Light-induced multistep oxidation of dinuclear manganese complexes for artificial photosynthesis

• Two dinuclear manganese complexes, [Mn2BPMP(mu-OAc)(2)] . ClO4 (1, where BPMP is the anion of 2,6-bis {[N,N-di(2-pyridinemethyl)amino]methyl}-4-methylphenol) and [Mn2L(mu-OAc)(2)] . ClO4 (2, where L is the trianion of 2,6-bis{[N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2-pyridinemethyl)amino]methyl}-4-methylphenol), undergo several oxidations by laser flash photolysis, using ruthenium(II)-tris-bipyridine (tris(2,2-bipyridyl)dichloro-ruthenium(II) hexahydrate) as photo-sensitizer and penta-amminechlorocobalt(III) chloride as external electron acceptor. In both complexes stepwise electron transfer was observed. In 1, four Mn-valence states from the initial Mn-2(II,II) to the Mn-2(III,IV) state are available. In 2, three oxidation steps are possible from the initial Mn-2(III,III) state. The last step is accomplished in the Mn-2(IV,IV) state, which results in a phenolate radical. For the first time we provide firm spectral evidence for formation of the first intermediate state, Mn-2(II,III) in 1 during the stepwise light-induced oxidation. Observation of Mn-2(II,III) is dependent on conditions that sustain the mu-acetato bridges in the complex, i.e., by forming Mn-2(II,III) in dry acetonitrile, or by addition of high concentrations of acetate in aqueous solutions. We maintain that the presence of water is necessary for the transition to higher oxidation states, e.g., Mn-2(III,III) and Mn-2(III,IV) in 1, due to a bridging ligand exchange reaction which takes place in the Mn-2(II,III) state in water solution. Water is also found to be necessary for reaching the Mn-2(IV,IV) state in 2, which explains why this state was not reached by electrolysis in our earlier work (Eur. J. Inorg. Chem (2002) 2965). In 2, the extra coordinating oxygen atoms facilitate the stabilization of higher Mn valence states than in 1, resulting in formation of a stable Mn-2(IV,IV) without disintegration of 2. In addition, further oxidation of 2, led to the formation of a phenolate radical (g = 2.0046) due to ligand oxidation. Its spectral width (8 mT) and very fast relaxation at 15 K indicates that this radical is magnetically coupled to the Mn-2(IV,IV) center.

Ämnesord

NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

Nyckelord

manganese

photosynthesis

ruthenium

electron paramagnetic resonance

oxygen-evolving complex

water-oxidizing complex

intramolecular electron-transfer

valence mniimniii complexes

microwave-power saturation

photosystem-ii

s2yz-center-dot state

functional models

tris-bipyridine

epr signals

Manganese

Photosynthesis

Ruthenium

Electron paramagnetic resonance

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