Elucidating the Iron‐Based Ionic Liquid [C4py][FeCl4]: Structural Insights and Potential for Nonaqueous Redox Flow Batteries

In this study, a low‐melting organic‐inorganic crystalline ionic liquid compound, N‐butyl pyridinium tetrachlorido ferrate (III) is described. The material can easily be synthesized using a one‐pot approach in an ionic liquid medium. Single‐crystal X‐ray diffraction confirms that the basic inorganic block is [FeCl4]−, which is counterbalanced by an N‐butyl pyridinium cation. The compound exhibits a melting point of 37.6 °C by differential scanning calorimetry, which is among the lowest values for a pyridinium‐based metal‐containing ionic liquid. The material shows promising electrochemical behavior at room temperature in both aqueous and nonaqueous solvents, and at elevated temperatures in its pure liquid state. Given its appreciable solubility in both water and acetonitrile, the compound can act as a redox‐active species in a supporting electrolyte for redox flow battery applications. These classes of low‐melting ionic solids with long‐range order and interesting electrochemical applications are potential candidates for a range of green energy storage and harvesting systems. A low‐melting organic‐inorganic crystalline solid [C4Py][FeCl4] is synthesized and a detailed structural, thermal, and optical analysis is performed. The relatively simple inorganic building block of [FeCl4]‐ provides interesting electrochemical properties in both water and acetonitrile. In a proof‐of‐concept study [C4Py][FeCl4] is shown to act as a redox‐active species as a negolyte for redox flow battery applications.