cation yielded inconclusive results, necessitating further investigation. Thus, elucidating the difference in behavior between these cations remains a challenge. Introduction According to the findings in Chapter 3, 1,3-dimethyl imidazolium cation was found to display the highest activity among the five cations investigated. Here, we will broaden the parameter space by focusing on the effect of C4 and C5-substituents. Imidazolium cations are conjugated acids of N-heterocyclic carbenes (NHCs) (Figure 4.1. a). For NHCs, the electronic features and consequently the conjugated acidity are controlled via electron donating or electron withdrawing characteristics of substituents at the N1, N3, or C4, C5 positions (Figure 4.1. b). In Chapter 3, we tuned the acidity of the imidazolium cation by altering the substituents at the N1 and N3 positions, affecting the C2 proton acidity and interaction with surface adsorbed CO2. So far, we have excluded steric effects, which could play a role if substituents become too bulky. This steric influence on the reactivity of C2-H is perceived to be much less by substitutions at the C4 and C5 positions5, 91. Besides, feasible synthetic routes are available for introducing various substituents at the C4 and C5 positions during construction of the imidazole ring. Figure 4.1. (a) Structure of 1,3-dimethyl imidazole-2-ylidenes (left) and its conjugate acid 1,3-dimethyl imidazolium cation (right). The acidity of the imidazolium cation is in direct relation with the basicity of the free carbene. (b) The electronic features of C2-carbene are tuned via substituents at N1, N3(R) and C4,C5 (R') positions.
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