Preparation and properties of an MnIV–hydroxide complex: proton and electron transfer at a mononuclear manganese site and its relationship to the oxygen evolving complex within photosystem II

Abstract

Photosynthetic water oxidation is catalyzed by a Mn₄O₅Ca cluster with an unprecedented arrangement of metal ions in which a single manganese center is bonded to a distorted Mn₃O₄Ca cubane-like structure. Several mechanistic proposals describe the unique manganese center as a site for water binding and subsequent formation of a high valent Mn–oxo center that reacts with a M–OH unit (M = Mn or Ca^II) to form the O–O bond. The conversion of low valent Mn–OH_n (n = 1, 2) to a Mn–oxo species requires that a single manganese site be able to accommodate several oxidation states as the water ligand is deprotonated. To study these processes, the preparation and characterization of a new monomeric Mn^IV–OH complex is described. The Mn^IV–OH complex completes a series of well characterized Mn–OH and Mn–oxo complexes containing the same primary and secondary coordination spheres; this work thus demonstrates that a single ligand can support mononuclear Mn complexes spanning four different oxidation states (II through V) with oxo and hydroxo ligands that are derived from water. Moreover, we have completed a thermodynamic analysis based on this series of manganese complexes to predict the formation of high valent Mn–oxo species; we demonstrated that the conversion of a Mn^IV–OH species to a Mn^V–oxo complex would likely occur via a stepwise proton transfer-electron transfer mechanism. The large dissociation energy for the Mn^IVO–H bond (∼95 kcal mol⁻¹) diminished the likelihood that other pathways are operative within a biological context. Furthermore, these studies showed that reactions between Mn–OH and Mn–oxo complexes lead to non-productive, one-electron processes suggesting that initial O–O bond formation with the OEC does not involve an Mn–OH unit.