McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States.
J Phys Chem B. 2020 Oct 8;124(40):8877-8887. doi: 10.1021/acs.jpcb.0c06374. Epub 2020 Sep 25.
Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO capture applications. The anions of AHA ILs play a significant role in tuning anion-CO 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 complexation. Here we investigate the role of the anion in cation-anion hydrogen bonding and ylide formation. Using CO uptake measurements, 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-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.
具有非质子杂环阴离子 (AHA) 的离子液体已被开发用于燃烧后 CO 捕获应用。AHA ILs 的阴离子在调节阴离子-CO 络合方面起着重要作用。此外，AHA 能够通过在阳离子和阴离子之间形成氢键，触发与位于鏻阳离子的α位置的酸性质子的离域，最终导致阳离子驱动的 CO 络合。在这里，我们研究了阴离子在阳离子-阴离子氢键和内鎓形成中的作用。使用 CO 吸收测量、P 核磁共振 (NMR)、衰减全反射-傅里叶变换红外 (ATR-FTIR) 氘交换平衡和速率、二维核 Overhauser 效应光谱 (2D NOESY) 和密度泛函理论计算，我们表明关键是阴离子上带负电荷的氮原子与α质子的接近程度，这不仅受阴离子碱性控制，还受立体位阻控制。因此，我们表明三乙基（辛基）鏻 3-甲基-5-三氟甲基吡唑啉比具有更碱性的 2-氰基吡咯烷和 3-三氟甲基吡唑啉阴离子的等效 [P] ILs 在与阳离子氢键和去质子化方面更有效。
