Photocatalytic oxidation of benzene to phenol using dioxygen as an oxygen source and water as an electron source in the presence of a cobalt catalyst

Abstract

Photocatalytic hydroxylation of benzene to phenol by dioxygen (O₂) occurs under visible light irradiation of an O₂-saturated acetonitrile solution containing [Ru^II(Me₂phen)₃]²⁺ as a photocatalyst, [Co^III(Cp*)(bpy)(H₂O)]²⁺ as an efficient catalyst for both the water oxidation and benzene hydroxylation reactions, and water as an electron source in the presence of Sc(NO₃)₃. The present study reports the first example of photocatalytic hydroxylation of benzene with O₂ and H₂O, both of which are the most green reagents, under visible light irradiation to afford a high turnover number (e.g., >500). Mechanistic studies revealed that the photocatalytic reduction of O₂ to H₂O₂ is the rate-determining step, followed by efficient catalytic hydroxylation of benzene to phenol with H₂O₂, paving a new way for the photocatalytic oxygenation of substrates by O₂ and water.

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Introduction

Phenol, which is an important precursor for many chemicals and industrial products (e.g., dyes, polymers, etc.), is currently produced from benzene by a three step cumene process.^1,2 Since the efficiency of the cumene process is low (∼5% yield of phenol) under severe conditions (e.g., high temperature, high pressure, and strong acidic conditions), it is highly desired to develop a one-step synthesis of phenol from benzene using homogeneous and heterogeneous inorganic catalysts.^3–5 Among various oxidants, hydrogen peroxide (H₂O₂) is frequently used as a green oxidant, which produces water or dioxygen (O₂) as products, in catalytic benzene hydroxylation.^6–13 O₂ is a more ideal oxidant than H₂O₂ because of its abundance in nature, low cost, and environmental benignity.^4,14,15 However, catalytic aerobic oxidation of benzene to phenol has required sacrificial reducing agents such as H₂, ascorbic acid, and NADH analogs.¹⁶ In addition, the catalytic aerobic oxidation of benzene without sacrificial reducing agents has so far required harsh conditions, such as high temperatures or UV light photoirradiation.^14,15,17,18 Moreover, although the photocatalytic hydroxylation of benzene to phenol using organic photocatalysts without overoxidation has been demonstrated,^19–22 the turnover numbers (TONs) of the catalytic hydroxylation of benzene to phenol are still low (e.g., 13).²⁰

We report herein an efficient photocatalytic hydroxylation of benzene to phenol (PhOH) using O₂ as an oxidant in the presence of Sc(NO₃)₃ and utilizing [Ru^II(Me₂phen)₃]²⁺ (Me₂phen = 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst, water as an electron source, and [Co^III(Cp*)(bpy)(H₂O)]²⁺ (Cp* = η⁵-pentamethylcyclopentadienyl and bpy = 2,2-bipyridine) as an efficient catalyst for water oxidation as well as benzene hydroxylation to attain a TON of greater than 500 (Scheme 1). Mechanistic aspects of the benzene hydroxylation reaction under the visible light irradiation conditions have been discussed as well.

Scheme 1 Mechanism of the photocatalytic hydroxylation of benzene.

Results and discussion

[Co^III(Cp*)(bpy)(H₂O)]²⁺ (1) and [Ru^II(Me₂phen)₃]²⁺ were synthesised according to the literature methods.^23,24 Visible light irradiation of an O₂-saturated solution of CH₃CN (MeCN) and H₂O (v/v = 23 : 2) containing a catalytic amount of 1 (1.0 μM), [Ru^II(Me₂phen)₃]²⁺ (1.0 mM), Sc(NO₃)₃ (100 mM), and benzene (1.0 M) at 298 K resulted in the formation of PhOH [eqn (1)],as shown in Fig. 1. PhOH is produced via the catalytic oxygenation of hydrogen peroxide (H₂O₂) with 1, which acts as not only a benzene oxygenation catalyst but also a water oxidation catalyst with [Ru^III(Me₂phen)₃]³⁺ produced by photoinduced electron transfer from the excited state of [Ru^II(Me₂phen)₃]²⁺ to O₂ in the presence of Sc³⁺, as shown in Scheme 1. It should be noted that the catalyst 1 acts as an efficient catalyst for water oxidation in the photocatalytic production of H₂O₂ in the presence of Sc(NO₃)₃, as reported previously.^23,25 [Ru^II(Me₂phen)₃]²⁺ acts as a more efficient photocatalyst compared to [Ru^II(bpy)₃]²⁺ for O₂ reduction by the excited state, because the one-electron oxidation potential of [Ru^II(Me₂phen)₃]²⁺* ( = −1.01 V vs. NHE) is more negative than that of [Ru^II(bpy)₃]²⁺* ( = −0.84 V vs. NHE).²³ The UV-vis spectrum of the photocatalyst, [Ru^II(Me₂phen)₃]²⁺, during the photocatalytic oxidation remained almost identical to that of the initial stage at 445 nm, indicating that the overall reactivity decreased with time due to oxidative degradation of the catalyst 1 (see Experimental section; Fig. S1, ESI†).

Fig. 1 Time courses of the products [PhOH (blue) and p-benzoquinone (red)] obtained in the photocatalytic oxidation of benzene by O₂ with [Ru^II(Me₂phen)₃]²⁺ (1.0 mM) in the presence of a catalytic amount of 1 (1.0 μM) in an O₂-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing Sc(NO₃)₃ (100 mM) and benzene (1.0 M) under photoirradiation (white light) at 298 K.

p-Benzoquinone was also produced during the photocatalytic oxidation of benzene by O₂, and the yield of p-benzoquinone increased as the reaction solution was irradiated for a longer time but the yield of PhOH remained constant (Fig. 1). The observation of the induction period for the formation of p-benzoquinone in Fig. 1 suggests that p-benzoquinone is produced by the further oxidation of PhOH. The production of p-benzoquinone from PhOH was independently confirmed (Fig. S2, ESI†). The TON for the production of both phenol and benzoquinone based on 1 was determined to be 500(20), where the TON of p-benzoquinone is counted three times because p-benzoquinone is the six-electron oxidized product whereas phenol is the two-electron oxidized product. When the reaction conditions were changed by increasing the concentration of the catalyst and decreasing the concentration of the benzene substrate, a much higher product yield of phenol (∼30%) based on benzene was obtained (Fig. 2) than that in Fig. 1. The quantum yield (QY) was estimated by eqn (2),

QY (%) = R/I × 100 (2)

where R (mol s⁻¹) is the PhOH production rate and I (einstein s⁻¹) is the rate of the number of incident photons. The quantum yield was determined to be 1.7(2)% from the amount of PhOH produced during the photocatalytic reaction under photoirradiation (λ = 440 nm) for 1 h (Fig. S3, ESI†). The photocatalytic reactivity increased on increasing the concentration of benzene as well as the concentration of [Ru^II(Me₂phen)₃]²⁺ (Fig. S4, ESI†). A negligible amount of PhOH was produced in the absence of 1, indicating that 1 is an essential component in the benzene hydroxylation reaction.

Fig. 2 Time courses of PhOH concentration produced in the photocatalytic oxidation of benzene under photoirradiation (white light) of an O₂-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing 1 (0.10 mM), [Ru^II(Me₂phen)₃]²⁺ (1.0 mM), and benzene (1.0 mM, blue circles) or benzene-d₆ (1.0 mM, red circles) in the presence of Sc(NO₃)₃ (100 mM) at 298 K. No p-benzoquinone was produced under the present experimental conditions.

The photocatalytic hydroxylation reaction proceeds as shown in Scheme 1. Visible light irradiation of [Ru^II(Me₂phen)₃]²⁺ resulted in the formation of H₂O₂ by oxidative quenching of the photoexcited state [Ru^II(Me₂phen)₃]²⁺* (* denotes the excited state) with O₂ in the presence of Sc(NO₃)₃ (Scheme 1, reaction pathways b and c), as reported for the case of [Ru^II(bpy)₃]²⁺.²⁶ The time courses of [Ru^III(Me₂phen)₃]³⁺ generation and H₂O₂ production were obtained to give the stoichiometry of the photochemical reaction, as shown in eqn (3) (Fig. 3).

Fig. 3 Time courses of the concentrations of [Ru^III(Me₂phen)₃]³⁺ (red) and H₂O₂ (blue) produced from H₂O and O₂ in the photocatalytic oxidation of [Ru^II(Me₂phen)₃]²⁺ by O₂ under photoirradiation (white light) of an air-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing [Ru^II(Me₂phen)₃]²⁺ (100 μM) in the presence of Sc(NO₃)₃ (100 mM) at 298 K.

The lifetime of [Ru^II(Me₂phen)₃]²⁺* and emission quenching have been determined to obtain the rate constants (k_et) of photoinduced electron transfer in the absence and presence of Sc(NO₃)₃ (Fig. S5 and Table S1, ESI†). The k_et values in both the absence and presence of Sc(NO₃)₃ are close to the diffusion-limited value, showing that Sc(NO₃)₃ does not affect the oxidative quenching of [Ru^II(Me₂phen)₃]²⁺* by O₂. The emission spectra of [Ru^II(Me₂phen)₃]²⁺ in the absence and presence of Sc(NO₃)₃ taken in a solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing different concentrations of O₂ indicate that the amount of O₂ in air is large enough for efficient H₂O₂ photogeneration (Fig. S5d, ESI†).

Visible light irradiation of the reaction solution without benzene results in the formation of H₂O₂, as shown in Fig. 4 (Scheme 1, reaction pathways a–c), and the production of H₂O₂ is obtained from the photooxidation of [Ru^II(Me₂phen)₃]²⁺ by O₂ [eqn (3)] and the catalytic water oxidation by [Ru^III(Me₂phen)₃]³⁺ in the presence of 1 [eqn (4)].²³ Thus, the overall reaction

is the photocatalytic oxidation of H₂O by O₂ to produce H₂O₂, as given by eqn (5).The rates of photocatalytic oxidation of H₂O by O₂ to produce H₂O₂ in the presence of 1 were also investigated with various concentrations of 1 at 298 K (Fig. S6, ESI†). The zeroth-order rate constants (k_obs) were obtained from the initial slopes of the plots of the amount of H₂O₂ produced vs. time, being proportional to the concentration of 1 (Fig. 4 and S6, ESI†).

Fig. 4 Time courses of the photocatalytic production of H₂O₂ from H₂O and O₂ with [Ru^II(Me₂phen)₃]²⁺ (0.10 mM) in the presence of a catalytic amount of 1 [0.50 mM (red), 1.0 mM (orange), 1.5 mM (green), and 2.0 mM (blue)] in an air-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing Sc(NO₃)₃ (100 mM) under photoirradiation (white light) at 298 K.

The catalytic water oxidation by [Ru^III(Me₂phen)₃]³⁺ with 1 to evolve O₂ [eqn (6); Scheme 1, reaction pathway a]

was independently confirmed, where O₂ was evolved, accompanied by the formation of [Ru^II(Me₂phen)₃]²⁺. The evolved O₂ concentrations were investigated by a Clark oxygen electrode, showing an increase in the amount of O₂ evolved after addition of [Ru^III(Me₂phen)₃]³⁺ (1.0 mM) to a solvent mixture of MeCN and H₂O (1.0 mL; v/v = 9 : 1) containing Sc(NO₃)₃ (100 mM) and various concentrations of 1 (Fig. 5), where the O₂ yield increased with an increasing concentration of 1 to approach 100% (1/4 of the initial concentration of [Ru^III(Me₂phen)₃]³⁺). The reaction rate was accelerated with an increasing concentration of 1 to account for oxidation of water by [Ru^III(Me₂phen)₃]³⁺ with 1 under acidic conditions due to the presence of Sc(NO₃)₃. Formation of [Ru^II(Me₂phen)₃]²⁺ accompanied by water oxidation was also confirmed by monitoring the UV-vis spectral changes (Fig. S7a, ESI†). The time courses of the oxidation of 1 by [Ru^III(Me₂phen)₃]³⁺ were obtained by spectral changes at λ = 445 nm due to [Ru^II(Me₂phen)₃]²⁺, showing that the rate of [Ru^II(Me₂phen)₃]²⁺ formation from [Ru^III(Me₂phen)₃]³⁺ increased with the increase of the concentration of 1 (Fig. S7b, ESI†).

Fig. 5 Time courses of O₂-evolution observed in the catalytic oxidation of H₂O by [Ru^III(Me₂phen)₃]³⁺ (1.0 mM) with 1 [10 μM (red), 50 μM (orange), 100 μM (green), and 200 μM (blue)] in the presence of Sc(NO₃)₃ (100 mM) in a deaerated solvent mixture (1.0 mL) of MeCN and H₂O (v/v = 9 : 1) at 298 K. The induction periods were observed due to the reaction time to oxidize 1 by [Ru^III(Me₂phen)₃]³⁺ to produce the catalytically active species.

Catalytic oxidation of benzene to phenol with H₂O₂ [eqn (7)]

has also been confirmed in a solvent mixture of MeCN and H₂O (23 : 2) containing Sc(NO₃)₃ in order to stabilize H₂O₂ with the same conditions for the overall photocatalytic reaction as shown in Fig. 6 (Scheme 1, reaction pathway d). The rate of benzene hydroxylation increased with increasing concentrations of 1 (Fig. 6) and H₂O₂ (Fig. S8, ESI†). A combination of the photocatalytic oxidation of H₂O by O₂ to produce H₂O₂ [eqn (5)] and the catalytic hydroxylation of benzene to phenol by H₂O₂ [eqn (7)] affords the overall photocatalytic oxidation of benzene to phenol by O₂ [eqn (1)]. The photocatalytic reactions of an O₂-saturated solution of MeCN and H₂O (v/v = 23 : 2) containing 1 (0.50 mM), [Ru^II(Me₂phen)₃]²⁺ (0.10 mM), and Sc(NO₃)₃ (100 mM) under photoirradiation (white light) were examined in the absence and presence of benzene to compare their reactivities (Fig. 7). The rate of H₂O₂ production from H₂O and O₂ in the absence of benzene was slightly higher than that of PhOH generation in the photocatalytic hydroxylation of benzene, indicating that H₂O₂ produced in the photocatalytic oxidation of H₂O by O₂ reacted with benzene efficiently in the presence of 1 to produce PhOH.²⁷ In fact, the rate of PhOH production (59 μM after 1 h) from H₂O₂ (1.0 mM) and benzene (1.0 M) with 1 (0.50 mM, red dots in Fig. 6) is much faster than that of PhOH production (estimated to be 7.5 μM after 1 h) in the photocatalytic reaction (Fig. 7), because the concentration of H₂O₂ produced in the photocatalytic oxidation of H₂O by O₂ is much smaller than 1.0 mM. Since the rate of PhOH production from H₂O₂ is proportional to the concentration of H₂O₂ (Fig. S8, ESI†), the rate of PhOH production in the photocatalytic hydroxylation of benzene by O₂ corresponds to that of PhOH production in the catalytic oxidation of benzene with 127 μM H₂O₂, which may be the steady state concentration during the photocatalytic hydroxylation of benzene.

Fig. 6 Time courses of the catalytic hydroxylation of benzene to PhOH with H₂O₂ (1.0 mM) in the presence of 1 [10 μM (black), 50 μM (blue), 100 μM (green), and 500 μM (red)] in a solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing benzene (1.0 M) and Sc(NO₃)₃ (100 mM) at 298 K.

Fig. 7 Time courses of the products from the photoinduced reaction under photoirradiation (white light) in an O₂-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing 1 (0.50 mM), [Ru^II(Me₂phen)₃]²⁺ (0.10 mM), and Sc(NO₃)₃ (100 mM) in the absence (blue) and presence (red) of benzene (1.0 M) at 298 K.

Then, ¹⁸O₂-labeling experiments were performed to confirm that the oxygen atom in the phenol product in eqn (1) derives from O₂ (Fig. 8). Indeed, Ph¹⁸OH (73%) was formed as the major product together with Ph¹⁶OH (27%) in the ¹⁸O₂-labeling experiments, indicating that the oxygen atom in the PhOH product obtained in the photocatalytic oxidation of benzene by H₂¹⁸O₂ derived from ¹⁸O₂ (98% ¹⁸O-enriched; Fig. 8). The formation of Ph¹⁶OH (27%) indicates that ¹⁶O₂ was produced by the oxidation of H₂¹⁶O by [Ru^III(Me₂phen)₃]³⁺, leading to the production of H₂¹⁶O₂ to oxidize benzene to produce Ph¹⁶OH instead of Ph¹⁸OH (Scheme 1, reaction pathway a). The ¹⁸O-labeling experiments were also performed by replacing H₂¹⁶O with H₂¹⁸O to support that the oxygen atom in the phenol product derives from O₂ rather than H₂O (Fig. 9). In this case, Ph¹⁶OH (66%) was the major product together with Ph¹⁸OH (34%), consistent with the ¹⁸O₂-labeling experiments described above.

Fig. 8 A comparison of the relative abundances of authentic PhOH (blue) and the PhOH product (red) yielded in the photocatalytic hydroxylation of benzene by 1 (0.10 mM) in the presence of [Ru^II(Me₂phen)₃]²⁺ (1.0 mM), Sc(NO₃)₃ (100 mM), and benzene (2.0 M) under photoirradiation (white light) in an ¹⁸O₂-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) at 298 K for 24 h. The peaks at m/z = 94 and 96 correspond to Ph¹⁶OH and Ph¹⁸OH, respectively.

Fig. 9 A comparison of the relative abundances of authentic PhOH (blue) and the produced PhOH (red) in the photocatalytic hydroxylation of benzene by O₂ with 1 (0.10 mM) in the presence of [Ru^II(Me₂phen)₃]²⁺ (1.0 mM), Sc(NO₃)₃ (100 mM), and benzene (2.0 M) under photoirradiation (white light) of an ¹⁶O₂-saturated solvent mixture of MeCN and H₂¹⁸O (v/v = 23 : 2) at 298 K for 24 h. The peaks at m/z = 94 and 96 correspond to Ph¹⁶OH and Ph¹⁸OH, respectively.

The photocatalytic mechanism of benzene hydroxylation to phenol by O₂ is summarized in Scheme 1. The photoexcitation of [Ru^II(Me₂phen)₃]²⁺ in an O₂-saturated solvent mixture of MeCN and H₂O (v/v = 23 : 2) containing Sc(NO₃)₃ resulted in electron transfer from the triplet excited state of [Ru^II(Me₂phen)₃]²⁺ to O₂ to produce [Ru^III(Me₂phen)₃]³⁺ and the O₂˙⁻–Sc³⁺ complex.^23,28 The strong binding of Sc³⁺ to O₂˙⁻, which was detected by EPR, inhibits the back electron transfer from the O₂˙⁻–Sc³⁺ complex to [Ru^III(Me₂phen)₃]³⁺, followed by disproportion with H₂O to produce H₂O₂.²³ H₂O₂ has attracted considerable attention as an ideal solar fuel for a one-compartment H₂O₂ fuel cell with a theoretical maximum output potential of 1.09 V, which is comparable to that of a hydrogen fuel cell (1.23 V).²³ [Ru^III(Me₂phen)₃]³⁺ can oxidize H₂O to O₂ with catalysis by 1. Benzene hydroxylation to PhOH was also catalysed by 1. As reported previously, the NMR peaks assignable to the bpy and Cp* of 1 remained the same after the photocatalytic production of H₂O₂, indicating that 1 acted as a homogeneous catalyst during the reaction.²³ However, the catalytically active intermediates such as the Co(IV)-oxo species²⁹ have yet to be detected in the catalytic benzene hydroxylation with H₂O₂. There was no deuterium kinetic isotope effect (KIE) for benzene (Fig. 2), indicating that C–H bond cleavage is not involved in the rate-determining step of the photocatalytic hydroxylation reaction.

Conclusions

In conclusion, we have shown that benzene is oxidized to phenol by dioxygen with a high TON (e.g., 500) under visible light irradiation of a reaction solution containing [Ru^II(Me₂phen)₃]²⁺ as a photocatalyst, [Co^III(Cp*)(bpy)(H₂O)]²⁺ as an efficient catalyst for both water oxidation and benzene hydroxylation, and water as an electron source in the presence of Sc(NO₃)₃. The combination of the photocatalytic H₂O₂ production by H₂O oxidation with O₂ and the catalytic hydroxylation of benzene to phenol with H₂O₂ demonstrated in this study has paved a new road towards direct oxygenation of substrates by O₂ as the most environmentally benign oxidant as well as oxygen source without any electron source except H₂O, which is also the most environmentally benign hydrogen source.

Experimental section

Materials

All solvents and chemicals were of reagent-grade quality, obtained commercially and used without further purification, unless otherwise noted. Ruthenium(III) chloride hydrate, ammonium hexafluorophosphate, n-butyllithium solution (2.7 M in heptane), n-pentane, and tetrahydrofuran were purchased from Aldrich Chemicals. The chemicals, such as 4,7-dimethyl-1,10-phenanthroline (Me₂phen), silver sulphate, lead dioxide, and cobalt(II) chloride, were purchased from Alfa Aesar. 2,2′-Bipyridine, 1,2,3,4,5-pentamethylcyclopentadiene, and Ti^IV(O)(tpyp) (tpyp = 5,10,15,20-tetra(4-pyridyl)porphyrinato anion) were obtained from Tokyo Chemical Industry Co., Ltd. Sc(NO₃)₃·4H₂O was supplied by Mitsuwa Chemicals Co., Ltd. ¹⁸O₂ gas (98% ¹⁸O-enriched) was purchased from ICON Services Inc. (Summit, NJ. USA). The purification of water (18.2 MΩ cm) was performed with a Milli-Q system (Millipore, Direct-Q 3 UV). Acetonitrile was dried according to published procedures and distilled prior to use.³⁰ The cobalt(III) starting complex, [Co^III(Cp*)(bpy)(H₂O)]²⁺ (1, Cp* = η⁵-pentamethylcyclopenta-dienyl and bpy = 2,2-bipyridine), and the tris(4,7-dimethyl-1,10-phenanthroline)ruthenium(II) complex, [Ru^II(Me₂phen)₃]²⁺, were prepared according to the published methods.^25,31

Instrumentation

UV-vis spectra were recorded on a Hewlett Packard 8453 diode array spectrophotometer equipped with a UNISOKU Scientific Instruments Cryostat USP-203A. Product analysis for the oxidation reactions was performed with an Agilent Technologies 6890N gas chromatograph (GC) and a Thermo Finnigan (Austin, Texas, U.S.A.) FOCUS DSQ (dual stage quadrupole) mass spectrometer interfaced with a Finnigan FOCUS gas chromatograph (GC-MS). The amount of evolved oxygen was recorded by a Clark-type oxygen electrode made by Hansatech Ltd.

Product analysis

The products formed in the oxidation of benzene by 1 and O₂ in the presence of [Ru^II(Me₂phen)₃]²⁺ and Sc(NO₃)₃ in a solvent mixture of MeCN and H₂O (v/v = 23 : 2) at 298 K were identified by GC and GC-MS by a comparison of the mass peaks and retention time of the products with respect to the authentic samples, and the product yields were determined by comparing the responsive peak areas of the sample products against standard curves prepared with known authentic compounds using the internal standard decane. The quantum yield (QY) of the photocatalytic hydroxylation of benzene has been determined under visible light irradiation of monochromatized light using a Compact Xenon Light Source (MAX-302; Asahi Spectra Co., Ltd). The amount of hydrogen peroxide produced was determined by spectroscopic titration with an acidic solution of a [Ti^IVO(tpypH₄)]⁴⁺ complex.³¹ [Ti^IVO(tpypH₄)]⁴⁺ (50 μM) was prepared by dissolving 3.4 mg of the TiO(tpyp) complex into water (100 mL) containing hydrochloric acid (50 mM). A small portion (100 μL) of the photocatalytic reaction solution was taken and diluted up to 1.0 mL with water. To 0.25 mL of the diluted sample, 0.25 mL of perchloric acid (4.8 M) and 0.25 mL of [Ti^IVO(tpypH₄)]⁴⁺ (50 μM) were added. The mixed solution was then allowed to stand for 5 min at room temperature. This sample solution was diluted up to 2.5 mL with water and used for the spectroscopic measurement. The absorbance at λ = 434 nm (A_S) due to [Ti^IVO(tpypH₄)]⁴⁺ was measured using a Hewlett Packard 8453 diode array spectrophotometer. In a similar manner, a blank solution was prepared by adding distilled water instead of the sample solution in the same volume with its absorbance designated as A_B. The difference in absorbance at 434 nm was determined as follows: ΔA₄₃₄ = A_B − A_S. Based on ΔA₄₃₄ and the volume of the solution, the amount of hydrogen peroxide was determined according to the literature.²³ A Clark-type oxygen electrode was used to obtain oxygen evolution data, and calibrated daily using Ar deoxygenated and oxygen saturated atmospheric solutions. Clark electrode experiments were performed by adding 10 μL of [Ru^III(Me₂phen)₃]³⁺ (100 mM) to an O₂-saturated solvent mixture (1.0 mL) of MeCN and H₂O (v/v = 9 : 1) containing 1 (10–200 μM) and monitoring the O₂ evolved.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by SENTAN projects from the Japan Science and Technology Agency (JST), JSPS KAKENHI (No. 16H02268) to S. F. and by the CRI (NRF-2012R1A3A2048842 to W. N.), GRL (NRF-2010-00353 to W. N.), and Basic Science Research Program (2017R1D1A1B03029982 to Y. M. L. and 2017R1D1A1B03032615 to S. F.) through the NRF of Korea.

Notes and references

1. M. Weber, M. Weber and M. Kleine-Boymann, Phenol. Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2004, vol. 26, p. 503.
2. (a) R. Molinari and T. Poerio, Asia–Pac. J. Chem. Eng., 2010, 5, 191; (b) R. J. Schmidt, Appl. Catal., A, 2005, 280, 89.
3. (a) L. Balducci, D. Bianchi, R. Bortolo, R. D’Aloisio, M. Ricci, R. Tassinari and R. Ungarelli, Angew. Chem., Int. Ed., 2003, 42, 4937; (b) J.-H. Yang, G. Sun, Y. Gao, H. Zhao, P. Tang, J. Tan, A.-H. Lu and D. Ma, Energy Environ. Sci., 2013, 6, 793.
4. S. Fukuzumi and K. Ohkubo, Asian J. Org. Chem., 2015, 4, 836.
5. (a) X. Cai, Q. Wang, Y. Liu, J. Xie, Z. Long, Y. Zhou and J. Wang, ACS Sustainable Chem. Eng., 2016, 4, 4986; (b) W. Wang, L. Shi, N. Li and Y. Ma, RSC Adv., 2017, 7, 12738.
6. (a) Y. Morimoto, S. Bunno, N. Fujieda, H. Sugimoto and S. Itoh, J. Am. Chem. Soc., 2015, 137, 5867; (b) Y. Aratani, Y. Yamada and S. Fukuzumi, Chem. Commun., 2015, 51, 4662.
7. (a) D. Wang, M. Wang and Z. Li, ACS Catal., 2015, 5, 6852; (b) X. Ye, Y. Cui, X. Qiu and X. Wang, Appl. Catal., B, 2014, 152–153, 383; (c) G. Wen, S. Wu, B. Li, C. Dai and D. S. Su, Angew. Chem., Int. Ed., 2015, 54, 4105.
8. (a) M. Yamada, K. D. Karlin and S. Fukuzumi, Chem. Sci., 2016, 7, 2856; (b) B. Xu, W. Zhong, Z. Wei, H. Wang, J. Liu, L. Wu, Y. Feng and X. Liu, Dalton Trans., 2014, 43, 15337; (c) L. Wu, W. Zhong, B. Xu, Z. Wei and X. Liu, Dalton Trans., 2015, 44, 8013.
9. (a) W. Ge, Z. Long, X. Cai, Q. Wang, Y. Zhou, Y. Xu and J. Wang, RSC Adv., 2014, 4, 45816; (b) C. Wang, L. Hu, Y. Hu, Y. Ren, X. Chen, B. Yue and H. He, Catal. Commun., 2015, 68, 1.
10. (a) P. Borah, X. Ma, K. T. Nguyen and Y. Zhao, Angew. Chem., Int. Ed., 2012, 51, 7756; (b) P. K. Khatri, B. Singh, S. L. Jain, B. Sain and A. K. Sinha, Chem. Commun., 2011, 47, 1610; (c) Y. Leng, J. Liu, P. Jiang and J. Wang, Chem. Eng. J., 2014, 239, 1.
11. (a) H. Wang, M. Zhao, Q. Zhao, Y. Yang, C. Wang and Y. Wang, Ind. Eng. Chem. Res., 2017, 56, 2711; (b) S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma, ACS Sustainable Chem. Eng., 2017, 5, 3637.
12. (a) L. Muniandya, F. Adama, A. R. Mohamed, A. Iqbal and N. R. A. Rahman, Appl. Surf. Sci., 2017, 398, 43; (b) J. Xu, Q. Jiang, T. Chen, F. Wu and Y.-X. Li, Catal. Sci. Technol., 2015, 5, 1504; (c) P. R. Makgwane and S. S. Raya, J. Mol. Catal. A: Chem., 2015, 398, 149.
13. (a) L. Carneiro and A. R. Silva, Catal. Sci. Technol., 2016, 6, 8166; (b) E. Gu, W. Zhong and X. Liu, RSC Adv., 2016, 6, 98406; (c) E. Gu, W. Zhong, H. Ma, B. Xu, H. Wang and X. Liu, Inorg. Chim. Acta, 2016, 444, 159; (d) B. Xu, W. Zhong, Z. Wei, H. Wang, J. Liu, L. Wu, Y. Feng and X. Liu, Dalton Trans., 2014, 43, 15337.
14. (a) S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji, T. Namba and F. Mizukami, Science, 2002, 295, 105; (b) K. Sato, T. Hanaoka, S. Niwa, C. Stefan, T. Namba and F. Mizukami, Catal. Today, 2005, 104, 260.
15. (a) A. Okemoto, Y. Tsukano, A. Utsunomiya, K. Taniya, Y. Ichihashi and S. Nishiyama, J. Mol. Catal. A: Chem., 2016, 411, 372; (b) B. B. Sarma, R. Carmieli, A. Collauto, I. Efremenko, J. M. L. Martin and R. Neumann, ACS Catal., 2016, 6, 6403.
16. (a) Z. Long, Y. Zhou, W. Ge, G. Chen, J. Xie, Q. Wang and J. Wang, ChemPlusChem, 2014, 79, 1590; (b) Z. Long, Y. Zhou, G. Chen, P. Zhao and J. Wang, Chem. Eng. J., 2014, 239, 19; (c) S. Shang, B. Chen, L. Wang, W. Dai, Y. Zhang and S. Gao, RSC Adv., 2015, 5, 31965.
17. Y. Aratani, K. Oyama, T. Suenobu, Y. Yamada and S. Fukuzumi, Inorg. Chem., 2016, 55, 5780.
18. (a) R. Hamada, Y. Shibata, S. Nishiyama and S. Tsuruya, Phys. Chem. Chem. Phys., 2003, 5, 956; (b) Z. Long, Y. Zhou, G. Chen, W. Ge and J. Wang, Sci. Rep., 2014, 4, 3651; (c) Z. Long, Y. Liu, P. Zhao, Q. Wang, Y. Zhou and J. Wang, Catal. Commun., 2015, 59, 1; (d) S. S. Acharyya, S. Ghosh, R. Tiwari, C. Pendem, T. Sasaki and R. Bal, ACS Catal., 2015, 5, 2850; (e) R. Bal, M. Tada, T. Sasaki and Y. Iwasawa, Angew. Chem., Int. Ed., 2006, 45, 448.
19. (a) T. D. Bui, A. Kimura, S. Ikeda and M. Matsumura, J. Am. Chem. Soc., 2010, 132, 8453; (b) Y. Ide, M. Torii and T. Sano, J. Am. Chem. Soc., 2013, 135, 11784; (c) Y. Ide, M. Matsuoka and M. Ogawa, J. Am. Chem. Soc., 2010, 132, 16762.
20. K. Ohkubo, T. Kobayashi and S. Fukuzumi, Angew. Chem., Int. Ed., 2011, 50, 8652.
21. K. Ohkubo, A. Fujimoto and S. Fukuzumi, J. Am. Chem. Soc., 2013, 135, 5368.
22. K. Ohkubo, K. Hirose and S. Fukuzumi, Chem.–Eur. J., 2015, 21, 2855.
23. S. Kato, J. Jung, T. Suenobu and S. Fukuzumi, Energy Environ. Sci., 2013, 6, 3756.
24. (a) Y. Isaka, S. Kato, D. Hong, T. Suenobu, Y. Yamada and S. Fukuzumi, J. Mater. Chem. A, 2015, 3, 12404; (b) Y. Isaka, K. Oyama, Y. Yamada, T. Suenobu and S. Fukuzumi, Catal. Sci. Technol., 2016, 6, 681.
25. (a) A. Das, V. Joshi, D. Kotkar, V. S. Pathak, V. Swayambunathan, P. V. Kamat and P. K. Ghosh, J. Phys. Chem. A, 2001, 105, 6945; (b) D. Hong, J. Jung, J. Park, Y. Yamada, T. Suenobu, Y.-M. Lee, W. Nam and S. Fukuzumi, Energy Environ. Sci., 2012, 5, 7606.
26. It should be noted, however, that the contribution from the catalytic hydroxylation of benzene with [Ru^III(bpy)₃]³⁺ in competition with H₂O oxidation cannot be ruled out.
27. (a) S. Fukuzumi and K. Ohkubo, Chem.–Eur. J., 2000, 6, 4532; (b) S. Fukuzumi, M. Patz, T. Suenobu, Y. Kuwahara and S. Itoh, J. Am. Chem. Soc., 1999, 121, 1605; (c) S. Fukuzumi, H. Ohtsu, K. Ohkubo, S. Itoh and H. Imahori, Coord. Chem. Rev., 2002, 226, 71.
28. W. L. F. Armarego and C. L. L. Chai, in Purification of Laboratory Chemicals, Pergamon Press, Oxford, U. K., 6th edn, 2009.
29. (a) S. Hong, F. F. Pfaff, E. Kwon, Y. Wang, M.-S. Seo, E. Bill, K. Ray and W. Nam, Angew. Chem., Int. Ed, 2014, 53, 10403; (b) B. Wang, Y.-M. Lee, W.-Y. Tcho, S. Tussupbayev, S.-T. Kim, Y. Kim, M. S. Seo, K.-B. Cho, Y. Dede, B. C. Keegan, T. Ogura, S. H. Kim, T. Ohta, M.-H. Baik, K. Ray, J. Shearer and W. Nam, Nat. Commun., 2016, 8, 14839.
30. C. Turro, J. M. Zaleski, Y. M. Karabatsos and D. G. Nocera, J. Am. Chem. Soc., 1996, 118, 6060.
31. C. Matsubara, N. Kawamoto and K. Takamura, Analyst, 1992, 117, 1781.

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Footnotes

† Electronic supplementary information (ESI) available: Table S1 and Fig. S1–S8. See DOI: 10.1039/c7sc02495a

‡ These authors contributed equally to this work.

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