N-Heterocyclic Carbenes (NHCs): An Introduction

*Ruchi Bharti, Monika Verma, Ajay Thakur and Renu Sharma*

## **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 [6–19].

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,

**Figure 1.** *First isolated NHC IAd (1c).*

different synthetic routes leading to N-heterocyclic carbenes, along with their applications, have also conversed.
