Synthesis of Triarylsulfonium Salts with Sterically Demanding Substituents and Their Alkaline Stability

As cationic functional groups with excellent alkaline resistance that are potentially applicable to building blocks of robust anion exchange membrane (AEM) materials for water splitting and fuel cell modules, we describe the synthesis of triarylsulfonium (TAS) salts bearing sterically demanding substituents by the reaction of arynes with diaryl sulfides/sulfoxides and by the Friedel-Crafts reaction of diaryl sulfoxides. The TAS cations possessing three substituted benzene rings, such as tris(2,5-dimethylphenyl)sulfonium and bis(2,5-dimethylphenyl)mesitylsulfonium, were effectively produced through the appropriate choice of reactions and reagents. The alkaline stability of the TAS cations thus obtained was evaluated from their time-course ¹H NMR spectra in 1 M KOH/CD₃OD, from which the alkaline resistance of the TAS cations increased dramatically as the steric bulkiness of the aromatic substituents attached to the TAS cations increased. Among them, bis(2,5-dimethylphenyl)mesitylsulfonium was found to exhibit 25 times higher alkaline resistance performance compared to benzyltrimethylammonium, a conventional quaternary ammonium cation. The decomposition mechanism of the TAS cations in the basic methanol media was studied in detail, and it was concluded that the decomposition occurred by the nucleophilic ipso-substitution by the methoxide anions.