Understanding the interactions between the bis(trifluoromethylsulfonyl)imide anion and absorbed CO2 using X-ray diffraction analysis of a soft crystal surrogate

The selective carbon dioxide (CO2) absorption properties of ionic liquids (ILs) are highly pertinent to the development of methods to capture CO2. Although it has been reported that fluorinated components give ILs enhanced CO2 solubilities, it has been challenging to gain a deep understanding of the interactions occurring between ILs and CO2. In this investigation, we have utilized the soft crystalline material [Cu(NTf2)(2)(bpp)(2)] (NTf2- = bis(trifluoromethylsulfonyl)imide, bpp = 1,3-bis-(4-pyridyl)propane) as a surrogate for single-crystal X-ray diffraction analysis to visualize interactions occurring between CO2 and NTf2-, the fluorinated IL component that is responsible for high CO2 solubility. Analysis of the structure of a CO2-loaded crystal reveals that CO2 interacts with both fluorine and oxygen atoms of NTf2- anions in a trans rather than cis conformation about the S-N bond. Theoretical analysis of the structure of the CO2-loaded crystal indicates that dispersion and electrostatic interactions exist between CO2 and the framework. The overall results provide important insight into understanding and improving the CO2 absorption properties of ILs. The selective CO2 absorption properties of ionic liquids make them attractive for CO2 capture purposes, but their tendency to crystallize poorly hampers structural investigations. Here a soft crystalline material is used as a surrogate for single-crystal X-ray diffraction analysis to visualize interactions between CO2 and NTf2-, the fluorinated component that is responsible for high CO2 solubility.