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

Xin Zheng,Yuh Hijikata,Shin-Ichiro Noro,Hiroyasu Sato,Jenny Pirillo,Kiyonori Takahashi,Katsuo Fukuhara,Takayoshi Nakamura

doi:10.1038/s42004-020-00390-1

Xin Zheng, Yuh Hijikata + Show 6 more

Open Access

https://doi.org/10.1038/s42004-020-00390-1

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Abstract

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.

Highlights

Ionic liquids (ILs) have received increasing attention in the past several decades owing to their wide variety of potential industrial applications[1,2,3]
The soft crystal 1 crystallized in the monoclinic space group P2/n, containing crystallographically independent one copper ion, two NTf2‒ anions and two bpp ligands
The 2D layers and NTf2‒ anions are densely packed through the above-mentioned weak coordinative interactions and weak hydrogen bonding interactions between the trifluoromethylsulfonyl groups of NTf2‒ and pyridyl groups of the bpp ligands (Supplementary Fig. 4)

Summary

Introduction

Ionic liquids (ILs) have received increasing attention in the past several decades owing to their wide variety of potential industrial applications[1,2,3]. We envisaged that it might be possible to design and fabricate soft crystals, which contain components that are the same as or similar to those present in ILs, and that might possess the ability to absorb CO2 in synchrony with stimuli promoted structural changes. In this way, it would be possible to utilize these substances as surrogates to determine the structures of and elucidate important interactions in ILs containing absorbed CO2 using standard single-crystal X-ray diffraction techniques. Crystal X-ray diffraction techniques, we elucidated the nature of interactions with the NTf2‒ anion that are responsible for CO2 absorption

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