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.metal-oxo complexes | water oxidation | inorganic chemistry | photosynthesis | oxygen-evolving complex P hotosynthetic water oxidation is an essential chemical reaction that is responsible for producing Earth's aerobic environment. Dioxygen production occurs at the active site of the enzyme photosystem II, referred to as the oxygen-evolving complex (OEC), which contains a unique Mn 4 CaO cluster (1, 2). Several features of the OEC are known, including an approximate arrangement of the metal ions within the cluster (Fig. 1A) (3, 4), its structural flexibility during turnover (5, 6), and its ability to store oxidizing equivalents via five photo-induced redox states (S i , i = 0-4 and known as the Kok cycle) (7). Substrate water molecules bind to the cluster and, upon oxidation, are coupled to produce dioxygen and 4 eq of protons. There is agreement that formation of the O-O bond occurs in the highest oxidized state of the Mn 4 CaO 5 cluster (S 4 ), after which the cluster reverts to the most reduced state, S 0 (1-6, 8-10). The transient S 4 state has eluded detection, making it difficult to experimentally probe the structural and physical requirements necessary to promote dioxygen production. Magnetic resonance and density functional theory (DFT) studies of the S 2 and S 3 states have been used to infer that the beginning and ending oxidation states of the Kok cycle are Mn 3 III Mn IV (S 0 ) and Mn 3 IV Mn V or Mn 3 IV Mn IV O • (S 4 ) (11-14). The location of the Mn V center within the cluster is not certain, yet one possibility is the dangling Mn A4 site, which is coordinated to anionic donors and is surrounded by a ne...

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear MnV–oxo complexes have tetragonal symmetry, producing low-spin species. A new MnV–oxo complex that is high-spin is now reported, which was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry. Titration experiments with [FeCp2]+ were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having Az = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope sensitive peaks at 737 cm−1 and 754 cm−1 assigned as a Fermi doublet centered at 746 cm−1(Δ18O = 31 cm−1). Kβ Mn X-ray emission spectra showed Kβ' and Kβ1,3 bands at 6475.92 and 6490.50 eV, which are characteristic of a high-spin MnV center.

An ultrathin overlayer of MgO on TiO2 is shown to drastically improve the stability of solid-state dye-sensitized solar cell using CuI as a hole conductor in addition to solar energy conversion efficiency.

Manganese–hydroxo species have been implicated in C–H bond activation performed by metalloenzymes, but the electronic properties of many of these intermediates are not well characterized. The present work presents a detailed characterization of three Mnn–OH complexes (where n = II, III, and IV) of the tris[(N′-tert-butylureaylato)-N-ethylene]aminato ([H3buea]3−) ligand using X- and Q-band dual mode electron paramagnetic resonance (EPR). Quantitative simulations for the [MnIIH3buea(OH)]2− complex demonstrated the ability to characterize similar MnII species commonly present in the resting states of manganese-containing enzymes. The spin states of the MnIII and MnIV complexes determined from EPR spectroscopy are S = 2 and 3/2, respectively, as expected for the C3 symmetry imposed by the [H3buea]3− ligand. Simulations of the spectra indicated the constant presence of two MnIV species in solutions of [MnIVH3buea(OH)] complex. The simulations of perpendicular- and parallel-mode EPR spectra allow determination of zero-field splitting and hyperfine parameters for all complexes. For the MnIII and MnIV complexes, density functional theory calculations are used to determine the isotropic Mn hyperfine values, to compare the excited electronic state energies, and to give theoretical estimates of the zero-field energy.

No abstract

L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming O K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.

No abstract

No abstract