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N-Heterocyclic Carbenes (NHCs): An Introduction

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Ruchi Bharti, Monika Verma, Ajay Thakur and Renu Sharma

Reviewed: 19 January 2022 Published: 01 March 2022

DOI: 10.5772/intechopen.102760

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Abstract

In 1991, the isolation and characterization of nitrogen heterocyclic carbene (NHCs) prompted the discovery of a new class of chemical compounds. NHCs have developed academic curiosity as one of the most potent tools in organic chemistry, exhibiting its utility in commercially relevant protocols. NHCs are cyclic compounds with a divalent carbon atom bonded to at least one nitrogen atom. The size of the carbene ring, the substituent moieties on the nitrogen atoms, and the extra atoms within the heterocycle can be changed to produce a variety of distinct NHCs with various electrical properties. They make excellent ligands in coordination chemistry because of their ability to act as donors and the consequent stable bonds with most transition metals. Free NHCs have also been used as organocatalysts in chemical reactions that require no metals. This chapter provides an outline of the N-Heterocyclic Carbenes in Contemporary Chemistry, including their general properties and highlighting the essential structural and electronic features of different NHCs along with their synthetic procedure.

Keywords

  • N-heterocyclic carbenes (NHCs)
  • metal-organic framework
  • precursor
  • coordination chemistry
  • organocatalysis

1. Introduction

A carbene is a divalent neutral carbon-bearing six electrons in the valance shell & is considered very reactive to be isolated. Over the last 150 years, chemists have been fascinated about the carbenes and attempted for its isolation but failed [1]. Usually, the carbenes have a brief life span and play the role of very reactive intermediates. However, N-Heterocyclic carbene, in which the carbene center is settled on an N-heterocyclic ring, possesses different traits. It was first investigated in the early 1960s by Wanzlick [2, 3]. Shortly after that, in 1968, Wanzlick and Ofele reported the first application of NHC, where they function as a ligand to make complexes with metal [4, 5]. Later, the first crystalline NHC IAd (Figure 1) was first isolated and identified in 1991 by Arduengo et al., who encouraged a plethora of research on the transition metal complexes with NHC [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

Figure 1.

First isolated NHC IAd (1c).

A broad range of NHCs is known with different carbene rings, substituting nitrogen atoms or additional heteroatom. The lifetime of NHCs is increased as the carbon is stabilized due to steric shielding. More generally, it can be said that this feature makes it a suitable fit ligand for coordination chemistry [20]. NHCs are also used as organocatalysts in metal-free chemical transformations [21]. This chapter discusses the basic outline of NHCs, including their standard structural features and properties, emphasizing various electronic and steric properties. In addition, different synthetic routes leading to N-heterocyclic carbenes, along with their applications, have also conversed.

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2. Chemistry of N-heterocyclic carbenes

2.1 Classes of NHCs and related stable carbenes

Many types of carbene compounds were labeled as NHCs in the past, and definitions of NHC based on their constitution are often subject to several interpretations [22]. N-heterocyclic carbenes can be defined as any chemical with a carbene center in a nitrogen-containing heterocyclic ring. A broad array of carbenes depending on substituents, size of the ring, and degree of stabilization of heteroatom are available, out of which a few important selected ones are shown in Figure 2. Another important classes of NHCs are imidazolinylidene (1), Tetrahydropyrimidinylidene (2), N,N-Diamidocarbene (DAC) (3), Benzimidazolylidene (4). These NHCs trigger small molecules like NH3 & can go for insertion into alkenes [23]. Out of these Imidazolylidene-derived NHCs (1) like IPr or IDipp (1a), IMes (1b), and IAd (1c) is the commonest NHCs that are mainly used as ligands with block elements. They function as catalysts in cross-coupling reactions or other significant organic–inorganic transformations [24, 25]. One more category of NHCs was reported by Bertrand et al. [26] where he reported only one nitrogen-containing NHCs like pyrrolidinylidene usually termed as cyclic on the basis of nature of the substituent present neighboring to the carbene center [27, 28].

Figure 2.

Some examples of important classes of NHC.

These compounds are more attracted to the π-electron cloud than other categories of NHCs and help stabilize delicate representative elements and organic free radicals [29]. Another category of NHCs with more than two nitrogen atoms in their heterocyclic framework is also present. Triazolylidenes (8) are examples of this class, widely used as organocatalysts for many transformations [30]. NHC classes having a different heteroatom like oxygen [oxazolylidene (6)], sulfur thiazolylidene (7) are also accessible. Different NHCs moieties can be obtained by developing the carbine center at different positions. In usual NHCs, the carbene center is generally present between the two nitrogen atoms as in imidazolylidenes 1, generating the carbene center at the 4-position. However, that species is labeled mesoionic or abnormal carbene (MIC or an NHC), which is not neutral, and it is not possible to draw its non-zwitterionic resonance structure (9) [31, 32]. It is termed as “remote” NHC (rNHC) (10) when the carbene center is away from the nitrogen atom [33].

All aforementioned act as NHCs, but some related non-NHCs have also been reported, exhibiting the same characteristics as shown in Figure 3 [34]. One of these is acyclic diamino carbenes (ADCs) (11) [35, 36, 37]. A carbene species 12 stabilized by adjoining silicon & phosphorus substituent was reported by Bertrand in 1988 and after 3 years free NHC was also isolated [38]. Cyclopropenylidene compounds (13) with exocyclic nitrogens were also synthesized by Bertrand [39] while “bent allene” species (14) have themselves been used as ligands [40]. These compounds are considered as NHCs stabilized acyclic carbene.

Figure 3.

Related classes of stable carbene.

2.2 Common structural features of NHCs

Nitrogen heterocycles are fundamental in defining the characteristics of NHC. Ranging from four-membered heterocycles, NHCs can be poly NHCs also, but the five-membered NHCs are most common (Figure 4), and all are based on imidazole and imidazolydiene [41]. Several structural features of imidazolylidene (1) are similar in all variants of NHCs, which helps stabilize the carbenes. Alteration in the structure and substituents of imidazolylidene develops diverse behavior in NHCs.

Figure 4.

Common five-membered heterocyclic rings.

In the basic structure of imidazolylidene, the carbene carbon is attached to the two nitrogen atoms of the heterocycle. The aliphatic/aromatic substituent on the nitrogen atom (s) is denoted as the N-substituent(s). The remaining positions, i.e., the 4- and 5-positions in imidazolylidenes, are referred to as the NHC backbone. The substituents present on the ring backbone do not contribute any steric effect at the carbene center and affect the electronic environment only. However, substituents present on the nitrogen of the ring greatly influence the steric properties of the carbene center (Figure 5).

Figure 5.

General structural features of NHCs(1c).

2.2.1 Stabilization of the carbene center

The steadiness of the carbene center in NHCs can be attributed to kinetic and thermodynamic factors. A carbene is carbon with an incomplete octet formed as an intermediate but does not undergo dimerization to form an alkene. Similarly, NHC bearing bulky aryl or alkyl substituent on nitrogen atom does not dimerize due to steric clashing and is termed the Wanzlick equilibrium. Electronic properties of NHC further contribute toward stability as they bear a singlet ground state despite classical carbene, which carries a triplet ground state. The lone pair of electrons of NHCs (singlet) is confined in sp2-hybridized orbital, which exists in the plane of the ring in the highest occupied molecular orbital and an empty p-orbital is lying perpendicular to it in the lowest unoccupied molecular orbital (Figure 6). As reported by Goddard et al. [42] singlet carbenes do not undergo dimerization easily, and adjacent nitrogen in NHCs further decreases the energy of HOMO, which causes sizeable singlet-triplet energy gaps. Interrelation among the lone pair of electrons of nitrogen present in p-orbital with empty LUMO (Mesomeric donation) also contributes to stabilizing the structure of the singlet ground-state.

Figure 6.

Stabilization of the carbene by adjacent ring nitrogens.

2.3 Attractive features of N-heterocyclic carbenes

NHCs have gained much application as ligands for transition metal catalysis due to their steric and electronic behavior leading toward complex stability.

2.3.1 Electronic character

N-Heterocyclic carbenes belong to the category of very electron-rich ligands, although their degree of π-acceptor power is still doubtful. The electron donation property of NHC depends on the nature/type of metal, the substituents, and the co-ligands present on the NHC ring corresponding to the metal [43, 44, 45, 46]. The capability to donate electrons can be calculated by comparing the stretching frequencies of CO ligands of complexes like LRh(CO)2Cl, [47], LIr(CO)2Cl [47] with L = NHC or LNi(CO)3 [48]. Hence, it is evident that N-heterocyclic carbenes are electron-rich ligands than the most basic trialkyl phosphines (Table 1) [49].

LigandѴCO (A1) (cm−1)ѴCO (E) (cm−1)
IMes2050.71969.8
SIMes2051.51970.6
IPr2051.51970.0
SIPr2052.21971.3
Icy2049.61964.6
PCy32056.41973
PPh32068.91990
PtBu32056.11971

Table 1.

IR values for the carbonyl stretching frequencies in LNi(CO)3 measured in CH2Cl2.

Furthermore, the NHCs have very similar levels of electron-donating ability as compared to the phosphines. The reason for this difference can be explained as the substituents of NHCs are exchanged only on the periphery of the ligand while for phosphines the different substituents are directly attached to the donor atom itself. Therefore, the finest way is to modify the electronic behavior of an NHC is to alter the type of the azole ring. In this way, it is rational to believe that the order of the electron-donating capability is benzimidazole < imidazole < imidazoline. It is easy to understand that components with +I and +M-effects increase the electron density of a NHC making it a better donor, while components with −I and −M-effects show the opposite behavior. However, some substituents have opposing electronic effects and complicate the situation. For example, halo groups (F−, Cl−, Br−, and I−) bound to the carbon atom exhibit the –I-effect due to their increased electronegativity but they also have the +M-effect as a result of three loan pairs for donation that need to be considered as well [50].

This electron-rich property of NHCs impacts many rudimentary levels of the catalytic process, e.g. smoothing the oxidative addition step. Hence, the complex of NHC with metal are suitable for cross-coupling reactions of non-activated aryl chlorides, which encounter the catalyst with a challenging oxidative addition step [51].

2.3.2 Sterics

NHCs are often used as phosphine mimics, but both structures are pretty different (Figure 7). Phosphine complexes have a cone-like structure where alkyl/aryl groups are pointed away from phosphorous. So, the steric properties of NHCs can be elucidated using Tolman’s ingenious cone angle descriptors [49].

Figure 7.

Shape of phosphines and NHC.

The topology of N-heterocyclic carbene is contrary to phosphene and is more complex to predict factors determining its steric effect. The shape of NHC is defined by the position of the alkyl/aryl group present on the nitrogen(s) of the heterocycle. NHCs have been featured as fence- or fan-like [52]. The side groups are bent toward the metal and wrap it by forming a pocket (Figure 7). The steric and electronic properties of NHCs can change via rotation around the metal-carbene bond, hence making it anisotropic.

2.3.3 NHCs as ligands

Most of the metals form a very stable bond with Nitrogen heterocyclic carbene [46, 53]. Whereas quite same bond dissociation energies have been noticed for unsaturated & saturated NHCs with the same steric impacts, phosphines generally form weaker bonds (Table 2) [28, 54].

Ligand%VBur for M-L (2.00 Å)BDE[kcal/mol] (theoretical) for L in Ni(CO)3L
IMes2641.1
SIMes2740.2
IAd3720.4
ItBu3724.0
PPh32726.7

Table 2.

Steric demand and bond strength of some important ligands.

Consequently, the equilibrium between the carbene metal complex and free carbene exists toward the complex compared to phosphines (Figure 8). It increases the lifetime of complex but still N-heterocyclic carbenes very sensitive & reactive for many electron-loving moieties. They need careful isolation and storage. The consequential unusual firmness of NHC-metal complexes has been explored in many demanding protocols such as coupling reactions [55, 56], polymerization [57, 58], transfer hydrogenation [59, 60, 61], photocatalysis [62, 63], and many other [64, 65, 66, 67, 68, 69, 70].

Figure 8.

Equilibrium of complexation.

However, escalating publications reveal that the bond between metal and carbene is not unreactive [53, 71, 72, 73, 74]. As seen during the migratory insertion of an NHC into the double bond of ruthenium-carbon [75] removal of alkyl imidazolium salts from NHC alkyl complexes via reductive elimination, [76] or the ligand substitution of NHC ligands by phosphines, [77, 78]. Additionally, during applications of palladium NHC complexes, the generation of palladium black is observed, which points toward the decomposition pathways.

2.3.4 NHC adducts-bonding

The researchers have investigated the bonding between carbene center and metals/non-metals over the past two decades [8, 15]. The main feature of this bonding is the donation of carbene lone pair of electrons into empty sigma orbital of metal/non-metal. It is evident in the graphical representations (Figure 9) of NHC-metal complexes (53, 54, and 55) that inspite of a double bond usually a single bond is shown. Therefore, in the deficiency of steric restrictions or chelation, the bond between metal and NHC can rotate easily. The π-orbital in the NHC’s complexes and metal is restricted within the NHC ring so indicated by a curved line between the heteroatoms of the ring.

Figure 9.

NHC-containing transition metal catalysts.

Metal donates its electrons in the carbene’s vacant p-orbital (LUMO) of carbene. The carbene can also donate electrons in the vacant π-orbitals of metal. The significance of each factor depends on the nature and geometry of another ligand on the complex and, most importantly nature of carbene itself. Using spectroscopic methods π-accepting ability of different NHCs can be measured and quantified [54, 79].

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3. Synthesis of NHCs

As a broad range of N-heterocyclic carbene is based on 5-membered rings, hence the simplest way to prepare them is via removal of a proton from the related azolium salts, such as imidazolium, pyrazolium, triazolium, tetrazolium, benzimidazolium, oxazolium, or thiazolium salts by using appropriate bases. The value of dissociation constant (pKa) of benzimidazolium and imidazolium salts was observed between 21 and 24, giving them a place in between the neutral carboxylic acid, acetone, and ethylacetate [9, 15, 80, 81]. Imidazolium salts are synthesized by following two routes. First method involves alkylation of existing imidazoles using appropriate electrophiles resulting in the generation of N-alkyl-substituted imidazolium salts. Whereas in other methods the imidazolium ring is synthesized by condensation reactions (Figures 10 and 11). As the attention for the synthesis of NHCs and imidazolium salts is increasing very rapidly, the methodologies related to its synthesis have been improved regularly.

  1. Reductive desulfurization of N-heterocyclic thiones: a variety of-heterocyclic carbene such as saturated, unsaturated, and benzannulated can be prepared by using this method. Imidazole-2-thione gets reduced to carbene by using potassium in boiling THF for 4 h (Figure 12). At the same time, benzimidazole-2-thiones undergo a reduction in the presence of Na/K alloy along with toluene within three weeks [82, 83, 84, 85, 86].

  2. α-Elimination or dehalogenation: Imidazolinium salts derivatives bearing bulky N, N′-substituents 23 undergo α-elimination reaction to form the NHCs 24 under the influence of thermal induction (Figure 13a) [30]. The corresponding alcohol elimination from 2-alkoxyimidazolidines 25 to give imidazolin-2-ylidenes of type 26 (Figure 13b) was outlined by Grubbs [87] after early unsuccessful attempts by Wanzlick and Kleiner [88]. Imidazoline-2-ylidenes 26 were also accessible via α-elimination of fluorinated aryls from 2-(fluorophenyl)imidazolines 27 (Figure 13c) [89, 90]. The α-elimination of acetonitrile from 29 to yield the benzimidazole-2-ylidene 30 has also been described (Figure 13d) [91]. Bertrand reported the dechlorination reaction among tetrahydropyrimidiniumchloride 31 and bis(trimethylsilyl)mercury leading to NHC 32 (Figure 13e) [92].

  3. Symmetric synthesis: For symmetric NHCs (36), synthesis formaldehyde (35) is treated with primary amine (33) and glyoxal (34) under strongly acidic conditions. Otherwise, the bisimine intermediate (34a) is reacted with electrophilic C1-fragments (chloromethyl ethyl ether or chloromethylpivalate) after isolation [81, 93, 94, 95]. During few crucial methodologies, the addition of stoichiometric amounts of AgOTfwas evidenced to be helpful (Figure 10) [96].

  4. Unsymmetrical synthesis: Unsymmetrically, NHCs are prepared by adding alkyls to monosubstitutedimidazoles (Figure 11) [96, 97, 98, 99, 100, 101, 102, 103]. The unsymmetrical imidazolium salt (39), so obtained is incorporating a counter anion (X−) to maintain the electric neutrality but careful selection of the counter anion is sensible as it affects the dissolution of the imidazolium salt. It is noteworthy that non-coordinating counter ions like OTf- or BF4-increase the solubility of the salts.

Figure 10.

Symmetric NHCs Synthesis.

Figure 11.

Unsymmetrical Synthesis of NHCs.

Figure 12.

Desulfurization of thiones.

Figure 13.

NHCs synthesis (a-e) by α-Elimination or dehalogenation.

In another protocol for synthesizing unsymmetrical NHCs, the α-hydroxy ketone (46) is reacted with the amine (R4-NH2) of choice under acidic conditions. Thus, by azeotropic removal of water, the α-aminoketones (47) are obtained, which are then N-formylated (48) before reaction with (CH3CO)2O in the presence of a aq. HBF4 or HClO4. The subsequent cyclization stops at the intermediate acetal stage. Also, the reaction of the oxazolinium adducts 49 with R2–NH2 goes on easily, generating hydroxylateimidazolinium salts bromoacetaldehyde diethyl acetal (53) (Figure 14) [104].

Figure 14.

Unsymmetrical synthesis of imidazolium salts.

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4. Conclusions

Since the 1960s, when the initial studies on NHC began, and later (1991) when Arduengo isolated the first NHC, the applications of these compounds are growing across various domains of chemistry. Nature itself has chosen an NHC as a part of vitamin B1 to achieve organocatalytic reactions in vivo. Over the past few years, NHCs have unfolded novel categories of enantioselective organic transformations. This chapter provides a brief outline of the fundamental properties and synthesis of NHCs by analyzing different methods used to quantify their steric & electronic properties.

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Acknowledgments

The authors are thankful to Chandigarh University for providing generous support for this work.

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Conflict of interest

The authors declare no conflict of interest.

References

  1. 1. Bertrand G. Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents. Boca Raton, Florida, United States: CRC Press; 2002
  2. 2. Benhamou L, Chardon E, Lavigne G, Bellemin-Laponnaz S, Cesar V. Synthetic routes to N-heterocyclic carbene precursors. Chemical Reviews. 2011;111(4):2705-2733
  3. 3. Wanzlick HW. Aspects of nucleophilic carbene chemistry. Angewandte Chemie International Edition in English. 1962;1(2):75-80
  4. 4. Wanzlick HW, Schönherr HJ. Direct synthesis of a mercury salt-carbene complex. Angewandte Chemie International Edition in English. 1968;7(2):141-142
  5. 5. Öfele K. 1, 3-Dimethyl-4-imidazolinyliden-(2)-pentacarbonylchromeinneuer Übergangsmetall-carben-komplex. Journal of Organometallic Chemistry. 1968;12(3):42-43
  6. 6. Arduengo AJ III, Harlow RL, Kline M. A stable crystalline carbene. Journal of the American Chemical Society. 1991;113(1):361-363
  7. 7. Crudden CM, Allen DP. Stability and reactivity of N-heterocyclic carbene complexes. Coordination Chemistry Reviews. 2004;248(21–24):2247-2273
  8. 8. Díez-González S, Nolan SP. Stereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding. Coordination Chemistry Reviews. 2007;251(5–6):874-883
  9. 9. Colacino E, Martinez J, Lamaty F. Preparation of NHC–ruthenium complexes and their catalytic activity in the metathesis reaction. Coordination Chemistry Reviews. 2007;251(5–6):726-764
  10. 10. Lee HM, Lee CC, Cheng PY. Recent development of functionalized N-heterocyclic carbene ligands: Coordination chemistry and catalytic applications. Current Organic Chemistry. 2007;11(17):1491-1524
  11. 11. Kühl O. The chemistry of functionalized N-heterocyclic carbenes. Chemical Society Reviews. 2007;36(4):592-607
  12. 12. Lin IJ, Vasam CS. Preparation and application of N-heterocyclic carbene complexes of Ag (I). Coordination Chemistry Reviews. 2007;251(5–6):642-670
  13. 13. Cavell K. N-heterocyclic carbenes/imidazolium salts as substrates in catalysis: The catalytic 2-substitution and annulation of heterocyclic compounds. Dalton Transactions. 2008;47:6676-6685
  14. 14. Marion N, Nolan SP. Well-defined N-heterocyclic carbenes− palladium (II) precatalysts for cross-coupling reactions. Accounts of Chemical Research. 2008;41(11):1440-1449
  15. 15. Jacobsen H, Correa A, Poater A, Costabile C, Cavallo L. Understanding the M (NHC)(NHC= N-heterocyclic carbene) bond. Coordination Chemistry Reviews. 2009;253(5–6):687-703
  16. 16. Wang F, Liu LJ, Wang W, Li S, Shi M. Chiral NHC–metal-based asymmetric catalysis. Coordination Chemistry Reviews. 2012;256(9–10):804-853
  17. 17. Liu W, Gust R. Metal N-heterocyclic carbene complexes as potential antitumor metallodrugs. Chemical Society Reviews. 2013;42(2):755-773
  18. 18. Nelson DJ, Nolan SP. Quantifying and understanding the electronic properties of N-heterocyclic carbenes. Chemical Society Reviews. 2013;42(16):6723-6753
  19. 19. Hu C, Li X, Wang W, Zhang R, Deng L. Metal-N-heterocyclic carbene complexes as antitumor agents. Current Medicinal Chemistry. 2014;21(10):1220-1230
  20. 20. Diez-Gonzalez S, Marion N, Nolan SP. N-heterocyclic carbenes in late transition metal catalysis. Chemical Reviews. 2009;109(8):3612-3676
  21. 21. Enders D, Niemeier O, Henseler A. Organocatalysis by N-heterocyclic carbenes. Chemical Reviews. 2007;107(12):5606-5655
  22. 22. Hopkinson MN, Glorius F. An overview of NHCs. N-Heterocyclic Carbenes in Organocatalysis. 1st ed. 2018. pp. 1-35
  23. 23. Moerdyk JP, Schilter D, Bielawski CW. N, N′-diamidocarbenes: Isolable divalent carbons with bona fide carbene reactivity. Accounts of Chemical Research. 2016;49(8):1458-1468
  24. 24. Shi S, Nolan SP, Szostak M. Well-defined palladium (II)–NHC precatalysts for cross-coupling reactions of amides and esters by selective N–C/O–C cleavage. Accounts of Chemical Research. 2018;51(10):2589-2599
  25. 25. Ohmiya H. N-heterocyclic carbene-based catalysis enabling cross-coupling reactions. ACS Catalysis. 2020;10(12):6862-6869
  26. 26. Lavallo V, Canac Y, Präsang C, Donnadieu B, Bertrand G. Stable cyclic (alkyl)(amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition metal catalysts: A quaternary carbon atom makes the difference. Angewandte Chemie International Edition. 2005;44(35):5705-5709
  27. 27. Soleilhavoup M, Bertrand G. Cyclic (alkyl)(amino) carbenes (CAACs): Stable carbenes on the rise. Accounts of Chemical Research. 2015;48(2):256-266
  28. 28. Rao B, Tang H, Zeng X, Liu LL, Melaimi M, Bertrand G. Cyclic (amino)(aryl) carbenes (CAArCs) as strong σ-donating and π-accepting ligands for transition metals. Angewandte Chemie International Edition. 2015;54(49):14915-14919
  29. 29. Martin CD, Soleilhavoup M, Bertrand G. Carbene-stabilized main group radicals and radical ions. Chemical Science. 2013;4(8):3020-3030
  30. 30. Enders D, Breuer K, Raabe G, Runsink J, Teles JH, Melder JP, et al. Preparation, structure, and reactivity of 1, 3, 4-Triphenyl-4, 5-dihydro-1H-1, 2, 4-triazol-5-ylidene, a new stable carbene. Angewandte Chemie International Edition in English. 1995;34(9):1021-1023
  31. 31. Aldeco-Perez E, Rosenthal AJ, Donnadieu B, Parameswaran P, Frenking G, Bertrand G. Isolation of a C5-deprotonated imidazolium, a crystalline “abnormal” N-heterocyclic carbene. Science. 2009;326(5952):556-559
  32. 32. Ghadwal RS. Carbon-based two-electron σ-donor ligands beyond classical N-heterocyclic carbenes. Dalton Transactions. 2016;45(41):16081-16095
  33. 33. Schuster O, Yang L, Raubenheimer HG, Albrecht M. Beyond conventional N-heterocyclic carbenes: Abnormal, remote, and other classes of NHC ligands with reduced heteroatom stabilization. Chemical Reviews. 2009;109(8):3445-3478
  34. 34. Melaimi M, Soleilhavoup M, Bertrand G. Stable cyclic carbenes and related species beyond diaminocarbenes. Angewandte Chemie International Edition. 2010;49(47):8810-8849
  35. 35. Vignolle J, Cattoen X, Bourissou D. Stable noncyclic singlet carbenes. Chemical Reviews. 2009;109(8):3333-3384
  36. 36. Igau A, Grutzmacher H, Baceiredo A, Bertrand G. Analogous. alpha.,. alpha.'-bis-carbenoid, triply bonded species: synthesis of a stable. lambda. 3-phosphino carbene-. lambda. 5-phosphaacetylene. Journal of the American Chemical Society. 1988;110(19):6463-6466
  37. 37. Kato T, Gornitzka H, Baceiredo A, Savin A, Bertrand G. On the electronic structure of (phosphine)(silyl) carbenes: Single-crystal X-ray diffraction and ELF analyses. Journal of the American Chemical Society. 2000;122(5):998-999
  38. 38. Dyker CA, Lavallo V, Donnadieu B, Bertrand G. Synthesis of an extremely bent acyclic allene (a “carbodicarbene”): A strong donor ligand. Angewandte Chemie International Edition. 2008;47(17):3206-3209
  39. 39. Igau A, Baceiredo A, Trinquier G, Bertrand G. [bis (diisopropylamino) phosphino] trimethylsilylcarbene: A stable nucleophilic carbene. Angewandte Chemie International Edition in English. 1989;28(5):621-622
  40. 40. Lavallo V, Canac Y, Donnadieu B, Schoeller WW, Bertrand G. Cyclopropenylidenes: From interstellar space to an isolated derivative in the laboratory. Science. 2006;312(5774):722-724
  41. 41. Cavallo L, Cazin CS. N-heterocyclic carbenes: An introductory overview. N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis. 2010;32:1-22
  42. 42. Carter EA, Goddard WA III. Relation between singlet-triplet gaps and bond energies. The Journal of Physical Chemistry. 1986;90(6):998-1001
  43. 43. Termaten AT, Schakel M, Ehlers AW, Lutz M, Spek AL, Lammertsma K. N-heterocyclic carbene functionalized iridium phosphinidene complex [Cp*(NHC) Ir= PMes*]: Comparison of phosphinidene, imido, and carbene complexes. Chemistry—A European Journal. 2003;9:3577-3582
  44. 44. Lee MT, Hu CH. Density functional study of N-heterocyclic and diaminocarbene complexes: Comparison with phosphines. Organometallics. 2004;23(5):976-983
  45. 45. Hu X, Castro-Rodriguez I, Meyer K. Dioxygen activation by a low-valent cobalt complex employing a flexible tripodal N-heterocyclic carbene ligand. Journal of the American Chemical Society. 2004;126(41):13464-13473
  46. 46. Cavallo L, Correa A, Costabile C, Jacobsen H. Steric and electronic effects in the bonding of N-heterocyclic ligands to transition metals. Journal of Organometallic Chemistry. 2005;690(24–25):5407-5413
  47. 47. Chianese AR, Li X, Janzen MC, Faller JW, Crabtree RH. Rhodium and iridium complexes of N-heterocyclic carbenes via transmetalation: Structure and dynamics. Organometallics. 2003;22(8):1663-1667
  48. 48. Dorta R, Stevens ED, Scott NM, Costabile C, Cavallo L, Hoff CD, et al. Steric and electronic properties of N-heterocyclic carbenes (NHC): A detailed study on their interaction with Ni (CO) 4. Journal of the American Chemical Society. 2005;127(8):2485-2495
  49. 49. Tolman CA. Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis. Chemical Reviews. 1977;77(3):313-348
  50. 50. Huynh HV. Electronic properties of N-heterocyclic carbenes and their experimental determination. Chemical Reviews. 2018;118(19):9457-9492
  51. 51. Littke AF, Fu GC. Palladium-catalyzed coupling reactions of aryl chlorides. Angewandte Chemie International Edition. 2002;41(22):4176-4211
  52. 52. Huang J, Schanz HJ, Stevens ED, Nolan SP. Stereoelectronic effects characterizing nucleophilic carbene ligands bound to the Cp* RuCl (Cp*= η5-C5Me5) moiety: A structural and thermochemical investigation. Organometallics. 1999;18(12):2370-2375
  53. 53. Dorta R, Stevens ED, Hoff CD, Nolan SP. Stable, three-coordinate Ni (CO)2 (NHC)(NHC= N-heterocyclic carbene) complexes enabling the determination of Ni− NHC bond energies. Journal of the American Chemical Society. 2003;125(35):10490-10491
  54. 54. Back O, Henry-Ellinger M, Martin CD, Martin D, Bertrand G. Angewandte Chemie International Edition. 2013;52(10):2939-2943
  55. 55. Biffis A, Gazzola L, Tubaro C, Basato M. Alkyne hydroarylation in ionic liquids catalyzed by palladium (II) complexes. ChemSusChem. 2010;3(7):834-839
  56. 56. Nirmala M, Saranya G, Viswanathamurthi P, Bertani R, Sgarbossa P, Malecki JG. Organonickel complexes encumbering bis-imidazolylidene carbene ligands: Synthesis, X-ray structure and catalytic insights on Buchwald-Hartwig amination reactions. Journal of Organometallic Chemistry. 2017;831:1-0
  57. 57. Kreisel KA, Yap GP, Theopold KH. A chelating N-heterocyclic carbene ligand in organochromium chemistry. Organometallics. 2006;25(19):4670-4679
  58. 58. Drent E, Van Broekhoven JA, Doyle MJ. Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones. Journal of Organometallic Chemistry. 1991;417(1–2):235-251
  59. 59. Farrell K, Müller-Bunz H, Albrecht M. Synthesis, isomerization, and catalytic transfer hydrogenation activity of rhodium (III) complexes containing both chelating dicarbenes and diphosphine ligands. Organometallics. 2015;34(24):5723-5733
  60. 60. Farrell K, Melle P, Gossage RA, Müller-Bunz H, Albrecht M. Transfer hydrogenation with abnormal dicarbene rhodium (III) complexes containing ancillary and modular poly-pyridine ligands. Dalton Transactions. 2016;45(11):4570-4579
  61. 61. Vivancos A, Albrecht M. Influence of the linker length and coordination mode of (Di) triazolylidene ligands on the structure and catalytic transfer hydrogenation activity of iridium (III) centres. Organometallics. 2017;36(8):1580-1590
  62. 62. Liu Y, Kjaer K, Fredin L, Chabera P, Harlang T, Canton S, et al. A heteroleptic ferrous complex with MesoionicBis (1, 2, 3-triazol-5-ylidene) ligands: Taming the MLCT excited state of iron (II). Chemistry: A European Journal. 2015;21(9):3628-3639
  63. 63. Liu Y, Persson P, Sundström V, Wärnmark K. Fe N-heterocyclic carbene complexes as promising photosensitizers. Accounts of Chemical Research. 2016;49(8):1477-1485
  64. 64. Desai SP, Mondal M, Choudhury J. Chelating bis-N-heterocyclic carbene–palladium (II) complexes for oxidative arene C–H functionalization. Organometallics. 2015;34(12):2731-2736
  65. 65. McCall AS, Wang H, Desper JM, Kraft S. Bis-N-heterocyclic carbene palladium (IV) tetrachloride complexes: Synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates. Journal of the American Chemical Society. 2011;133(6):1832-1848
  66. 66. Meyer D, Ahrens S, Strassner T. Platinum (IV) complexes with chelating N-heterocyclic carbene ligands. Organometallics. 2010;15(29):3392-3396
  67. 67. Prokopchuk EM, Puddephatt RJ. Hydrido (methyl) carbene complex of platinum (IV). Organometallics. 2003;22(3):563-566
  68. 68. Lindner R, Wagner C, Steinborn D. Synthesis of Trimethylplatinum (IV) complexes with N, N-and N, O-heterocyclic carbene ligands and their reductive C− C elimination reactions. Journal of the American Chemical Society. 2009;131(25):8861-8874
  69. 69. Arnold PL, Sanford MS, Pearson SM. Chelating N-heterocyclic carbene alkoxide as a supporting ligand for PdII/IV C−H bond functionalization catalysis. Journal of the American Chemical Society. 2009;131(39):13912-13913
  70. 70. Aznarez F, Iglesias M, Hepp A, Veit B, Miguel PJ, Oro LA, et al. Iridium (III) complexes bearing chelating bis-NHC ligands and their application in the catalytic reduction of imines. European Journal of Inorganic Chemistry. 2016;28(2016):4598-4603
  71. 71. Galan BR, Gembicky M, Dominiak PM, Keister JB, Diver ST. Carbon monoxide-promoted carbene insertion into the aryl substituent of an N-heterocyclic carbene ligand: Buchner reaction in a ruthenium carbene complex. Journal of the American Chemical Society. 2005;127(45):15702-15703
  72. 72. Allen DP, Crudden CM, Calhoun LA, Wang R. Irreversible cleavage of a carbene–rhodium bond in Rh-N-heterocyclic carbene complexes: Implications for catalysis. Journal of Organometallic Chemistry. 2004;689(20):3203-3209
  73. 73. Danopoulos AA, Tsoureas N, Green JC, Hursthouse MB. Migratory insertion in N-heterocyclic carbene complexes of palladium; an experimental and DFT study. Chemical Communications. 2003;6:756-757
  74. 74. Jazzar RF, Macgregor SA, Mahon MF, Richards SP, Whittlesey MK. C− C and C− H bond activation reactions in N-heterocyclic carbene complexes of ruthenium. Journal of the American Chemical Society. 2002;124(18):4944-4945
  75. 75. Becker E, Stingl V, Dazinger G, Puchberger M, Mereiter K, Kirchner K. Facile migratory insertion of a N-heterocyclic carbene into a ruthenium− carbon double bond: A new type of reaction of a NHC ligand. Journal of the American Chemical Society. 2006;128(20):6572-6573
  76. 76. McGuinness DS, Saendig N, Yates BF, Cavell KJ. Kinetic and density functional studies on alkyl-carbene elimination from pdii heterocyclic carbene complexes: A new type of reductive elimination with clear implications for catalysis. Journal of the American Chemical Society. 2001;123(17):4029-4040
  77. 77. Simms RW, Drewitt MJ, Baird MC. Equilibration between a phosphine− cobalt complex and an analogous complex containing an N-heterocyclic carbene: The thermodynamics of a phosphine− carbene exchange reaction. Organometallics. 2002;21(14):2958-2963
  78. 78. Titcomb LR, Caddick S, Cloke FGN, Wilson DJ, McKerrecher D. Unexpected reactivity of two-coordinate palladium–carbene complexes; synthetic and catalytic implications. Chemical Communications. 2001;15:1388-1389
  79. 79. Liske A, Verlinden K, Buhl H, Schaper K, Ganter C. Determining the π-acceptor properties of N-heterocyclic carbenes by measuring the 77Se NMR chemical shifts of their selenium adducts. Organometallics. 2013;32(19):5269-5272
  80. 80. Boehme C, Frenking G. Electronic structure of stable carbenes, silylenes, and germylenes. Journal of the American Chemical Society. 1996;118(8):2039-2046
  81. 81. Arduengo AJ III, Dias HR, Harlow RL, Kline M. Electronic stabilization of nucleophilic carbenes. Journal of the American Chemical Society. 1992;114(14):5530-5534
  82. 82. Denk MK, Thadani A, Hatano K, Lough AJ. Steric stabilization of nucleophilic carbenes. Angewandte Chemie International Edition in English. 1997;36(23):2607-2609
  83. 83. Hahn FE, Paas M, Le Van D, Lügger T. Simple access to unsymmetrically substituted, saturated N-heterocyclic carbenes. Angewandte Chemie International Edition. 2003;42(42):5243-5246
  84. 84. Hahn FE, Paas M, Le Van D, Fröhlich R. Spirocyclicdiaminocarbenes: Synthesis, coordination chemistry, and investigation of their dimerization behavior. Chemistry—A European Journal. 2005;11(17):5080-5085
  85. 85. Glorius F, Altenhoff G, Goddard R, Lehmann C. Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands. Chemical Communications. 2002;22:2704-2705
  86. 86. Kuhn N, Kratz T. Synthesis of imidazol-2-ylidenes by reduction of imidazole-2 (3H)-thiones. Synthesis. 1993;1993(06):561-562
  87. 87. Scholl M, Ding S, Lee CW, Grubbs RH. Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with 1, 3-dimesityl-4, 5-dihydroimidazol-2-ylidene ligands. Organic Letters. 1999;1(6):953-956
  88. 88. Wanzlick HW, Kleiner HJ. Nucleophilic carbene chemistry Representation of bis[1.3-diphenyl-imidazolidinylidene-(2)]. Applied Chemistry. 1961;73(14):493
  89. 89. Nyce GW, Csihony S, Waymouth RM, Hedrick JL. A general and versatile approach to thermally generated N-heterocyclic carbenes. Chemistry—A European Journal. 2004;10(16):4073-4079
  90. 90. Blum AP, Ritter T, Grubbs RH. Synthesis of N-heterocyclic carbene-containing metal complexes from 2-(pentafluorophenyl) imidazolidines. Organometallics. 2007;26(8):2122-2124
  91. 91. Korotkikh NI, Raenko GF, Pekhtereva TM, Shvaika OP, Cowley AH, Jones JN. Stable carbenes. Synthesis and properties of benzimidazol-2-ylidenes. Russian Journal of Organic Chemistry. 2006;42(12):1822-1833
  92. 92. Otto M, Conejero S, Canac Y, Romanenko VD, Rudzevitch V, Bertrand G. Journal of the American Chemical Society. 2004;126:1016-1017
  93. 93. Arduengo AJ III, Krafczyk R, Schmutzler R, Craig HA, Goerlich JR, Marshall WJ, et al. Imidazolylidenes, imidazolinylidenes and imidazolidines. Tetrahedron. 1999;55(51):14523-14534
  94. 94. Herrmann WA, Köcher C, Gooßen LJ, Artus GR. Heterocyclic carbenes: A high-yielding synthesis of novel, functionalized N-heterocyclic carbenes in liquid ammonia. Chemistry–A European Journal. 1996;2(12):1627-1636
  95. 95. Glorius F, Altenhoff G, Goddard R, Lehmann C. Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands. Chemical Communications. 2002;22:2704-2705
  96. 96. Wang AE, Xie JH, Wang LX, Zhou QL. Triaryl phosphine-functionalized N-heterocyclic carbene ligands for Heck reaction. Tetrahedron. 2005;61(1):259-266
  97. 97. Perry MC, Cui X, Powell MT, Hou DR, Reibenspies JH, Burgess K. Optically active iridium imidazole-2-ylidene-oxazoline complexes: Preparation and use in asymmetric hydrogenation of aryl alkenes. Journal of the American Chemical Society. 2003;125(1):113-123
  98. 98. César V, Bellemin-Laponnaz S, Gade LH. Direct coupling of oxazolines and N-heterocyclic carbenes: A modular approach to a new class of c− n donor ligands for homogeneous catalysis. Organometallics. 2002;21(24):5204-5208
  99. 99. Loch JA, Albrecht M, Peris E, Mata J, Faller JW, Crabtree RH. Palladium complexes with tridentate pincer bis-carbene ligands as efficient catalysts for C−C coupling. Organometallics. 2002;21(4):700-706
  100. 100. Yang C, Lee HM, Nolan SP. Highly efficient Heck reactions of aryl bromides with n-butyl acrylate mediated by a palladium/phosphine−imidazolium salt system. Organic Letters. 2001;3(10):1511-1514
  101. 101. Gardiner MG, Herrmann WA, Reisinger CP, Schwarz J, Spiegler M. Dicationic chelating N-heterocyclic carbene complexes of palladium: New catalysts for the copolymerisation of C2H4 and CO. Journal of Organometallic Chemistry. 1999;572(2):239-247
  102. 102. Herrmann WA, Goossen LJ, Spiegler M. Chiral oxazoline/imidazoline-2-ylidene complexes. Organometallics. 1998;17(11):2162-2168
  103. 103. Herrmann WA, Gooβen LJ, Spiegler M. Functionalized imidazoline-2-ylidene complexes of rhodium and palladium. Journal of Organometallic Chemistry. 1997;547(2):357-366
  104. 104. Fürstner A, Alcarazo M, César V, Lehmann CW. Convenient, scalable and flexible method for the preparation of imidazolium salts with previously inaccessible substitution patterns. Chemical Communications. 2006;20:2176-2178

Written By

Ruchi Bharti, Monika Verma, Ajay Thakur and Renu Sharma

Reviewed: 19 January 2022 Published: 01 March 2022

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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