High-Rate Hydrogen Generation by Direct Sunlight Irradiation with a

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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High-Rate Hydrogen Generation by Direct Sunlight Irradiation with a Triruthenium Complex Tung-Kung Wu,*,†,‡ Yi-Ting Chen,† Chun-Sheng Peng,† Jia-Hoa Lin,† Jacek Gliniak,† Hsin-Fang Chan,† Chin-Hao Chang,† Chuen-Ru Li,† Jen-Shiang K. Yu,† and Jui-Nien Lin† †

Department of Biological Science and Technology and ‡Center for Emergent Functional Matter Science, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan, Republic of China

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S Supporting Information *

ABSTRACT: Herein we report a biomimetic triruthenium catalyst that, when under direct sunlight irradiation, facilitates high-rate H2 production from formic acid (FA) dehydrogenation. The system consists of 2 μmol of catalyst and 6 μmol of tri-otolylphosphine in 1 mL of dimethylformamide (DMF) and 4 mL of FA/triethylamine (TEA; 5:2). With 0.4 mM catalyst loaded, a high turnover frequency of 1.15 × 106 h−1 was detected when under direct sunlight irradiation. In an experiment with 0.2 mM catalyst loaded, more than 140 L of H2 (280 L of H2 + CO2) was produced, and a turnover number of approximately 2.78 × 106 was obtained within 5 h without decline in H2 generation activity, making it suitable for high-rate H2 production.

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3.7 s−1 and a turnover number (TON) of 2.16 million, was reported.5b So far, the best performance in FA dehydrogenation was achieved with a ruthenium PNP-pincer catalyst, obtaining a TOF of 257000 h−1.5e On the other hand, heterogeneous catalysts with advantages of separability and reusability have also been developed for FA dehydrogenation.7c−e,8,9 A heterogeneous catalyst, which consists of immobilized palladium nanoparticles on carbon nanospheres, exhibiting a TOF of 7256 h−1 was reported by Zhu et al.9e The generated carbon dioxide (CO2) can be separated from H2 with sorbents/solvents, membranes, or cryogenic methods. In addition, emitted CO2 from FA dehydrogenation can be recycled through carbon capture and sequestration processes in industry or reduced to FA, formaldehyde, methanol, or methane using transition-metal complexes.12 Recently, a diruthenium-based biomimetic H-cluster system, based on the H-cluster of the [FeFe]-H2ases, was reported and applied to a photocatalytic H2 production study in the presence of covalently and noncovalently linked phosphine ligands and FA.11 A high catalytic activity, represented by a TOF of 15840 h−1, was obtained with less than 50 ppm of the catalyst present.11b With a goal of developing a system capable of high-rate H2 production, we explored a new design involving the synthesis of an amido−bis(S) ligand-substituted triruthenium biomimetic H-cluster to act as a photocatalyst. This catalyst, when combined with a Fresnel lens to concentrate sunlight, is capable of catalyzing H2 generation from FA dehydrogenation. The photocatalytic H2 generation rate was

INTRODUCTION Hydrogen (H2), an energy carrier that produces only the benign combustion product of water, is a highly compelling source of clean and sustainable energy. In the quest for highrate H2 generation, researchers have investigated a multitude of organic, inorganic, biohybrid, or bioinspired catalysts to catalyze water splitting or biomass conversion to H2.1−4 Maintaining a sustainable supply of H2 from aqueous media, with the exception of electrolysis of water, remains a challenge because of low turnover numbers and frequencies. To date, significant progress has been achieved in developing better catalysts for the release of H2 from biomass originating from methanol, ethanol, and formic acid (FA), which can be easily handled, stored, and transported in their liquid forms under ambient conditions.5−11 FA has gradually become an attractive H2 storage compound owing to its various advantages including (1) its significant H2 content at 4.4 wt %, (2) kinetic stability of FA at room temperature, (3) ease of procurement from the chemicals industry or biomass fermentation processes, and (4) superb H2 atom (100%) over that of methanol (66%) and methane (50%), the latter two of which can lose H2 as water.12 Homogeneous systems containing iridium, rhodium, and ruthenium complexes have been investigated for H 2 production from FA dehydrogenation.5−7,10 For example, a binuclear ruthenium phosphine complex showed selective H2 production with a turnover frequency (TOF) of ∼500 h−1 at ambient temperature.6b,10a In 2014, Himeda’s group reported an iridium azole-containing complex with a TOF of 34000 h−1 at 80 °C.6c Recently, a robust, reusable iridium catalyst that enables H2 production from neat FA, with a maximum TOF of © XXXX American Chemical Society

Received: October 14, 2018

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DOI: 10.1021/acs.inorgchem.8b02888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. H2 Generation Reaction Catalyzed by an Amido−Bis(S) Ligand-Substituted Triruthenium Complex under Direct Sunlight Irradiation

Table 1. Influence of Light, Catalyst-to-P-Ligand Ratio, Temperature Effect, and Wavelength on the H2 Generation Efficiency of 1a entry

catalyst (μmol)

P(o-tol)3 (μmol)

temp (°C)

500 W lamp/filter

TON (min)b

TOP (h−)c

H2 conversion efficiency (%)d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

2 2 2 2 2 2 2 2 2 2 2 1 2 2 2 2 2 2 2

6 6 6 6 6 6

90

−/− +/− +/− +/− +/− +/− +/− +/− +/− +/− +/− +/− +/1280 nm long pass +/280−397 nm bandpass +/301−384 nm bandpass +/355−469 nm bandpass +/1400 nm long pass −/− sunlight/−

2912(70) 191 267(60) 868(60) 3105(60) 11189(60) 1264(70) 24789(40) 14001(60) 22122(60) 16789(60) 48147 24655(60) 2725(60) 9288(60) 6070(60) 3008(60) 29684(60) 38333(2)

2496 164 267 868 3105 11189 1083 32736 14001 22122 16789 41269 24655 2725 9288 6070 3008 29684 1149996

6 0.4 1.1 3.5 12 45 2.6 99 56 89 67 96 99 37 24 9.0 11 69 90

6 2 4 8 3 6 6 6 6 6 6 6

60 70 80 85 90 90 90 90 90 90 90 90 90 90 90 153 153

a The reaction mixtures were photoirradiated in 4 mL of 5HCO2H·2NEt3 and 1 mL of DMF via a 500 W Xe lamp. bTON = number of moles of FA that 1 mol of complex 1 can convert before becoming inactivated. cTOF = TON per unit time. dH2 conversion efficiency (%) = number of moles of H2 produced at certain time point/number of moles of FA input at the beginning.

elution with hexane/ethyl acetate (4:1, v/v) to obtain a deep-orange solid, 1, with 28.7% yield (64.1 mg). Complex 1 was crystallized from hexane/ethyl acetate (9:1) in a 0.5-cm-diameter glass tube for several days at 4 °C. The crystal structure was determined by X-ray crystallography at the instrumentation center of National Taiwan Normal University. Crystallographic data were recorded at 200 K on a Bruker Nonius-Kappa CCD X-ray diffractometer. 1H NMR (600 MHz, DMF-d7, 297 K): 1.53 (9H), 2.46−2.52 (2H), 3.02−3.05 (1H), 3.59−3.62 (1H), 4.15 (1H). 13C NMR (600 MHz, DMF-d7, 297 K): 27.23−29.94 (CH3), 34.06−34.89 (CH2), 61.27 (CH), 162.05 (CO), 184.47−202.20 (CO). FT-IR (powder, νCO): 1706, 1927, 1973, 1986, 1996, 2002, 2024, 2045, 2084 cm−1. ESI-MS: 776 ([M]−), 718 ([M −

preliminary evaluated using a 500 W xenon (Xe) lamp and with direct sunlight irradiation (Scheme 1).

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EXPERIMENTAL SECTION

Synthesis and Structure Determination of [Ru3(CO)9(μSCH2CH(NCO2(C(CH3)3))CH2S)] (1). Commercial Ru3(CO)12 (0.32 mmol, 204 mg) was dissolved in tetrahydrofuran (70 mL), followed by the addition of 1.3 equiv of tert-butyl (1,2-dithiolan-4-yl)carbamate under argon (Ar). The mixture was refluxed at 70 °C until its color changed from orange to deep red. Subsequently, the crude compound was added to 0.5 g of Celite and dried with a rotary evaporator. The sample was loaded onto a silica gel column and purified through B


Article

Inorganic Chemistry

Figure 1. Synthesis and structure of 1. A view of the structure of 1 shown by a thermal ellipsoid plot. H atoms were omitted for clarity. 2CO]−), 690 ([M − 3CO]−). High-resolution MS. Calcd for C17H14NO11Ru3S2: m/z 777.7167 ([M]−). Found: m/z 777.7140. UV−Vis, IR, and NMR Characterization. UV−vis spectra were measured with an Agilent 8453 UV−vis spectrometer. Attenuatedtotal-reflectance infrared (ATR-IR) spectra were measured with a Shimadzu IRTracer-100 Fourier transform infrared (FT-IR) spectrophotometer. NMR spectra were measured with a Bruker Avance600 NMR spectrometer. Spectroscopic Monitoring of Photocatalytic H2 Generation Experiments. To monitor the photocatalytic H2 reaction by UV−vis, the solution system was photoirradiated under a 500 W Xe lamp. Fractions of the sample were taken at 0, 5, 10, 15, 20, and 25 min and measured by UV−vis to determine the reaction profiles. To monitor the reaction by FT-IR, the solution was photoirradiated under a 500 W Xe lamp and fractions (300 μL for each reaction) were collected after 0, 10, 20, and 30 min of photoirradiation and then measured by an IR spectrometer in the range from 700 to 4000 cm−1. To monitor the reaction by 1H NMR, 24 mM catalyst and 72 mM P(o-tol)3 were used in solution, and the reaction was monitored at 0, 2, 4, 6, and 8 min after photoirradiation. Photocatalytic H2 Generation with 500 W Xe Lamp Photoirradiation. For photocatalytic H2 generation using a 500 W Xe lamp, a Schlenk tube containing 4 mL of FA/triethylamine (TEA; 5:2) with 1 (1 or 2 μmol) and P(o-tol)3 (3 or 6 μmol) dissolved in 1 mL of dimethylformamide (DMF) was stirred at 90 °C under a 500 W Xe lamp for 1.5 h. At time intervals of 10 min, 500 μL of the gas in the headspace of the sample tube was harvested using a gastight syringe and analyzed on a Bruker 450 gas chromatograph instrument equipped with a Carboxen 1010 PLOT fused-silica capillary column (Supelco, 30 m × 0.53 mm) to measure the volume of H2 at 40 °C oven temperature and with Ar as the carrier gas. The amount of H2 was determined and calculated by the external standard method. In a separate experiment, the amount of TEA after the reaction was measured. It was found that ∼97% TEA (as a measure of the total volume minue 1 mL of DMF solvent) was left in the solution in addition to a few droplets on the side wall of the vessel after the reaction. To determine the effects from the influence of light, catalystto-P-ligand ratio, temperature effect, and incident light wavelength on the H2 generation rate of 1, the reaction conditions, as described in Table 1, were monitored by gas chromatography with a thermal conductivity detector (GC-TCD) at different time intervals. Photocatalytic H2 Generation under Direct Sunlight Irradiation in Conjunction with a Fresnel Lens. For photocatalytic H2 generation under direct sunlight irradiation, the solution was placed in a 2 L two-necked glass tube. One opening of the flask was sealed with a silicone rubber septum, and the other opening was equipped with a rubber tube leading to an inverted bottle filled with water, the opening of which was immersed in a larger container of water. Photocatalytic experiments were performed using solar light focused by a Fresnel lens on the reaction tube. The tilt of the lens surface was adjusted to be perpendicular to the incident angle of the solar irradiation. Under direct sunlight irradiation, the reaction completed within 2−5 min and obtained more than 95% H2 conversion. The temperature of the reaction was recorded using two methods: (1) by recording the temperature at the focus point of

the Fresnel lens, which is at the outer surface of the glass tube; (2) by measuring the solution temperature inside the glass tube. The temperature at the focus point of the Fresnel lens was recorded at approximately 450 °C. On the other hand, the solution temperature was recorded to reach approximately 153 °C, the boiling point of DMF, during the photocatalytic reaction. Upon photoirradiation, the volume of generated gas was determined by collecting it over water and measuring the volume of displaced water. Water was dyed red to simplify the observation.

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RESULTS AND DISCUSSION Synthesis and Characterization of Complex 1. The amido-bis(S) ligand-substituted triruthenium complex 1 was synthesized through the reaction of tert-butyl (1,2-dithiolan-4yl)carbamate with Ru3(CO)12 and characterized by UV−vis, 1 H and 13C NMR, and IR spectroscopies and single-crystal Xray crystallography (Figures 1 and S1 and Tables S1 and S2). The UV−vis absorption spectrum of 1 in ethyl acetate exhibited characteristic absorption bands at 254/301/356/ 439 nm. The electronic absorption bands of 1 observed in the UV region could be designated as a π → π* transition and a metal-to-ligand charge-transfer (MLCT) transition, respectively, whereas the broad and low-energy bands at 356/439 nm observed for 1 could be assigned to σ → σ* transitions in metal−metal d orbitals. The ATR-IR spectrum of 1 exhibited six ν(CO) bands from approximately 1988 to 2048 cm−1, which are analogous to the characteristic terminal metal carbonyl group of Ru3(CO)12 and two distinctive ν(CO) bands at 2084 and 1927 cm−1, respectively. The ν(CO) bands at 2084 and 1927 cm−1 are analogous to those of complexes [Ru 2 (CO) 6 (μ-SCH 2 CH 2 CH 2 S)] and [Ru 2 (CO) 5 (μSCH2CH2CH2S)P(o-tol)3], respectively.11 In addition, a band at 1706 cm−1, corresponding to the carbamate ν(NCOO−) of 1, was also identified. The Ru(1)−Ru(3) distance (2.8021 Å) is slightly shorter than that of Ru(2)− Ru(3) (2.8744 Å), indicating that the Ru−Ru distance is affected by the attachment of the carbamate group NCOO to the Ru. Similarly, the bridging Ru(1)−S(2) (2.4200 Å) distance is shorter than that of the bridging Ru(1)−S(1) (2.4457 Å), whereas no apparent difference was observed between Ru(2)−S(1) (2.4279 Å) and Ru(2)−S(2) (2.4212 Å) (Table S2). The Ru−N bond length in 1 is 2.2627(19) Å, which is somewhat longer than the average Ru−N bond length of 2.05 Å. The latter is consistent with a Ru−N(sp3) dative bond.13 Photocatalytic H2 Generation of 1. The photocatalytic FA dehydrogenation activity of 1 was evaluated by either (1) heating at 90 °C, (2) photoirradiating under a 500 W Xe lamp, (3) photoirradiating under a 500 W Xe lamp and heating between 60 and 90 °C, or (4) photoirradiating under a 500 W C


Article

Inorganic Chemistry Xe lamp and heating at 90 °C with a wavelength filter (Table 1). The reaction system consisted of 4 mL of FA/TEA (5:2, mol/mol) with 1 and tri-o-tolylphosphine [P(o-tol)3] dissolved in 1 mL of DMF. A 5:2 ratio of FA/TEA was chosen because it is known that amines are employed as valuable cocatalysts and solvents for the catalytic decomposition of FA and a higher concentration of amine is beneficial for H2 production.10b,14 The evolved gas was harvested using a gastight syringe and applied to a gas chromatography system for qualitative and quantitative characterization of H2 generation, as previously described.11 An equimolar mixture of H2 and CO2 with no detectable traces of CO was detected. In addition, stoichiometric amounts of TEA were recovered after the reaction, supporting the role of TEA as cocatalysts or solvents. The catalytic activity of 1 was low when the reaction was heated at 90 °C (TOF of 2496 h−1; Figure 2 and Table 1, entry 1) or

photoirradiation, the measured quantum yield is found to be about 2.48. These results indicate the importance of both heat and UV wavelength on the catalytic activity of H2 generation. One important feature regarding the practical applications of the system is the applicability of the catalyst to direct sunlight irradiation with high activity and stability. To test the feasibility of the catalyst for further real-world applications, the system was operated under direct sunlight irradiation in conjunction with a Fresnel lens that concentrated the sunlight. The control experiment in the absence of catalysts showed no detectable production of H2, thus verifying that the catalyst is essential for the photocatalytic generation of H2 under direct sunlight irradiation. A dark control experiment was also conducted by heating the system at 153 °C in the absence of light, resulting in a TOF of 29684 h−1 (Table 1, entry 18). When 0.4 mM catalyst was loaded and the system was irradiated under direct sunlight, a high TOF of 1.15 × 106 h−1 was detected (Figure 2, Table 1, entry 19, and Video S1).15 To determine the TONs, reactions were performed using a batch addition of FA/TEA. In an experiment with 0.2 mM catalyst loading, more than 140 L of H2 (280 L of H2 + CO2) was produced and a TON of approximately 2.78 × 106 was obtained within 5 h without a decline in the H2 generation activity. Only a slight nonlinearity of the H2 generation activity, attributed to adventitious shading of the sunlight, was observed (Figure 3). To the best of our knowledge, these results represent the highest photocatalytic H2 generation activity and stability of a biomimetic H-cluster catalyst obtained thus far.

Figure 2. Photocatalytic H2 generation rate for 1 as a catalyst using (1) 90 °C (□, Table 1, entry 1), (2) a 500 W Xe lamp (○, Table 1, entry 2), (3) a 500 W Xe lamp + 90 °C (△, Table 1, entry 12), (4) direct sunlight irradiation (▽), or (5) direct sunlight irradiation without a catalyst (*).

photoirradiated under a 500 W Xe lamp without temperature control (TOF of 164 h−1; Figure 2 and Table 1, entry 2). A synergistic effect was observed when 1 was photoirradiated with a 500 W Xe lamp, and the temperature was increased from 60 to 90 °C (Table 1, entries 3−9).11 We next evaluated the effect of the 1-to-P(o-tol)3 ratio on H2 generation in the reaction by changing the ratio from 1:0, 1:1, 1:2, 1:3, to 1:4 (Table 1, entries 7−12). As shown in Table 1, the 1:3 ratio exhibited the highest TOF, whereas a higher P(o-tol)3 ratio inhibited the catalyst activity. A high H2 generation rate with a TON of 48147 and a TOF of 41269 h−1, combined with a 96% H2 conversion, was obtained at 70 min (Figure 2 and Table 1, entry 12). To investigate the effect of the incident light wavelength, different optical wavelength filters [280 nm long pass, 280−397 nm (T = 346 nm) bandpass, 301−384 nm (T = 352 nm) bandpass, 355−469 nm (T = 397 nm) bandpass, and 400 nm long pass] were tested for H2 generation effectiveness. The results showed that 99%, 37%, 24%, 9%, and 11% FA were consumed in 1 h for a 280 nm long pass, 280−397, 301−384, and 355−469 nm bandpasses, and a 400 nm long pass, respectively (Table 1, entries 13−17). Furthermore, by calculating the total number of moles of photons absorbed at 370 nm and the net number of moles of H2 generated by

Figure 3. Continuous H2 generation of 1 under direct sunlight irradiation.

Mechanistic Studies of Photocatalytic H2 Generation. To characterize active components involved in FA dehydrogenation by 1, thin-layer chromatography (TLC) was first applied to analyze the reaction profile (Figure S4). The TLC analysis of the reaction profile revealed that two new compounds were generated during the reaction. These two compounds were isolated and characterized by X-ray crystallography and found to be [Ru2(CO)6(μ-SCH2CH(NHCO 2 (C(CH 3 ) 3 ))CH 2 S)] (2) and [Ru 2 (CO) 5 (μSCH2CH(NHCO2(C(CH3)3))CH2S)P(o-tol)3] (3) (Figures 4 and S2 and S3 and Tables S3−S5). An amido−bis(S) ligandsubstituted [FeFe] H-cluster model complex with a structure similar to that of complex 2 was previously reported by Weigand’s group.16 In separate experiments where 2 and 3 D


Article

Inorganic Chemistry

Figure 4. Structures of 2 and 3 shown by thermal ellipsoid plots. H atoms were omitted for clarity.

δH = −12.40, −12.65, −16.90, −18.58, and −19.09 ppm (Figure S7, lanes 4−6). On the basis of these observations, it was concluded that the signals at δH = −12.40, −12.65, −16.90, −18.58, and −19.09 ppm were mainly derived from 1 and the signals at δH = −12.93 and −13.13 ppm were derived from 2. The observation that 1 + P(o-tol)3 + FA/TEA also exhibits spectral profiles of 2 + P(o-tol)3 + FA/TEA indicates that a conversion from 1 to 2 occurs during these two reactions. In parallel, 1 + P(o-tol)3 + NaBH4, in which the latter is a characteristic hydride source, was photoirradiated in the absence and presence of FA (Figure S7, lanes 7−9). As expected, with the exception of two signals at δH = −12.93 and −13.13 ppm, patterns similar to those of 1 + P(o-tol)3 + FA/ TEA were detected for 1 + P(o-tol)3 + NaBH4 in the absence of FA, supporting the proposal for the formation of a ruthenium hydride species. Alternatively, when NaBH4 was added to 1 and photoirradiated in the presence of FA, no corresponding 1H NMR signals were observed, but vigorous bubbling did occur. The bubbles were characterized as H2 by GC-TCD, demonstrating H2 formation through the combination of hydrides and protons. On the basis of the results described above, a proposed reaction profile for photocatalytic H2 generation by 1 can be put forth and is shown in Scheme 2. Upon photoirradiation, 1 is converted to 2. As expected, upon photoirradiation and in the presence of P(o-tol)3, substitution of the carbonyl group with P(o-tol)3 to generate 3 and interconversion between 2 and 3 occurred.11 This is supported by the TLC profiles and 1 H NMR and IR spectra. Consistent with these observations, when 2 was photoirradiated in the presence of P(o-tol)3, the amounts of 3 increased upon prolonged photoirradiation (vice versa; Figure S4). Subsequently, in the presence of FA and upon photoirradiation, the carbonyl group of 3 was replaced with a formate or a hydride, as supported by detection of the hydride signals by 1H NMR and IR reaction profile monitoring. Rearrangement of the formate group and the evolution of CO2 generate a transient complex (I), which then

were photoirradiated under direct sunlight irradiation, TOFs comparable to that of 1 were obtained. No apparent decline in activity was detected with long-term photoirradiation, indicating that the change of the structure from 1 to 2 or 3 does not affect the active conformation of the catalysts or H2 generation activity. Next, we compared 1- and 2-catalyzed photocatalytic FA dehydrogenation reactions by UV−vis, IR, and 1H NMR spectrometry. Monitoring the reaction using UV−vis spectroscopy showed a bathochromic band at 366 nm that increased in intensity over time in both reactions, indicating coordination of FA or P(o-tol)3 to the system through replacement of the coordinated CO (Figure S5).11 In addition, in-situ-monitored IR spectra of the 1 + P(o-tol)3 + FA/TEA photocatalytic reaction showed two new stretching bands at 1593 and 2031 cm−1, assignable to the Ru−H and Ru−OCOH stretching frequencies, and the intensity of the ν(CO) band at 1951 cm−1, which is characteristic of the CO−Ru−P(o-tol) 3 stretching frequency, increased with prolonged photoirradiation (Figure S6).11,17 In-situ-monitored IR spectra of the 2 + P(o-tol)3 + FA/TEA photocatalytic reaction also revealed a decrease of the ν(CO) stretching band intensity at 2086 cm−1 and an increase of the ν(CO) band at 1944 cm−1, which was shifted from 1953 cm−1 (Figure S6). These results supported the covalently linked P(o-tol)3 onto ruthenium of complex 1 or 2.11b 1 H NMR characterization of the photoirradiation reaction profile of 1 + P(o-tol)3 + FA/TEA showed distinctive peaks between δH = −6.5 and −20.0 ppm, assignable to the hydride or formate on the ruthenium (Figure S7).15,18 As shown in Figure S7, lanes 1−3, apparent signals at δH = −12.40, −12.65, −12.93, −13.13, −16.90, −18.58, and −19.09 ppm were detected for 1 + P(o-tol)3 + FA/TEA photocatalytic reactions. Subsequently, when the photoirradiation of 2 + P(o-tol)3 + FA/TEA was monitored by 1H NMR spectroscopy, peaks similar to those of 1 + P(o-tol)3 + FA/TEA between δH = −6.5 and −20.0 ppm were detected, with the exception of signals at E


Article

Inorganic Chemistry Scheme 2. Proposed H2 Generation Reaction Catalyzed by 1 + P(o-tol)3 + FA/TEA

undergoes a protonation process to yield a complex (II). It follows release of H2, reincorporation of the formate anion, and the evolution of CO2 regeneration of the active complex (I), and the cycle begins again.

In nature, hydrogenases (H2ases) reversibly catalyze the formation of H2 from protons and electrons and the oxidation of H2 into protons and electrons. In the H-cluster of the [FeFe]-H2ases, the dithiolate ligand was found to exist in the F


Article

Inorganic Chemistry form −SCH2XCH2S− (X = NH, azadithiolate, adt2−) and be a key component in relaying H+ to the active site for H2 production.19 The respective NH group has been supposed to act as a catalytic base.18 In parallel, lessons from synthetic modeling of [FeFe]-H-clusters have shown that the H+reduction catalyst [Fe2(adt)(CO)2(dppv)2] is 104 times more active than its propanedithiolate (pdt2−) or oxadithiolate (odt2−) congener. This is because the basicity of the amine is presumably tuned to both protonate and deprotonate Fe−H, whereas the O atom in odt2− is insufficiently basic and pdt2− has no acid/base chemistry capabilities for the abovementioned tasks.19,20 Moreover, mononuclear mimicking compounds, such as mononuclear nickel, iron, and ruthenium diphosphine complexes bearing the second coordination sphere pendant amine moiety, have also demonstrated the important role of the amine moiety as a proton relay in promoting the reaction rate for electrocatalysis.21 In addition, the Ru−N bond length of 2.2627(19) Å is longer than the Ru−N(sp3) dative bond, which is approximately 2.05 Å on average.13 Moreover, the presence of steric bulk in the aminefunctionalized bridge forces a strong steric repulsion between the bridge and the apical ligands of the ruthenium. Thus, a combination of the amine-functionalized bridge and a sterically demanding substituent on the bridge may facilitate the conversion of hydride and proton into H2.

ORCID

Tung-Kung Wu: 0000-0002-6478-7768 Jen-Shiang K. Yu: 0000-0003-1552-9436 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval for the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ministry of Science and Technology of the Republic of China (Contract MOST 106-2113-M-009-009) and the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan for financial support. We are grateful to Prof. Ya-Ting Kao, Prof. Chien-Chung Cheng, and Prof. Hsiu-Fu Hsu for providing helpful discussions.

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CONCLUSION A biomimetic H2 generation system was obtained through direct-sunlight-irradiated photocatalytic FA dehydrogenation. The active species isolation and structure determination strongly indicate the importance of the amido−bis(S) ligandsubstituted group and the steric effects of the aminefunctionalized bridge of the catalyst for reactivity. Finally, the advantages of high TOFs and TONs with low catalyst loading, the robust nature of the catalyst, and the use of direct sunlight irradiation make this biomimetic system applicable for highrate photocatalytic H2 production.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02888. Experimental section, NMR, UV−vis, IR and MS spectra, TLC profile, crystal data and structure refinement, and selected bond lengths and angles (PDF) Video S1. Photocatalytic H2 generation under direct sunlight irradiation (AVI) Accession Codes

CCDC 1856607, 1856609, and 1878880 contain the supplementary crystallographic data for this paper. These data 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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