and Electrocatalytic Hydrogen Production Using ... - ACS Publications

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Photo- and Electrocatalytic Hydrogen Production Using Valence Isomers of N2S2‑Type Nickel Complexes Satoshi Inoue,† Manabu Mitsuhashi,† Takeshi Ono,† Yin-Nan Yan,† Yusuke Kataoka,‡ Makoto Handa,‡ and Tatsuya Kawamoto*,† †

Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan Department of Material Science, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan

‡

S Supporting Information *

ABSTRACT: Three Schiff-base-type nickel(II) complexes (1a−3a) and the corresponding noninnocent-type complexes (1b−3b) were synthesized, and the equilibria between these valence isomers were observed in tetrahydrofuran (THF) at room temperature. The electronic state of the noninnocent-type nickel complex was also confirmed by isolation of the one-electron-reduced species. The catalytic ability for the photogeneration of hydrogen from water was examined about 1a−3a and 1b−3b in the presence of a photosensitizer and a sacrificial electron donor. Then, a Schiff-base-type complex with chlorine atoms (2a) and a noninnocent-type complex with methyl groups (3b) on the pendant phenyl rings being present as the minor species in THF exhibited high activity of over 400 turnover numbers. The dynamic light scattering and transmission electron microscopy measurements suggested the formation of NiSx-like aggregate species under photocatalytic conditions. The electrocatalytic activities of the nickel complexes for hydrogen production were also investigated, and a plausible reaction mechanism was proposed on the basis of a combined electrochemical and density functional theory study.

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the formation of stable iminosemiquinone radical ligands.9 Furthermore, Eisenberg and co-workers described that a cobalt dithiolene complex, which is a representative noninnocent-type complex, shows high catalytic activity in the photoreduction of aqueous protons.10 Such catalytic behavior of the dithiolene complex, as well as nickel and cobalt complexes with redoxactive nitrogen-donor ligands,11 confirms the high potential of noninnocent-type metal complexes as catalysts in the photoreduction of water.12 Herein we report on photoinduced hydrogen production using Schiff-base-type nickel(II) complexes and the corresponding valence isomers, noninnocenttype complexes, as catalysts in a photocatalytic system employing the PS [Ir(ppy)2(bpy)]+ pioneered by Bernhard et al.13 and triethanolamine (TEOA) as the SED, as well as the electrocatalytic behavior of these complexes.

INTRODUCTION The production of molecular hydrogen from water using solar energy is a really challenging task because hydrogen could become an important alternative fuel in the future.1 The reduction of protons to hydrogen using electrons from an external source has been achieved with complexes, based on inexpensive and common metals such as iron or nickel, inspired by the hydrogenase enzymes as electrocatalysts.2 On the other hand, the typical water photoreduction system for hydrogen production consists of a photosensitizer (PS), a water-reduction catalyst (WRC), and a sacrificial electron donor (SED).3 However, a number of photocatalytic systems contain noble metals like colloidal platinum as the catalyst,4 and hence the development of novel active catalysts comprised of cheap and earth-abundant transition metals is a promising approach to a more efficient catalytic system.5 In order to account for the unique electronic and optical properties, transition-metal complexes with the so-called noninnocent ligands have been widely investigated over the past 5 decades.6 Such complexes based on redox-noninnocent ligands have received a renaissance in the application of abundant metals for cooperative catalysis.7 For example, Chirik et al. showed that a two-electron oxidative addition process can occur without changing the oxidation state of the metal center in iron(II)-catalyzed [2π + 2π] cycloaddition reaction.8 Soper et al. reported the metal-centered oxidation addition steps with © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Materials and Methods. Reagents and solvents were purchased from Tokyo Chemical Industry, Wako Pure Chemical Industries, Sigma-Aldrich Japan, Kishida Chemical, Tanaka Kikinzoku Kogyo, or Strem Chemicals and used without further purification. All of the synthetic reactions except [Ir(ppy)2(bpy)]PF6 were carried out under a nitrogen atmosphere with Schlenk techniques. 2-PhenylbenzothiazoReceived: May 18, 2017 Published: September 22, 2017 12129

DOI: 10.1021/acs.inorgchem.7b01244 Inorg. Chem. 2017, 56, 12129−12138

Article

Inorganic Chemistry line (L1), bis[2-(phenylmethyleneamino)benzenethiolato]nickel(II) (1a), and [bis-2,2′-(1,2-diphenylethylenediimine)benzenethiolate]nickel(II) (1b) were synthesized following previously reported procedures.14 [Ir(ppy)2(bpy)]PF6 was synthesized following a procedure slightly modified from that reported by Bernhard et al.13 CHN elemental analyses were performed by a PerkinElmer 2400 series II CHNS/O analyzer. NMR spectra were collected on a JEOL ECA400 NMR spectrometer using tetramethylsilane as an internal standard and processed using a FT-NMR version 4 data processor. UV−vis− near-IR (NIR) spectra were recorded on a Jasco V-570 spectrophotometer. Single-crystal X-ray data were collected on a Rigaku VariMax with Saturn diffractometer. Crystals were placed in the loop with paratone oil and mounted on the X-ray diffractometer. Structures were solved using SIR2008 and refined using SHELXL-97 on CrystalStructure 4.1. Emission spectra were recorded on a Jasco FP-8300 spectrofluorometer. Electron-spin resonance (ESR) spectra were collected on a JEOL JES-RE2X ESR spectrometer. Cyclic voltammetry was performed in tetrahydrofuran (THF) containing 0.10 M tetrabutylammonium perchlorate (nBu4NClO4) as an electrolyte with a CH Instruments model 620A electrochemical analyzer using a glassy carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode (SCE) at a scan rate of 50 mV s−1. Before measurement, the sample solution was purged well with nitrogen gas. Dynamic light scattering (DLS) measurements were performed by an Otsuka Electronics Fiber-Optics FPAR-1000AS particle analyzer. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2000EX transmission electron microscope operated at an accelerating voltage of 100 kV. The sample was placed on a Formvar film-coated copper grid and dried in vacuo overnight prior to observation. Synthetic Procedure of Bis[2-(phenylmethyleneamino)benzenethiolato]nickel(II) (1a). 1a was synthesized following a previously reported procedure.14 Anal. Calcd for C26H20N2NiS2: C, 64.61; H, 4.17; N, 5.80. Found: C, 64.65; H, 4.20; N, 5.81. 1H NMR (400 MHz, CDCl3): δ 6.22 (2H, d with fine coupling, J = 8.0 Hz, ArH), 6.67 (2H, ddd, J = 8.0, 7.2, and 1.2 Hz, ArH), 7.02 (2H, ddd, J = 7.6, 7.2, and 1.1 Hz, ArH), 7.23 (4H, dd, J = 7.6 and 7.4 Hz, ArH), 7.40 (2H, dd, J = 7.6 and 1.2 Hz, ArH), 7.41 (2H, td, J = 7.4 and 1.1 Hz, ArH), 7.78 (2H, s, NCH), 8.79 (4H, d with fine coupling, J = 7.6 Hz, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1142 (137), 832 (2246), 468 (3721). Synthetic Procedure of [Bis-2,2′-(1,2-diphenylethylenediimine)benzenethiolate]nickel(II) (1b). 1b was synthesized following a previously reported procedure.14 Anal. Calcd for C26H20N2NiS2· 0.2CH2Cl2: C, 62.90; H, 4.11; N, 5.60. Found: C, 62.73; H, 4.13; N, 5.48. 1H MNR (400 MHz, CDCl3): δ 6.34 (2H, s, CHN), 7.01 (2H, ddd, J = 8.7, 7.0, and 1.3 Hz, ArH), 7.14 (2H, dd, J = 8.0 and 7.0 Hz, ArH), 7.23−7.29 (8H, m, ArH), 7.33 (4H, dd, J = 7.9 and 1.7 Hz, ArH), 7.71 (2H, d with fine coupling, J = 8.0 Hz, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1143 (3180), 983 (2605), 833 (38743), 586 (1420), 539 (1369), 471 (1225), 354 (4296), 324 (5500). Synthetic Procedure of 2-(3,5-Dichlorophenyl)benzothiazoline (L2). To 2-aminobenzenethiol (1.03 g, 8.21 mmol) dissolved in 20 mL of ethanol was added 3,5-dichlorobenzaldehyde (1.43 g, 8.18 mmol). The mixture was refluxed under a nitrogen atmosphere for 2 h, and then the reaction solution was concentrated to oil using a rotary evaporator. After 30 mL of hot hexane was added to the obtained yellow oil, the mixture was cooled with vigorous stirring until a solid was formed. The resulting white solid was collected by filtration, washed with hexane (3 × 20 mL), and dried in vacuo. Yield: 1.55 g, 62.7%. Anal. Calcd for C13H9Cl2NS: C, 55.33; H, 3.21; N, 4.96. Found: C, 55.40; H, 3.11; N, 5.02. 1H NMR (400 MHz, acetone-d6): δ 6.29 (br s, 1H, NH), 6.49 (d, J = 3.1 Hz, 1H, bt-H2), 6.70 (ddd, J = 7.6, 7.5, and 1.1 Hz, 1H, bt-H6), 6.75 (dd, J = 7.8 and 1.1 Hz, 1H, btH4), 6.94 (ddd, J = 7.8, 7.6, and 1.2 Hz, 1H, bt-H5), 7.03 (d with fine coupling, J = 7.5 Hz, 1H, bt-H7), 7.42 (t, J = 1.9 Hz, 1H, ph-Hp), 7.53 (d with fine coupling, J = 1.9 Hz, 2H, ph-Ho). Synthetic Procedure of Bis[2-(3,5dichlorophenylmethyleneamino)benzenethiolato]nickel(II) (2a). L2

(782 mg, 2.77 mmol) and nickel(II) acetate tetrahydrate (347 mg, 1.39 mmol) were added to 40 mL of ethanol purged with nitrogen gas. The mixture was refluxed under a nitrogen atmosphere for 30 min and then cooled to room temperature. The resulting dark-reddish-brown powder was collected by filtration and dried in vacuo. Yield: 655 mg, 75.8%. Anal. Calcd for C26H16Cl4N2NiS2: C, 50.28; H, 2.60; N, 4.51. Found: C, 50.21; H, 2.36; N, 4.48. 1H NMR (400 MHz, CDCl3): δ 6.39 (2H, d, J = 8.1 Hz, ArH), 6.81 (2H, ddd, J = 8.1, 7.1, and 1.2 Hz, ArH), 7.09 (2H, ddd, J = 7.9, 7.1, and 1.0 Hz, ArH), 7.38 (2H, t, J = 1.9 Hz, ArH), 7.43 (2H, dd, J = 7.9 and 1.2 Hz, ArH), 7.87 (2H, s, NCH), 8.69 (4H, s, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1152 (117), 837 (3042), 521 (3337). Synthetic Procedure of {Bis-2,2′-[1,2-di(3,5-dichlorophenyl)ethylenediimine]benzenethiolato}nickel(II) (2b). A suspension of 2a (150 mg, 0.242 mmol) in 40 mL of toluene purged with nitrogen gas was refluxed under a nitrogen atmosphere for 30 min and then cooled to room temperature. The solvent was removed by rotary evaporation, and the resulting residue was dried in vacuo. This crude product was dissolved in a minimum amount of dichloromethane and purified by column chromatography on silica gel using dichloromethane/hexane (1:1, v/v) as the eluent. The first dark-blue fraction was collected and evaporated to dryness using a rotary evaporator. The resulting darkblue powder was dried in vacuo. Yield: 115 mg, 76.4%. Black prism crystals suitable for single-crystal X-ray analysis were obtained by the slow evaporation of a 1-butanol/dichloromethane (1:4, v/v) mixed solvent. Anal. Calcd for C26H16Cl4N2NiS2: C, 50.28; H, 2.60; N, 4.51. Found: C, 50.38; H, 2.37; N, 4.38. 1H NMR (400 MHz, CDCl3): δ 6.19 (2H, s, CHN), 7.10−7.17 (8H, m, ArH), 7.21 (2H, ddd, J = 8.2, 6.3, and 1.7 Hz, ArH), 7.27 (2H, t, J = 1.9 Hz, ArH), 7.76 (2H, d, J = 8.2 Hz, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1151 (2701), 989 (2535), 839 (39594), 597 (1328), 545 (1231), 476 (1273), 355 (4375), 326 (5475). Synthetic Procedure of 2-(3,5-Dimethylphenyl)benzothiazoline (L3). To 2-aminobenzenethiol (1.10 g, 8.78 mmol) dissolved in 20 mL of ethanol was added 3,5-dimethylbenzaldehyde (1.18 mL, 8.78 mmol). The mixture was refluxed under a nitrogen atmosphere for 2 h, and then the reaction solution was concentrated to oil using a rotary evaporator. After the obtained yellow oil was dissolved in 30 mL of hot hexane, the solution was cooled with vigorous stirring until a solid was formed. The resulting white solid was collected by filtration, washed with cold hexane (3 × 20 mL), and dried in vacuo. Yield: 0.503 g, 23.7%. Anal. Calcd for C15H15NS: C, 74.64; H, 6.26; N, 5.80. Found: C, 74.57; H, 6.21; N, 5.85. 1H NMR (400 MHz, acetone-d6): δ 2.27 (d, J = 0.5 Hz, 6H, Me), 6.00 (br s, 1H, NH), 6.42 (d, J = 2.7 Hz, 1H, bt-H2), 6.64 (ddd, J = 7.5, 7.5, and 1.2 Hz, 1H, bt-H6), 6.66 (d with fine coupling, J = 7.8 Hz, 1H, bt-H4), 6.89 (ddd, J = 7.8, 7.5, and 1.2 Hz, 1H, bt-H5), 6.95 (t with coupling, J = 0.6 Hz, 1H, ph-Hp), 6.98 (d with fine coupling, J = 7.5 Hz, 1H, bt-H7), 7.16 (dd with fine coupling, J = 0.6 and 0.5 Hz, 2H, ph-Ho). Synthetic Procedure of Bis[2-(3,5dimethylphenylmethyleneamino)benzenethiolato]nickel(II) (3a). To L3 (359 mg, 1.07 mmol) dissolved in 20 mL of ethanol was added nickel(II) acetate tetrahydrate (132 mg, 0.531 mmol). The mixture was refluxed under a nitrogen atmosphere for 1 h and then cooled to room temperature. The resulting brown powder was collected by filtration, washed with ethanol (3 × 20 mL), and dried in vacuo. Yield: 255 mg, 89.1%. Brown plate crystals suitable for singlecrystal X-ray analysis were obtained by the slow evaporation of a 1butanol/dichloromethane (1:4, v/v) mixed solvent. Anal. Calcd for C30H28N2NiS2: 66.80; H, 5.23; N, 5.19. Found: C, 66.59; H, 5.16; N, 5.14. 1H NMR (400 MHz, CDCl3): δ 2.08 (12H, s, Me), 6.26 (2H, d, J = 8.0 Hz, ArH), 6.70 (2H, ddd, J = 8.0, 7.2, and 1.2 Hz, ArH), 7.00 (2H, ddd, J = 7.8, 7.2, and 1.1 Hz, ArH), 7.01 (2H, s, ArH), 7.41 (2H, dd, J = 7.8 and 1.2 Hz, ArH), 7.73 (2H, s, CHN), 8.43 (4H, br s, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 467 (4694). Synthetic Procedure of {Bis-2,2′-[1,2-di(3,5-dimethylphenyl)ethylenediimine]benzenethiolato}nickel(II) (3b). A suspension of 3a (708 mg, 1.31 mmol) in 100 mL of toluene was refluxed for 1 h under a nitrogen atmosphere and then cooled to room temperature. After the solvent was removed by rotary evaporation, the resulting residue was 12130


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Inorganic Chemistry

5A stainless columns (3.0 mm i.d. × 3.0 m × 2) and a thermal conductivity detector. A total of 10 mL of sample solution containing 40 μM WRC (1a, 1b, 2a, 2b, 3a, or 3b) and 2.5 mM [Ir(ppy)2(bpy)]PF6 in a THF/ H2O/TEOA (9:3:1, v/v/v) mixed solvent was prepared. The solution was degassed by freeze−pump−thaw cycling with Schlenk techniques and then transferred into a cell for photoreaction in a glovebox. After the cell was attached to a closed circular system, the headspace of the system was purged three times with argon gas and finally filled with ca. 300 Torr of argon gas. The photoreaction progressed by irradiation of the sample solution using a 500 W xenon lamp with magnetic stirring for 24 h at 25 °C. During the reaction, the amount of evolved hydrogen in the closed circular system was quantitated with gas chromatography per 1 h. Cyclic Voltammetry in the Presence of Acetic Acid. Measurement was performed with a CH Instrument model 620A electrochemical analyzer using a glassy carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode at a scan rate of 50 mV s−1. A sample solution containing 1.0 mM complex and 0.10 M nBu4NClO4 as an electrolyte in dry THF was prepared. To the solution was added using a microsyringe an aliquot of acetic acid, and then the mixture was purged well with nitrogen gas before measurement. Quenching Experiments. Quenching experiments were carried out under conditions similar to those of the photocatalytic water reduction reactions. When TEOA was used as a quencher, a 40 μM [Ir(ppy)2(bpy)]PF6 solution was prepared in 12:1 (v/v) THF/TEOA. When 1b was used as a quencher, the sample solution containing 40 μM 1b and 40 μM [Ir(ppy)2(bpy)]PF6 was prepared in dry THF. All sample solutions were degassed by freeze−pump−thaw cycling with Schlenk techniques before these solutions were placed in a quartz cuvette. The emission intensities were monitored by exciting the samples at 337 nm using a Jasco FP-8300 spectrofluorometer. Density Functional Theory (DFT) Calculations. All DFT calculations were carried out with Gaussian 09 (revision C.02).15 The geometry optimization, vibrational frequency, and single-point energy calculations were performed using the ωB97XD functional with an SVP basis for the nickel atom, an aug-cc-pVDZ basis for the nitrogen and sulfur atoms, and a cc-pVDZ basis for the other atoms. Solvent effects were described with the conductor-like polarizable continuum model method. The methods used for computing the redox potentials and pKa values from the calculated Gibbs free energy differences were applied with an established method developed by Noodleman et al.16

dissolved in a minimum amount of dichloromethane and purified by column chromatography on silica gel using dichloromethane as the eluent. The first bluish-black fraction was collected and evaporated to dryness using a rotary evaporator. The resulting black powder was dried in vacuo. Yield: 20.7 mg, 2.93%. Anal. Calcd for C30H28N2NiS2: C, 66.80; H, 5.23; N, 5.19. Found: C, 66.66; H, 5.08; N, 5.12. 1H NMR (400 MHz, CDCl3): δ 2.20 (12H, s, Me), 6.21 (2H, s, CHN), 6.86 (2H, s, ArH), 6.89 (4H, s, ArH), 7.01 (2H, ddd, J = 8.8, 6.7, and 1.2 Hz, ArH), 7.14 (2H, ddd, J = 8.3, 6.7, and 0.8 Hz, ArH), 7.26 (2H, d, J = 8.8 Hz, ArH), 7.71 (2H, d, J = 8.3 Hz, ArH). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1142 (2938), 983 (2352), 831 (35672), 585 (1368), 538 (1339), 467 (1214), 353 (4006), 321 (5287). Chemical One-Electron Reduction of 1b with [CoCp2] ([CoCp2][1b]). A suspension of cobaltocene (CoCp2; 98.4 mg, 0.520 mmol) and 1b·0.2CH2Cl2 (261 mg, 0.521 mmol) in 30 mL of dichloromethane was stirred at room temperature under a nitrogen atmosphere for 1 h. The resulting deep-green solid was collected by filtration and dried in vacuo to give a deep-green powder of [CoCp2][1b]·CH2Cl2. Yield: 319 mg, 81.0%. Deep-green block crystals suitable for single-crystal X-ray analysis were obtained by the slow evaporation of dichloromethane. Anal. Calcd for C36H30CoN2NiS2·CH2Cl2: C, 58.68; H, 4.26; N, 3.70. Found: C, 58.84; H, 4.41; N, 3.60. 1H NMR (400 MHz, acetone-d6): δ 5.81 (br s, [CoCp2]+). UV−vis−NIR [THF; λmax/mn (ε/M−1 cm−1]: 1402 (1007), 903 (135541), 696 (3290), 395 (4841). Chemical One-Electron Reduction of 1b with [CoCp*2 ] ([CoCp*2][1b]). A suspension of decamethylcobaltocene (CoCp*2; 165 mg, 0.502 mmol) and 1b·0.2CH2Cl2 (252 mg, 0.503 mmol) in 30 mL of dichloromethane was stirred at room temperature under a nitrogen atmosphere for 1 h. The resulting deep-green suspension was filtered for the removal of an insoluble solid, and the deep-green filtrate was rotavaped to dryness. The obtained deep-green solid was dried in vacuo. Yield: 414 mg, 93.6%. Anal. Calcd for C46H50CoN2NiS2·0.8CH2Cl2: C, 63.83; H, 5.91; N, 3.18. Found: C, 63.80; H, 5.96; N, 3.23. 1H NMR (400 MHz, acetone-d6): δ 1.82 (s, [CoCp*2]+). UV−vis−NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1402 (517), 895 (9399), 699 (2378), 404 sh (3921). Chemical One-Electron Reduction of 2b with [CoCp2] ([CoCp2][2b]). A suspension of CoCp2 (90.4 mg, 0.478 mmol) and 2b (297 mg, 0.478 mmol) in 30 mL of dichloromethane was stirred at room temperature under a nitrogen atmosphere for 2 h. The resulting deepgreen solid was collected by filtration and dried in vacuo to give a deep-green powder of [CoCp2][2b]·0.2CH2Cl2. Yield: 329 mg, 83.3%. Black block crystals suitable for single-crystal X-ray diffraction analysis were obtained by layering diethyl ether over an N,N-dimethylformamide solution. Anal. Calcd for C36H26Cl4CoN2NiS2·0.2CH2Cl2: C, 52.56; H, 3.22; N, 3.39. Found: C, 52.64; H, 3.02; N, 3.40. UV−vis− NIR [THF; λmax/nm (ε/M−1 cm−1)]: 1404 (816), 916 (14276), 698 (3491), 389 (5133). Chemical One-Electron Reduction of 2b with [CoCp*2 ] ([CoCp*2][2b]). A suspension of CoCp*2 (108 mg, 0.328 mmol) and 2b (101 mg, 0.162 mmol) in 20 mL of dichloromethane was stirred at room temperature under a nitrogen atmosphere for 1 day. The resulting deep-green suspension was filtered for the removal of an insoluble solid, and the dark-green filtrate was rotavaped to dryness. The obtained dark-green solid was dried in vacuo. Yield: 185 mg, 81.5%. Black plate crystals suitable for single-crystal X-ray diffraction analysis were obtained by the slow evaporation of a toluene/ dichloromethane (1:4, v/v) mixed solvent. Anal. Calcd for C46H46Cl4CoN2NiS2·C20H30ClCo·CH2Cl2: C, 57.47; H, 5.62; N, 2.00. Found: C, 57.13; H, 5.38; N, 2.06. 1H NMR (400 MHz, CDCl3): δ 1.72 (s, [CoCp*2]+). Photocatalytic Water Reduction to Molecular Hydrogen. For photocatalytic reaction, all of the photoreactions were carried out in a closed circular system equipped with single auto gas sampler produced by Makuhari Rikagaku Garasu Inc., and the irradiation was carried out with an Ushio Optical ModuleX 500 W xenon lamp with a long-pass filter (λ > 400 nm). Evolved hydrogen was quantitated using a Shimadzu GC-8A gas chromatograph equipped with molecular sieve

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RESULTS AND DISCUSSION Schiff-base-type (1a−3a) and noninnocent-type (1b−3b) nickel complexes were prepared according to the previously reported methods by one of the present authors (Figure 1).14

Figure 1. Complexes of interest in this study.

It has been reported that the noninnocent-type cobalt complex with the same tetradentate ligand as that of 1b prepared previously by the author contains a cobalt(III) ion as a metal center (eq 1).17 Therefore, the corresponding nickel complexes were once again characterized by electronic absorption, 1H NMR, ESR, and X-ray crystallography in the present work. 12131


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Inorganic Chemistry

summarized in Table S3. These equilibrium states can be attributed to the electronic effects of the substituents; that is, the noninnocent-type complex 2b, which can be easily reduced compared with 2a (vide infra), dominates the dynamic equilibrium between 2a and 2b with chlorine atoms of electron-withdrawing substituents. In contrast, for 3a and 3b with methyl groups of electron-donating substituents, the Schiff-base-type complex 3a, which is hardly reduced compared with 3b (vide infra), dominates the equilibrium. The electronic state of the noninnocent-type nickel complex was further confirmed by isolation of the one-electron-reduced species. The reduced species [CoCp2][1b] and [CoCp2][2b] were newly obtained by reduction of the neutral species 1b and 2b with cobaltocene, and the structures were determined by Xray crystallography (Figures 2 and S10 and Table S1).

[CoIII(L•• 2 −)]+ ⇄ [CoIII(L•3 −)] ⇄ [CoIII(L4 −)]− ⇄ [CoII(L4 −)]2 −

(1)

The noninnocent-type complexes 1b−3b show characteristic absorption peaks at 833, 839, and 831 nm, respectively, which are assigned to the intraligand charge-transfer (ILCT) transition (Figure S1), and also a number of sharp signals in the aromatic region in 1H NMR spectra (Figure S2).18 Attempts to obtain suitable crystals of 2b and 3b for X-ray study resulted in crystals of the desired 2b and Schiff-base complex 3a through the undesirable isomerization of 3b, respectively. The bond lengths and angles in 3a are typical of Schiff-base nickel(II) complexes as well as 1a (Figure S3),14 and it has an almost planar structure with a dihedral angle of 14.75° (Figure S4 and Table S1). As shown in 1b,14 complex 2b has a more closely planar structure with a dihedral angle of 2.60° (Figures S5 and S6 and Table S1) and its metric parameters resemble those of 1b (Tables 1 and S2). Thus, complexes 1b−3b are best described as singlet diradicals like bis(benzene-1,2-thiolato)nickel(II) and bis(2aminobenzenethiolato)nickel(II) complexes.18 Table 1. Selected Bond Lengths (Å) for 1a, 1b, and [CoCp2][1b]a Ni1−S1 Ni1−S2 Ni1−N1 Ni1−N2 S1−C1 S2−C14 N1−C2 N2−C15 C1−C2 C1−C6 C2−C3 C3−C4 C4−C5 C5−C6 C14−C15 C14−C19 C15−C16 C16−C17 C17−C18 C18−C19 a

1a

1b

[CoCp2][1b]

2.1827(4) 2.1827(4)#1 1.9249(12) 1.9249(12)#1 1.7629(15) 1.7629(15)#1 1.4353(16) 1.4353(16)#1 1.399(2) 1.395(2) 1.383(2) 1.390(2) 1.387(3) 1.384(2) 1.399(2)#1 1.395(2)#1 1.383(2)#1 1.390(2)#1 1.387(3)#1 1.384(2)#1

2.1239(6) 2.1264(7) 1.822(2) 1.8110(18) 1.720(3) 1.715(2) 1.344(3) 1.355(3) 1.426(3) 1.404(4) 1.415(4) 1.373(4) 1.404(4) 1.368(4) 1.428(3) 1.406(4) 1.413(3) 1.360(4) 1.415(4) 1.363(4)

2.1457(8) 2.1478(9) 1.823(2) 1.818(2) 1.748(3) 1.752(3) 1.368(4) 1.370(4) 1.422(4) 1.393(4) 1.411(4) 1.390(4) 1.397(5) 1.379(5) 1.417(4) 1.387(5) 1.404(4) 1.388(5) 1.385(5) 1.387(5)

Figure 2. Structure of [CoCp2][1b] (left, top view; right, side view). All data were collected at 200 K, and atoms are drawn as 50% probability thermal ellipsoids. A solvated CH2Cl2 molecule and hydrogen atoms are omitted for clarity. Inset value: Dihedral angle (deg) composed of Ni−S−N planes.

The metric parameters of the C−N and C−S bonds can often be used to distinguish between a closed-shell tetraanion ligand and an open-shell dianion ligand,17,18,20 and those in [CoCp2][1b] show bond distances that are intermediate between them (Table 1). The monoanionic species [CoCp2][1b] displays an intense intervalence charge-transfer (IVCT) band between a closed-shell dianion and an open-shell monoanion in two N,S-chelating moieties at 903 nm with the intensity of approximately half of that observed for the ILCT band of the neutral species in the absorption spectrum of a THF solution (Figure S11).18 Furthermore, an attempt to prepare a two-electron-reduced species by the reduction of 2b with 2 equiv of CoCp*2 afforded instead a one-electronreduced species with only one [CoCp*2]+ counterion ([CoCp*2][2b]) that possesses a one-dimensional chain structure based on π-associated donor−acceptor interactions (Figures S12 and S13). The ESR spectra of [CoCp2][1b], [CoCp2][2b], and [CoCp*2][2b] exhibit anisotropic signals that are characteristic for monoanionic nickel(III) complexes with d7 electron configuration (Figures S14−S16).21 Especially, the rhombic signal of [CoCp*2][2b] means that one unpaired electron is localized on the nickel(III) center. In contrast, [CoCp*2][1b] generated by the reduction of 1b with 1 equiv of CoCp*2 shows an isotropic signal with a g value similar to that of a typical radical, which characterizes [CoCp*2][1b] as a monoanionic nickel(II) complex with closed-shell dianion and open-shell monoanion moieties (Figure S17). In addition, the cycliv voltammograms of 1b−3b (vide infra) show only two redox couples in the range of +1 to −2 V, different from that of

Symmetry operators: #1, −x + 1, y, −z + 1/2.

The Schiff-base-type complexes (1a−3a) are expected to be reversibly converted to the corresponding noninnocent-type complexes (1b−3b) based on a C−C bond formation/ breaking.19 Actually, the relative formation ratio of Schiffbase-type complex 1a without any special substituents on the pendant phenyl rings and the corresponding noninnocent-type complex 1b in THF is ca. 16:84 in an equilibrium state (Figure S7). In addition, those of Schiff-base-type complex 2a and noninnocent-type complex 2b with 3,5-dichlorophenyl groups and Schiff-base-type complex 3a and noninnocent-type complex 3b with 3,5-dimethylphenyl groups are ca. 13:87 and 88:12, respectively (Figures S8 and S9). These rate constants between Schiff-base- and noninnocent-type complexes were estimated by time-dependent UV−vis−NIR spectroscopy and 12132


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hydrogen are the highest at a concentration of 40 μM 1b, achieving a TON in excess of 200 mol of hydrogen (mol of catalyst)−1 after 15 h. In addition, when a concentration of [Ir(ppy)2(bpy)]+ is varied in the presence of 40 μM 1b, the TON reaches a maximum of 265 at 2.5 mM [Ir(ppy)2(bpy)]+ (Figure S19, inset table). Although we conducted the photocatalytic water reduction using a 0.1 M phosphate buffer aqueous solution instead of H2O, the TON value per mole of 1b used as a catalyst was only 71. A decrease in the pH led to the protonated form of TEOA being an ineffective SED.22 These experiments demonstrate that the most effective system consists of 40 μM 1b and 2.5 mM [Ir(ppy)2(bpy)]+ in a THF/ H2O/TEOA (9:3:1, v/v/v; total 10 mL). Under these optimum conditions, the catalytic ability of three Schiff-base-type nickel(II) complexes (1a−3a) and the corresponding noninnocent-type complexes (1b−3b) for photocatalytic hydrogen production was investigated. The Schiff-base-type complex 2a with chlorine atoms and the noninnocent-type complex 3b with methyl groups on the pendant phenyl rings, which are minor species in equilibria, show a higher TON of over 400, and the noninnocent-type complex 2b with chlorine atoms, which is the major species, the lowest TON of 149 (Figure 3, inset table).

the noninnocent-type cobalt complex (Figure S18 and Table S4). These results lead us to the conclusion that the oneelectron-reduced species of [NiII(L•• 2−)] exists as a resonance between two states of [NiIII(L4−)]− and [NiII(L• 3−)]−, and the electronic states of the reduced species of 1b−3b are given in eq 2.

Photocatalytic Proton Reduction. In evaluating the catalytic properties of nickel complexes, we first examined the photoreduction of water to generate hydrogen using the noninnocent-type complex 1b as a WRC, [Ir(ppy)2(bpy)]+ as a PS, and TEOA as a SED in THF/H2O (3:1, v/v) with λ > 400 nm, and the evolved hydrogen gas was periodically monitored by an online gas chromatograph (Shimadzu GC-8A) with a thermal conductivity detector (MS-5A; argon carrier). Control experiments show that 1b serves as effective reduction catalyst for the photogeneration of hydrogen in the THF/H2O mixed solvent, although a [Ir(ppy)2(bpy)]+ solution without the addition of 1b exhibits modest photocatalytic activity for hydrogen production (Table 2). The amounts of generated hydrogen and turnover numbers (TONs) relative to the catalyst of systems containing [Ir(ppy)2(bpy)]+ and TEOA together with various concentrations of 1b in THF/H2O are shown in Figure S19. When a concentration of [Ir(ppy)2(bpy)]+ is fixed, both the initial rate of hydrogen production and the total amount of produced Table 2. Performance of WRC under Various Conditionsa WRC 30 μM 1b 40 μM 1b 50 μM 1b 40 μM 1b 40 μM 1b 40 μM 1a 40 μM 1bb 40 μM 2a 40 μM 2b 40 μM 3a 40 μM 3b 40 μM Ni(ClO4)2 40 μM NiCl2 40 μM Ni(OAc)2 40 μM Ni(OAc)2c 40 μM Ni(acac)2

[PS] (mM)

evolved hydrogen (μmol)

conversion (%)

WRC TON

WRC TOFmax (h−1)

2.5 2.5 2.5 2.5 0.50 1.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

24 64 106 116 29 61 95 7.1 164 59 145 166 33

0.019 0.050 0.083 0.091 0.023 0.048 0.074 0.0056 0.13 0.046 0.11 0.13 0.026

212 265 230 74 154 239 18 414 149 364 416 82

52 68 60 19 39 30 10 118 19 95 102 22

2.5

41

0.032

102

61

2.5

49

0.039

122

51

2.5

93

0.073

234

89

2.5

52

0.041

132

53

Figure 3. Performance of various WRCs for photocatalytic hydrogen evolution in a system (total volume of solution: 10 mL) containing 40 μM WRC and 2.5 mM [Ir(ppy)2(bpy)]PF6 (PS) in a 9:3:1 (v/v/v) THF/H2O/TEOA (SED) mixed solvent upon irradiation with a 500 W xenon lamp (λ > 400 nm) at room temperature (ca. 298 K). Inset table: μmol values for the amount of evolved hydrogen, conversion, WRC TON, and WRC TOFmax.

The emission spectra of [Ir(ppy)2(bpy)]+ in the presence of TEOA and in the presence of 1b as a quencher are shown in Figures S20 and S21, respectively. Under conditions similar to those of the photocatalytic water reduction reactions, TEOA quenched the emission at around 590 nm of [Ir(ppy)2(bpy)]+ more effectively than 1b, indicating that the photocatalytic reactions for hydrogen evolution are provided through the reductive quenching of [Ir(ppy)2(bpy)]+* by TEOA. Mechanisms of Electro- and Photocatalytic Hydrogen Production. Recently, Eisenberg and co-workers reported that the noninnocent-type nickel complex Ni(abt)2 (abt = 2aminobenzenethiolate) having S and NH donor sets shows excellent activity for light-driven hydrogen production.23 In the Article, they pointed out that the excellent activity is correlated with the facial reduction of the nickel complex by PS as shown in the electrocatalytic wave at less negative potential. In order to compare the electrocatalytic activity of 1a−3a and 1b−3b, the cyclic voltammograms were recorded in THF solutions with different concentrations of acetic acid. In the absence of acids, Schiff-base complexes 1a−3a each display one quasi-

a

In a system (total volume of solution: 10 mL) containing WRC and [Ir(ppy)2(bpy)]PF6 (PS) in a 9:3:1 (v/v/v) THF/H2O/TEOA (SED) mixed solvent upon irradiation with a 500 W xenon lamp (λ > 400 nm) at room temperature (ca. 298 K). bIn the presence of ca. 1 mL of mercury. cIn the presence of ca. 80 μM of L1. 12133


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Inorganic Chemistry reversible redox couple due to NiII/I at −1.09, −0.96, and −1.13 V, respectively (Figure S22 and Table S4).24 All noninnocenttype complexes 1b−3b exhibit two quasi-reversible couples that are attributed to 1b0/1b− and 1b−/1b2−, 2b0/2b− and 2b−/ 2b2−, and 3b0/3b− and 3b−/3b2− (Figure S23 and Table S4) at −0.19 and −1.09 V, −0.05 and −0.96 V, and −0.21 and −1.12 V, respectively. The second redox potentials of 1b−3b are in good agreement with the reduction potentials in the corresponding Schiff-base complexes. This result suggests that the two-electron-reduced species of noninnocent-type complexes contain nickel atoms in the NiI oxidation state (eq 2). These cyclic voltammetry data indicate that a reduction of the PS [Ir(ppy)2(bpy)]+ (Figure S24 and Table S4) is sufficient to transfer an electron into the WRCs (1a−3a and 1b−3b). The reduction potentials of 2a and 2b are more positive than those of 1a and 1b, respectively, which is consistent with the electronwithdrawing capability of chlorine atoms on pendant phenyl rings in 2a and 2b, and those of 3a and 3b show quite the opposite tendency for 1a and 1b because of the electrondonating property of the methyl substituents. The sequential additions of acetic acid to the THF solution of 2a, which shows higher activity as a catalyst for the photoreduction reaction of water, caused obvious increases in the current heights at more negative potentials than the NiII/I reduction potential (Figure 4) without proton reduction at the glassy carbon observed between −1.4 and −1.6 V.25

Figure 5. Cyclic voltammograms of 1.0 mM 3b in THF containing 0.10 M nBu4NClO4 as the electrolyte with increasing equivalents of acetic acid using a glassy carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode at a scan rate of 50 mV s−1. Inset: Plot of ic/ip versus acetic acid equivalents.

innocent-type nickel complex 3b and Ni(abt)2 can be understood based on a comparison of the reaction mechanisms (vide infra); that is, protonation of the noninnocent ligand in 3b does not occur effectively in contract to Ni(abt)2, in which nitrogen atoms can easily be protonated. Ni(abt)2 can evolve hydrogen at less negative potential with a proton-coupled electron-transfer event, and it is just acting as a cooperative catalyst. To confirm which is a key intermediate, the hydride or sulfur-protonated complex, we carried out DFT calculations. Our calculations for 1b provide insight into the reaction mechanism as well as the physicochemical properties of the intermediates (Figure S25). On the basis of the agreement between our calculated values (+0.07 and −1.09 V) and the experimental values (−0.11 and −1.05 V) for the reduction potentials, we deduce that the subsequent reduction occurs at a potential of −2.39 V vs SCE. As seen about −1.0 V in Figure S26, ligand protonation of the doubly reduced species [Ni(ddbt)]2− ([1b]2−) can shift the reduction potential because the complex is more easily reduced as a result of the additional positive charge. Subsequently, the ligand-protonated species [Ni(H-ddbt)]2−, which is formed at a potential as low as −2.39 V, undergoes intramolecular proton transfer to form nickel(II) hydride species [H-Ni(ddbt)]2−, which is more stable by 4.97 kcal mol−1 than [Ni(H-ddbt)]2−. Following proton transfer in [Ni(H-ddbt)]2−, the formed [H-Ni(ddbt)]2− is reduced to [HNi(ddbt)]3−. The calculated reduction potential is −2.06 V versus SCE. Furthermore, [H-Ni(ddbt)]3− is protonated to [HNi(H free-ddbt)] 2− (pK a = 29.06) because a higher pK a corresponds to thermodynamically more favorable protonation. Figure S27 depicts the optimized structure of [H-Ni(Hfreeddbt)]2−. Hydrogen evolution from this state affords the doubly reduced species [Ni(ddbt)]2− and completes the catalytic cycle (Scheme 1). Thus, the noninnocent ligand in 1b plays a nonessential role in the catalytic cycle in contrast to that in Ni(abt)2.23 The catalytic waves for the present complexes appear at more negative potentials (Figures 4, 5, and S26) than −1.25 V of the redox couple for [Ir(ppy)2(bpy)]+ (Figure S24 and Table S4), which means that the photocatalytic reaction progresses with a mechanism different from the one proposed by electrochemical analysis and DFT calculations. The complex [CoCp2][1b] corresponding to a precursor in Scheme 1 displays an IVCT band (Figure S11), but the UV−vis−NIR spectrum of

Figure 4. Cyclic voltammograms of 1.0 mM 2a in THF containing 0.10 M nBu4NClO4 as the electrolyte with increasing equivalents of acetic acid using a glassy carbon working electrode, a platinum wire auxiliary electrode, and a saturated calomel reference electrode at a scan rate of 50 mV s−1. Inset: Plot of ic/ip versus acetic acid equivalents.

This result indicates that catalysis proceeds with two-electron reduction of the nickel complex followed by the formation of nickel hydride or a sulfur-protonated complex. The additions of acetic acid to 3b with high activity as well as 2a also gave clear electrocatalytic waves at a similar electrical potential (Figure 5). This occurrence must involve protonation at the metal center or sulfur donor atom, as previously postulated for dithiolene complexes.2n,25b,26 Thus, the reaction mechanism for 3b is presumed to be essentially the same as that of 2a, even though one-electron-reduced species of 3b correspond to the neutral complex 2a on the reaction mechanism. Furthermore, the linear plots of ic/ip versus acid concentration (Figures 4 and 5, inset) indicate a second-order dependence of the catalytic reaction on acid (ic is the catalytic current and ip is the peak current observed for the catalyst in the absence of acid).2a The difference in the catalytic activity between both the non12134


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Inorganic Chemistry Scheme 1. Proposed Mechanism of Electrocatalytic Molecular Hydrogen Formation

In conclusion, the catalytic activities of three Schiff-base-type complexes and three noninnocent-type complexes, which can be interconverted into each other, were examined in a photocatalytic water reduction system containing a PS, [Ir(ppy)2(bpy)]+, and a SED, TEOA, together with the electrocatalytic hydrogen formation reactions. All of the complexes function as useful catalysts, and especially the Schiff-base-type complex with chlorine atoms and the noninnocent-type complex with methyl groups on the pendant phenyl rings, which are minor species in equilibria between the valence isomers, exhibited higher activity. This can be correlated with the electronic effects of the substituents for generation of the in situ forming NiSx-like aggregate species as active species in the photocatalytic reaction.

[CoCp2][1b], which was allowed to stand overnight after irradiation for only 1 h under photocatalytic conditions, did not show such a strong absorption band (Figure S28). This result implies that [CoCp2][1b] was degraded by photoirradiation. Indeed, the DLS data indicate that particles with an average size of approximately 1 μm were formed after irradiation for 5 h under the same photocatalytic conditions (Figure S29). In addition, the addition of mercury to this catalytic system significantly reduced the activity, suggesting that hydrogen evolution from water can be achieved by colloidal catalysts (Table 2). However, Ni(ClO4)2, NiCl2, Ni(OAc)2, and Ni(acac)2, expected to form colloidal nickel by photoirradiation, exhibited much lower catalytic activity in the same catalytic system (Table 2). Furthermore, Ni(OAc)2 in the presence of 2 equiv of L1 showed higher catalytic activity similar to that with 1a prepared separately from Ni(OAc)2 and L1. As shown in Figure S30, the observation by TEM of the reaction solution after visible-light irradiation for 5 h to the system containing 1b reveals the existence of particles similar to NiSx that exhibits activity for photocatalytic hydrogen production.27 The particle sizes are compared with those obtained using the DLS. Thus, complex catalysts in this catalytic system appear to be degraded into any aggregate species that can be derived directly from nickel ion and ligand. Although the rates of isomerization between Schiff-base- and noninnocent-type complexes (Figures S7−S9) are much slower compared to the formation of aggregate species, as seen in the TOF values (Figure 3, inset table), 2a and 2b displaying a significant difference in the catalytic activity may be related to the fact that 2a is easily transformed to 2b; that is, 2a can be more easily rearranged to such aggregate species under photocatalytic conditions and show higher catalytic activity. In contrast, 2b, which has the least negative reduction potential, is considered to be unfavorable for the formation of such active species and shows the lowest catalytic activity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01244. UV−vis−NIR, 1H NMR, X-band solid-state ESR, and emission spectra, structures of 1a, 1b, 2b, 3a, [CoCp2][2b], and [CoCp*2][2b], cyclic voltammograms, performance of 1b as the WRC for photocatalytic hydrogen evolution, physicochemical properties of the intermediates, optimized structure of [H-Ni(Hfree-ddbt)]2−, particle-size distribution, TEM images, isomerization and isomerization rate constants between noninnocentand Schiff-base-type complexes, and crystal data and structure refinement, selected bond lengths, and redox potentials (PDF) Accession Codes

CCDC 1547333−1547337 and 1562134−1562135 contain the supplementary crystallographic data for this paper. These data 12135


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(k) Martin, M. G. M. B.; Vidotti, M.; Nunes, F. S. Synthesis, characterization and electrocatalysis of mono- and di-nickel tetraiminodiphenolate macrocyclic complexes as active site models of [NiFe]-hydrogenases. Int. J. Hydrogen Energy 2012, 37, 14094− 14102. (l) Wiedner, E. S.; Appel, A. M.; DuBois, D. L.; Bullock, R. M. Thermochemical and Mechanistic Studies of Electrocatalytic Hydrogen Production by Cobalt Complexes Containing Pendant Amines. Inorg. Chem. 2013, 52, 14391−14403. (m) Fang, M.; Engelhard, M. H.; Zhu, Z.; Helm, M. L.; Roberts, J. A. S. Electrodeposition from Acidic Solutions of Nickel Bis(benzenedithiolate) Produces a Hydrogen-Evolving Ni−S Film on Glassy Carbon. ACS Catal. 2014, 4, 90−98. (n) Solis, B. H.; Hammes-Schiffer, S. Proton-Coupled Electron Transfer in Molecular Electrocatalysis: Theoretical Methods and Design Principles. Inorg. Chem. 2014, 53, 6427−6443. (o) Zhang, P.; Wang, M.; Yang, Y.; Yao, T.; Sun, L. A, Molecular Copper Catalyst for Electrochemical Water Reduction with a Large HydrogenGeneration Rate Constant in Aqueous Solution. Angew. Chem., Int. Ed. 2014, 53, 13803−13807. (p) Chen, L.; Chen, G.; Leung, C.-F.; Yiu, S.-M.; Ko, C.-C.; Anxolabéhère-Mallart, E.; Robert, M.; Lau, T.-C. Dual Homogeneous and Heterogeneous Pathways in Photo- and Electrocatalytic Hydrogen Evolution with Nickel(II) Catalysts Bearing Tetradentate Macrocyclic Ligands. ACS Catal. 2015, 5, 356−364. (3) (a) Kalyanasundaram, K.; Kiwi, J.; Grätzel, M. Hydrogen Evolution from Water by Visible Light, a Homogeneous Three Component Test System for Redox Catalysis. Helv. Chim. Acta 1978, 61, 2720−2730. (b) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Hydrogen Generation by Visible Light Irradiation of Aqueous Solutions of Metal Complexes. An approach to the photochemical conversion and storage of solar energy. Helv. Chim. Acta 1979, 62, 1345−1384. (c) Teets, T. S.; Nocera, D. G. Photocatalytic hydrogen production. Chem. Commun. 2011, 47, 9268−9274. (4) (a) Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141−145. (b) Esswein, A. J.; Nocera, D. G. Hydrogen Production by Molecular Photocatalysis. Chem. Rev. 2007, 107, 4022−4047. (c) Lei, P.; Hedlund, M.; Lomoth, R.; Rensmo, H.; Johansson, O.; Hammarström, L. The Role of Colloid Formation in the Photoinduced H2 Production with a RuII-PdII Supramolecular Complex: A Study by GC, XPS, and TEM. J. Am. Chem. Soc. 2008, 130, 26−27. (d) Du, P.; Schneider, J.; Li, F.; Zhao, W.; Patel, U.; Castellano, F. N.; Eisenberg, R. Bi- and Terpyridyl Platinum(II) Chloro Complexes: Molecular Catalysts for the Photogeneration of Hydrogen from Water or Simply Precursors for Colloidal Platinum? J. Am. Chem. Soc. 2008, 130, 5056−5058. (e) Yamauchi, K.; Masaoka, S.; Sakai, K. Evidence for Pt(II)-Based Molecular Catalysis in the Thermal Reduction of Water into Molecular Hydrogen. J. Am. Chem. Soc. 2009, 131, 8404− 8406. (f) White, T. A.; Whitaker, B. N.; Brewer, K. J. Discovering the Balance of Steric and Electronic Factors Needed To Provide a New Structural Motif for Photocatalytic Hydrogen Production from Water. J. Am. Chem. Soc. 2011, 133, 15332−15334. (5) (a) DuBois, M. R.; DuBois, D. L. The roles of the first and second coordination spheres in the design of molecular catalysts for H2 production and oxidation. Chem. Soc. Rev. 2009, 38, 62−72. (b) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. Making Hydrogen from Water Using a Homogeneous System Without Noble Metals. J. Am. Chem. Soc. 2009, 131, 9192− 9194. (c) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42, 1995−2004. (d) Kluwer, A. M.; Kapre, R.; Hartl, F.; Lutz, M.; Spek, A. L.; Brouwer, A. M.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for molecular hydrogen evolution. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 10460−10465. (e) Streich, D.; Astuti, Y.; Orlandi, M.; Schwartz, L.; Lomoth, R.; Hammarström, L.; Ott, S. High-Turnover Photochemical Hydrogen Production Catalyzed by a Model Complex of the [FeFe]-Hydrogenase Active Site. Chem. - Eur. J. 2010, 16, 60−63. (f) Probst, B.; Rodenberg, A.; Guttentag, M.; Hamm, P.; Alberto, R. A Highly Stable Rhenium-Cobalt System for Photocatalytic H 2 Production: Unraveling the Performance-Limiting Steps. Inorg.

can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tatsuya Kawamoto: 0000-0002-3572-1225 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor A. Kameyama and Associate Professor Y. Azumi for their kind help with DLS and TEM measurements, respectively. This work was supported by JSPS Kakenhi Grant 25620145 and also by the Strategic Research Base Development Program for Private Universities of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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