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

1. Rupal Guptaa, 
2. Taketo Taguchib, 
3. Benedikt Lassalle-Kaiserc, 
4. Emile L. Bominaara, 
5. Junko Yanoc, 
6. Michael P. Hendricha, and 
7. A. S. Borovikb,1

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Significance

Metal complexes with terminal oxido ligands are important in a wide variety of transformations, including a high valent manganese-oxido unit that is involved in the O–O bond-forming step in photosynthetic water oxidation. Theoretical proposals suggest that a MnIV–oxyl radical species is present, yet such species have not been observed experimentally. Using a combination of experimental measurements and theoretical calculations, we show here that the bonding within the Mn–oxido unit is best described as highly covalent, with 0.45 spins on the oxido ligand. These findings offer a counter explanation for the putative high valent manganese species in photosynthesis as an energetically accessible, high-spin MnV–oxido unit instead of a MnIV–oxyl radical species.

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 MnV–oxo complex and not a MnIV–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 MnV–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.

ボロヴィックがいつも使っているbuea配位子を有するMn錯体の、五価から三価のオキソ錯体の分光をやっています。（この配位子は、三回対称性を持つために、四回対称系の錯体系よりも金属ーオキソ間の結合次数が下がる傾向にあることもポイントです。PNAS同号、解説記事参照、詳しく知りたい人は森本まで。）

Mn(V)-オキソ錯体(O2-)は、電荷のシフトによって、Mn(IV)-オキシル錯体(O•–)の状態の極限構造を取る事が出来ますが、この錯体だとMn(V)(O2-)キャラクターが強いようです。

さらに、酸素17ラベル化したオキソ錯体を用いることで、酸素原子上のスピン密度を調べており、この錯体では、0.45個分と比較的大きなスピン密度がオキソ配位子上にある事を明らかにしています。