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 H 2 O afforded the evolution of O 2 and the formation of the corresponding hydroquinone derivatives (X-QH 2 ) 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.

In nature, water is oxidized by plastoquinone to evolve O2 and form plastoquinol in Photosystem II (PSII), whereas NADP+ is reduced by plastoquinol to produce NADPH and regenerate plastoquinone in Photosystem I (PSI), using homogeneous molecular photocatalysts. However, water splitting to evolve H2 and O2 in a 2:1 stoichiometric ratio has yet to be achieved using homogeneous molecular photocatalysts, remaining as one of the biggest challenges in science. Herein, we demonstrate overall water splitting to evolve H2 and O2 in a 2:1 ratio using a two liquid membranes system composed of two toluene phases, which are separated by a solvent mixture of water and trifluoroethanol (H2O/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [p-benzoquinone derivatives (X-Q)] in toluene and an iron­(II) complex as a molecular oxidation catalyst in H2O/TFE (3:1 v/v), which evolves a stoichiometric amount of O2 and forms plastoquinol analogs (X-QH2) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH2, 9-mesityl-10-methylacridinium ion (Acr+-Mes) as a photocatalyst, and a cobalt­(III) complex as an H2 evolution catalyst in H2O/TFE (3:1 v/v), which evolves a stoichiometric amount of H2 and forms X-Q under photoirradiation. When a PSII model system is combined with a PSI model system with two glass membranes and two liquid membranes, photocatalytic water splitting with homogeneous molecular photocatalysts is achieved to evolve hydrogen and oxygen with the turnover number (TON) of >100.

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH 2 ) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H 2 ) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH 2 ) as a hydrogen source, 9mesityl-10-methylacridinium ion (Acr + -Mes) as a photoredox catalyst, and a cobalt(III) complex, Co III (dmgH) 2 pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H 2 and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H 2 evolution from tetrachlorohydroquinone (Cl 4 QH 2 ) with Acr + -Mes and Co III (dmgH) 2 pyCl and H 2 O in deaerated MeCN was determined to be 10%. Photocatalytic H 2 evolution is started by electron transfer (ET) from Cl 4 QH 2 to the triplet ET state of Acr + -Mes to produce Cl 4 QH 2•+ and Acr • -Mes with a rate constant of 7.2 × 10 7 M −1 s −1 , followed by ET from Acr • -Mes to Co III (dmgH) 2 pyCl to produce [Co II (dmgH) 2 pyCl] − , accompanied by the regeneration of Acr + -Mes. On the other hand, Cl 4 QH 2•+ is deprotonated to produce Cl 4 QH • , which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co II (dmgH) 2 pyCl] − to produce a cobalt(III) hydride complex, [Co III (H)(dmgH) 2 pyCl] − , which reacts with H + to evolve H 2 , accompanied by the regeneration of Co III (dmgH) 2 pyCl. The formation of [Co II (dmgH) 2 pyCl] − was detected by electron paramagnetic resonance measurements.

We report the synthesis, characterization, and electron-transfer (ET) oxidation reactivity of a chromium(V)−oxo TAML cation radical complex binding Sc 3+ ion, {[Cr V (O)(TAML •+ )]-Sc 3+ } 3+ (2-Sc). Its precursors, such as [Cr V (O)(TAML)] − (1) and {[Cr V (O)(TAML)]-Sc 3+ } 2+ (1-Sc), were also characterized spectroscopically and/or structurally. In ET and oxygen atom transfer (OAT) reactions, while 1 and 1-Sc were sluggish oxidants, 2-Sc was a highly reactive oxidant with an extremely small reorganization energy. For example, in ET oxidation reactions, nanosecond laser-induced transient absorption measurements were performed to examine the fast ET from electron donors (e.g., ferrocene derivatives) to 2-Sc, affording a small reorganization energy (λ = 0.26 eV) of ET, which is even much smaller than the λ values reported in the ET reduction of heme Compound I (Cpd I) models and non-heme metal−oxo complexes. Such a small reorganization energy is ascribed to the TAML ligand centered ET reduction of 2-Sc. The λ value of 0.26 eV was also obtained in the electron self-exchange reaction between 2-Sc and 1-Sc. In OAT reactions, the rate constants of the sulfoxidation of thioanisole derivatives by 2-Sc at −40 °C were much greater than those reported in the oxidation of thioanisoles by heme Cpd I and non-heme metal−oxo complexes. The reactivity of 2-Sc in hydrogen atom transfer (HAT) reactions is also discussed briefly. To the best of our knowledge, this Cr(V)-oxo TAML cation radical complex binding Sc 3+ ion, {[Cr V (O)(TAML •+ )]-Sc 3+ } 3+ , with an extremely small reorganization energy is one of the most powerful high-valent metal−oxo oxidants in ET and OAT reactions.

Photocatalytic oxygenation of hexamethylbenzene occurs under visible-light irradiation of an O 2saturated acetonitrile solution containing a cobalt porphyrin complex Co II (TPP) (TPP 2− = tetraphenylporphyrin dianion), water, and triflic acid (HOTf) via a onephoton−two-electron process, affording pentamethylbenzyl alcohol and hydrogen peroxide as products with a turnover number of >6000; in this reaction, H 2 O and O 2 were used as an oxygen source and a two-electron oxidant, respectively. The photocatalytic mechanism was clarified by means of electron paramagnetic resonance, timeresolved fluorescence, and transient absorption measurements as well as 18 O-labeling experiments with H 2 18 O and 18 O 2 . To the best of our knowledge, we report the first example of efficient photocatalytic oxygenation of an organic substrate by a metal complex using H 2 O as an oxygen source and O 2 as a two-electron oxidant.