VI. Discussion of acidity trends in C4, C5-substituted cations As mentioned earlier, imidazolium ions are converted into carbenes when treated with a sufficiently strong base. In the corresponding N-heterocyclic carbenes, the carbene moiety is stabilized by lone pairs from the adjacent nitrogen atoms. Substituents at the imidazolium ring play an important role in stabilizing the carbene. The first air-stable carbene, 1,3-dimesityl-4,5dichloroimidazol-2-ylidene, owed its stability to the presence of chlorine atoms at C4 and C5. Its stability was attributed to the inductive electron-withdrawing effect of the chlorine substituents, which reduce electron density on the C2 carbon atom that bears the carbene lone pair100. As electron withdrawing substituents stabilize carbenes generated from imidazolium ions, it may be expected that such substituents also facilitate proton removal from the imidazolium C2 and thus lead to more acidic C2 hydrogen atoms. When comparing VDD atomic charges101 calculated for MM and DiCl, a higher positive charge is found at C2 of DiCl and also its C2 hydrogen atom bears a higher positive charge (Table S4.1). This suggests that C2–H of the electron-poor DiCl will be more acidic than C2–H of MM. DiPh shows atomic charges at C2 and at the C2 hydrogen atom that are lower than the charges at DiCl and more similar to MM, suggesting a closer C2–H acidity for MM and DiPh cations. Although the overall charge density of the ring, including C4 and C5, displays a larger localized positive charge for DiPh compared to MM cation (0.266 for DiPh versus 0.243 for MM) which should Table S4.1. VDD Atomic Charges of the Imidazolium Ions
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