Ring-Shaped Polyoxometalate Built by {Mn ₄ PW ₉ } and PO ₄ Units for Efficient Visible-Light-Driven Hydrogen Evolution

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Aug 2021Ring-Shaped Polyoxometalate Built by {Mn4PW9} and PO4 Units for Efficient Visible-Light-Driven Hydrogen Evolution Hai-Lou Li, Mo Zhang, Chen Lian, Zhong-Ling Lang, Hongjin Lv and Guo-Yu Yang Hai-Lou Li MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 , Mo Zhang MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 , Chen Lian MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 , Zhong-Ling Lang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Polyoxometalate Science, College of Chemistry, Northeast Normal University, Changchun, Jilin 130024 , Hongjin Lv *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 and Guo-Yu Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488 https://doi.org/10.31635/ccschem.020.202000403 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail A novel mixed-valence, ring-shaped multinuclear Mn-containing polyoxometalate, [H2N(CH3)2]15NaH8-[MnIII3MnIV(μ3-O)3(OAc)PO4(B-α-PW9O34)]4·36H2O ( 1) was made and systematically characterized using various spectroscopic and computational techniques. Its polyoxoanion can be described as a tetramer made of four [MnIII3MnIV(μ3-O)3(OAc)(B-α-PW9O34)]3− ({MnIII3MnIV(PW9)}) clusters and four PO4 linkers. Significantly, this structurally new complex 1 can efficiently catalyze hydrogen evolution with 23 μmol H2 gas after 12 h of visible-light irradiation using a three-component system. We propose a possible catalytic hydrogen-evolving mechanism based on both experimental results and density functional theory (DFT) calculations. Download figure Download PowerPoint Introduction Polyoxometalates (POMs) have attracted considerable research interest due to their structural diversity and potential applications in medicine, catalysis, materials science, photochemistry, molecular magnetism, and so forth.1–7 The versatile lacunary POM fragments generated by removing one or more skeletal MO6 groups can serve as multidentate O-donor inorganic ligands to construct mono- or multinuclear transition-metal-substituted polyoxometalates (TMSPs).8–15 Currently, the design and preparation of high-nuclearity TMSPs remain one of the most interesting topics in synthetic chemistry. To date, various high-nuclearity TMSPs have been made such as [H56Fe28P8W48O248]28−,16 [{Co4(OH)3PO4}4(PW9O34)4]28−,17 [Cu20Cl(OH)2(H2O)12(P8W48O184)]25−,18 [H6Ni20P4W34(OH)4O136(enMe)8(H2O)6]6−,19 [{Ni6(Tris)(en)3(BTC)1.5(PW9O34)}8]36−,1 and [Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4(GeW8O31)2]32−.11 Among categories of TMSPs, the assembly of multinuclear Mn-containing POMs (Mn-POMs) has continuously developed into one of the most intriguing research subjects considering their extraordinary performance in the field of molecular magnetism and more recently photocatalysis.8,20 Until now, a number of Mn-POMs using different vacant POM ligands are well documented; however, there are far fewer examples of high-nuclearity Mn-POMs bearing more than 10 manganese atoms. Representative examples of these structurally, magnetically, and catalytically interesting Mn-POMs include [MnIV2MnIII6MnII4(μ3-O)6(μ-OH)4(H2O)2(CO3)6(SiW6O26)2]18−,21 [MnIII13MnIIO12(PO4)4(PW9O34)4]31−,22 [{MnIII3MnIV4O4(OH)2(OH2)}2(W6O22)(H2W8O32)2(H4W13O46)2]26−,23 [MnIII10MnII6O6(OH)6(PO4)4(SiW9O34)4]28−, [MnIII4MnII12(OH)12PO4)4(SiW9O34)4]28−,24 [MnII19(OH)12(SiW10O37)6]28−,25 [MnIII20(OH)8(H2O)8(O3P(CH2)6PO3)6(A-PW9O34)8]44−,26 [(P8W48O184){(P2WIII14Mn4O60)(P2W15MnIII3O58)2}4]144−,27 and [{MnIV24MnIII12O28(H2O)23}2(W24O120)2]40−28 ( Supporting Information Table S1). Therefore, the construction of high-nuclearity Mn-POMs remains an interesting field to explore. In addition to the syntheses of new high-nuclearity Mn-POMs, studies on potential catalytic properties of Mn-POMs attract tremendous attention, especially in the field of catalytic renewable energy exploration.29 The Mn-POMs containing {Mn4O4} cubane moieties have been investigated as excellent candidates for photocatalysis, and they can potentially serve an essential role in the exploration of clean energy sources. Literature reports show that the {Mn4O4} cubane moiety may work as a structural model of the oxygen-evolving center, {Mn4O5Ca}, in the natural photosystem II; its atomic structure was solved by using an advanced crystallographic technique.29–32 However, in the case of high-nuclearity Mn-POMs, these reports have used Mn-POMs for catalyzing water oxidation reactions under chemical or photochemical conditions,33–36 but little has been reported for proton reduction to generate hydrogen fuel.37,38 Herein, we report a novel mixed-valence, ring-shaped multinuclear Mn-containing tetramer, [H2N(CH3)2]15NaH8[MnIII3MnIV(μ3-O)3(OAc)(PO4)(B-α-PW9O34)]4·36H2O ( 1), containing four {B-α-PW9O34}-stabilized {Mn4O4} cubane moieties connected by four PO4 linkers and explore its catalytic property for visible-light-driven hydrogen production. Results and Discussion Black strip crystal of 1 was hydrothermally made by combining Na9[A-PW9O34]·7H2O, Na2CO3, MnCl2·4H2O, [H2N(CH3)2]Cl, and KMnO4 in a molar ratio of 1.00∶3.97∶17.25∶20.88∶2.71. The addition of KMnO4 worked as a source of high-valence MnIV leading to the formation of 1 (experimental details are reported in Supporting Information; Figures S1–S4). Parallel experiments revealed the presence of [H2N(CH3)2]Cl during the synthetic process is vitally important for producing target cluster 1. The absence or replacement of [H2N(CH3)2]Cl with other ammonium salts (e.g., N(CH3)4Br, N(C4H9)4Br, etc.) does not yield the final product. Single-crystal X-ray diffraction shows that 1 crystallizes in the monoclinic space group C2/c ( Supporting Information Table S2). Its molecular structure contains one unique ring-shaped tetrameric polyoxoanion [MnIII3MnIV(μ3-O)3(OAc)(PO4)(B-α-PW9O34)]424− ( 1a), 1 Na+, 15[H2N(CH3)2]+, and 36H2O. Bond valence sum (BVS) calculations reveal all W and P atoms in 1a are in the +6 and +5 oxidation states ( Supporting Information Table S3), respectively.39 The BVS values for Mn1–Mn8 atoms are 3.801, 3.222, 3.142, 3.211, 3.146, 3.249, 3.779, and 3.179, respectively, suggesting that Mn2–Mn6 and Mn8 are in the +3 oxidation states, whereas Mn1 and Mn7 are in the +4 oxidation states. Moreover, the oxidation states of the Mn centers were also investigated by X-ray photoelectron spectroscopy (XPS), which shows the ratio of the integrated areas of MnIV and MnIII is approximately 1∶3 ( Supporting Information Figure S4). The XPS ratio is consistent with BVS calculations, and each 1a contains four MnIV and 12 MnIII centers.40 The structure of 1a (Figure 1b) has a ring shape with four {MnIII3MnIV(PW9)} subunits connected by four PO4 linkers (Figure 1a). Such connection with PO4 acting as pure inorganic μ2-bridging units is rare and distinctive from previous reports where the participation of inorganic PO4 groups usually stabilize the high nuclear cores of the structure.41–47 Particularly, the tetrahedral PO4 and [B-α-PW9O34]9− ({PW9}) fragments (Figure 2d) are generated in situ from the isomerization of the [A-α-PW9O34]9− precursor (Figure 2c) during hydrothermal reaction. Each {MnIII3MnIV(PW9)} subunit is composed of a typical [MnIII3MnIV(μ3-O)3(OAc)]6+ ({Mn4}) cubane cluster (Figure 2a) anchored to a trivacant Keggin {PW9} unit via six μ-O and one μ4-O atoms (Figure 1; Supporting Information Figures S5a and S5b). Figure 1 | (a) Combined polyhedral and ball-and-stick representation of four {MnIII3MnIV(μ3-O3)(OAc)(B-α-PW9O34)} subunits and four PO4 groups. (b) The polyoxoanion 1a. Color codes for polyhedral and atoms: WO6, red; PO4, purple; MnIIIO6, yellow; MnIVO6, orange; C, black. Download figure Download PowerPoint Figure 2 | (a) The {MnIII3MnIV(μ3-O3)(OAc)} cluster. (b) The ring of {MnIII3MnIV(μ3-O3)(OAc)(PO4)}4 cluster. (c) The trivacant Keggin [A-α-PW9O34]9– precursor. (d) The trivacant Keggin [B-α-PW9O34]9– unit in 1a. (e) The polyoxoanion 1a. A: –x, y, 0.5–z. Color codes for polyhedral and atoms: WO6, red; P/PO4, purple; MnIII, yellow; MnIV, orange; O, red; C, black. Download figure Download PowerPoint In each {Mn4} cubane, four Mn atoms exhibit six-coordinated octahedral geometry [MnIII−O: 1.883(11)−2.287(10) Å and MnIV−O: 1.896(9)−2.012(8) Å]; these Mn atoms occupy four vertexes of a simplified tetrahedron with MnIII−MnIII and MnIII−MnIV distances of 3.180−3.255 Å and 2.828−2.969 Å, respectively ( Supporting Information Figure S5c). Each PO4 group in 1a acts as an exclusively pure inorganic μ2-linker bridging two adjacent {MnIII3MnIV(PW9)} subunits (Figure 1 and 2b). To the best of our knowledge, this represents the first example of ring-shaped multinuclear Mn-POMs using pure inorganic PO4 linker in POM chemistry.48 Alternatively, the skeletal polyoxoanion 1a can be perceived as a unique [MnIII12MnIV4(μ3-O)12 (OAc)4P4O16]12+ ({Mn16P4}) ring surrounded by four evenly-distributed {PW9} fragments via 24 μ2-O and 4 μ4-O atoms (Figure 2e). In the {Mn16P4} ring, each PO4 group connects two {Mn4} cubanes via four O atoms (Figure 1b, 2b, 2e; Supporting Information Figures S6a and S6b), and the {Mn4} cubane is the node of the ring, which is additionally stabilized by the OAc– group apart from the coordinating O atoms from {PW9} and PO4 units. Interestingly, a 16-membered ring (dimensions: 7.8 × 7.8 Å2) is formed with 4 MnIV (Mn1, Mn7, Mn1A, and Mn7A), 4 P, and 8 O atoms from PO4 bridging linkers ( Supporting Information Figure S6c). Additionally, the packing of 1a viewed along the b-axis ( Supporting Information Figure S7a) can be simplified as a three-dimensional assembly ( Supporting Information Figure S7b), in which 1a is arranged in the –AAA– mode along the a-, b-, or c-axis. The visible-light-driven catalytic activity of 1 for hydrogen evolution was examined using a well-established three-component system; the amount of hydrogen produced was quantified using gas chromatography with a TCD (Thermal Conductivity Detector).49 Photolysis of a deaerated solution containing catalyst 1 (20 μmol/L), [Ir(ppy)2(dtbbpy)]+ (0.2 mmol/L), triethanolamine (TEOA) (0.25 mol/L), and H2O (2 mol/L) in 6 mL CH3CN/DMF (1∶3 v/v) using a blue light-emitting diode (LED) (λ = 450 nm, 45 mW/cm2) at 15 degC; efficiently produced hydrogen (Figure 3a); the concomitant reduction of catalyst 1 was observed as indicated by the color change of solution from yellow to green ( Supporting Information Figures S8 and S9). No such color change was detected in the absence of catalyst 1. In addition, cyclic voltammograms of the TBA+ salt of 1 in deaerated CH3CN/DMF (1∶3 v/v) solution showed quasireversible redox waves in the range of –1.5 to 1.5 V versus SCE. The positive domain contains the MnIV/III and MnIII/II redox peaks at potential of 1.14 and 0.78 V versus SCE, respectively. When further reduction to the negative domain, the CV shows the redox waves of W-based reductions ( Supporting Information Figure S10a). Addition of trifluoroacetic acid to the CV solution leads to substantial current enhancement, indicating the catalytic activity of reduced 1 for proton reduction ( Supporting Information Figure S10b). Upon exposure to 450 nm visible light, H2 evolution increases linearly with time, while no H2 is produced in the dark. After 12 h of irradiation, approximately 23 μmol H2 gas is obtained, corresponding to a turnover number (TON) of ∼192, which is the highest value, to the best of our knowledge, among known Mn-POMs catalyzed H2-evolving systems.37,38 Such interesting hydrogen-evolving performance encourages us to further explore the catalytic activity of 1 under various photolysis conditions. Control experiments show that the absence of any essential component (catalyst 1, TEOA, [Ir(ppy)2(dtbbpy)]+, or H2O) yields negligible H2 evolution (Figure 3a). Additional control experiments using the {PW9} fragment or 16 equivalents of MnCl2 as catalyst result in much lower H2 production (∼4 or 1 μmol after 12 h, respectively). Furthermore, we also varied the concentration of catalyst 1, [Ir(ppy)2(dtbbpy)]+, and TEOA under otherwise identical conditions. Both increasing the concentration of [Ir(ppy)2(dtbbpy)]+ (from 0.1 to 0.4 mmol/L) and TEOA (from 0.05 to 0.25 mol/L) led to increased hydrogen production ( Supporting Information Figures S11a and S11b). Notably, at a constant concentration of [Ir(ppy)2(dtbbpy)]+ and TEOA, the H2 yield first increased when varying catalyst 1 from 10 to 20 μmol/L, and then decreased upon increasing 1 to 30 μmol/L ( Supporting Information Figure S11c). Such phenomenon might be attributed to the shielding of photons absorbed by the [Ir(ppy)2(dtbbpy)]+ photosensitizer because catalyst 1 can absorb 450 nm light. Additionally, the control experiment "mercury poison" test has almost no effect on the photocatalytic performance ( Supporting Information Figure S12), indicating that the catalyst does not form nickel nanoparticles during the photocatalytic process. The stability of 1 during the reaction was further verified by comparing IR spectra of the isolated catalyst before and after photocatalysis ( Supporting Information Figure S13), and such good agreement between the spectra confirmed that the structure of 1 did not change during the catalytic reaction. Figure 3 | (a) Photocatalytic H2 evolution under different conditions. (b) The decay curves of [Ir(ppy)2(dtbbpy)]+ (0.2 mmol/L) (black curve), [Ir(ppy)2(dtbbpy)]+ (0.2 mmol/L) + 1(50 μmol/L) (green curve), and [Ir(ppy)2(dtbbpy)]+ (0.2 mmol/L) + TEOA (0.25 mol/L) (blue curve). The red curves are best fits from single exponential decay. (c and e) The emission spectra of [Ir(ppy)2(dtbbpy)]+ (0.2 mmol/L) as a function of added 1 and TEOA. (d and f) Stern–Volmer plots for emission quenching of [Ir(ppy)2(dtbbpy)]+* by 1 and TEOA; the calculated quenching rate constants are 3.1 × 1011 (mol/L)–1 s–1 and 1.0 × 107 (mol/L)–1 s–1, respectively. Download figure Download PowerPoint Previous studies reported that the excited state photosensitizer can either be reductively quenched by an electron donor or oxidatively quenched by an electron acceptor.50–52 In our system, both emission quenching and time-resolved luminescence decay techniques were utilized to characterize the quenching mechanism. Experimental results show that the quenching of [Ir(ppy)2(dtbbpy)]*+ luminescence can be accelerated by catalyst 1 and TEOA (Figures 3b, 3c, and 3e). The Stern–Volmer (SV) plot of the emission quenching data reveals an apparent rate constant of 3.1 × 1011 and 1.0 × 107 (mol/L)–1 s–1 for catalyst 1 and TEOA, respectively (Figures 3d and 3f). Exponential fitting of the [Ir(ppy)2(dtbbpy)]*+ luminescence decay kinetics for dye only or in the presence of catalyst 1 and TEOA yields lifetimes of approximately 96, 81, and 50 ns, respectively. These results indicate that both oxidative and reductive quenching of the excited state of [Ir(ppy)2(dtbbpy)]*+ exist in our catalytic systems, consistent with literature reports.53–55 Based on the above experimental information, the mechanism of this visible light-induced photocatalytic H2 evolution is proposed in Supporting Information Scheme S1. Density functional theory (DFT) calculations were carried out at B3LYP/PCM(H2O)/[6-31G(d,p)/LANL2DZ(Mn&W)] level to reveal the driving force of 1 toward hydrogen evolution reaction (HER).56–61 Given that {MnIII3MnIV(μ3-O)3(OAc)(B-α-PW9O34)} is the basic building block of the whole compound, herein we selected the [MnIII3MnIV(μ3-O)3(OAc)(PO4)2(B-α-PW9O34)]9– (Mn4O3; Supporting Information Figure S14) as the working model (Figure 4a). An S = 9/2 ground state was set in the calculation by consulting the previous experimental reports, and computational results allocated a α-spin density of 3.74, 3.68, and 3.85|e| on the three MnIII centers (Mn2/3/4 respectively as labeled in Supporting Information Figure S15), whereas the MnIV center (Mn1) has a 2.64|e| β-spin distribution.23 Notably, the top singly unoccupied MOs of the ground state are mostly Mn core orbitals ( Supporting Information Figure S16), suggesting that photo-reduction would preferably occur on these sites. As proposed by Hinnemann et al.,62 a good HER catalyst should be able to trap protons and also desorb H2, thus requiring the H adsorbed on the active site to have a ΔGH* close to zero. The molecular electrostatic potential (MEP; Supporting Information Figure S17) analysis for the model revealed an interesting distribution of the most negative partial atomic charges at μ3-O of the Mn4O3 core, indicating that protons are preferably captured on it. In contrast, release of OAc ligand from the Mn4O3 core is highly exothermic, resulting in H attack on the five-coordinated Mn to obtain the Mn-H intermediate. We hypothesized that the H2 evolution occurs on the full oxide state [MnIII3MnIV(μ3-O)3(PO4)2(B-α-PW9O34)]8–; however, this is largely limited on both μ3-O and Mn due to the extremely high H adsorption energy. To identify the real active state toward HER (Figure 4b), we computed the ΔEads of H on different reduced states of Mn4O3. The adsorption ability calculated is consistently more favorable on O than on Mn. Conversely, the strong adsorption on μ3-O based on 1e- and 2e-Mn4O3 makes the H desorption a challenge. When the model reaches three-electron reduction, the H adsorption on both Mn and μ3-O approaches zero respectively, giving a balance between desorption and adsorption. As such, the three-electron-reduced Mn4O3 would represent the real active center for the hydrogen evolution, which is consistent with both photolysis and cyclic voltammetry experiments ( Supporting Information Figures S8–S10). Particularly, the H was very likely adsorbing on the μ3-O initially and subsequently transferring to the neighboring Mn to form the active Mn-H. Figure 4 | (a) Computational model and representation of the possible H adsorption sites (S1 and S2). (b) The adsorption energies for H on μ3-O (H-S1) and Mn (H-S2) dependent on different reduced state of {MnIII3MnIV(μ3-O)3(OAc)(PO4)2(B-α-PW9O34)}9–. Download figure Download PowerPoint Conclusion A ring-shaped multinuclear Mn-substituted POM was made by a hydrothermal method. Four μ2-PO4 groups act as unique connectors to bridge four {MnIII3MnIV(PW9)} units, which is a rare occurance in POM chemistry and exhibits good photocatalytic hydrogen evolution reaction activity. This work not only enriches the family of multinuclear Mn-POMs but also paves the way for developing the potential application of Mn-POMs in the solar energy conversion. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflicts of interests. Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC; nos. 21831001, 21571016, 91122028, 20725101, and 21871025). References 1. Zheng S. T.; Zhang J.; Li X. X.; Fang W. H.; Yang G. Y.Cubic Polyoxometalate–Organic Molecular Cage.J. Am. Chem. Soc.2010, 132, 15102–15103. Google Scholar 2. Wang Y.-J.; Wu S.-Y.; Sun Y.-Q.; Li X.-X.; Zheng S.-T.Octahedron-Shaped Three-Shell Ln14-Substituted Polyoxotungstogermanates Encapsulating a W4O15 Cluster: Luminescence and Frequency Dependent Magnetic Properties.Chem. Commun.2019, 55, 2857–2860. Google Scholar 3. Ma P. T.; Hu F.; Wang J. P.; Niu J. 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