Twofold Oxidized and Twofold Protonated Redox-Active Guanidine: An Ultimate Intermediate in Proton-Coupled Electron-Transfer Reactions

Due to their high Bronsted basicity and redox properties, guanidino-functionalized aromatics (GFAs) are privileged proton-coupled electron-transfer (PCET) reagents. Herein we show that the GFA 1,2,4,5-tetrakis(tetramethylguanidino)-benzene (1), after being oxidized to the green dicationic state (1(2+)), can bind one or two protons, resulting in the blue monoprotonated trication (1+H)(3+) and the orange diprotonated tetracation (1+2H)(4+), realizing the first purely organic PCET reagent that can be diprotonated in its oxidized state. With trifluoroacetate counterions, low-barrier hydrogen-bonding is observed, allowing to shift protons reversibly in dependence of the temperature and solvent polarity between 1(2+) and trifluoroacetate. The protonation of 1(2+) has dramatic effects on its oxidation capabilities.