Fluoride-Based Deep Eutectic Solvents with Amide Dual-Hydrogen-Bond Donors

Developing F^--containing electrolytes is crucial for electrochemical and chemical fluorination. However, balancing the F^- concentration and electrochemical stability of the electrolytes remains a challenge. In this study, fluoride-based deep eutectic solvents (F-DESs) were obtained by using amide hydrogen-bond donors (HBDs) containing dual N-H bonds. The obtained F-DES, [TMA]F·3.5[1,3-DMU], was prepared by facilely mixing solid compounds of tetramethylammonium fluoride ([TMA]F) and 1,3-dimethylurea (1,3-DMU), resulting in a high F^- concentration (2.6 mol dm^-3) and a wide electrochemical window (3.1 V) at room temperature. The electrochemical window was much wider than that of [TMA]F·3.5[EG] (EG, ethylene glycol) as another F-DES with an alcohol HBD (1.9 V). Moreover, [TMA]F·3.5[1,3-DMU] exhibited an ionic conductivity that was 2 orders of magnitude higher than that of [TMA]F·3.5[1,3-DMTU] (1,3-DMTU, 1,3-dimethylthiourea) around room temperature because of the bifurcated hydrogen bonds between the dual N-H bonds of 1,3-DMU and one F^-. Thus, [TMA]F·3.5[1,3-DMU] was demonstrated to be applicable to electrochemical fluorination.