Characterization of Monomeric Mn<sup>II/III/IV</sup>–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

Gupta, Rupal; Taguchi, Takayuki; Borovik, A. S.; Hendrich, Michael P.

doi:10.1021/ic401681r

Cited by 51 publications

(73 citation statements)

References 43 publications

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“…Our analysis of the EPR data showed that two distinct Mn IV –OH species are formed, a result that is similar to what we reported for [Mn IV H 3 buea(OH)]. 30,45 The largest difference between the two species for [Mn IV H 2 pout(OH)] is in their E/D values: species 1 ( E/D = 0.33) is significantly more rhombic than species 2 ( E/D = 0.17). The effective g-tensors for the |±1/2> and excited |±3/2> doublets for the two species are reported in Table 3 and indicated on Figure 8.…”

Section: Resultssupporting

confidence: 90%

“…The EPR spectrum of [Mn II H 2 pout(OH)] 2- (Figure 6) in perpendicular-mode showed signals over a wide magnetic field range. The spectra are complicated by resonances from multiple overlapping transitions as has been described previously for [Mn II H 3 buea(OH)] 2- 45. The simulation overlaid on the experimental spectrum in Figure 6 is for a S = 5/2 spin center using the parameters of described in Table 2.…”

Section: Resultsmentioning

confidence: 97%

“…The zero-field parameters are similar to the previously characterized [Mn II H 3 buea(OH)] 2- and, as expected, [Mn II H 2 pout(OH)] 2- showed an increased in rhombic parameter E/D owing to the unsymmetrical nature of the tripodal ligand. 45 …”

Section: Resultsmentioning

confidence: 99%

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Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

et al. 2018

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Hydrogen-bonds (H-bonds) within the secondary coordination sphere are often invoked as essential noncovalent interactions that lead to productive chemistry in metalloproteins. Incorporating these types of effects within synthetic systems has proven a challenge in molecular design that often requires the use of rigid organic scaffolds to support H-bond donors or acceptors. We describe the preparation and characterization of a new hybrid tripodal ligand ([H2pout]3–) that contains two mono-deprotonated urea groups and one phosphinic amide: the urea groups serve as H-bond donors while the phosphinic amide group serves as a single H-bond acceptor. The [H2pout]3– ligand was utilized to stabilizes a series of Mn–hydroxido complexes in which the oxidation state of the metal center ranges from 2+ to 4+. The molecular structure of MnIII–OH complex demonstrates that three intramolecular H-bonds involving the hydroxido ligand are formed. Additional evidence for the formation of intramolecular H-bonds was provided by vibrational spectroscopy in which the energy of the O–H vibration supports its assignment as an H-bond donor. The step-wise oxidation of [MnIIH2pout(OH)]2- to its higher oxidized analogs was further substantiated by electrochemical measurements and results from electronic absorbance and electron paramagnetic resonance spectroscopies. Our findings illustrate the utility of controlling both the primary and secondary coordination spheres to achieve structurally similar Mn–OH complexes with varying oxidation states.

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confidence: 90%

Section: Resultsmentioning

confidence: 97%

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Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

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“…(Table 2). Attempts to prepare the analogous Mn II -oxo complex were unsuccessful because the oxido ligand is too basic (22), but we have previously reported the isolation of [Mn II H 3 buea(OH)] 2− , which has an experimental value of jA FC j = 250 MHz (32). We examined the changes in A FC for this series of manganese complexes spanning four consecutive oxidation states with nearly identical molecular structures.…”

Section: Resultsmentioning

confidence: 99%

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

Gupta

Taguchi

Lassalle‐Kaiser

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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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...

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“…5c A detailed analysis of the EPR properties for the three Mn–OH complexes has been described in a separate report. 14 …”

Section: Results ‡mentioning

confidence: 99%

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

et al. 2014

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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.

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Characterization of Monomeric Mn^II/III/IV–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

Cited by 51 publications

References 43 publications

Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

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

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Characterization of Monomeric MnII/III/IV–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

Cited by 51 publications

References 43 publications

Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

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

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Product

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Characterization of Monomeric Mn^II/III/IV–Hydroxo Complexes from X- and Q-Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

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