192.217(3) |Solid|Nonmetals|Primordial}} | {{element cell|78|platinum |Pt|195.084(9) |Solid|Nonmetals|Primordial}} | {{element cell|79|gold |Au|196.966569(4) |Solid|Nonmetals|Primordial}} | {{element cell|80|mercury |Hg|200.59(2) |Liquid|Nonmetals|Primordial|mercury (element)}} | {{element cell|81|thallium |Tl|204.3833(2) |Solid|Poor metals|Primordial}} | {{element cell|82|lead |Pb|207.2(1) |Solid|Poor metals|Primordial}} | {{element cell|83|bismuth |Bi|208.98040(1) |Solid|Poor metals|Primordial}} | {{element cell|84|polonium |Po|[208.9824] |Solid|Poor metals|Natural radio}} | {{element cell|85|astatine |At|[209.9871] |Solid|Poor metals|Natural radio}} | {{element cell|86|radon |Rn|[222.0176] |Gas |Poor metals|Natural radio}} |- ! [[Period 7 element|7]] | {{element cell|87|francium |Fr|[223.0197] |Solid|Poor metals|Natural radio}} | {{element cell|88|radium |Ra|[226.0254] |Solid|Poor metals|Natural radio}} | {{element cell|**|actinoid |Actinoids||Solid|Poor metals|Undiscovered}} | {{element cell|104|rutherfordium|Rf|[263.1125] |Solid|Poor metals|Synthetic}} | {{element cell|105|dubnium |Db|[262.1144] |Solid|Poor metals|Synthetic}} | {{element cell|106|seaborgium |Sg|[266.1219] |Solid|Poor metals|Synthetic}} | {{element cell|107|bohrium |Bh|[264.1247] |Solid|Poor metals|Synthetic}} | {{element cell|108|hassium |Hs|[269.1341] |Solid|Poor metals|Synthetic}} | {{element cell|109|meitnerium |Mt|[268.1388] |Solid|Poor metals|Synthetic}} | {{element cell|110|darmstadtium |Ds|[272.1463] |Solid|Poor metals|Synthetic}} | {{element cell|111|roentgenium |Rg|[272.1535] |Solid|Poor metals|Synthetic}} | {{element cell|112|copernicium |Cn|[277] |Liquid|Poor metals|Synthetic}} | {{element cell|113|ununtrium |Uut|[284] |Solid|Poor metals|Synthetic}} | {{element cell|114|ununquadium |Uuq|[289] |Solid|Poor metals|Synthetic}} | {{element cell|115|ununpentium |Uup|[288] |Solid|Poor metals|Synthetic}} | {{element cell|116|ununhexium |Uuh|[292] |Solid|Poor metals|Synthetic}} | {{element cell|117|ununseptium |Uus|[291]‡ |Solid|Poor metals|Synthetic}} | {{element cell|118|ununoctium |Uuo|[293]‡ |Solid|Poor metals|Synthetic}} |- | colspan="21"|
|- | colspan="4" style="text-align:right"|*'''[[Lanthanoid]]s''' | {{element cell|57|lanthanum |La|138.90547(7)|Solid|Poor metals|Primordial}} | {{element cell|58|cerium |Ce|140.116(1) |Solid|Poor metals|Primordial}} | {{element cell|59|praseodymium|Pr|140.90765(2)|Solid|Poor metals|Primordial}} | {{element cell|60|neodymium |Nd|144.242(3) |Solid|Poor metals|Primordial}} | {{element cell|61|promethium |Pm|[144.9127] |Solid|Poor metals|Synthetic}} | {{element cell|62|samarium |Sm|150.36(2) |Solid|Nonmetals|Primordial}} | {{element cell|63|europium |Eu|151.964(1) |Solid|Nonmetals|Primordial}} | {{element cell|64|gadolinium |Gd|157.25(3) |Solid|Poor metals|Primordial}} | {{element cell|65|terbium |Tb|158.92535(2)|Solid|Poor metals|Primordial}} | {{element cell|66|dysprosium |Dy|162.500(1) |Solid|Poor metals|Primordial}} | {{element cell|67|holmium |Ho|164.93032(2)|Solid|Poor metals|Primordial}} | {{element cell|68|erbium |Er|167.259(3) |Solid|Poor metals|Primordial}} | {{element cell|69|thulium |Tm|168.93421(2)|Solid|Poor metals|Primordial}} | {{element cell|70|ytterbium |Yb|173.04(3) |Solid|Nonmetals|Primordial}} | {{element cell|71|lutetium |Lu|174.967(1) |Solid|Poor metals|Primordial}} |- | colspan="4" style="text-align:right"|**'''[[Actinoid]]s''' | {{element cell|89|actinium |Ac|[227.0277] |Solid|Poor metals|Natural radio}} | {{element cell|90|thorium |Th|232.03806(2)|Solid|Poor metals|Primordial}} | {{element cell|91|protactinium|Pa|231.03588(2)|Solid|Poor metals|Natural radio}} | {{element cell|92|uranium |U |238.02891(3)|Solid|Nonmetals|Primordial}} | {{element cell|93|neptunium |Np|[237.0482] |Solid|Poor metals|Natural radio}} | {{element cell|94|plutonium |Pu|[244.0642] |Solid|Poor metals|Natural radio}} | {{element cell|95|americium |Am|[243.0614] |Solid|Poor metals|Synthetic}} | {{element cell|96|curium |Cm|[247.0703] |Solid|Poor metals|Synthetic}} | {{element cell|97|berkelium |Bk|[247.0703] |Solid|Poor metals|Synthetic}} | {{element cell|98|californium |Cf|[251.0796] |Solid|Poor metals|Synthetic}} | {{element cell|99|einsteinium |Es|[252.0830] |Solid|Poor metals|Synthetic}} | {{element cell|100|fermium |Fm|[257.0951] |Solid|Poor metals|Synthetic}} | {{element cell|101|mendelevium|Md|[258.0984] |Solid|Poor metals|Synthetic}} | {{element cell|102|nobelium |No|[259.1011] |Solid|Poor metals|Synthetic}} | {{element cell|103|lawrencium |Lr|[262.110] |Solid|Poor metals|Synthetic}} |} *'''Green box''' = Carbene complex with element known. *'''Grey box''' = No carbene complex with element known. Figure: Periodic Table featuring elements that have formed stable carbenes complexes.
Stable carbenes are believed to behave in a similar fashion to [[organophosphine]]s in their co-ordination properties to metals. These [[ligand]]s are said to be good σ-donors through the carbenic [[lone pair]], but poor π-acceptors due to internal [[ligand]] [[back-donation]] from the [[nitrogen]] atoms adjacent to the carbene centre, and so are able to co-ordinate to even relatively electron deficient metals. Enders {{cite journal|author = D. Enders, H. Gielen, G. Raabe, J. Runsink, and J. H. Teles|journal = [[Chem. Ber.]]|year = 1996|volume = 129|pages = 1483|doi = 10.1002/cber.19961291213|title = Synthesis and Stereochemistry of the First Chiral (Imidazolinylidene)- and (Triazolinylidene)palladium(II) Complexes}} and Hermann {{cite journal|title = Metal Complexes of N-Heterocyclic Carbenes - A New Structural Principle for Catalysts in Homogeneous Catalysis|journal = [[Angew. Chem. Int. Ed. Engl.]]|volume 34|issue = 21|year = 1995|pages = 2371–2374|author = Wolfgang A. Herrmann, Martina Elison, Jakob Fischer, Christian Köcher, Georg R. J. Artus|doi = 10.1002/anie.199523711|volume = 34}} {{cite journal|title = Chiral Heterocylic Carbenes in Asymmetric Homogeneous Catalysis|journal = [[Angew. Chem. Int. Ed. Engl.]]|volume = 35|issue = 23-24|year = 1996|pages = 2805–2807|author = Wolfgang A. Herrmann, Lukas J. Goossen, Christian Köcher, Georg R. J. Artus|doi = 10.1002/anie.199628051}} have shown that these carbenes are suitable replacements for [[phosphine]] [[ligands]] in several [[catalytic cycles]]. Whilst they have found that these ligands do not activate the metal catalyst as much as phosphine ligands they often result in more robust catalysts. Several catalytic systems have been looked into by Hermann and Enders, using catalysts containing imidazole and triazole carbene ligands, with moderate success. Grubbs {{cite journal|author = M. Scholl, T. M. Trnka, J. P. Morgan, and R. H. Grubbs|journal = [[Tetrahedron Lett.]]|year = 1999|volume = 40|pages = 2247|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}} has reported replacing a phosphine ligand (PCy3) with an imidazol-2-ylidene in the [[olefin metathesis]] catalyst RuCl2(PCy3)2CHPh, and noted increased ring closing metathesis as well as exhibiting “a remarkable air and water stability”. Molecules containing two and three carbene moieties have been prepared as potential [[bidentate]] and [[tridentate]] carbene ligands. ===Carbenes in organometallic chemistry & catalysis=== Carbenes can be stabilised as [[organometallic chemistry|organometallic]] species. These [[transition metal carbene complex]]es fall into two categories: *[[Ernst Otto Fischer|Fischer]] carbenes in which carbenes are tethered to a metal and an [[electron-withdrawing group]] (usually a carbonyl), *[[Richard R. Schrock|Schrock]] carbenes; in which carbenes are tethered to a metal and an [[electron-donating group]]. The reactions that such carbenes participate in are very different from those in which organic carbenes participate. ===Triplet state carbene chemistry=== Persistent triplet state carbenes are likely to have very similar reactivity as other non-persistent triplet state [[carbenes]]. ==Physical properties== [[Image:Carbene 13C NMR.png|left|150px|Carbene peak in 13NMR]] 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. One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the 13C-[[NMR]] spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the 13C-[[NMR]] spectrum. An example is shown left for a cyclic diaminocarbene which has a carbenic peak at 238 ppm. A study of ligand donor strengths via 13C chemical shifts of palladium(II)-carbene complexes has been published by [[H. V. Huynh]] and others. The use of 13C-labeled carbene precursors afforded NMR data for phosphine complexes, which are known to isomerize from the ''trans''- to the ''cis''-configuration. {{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 ''et al.''| volume = 28|issue = 18|pages = 5395–5404}} ==Applications== [[Image:Grubbs'-2G-3D-balls.png|right|thumb|150px|A second generation Grubbs' catalyst.]] Some persistent carbenes are used as [[ligand|ancillary ligand]] in [[organometallic]] chemistry. Recently there have been practical application of these carbenes as metal [[ligands]] in catalysis, e.g. in the [[ruthenium]]-based [[Grubbs' catalyst]] and [[palladium]]-based catalysts for cross-coupling reactions. S. P. Nolan [editor] (2006). [http://books.google.com/books?id=9JOG1g9iFrwC&printsec=frontcover N-Heterocyclic carbenes in synthesis], Wiley-VCH ISBN 3527314008 F. Glorius [editor] (2007) [http://books.google.com/books?id=Fv2orfXKVnoC&pg=PP1 N-Heterocyclic carbenes in transition metal catalysis], Springer ISBN 3540369295 . ==Preparation methods== Stable carbenes are very reactive [[molecules]] and so it is important to consider the reaction conditions carefully when attempting to prepare these molecules. Stable carbenes are strongly [[Base (chemistry)|basic]] (the [[pKa]] value of the [[conjugate acid]] of an imidazol-2-ylidene was measured at ca. 24) {{cite journal|author = R. W. Alder, P. R. Allen, and S. J. Williams|journal = [[J. Chem. Soc., Chem. Commun.]]|year = 1995|pages = 1267|doi = 10.1039/c39950001267|title = Stable carbenes as strong bases}} and react with [[oxygen]]. Clearly these reactions must be performed under a dry, inert atmosphere, avoiding protic solvents or compounds of even moderate [[acidity]]. Furthermore, one must also consider the relative stability of the starting materials. Whilst imidazolium salts are stable to [[nucleophilic]] addition, other non-aromatic salts are not (i.e. [[formamidinium]] salts). {{cite journal|title = Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes|journal =Perkin Trans. 1|author = Roger W. Alder, Michael E. Blake, Simone Bufali, Craig P. Butts, A. Guy Orpen, Jan Schütz and Stuart J. Williams|year = 2001|pages = 1586–1593|doi = 10.1039/b104110j}} Consequently in these cases, strong unhindered nucleophiles must be avoided whether they are generated in ''situ'' or are present as an impurity in other reagents (e.g. LiOH in BuLi). Several approaches have been developed in order to prepare stable carbenes, these are outlined below. ===Deprotonation=== [[Deprotonation]] of carbene precursor salts with strong bases has proved a reliable route to almost all stable carbenes: [[Image:deprotonation1.png|center|thumb|600px|Deprotonation of precursor salts to give stable carbenes.]] Imidazol-2-ylidenes and dihydroimidazol-2-ylidenes, in particular, have been prepared by the deprotonation of the respective [[imidazolium]] and [[dihydroimidazolium]] salts. The acyclic carbenes and the tetrahydropyrimidinyl based carbenes were prepared by deprotonation using strong homogeneous bases. Several bases and reaction conditions have been employed with varying success. The degree of success has been principally dependent on the nature of the [[wiktionary:Precursor|precursor]] being deprotonated. The major drawback with this method of preparation is the problem of isolation of the free carbene from the metals ions used in their preparation. ====Metal hydride bases==== One might believe that sodium or [[potassium hydride]] would be the ideal base for deprotonating these precursor salts. The hydride should react irreversibly with the loss of [[hydrogen]] to give the desired carbene, with the [[inorganic]] by-products and excess hydride being removed by filtration. In practice this reaction is often too slow in suitable solvents (e.g. THF) due to the relative insolubility of the metal hydride and the salt. The addition of soluble "[[catalysts]]" ([[Dimethyl sulfoxide|DMSO]], [[Tert-Butanol|''t''-BuOH]]) considerably improves the rate of reaction of this heterogeneous system, via the generation of tert-butoxide or [[dimsyl sodium|dimsyl anion]]. However, these catalysts have proved ineffective for the preparation of non-imidazolium adducts as they tend to act as nucleophiles towards the precursor salts and in so doing are destroyed. The presence of [[hydroxide]] ions as an impurity in the metal hydride could also destroy non-aromatic salts. Deprotonation with [[sodium]] or [[potassium]] hydride in a mixture of liquid [[ammonia]]/THF at -40 °C has been reported to work well by Hermann and others for imidazole based carbenes. Arduengo and co-workers managed to prepare a dihydroimidazol-2-ylidene using NaH. However, this method has not been applied to the preparation of diaminocarbenes. ====Potassium tert-butoxide==== Arduengo and co-workers have used [[potassium tert-butoxide]] without the addition of a metal hydride to deprotonate precursor salts. ====Alkyllithiums==== The use of [[alkyllithiums]] as strong bases has not been extensively studied, and have been unreliable for deprotonation of precursor salts. With non-aromatic salts, n-BuLi and PhLi can act as nucleophiles whilst t-BuLi can on occasion act as a source of hydride, reducing the salt with the generation of [[isobutene]]: [[Image:Basebutyllithium.png|center|thumb|600px|Reduction of [[formamidinium]] salts with tert-butyllithium]] ====Lithium amides==== Lithium amides like the [[Lithium diisopropylamide|diisopropylamide (LDA)]] and the ([[lithium tetramethylpiperidide|tetramethylpiperidide (LiTMP)]]) generally work well for the deprotonation of all types of salts, providing that not too much [[lithium hydroxyde|LiOH]] is present in the [[n-Butyl-lithium|''n''-butyl-lithium]] used to make the lithium amide. Titration of lithium amide can be used to determine the amount of hydroxide in solution. ====Metal hexamethyldisilazides==== The deprotonation of precursor salts with metal [[hexamethyldisilazides]] works very cleanly for the deprotonation of all types of salts, except for unhindered formamidinium salts, where this base can act as a nucleophile to give a triaminomethane adduct. ===Metal free carbene preparation=== [[Image:Kcarbene.png|right|300px|Stable carbenes readily coordinate to metals; in this case a diaminocarbene co-ordinates to [[KHMDS]] to form a complex.]] The preparation of stable carbenes free from metal cations has been keenly sought to allow further study of the carbene species in isolation from these metals. Separating a carbene from a carbene-metal complex can be problematic due to the stability of the complex. Accordingly, it is preferable to make the carbene free from these metals in the first place. Indeed, some metal ions, rather than stabilising the carbene, have been implicated in the catalytic dimerisation of unhindered examples. Shown right is an X-ray structure showing a complex between a diaminocarbene and potassium [[HMDS]]. This complex was formed when excess [[potassium bis(trimethylsilyl)amide|KHMDS]] was used as a strong base to deprotonate the [[formamidinium]] salt. Removing lithium ions resulting from deprotonation with reagents such as [[LDA]] can be especially problematic. Potassium and sodium salt by-products tend to precipitate from solution and can be removed. Lithium ions may be chemically removed by binding to species such as [[kryptane]]s or [[crown ether]]s. Metal free carbenes have been prepared in several ways as outlined below: ====Dechalcogenation==== Another approach of preparing carbenes has relied on the [[desulfurisation]] of [[thiourea]]s with molten [[potassium]] in boiling [[THF]]. {{cite journal|author = N. Kuhn and T. Kratz|journal = [[Synthesis (journal)|Synthesis]]|year = 1993|pages = 561|doi = 10.1055/s-1993-25902|title = Synthesis of Imidazol-2-ylidenes by Reduction of Imidazole-2(3''H'')-thiones|volume = 1993}} A contributing factor to the success of this reaction is that the byproduct, [[potassium sulfide]], is insoluble in the solvent. The elevated temperatures suggest that this method is not suitable for the preparation of unstable dimerising carbenes. A single example of the [[deoxygenation]] of a [[urea]] with a [[fluorene]] derived carbene to give the tetramethyldiaminocarbene and fluorenone has also been reported: {{cite journal|title = Carbene-to-Carbene Oxygen Atom Transfer|author = D. Kovacs, M. S. Lee, D. Olson, and J. E. Jackson|journal = [[J. Am. Chem. Soc.]]|year = 1996|volume = 118|pages = 8144|doi = 10.1021/ja961324j}} [[Image:dechalcogenation.png|center|thumb|600px|Preparation of carbenes by dechalcogenation.]] The [[desulfurisation]] of [[thiourea]]s with molten [[potassium]] to give imidazol-2-ylidenes or diaminocarbenes has not been widely used. The method was used to prepare dihydroimidazole carbenes.. ====Vacuum pyrolysis==== Vacuum pyrolysis, with the removal of neutral volatile by-products (CH3OH, CHCl3), has been used to prepare dihydroimidazole and triazole based carbenes: [[Image:vaccumpyrolysis.png|center|thumb|600px|Preparation of carbenes via vacuum pyrolysis.]] Historically the removal of chloroform by [[vacuum pyrolysis]] of '''d''' adducts was used by Wanzlick in his early attempts to prepare dihydroimidazol-2-ylidenes but this method is not widely used. The Enders laboratory has used vacuum pyrolysis of a '''c''' adduct to generate a triazolium-5-ylidene '''c'''. ====Bis(trimethylsilyl)mercury==== [[Bis(trimethylsilyl)mercury]] (CH3)3Si-Hg-Si(CH3)3 reacts with chloro-[[iminium]] and chloro-[[amidinium]] salts to give a metal-free carbene and elemental [[Mercury (element)|mercury]]. {{cite journal |title = Mono- and Diaminocarbenes from Chloroiminium and -amidinium Salts: Synthesis of Metal-Free Bis(dimethylamino)carbene |author = Michael Otto, Salvador Conejero, Yves Canac, Vadim D. Romanenko, Valentyn Rudzevitch, and Guy Bertrand |journal = [[J. Am. Chem. Soc.]] |volume = 126 |issue = 4 |pages = 1016–1017 |year = 2004 |doi = 10.1021/ja0393325 |pmid = 14746458}} For example, (CH3)3Si-Hg-Si(CH3)3 + R2N=C(Cl)-NR2+ Cl- → R2N-C:-NR2 + Hg(l) + (CH3)3Si-Cl ====Photochemical decomposition==== Persistent triplet state carbenes have been prepared by [[photochemical]] decomposition of a [[diazomethane]] product via the expulsion of [[nitrogen]] gas, at a wavelength of 300 nm in benzene. ===Purification=== [[Image:Air-free sublimation.png|right|300px|Sublimation of a carbene]] Stable carbenes are very reactive, and so the minimum amount of handling is desirable using [[air-free technique]]s. However, provided rigorously dry, relatively non-acidic and air-free materials are used, stable carbenes are reasonably robust to handling ''per se''. By way of example, 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]]. Stable carbenes are readily soluble in non-polar solvents such as hexane, and so typically [[recrystallisation]] of stable carbenes can be difficult, due to the unavailability of suitable non-acidic polar solvents. Air-free [[Sublimation (chemistry)|sublimation]] as shown right can be an effective method of purification, although temperatures below 60 °C under high vacuum are preferable as these carbenes are relatively volatile and also could begin to decompose at these higher temperatures. Indeed, sublimation in some cases can give single crystals suitable for X-ray analysis. However, strong complexation to metal ions like [[lithium]] will in most cases prevent sublimation. ==References== {{reflist|2}} ==Further reading== Reviews on persistent carbenes: *[http://books.google.com/books?id=ElV85eHbubQC&pg=PA153&dq=Alder+Bertrand&lr=&sig=wNCxFAU9c83Sa-o5Ck7KCkN6LnA Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, (Chapter 4, Hideo Tomioka (triplet state); Chapter 5 (singlet state), Roger W. Alder) - ed. Guy Bertrand] *[http://books.google.com/books?id=wp0C10qO0A8C&pg=PA329&dq=moss+jones+platz+Bertrand&lr=&source=gbs_toc_s&cad=1&sig=PcsZRp-ha0h09pG06s4Ip11Q4K8 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. Engl.|year=1996|volume=30|issue=6|pages=674–676|doi=10.1002/anie.199106741}} [[Category:Functional groups]] [[Category:Carbenes]] [[Category:Organometallic chemistry]] [[es:Carbeno persistente]]