**1. Introduction**

Carbon in its low coordinate divalent state is known as "carbene." It is a well-known transient species in many catalytic reactions. Initially, the carbenes were considered to be very reactive and difficult to isolate. However, the carbenes were first stabilized as metal carbene complexes, which have found numerous applications in many important organometallic reactions. The challenge of isolating a stable metal-free carbene was ended in 1991, through the pioneering works of Arduengo et al. It was for the first time a stable metal-free N-heterocyclic carbene (NHC) was synthesized and isolated in the form of 1,3-di(adamantyl)imidazol-2 ylidene (IAd) by deprotonation of its 1,3-imidazolium salt precursor [1]. Thus, it enabled the easy synthesis of NHCs using simple synthetic routes and thereby extended its scope in synthetic organic chemistry. One such celebrated success is the role of NHCs as ligands in olefin metathesis [2].

1,3-Imidazolium salts are simple and stable precursors of NHCs. They are a good source for *in situ* production of NHCs either by heating or upon reaction with a base. Chemically, these NHCs are nitrogen-containing heterocyclic compounds with a

#### **Figure 1.**

*Schematic representation of σ-withdrawing and π-donating electronic effects through the adjacent nitrogen atoms with the carbene carbon* via *inductive and mesomeric effects, respectively.*

divalent carbene center [3]. Due to the available lone pair of electrons on the carbon, the carbene in these NHCs were often compared with phosphine ligands and validated for its effectiveness. The stability of this electron-rich carbene and its unique electronic properties are primarily due to the presence of two adjacent nitrogen atoms, which will have a push-pull electronic influence at the carbene center (**Figure 1**). The effect is synergistic in nature, where the N atom in the NHC withdraws the σ-electrons inductively and donates π-electrons to the imidazolium ring mesomerically. The inductive effect of the N-atom lowers the highest occupied molecular orbital (HOMO) energy level occupied by the *p*-orbital and thereby increases the energy gap between HOMO and lowest unoccupied molecular orbital (LUMO) levels of NHC. Hence, the second electron gets paired in the HOMO level resulting in the singlet form of carbene. Furthermore, the N-atom mesomerically donates a pair of non-bonding electrons from its *p*-orbital and overlaps with the empty *p*-orbitals of the C-atom, which reinforces the stability of the singlet carbene. Moreover, the cyclic bent structure of NHCs forces the carbene to a singlet state, whereas in acyclic carbenes, it exists in the triplet state [3–8]. Singlet NHCs are capable of forming stable and stronger bonds with metals due to their good σ-donor and π-acceptor properties [9, 10], especially with electron-deficient metal atoms they act as good π-donors [11]. These unique electronic properties will equip NHCs to have similar or sometimes better reactivity than phosphines; hence, they are considered to be one of the important ligands in catalysis.

In addition to the unique electronic features, the structural diversity of the NHCs was another important aspect that attracted the attention of chemists. The scope for variation of the ring size from 4 to 7-membered ring structures and the possibilities for substituent variations either at the N-atom or on the C-atom of the ring have tremendously increased the structural diversity of NHCs. Dimer, trimer, and poly NHCs are also known in the literature. Thus, the relatively easy access to structurally diverse NHCs and its flexible metal coordination has resulted in a large number of NHC-metal complexes. Based on the metal coordination to the NHC unit, they have been classified as normal, abnormal, and remote NHCs (**Figure 2**). If it binds to the metal through the C2 atom, then it is known as normal NHC;

**Figure 2.** *Representative structures of (a) normal NHC; (b) abnormal NHC; and (c) remote NHC.* *Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

**Figure 3.** *Different synthetic strategies for making NHC precursors [14].*

similarly if it is through the C4 atom, it is known as abnormal NHC. If there is an absence of a heteroatom in the α-position of the carbenic coordination center, it is known as remote NHC. Compared with normal NHCs, the abnormal NHCs have stronger donor ability and hence have direct impact on its catalytic efficacy [12].

The typical synthesis of NHCs were often accomplished by deprotonation of azolium salts, such as imidazolium, thiazolium, and triazolium. The focus of this chapter is primarily on the imidazolium-based NHCs. In the synthesis of imidazolium-based NHCs, there are three key components, which form the NHC framework. They are (1) carbon backbone, (2) pre-carbenic unit, and (3) amino unit [13]. Different synthetic strategies were evolved based on the construction of these three key components (**Figure 3**). Apart from the conventional synthetic methods, other methods such as electrochemical, microwave, and solvent free synthetic methods were also known [15–17]. Recently, the incorporation of one or more NHC units into a single carbon backbone was also reported, and these multidentate ligands were used to make poly NHC-metal complexes. These poly NHCs are capable of exhibiting different steric and electronic properties than the normal NHCs. Moreover, due to the inherent structural chirality, they are often studied for applications in asymmetric catalysis [18–23].

Considering the scope and potential applications of imidazolium-based NHCs *via* structural refinement, it is essential to understand the factors that will control its properties and catalytic efficiency. Therefore, the following sections elaborates the methods to assess the steric and electronic properties of the NHCs.
