Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

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• 作者：Lanka D. Wickramasinghe ; Rongwei Zhou ; Ruifa Zong ; Pascal Vo ; Kevin J. Gagnon ; Randolph P. Thummel
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：October 21, 2015
• 年：2015
• 卷：137
• 期：41
• 页码：13260-13263
• 全文大小：323K
• ISSN：1520-5126

文摘

The tetradentate ligand, 2-(pyrid-2鈥?yl)-8-(1鈥?10鈥?phenanthrolin-2鈥?yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp² center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl₃, a 渭-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl₃. Both structures are verified by X-ray analysis. While the Fe^III(dpa) complex shows two reversible one-electron oxidation waves, the Fe^III(ppq) complex shows a clear two-electron oxidation associated with the process H₂O鈥揊e^IIIFe^III 鈫?H₂O鈥揊e^IVFe^IV 鈫?O鈺怓e^VFe^III. Subsequent disproportionation to an Fe鈺怬 species is suggested. When the Fe^III(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h^鈥?. Under the same conditions the mononuclear Fe^III(dpa) complex also evolves oxygen with TOF = 842 h^鈭?.