Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO 2 capture applications. The anions of AHA ILs play a significant role in tuning anion−CO 2 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 CO 2 complexation. Here we investigate the role of the anion in cation− anion hydrogen bonding and ylide formation. Using CO 2 uptake measurements, 31 P 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-5trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P 2228 ] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.

The transport properties, thermal properties, and CO2 solubility for several ionic liquids (ILs) with triethyl­(octyl)­phosphonium cations and a variety of CO2-reactive aprotic N-heterocyclic anions (AHAs) are reported in this work. Eleven new ILs were designed and synthesized. They were characterized in terms of their melting points, glass transition temperatures, decomposition temperatures, viscosities and densities (where possible), as well as their CO2 capacity as a function of pressure. Of the 11, 3 were solid at room temperature, 1 was a room-temperature liquid which remained liquid upon reaction with CO2, and 7 others were liquids that crystallized at room temperature upon reaction with CO2, so experimentation at elevated temperatures was required. The CO2 uptake isotherms for seven of the ILs, at temperatures ranging from 49 to 64 °C and pressures from 0 to 80 kPa, were fit to a Langmuir model. The CO2 solubility for several of these ILs was among the highest reported at these temperatures and pressures for AHA ILs in the literature, but they have lower thermal stability and higher viscosity than other promising AHA ILs.