133.67, 126.19 (NTf2), 123.00 (NTf2), 119.81 (NTf2), 118.78, 116.62 (NTf2), 115.79, 56.55 (OCH3), 35.50 (1+3-C). 19F NMR (376 MHz, CD3CN): −80.21. 1,3,4,5-Tetramethylimidazolium iodide. 1H NMR (400 MHz, CD3CN): 8.62 (s, 1H, 2-H), 3.69 (s, 6H, 1+3-CH3), 2.19 (s, 6H, 4+5-CH3). 13C NMR (101 MHz, DMSO-d6): 134.66 (2-C), 126.71 (4+5C), 33.35 (1+3-C), 7.91 (4+5-CH3). 1,3,4,5-Tetramethylimidazolium bis(trifluoromethane)sulfonamide. 1H NMR (400 MHz, CD3CN): 8.22 (s, 1H, 2-H), 3.65 (s, 6H, 1+3-CH3), 2.19 (s, 6H, 4+5-CH3). 13C NMR (101 MHz, CD3CN): 135.48 (2-C), 128.85 (4+5-C), 126.05 (NTf2), 122.87 (NTf2), 119.68 (NTf2), 116.49 (NTf2), 34.44 (1+3-C), 8.62 (4+5-CH3). 19F NMR (376 MHz, CD3CN): −80.23. V. Determining the pKa of DiPh cation Information on the acidity of imidazolium cations can be obtained by exposing such cations to anions with a known basicity. If its basicity is sufficiently high, the anion will acquire a proton from the imidazolium cation. This process can be monitored using 1H NMR spectroscopy 70. Here, the indenyl anion was selected to act as a base and indicator anion. DMSO-d6 was deprotonated using potassium hydride, and to the resulting potassium dimsyl (CH3S(=O)- CH2K) solution, indene was added, yielding a green solution of indenylpotassium in DMSOd6. Indene is known to have a pKa of 20.1 in DMSO98. Treatment of DiPh with a 1:1 mixture of indene (1 eq.) and indenylpotassium (1 eq.) led to a rapid color change from green to dark purple, indicating the formation of DiPh carbene. A 1H NMR spectrum of this reaction mixture (Figure S4.19, bottom) showed a strong decrease in signal intensity of peaks at δ = 5.8, 6.27 and 7.15 ppm, associated with the indenyl anion99. This demonstrates that the pKa of DiPh has a value lower than 20.1. Interestingly, since the DMSO pKa value of MM is 22.0 70, we can conclude that the C2-proton acidity of DiPh is higher than that of MM.
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