**1. Introduction**

Carbenes are divalent carbon compounds which are generally highly reactive organic intermediates with six valence electrons having the general formula R2 C or R1 R2 C [1, 2]. Carbenes are classified as either singlets or triplets, depending upon their electronic structure. Most carbenes are very short lived, although persistent carbene are also known.

Carbene generally have either linear (as an extreme case) or bent geometry with sp2 hybridized central carbon atom. These structures are related to electronically different orbital coupled states of central carbon atom in carbene. The orbital coupling between sp-hybrid orbitals and other two energetically degenerated porbitals results linear geometry of carbene (**Figure 1**). In other case, the hybrid sp2

#### **Figure 1.**

*Geometry (linear and bent) and hybridization of carbine; Single head arrow indicates electron.*

orbitals (δ orbitals) coupled with a p orbital (p-π orbital) promotes bent geometry of carbine (**Figure 1**).

The arrangement of two nonbonding electrons in carbene is extremely important to reactivity of carbene. The two different electronic states are obtained from different arrangement of two nonbonding electrons in carbene. These electronic states are related to triplet and singlet state of carbene.

#### **1.1 Triplet state of carbene**

From two nonbonding electrons, one electron occupies in empty δ orbitals and another electron resides in empty pπ with parallel spin orientation (*δ*<sup>1</sup> p*π*1: 3B1). If electronic spin orientation is antiparallel, then the carbene is no longer triplet carbene (*δ*<sup>1</sup> p*π*1: 1B1) (**Figure 2**).

#### **1.2 Singlet state of carbene**

When the two nonbonding electrons occupy as a lone-pair in the emptyδ orbital, then the pπ orbital is being vacant (*δ*<sup>2</sup> p*π*0: 1A1 state). If these two nonbonding electrons are present in pπ orbital as a lone pair with empty δ-orbital, then 1A1 state is also created with *δ*0p*π*2electronic configuration. Interestingly, the *δ*<sup>2</sup> p*π*<sup>0</sup> (1A1 state) is considered as a more stable state than the *δ*0p*π*<sup>2</sup> (another 1A1 state) where the lone-pair occupies the pπ orbital (**Figure 2**).

The stability of singlet state (*δ*<sup>2</sup> p*π*<sup>0</sup> (1A1 state)) is explained on the basis of significant energy difference between δ and pπ orbital (>2.0 eV).1 Singlet carbenes show the amphiphilic behavior (nucleophilic and electrophilic character) due to the presence of a sp2 hybridized lone-pair and of a vacant p-orbital.

Initially, carbene were so reactive that they were only considered as reaction intermediates or transition states. They could not be isolated and were only indirectly studied, often by trapping them in the presence of suitable reagents. However, now carbene can be stabilized and isolated by forming complexes with transition metals. They act as ligands for organometallic complexes. Two types of carbene-metal complex are known and they are Fischer and Schrock-type complexes (**Figure 3**).

**Figure 2.** *Triplet and singlet carbenes: electronic configurations. Arrows: electrons.*

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

#### **Figure 3.**

*General structure of Fisher and Schrock Carbene with examples.*

#### **Figure 4.**

*Synthesis of Fischer carbene and their application in organic synthesis.*

The Fischer carbene, which were first described in 1960s, form complexes with low valent or lower oxidation state of metal and are versatile reagents for organic synthesis due to presence of electrophilic carbon center (**Figure 4**). The Schrocktype compounds (first reported in the early 1970s) play an important role in olefin metathesis due to present of nucleophilic carbon center (**Figure 5**).

#### **2. Types of N-heterocyclic carbene (NHC) ligands**

The structure of carbene depends upon the nature of ligands.

**Figure 5.** *Synthesis of Schrock's carbene and their application as catalyst.*

**Figure 6.** *A 4-membered N-heterocyclic carbene ligand 2,6-diisopropyl-substituted substituents.*

#### **2.1 4-Membered NHC**

Grubbs et al. was the first group to develop the 4-membered NHC [3] (**Figure 6**). It was found that for the isolation of carbene carbon steric shielding was very much important. The 2, 6-diisopropyl-substituted constituents led to the successful isolation of the free NHC [3, 4]. The vibrational, ν(CO) values of the corresponding Rhodium dicarbonyl complex (ν(CO) in toluene: 2080 and 1988 cm<sup>1</sup> ) show that it's σ-electron donating properties is slightly less than the dihydroimidazol-2-ylidene analogue [5].

#### **2.2 5-Membered NHC**

5-membered ring systems were most and widely reported NHC carbene so far [6–11]. This is due to the fact that 5-membered ring system is sterically more stable and hence provide the extra stability to the NHC as well as improving its catalytic

*Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

**Figure 7.**

*Imidazol-2-ylidenes and imidazolidin-2-ylidenes.*

properties [8, 9]. Some of the scaffolds used for the preparation of 5-membered NHC were imidazole-2-ylidenes Imes (**L2**), IPr (**L3**), Icy (**L4**), ItBu (**L5**), and IAd (**L6**) and the imidazolidin-2-ylidenes SIMes (**L7**) and SIPr (**L8**) presented in **Figure 7**.

IBIox system of NHC ligands have been well explored and readily derived from bioxazolines (**Figure 8)** (**L9–L13**) [5, 6] probably due to the two reasons. (1) The 4,5-dioxygen substitution affects the ligand's electronic properties. The electron donating capability is similar to electron-rich phosphines like PtBu3, but slightly less electron-rich than other imidazolium-based N-heterocyclic carbene. It is fascinating that all IBiox ligands have the similar or same electronic properties. There are several salient features among the ligands (**L10–L13**):


**Figure 8.** *Some of the most widely applicable 5-membered NHC.*

#### **Figure 9.**

*Important feature for the IBiox NHC ligands.*

A salient feature of these carbenes (NHC ligands) is that steric bulkiness of the ligands can be modified according to the uses without affecting the electronic character of carbene which is an ideal criterion for selection of ligands (**Figure 9**). It is an important and a unique property for such monodentate ligands as compared to monodentate phosphines where increasing the size of the phosphine ligands can affect both their steric and electronic properties.

N-heterocyclic carbene based on Benzimidazolium (**Figure 8**) **L14–L16** [12–16] and **L17** [17] are an important as well as interesting classes of carbenes, though less commonly explored classes of NHC. The synthetic challenges limited the scope to only three electronically different ligands (**L14–L16**) and no sterically tunable benzimidazolium-derived N-heterocyclic carbene [18, 19].

Weiss et al. was the first person to develop and introduce the Bipyridocarbene (**L18** and **L19)** which is a highly electron-rich NHC (**Figure 8**) [20, 21]. This is evident from the strong high-field shift of its carbene signal (196 ppm) in the 13C NMR spectrum [22]. But the instability of this compound limited its application in catalysis. On the other hand, Kunz et al. showed that tert-butyl substitution can lead to the formation of more stable NHC (**L19)** and also reported for the first time, the X-ray structural analysis of these types of carbene [23]. Later on, Lassaletta et al. [8] and Glorius et al. [7] independently developed imidazo [1,5-a] pyridine-3-ylidenes. These can be viewed as benzannulated imidazolin-2-ylidenes **L2–L6.** These ligands form electron-rich carbene as seen in IR spectra. The ν(CO) for cis-(CO)2 RhCl with R1, R2 = Me was found to be 2079 and 2000 cm<sup>1</sup> .

The structures of some other interesting carbene ligands (**L20**) and (**L21**) based on imidazolium backbone are also shown. These ligands showed different reactivity in the palladium-catalysed α-arylation of propiophenone because of their structural features.

#### **2.3 6- and 7-Membered NHC**

6 or 7 membered ring carbenes of N-heterocyclic such as 1,3-disubstituted pyrimidin- 2-ylidenes **L23** [24–27], perimidine-based carbene **L24** [28], **L25–L27** [29] or chiral 7-membered NHC **L28** [30] have only rarely been reported (**Figure 10**). The different electronic properties of NHCs are due to the different backbone structures and in the topology of the substituents on the NHC. Richeson et al. validated this by incorporating a naphthyl ring system in ligand **L25**. This modification changed the shape of the NHC [28]. The value of the N-C carbene–N *Recent Development of Carbenes: Synthesis, Structure, Photophysical Properties… DOI: http://dx.doi.org/10.5772/intechopen.101413*

**Figure 10.** *Some most and widely applicable six and seven-membered NHC.*

bond angle increased from 100 to 110° in 5 membered to 115.3° in 6 membered ring. The carbene N–R angle α is also reduced from 122 to 123° in (**L2–L6)** and (**L7** & **L8**) to 115.5° in **L24**, which had a steric influence of the N-substituents on the carbene carbon. Based on the ν(CO) values of the corresponding cis-(CO)2RhCl complex, ligand **L24** is an even stronger electron donor than the dihydroimidazol-2- ylidenes **L7** & **L8**, but weaker than the acyclic carbene C(NiPr2)2.

Borazines, also known as "inorganic benzene" and isoelectronic with benzene are excellent scaffolds for highly stable heterocycles. When the borane moiety is "exchanged" with an iso-electronic carbene moiety one can obtain NHC **L25–L27**. There have been reports on the synthesis of such stable complexes of these ligands but their catalytic properties have been not explored yet.

The first synthesis of a 7-membered NHC ligand was reported by Stahl et al. very recently [29, 30]. Even though NHC **L28** could not be isolated as a free carbene, palladium complexes of **L28** were isolated and their structures fully characterized. Ligand **L28** is C2 symmetric and because of a torsional twist it shows the Möbiusaromatic character of the 8π-electron carbene heterocycles [31].

#### **2.4 Bi- and multi-dentate NHC**

In addition to these monodentate ligands, several multi-dentate ligands have been synthesized and used for various applications. The rigid bidentate benzimidazole-based N-heterocyclic carbene was used in the synthesis of conjugated organometallic polymers which show interesting electronic and mechanical properties [32]. Another application of such bidentate NHC was the formation of stable chelate complexes. One such palladium-NHC complex was used in the catalytic conversion of methane to methanol [33]. The stability of the complexes is a pre-requirement for such applications as the reaction takes place in an acidic medium (trifluoroacetic acid) at high temperatures (80°C) in the presence of strong oxidizing agents like potassium peroxodisulfate.

Similar stable metal-chelate complexes were reported using tri- and tetradentate ligands. Iron (III) and chromium (III) form complexes of the structure [M(L29)2] <sup>+</sup> with the tripodal tricarbene ligand **L29** (**Figure 11**) [34].

On the other hand, the development of macrocyclic ligands was found to be challenging. Hahn et al. successfully synthesized tetracarbene ligands having crown

**Figure 11.** *A tridentate ligand (L29).*

ether topology in a template-controlled synthetic approach.Initially, a transition metal complex with four unsubstituted benzimidazol-derived NHC **L14** to **L16** (R, X = H) was formed. The carbene ligands are not stable when separated from the transition metal. Then, the carbene ligands were linked by a template-controlled cyclization of alkyl or aryl isocyanides and finally, the desired product was synthesized.
