Hydrogen Evolution Catalyzed by a Metal-Free Corrole: an Experimental and Theoretical Mechanistic Study

Free-base 5,10,15-tris(pentafluorophenyl)corrole [C-3H] has been shown to be electrocatalytically active towards hydrogen evolution in acetonitrile under acidic conditions. We observed hydrogen gas generation at -1.22 V vs Fc/Fc(+) in acetonitrile when [C-3H] was used as a catalyst and p-toluenesulfonic acid was used as the proton source, with an overpotential of 0.57 V. This activity is comparable to other metalated and metal-free macrocycles. From a combination of experimental observations and density functional theory calculations, a mechanistic pathway was obtained when a strong acid was used as the proton source (p-toluenesulfonic acid). The favored mechanism for hydrogen gas generation when p-toluenesulfonic acid is used is a protonation, reduction, reduction, protonation (CEEC) pathway. On the other hand, when a weaker acid (benzoic acid) was used, the metal-free corrole could not catalyze the proton reduction reaction.