Understanding the interactions between the bis(trifluoromethylsulfonyl)imide anion and absorbed CO₂ using X-ray...

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.