In the last two decades, CO₂ emission has caused a lot of environmental problems. To mitigate the concentration of CO₂ in the atmosphere, various strategies have been implemented, one of which is the use of N-heterocyclic carbenes (NHCs) and related complexes to accomplish the capture, fixation, and activation of CO₂ effectively. In this review, we summarize CO₂ capture, fixation, and activation by utilizing NHCs and related complexes; homogeneous reactions and their reaction mechanisms are discussed. Free NHCs and NHC salts can capture CO₂ in both direct and indirect ways to form imidazolium carboxylates, and they can also catalyze the reaction of aromatic aldehydes with CO₂ to form carboxylic acids and derivatives. Moreover, associated with transition metals (TMs), NHCs can form NHC–TM complexes to transform CO₂ into industrial acid or esters. Non-metal–NHC complexes can also catalyze the reactions of silicon and boron complexes with CO₂. In addition, catalytic cycloaddition of epoxides with CO₂ is another effective function of NHC complexes, and NHC ionic liquids perform excellently in this aspect.

The mechanism of the 1,3-dipolar cycloaddition reaction of azidotrimethylsilane (ATS) onto nanographene (NG) is thoroughly investigated at the B3LYP/6-31G(d,p) level. Calculations reveal that the reaction occurs through a two-step reaction mechanism. The first step is the chemical adsorption and the second one is the decomposition of the thereby formed nitride upon thermal activation, giving rise to an N-bridged product ultimately. The latter is the rate-determining step. Two possible pathways are compared to show that the [3+2] channel is favored over the [3+4] channel. The former is a symmetric synchronous process, whereas the latter follows an asymmetric concerted way, which can be rationalized by means of the frontier molecular orbital (FMO) theory. The reactivity of NG is discussed in detail, revealing that it is the electron density at the functionalization site which dominates the reactivity rather than the energetic effect. As a result, the edge area is calculated to be much more reactive than the centre.

The mechanism of the CO₂ splitting reaction with N-heterocyclic carbene (NHC) as a catalyst and cinnamic aldehydes as the oxygen acceptor was thoroughly investigated at the B3LYP/6-31G(d,p) level. Owing to its nucleophilicity, NHC can initiate the reaction through two distinct channels (I and II) by nucleophilic attack. The results reveal that without the assistance by H₂O, channel II is slightly more favorable than channel I, and the reaction path II-D is the optimum reaction path. However, the reaction is difficult to conduct at room temperature because of its high activation energy barrier. Although the results suggest that most of the high activation energy barriers involved in proton transfer decrease if the reaction is assisted by H₂O, all highest energy barriers for all reaction channels are still above 40 kcal mol⁻¹. This indicates that this reaction cannot be performed at room temperature, which is also proven by experiments.