Figure 3.

(a) The water oxidation centre in photosystem II (Ferreira et al. 2004) and (b) the Hangman porphyrin (I) assemble the oxygen of two waters for coupling, (II) activate the water to oxo by PCET and (III) position a high-valent oxo along the reactive metal hydroxide vector. Though the resting state of the Hangman is a Fe^III–OH⋯H₂O complex, the Hangman porphyrin is prepared by the introduction of an Fe^II into the porphyrin core and the assembly of two waters. Production of the Compound I intermediate of the Hangman thus results from an overall (4e⁻,4H⁺) process.