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 Co(II)H is exclusively involved in the generation of H(2) from weak acids. In a stronger acid, a Co(III)H species is observed electrochemically, but it still needs to be further reduced to Co(II)H before H(2) generation occurs. Overpotentials for H(2) generation are lowered as a result of the hangman effect.