Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

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• 作者：Dooshaye Moonshiram ; Yuliana Pineda-Galvan ; Darren Erdman ; Mark Palenik ; Ruifa Zong ; Randolph Thummel ; Yulia Pushkar
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：December 7, 2016
• 年：2016
• 卷：138
• 期：48
• 页码：15605-15616
• 全文大小：680K
• ISSN：1520-5126

文摘

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O–O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond–second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [Ru^II(NPM)(4-pic)₂(H₂O)]²⁺ (NPM = 4-t-butyl-2,6-di(1′,8′-naphthyrid-2′-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, g_x = 2.30, g_y = 2.18, and g_z = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the Ru^IV═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [Ru^III(NPM-NO)(4-pic)₂(H₂O)]³⁺ and [Ru^III(NPM-NO,NO)(4-pic)₂]³⁺ complexes. A proposed [Ru^V(NPM)(4-pic)₂═O]³⁺ intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O–O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N–O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N–O ligand in promoting water oxidation catalysis.