title: 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 CaII) to form the O-O bond. The conversion of low valent Mn-OHn(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 MnIV-OH complex is described. The MnIV-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 MnIV-OH species to a MnV-oxo complex would likely occur via a stepwise proton transfer-electron transfer mechanism. The large dissociation energy for the MnIVO-H bond (~95 kcal/mol) 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.
authors.affiliation: Department of Chemistry, University of California-Irvine, 1102 Natural Sciences II, Irvine, CA 92697-2025, USA.
authors.forename: Taketo
authors.initials: T
authors.lastname: Taguchi
authors.affiliation: Department of Chemistry, Benedictine College, Lisle, IL 60532. kstone@ben.edu.
authors.forename: Kari L
authors.initials: KL
authors.lastname: Stone
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213.
authors.forename: Rupal
authors.initials: R
authors.lastname: Gupta
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors.forename: Benedikt
authors.initials: B
authors.lastname: Kaiser-Lassalle
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors.forename: Junko
authors.initials: J
authors.lastname: Yano
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hendrich
authors.affiliation: Department of Chemistry, University of California-Irvine, 1102 Natural Sciences II, Irvine, CA 92697-2025, USA.
authors.forename: A S
authors.initials: AS
authors.lastname: Borovik