Photodriven Oxidation of Water by Plastoquinone Analogs with a Nonheme Iron Catalyst

Photoirradiation of an acetonitrile solution containing p-benzoquinone derivatives (X-Q) as plastoquinone analogs, a nonheme iron(II) complex, [(N4Py)Fe^II]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), and H₂O afforded the evolution of O₂ and the formation of the corresponding hydroquinone derivatives (X-QH₂) quantitatively. During the photodriven oxidation of water by X-Q, [(N4Py)Fe^II]²⁺ was oxidized by the excited state of X-Q to produce the iron(IV)-oxo complex ([(N4Py)Fe^IV(O)]²⁺) quantitatively. The concentration of [(N4Py)Fe^IV(O)]²⁺ remained virtually the same during the repeated cycles of photodriven oxidation of water by X-Q. [(N4Py)Fe^IV(O)]²⁺ was further oxidized by the excited state of X-Q to [(N4Py)Fe^V(O)]³⁺ this Fe^V-oxo species is proposed as an active oxidant that affects the water oxidation. The photocatalytic mechanism of the water oxidation by X-Q with [(N4Py)Fe^II]²⁺ was clarified by detecting intermediates using various spectroscopic techniques, such as transient absorption and electron paramagnetic resonance measurements. To the best of our knowledge, the present study reports the first example of a functional model of Photosystem II (PSII) using X-Q as plastoquinone analogs in the photocatalytic oxidation of water.