**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


**Table 1.**

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

co-ligands present on the NHC ring corresponding to the metal [43–46]. 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 electronrich ligands than the most basic trialkyl phosphines (**Table 1**) [49].

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.*

#### *N-Heterocyclic Carbenes (NHCs): An Introduction DOI: http://dx.doi.org/10.5772/intechopen.102760*

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].

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 [59–61], photocatalysis [62, 63], and many other [64–70].

However, escalating publications reveal that the bond between metal and carbene is not unreactive [53, 71–74]. 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


#### **Table 2.**

*Steric demand and bond strength of some important ligands.*

**Figure 8.** *Equilibrium of complexation.*

of palladium NHC complexes, the generation of palladium black is observed, which points toward the decomposition pathways.
