Hydrogen Generation by Hangman Metalloporphyrins

A cobalt(II) hangman porphyrin with a xanthene backbone and a carboxylic acid hanging group catalyzes the electrochemical production of hydrogen from benzoic and tosic acid in acetonitrile solutions. We show that (CoH)-H-II is exclusively involved in the generation of H-2 from weak acids. In a stronger acid, a (CoH)-H-III species is observed electrochemically, but it still needs to be further reduced to (CoH)-H-II before H-2 generation occurs. Overpotentials for H-2 generation are lowered as a result of the hangman effect.