Understanding the interactions between the bis(trifluoromethylsulfonyl)imide anion and absorbed CO 2 using X-ray diffraction analysis of a soft crystal surrogate.

The selective carbon dioxide (CO 2 ) absorption properties of ionic liquids (ILs) are highly pertinent to the development of methods to capture CO 2 . Although it has been reported that fluorinated components give ILs enhanced CO 2 solubilities, it has been challenging to gain a deep understanding of the interactions occurring between ILs and CO 2 . In this investigation, we have utilized the soft crystalline material [Cu(NTf 2 ) 2 (bpp) 2 ] (NTf 2 ‒  = 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 CO 2 and NTf 2 ‒ , the fluorinated IL component that is responsible for high CO 2 solubility. Analysis of the structure of a CO 2 -loaded crystal reveals that CO 2 interacts with both fluorine and oxygen atoms of NTf 2 ‒ anions in a trans rather than cis conformation about the S-N bond. Theoretical analysis of the structure of the CO 2 -loaded crystal indicates that dispersion and electrostatic interactions exist between CO 2 and the framework. The overall results provide important insight into understanding and improving the CO 2 absorption properties of ILs.