Heptanuclear CoII5CoIII2 Cluster as Efficient Water Oxidation Catalyst

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Heptanuclear CoII5CoIII2 Cluster as Efficient Water Oxidation Catalyst Jia-Heng Xu,†,§ Ling-Yu Guo,†,§ Hai-Feng Su,‡,§ Xiang Gao,† Xiao-Fan Wu,† Wen-Guang Wang,*,† Chen-Ho Tung,† and Di Sun*,† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China ‡ State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China S Supporting Information *

ABSTRACT: Inspired by the transition-metal-oxo cubical Mn4CaO5 in photosystem II, we herein report a disc-like heptanuclear mixed-valent cobalt cluster, [CoII5CoIII2(mdea)4(N3)2(CH3CN)6(OH)2(H2O)2·4ClO4] (1, H2mdea = N-methyldiethanolamine), for photocatalytic oxygen evolution. The topology of the Co7 core resembles a small piece of cobaltate protected by terminal H2O, N3−, CH3CN, and multidentate N-methyldiethanolamine at the periphery. Under the optimal photocatalytic conditions, 1 exhibits water oxidation activity with a turnover number (TON) of 210 and a turnover frequency (TOFinitial) of 0.23 s−1. Importantly, electrospray mass spectrometry (ESI-MS) was used to not only identify the possible main active species in the water oxidation reaction but also monitor the evolutions of oxidation states of cobalt during the photocatalytic reactions. These results shed light on the design concept of new water oxidation catalysts and mechanism-related issues such as the key active intermediate and oxidation state evolution in the oxygen evolution process. The magnetic properties of 1 were also discussed in detail.

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INTRODUCTION Given energy and environment crisis issues, the conversion of solar energy into renewable chemical fuels in biomimetic ways is highly significant, and is the source of continuous interest.1,2 Naturally, green plants and algae can easily realize oxygen evolution by sunlight at mild conditions via the so-called Kok cycle. The oxygen evolution center (OEC) constitutes a CaMn4O5 cluster as revealed by X-ray structure studies.3 Chemists benefited from this structural motif and devoted massive efforts to mimic cubical tetranuclear metal clusters as molecular water oxidation catalysts (WOCs).4 In 2015, tailored [Mn4CaO4(tBuCO2)8(tBuCO2H)2(py)], by far the most similar to the Mn4CaO5 cluster in photosystem II, proved to have almost the same level of ingenuity of chemists as nature.5 Although a few oxygen-evolving complexes (OECs) based on earth-abundant metals such as Mn,6 Fe,7 Co,8 Ni,9 and Cu10 have been widely developed, the exploration of efficient and well-defined molecular catalysts for water oxidation as well as the underlying reaction mechanism remains relevant and challenging. Cobalt-based molecular WOC, a strong competitor to the noble-metal-based catalyst,11 was recognized long ago.12 Homogeneous cobalt-based WOCs13 are particularly appealing owing to their controllable electrochemical behaviors, accessible catalysis mechanism, and higher activities compared with the heterogeneous WOCs. Of interest from these are clusters containing a cubical Co4O4 core.14 Dismukes et al. comparably © XXXX American Chemical Society

studied the water oxidation performances driven by six cobalt clusters with four different CoxOy cores, and pointed out that the Co4O4 motif is the best one.15 Cobalt exhibits +2 and +3 valences and forms diverse clusters with a variety of ligands under different assembly conditions.16 However, most of the cobalt clusters have not been employed as WOCs for photocatalytic water oxidation. Thus, developing the pristine Co-based WOCs and investigating the real active species responsible for O2 evolution are incredibly important and meaningful in the expectation of the rational design of WOCs with new CoxOy core and the exploration of structure−property correlations. To date, the structural motifs found in active Co-based WOCs are CoIII4O4,17 CoII4O4,18 CoII3LnO4,19 CoII7O6,20 and Co16O16;21 however, a mixed-valent molecular cobalt cluster has rarely been reported as a WOC. The only example reported by Ding is an inorganic POM-sandwiched cluster. The comparison of catalysis activities between all-CoII and all-CoIII clusters sheds lights on the importance of the mixed-valent metal centers in the catalytic stability and efficiency.22 The mixed-valent polynuclear cobalt clusters could be promising WOCs with more rugged and efficient characteristics, because the similar stepwise oxidation of multimetal centers in the Kok cycle23 avoids the harsh oxidation process that leads to the Received: November 7, 2016

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DOI: 10.1021/acs.inorgchem.6b02698 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry decomposition of the WOC. Meanwhile, the pre-existing CoIII center saves an oxidation step, thermodynamically favoring the oxygen-evolving process. With these in mind, herein, we synthesized a new disc-like heptanuclear mixed-valent cobalt cluster, [CoII5CoIII2(mdea)4(N3)2(CH3CN)6(OH)2(H2O)2·4ClO4] (1, H2mdea = Nmethyldiethanolamine), and the crystal structure was determined by X-ray crystallography. Its structure resembles a small piece of cobaltate with six defective Co3O4 cubanes condensed together by face-sharing. It shows remarkable water oxidation activity under visible light irradiation. Importantly, certain active species and the evolutions of oxidation states of cobalt during the photocatalytic reactions were revealed by electrospray mass spectrometry (ESI-MS). The present work provides the design concept for efficient and stable WOCs and the mechanism-related issues such as the key active intermediate and the oxidation state evolution in the oxygen production process.

cluster possesses a disc-like structure comprising a central Co ion, linked to six peripheral Co ions. All seven Co ions are coplanar (Figure 2a). Alternatively, the disc-like core can also

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RESULTS AND DISCUSSION Synthesis. The most common oxidation states of the cobalt are +2 and +3. According to the standard potentials of the [Co(H2O)6]3+/[Co(H2O)6]2+ redox couple, cobaltIII provides the most oxidizing system (E0 = 1.84 V vs NHE, acidic solution). In the absence of strong multidentate chelating ligands that stabilize CoIII, oxidation of CoII is not very preferred, and CoIII is prone to be reduced by a trace of water.24 The initial synthetic challenge was the selection of a suitable ligand to stabilize cobalt(III). The mixed-valent cluster, 1, was synthesized via room-temperature solution reaction of CoII salt and multidentate chelating N-methyldiethanolamine ligand. Note that the screw-cap vial was not completely sealed, and thus, the CoII ions in the final cluster could be oxidized by the O2 in air. Structure of 1. Cluster 1 was isolated as olive-green, moisture-stable crystals. The crystallographic data and selected structural parameters for 1 are given in Tables S1 and S2, respectively. Single-crystal X-ray analyses revealed that 1 is a cationic cluster consisting of seven cobalt atoms totally in octahedral coordination environments. It crystallized in the monoclinic P21/n space group with a half of a cluster and two perchlorates in the asymmetric unit. As shown in Figure 1, the

Figure 2. (a) and (b) Core structure of CoII5CoIII2, mutually orthogonal views. For clarity, all labels as well as carbon and hydrogen atoms have been omitted. (Color legend: purple, CoII; green, CoIII; red, O; blue: N).

be seen as six defective cubanes fused together by face-sharing in a round fashion, each sharing two faces. The Co7 disc is held together by two μ3-OH−, two μ2-N3−, and two O atoms of mdea2− ligands. The Co3 is on the inversion center and exhibits an O6 environment (2 OH− and 4 mdea) in distorted octahedral geometry (Co3−O: 2.033(3)−2.132(4) Å), whereas the other three are coordinated by both N and O from ligand, anion, and solvent. The average bond lengths around Co4 are 1.94 Å shorter than those around Co1−Co3, indicating the higher oxidation state of Co4. Considering the charge neutrality, two of the seven cobalt ions should be +3, and the remaining are +2. Bond valence sum (BVS) calculations also supported this attribution (Co1−Co4: 2.440, 1.992, 2.094, and 3.558; Table S3). Each Co4 ion is in a tight-fitting hexacoordinated sphere, finished by two μ3-ηO3:η O2:ηN1 mdea2− ligands, which sit above and below the plane defined by 7 Co atoms (Figure 2b). In the disc, the Co−O−Co angles fall in the range 92.59(14)−108.30(18)°, and the adjacent CoII···CoII and CoII···CoIII distances are 3.169(2) - 3.185(4) and 3.096(7) - 3.127(6) Å. Peripheral ligation of the disc is furnished by μ2-η N1:ηN1 N3− with a Co−N−Co angle of 98.0(2)°. Water and CH3CN act as terminal ligands to finish the overall cluster. There are hydrogen bonds between water and perchlorate, and OH− and perchlorate and no typical supramolecular interactions are observed between adjacent clusters. To the best of our knowledge, there are several disc-like Co7 clusters reported based on a variety of ligands and different oxidation states (Co I I 7 , Co I I 6 Co I II , Co I I 4 Co I II 3 and

Figure 1. ORTEP representation (thermal ellipsoids set at 30% probability) of the Co7 disc inner core. (Color legend: purple, CoII; green, CoIII; red, O; black, C; blue: N.) B


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Inorganic Chemistry CoII3CoIII4),16c,25 however, CoII5CoIII2 cluster has been unknown. Electrochemical Properties of 1. Electrochemical characterization of the mixed-valent cobalt cluster 1 in DMF (N,N′dimethylformamide) displays an irreversible wave at around 0.95 V (vs NHE), which is attributed to CoII/CoIII in the cluster. The oxidation wave decreased slightly with the increase of the scan rate from 20 to 100 mV s−1 (Figure 3a). As a

evolution is quickly initiated after visible light irradiation for about 3 s, and the amount of O2 almost linearly increases during the first 20 s (Figure 4a). Changes in pH value have very

Figure 4. O2 evolution catalyzed by 1 at different (a) pH values, (b) [Ru(bpy)3]Cl2 concentrations, (c) K2S2O8 concentrations, and (d) catalyst concentrations. Conditions: 1.0 mM [Ru(bpy)3]Cl2 for parts a, c, and d; 0−1.0 mM [Ru(bpy)3]Cl2 for part b; 9 mM K2S2O8 for parts a, b, and d, 0−9 mM K2S2O8 for part c; 25 μM 1 for parts a−c, 2.5−25 μM 1 for part d; 0.2 M borate buffer, initial pH 9.0 for parts b−d, initial pH 7.4, 8.0, and 9.0 for part a, irradiated by a 450 nm LED light source.

little influence on the initial rate of the O2 evolution process, as reflected by the initial TOF: 1.86 s−1 at pH = 7.4, 1.75 s−1 at pH = 8.0, and 1.98 s−1 at pH = 9.0. Further studies revealed that the O2 evolution process was affected by the concentrations of [Ru(bpy)3]Cl2, K2S2O8, and 1 (Figure 4b−d). In general, more O2 was produced with the increased concentrations of [Ru(bpy)3]Cl2, K2S2O8, and 1. The results of control experiments indicated that the photosensitizer, sacrifice oxidant agent and catalyst 1 were all responsible for O2 production. Without of the any above components, no O2 was detected under otherwise identical conditions. Scaled-Up Photocatalytic O2 Production. The largest amount of produced O2 was determined by GC (gas chromatograph) analysis. We carried out a scale-up reaction in a Pyrex tube. In a typical experiment, 10.0 mL of 0.2 M borate buffer solution (initial pH 9.0) containing 25 μM catalyst, 1.0 mM [Ru(bpy)3]Cl2, and 9 mM K2S2O8 was irradiated by a 450 nm LED lamp. The produced O2 in the headspace was identified and analyzed by GC. Figure S6 shows the O2 evolution produced by 1 over the time. 1 is indeed active toward water oxidation, and the O2 generation gradually ceases after ca. 900 s. The largest amount of O2 generated after the photocatalysis reaction is 10.65 μmol, and a TON of 43 is obtained. On the basis of the amount of K2S2O8 used, the yield of O2 was 23.7%. When [Ru(bpy)3]Cl2 was replaced by [Ru(bpy)3](ClO4)2, a higher TON of 88 was obtained. Decrease of the concentration of 1 from 25 μM to 5 μM also causes a decrease in the total amount of O2 involved (from 22 to 10 μmol) The TON is, however, increased from 88 to 210 (Figure 5). Stability Tests of 1 in Photocatalysis Reaction. The identification of the real active species in the water oxidation

Figure 3. (a) Cyclic voltammograms (CVs) of 0.5 mM 1 in 0.1 M TBAPF6/DMF solution at a different scan rate. (b) Cyclic voltammograms (CVs) of 0.5 mM 1 in 0.1 M TBAPF6/DMF/H2O solution. Conditions: scan rate 20−100 mV s−1 for (a) and 100 mV s−1 for (b), GC working electrode, Ag/Ag+ reference electrode, and Pt wire counter electrode.

different amount of H2O was added into the DMF solution of 1, the anode current increased following the CoII/CoIII oxidation wave (Figure 3b). The anode current reaches a plateau with the addition of water from 200 to 600 μL. No noticeable increase was observed in the absence of 1; such increases in anode current are due to the catalytic H2O oxidation by 1. The electrochemical data of 1 in borate solution (0.5 mM; pH = 9) were shown in Figure S5, and an obvious current enhancement is also observed above the background. Photochemical O2 Production Performance Monitored by a Clark Electrode. Cyclic voltammograms (CV) of 0.5 mM 1 in 0.1 M TBAPF6/DMF/H2O solution suggest the potential ability of 1 as WOC. Then, the catalytic activity of 1 for water oxidation was evaluated under photochemical conditions. A 2.0 mL degassed borate buffer solution (initial pH = 7.4, 8.0 and 9.0) containing 25 μM 1, 1.0 mM [Ru(bpy)3]Cl2, and 9.0 mM K2S2O8 was irradiated with a 450 nm LED lamp. A Clark-type oxygen electrode was used to in situ monitor the amount of evolved O2 dissolved in solution. O2 C


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Inorganic Chemistry

of species that are not easily seen in the solid state, therefore revealing the mechanism.16d,28 Recently, the ESI-MS in combination with X-ray crystallography revealed an important assembly mechanism of a Mn19 brucite disc by our group.29 In that work, we were able to gather abundant information on assembly intermediates that were only found to be present during the reaction process. Although current studies include using ESI-MS to identify the molecule integrity during the photocatalysis reaction,19 there have been fewer studies involving the identification of the chemical state of WOCs by ESI-MS during the oxygen-evolving process.30 Cluster 1 is soluble in water; thus, the positive-mode ESI-MS was performed in its aqueous solution. As shown in Figure 7a,

Figure 5. Total amount of O2 evolved (black line) and TON (blue line) of 1 at different catalyst concentration from scaled up photocatalytic reaction. Conditions: 5−25 μM catalyst, 1.0 mM [Ru(bpy)3](ClO4)2, 9.0 mM K2S2O8, 0.2 M borate buffer (10.0 mL, initial pH 9.0), irradiated by a 450 nm LED light source.

reaction is very important though challengeable.13f In some cases, the water oxidation process proved to be catalyzed by CoO x formed by the release of Co ions form the precatalysts.13h,26 It is essential to rule out the Co ions leached during the photocatalytic process. The control experiments were conducted by using chelating ligand, i.e., 2,2′-bipyridine (bpy). When a trace of Co ions were leached in solution, bpy could immediately chelate at the Co center to inhibit the O2 evolution as reported for the [(TPA)CoIII(μ-OH)(μ-O2)CoIII(TPA)](ClO4)3 system.27 Figure 6 shows the O2 evolution

Figure 7. (a) Positive-mode ESI-MS of 1 dissolved in water. (b) Negative-mode ESI-MS of 1 in borate buffer solution after photocatalysis reaction. Inserted: Comparison of the experimental (red spectrum) and simulated (superimposed black spectrum) isotopic envelopes.

Figure 6. O2 evolution catalyzed by 1 in the presence of different amounts of bpy. Conditions: 1.0 mM [Ru(bpy)3]Cl2, 9 mM K2S2O8, 25 μM catalyst, 0−3 equiv bpy, 0.2 M borate buffer (initial pH 9.0), and irradiation by a 450 nm LED light source.

catalyzed by 1 in the presence of different amounts of bpy. The system retains high O2 evolution activity after 3 equiv of bpy is added, which suggests that Co ions are not the predominant active species in this system. We also tried to chelate possible Co ions in solution by stronger ethylenediaminetetraacetic acid (EDTA) as chelator (Figure S7), which also supported the above conclusion. Identification the Main Active Species and Evolution of the Oxidation State of Cobalt by ESI-MS. Understanding the mechanism of water oxidation is significant including identification of the genuine active species and observation of a change of the oxidation state of metal centers in a redox reaction. In recent years, electrospray ionization mass spectrometry (ESI-MS) has become a well-established technique widely used in organic and coordination chemistry fields regarding the recognition of the presence and time scales

we observed two grouped peaks in the m/z ranges of 500−580 and 720−900, corresponding to doubly charged Co5 species (1a−d) and singly charged Co4 species (1e−g), respectively. By matching the experimental and simulated isotope patterns, we could assign the formulas of 1a−d as [CoIIICo4II(mdea)(CH3CN)5(N3)7(H2O)8]2+, [CoIIICo4II(mdea)(CH3CN)5(N3)7(H2O)9]2+, [CoIIICo4II(mdea)(CH3CN)7(N3)5(OH)2(H2O)8]2+, and [CoIIICo4II(mdea)(CH3CN)7(N3)5(OH)2(H2O)9]2+. For four Co5 species (1a− d), they were composed of one CoIII and four CoII based on the charge consideration, whereas there were one CoIII and three CoII for Co4 species (1e−g). The detailed identifications for them were listed in Table 1. No species related to Co7 in parent 1 was detected, indicating cluster 1 is unstable in water and decomposed to some low nuclearity cobalt cluster species. D


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Inorganic Chemistry Table 1. Identification of the Key Species Found in ESI-MS Before and After Photocatalysis Reaction species

identification

sim

exp

1a 1b 1c 1d 1e 1f 1g 1h 1i

[CoIIICo4II(mdea)(CH3CN)5(N3)7(H2O)8]2+ [CoIIICo4II(mdea) (CH3CN)5(N3)7(H2O)9]2+ [CoIIICo4II(mdea)(CH3CN)7(N3)5(OH)2(H2O)8]2+ [CoIIICo4II(mdea) (CH3CN)7(N3)5(OH)2(H2O)9]2+ [CoIIICo3II(mdea)3(CH3CN)(N3)(H2O)6]+ [CoIIICo3II(mdea)3(CH3CN)(H2O)6(ClO4)]+ [CoIIICo3II(mdea)3(CH3CN)(H2O)7(ClO4)]+ [Co2IIICo2II(N3)3(OH)8(H2O)4]− [Co3IIICoII(N3)3(OH)9(H2O)4]−

527.513 536.518 543.533 552.538 778.068 835.008 853.018 569.824 586.827

527.530 536.534 543.539 552.543 778.075 835.015 853.025 569.819 586.816

five high spin Co2+ ions and two high spin Co3+ ions with orbital angular momentum L = 0 summed up (5 × 1.87 emu K/mol + 2 × 3.00 emu K/mol = 15.4 emu K/mol). A small difference between the measured and calculated product χT can be attributed to incomplete quenching of the orbital angular momentum L. Below 100 K a weak temperature dependence of the product χT for 1 is observed. There are two possible sources for the measured temperature variations: a weak magnetic interaction between the magnetic moments in the molecule or a zero-field splitting. However, as no local maximum or steep increase of the susceptibility was detected, we can conclude that the magnetic interaction, if any, is very small and is restricted only between intramolecular ions. A similar conclusion can be made by observing the measured isothermal magnetization at 2 K in Figure 9. The M(H) curve

These degraded species may be key active intermediates in the oxygen-evolving process. Moreover, we further investigated the solution species after photocatalysis reaction in borate buffer solution. As shown in Figure 7b, only two singly charged peaks (1h and 1i) centered at m/z = 569.819 and 586.816 were observed in the m/z range of 500−800, which were formulated to [Co 2 III Co 2 II (N 3 ) 3 (OH) 8 (H 2 O) 4 ] − and [Co 3 III Co II (N3)3(OH)9(H2O)4]−. After photocatalysis reaction, we found that the organic ligands were lost from the cobalt clusters, and simultaneously, more high valent Co centers formed as potential main active species participating in the oxygen-evolving process, which also coincided with the mechanism of the transformation of the precursor during the water oxidation process with Cp*IrIII complexes (Cp* = pentamethyl-cyclopentadienyl, C5Me5−).31 Although the parent cluster 1 is not stable in solution, the possible active species and valence evolution of Co atoms were disclosed by ESI-MS, which gave us better understanding on mechanism-related issues in the oxygen evolution process using such a higher level molecule as catalyst. Magnetism of 1. Magnetic properties of 1 were investigated using a Quantum Design MPMS3 SQUID magnetometer. All presented data were corrected for temperature independent diamagnetic contribution of inner shell electrons as obtained from Pascal’s tables.32 Magnetic susceptibility was measured between 2 and 300 K in a magnetic field of 1 kOe. The χ versus T and χT versus T curves are shown in Figure 8. In the 150−300 K range, the product χT is almost constant revealing a paramagnetic behavior of noninteracting magnetic moments. For 1 the measured product χT of 17.8 emu K/mol is similar to the theoretical value in the case of products χT for

Figure 9. Isothermal magnetization curves at 2 K.

of is not saturated even in the maximal magnetic field of 50 kOe. This might be an indication of a weak antiferromagnetic interaction between the cobalt ions in 1. No hysteresis was detected in 1. In order to estimate the interaction parameter J in 1 a Curie−Weiss fit χ = C/(T − θ) was applied for 1 in the high temperature range. The obtained Curie−Weiss temperature was θ = −13 K, and the interaction parameter J between the neighboring spin was estimated applying the result of molecular field approximation:33

J 3θ = kB zS(S + 1) Here, z is the number of the nearest neighbors, and kB is the Boltzmann constant. Taking into account S = 3 for Co2+ and z = 3, the interaction parameter J = −0.8 cm−1. The calculated interaction parameter J can be understood only as an upper

Figure 8. Effective magnetic moment (black line) and susceptibility (blue line) of 1. E


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Inorganic Chemistry

Faraday Discuss. 2012, 155, 357−376. (e) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial Photosynthesis for Solar Water-Splitting. Nat. Photonics 2012, 6, 511−518. (2) (a) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863−12001. (b) Zhang, T.; Lin, W. Metal-Organic Frameworks for Artificial Photosynthesis and Photocatalysis. Chem. Soc. Rev. 2014, 43, 5982−5993. (c) Du, P.; Eisenberg, R. Catalysts Made of Earth-Abundant Elements (Co, Ni, Fe) for Water Splitting: Recent Progress and Future Challenges. Energy Environ. Sci. 2012, 5, 6012−6021. (d) Karlsson, E. A.; Lee, B.L.; Åkermark, T.; Johnston, E. V.; Kärkäs, M. D.; Sun, J.; Hansson, O.; Bäckvall, J.-E.; Åkermark, B. Photosensitized Water Oxidation by Use of a Bioinspired Manganese Catalyst. Angew. Chem., Int. Ed. 2011, 50, 11715−11718. (3) (a) Zouni, A.; Witt, H.-T.; Kern, J.; Fromme, P.; Krauss, N.; Saenger, W.; Orth, P. Crystal Structure of Photosystem II from Synechococcus Elongatus at 3.8 Å Resolution. Nature 2001, 409, 739− 743. (b) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Architecture of the Photosynthetic Oxygen-Evolving Center. Science 2004, 303, 1831−1838. (c) Umena, Y.; Kawakami, K.; Shen, J.R.; Kamiya, N. Crystal Structure of Oxygen-Evolving Photosystem II at a Resolution of 1.9 Å. Nature 2011, 473, 55−60. (4) (a) Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Polyoxometalate Water Oxidation Catalysts and the Production of Green Fuel. Chem. Soc. Rev. 2012, 41, 7572−7589. (b) Sartorel, A.; Carraro, M.; Scorrano, G.; Zorzi, R. D.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. Polyoxometalate Embedding of a Tetraruthenium(IV)-oxo-core by Template-Directed Metalation of [γ-SiW10O36]8‑: A Totally Inorganic Oxygen-Evolving Catalyst. J. Am. Chem. Soc. 2008, 130, 5006−5007. (c) Geletii, Y. V.; Botar, B.; Kögerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. An All-Inorganic, Stable, and Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation. Angew. Chem., Int. Ed. 2008, 47, 3896−3899. (d) Al-Oweini, R.; Sartorel, A.; Bassil, B. S.; Natali, M.; Berardi, S.; Scandola, F.; Kortz, U.; Bonchio, M. Photocatalytic Water Oxidation by a Mixed-Valent MnIII3MnIVO3 Manganese Oxo Core that Mimics the Natural Oxygen-Evolving Center. Angew. Chem., Int. Ed. 2014, 53, 11182−11185. (e) Han, X.-B.; Li, Y.-G.; Zhang, Z.-M.; Tan, H.-Q.; Lu, Y.; Wang, E.-B. Polyoxometalate-Based Nickel Clusters as Visible Light-Driven Water Oxidation Catalysts. J. Am. Chem. Soc. 2015, 137, 5486−5493. (f) Wei, J.; Feng, Y.; Zhou, P.; Liu, Y.; Xu, J.; Xiang, R.; Ding, Y.; Zhao, C.; Fan, L.; Hu, C. A Bioinspired Molecular Polyoxometalate Catalyst with Two Cobalt(II) Oxide Cores for Photocatalytic Water Oxidation. ChemSusChem 2015, 8, 2630−2634. (g) Schwarz, B.; Forster, J.; Goetz, M. K.; Yücel, D.; Berger, C.; Jacob, T.; Streb, C. Visible-LightDriven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster. Angew. Chem., Int. Ed. 2016, 55, 6329−6333. (5) Zhang, C.; Chen, C.; Dong, H.; Shen, J.-R.; Dau, H.; Zhao, J. A Synthetic Mn4Ca-Cluster Mimicking the Oxygen-Evolving Center of Photosynthesis. Science 2015, 348, 690−693. (6) (a) Dismukes, G. C.; Brimblecombe, R.; Felton, G. A. N.; Pryadun, R. S.; Sheats, J. E.; Spiccia, L.; Swiegers, G. F. Development of Bioinspired Mn4O4-Cubane Water Oxidation Catalysts: Lessons from Photosynthesis. Acc. Chem. Res. 2009, 42, 1935−1943. (b) Najafpour, M. M.; Ehrenberg, T.; Wiechen, M.; Kurz, P. Calcium Manganese(III) Oxides (CaMn2O4·xH2O) as Biomimetic OxygenEvolving Catalysts. Angew. Chem., Int. Ed. 2010, 49, 2233−2237. (7) Wang, M.; Chen, L.; Sun, L. Recent Progress in Electrochemical Hydrogen Production with Earth-Abundant Metal Complexes as Catalysts. Energy Environ. Sci. 2012, 5, 6763−6778. (8) (a) Hu, X.; Brunschwig, B. S.; Peters, J. C. Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes. J. Am. Chem. Soc. 2007, 129, 8988−8998. (b) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42, 1995−2004.

limit for the interaction strength, as the zero-field splitting was completely ignored in our calculation.

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CONCLUSIONS In summary, we isolated and structurally characterized a new mixed-valent disc-like heptanuclear cobalt cluster. The photocatalytic activity of 1 toward water oxidation was evaluated in visible light irradiation reactions. Our results reveals that 1 is a competent molecular catalyst for O2 evolution from water under the photocatalytic conditions with a TON of 210 and a TOFinitial of 0.23 s−1. What’s more, electrospray mass spectrometry (ESI-MS) was used to (i) unmask the potential active species in water oxidation reaction and (ii) monitor the evolutions of oxidation states of cobalt during the photocatalytic reactions. The magnetic behaviors of 1 were also discussed in detail. These developments highlight the design concept of synthetic water oxidation catalysts and mechanismrelated issues such as the key active intermediate and oxidation state evolution in the oxygen evolution process using such a higher level molecule as catalyst.

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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.6b02698. Detailed synthesis procedure, crystallographic table, BVS calculation, and IR and TGA data (PDF) Crystallographic data comparable to CCDC file 1515492 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Di Sun: 0000-0001-5966-1207 Author Contributions §

J.-H.X., L.-Y.G., and H.-F.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 21571115 and 21201110), the Natural Science Foundation of Shandong Province (ZR2014BM027), Young Scholars Program of Shandong University (2015WLJH24), and the Fundamental Research Funds of Shandong University (104.205.2.5). We thank Prof. Zvonko Jagličić in University of Ljubljana for the magnetism analysis for 1.

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