title: High-spin Mn-oxo complexes and their relevance to the oxygen-evolving complex within photosystem II.
abstract: The structural and electronic properties of a series of manganese complexes with terminal oxido ligands are described. The complexes span three different oxidation states at the manganese center (III-V), have similar molecular structures, and contain intramolecular hydrogen-bonding networks surrounding the Mn-oxo unit. Structural studies using X-ray absorption methods indicated that each complex is mononuclear and that oxidation occurs at the manganese centers, which is also supported by electron paramagnetic resonance (EPR) studies. This gives a high-spin Mn(V)-oxo complex and not a Mn(IV)-oxy radical as the most oxidized species. In addition, the EPR findings demonstrated that the Fermi contact term could experimentally substantiate the oxidation states at the manganese centers and the covalency in the metal-ligand bonding. Oxygen-17-labeled samples were used to determine spin density within the Mn-oxo unit, with the greatest delocalization occurring within the Mn(V)-oxo species (0.45 spins on the oxido ligand). The experimental results coupled with density functional theory studies show a large amount of covalency within the Mn-oxo bonds. Finally, these results are examined within the context of possible mechanisms associated with photosynthetic water oxidation; specifically, the possible identity of the proposed high valent Mn-oxo species that is postulated to form during turnover is discussed.
authors: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213;
authors: Rupal
authors: R
authors: Gupta
authors: Department of Chemistry, University of California, Irvine, CA 92697; and.
authors: Taketo
authors: T
authors: Taguchi
authors: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors: Benedikt
authors: B
authors: Lassalle-Kaiser
authors: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213;
authors: Emile L
authors: EL
authors: Bominaar
authors: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors: Junko
authors: J
authors: Yano
authors: Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213;
authors: Michael P
authors: MP
authors: Hendrich
authors: Department of Chemistry, University of California, Irvine, CA 92697; and aborovik@uci.edu.
authors: A S
authors: AS
authors: Borovik
mesh_terms: D015502:Absorptiometry, Photon; D004578:Electron Spin Resonance Spectroscopy; D008345:Manganese / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D010100:Oxygen
keywords: inorganic chemistry; metal–oxo complexes; oxygen-evolving complex; photosynthesis; water oxidation