Cation-Anion and Anion-CO₂ Interactions in Triethyl(octyl)phosphonium Ionic Liquids with Aprotic Heterocyclic Anions...

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO₂ capture applications. The anions of AHA ILs play a significant role in tuning anion-CO₂ complexation. In addition, AHAs are able to trigger the abstraction of acidic protons located at the α position of phosphonium cations by forming hydrogen bonds between cations and anions, eventually leading to cation-driven CO₂ complexation. Here we investigate the role of the anion in cation-anion hydrogen bonding and ylide formation. Using CO₂ uptake measurements, ³¹P nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) deuterium exchange equilibrium and rates, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and density functional theory calculations, we show that the key is the proximity of the negatively charged nitrogen atoms on the anion to the α protons, which is governed not just by anion basicity but by sterics. Thus, we show that triethyl(octyl)phosphonium 3-methyl-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P₂₂₂₈] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.