Photocatalytic Hydrogen Production from Water by Noble-Metal-Free

Aug 26, 2010 - Hydrogen evolution was observed from the noble-metal-free catalyst systems, comprising Rose ... studies of hydrogen-evolving molecular ...
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J. Phys. Chem. C 2010, 114, 15868–15874

Photocatalytic Hydrogen Production from Water by Noble-Metal-Free Molecular Catalyst Systems Containing Rose Bengal and the Cobaloximes of BFx-Bridged Oxime Ligands Pan Zhang,† Mei Wang,*,† Jingfeng Dong,† Xueqiang Li,† Feng Wang,‡ Lizhu Wu,‡ and Licheng Sun*,†,§ State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Centre on Molecular DeVices, Dalian UniVersity of Technology, 116012 Dalian, China, Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Beijing 100190, China, and Department of Chemistry, Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: August 12, 2010

Hydrogen evolution was observed from the noble-metal-free catalyst systems, comprising Rose Bengal, BFxbridged cobaloximes, and triethylamine, in an aqueous solution under irradiation of visible light. Two types of BFx-bridged cobaloximessnamely, the annulated cobaloximes [Co(dmgBF2)2(H2O)2] (1, dmgBF2 ) (difluoroboryl)dimethylglyoximate anion) and [Co(dpgBF2)2(H2O)2] (2, dpgBF2 ) (difluoroboryl)diphenylglyoximate anion), and the clathrochelated cobaloximes [Co(dmg(BF)2/3)3](BF4) (3) and [Co(dpg(BF)2/3)3](BF4) (4)swere used as catalysts. Among the four cobalt complexes, complex 1 displayed the highest hydrogenevolving efficiency, with turnovers up to 327. Complexes 2 and 4 that bear the diphenylglyoximate ligands exhibited much lower efficiencies as compared with their analogues 1 and 3 that have the dimethylglyoximate ligands. The hydrogen-evolving efficiency of the annulated cobalt(II) complex 1 that contains two labile axial ligands is more than three times as high as that of the encapsulated cobalt(III) complex 3 that has a single macrobicyclic ligand. The different pathways for formation of the cobalt(I) species from these two types of cobaloximes are discussed on the basis of the results obtained from fluorescence and laser flash photolysis spectroscopic studies. 1. Introduction The conversion of solar energy into conveniently usable energy in the form of molecular hydrogen by photoinduced water splitting is one of the most promising approaches toward production of renewable and clean energy.1-3 In recent years, studies of hydrogen-evolving molecular catalyst systems have attracted more and more attention. A number of single- and multicomponent molecular catalysts have been reported to be active for photochemical hydrogen production.4-8 Aiming at a highly efficient and low-cost production of hydrogen, much progress has been achieved in developing and understanding of photocatalytic hydrogen production with proton reduction catalysts based on first-row transition metals, such as FeFe hydrogenase active site models,9-13 cobaloximes,14-17 and the tris(bipyridine)cobalt complex.18,19 However, these reported systems usually require precious metal-based photosensitizers, such as RuII-,9,10,18 IrIII-,15,19 PtII-,17 and ReI-based photosensitizers.15,16 Only a few examples of completely noble-metal-free catalytic systems for photoinduced hydrogen generation have been reported.10,20-22 Very recently, coordinately assembled zinc porphyrin-diiron photocatalysts were reported by Kluwer and us independently,10,20 and a covalently linked porphyrin-diiron photocatalyst was reported by Song and co-workers.21 These molecular devices are active for visible-light-driven hydrogen evolution, but the turnover numbers (TONs) are relatively low (0.2-4). In addition to porphyrin-derived chromophores, xanthene dyes are the other * To whom correspondence should be addressed. (M.W.) Fax: +86411-83702185. E-mail: [email protected]. (L.S.) Fax: +46-8-7912333. E-mail: [email protected]. † Dalian University of Technology. ‡ Technical Institute of Physics and Chemistry. § Royal Institute of Technology.

Figure 1. Structures of RB, BF2-annulated bis(glyoximato)cobalt complexes (1, 2), and BF-clathrochelated tris(glyoximato)cobalt complexes (3, 4).

type of inexpensive and promising organic photosensitizers. Although they have been used in several heterogeneous hydrogen-evolving systems with colloidal platinum as catalyst,23,24 there is only one recent report, made by Eisenberg and co-workers,22 using xanthene dyes as photosensitizers in combination with [Co(dmgH)2(Py)Cl] (dmgH ) dimethylglyoximate monoanion) as catalyst and triethanolamine (TEOA) as a sacrificial electron donor for homogeneous production of hydrogen from aqueous protons. These noble-metal-free catalyst systems display high efficiency for hydrogen production, with TONs up to 900 in the presence of an excess of the dmgH2 free ligand. We have recently investigated the photocatalytic production of hydrogen from water, using Rose Bengal (tetrachlorotetraiodofluorescein, RB, Figure 1) as photosensitizer and the cobaloxime complexes that contain BFx-bridged glyoximate ligands as catalysts. It has been reported that RB is an efficient photosensitizer in several hydrogen generation systems containing Pt catalysts.23-26 Considering the high quantum yield (0.86)

10.1021/jp106512a  2010 American Chemical Society Published on Web 08/26/2010

Photocatalytic Hydrogen Production from H2O and long lifetime (104 µs) of the triplet excited state (3*RB) in an aqueous solution26 as well as the triplet energy (1.83 eV) and the ground-state oxidation (0.94 V vs NHE) and reduction potentials (-0.51 V vs NHE),24,25 RB could be a good candidate for a photosensitizer for the light-driven water reduction catalyzed by cobalt complexes. The BFx-bridged cobaloximes are attractive molecular catalysts due to their special stability and activity in electro- and photochemical hydrogen evolution.14,15,27-31 Previous reports have shown that replacing the H bridges with BFx bridges in cobaloxime complexes could not only enhance the coordination stability of the ligands but also make the reduction potentials of the cobalt complexes more positive.32 Two kinds of previously reported cobaloximes containing BFx bridges (namely, BF2-annulated bis(glyoximato)cobalt complexes (1 and 2, Figure 1) and BF-clathrochelated tris(glyoximato)cobalt complexes (3 and 4)) were used as catalysts. Among the four cobalt complexes, complex 1 is apparently more efficient than the other three complexes for photoinduced hydrogen generation, with TONs up to 327 when RB was used as the photosensitizer in a 10% triethylamine (TEA) aqueous solution under optimal conditions. The mechanisms of the photoinduced hydrogen-evolving reactions catalyzed by these two types of cobaloxime complexes are discussed on the basis of fluorescence and flash photolysis spectroscopic studies. 2. Experimental Section 2.1. Reagents and Instruments. Commercially available RB (disodium salt), dimethylglyoxime, and diphenylglyoxime were purchased from Alfa Aesar and used without further purification. Triethylamine and all solvents were distilled before use. Cobaloximes [Co(dmgBF2)2(OH2)2] (1),27 [Co(dpgBF2)2(OH2)2] (2),33 [Co(dmg(BF)2/3)3](BF4) (3), and [Co(dpg(BF)2/3)3](BF4) (4)31 were prepared according to the literature procedures. UV-vis absorption measurements were carried out on a Jasco-V-530 spectrophotometer. Photoluminescence spectra were recorded using a Spex Fluorolog fluorimeter by exciting the sample at 420 nm. The pH of the solution was determined by a PHS-25 pH meter and adjusted by addition of hydrochloric acid or sodium hydroxide as required. The fluorescence decay times were obtained with a nanosecond laser (λexc: 460 nm) time-correlated single photon counting setup (FluoroMax-4 spectrofluorometer, Horiba Jobin Yvon Corp). The instrument response function of the laser SPC system has a fwhm of 1 ns and the time resolution is estimated at 200 ps. The time ranges are 0.055 ns/channel, in 4096 effective channels. The estimated reproducibility is around 2% for the nanosecond decays. Nanosecond transient absorption measurements were performed on a LP-920 laser flash photolysis setup (Edinburgh). The excitation pulses were obtained from the unfocused second harmonic (532 nm, 7 ns fwhm) output of a Nd:YAG laser (Continuum surelite II), and the probe light was provided by a 75 W Xe arc lamp. The laser and analyzing light beam passed perpendicularly through a 1 cm quartz cell. The signals were detected by Tektronix TDS 3012B oscilloscope and R928P photomultiplier and, finally, analyzed by Edinburgh analytical software (LP900). All samples used in flash photolysis experiments were bubbled with argon for 20 min prior to the measurement. 2.2. General Procedure for Photocatalysis. In a typical experiment, 5 mL of a 10% TEA aqueous solution containing RB (2 mg, 2 µmol, 4 × 10-4 M) and the cobalt complex 1 (0.8 mg, 2 µmol, 4 × 10-4 M) was added to a Schlenk bottle with magnetic stirring under N2 atmosphere. The solution was then

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Figure 2. Influence of the pH on photoinduced hydrogen production from the system comprising 1 (4 × 10-4 M) and RB (4 × 10-4 M) in a 10% TEA aqueous solution in 5 h of irradiation.

free-pump-thaw degassed three times and irradiated using a Xe lamp (500 W) with a Pyrex glass filter (λ > 400 nm). The gas phase of the reaction system was analyzed by a GC 7890T instrument with a thermal conductivity detector, a 5 Å molecular sieve column (2 mm × 2 m), and with N2 as carrying gas. 3. Results and Discussion 3.1. Optimization of Reaction Conditions for Photocatalytic Hydrogen Production by RB and Complex 1. When RB and 1 were dissolved in a 10% TEA aqueous solution, the color of the solution turned to dark blue due to the formation of the intermediate CoI species under irradiation. The formation rate of the CoI species was apparently influenced by the pH of the reaction solution. To determine the effect of pH on the formation rate of the reduced complex 1, the time that was needed for observation of the maximum intensity of the typical absorption at 610 nm for the CoI species was measured.34 In a 10% TEA aqueous solution containing RB and 1 at pH 9, the absorption of the CoI species reached the maximum intensity after 330 s irradiation. When the pH was enhanced to 10 and then to 11, the time needed was shortened to 20 and less than 5 s, respectively, indicating that the formation of the CoI species occurred faster in a more basic solution. This result suggests that the desired electron transfer (ET) among the three components, RB, complex 1, and TEA could be facilitated by enhancing the pH of the medium.35 Figure 2 shows that the pH of the reaction solution also has an apparent effect on hydrogen generation. The maximum hydrogen generation efficiency was achieved at pH 10 with complex 1 as catalyst. The turnover number based on complex 1 was 21 in a 10% TEA aqueous solution in 5 h irradiation. Although the formation of the CoI species can be accelerated at a more basic solution, the amount of hydrogen generation decreased sharply when the pH of the solution turned more basic than 10. One of the possible reasons for this phenomenon is that the reduced concentration of protons is not favorable for protonation of the CoI species to form the CoIII hydride, which is believed to be one of the crucial intermediates for proton reduction in the similar system.22,34,36 When the pH of the solution was less basic than 10, the amount of hydrogen generation also decreased, due to the fact that the rate of CoI species formation is apparently slowed, and it is difficult for TEA to be oxidized at the lower pH to provide electrons. With an increase in the concentration of TEA from 10% to 12.5% or decrease of it to 7.5%, the TON based on 1 was reduced from 21 to 8 and 12, respectively, in 5 h irradiation. When TEOA was used in place of TEA, only a small amount of hydrogen was evolved with 2 turnovers based on complex 1 in a 10% TEOA aqueous solution at pH 10 in 5 h of irradiation. After optimization of the concentration of TEOA and the pH

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TABLE 1: Influence of the CH3CN/H2O Ratio on Photoinduced Hydrogen Productiona CH3CN/H2O ratio (V/V)

amt of H2 (µmol)

TON

water 1:9 1:4 1:2 1:1 2:1

41 156 214 236 215 191

21 78 107 118 108 96

a Conditions: a 10% TEA solution of 1 (4 × 10-4 M) and RB (4 × 10-4 M) at pH 10 and 5 h of irradiation.

TABLE 2: Influence of the Catalyst Concentration on Photoinduced Hydrogen Productiona concn of 1 (mM)

amt of H2 (µmol)

TON

0.05 0.075 0.1 0.2 0.4

53 94 164 196 236

212 248 327 196 118

a Conditions: RB (4 × 10-4 M) and 1 in a CH3CN/H2O (1:2) solution with 10% TEA at pH 10 and 5 h of irradiation.

of the medium, the hydrogen evolved reached 10 turnovers in a 5% TEOA aqueous solution at pH 7 in 6 h of irradiation, which is lower than that obtained by using TEA as an electron donor. Other potential electron donors, such as ascorbate and EDTA, were also tested, but no hydrogen evolved from the catalyst system was detected by GC analysis. Addition of acetonitrile to the TEA aqueous solution apparently shortened the time needed for observation of the maximum intensity of the typical absorption of the CoI species at 610 nm, from 20 s in an aqueous solution to less than 5 s in CH3CN/ H2O (1:2 V/V). Accordingly, the efficiency of the hydrogen generation was greatly improved (Table 1), presumably resulting from the better solubility of TEA in the mixture of acetonitrile/ water and the coordination of acetonitrile molecules to the cobalt center of complex 1.17,29 The TON was enhanced to 118 for complex 1 when the ratio of acetonitrile/water was 1 to 2 (V/ V). The TONs of hydrogen evolution are five to six times as high as that obtained for complex 1 in water, whereas in acetone/ water (1/2, V/V) and ethanol/water (1/2, V/V) mixed solutions, 23 and 25 turnovers were obtained, respectively, for the system of 1, RB, and TEA in 5 h of irradiation. It was reported that the energy, the quantum yield, and the lifetime of the triplet excited state of RB are markedly influenced by solvents.37 The efficiency of hydrogen generation was decreased with further increase of the ratio of acetonitrile to water, probably because the decrease of the static dielectric constant of the solution could slow down the intermolecular ET rate.38 The influence of the loading amount of the Co-based catalyst on hydrogen evolution was explored. The efficiency of hydrogen evolution for complex 1 grew from 118 to 327 turnovers as the concentration of 1 was decreased from 0.4 to 0.1 mM, whereas other reaction conditions were kept unchanged. A further decrease in the concentration of 1 resulted in apparent drops of the hydrogen-evolving efficiency of the catalyst (Table 2). 3.2. Influence of the Structures of Cobaloxime Complexes on Photocatalytic Hydrogen Production. Changing the methyl groups in the glyoximate ligands of 1 to the phenyl groups makes the reduction potential of 2 more positive than 1,32 but even so, the TONs of hydrogen evolution drastically decreased to 25 (Figure 3) when complex 2 was used as catalyst under the optimal conditions. A similar influence of the substituents

Figure 3. Hydrogen production from the systems containing RB (4 × 10-4 M) and a cobaloxime catalyst (1 × 10-4 M) in a 10% TEA acetonitrile aqueous solution at pH 10.

in cobaloxime catalysts on electro- and photochemical hydrogen generation has been previously reported for other cobaloxime catalytic systems.14,31 The other type of cobaloximes, BF-bridged clathrochelated cobalt complexes 3 and 4, was reported to be efficient catalysts in electrochemical hydrogen generation.31 We studied the catalytic performance of such cobaloxime complexes for photoinduced hydrogen generation. The optimal conditions for complexes 3 and 4 are the same as those for 1 and 2. With 3 and 4 as catalysts in the presence of RB and TEA in CH3CN/ H2O (1:2), the TONs of hydrogen evolution were 96 and 17 for 3 and 4 (Figure 3), respectively. The catalytic results show that the nature of the substituents on the periphery of glyoximate ligands has a significant effect on the photocatalytic hydrogen production. Both complexes 2 and 4 that bear the diphenylglyoximate ligands displayed much lower efficiencies as compared with their analogues 1 and 3 that have the dimethylglyoximate ligands. In another aspect, the hydrogen-evolving efficiency of the annulated CoII complex 1 containing labile axial ligands is more than three times as high as that of the encapsulated CoIII complex 3 having a single macrobicyclic ligand. The different hydrogenevolving activities of cobaloximes 1 and 3 may be caused by the steric effect of the bis- and tris(glyoximato) ligands as well as the different reduction pathways of the CoII and CoIII complexes to the CoI species. The hydrogen-evolving efficiency based on the BF2-bridged cobaloxime (327 turnovers vs Co and 82 turnovers vs RB in 5 h irradiation) for the system comprising RB and 1 in a 10% TEA acetonitrile aqueous solution is higher than or comparable to that previously reported for the other cobaloxime-based multicomponent catalyst systems in the absence of free glyoximate ligands, such as the system of RB, [Co(dmgH)2(Py)Cl], and TEOA in CH3CN/H2O (1:1) (∼55 turnovers vs Co and 270 turnovers vs RB in 5 h of irradiation);22 the similar system of eosin Y, [Co(dmgH)2(Py)Cl], and TEOA in CH3CN/H2O (1:1) (∼72 turnovers vs Co and 360 turnovers vs eosin Y in 5 h irradiation); and the system of [ReBr(phen)(CO)3], [Co(dmgBF2)2(H2O)2], and TEA in acetone (273 turnovers vs both Re and Co in 15 h of irradiation).15 As shown in Figure 3, hydrogen production levels off after 5 h of irradiation, no matter which catalyst is used. The experiment with readdition of the catalyst (0.5 µmol) to the system could not regenerate hydrogen under irradiation, whereas the parallel experiment with readdition of RB (2.0 µmol) showed a recovered but apparently lowered activity for hydrogen generation in action of light (Figure 4). It was found that the absorption of RB around 550 nm disappeared in the UV-vis spectrum after the 5 h photocatalytic reaction (Figure S1), which suggests that the deactivation of the system may result mainly from the photodegradation of the organic dye.22,23

Photocatalytic Hydrogen Production from H2O

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Figure 4. Hydrogen generation (a) from the system containing Rose Bengal (4 × 10-4 M) and complex 1 (1 × 10-4 M) in a 10% TEA acetonitrile aqueous solution at pH 10 and (b) readdition of RB (2 µmol) to the solution after 6 h of irradiation.

Figure 5. Kinetic traces at 620 nm obtained following flash photolysis of a deoxygenated acetonitrile solution that contains (a) RB (1 × 10-5 M) and (b) RB (1 × 10-5 M) + 1 (5 × 10-5 M). The decay curves are normalized.

TABLE 3: Redox Potentials of Cobaloxime Complexes 1-4 and the Free Energies for the ET Reaction complex 1 2 3 4

CoIII/CoII E1/2 (V vs NHE)a

CoII/CoI E1/2 (V vs NHE)a

∆G1b

∆G2c

∆G3d

0.62 0.70

-0.3129 -0.0429 -0.4231 -0.1431

-0.97 -1.20 -0.82 -1.10

-0.58 -0.85 -0.47 -0.75

-0.20 -0.47 -0.09 -0.38

a In acetonitrile. b The free energy of the formation of Co(I) species from 1RB* to the Co complex was calculated from the equation, ∆G1 ) E(RB+/RB) - E(CoII/CoI) - E00(1*RB). c The free energy of the formation of CoI species from 3RB* to the Co complex was calculated from the equation, ∆G2 ) E(RB+/RB) E(CoII/CoI) - E00(3*RB). d The free energy of the formation of Co(I) species from RB•- to the Co complex was calculated from the equation, ∆G3 ) E (RB•-/RB) - E(CoII/CoI).

3.3. Mechanisms for the Intermolecular ET and Hydrogen Production. The redox potentials of 1-4 are given in Table 3 to evaluate the driving force of the photoinduced ET reaction. According to the reduction potentials of 1-4, the ground-state redox potentials of RB (E1/2(RB•+/RB) ) 0.94 V and E1/2(RB•-/ RB) ) -0.51 V vs NHE),39,40 and the excited state energies of 1* RB (2.18 eV) and 3*RB (1.83 eV),26 the free energy ∆G for formation of the CoI species can be calculated according to the Weller-equation.41 The negative ∆G1, ∆G2, and ∆G3 suggest that the ET from 1*RB, 3*RB, and the reduced RB•- species to the cobaloximes are thermodynamically feasible for these catalyst systems. The possible pathways for the initial ET in the catalyst system were studied by fluorescence spectroscopy and laser flash photolysis technique. It was found that addition of TEA or complex 1 or 2 to the aqueous solution of RB did not have any effect on the fluorescence emission intensity and the lifetime of the singlet excited state (1*RB), suggesting that 1*RB is not involved in the ET. To examine the possibility of the triplet state’s 3*RB being responsible for the hydrogen generation, complex 1 was taken as an instance for the 3*RB lifetime studies. The lifetime of 3*RB was significantly shortened from 78 µs in the absence of 1 to 13.2 µs upon addition of 1 (0.05 mM) in acetonitrile (Figure 5), indicating the occurrence of the oxidative quenching of 3*RB by 1. The 3*RB could also be reductively quenched by the electron donor TEA, resulting in formation of the RB•- radical anion, which was characterized by a typical absorption band at around 425 nm.26 When complex 1 was added to the photogenerated RB•- solution, the lifetime (1.4 ms) of the RB•- species was shortened to 76 µs (Figure 6). This result indicates that the RB•- species can reduce complex 1 to the CoI species. The decays of 3*RB at 620 nm at the different concentrations of TEA and 1 in acetonitrile, respectively, provide more

Figure 6. Kinetic traces at 425 nm obtained following flash photolysis of a deoxygenated acetonitrile solution that contains (a) RB (1 × 10-5 M) + TEA (1 × 10-3 M) and (b) RB (1 × 10-5 M) + TEA (1 × 10-3 M) + 1 (5 × 10-5 M). The decay curves are normalized.

SCHEME 1: Possible Initial Intermolecular ET Processes in the System Containing RB, the CoII Complex (1 or 2), and TEA in Irradiation

kinetic information. By varying the concentration of 1 from 0.005 to 0.05 mM, the rate constant for the oxidative quenching of 3*RB by complex 1 is determined to be 1.1 × 109 M-1 s-1 by flash photolysis technique, which is 3 orders of magnitude faster than that (3.9 × 106 M-1 s-1) for reductive quenching of 3*RB by TEA with the concentration of TEA varying from 0.005 to 1.0 mM. These rate constants show that the oxidative quenching of 3*RB by complex 1 is preferred over the reductive quenching by TEA. However, the concentration of TEA is much higher than that of 1 in the photocatalytic system; therefore, both the ET from the 3* RB and RB•- species to complex 1, shown as paths II and

Figure 7. Fluorescence spectra of RB (2.5 × 10-6 M), RB + an equiv of 3, and RB + an equiv of 4 in acetonitrile.

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SCHEME 2: Possible Initial Intermolecular ET Processes in the System Containing RB, the CoIII Complex (3 or 4), and TEA in Irradiation

III in Scheme 1, are possible for the formation of the CoI species from 1 in this hydrogen-evolving system. Unexpectedly, it was found that addition of 3 or 4 to the RB solution caused an apparent decrease in the fluorescence emission intensity of RB. When 1 equiv of 3 was added to the acetonitrile solution of RB, the fluorescence intensity of RB was quenched by ∼90% (Figure 7). The fluorescence lifetime of 1*RB was shortened from 2.7 ns in the absence of 3 to 1.3 ns upon addition of 3 (1 × 10-5 M) in acetonitrile, indicating the possibility of the direct electron transfer from

Figure 8. Kinetic traces at 620 nm obtained following flash photolysis of a deoxygenated acetonitrile solution that contains (a) RB (1 × 10-5 M) and (b) RB (1 × 10-5 M) + 3 (5 × 10-5 M). The decay curves are normalized.

Figure 9. Kinetic traces at 425 nm obtained following flash photolysis of a deoxygenated acetonitrile solution that contains (a) RB (1 × 10-5 M) + TEA (1 × 10-3 M) and (b) RB (1 × 10-5 M) + TEA (1 × 10-3 M) + 3 (5 × 10-5 M). The decay curves are normalized.

Zhang et al. 1*

RB to 3. This result suggests that the 1*RB may be involved in electron transfer and hydrogen generation, which is different from the systems containing the cobaloximes 1 and 2 of bis(glyoximato) ligands. In addition to the possibility for the occurrence of ET from 1* RB to 3 shown by the fluorescence spectra (path I in Scheme 2), the electron transfers from 3*RB and RB•- to 3 are also thermodynamically feasible for the formation of the CoI species (paths II and III in Scheme 2), which were studied by flash photolysis. The transient absorption of 3*RB at 620 nm shows that the lifetime (78 µs) of 3*RB is significantly shortened to 6.2 µs when 3 (0.05 mM) is added (Figure 8). The decay of the characteristic absorption of RB•- at 425 nm indicates that the lifetime (1.4 ms) of RB•-, generated by the reductive quenching of 3*RB by TEA in an acetonitrile solution, is shortened to 52 µs upon addition of 3 (Figure 9), evincing that the RB•- is also capable of reducing complex 3 to a CoI species. By varying the concentration of 3 from 0.005 to 0.05 mM, the rate constant for the oxidative quenching of 3*RB by 3 is determined to be 2.6 × 109 M-1 s-1, which is more than twice that by 1. The faster electron transfer rate from 3*RB to 3 than to 1 might be caused by the electrostatic interaction between RB and the cationic complex 3 or by the different initial reduction processes for 3 (CoIII to CoII) and 1 (CoII to CoI). These spectroscopic experiments suggest that all three pathways shown in Scheme 2 are possible for the initial electron transfer in the systems comprising RB, the encapsulated CoIII complex (3 or 4), and TEA. The mechanism for hydrogen production catalyzed by the annulated cobaloximes has been well documented.42,43 Formation of a CoI species is considered to be one of the essential processes in the proposed mechanism for both electro- and photochemical hydrogen generation catalyzed by the cobaloxime complexes. The formation of the CoI species in the system of RB and 1 was observed by time-dependent UV-vis spectroscopy in a TEA aqueous solution at pH 9. Figure 10 shows the UV-vis absorptions at ∼457 and 610 nm, ascribed to the CoII and CoI species, respectively. The plot of the intensities of the absorptions at 457 and 610 nm vs irradiation time for the system of RB and 1 in a 10% TEA aqueous solution indicates that the consumption of the CoII complex and the formation of a CoI species occur simultaneously (Figure S2). The further reactions of the CoI species to evolve molecular hydrogen and regenerate the starting photocatalyst for the system of RB, the cobaloxime complex, and TEA could be similar with the mechanism well-described in the literature.22,34,36 The CoI species is protonated to form the CoIII hydride, which is proposed to be the rate-limiting step.42 The generation of hydrogen from the CoIII hydride may undergo by subsequent reduction and

Figure 10. Time-dependent UV-vis spectra of the solution comprising RB (1 × 10-4 M) and 1 (5 × 10-4 M) in a 10% TEA aqueous solution at pH 9. (left) Decrease of the absorption intensity of the CoII complex and (right) increase of the absorption intensity of the CoI species during the initial 5 min of irradiation.

Photocatalytic Hydrogen Production from H2O further protonation, leading to heterolytic cleavage of the CoII-H bond (a heterolytic route), or by a bimolecular reaction of two CoIII or CoII hydrides with homolytic cleavage of the CoIII-H or CoII-H bond (a homolytic route). Very recent reports indicate that the homolytic route is favored over the heterolytic route for the annulated cobaloxime catalysts.42,43 The basic condition (pH 10) in our system should disfavor the further reaction of the CoII-H species with protons to form hydrogen by the heterolytic route. The homolytic route releases hydrogen accompanied by regeneration of the CoII or CoI cobaloxime to fulfill a catalytic cycle. The mechanism of electro- and photochemical hydrogen evolution catalyzed by the clathrochelated cobaloxime complexes has not been clear up to now. Owing to the steric factors, it is difficult for the CoI species derived from the clathrochelated cobaloxime complexes (3 and 4) to be protonated at the well-encapsulated and coordinately saturated metal center, as compared with the annulated cobaloximes. It might be a major reason for the lower hydrogen-evolving efficiency of the clathrochelated cobaloximes, although the influence of different initial reduction processes cannot be excluded. For steric reasons, as well, a bimolecular homolytic route seems improbable for clathrochelated cobaloximes, as reported in the literature.35 However, as a consequence of reduction or protonation of the metal ion, the structural changes around the metal ion might happen, which could lead to such cooperative action. More studies are needed to understand the mechanistic routes for hydrogen generation by clathrochelated cobaloximes. 4. Conclusion The noble-metal-free systems, containing RB, TEA, and two types of cobaloxime complexes, are active for visible-lightdriven hydrogen production from water, with turnovers up to 327 vs Co under optimal conditions. The catalytic results give us two hints: first, the nature of the substituents on the periphery of glyoximate ligands has a significant effect on the photocatalytic hydrogen production; and second, the disassociation of a labile ligand at the metal center might be important for the photoinduced hydrogen generation catalyzed by cobaloxime complexes. On the basis of the results obtained by fluorescence and flash photolysis spectroscopic studies, we assumed that for the system comprising RB, TEA, and the BF2-bridged cobaloxime complex (1 or 2), only 3*RB is involved in hydrogen production. The CoI species was formed either by ET from the 3* RB or from the reduced RB•- species. In contrast, for the systems with the BF-bridged clathrochelated cobaloximes (3 and 4) as catalysts, not only 3*RB but also 1*RB are involved in the electron transfer processes. Three initiating pathways (namely, the electron transfers from 1*RB, 3*RB, and RB•- to the cobalt centers of 3 and 4) are possible for the hydrogenevolving reaction. Acknowledgment. We are grateful to the Natural Science Foundation of China (Grant no. 20633020), the Basic Research Program of China (Grant No. 2009CB220009), the Program for Innovative Research Team of Liaoning Province (Grant No. 2006T025), the Swedish Energy Agency, the Swedish Research Council, and the K & A Wallenberg Foundation for financial support of this work. Supporting Information Available: Two figures showing (1) UV absorption spectra of the system containing RB (2 × 10-5 M) in a 10% TEA aqueous solution at pH 10 before and

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15873 after irradiation and (2) a plot of the intensities of the absorptions at 457 nm for CoII species and 610 nm for CoI species vs irradiation time for the system of RB (1 × 10-4 M) and 1 (5 × 10-4 M) in a 10% TEA aqueous solution at pH 9 within 10 min. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Armaroli, N.; Balzani, V. Angew. Chem., Int. Ed. 2007, 46, 52– 66. (2) Lubitz, W.; Tumas, B. Chem. ReV. 2007, 107, 3900–3903. (3) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799–6801. (4) Esswein, A. J.; Nocera, D. G. Chem. ReV. 2007, 107, 4022–4047. (5) Rau, S.; Walther, D.; Vos, J. G. Dalton Trans. 2007, 915–919. (6) Fukuzumi, S. Eur. J. Inorg. Chem. 2008, 1351–1362. (7) Tinker, L. L.; McDaniel, N. D.; Bernhard, S. J. Mater. Chem. 2009, 19, 3328–3337. (8) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. Dalton Trans. 2009, 6458– 6467. (9) Na, Y.; Wang, M.; Pan, J.; Zhang, P.; Åkermark, B.; Sun, L. Inorg. Chem. 2008, 47, 2805–2810. (10) 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. Proc. Natl. Acad. Sci. 2009, 106, 10460–10465. (11) Streich, D.; Astuti, Y.; Orlandi, M.; Achwartz, L.; Lomoth, R.; Ott, S. Chem.sEur. J. 2010, 16, 60–63. (12) Wang, W.-G.; Wang, F.; Wang, H.-Y.; Si, G.; Tung, C.-H.; Wu, L.-Z. Chem.sAsian J. 2010, 5, 1796–1803. (13) Ga¨rtner, F.; Sundararaju, B.; Surkus, A. E.; Boddien, A.; Loges, B.; Junge, H.; Dixneuf, P. H.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 9962–9965. (14) Fihri, A.; Artero, V.; Razavet, M.; Baffert, C.; Leibl, W.; Fontecave, M. Angew. Chem., Int. Ed. 2008, 47, 564–567. (15) Fihri, A.; Artero, V.; Pereira, A.; Fontecave, M. Dalton Trans. 2008, 5567–5569. (16) Probst, B.; Kolano, C.; Hamm, P.; Alberto, R. Inorg. Chem. 2009, 48, 1836–1843. (17) Du, P.; Schneider, J.; Luo, G.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2009, 48, 4952–4962. (18) Krishnan, C. V.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1985, 107, 2005–2303. (19) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502–7510. (20) Li, X.; Wang, M.; Zhang, S.; Pan, J.; Na, Y.; Liu, J.; Åkermark, B.; Sun, L. J. Phys. Chem. B 2008, 112, 8198–8202. (21) Song, L.-C.; Wang, L.-X.; Tang, M.-Y.; Li, C.-G.; Hu, Q.-M. Organometallics 2009, 28, 3834–3841. (22) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. J. Am. Chem. Soc. 2009, 131, 9192–9194. (23) Shimidzu, T.; Iyoda, T.; Koide, Y. J. Am. Chem. Soc. 1985, 107, 35–41. (24) Zhang, X.; Jin, Z.; Li, Y.; Li, S.; Lu, G. J. Phys. Chem. C 2009, 113, 2630–2635. (25) Mau, A. W.-H.; Johanssen, O.; Sasse, W. H. F. Photochem. Photobiol. 1985, 41, 503–509. (26) Mills, A.; Lawrence, C.; Douglas, P. J. Chem. Soc. Faraday Trans. 1986, 82, 2291–2303. (27) Bakac, A.; Brynildson, M. E.; Espenson, J. H. Inorg. Chem. 1986, 25, 4108–4114. (28) Baffert, C.; Artero, V.; Fontecave, M. Inorg. Chem. 2007, 46, 1817– 1824. (29) Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc. 2007, 129, 8988–8998. (30) Jacques, P. A.; Artero, V.; Pecaut, J.; Fontecave, M. Proc. Natl. Acad. Sci. 2009, 106, 20627–20632. (31) Pantani, O.; Naskar, S.; Guillot, R.; Millet, P.; AnxolabehereMallart, E.; Aukauloo, A. Angew. Chem., Int. Ed. 2008, 47, 9948–9950. (32) Hu, X.; Cossait, B. M.; Brunschwig, B. S.; Lewis, N. S.; Peters, J. C. Chem. Commun. 2005, 4723–4725. (33) Tovrog, B. S.; Kitko, D. J.; Drago, R. S. J. Am. Chem. Soc. 1976, 98, 5144–5153. (34) Hawecher, J.; Lehn, J.-M.; Ziessel, R. NouV. J. Chim. 1983, 7, 271– 277. (35) Islam, S. D.-M.; Konishi, T.; Fujitsuka, M.; Ito, O.; Nakamura, Y.; Usui, Y. Photochem. Photobiol. 2000, 71, 675–680. (36) Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 129, 12576–12577. (37) Fleming, G. R.; Knight, A. W. E.; Morris, J. M.; Morrison, R. J. S.; Robinson, G. W. J. Am. Chem. Soc. 1977, 99, 4306–4311.

15874

J. Phys. Chem. C, Vol. 114, No. 37, 2010

(38) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378–10388. (39) Lambert, C.; Sarna, T.; Truscott, T. G. J. Chem. Soc. Faraday Trans. 1990, 86, 3879–3881. (40) Sarna, T.; Zajac, J.; Bowman, M. K.; Truscott, T. G. J. Photochem. Photobiol., A 1991, 60, 295–230. (41) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401–449.

Zhang et al. (42) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Acc. Chem. Res. 2009, 42, 1995–2004. (43) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2010, 132, 1060–1065.

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