EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II

Huang, Ping; Kurz, Philipp

doi:10.1007/bf03166262

Cited by 8 publications

(10 citation statements)

References 50 publications

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“…41,50-52 Especially EPR is a powerful technique in manganese chemistry, as the strong coupling of the 55 Mn nuclei (I = 5/2) with unpaired electrons of the complex results in EPR signal patterns which are characteristic for distinct manganese redox states and complex nuclearities. 53 For example, the electrochemical oxidation of 1 (an EPR silent compound) at +0.69 V vs. Fc/Fc + in acetonitrile containing 0.5 M water leads to the formation of a species for which the 16-line Xband EPR spectrum shown in Fig. 3 is detected.…”

Section: Manganese Based Catalysts For Oxygen Evolution and Water Oxi...mentioning

confidence: 99%

Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

2012

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One of the most fundamental processes of the natural photosynthetic reaction sequence is the light-driven oxidation of water to molecular oxygen. In vivo, this reaction takes place in the large protein ensemble Photosystem II, where a μ-oxido-Mn(4)Ca- cluster, the oxygen-evolving-complex (OEC), has been identified as the catalytic site for the four-electron/four-proton redox reaction of water oxidation. This Perspective presents recent progress for three strategies which have been followed to prepare functional synthetic analogues of the OEC: (1) the synthesis of dinuclear manganese complexes designed to act as water-oxidation catalysts in homogeneous solution, (2) heterogeneous catalysts in the form of clay hybrids of such Mn(2)-complexes and (3) the preparation of manganese oxide particles of different compositions and morphologies. We discuss the key observations from the studies of such synthetic manganese systems in order to shed light upon the catalytic mechanism of natural water oxidation. Additionally, it is shown how research in this field has recently been motivated more and more by the prospect of finding efficient, robust and affordable catalysts for light-driven water oxidation, a key reaction of artificial photosynthesis. As manganese is an abundant and non-toxic element, manganese compounds are very promising candidates for the extraction of reduction equivalents from water. These electrons could consecutively be fed into the synthesis of "solar fuels" such as hydrogen or methanol.

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Section: Manganese Based Catalysts For Oxygen Evolution and Water Oxi...mentioning

confidence: 99%

Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

2012

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“…[21,22] These features are incorporated in a series of our synthetic dinuclear manganese complexes, where we have used bridging acetato groups together with pyridine rings as ligands for the manganese centers. [23] We present here the detailed study of the redox behavior of a dimeric manganese compound, [Mn…”

Section: Introductionmentioning

confidence: 99%

Redox Reactions of a Dinuclear Manganese Complex – the Influence of Water

et al. 2008

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The redox properties of the dinuclear manganese complex [Mn III,III 2 L(µ-OAc) 2 ] + (1) (where L is the trianion of the heptadentate ligand 2,6-bis{[(3,5-di-tert-butyl-2-hydroxybenzyl)-(2-pyridylmethyl)amino]methyl}-4-methylphenol) were studied in acetonitrile solutions containing different concentrations of water. Electrochemical reactions as well as reactions with different chemical and photochemical redox reagents were monitored, using a variety of analytical techniques, namely cyclic voltammetry, UV/Vis spectroelectrochemistry, and EPR spectroscopy. We found that even small concentrations of water influence the compound's redox behaviour significantly, especially the oxidation reactions. As a consequence, the presence of water reduces the overall potential span needed to reach the highest oxidation state observed

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“…Under reducing conditions, no stable reduced reaction product retaining a Mn 2 -core could be identified. We thus conclude that potentially formed Mn 2 II,III -or Mn 2 II,II -intermediates are unstable (both would show characteristic EPR-signals 59 ) and immediately react further to yield mononuclear Mn II -decomposition and/or Mn 2 III,IV -disproportionation products.…”

Section: Redox Reaction Pathways Of Complexmentioning

confidence: 86%

“…It allows the qualitative and quantitative analysis of Mn oxidation states, nuclearity and in some cases even the geometry of manganese complexes. 17,[58][59][60] No EPR signal is detected for a frozen solution of complex 1 in acetonitrile at 5 K (Fig. 6, solid line).…”

Section: Epr Spectroscopymentioning

confidence: 97%

“…This is expected for antiferromagnetically coupled Mn 2 III,III systems, which are often EPR silent, but this depends on the anisotropies of their zero field splittings. 59 To measure EPR spectra of the products obtained by an oxidation of complex 1, we carried out bulk electrolyses of 1 in acetonitrile at the same potentials as probed before by UV/Visspectroelectrochemistry. After different electrolysis times, samples were taken from such solutions, immediately frozen and then analysed by EPR.…”

Section: Epr Spectroscopymentioning

confidence: 99%

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A manganese oxido complex bearing facially coordinating trispyridyl ligands – is coordination geometry crucial for water oxidation catalysis?

et al. 2012

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In this work the synthesis of the novel manganese complex [Mn(2)(III,III)(tpdm)(2)(μ-O)(μ-OAc)(2)](2+) (1) is reported, containing two manganese centres ligated to the unusual, facially coordinating, all-pyridine ligand tpdm (tris(2-pyridyl)methane). The geometric and electronic properties of complex 1 were characterised by X-ray crystallography, vibrational (IR and Raman) and optical spectroscopy (UV/Vis and MCD). Cyclic voltammograms of 1 showed a quasi-reversible oxidation event at 950 mV and an irreversible reduction wave at -250 mV vs. Ag/Ag(+). The redox behaviour of the compound was investigated in detail by UV/Vis- and X-band EPR-spectroelectrochemistry. Both electrochemical (+1200 mV) and chemical (tBuOOH) oxidations transform 1 into the singly oxidized di-μ-oxido species [Mn(2)(III,IV)(tpdm)(2)(μ-O)(2)(μ-OAc)](2+). Further electrochemical oxidation at the same potential results in the removal of a second electron to obtain a Mn(2)(IV,IV)-species. The ability of compound 1 to evolve O(2) was studied using different reaction agents. While reactions with both hydrogen peroxide and peroxomonosulfate yield O(2), homogeneous water-oxidation using Ce(IV) was not observed. Nevertheless, the oxidation reactions of 1 are very interesting model processes for oxidation state (S-state) transitions of the natural manganese water-oxidation catalyst in photosynthesis. However, despite its favourable coordination geometry and multielectron redox chemistry, complex 1 fails to be a catalytically active model for natural water-oxidation.

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EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II

Cited by 8 publications

References 50 publications

Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

Redox Reactions of a Dinuclear Manganese Complex – the Influence of Water

A manganese oxido complex bearing facially coordinating trispyridyl ligands – is coordination geometry crucial for water oxidation catalysis?

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