Studies on the Reactivity of a Bis–Mesityl Imidazolyl Carbene Intermediate toward Carbon Dioxide and Stability of the Resulting Carboxylate

1,3–bis(2,4,6–trimethylphenyl)imidazolium carboxylate, an adduct between CO₂ and the N–heterocyclic carbene (NHC), 1,3–bis(2,4,6–trimethylphenyl)–1,3–dihydro–2H–imidazol–2–ylidene, was synthesized to study the reactivity of CO₂ after binding to the carbene intermediate. Nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, X–ray powder diffraction (XRD), gas chromatography (GC) and thermal gravimetric analysis (TGA) were employed to characterize the final imidazolium carboxylate. GC was specifically used to study the dissociation of the CO₂ adduct. The structure of the synthesized zwitterion was confirmed via ¹H and ¹³C NMR, where adduct formation generated a new peak in the ¹³C NMR spectrum. IR spectroscopic data showed a significant characteristic peak for C=O stretch at around 1670 cm^–1. TGA spectra showed that the zwitterion‟s weight loss of 13% at 155 °C which is the percent weight of CO₂ . The GC study of CO₂ , which was released after treating the imidazolium carboxylate with 5% H₂O in CH₃CN, enabled the possibility of the reversibility of CO₂^– NHC adduct formation. The stability and air sensitivity of the imidazolium carboxylate were tested in polar, nonpolar, acidic, basic, and mixed solvents via simple effervescence tests and GC. The hydrolytic stability of the imidazolium carboxylate was examined. The bis-mesityl carboxylate showed reasonable stability in water, in contrast to carboxylates with smaller alkyl substituents, but admixture with organic solvent would cause it to break down into the corresponding imidazolium bicarbonate. After exposure to H_{2 (g)} and heat, there was evidence for the reductive conversion of the carboxylate into imidazolium formate. This suggests the application of the mesityl imidazolyl carbene as an organic catalyst for CO₂ reduction.