Functional Manganese Model Chemistry Relevant to the Oxygen-Evolving Complex of Photosystem II: Oxidation of a Mn(III,IV) Complex Coupled to Deprotonation of a Terminal Water Ligand

Abstract

A comparison of the pH-dependent oxidation of two structurally similar dinuclear manganese complexes, one that contains terminal water ligands and one that does not, shows that deprotonation of a terminal water ligand is coupled to the Mn(III,IV)/Mn(IV,IV) oxidation. Analysis of the data reveals that the pKa of the terminal water bound to the Mn(IV,IV) complex is approximately 1.8. Proton-coupled electron transfer makes the Mn(III,IV) complex with terminal water ligands progressively easier to oxidize by 59 mV per pH unit when the pH is above 1.8. The presence of terminal water ligands in the oxygen-evolving complex may play an important role in the ``redox leveling'' effect in which all of the redox steps in the four-electron cycle of the oxygen-evolving complex occur over a narrow range of potential.