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Jan 21, 2016 - 305-8571, Japan. ‡. Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, ...
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Homogeneous Photocatalytic Water Oxidation with a Dinuclear CoIII−Pyridylmethylamine Complex Tomoya Ishizuka,† Atsuko Watanabe,† Hiroaki Kotani,† Dachao Hong,† Kenta Satonaka,† Tohru Wada,‡ Yoshihito Shiota,§ Kazunari Yoshizawa,§,⊥ Kazuaki Ohara,¶ Kentaro Yamaguchi,¶ Satoshi Kato,∥ Shunichi Fukuzumi,*,∥,⧫,∞ and Takahiko Kojima*,† †

Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan ‡ Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan § Institute for Materials Chemistry and Engineering, Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan ⊥ Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Nishi-ku, Kyoto 615-8520, Japan ¶ Faculty of Pharmaceutical Science at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan ∥ Department of Material and Life Science, Graduate School of Engineering, Osaka University, and ALCA, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan ⧫ Faculty of Science and Technology, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Tempaku, Nagoya, Aichi 468-8502, Japan ∞ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: A bis-hydroxo-bridged dinuclear CoIII-pyridylmethylamine complex (1) was synthesized and the crystal structure was determined by X-ray crystallography. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S2O82−) as an oxidant with [RuII(bpy)3]2+ (bpy = 2,2′bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O2 formation without forming catalytically active Co-oxide (CoOx) nanoparticles. In the water-oxidation process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-μ-oxyl CoIII complex (2), which has been suggested by DFT calculations. Catalytic water oxidation by 1 using [RuIII(bpy)3]3+ as an oxidant in a H216O and H218O mixture was examined to reveal an intramolecular O−O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O2 evolution.

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INTRODUCTION Catalytic oxidation of water is an indispensable reaction for maintaining the global life cycle, as well as the production of energy sources, as can be seen in nature in photosynthesis. The photosynthetic water oxidation has been clarified to be a metalcatalyzed multistep reaction cascade, involving an oxo-bridged manganese cluster as the responsible reaction site.1 Inspired by the reaction, water-oxidation catalysts based on transition-metal complexes, by which two water molecules are oxidized to evolve molecular oxygen (eq 1), have been prepared to examine their catalytic performance.2 2H 2O → O2 + 4H+ + 4e−

workers on water oxidation using a dinuclear ruthenium complex,5,6 many catalytic water oxidation systems based on metal complexes have been developed by employing chemical oxidants, as represented by (NH4)2[CeIV(NO3)6] (CAN) and electrochemical methods.7−14 Among various transition-metal complexes utilized as water-oxidation catalysts, cobalt complexes have recently been drawing much interest, because of the merits of the natural abundance and the high catalytic activity as water-oxidation catalysts.15−17 On the other hand, photocatalytic water oxidation also has been studied intensively, since photocatalytic water oxidation is of significant importance to obtain electrons from water as an energy source using photon energy, as observed in photosynthesis.18,19 However, there are a few examples showing sufficient stability under photoirradiation.20 In order to develop photocatalytic water oxidation, it is

E° = +1.229 V (vs NHE) (1)

Artificial efficient catalytic water oxidation has been required for the development of fuel cells for the next generation3 and electron uptake for running reductive processes in the production of organic resources, as observed in natural photosynthesis.4 Since the pioneering work by Meyer and co© XXXX American Chemical Society

Received: October 10, 2015

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DOI: 10.1021/acs.inorgchem.5b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry indispensable to provide a robust catalyst under conditions required for the photocatalytic reactions. As for the O−O bond formation in the course of O2 evolution, two types of reaction mechanisms have been proposed: (a) intermolecular electrophilic attack of a metaloxo complex to a water molecule and (b) radical coupling between metal-oxo complexes with odd numbers of d electrons.21 Thus far, reaction mechanisms of Co-catalyzed water oxidation have yet to be clarified, especially the O−O bond forming step, although an intermolecular nucleophilic addition of H2O to Co(IV)-oxo species of the type (a) mentioned above has been recently proposed, on the basis of rapid-scan Fourier transform infrared (FTIR) measurments.22 We report herein that a bis-μ-hydroxo-CoIII-TPA dinuclear complex (1) (TPA = tris(2-pyridylmethyl)amine) acts as an efficient homogeneous water-oxidation catalyst. Complex 1 stands intact, even after catalytic water oxidations. The robustness of 1 allowed us to construct an efficient photocatalytic water oxidation system in high quantum yield and enabled us to clarify the mechanism of the O−O bond formation and O2 evolution.

Figure 1. An ORTEP drawing of the cation part of 1. All the thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms except OH protons and counteranions (ClO4−) are omitted for clarity.

to those for CoIII2(μ-OH)2 diamond cores reported in the literature.20b,26 The two equatorial pyridine rings of a TPA ligand in 1 formed π−π stacking pairs with those of the other TPA ligand and the mean distance between the N2- and N3′pyridines was 3.25 Å (see Figure S2 in the SI). This hydrophobic intramolecular π−π interaction probably contributes to the stabilization of the dimeric structure in an aqueous solution. To determine the pKa values of the μ-hydroxo ligands and to clarify the equilibrated structural changes of 1, depending on the solution pH, pH titration experiments were conducted in the pH range of 5−12.5 by using UV−vis spectroscopy in a Britton−Robinson (B-R) buffer solution.27 Complex 1 exhibited pH-dependent spectral change as shown in Figure 2. The reversible pH-dependent UV−vis spectral changes of 1

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RESULTS AND DISCUSSION Synthesis and Characterization of a Dinuclear Bis-μhydroxo CoIII(TPA) Complex (1). The synthesis of [Co2(μOH)2(TPA)2](ClO4)4 (1·(ClO4)4) was performed by treatment of [CoIIICl2(TPA)](ClO4)23 with slightly excess amount of AgClO4 in an aqueous solution and the purification was done by recrystallization from acetone/ethyl formate to give red-orange crystals in 43% yield (see Scheme 1). Scheme 1. Synthesis of Complex 1

Spectroscopic characterization of 1 was performed by 1H NMR spectroscopy and ESI-TOF-MS spectrometry and elemental analysis (EA).24 FTICR CSI-MS of 1 in H2O exhibited a peak cluster at m/z = 1029.022, which was ascribed to [1 + 3ClO4]+ (calcd. 1029.023) (see Figure S1 in the Supporting Information (SI)). This observation clearly indicates that complex 1 maintains the dinuclear structure, even in water at neutral pH. The structure of 1 was explicitly determined by X-ray diffraction (XRD) analysis. Complex 1 was crystallized into a triclinic lattice and the asymmetric unit involved a half of the cationic part of 1, two perchlorate ions as counteranions and an acetone molecule as a co-crystallized solvent molecule (Figure 1).24 The bond lengths around the metal center were 1.921(3) Å for Co−O1, 1.924(3) Å for Co− O1′, 1.930(3) Å for Co−N1, 1.930(4) Å for Co−N2, 1.940(3) Å for Co−N3, and 1.892(4) Å for Co−N4, respectively, which were in the typical range for those of CoIII complexes reported in the literature.25 The atom separations between Co···Co′ and O1···O1′ are 2.944(1) and 2.473(4) Å, respectively, in a CoIII2(μ-OH)2 diamond core: those values are also comparable

Figure 2. UV-vis spectra of 1 (1.0 mM) in B-R buffer at pH 5.4 (orange, {[Co(TPA)]2(μ-OH)2}4+), pH 8.5 (blue, {[Co(TPA)]2(μO)(μ-OH)}3+), and pH 11.7 (green, {[Co(TPA)]2(μ-O)2}2+), respectively.

having a Co2(μ-OH)2 structure allowed us to propose the structures of the complexes in the protonation−deprotonation equilibrium, as depicted in Scheme 2 (see also Table S1 in the SI).28 The two absorption peaks at 355 and 502 nm observed at pH 5.4, which can be assigned to d−d transitions of the CoIII centers, are comparable to those of reported complexes having dimeric Co2(μ-OH)2 core structure.28 The solution pH was increased, and when the pH reached 8, the first spectral change was observed, where both of the d−d bands showed slight red shifts to 365 and 508 nm, respectively. At pH ∼10, a further bathochromic shift was observed: At pH 11.7, the lower-energy band shifted to 515 nm; however, the B

DOI: 10.1021/acs.inorgchem.5b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Protonation−Deprotonation Equilibrium of 1 in a Britton−Robinson (B-R) Buffer

presence of 1 should be indispensable for the photocatalytic water oxidation as the catalyst. The highest yield of dioxygen (O2) against the oxidant was achieved using [1] = 0.4 mM, [[Ru(bpy)3]2+] = 0.1 mM, and [S2O82−] = 3 mM, when 0.12 mL of O2 was evolved, and the yield was determined to be 72%, on the basis of the terminal oxidant used (see Figure S4a in the SI). The highest turnover number (TON) of the photocatalytic water oxidation reached 742, when the reaction was conducted in the presence of 1 (2.5 μM) in the buffer (5 mL) and the photosensitizer, [Ru(bpy)3]2+ (3.1 nmol × 4) and the oxidant, S2O82− (0.25 mmol × 4), were added separately four times at an interval of 1.5 h, when, in total, 0.21 mL of oxygen evolution was observed (Figure 5).32 Under the present photocatalytic conditions, the amount of photochemically generated oxidant, [RuIII(bpy)3]3+, is much less than that of complex 1 and thus can react rapidly with the catalyst within its lifetime at pH 9.3. The robustness of the catalyst under photocatalytic conditions was clearly demonstrated by the recovery of the catalytic activity upon further addition of the oxidant and the photosensitizer, as shown in Figure 5. Since the catalyst (1) is stable under the present experimental conditions, the procedure allows us to gain the high TON via successive addition of the oxidant and the photosensitizer. The quantum yield of the catalytic reaction was determined by a standard actinometer method33 to be 44%, using initial oxygen evolution, as depicted in Figure S5 in the SI. 1 H NMR spectra were measured in the course of the photocatalytic water oxidation under the same conditions in a deuterated borate buffer solution at pD 9.3 (see Figure S6 in the SI). Peak integration intensities relative to that of 4,4dimethyl-4-silapentane-1-sulfonic acid (DSS) as an internal standard were monitored for signals assigned to the methylene protons of TPA at 5.37 ppm, that of the proton at the 3position of the axial pyridine ring at 7.17 ppm, and that of the proton at the 6-position of the axial pyridine moiety of TPA at 9.16 ppm. The integration ratios of the three signals were not changed during the reaction for 2 h (see Figure S6g). The results clearly demonstrated that the catalyst was intact. In addition, judging from the results of dynamic light scattering (DLS) measurements, as shown in Figure S7 in the SI, no formation of nanoparticles was observed under the photocatalytic conditions; however, the measurement of a solution containing [RuII(bpy)3]2+ and Na2S2O8 exhibited particles ca. 40 nm in size (see Figure S7b), which were probably derived from decomposed products of [RuIII(bpy)3]3+ formed in the photochemical process. Thereby, together with the result shown in Figure 4, any chemical species derived from [RuII(bpy)3]2+, which may form certain nanoparticles observed in DLS measurements (recall Figure S7), may be inactive for the water oxidation. We propose a catalytic cycle of the photocatalytic water oxidation with 1 in the presence of [Ru(bpy)3]2+ and S2O82− at

higher-energy band was unchanged (see Figure 2 and Table S1). The curve-fitting analysis was made using the absorbance changes at 365 nm for pKa1 and at 510 nm for pKa2, allowing us to determine the pKa value for each step as pKa1 = 6.8 and pKa2 = 10.9 (see Figure 3 and Scheme 2).29

Figure 3. Plots of the absorbance changes against the solution pH: (a) pH 5.6−8.8 and (b) pH 8.8−12.2.

Photocatalytic Oxidation of Water with 1 Using Persulfate as an Oxidant. Photocatalytic oxidation of water with 1 as the catalyst was conducted under irradiation with a xenon lamp (equipped with a 20 nm band-path filter centered at 420 nm) in a borate buffer (50 mM, 5 mL, pH 9.3) in the presence of [RuII(bpy)3](ClO4)2 (0.1 mM) as a photosensitizer and Na2S2O8 (25 mM) as a sacrificial oxidant (blue line in Figure 4).30,31 Under the present experimental conditions,

Figure 4. Time-courses of the O2 evolution from the photocatalytic system of 1 in borate buffer (50 mM, pH 9.3) under light illumination (λ = 420 nm), monitored by an O2-fluorescence sensor (blue line) and a control experiment in the absence of 1 (gray line): [1] = 0.4 mM, [[RuII(bpy)3]2+] = 0.1 mM, [S2O82−] = 25 mM.

complex 1 exists as a μ-oxo-μ-hydroxo dinuclear CoIII complex, [CoIII(μ-O)(μ-OH)(TPA)2]3+, on the basis of the pKa1 (6.8) and pKa2 (10.9) values (see Scheme 2).29 In order to clarify the responsible species in the photocatalytic water oxidation, we examined photocatalytic water oxidation using [RuII(bpy)3]2+ as a photosensitizer and Na2S2O8 as an oxidant in the absence of 1. The time profile of O2 evolution for the blank test is depicted as the gray line in Figure 4. The amount of O2 evolved was more than 10 times larger in the presence of 1, compared to that in the absence of 1. Thus, we concluded that the C

DOI: 10.1021/acs.inorgchem.5b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Time profile of repeated O2 evolution under the photocatalytic conditions in the presence of 1 (2.5 μM) in borate buffer (50 mM, pH 9.3, 5 mL). On the first cycle, were added [RuII(bpy)3]2+ (3.1 nmol) as a photosensitizer and S2O82− (0.25 mmol) as the oxidant. Before the next cycle, the same amounts of [RuII(bpy)3]2+ and S2O82− were further added, breaking the seal of the reaction vessel.

pH 9.3, as depicted in Scheme 3. At pH 9.3, 1 should exist as the monodeprotonated 1−H+, as described in Scheme 2. Scheme 3. A Proposed Catalytic Cycle of Photocatalytic Water Oxidation with 1, Using Persulfate as an Oxidant at pH 9.3a

Figure 6. (a) Cyclic voltammograms of 1 (0.44 mM) at pH 6.0 (pink), pH 7.9 (orange), pH 9.5 (light-green), pH 10.1 (turquoise), and pH 12.0 (blue) in B-R buffer at room temperature. The black solid line indicates the blank curve given by the B-R buffer (pH 6.0) in the absence of the Co catalyst. Scan rate = 100 mV s−1. (b) Plot of potential (measured at 0.1 mA) vs pH.

a

calculated to be −59 mV/pH. Upon increasing the solution pH (7.5−11.5), the gradient became smaller to be −27.5 mV/pH. In the range over pH 11.5, the slope was nearly flat and the pH dependence of the current was lost. These observations indicate that the species participating in the catalytic water oxidation are dependent on the solution pH, as the chemical species in solution change, depending on the solution pH, as described in Scheme 2. In the pH range of 5.0−7.5, where the [CoIII2(μ-OH)2] complex (1) is the major species, the slope of the potential against pH here indicates that the rate-determining step (RDS) of the electrochemical reaction should be a proton-coupled electron transfer (PCET) process, in which proton and electron are transferred in the e−:H+ = 1:1 ratio, which is assumed to be 2e− oxidation coupled with two-proton release. At higher pH (7.5−11.5), complex 1 is mainly converted to [CoIII2(μ-O)(μOH)(TPA)2]3+ (1−H+; see Scheme 2) by deprotonation of one of the μ-hydroxo ligands of 1 and the catalytic oxidation also changes into a PCET process with the e−: H+ = 2:1 ratio, judging from the slope of the potential dependency on the pH. In the pH range over 11.5, since the species in the solution should be [Co2(μ-O)2(TPA)2]2+ (1−2H+; see Scheme 2), the reaction changes to a simple electron transfer without coupling with proton transfer. Water Oxidation with 1 Using [RuIII(bpy)3]3+ as an Oxidant. To confirm that the wave observed in CV was derived from the catalytic water oxidation process, oxidation of 1 was conducted using a chemical oxidant, [RuIII(bpy)3]3+ (bpy = 2,2′-bipyridine) (E1/2 = +1.26 V vs NHE in H2O),36 in the presence of a catalytic amount of 1 in B-R buffer at pH 7.8.

The TPA ligand in each CoIII complex is omitted for clarity.

Photochemically formed [RuIII(bpy)3]3+ oxidizes 1−H+ to evolve O2 and to regenerate 1−H+. For completing the reaction, 4 mol equiv of [RuIII(bpy)3]3+ and the corresponding 2 mol equiv of S2O82− are required. PCET Behaviors of 1 in Water. To gain mechanistic insights into the photocatalytic water oxidation using 1 as the catalyst, a cyclic voltammogram (CV) of 1 was measured with a disk electrode of a glassy carbon plate as a working electrode in B-R buffer,34 where pH was set to be 7.9 by the addition of an aqueous NaOH solution. The CV showed a significant rise from the baseline, starting at +1.25 V vs NHE (see orange line in Figure 6a and Figure S9 in the SI). This rise can be ascribed to the catalytic current derived from the water oxidation and the catalytic current was observed in the pH range of 5.0−12.5 (see Figure 6a and Figure S10 in the SI). Electrochemical oxidation of water in the presence of 1 (0.4 mM) was confirmed by bulk electrolysis of the B-R buffer solution of 1 at pH 7.8 and +1.74 V vs NHE for 2 × 104 s to detect O2 evolution, as shown in Figure S11 in the SI. In the pH 2σ(I)) and wR2 = 0.1814, GOF = 1.048. CSI-MS Measurements.59 The water solution of Co complex (1.2 mg/mL) was investigated by CSI-MS under the following equipment and conditions. CSI-MS measurement was performed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) (Apex-Qe 9.4 T, Bruker Daltonics, Inc., Billerica, MA, USA) equipped with a CSI device (Bruker). The heater of the desolvation assembly was turned off to maintain room temperature around the spray chamber in the ionization process and the temperature of nebulizer gas N2 was set at 35 °C. General measurement conditions were as follows: positive mode, capillary voltage, 4.5 kV; dry gas flow rate, 5.0 L/min; nebulizer gas flow rate, 1.5 L/min; and sample flow rate, 120 μL/h. Determination of pKa Values. The spectroscopic titrations were performed to determine the pKa values of 1 in a 0.1 M B-R buffer at 298 K. A small amount of 15 M NaOH aqueous solution or 70% HClO4 aqueous solution were added to a 0.1 M B-R buffer solution containing 1 (1.0 mM) to adjust pH values. A plot of absorbance at a particular wavelength versus pH was fitted using eq 2 to obtain Figure 3:

⎛ A −A ⎞ 0 ⎟ A = A 0 + K a⎜ ∞ −pH ⎝ K a + 10 ⎠

the fitting curve as the pH value when the absorbance was the average value of A0 and A∞. For the determination of each pKa value, the absorbance at following wavelength was used: 365 nm for pKa1 and 510 nm for pKa2. Electrochemical Measurements. Cyclic and square-wave voltammetries were performed on an ALS 710D electrochemical analyzer by using a glassy carbon electrode as a working electrode, an Ag/AgNO3 or Ag/AgCl as a reference electrode, and a Pt wire as an auxiliary electrode. Cyclic voltammograms of 1 (0.4 mM) were obtained in 0.1 M B-R buffer solutions in the range of 2.0 < pH < 12.0. The pH values of the solution were controlled using 15 M NaOH aqueous solutions. The potentials relative to Ag/AgNO3 were converted to those relative to NHE by adding 0.53 V.60 Catalytic Oxygen Evolution. An aliquot (2.0 mL) of 0.1 M B-R buffer (pH 7.8) containing 1 (4.4 × 10−8 mol) was injected into the reaction vessel (4.9 mL) containing [RuIII(bpy)3](ClO4)3 (3.4 × 10−6 mol) and a magnetic stirring bar. After 5 min, 100 μL of gas in headspace of the reaction vessel was taken by using a gas-tight syringe and analyzed by gas chromatography (GC). The yield of O2 was determined with a Shimadzu GC-17A gas chromatograph (Ar carrier, a capillary column with molecular sieves (Agilent Technologies, Model 19095P-MS0, 30 m × 0.53 mm) at 313 K) equipped with a thermal conductivity detector at Osaka University (Osaka, Japan). The total amount of evolved O2 was determined based on a calibration curve prepared for various O2 concentrations in argon gas. CO2 Detection by GC. A borate buffer solution (50 mM, pH 9.4, 5.0 mL) containing complex 1 (0.4 mM), Na2S2O8 (3.0 mM) and [Ru(bpy)3](ClO4)2 (0.10 mM) was flushed with argon gas for 20 min. The solution was then irradiated with a xenon lamp (Asahi Spectra, Model MAX-301) through a band-path filter (λ = 420 nm). After each reaction time (15, 30, 45, 60, 90, and 120 min), 100 μL of argon gas was injected into the vial, and then the same volume of gas in the headspace of the vial was sampled by a gas-tight syringe and quantified by a Shimadzu Model GC-2014 gas chromatograph [argon carrier gas, packed column with activated charcoal (3.0 mm × 3.0 m, 60−80 mesh) at 353 K] equipped with a thermal conductivity detector (TCD). The limit of CO2 detection for the gas chromatograph with argon carrier gas was calculated to be 400 ppm, based on a calibration using pure CO2. Electrochemical Water Oxidation by 1. Bulk electrolysis was performed in a two-compartment electrochemical cell (42 mL) with a Nafion perfluorinated membrane (Sigma−Aldrich Co. LLC.), an ITO electrode (BAS, Inc., 8 mm × 27 mm × 0.5 mm) as a working electrode, an Ag/AgCl (saturated KCl) reference electrode, and a Pt mesh as a counter electrode. Electrochemical water oxidation was conducted at +1.7 V (vs NHE) in an Ar-saturated borate buffer solution (pH 9.4, 50 mM; 15 mL) containing complex 1 (0.40 mM) and 0.10 M Na2SO4 as a supporting electrolyte at room temperature. Typically, a 0.8 mm × 15 mm piece of ITO electrode was immersed in the buffer solution and rinsed with water before use. After 1 h electrolysis, the evolved O2 in the headspace was sampled by a gastight syringe (100 μL) and quantified by GC. The ITO electrode used for the electrolysis with complex 1 was rinsed with water. Control experiments were performed with the used ITO electrode or a pristine ITO electrode in a fresh buffer solution without complex 1. A cobaltoxide-deposited ITO electrode for SEM measurements was prepared by applying +1.7 V (vs NHE) on an ITO-coated glass for 1 h in a borate buffer (pH 9.4, 50 mM) containing Co(ClO4)2 (20 μM). Isotope-Labeling Experiments. An aliquot (1.0 mL) of a deaerated 500 μL of H218O solution (83.8% 18O) containing 1 (5.0 × 10−8 mol) and 500 μL of a 0.1 M B-R buffer (pH 7.6) was injected to reaction vessel (2 mL) containing [RuIII(bpy)3] (ClO4)3 (1.5 × 10−6 mol). The air in the headspace of a sealed reaction vessel was replaced with helium before the reaction by bubbling helium gas through a syringe. After 15 min, 50 μL of the gas in the headspace of the reaction vessel was taken using a gas-tight syringe and analyzed by GC-MS. The ratio of 16O2, 16O18O, and 18O2 was determined on the basis of the intensity of mass spectrum (m/z = 32, 34 and 36) obtained on a Shimadzu Model GC-17A gas chromatograph (helium carrier, TC-FFAP column (GL Science, Model 1010-15242) at 313 K)

(2)

where A0 is the absorbance of a starting complex and A∞ is the absorbance of a product complex. The pKa value was determined from H

DOI: 10.1021/acs.inorgchem.5b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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equipped with a mass spectrometer (Shimadzu, Model QP-5000) at Osaka University (Osaka, Japan). Kinetic Measurements. Kinetic analysis on the reactions of [RuIII(bpy)3]3+ with 1 was made based on the spectral changes measured on a UNISOKU Model RSP-2000 stopped-flow spectrometer equipped with a multichannel photodiode array or an Agilent Model 8453 photodiode-array spectrophotometer. The second-order rate constants were determined from the corresponding second-order plots (1/[RuII(bpy)3]2+ vs time), where the concentration of [RuII(bpy)3]2+ was calculated from Abs/(ε460 nm of [RuII(bpy)3]2+, 14500 M−1 cm−1) . Photocatalytic Oxidation. Catalyst 1 (2.5−400 μM) and 0.25 equiv of [RuII(bpy)3](ClO4)2 (1.25−100 μM) were added to a borate buffer solution (pH 9.3, 5.0 mL) containing Na2S2O8 (3−1000 mM). The solution was then irradiated with a xenon lamp (Asahi Spectra, MAX-301) through a band-path filter centered at 420 nm at room temperature. Evolved oxygen gas in a headspace was quantified with a fluorescence oxygen sensor (Ocean Photonics, Neo Fox with the FOXY Formulation). Quantum Yield Determination. The quantum yield of the photocatalytic water oxidation in the presence of 1 was determined by a standard method using an actinometer (potassium ferrioxalate)33 in a borate buffer (pH 9.3) with photoirradiation at 420 nm. The oxygen evolution was monitored with optical oxygen sensor to determine the time course of the evolution, and the data in the initial stage, during which the time course exhibited linear change, were used to determine the quantum yield. The quantum yield was calculated with eq 3, Φ=

ΔO2 × 2 × 100 I

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S. Fukuzumi). *E-mail: [email protected] (T. Kojima). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid (Nos. 24655044, 24245011, 25107508, 15H00861 to T.K.) from Japan Society for the Promotion of Science (JSPS) and an ALCA project from JST, Japan (to S.F.). T.K. also appreciates financial supports from The Asahi Glass Foundation and The Mitsubishi Foundation. D.H. gratefully acknowledges support from JSPS by Grant-in-Aid (No. 15J04635) for JSPS fellowship for young scientists.

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REFERENCES

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(3)

where I is the number of absorbed photon per second (see Figure S5a in the SI) and ΔO2 is the amount of evolved oxygen per second, which was determined from the initial slope for the first 50 min in Figure S5b in the SI. As shown in Scheme 3, one photon forms two [RuIII(bpy)3]3+; thus, the evolution of one O2 molecule requires two photons. In this experiment, I and ΔO2 were determined to be 9.61 × 10−10 mol s−1 and 2.1 × 10−10 mol s−1, respectively. Computational Method. Energy calculations for 2 in the triplet and singlet states were carried out using unrestricted density functional theory (UDFT) implemented in Gaussian 09.61 Geometry optimizations were performed with the B3LYP method.62,63 The ⟨s2⟩ values of the optimized structures at the B3LYP level were calculated to be 2.00 in the triplet state, 1.01 in the open-shell singlet state, and 0.00 in the closed-shell singlet state. The open-shell singlet state was computed using the broken-symmetry approach. The stability of optimized geometries was confirmed with vibrational analyses, and no imaginary frequency was found. For the Co atom, the (14s9p5d)/[9s5p3d] primitive set of Wachters−Hay64,65 with one polarization f-function (α = 1.117)66 and for the H, C, N, and O, the D95** basis set67 were used.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02336. Summary of absorption maxima, FTICR CSI-MS spectrum, ORTEP drawings of complex 1, O2 evolution profiles, 1H NMR spectra, DLS results, cyclic voltammograms, gas chromatograms, time profiles of the absorbance changes of [RuIII(bpy)3]3+ at 460 nm, ESITOF-MS spectrum, SEM images of ITO electrodes, and DFT-optimized structures (PDF) Cartesian coordinates of the DFT-optimized structure of complex 2 and the μ:η2,η2-peroxo complex (Tables S3− S5) (PDF) Crystallographic data for complex 1 (CIF) I

DOI: 10.1021/acs.inorgchem.5b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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(21) Betley, T. A.; Wu, Q.; Van Voorhis, T.; Nocera, D. G. Inorg. Chem. 2008, 47, 1849. (22) (a) Zhang, M.; de Respinis, M.; Frei, H. Nat. Chem. 2014, 6, 362. (b) Davenport, T. C.; Ahn, H. S.; Ziegler, M. S.; Tilley, T. D. Chem. Commun. 2014, 50, 6326. (23) Massoud, S. S.; Jeanbatiste, R. R.; Nguyen, T. M.; Louka, F. R.; Gallo, A. A. J. Coord. Chem. 2007, 60, 2409. (24) See the Experimental Section. (25) (a) Cheyne, S. E.; McClintock, L. F.; Blackman, A. G. Inorg. Chem. 2006, 45, 2610. (b) Min, K. S.; DiPasquale, A. G.; Golen, J. A.; Rheingold, A. L.; Miller, J. S. J. Am. Chem. Soc. 2007, 129, 2360. (26) (a) Sumner, C. E., Jr. Inorg. Chem. 1988, 27, 1320. (b) Dimitrou, K.; Folting, K.; Streib, W. E.; Christou, G. J. Am. Chem. Soc. 1993, 115, 6432. (27) Wang, R.; Vos, J. G.; Schmehl, R. H.; Hage, R. J. Am. Chem. Soc. 1992, 114, 1964. (28) Meloon, D. R.; Harris, G. M. Inorg. Chem. 1977, 16, 434. (29) During the reviewing process, one reviewer suggested that if there was equilibrium between the dinuclear and mononuclear forms of 1, the pKa values were dependent on the concentration. Thus, we performed the pKa determination with a more diluted solution of 1; as a result, the pKa values of 1 are independent of the concentration. Therefore, complex 1 maintains the dinuclear form in the pH range. (30) Hong, D.; Yamada, Y.; Nagatomi, T.; Takai, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 19572. (31) Photocatalytic water oxidation reaction with use of Co(ClO4)2 instead of 1 was performed in a borate buffer (50 mM, pH 9.4) in the presence of persulfate as an oxidant and [Ru(bpy)3]2+ as a photosensitizer. The amount of dioxygen evolved was comparable to that with use of 1 as a catalyst (Figure S3 in the Supporting Information (SI)). However, in the case of Co(ClO4)2 as the catalyst precursor, Co ions immediately formed Co clusters as responsible species to afford black suspension. On the other hand, complex 1 is robust and intact under the catalytic conditions as reflected on the NMR spectrum and DLS data after the reaction. Thus, the solution of 1 can be repeatedly utilized for photocatalytic water oxidation by further addition of the oxidant as depicted in Figure 5. (32) Apparent decrease of catalytic activity in Figure 5 was derived from increased viscosity of the solution, because of the successive addition of excess Na2S2O8. (33) Langford, C. H.; Holubov, C. A. Inorg. Chim. Acta 1981, 53, L59. (34) CV of 1 in MeCN also showed an irreversible redox wave at +2.30 V (vs NHE), ascribable to an oxidation process of 1. Addition of water to the MeCN solution gave a rise of the catalytic wave probably derived from the water oxidation (Figure S8 in the SI). (35) Dogutan, D. K.; McGuire, R.; Nocera, D. G. J. Am. Chem. Soc. 2011, 133, 9178. (36) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815. (37) At pH 7.7, [RuIII(bpy)3]3+ gradually decomposed at the rate constant of 0.004 s−1 (Figure S13 in the SI). This is the reason for the low O2 yield and the low TON under the reaction conditions. (38) The μ-hydroxo ligands should be 16OH not 18OH in the first catalytic cycle, because of the substitution inert character of the CoIIIcenter, In fact, ESI-TOF-MS spectrum of 1 dissolved in H218O indicates no substitution of the μ-16OH ligands by 18OH (Figure S14 in the SI). (39) If the bridging hydroxo ligands of 1 were substitution-inert and the O−O bond formation occurred between a bridging oxyl ligand and a solvent water molecule after oxidation of 1, the calculated isotopic ratio of the evolved oxygen should be 16O2:16O18O:18O2 = 23:51:26. On the other hand, if the bridging hydroxo ligands of 1 were substituted very fast, the calculated isotopic ratio of the evolved oxygen should be 16O2:16O18O:18O2 = 18:49:34, regardless of the O−O bond formation mechanism (Figure S15 in the SI). Both scenarios cannot explain the experimental result depicted in Figure 7. J

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Inorganic Chemistry (40) McGrail, B. T.; Pianowski, L. S.; Burns, P. C. J. Am. Chem. Soc. 2014, 136, 4797. (41) The 1H NMR signal ascribable to the protons at the 6-position of the axial pyridine ring of the TPA ligand was downfield-shifted from δ 8.56 ppm to δ 8.48 ppm after the catalytic reaction. To account for the downfield shift, the 1H NMR spectrum of 1 in D2O at pD 1 acidified with DCl was measured; the signal observed at 8.56 ppm at pH 7 shifted downfield to 8.49 ppm at pH 1 (Figure S17 in the SI). During the water oxidation with [Ru(bpy)3]3+ as an oxidant in the experiment of Figure S16 in the SI, the solution pH was changed to be acidic, because the reaction was done in nonbuffered D2O. (42) We measured dynamic light scattering (DLS) of the solution after the catalytic reaction and no scattering signals were detected above the lower detection limit (ca. 1 nm). This indicates that the solution includes no nanoclusters of cobalt oxides [(CoOx)n] with the size over 1 nm (Figure S18 in the SI). (43) Radaram, B.; Ivie, J. A.; Singh, W. M.; Grudzien, R. M.; Reibenspies, J. H.; Webster, C. E.; Zhao, X. Inorg. Chem. 2011, 50, 10564. (44) Higher O2 evolution observed in the electrolysis with the used ITO electrode as the working electrode than that with the pristine ITO electrode can be ascribed to the physically adsorbed 1 on the surface of the ITO electrode that was not removed by rinsing with water or impurity in the buffer solution, because no cobalt oxide particles were observed by SEM measurements (Figure S19 in the SI). (45) (a) Blakemore, J. D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.; Brudvig, G. W.; Crabtree, R. H. Chem. Sci. 2011, 2, 94. (b) Hong, D.; Mandal, S.; Yamada, Y.; Lee, Y.-M.; Nam, W.; Llobet, A.; Fukuzumi, S. Inorg. Chem. 2013, 52, 9522. (c) Chen, G.; Chen, L.; Ng, S.-M.; Man, W.-L.; Lau, T.-C. Angew. Chem., Int. Ed. 2013, 52, 1789. (d) Chen, G.; Chen, L.; Ng, S.-M.; Lau, T.-C. ChemSusChem 2014, 7, 127. (e) Najafpour, M. M.; Moghaddam, A. N.; Dau, H.; Zaharieva, I. J. Am. Chem. Soc. 2014, 136, 7245. (46) In order to verify that CoII ions derived from decomposition of 1 should not be active species in the water-oxidation catalysis, we performed CV experiments of 1 in the presence of H4EDTA in a phosphate buffer (50 mM, pH 8.0) (Figure S25 in the SI). As a result, no decrease of the catalytic current was observed at +1.7 V (vs NHE) by addition of H4EDTA (0−1 equiv). A slight increase of the catalytic current at +1.7 V (vs NHE) observed upon addion of 1 equiv of H4EDTA indicates that the H4EDTA oxidation occurs at such a high potential. The result demonstrates that no Co2+ ions were released into the reaction solution during the electrochemical water oxidation with 1. (47) For instance, see: Su, X.-J.; Gao, M.; Jiao, L.; Liao, R.-Z.; Siegbahn, P. E. M.; Cheng, J.-P.; Zhang, M.-T. Angew. Chem., Int. Ed. 2015, 54, 4909. (48) To visualize the spin distributions on 2, spin natural orbitals corresponding to singly occupied molecular orbitals (SOMOs) were calculated in the open-shell singlet state. The broken-symmetry approach provides the antiferromagnetic state between the two radicals (Figure S26 in the SI). Consequently, both of the unpaired electrons were localized on one of the two μ-oxo ligands. This situation is favorable for intramolecular radical coupling between the two oxo-ligands, rather than oxidation of the ancillary ligands as undesired side-reactions. (49) Li, X.; Clatworthy, E. B.; Masters, A. F.; Maschmeyer, T. Chem.Eur. J. 2015, 21, 16578. (50) During the preparation of this manuscript, complex 3 was independently reported by Thapper’s group: Wang, H.-Y.; Mijangos, E.; Ott, S.; Thapper, A. Angew. Chem., Int. Ed. 2014, 53, 14499. In contrast to the proposal by Wang et al., we could not detect [{(TPA) CoII(OH)}(μ-O){CoIII(OH)(TPA)}], which was denoted as ‘complex 5’ in the Thapper’s paper, as the actual catalyst during the water oxidation; because, after the catalytic reaction, the 1H NMR signals of 1 were intact. (51) Bogucki, R. F.; McLendon, G.; Martell, A. E. J. Am. Chem. Soc. 1976, 98, 3202.

(52) Wasylenko, D. J.; Ganesamoorthy, C.; Borau-Garcia, J.; Berlinguette, C. P. Chem. Commun. 2011, 47, 4249. (53) As a preliminary experiment, we examined an oxidation reaction of 3 with 2 equiv of [Ru(bpy)3]3+ as an oxidant at pH 6 (Figure S27 in the SI). As a result, we could observe two-step oxidation reactions, consisting of a fast primary step due to the 1e− oxidation of 3 to afford a putative superoxo intermediate, followed by a slow subsequent 1e− oxidation step that should be the oxidation of the superoxo intermediate. The two-step oxidation of 3 is in agreement with the observation in the CV measurement on 3 (ref 50). The rate constant for the second step was determined to be 8.0 × 103 M−1 s−1 at 298 K, which was much larger than that of the oxidation of 1 (46 M−1 s−1), although the rate constant of the first process was too large to be determined under the same conditions. The results support that the oxidation of 1 to afford the bis-μ-oxyl dinuclear CoIII complex, 2, is the RDS of the water oxidation catalyzed by 1 as reflected on the catalytic currents shown in Figure 6. (54) Anderegg, G.; Wenk, F. Helv. Chim. Acta 1967, 50, 2330. (55) Britton, H. T. K.; Robinson, R. A. J. Chem. Soc. 1931, 1456. (56) Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. Nippon Kessho Gakkaishi 2009, 51, 218. (57) Sheldrick, G. M. SIR97 and SHELX97, Programs for Crystal Structure Refinement; University of Göttingen, Göttingen, Germany, 1997. (58) Sluis, P. V. P.; Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, A46, 194. (59) Yamaguchi, K. J. Mass Spectrom. 2003, 38, 473. (60) Nakanishi, T.; Ohkubo, K.; Kojima, T.; Fukuzumi, S. J. Am. Chem. Soc. 2009, 131, 577. (61) Gaussian 09, Revision C01; Gaussian, Inc.: Wallingford, CT, 2009. For the complete list of authors, see the SI. (62) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (63) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (64) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (65) Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (66) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (67) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry, Vol. 3; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1976; pp 1− 28.

K

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