Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light

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Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light-Driven Hydrogen Evolution and Oxidative Dehydrogenation Wenlong Sun, Bowen An, Bo Qi, Tao Liu, Meng Jin, and Chunying Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00350 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light-Driven Hydrogen Evolution and Oxidative Dehydrogenation Wenlong Sun†, Bowen An†, Bo Qi†, Tao Liu†, Meng Jin† and Chunying Duan†,‡ * †Chemical School of Zhang Dayu State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian 116024, China ‡Collaborative Innovation Center of Chemical Science and Engineering Tianjin 300071, China

KEYWORD: polyoxometalate, metal organic framework, dehydrogenation, hydrogen evolution, photocataysis, electron transfer

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Abstract: Dehydrogenation of the tetrahydroisoquinoline derivatives coupled with hydrogen production is important for hydrogen-storage applications. Herein, we formulated a new system that embedded Dawson-type polyoxometalates as efficient photosensitizers into the pores of redox active coordination polymers for the light driven photocatalytic oxidative Mannich reaction and hydrogen evolution. In the designed Co-POM polymer, Uv light excitation gives the excited state of the Dawson-type polyoxometalate firstly to oxidize electron donors or substrates; the reduced formed (i.e., heteropolyblue) adsorbs visible light to achieve a new excited state, which reduced the cobalt redox sites and facilitates hydrogen evolution reaction. The photosensitizer recovered to the ground-state, completing the catalytic cycle. Under the optimized conditions, Co-POM enabled the hydrogen evolution and dehydrogenation of tetrahydroisoquinoline without the presence of any other additives. The high catalytic efficiency and robustness indicated the advantages of the combining functional polyoxometalate-based catalysts and porous characters of the coordination polymers for the development of highly active heterogeneous catalysts.


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INTRODUCTION The catalytic dehydrogenation of N-heterocycle coupled with H2 evolution are fundamentally important organic transformations.1-6 In particular, the dehydrogenation of tetrahydroisoquinoline and tetrahydroquinolines derivatives coupled with hydrogen production has attracted much research attention7-12 because of the importance of these compounds in development of oxidation and reduction coupling reactions as well as potential application as hydrogen storage materials.13-18 Recent researchers pioneered the homogeneous investigations with iridium and ruthenium complexes or the use of organic borane compounds as the oxidation and reduction catalysts, which leads to reversible dehydrogenation reactivity.19-20 In these studies, the use of relatively harsh conditions suggests that new highly efficient systems for reversible dehydrogenation under benign reaction conditions is highly demanded, especially for the systems utilizing light as a clean energy source without the assistant of noble metal complexes. Polyoxometalates are well known photosensitive and redox materials that have the potential to realize multi electron transfer reactivity as light driven hydrogen evolution reaction and water splitting catalysts.21-28 Imbedding of POMs in the pores of frameworks has reemerged as a promising approach to stabilize and heterogenize these redox active catalysts for photocatalytic hydrogen evolution and oxidative electron transfer reactions.29-31 This approach offers several advantages, including the mild synthetic conditions, a wide range of structural motifs and the easy handled heterogeneous manner of these photocatalytic transformation, comparing to literature results relying on the use of [Ru(bpy)3]2+ (bpy = 2,2-bipyridine) or [Ir(ppy)2(bpy)]+(ppy = 2-phenylpyridine) derivatives as photosensitizers.29-31


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Scheme 1. Diagram illustrating the electron-transfer pathways that combine both the photocatalytic oxidation and reduction in a one-pot transformation.

Bearing all these in our mind, we report herein the first example of polyoxometalate-based polymers that applied to accelerate both dehydrogenation based on tetrahydroisoquinoline coupling under mild reaction condition without the presence of any other additives. The molecular design included the incorporating of the Wells-Dawson P2W18O626- as a photosensitizer unit into the pores the well established redox active of cobalt organic polymers. We envisioned that the unique photoredox properties of P2W18O626- anion enabled the heteropolyacid to function as a catalyst to directly oxidize the substrates via ultraviolet excitation. The resulting heteropolyblue could be excited by visible light to reach a new excited state and further reduce the redox active cobalt sites (Scheme 1). The water coordinated cobalt centers with suitable redox potential reduce the proton for hydrogen evolution, completing the entire catalytic cycle. Most importantly, the imbedding of photosensitizer within the pores of redox


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active porous coordination polymers enforces the close proximity of the catalytic require units (Figure 1a), enable the new polymers to directly split tetrahydroisoquinoline into hydrogen and 3,4-dihydroisoquinoline under irradiation without the presence of any other additives. EXPERIMENTAL SECTION Materials. All solvents and chemical materials for syntheses were purchased from commercial sources and used as received without further purification. Instrumentation. The elemental analyses were of C, H and N conducted on a Vario EL III elemental analyzer, and that of W and Co were analyzed on a Jarrel-AshJ-A1100(ICP) atomic emission spectrometer. The X-ray powder diffraction (XRPD) patterns were recorded with a Bruker AXS D8 Advance diffractometer instrument with Cu Kα radiation (λ=1.54056 Å) in the angular range 2θ = 5–50° at 293K. Thermogravimetric analysis (TGA) was carried out at a ramp rate of 10℃/min in a nitrogen flow with a Mettler-Toledo TGA/SDTA851 instrument. Scanning electron microscopy (SEM) images were taken with a NOVA Nano SEM 450 microscope. The generated photoproduct of H2 was characterized by GC 7890T instrument analysis using a 5Å molecular sieve column (0.6m×3mm), thermal conductivity detector, and nitrogen used as carrier gas. the samples were irradiated by a 300 W Xenon Lamp (CEL-HXUV300). W element valence was analyzed on an XPS spectrum. IR spectra were recorded as KBr pellets on a NEXUS instrument. Solid UV-vis spectra were recorded on a U-4100 spectrometer. Liquid UV-vis spectra were performed on a TU-1900 spectrophotometer. Solid fluorescent spectra were measured on a JASCO FP-6500 instrument. ZAHNER ENNIUM electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with a modified carbon paste electrode (CPE) as a working electrode, a twisted platinum wire as counter electrode, and a commercial Ag/AgCl as a


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reference electrode. 1H NMR spectra were measured on a Varian INOVA 400/500 MHz spectrometer. Analysis of yield in the organic reaction head space was performed using Agilent 7820A gas-chromatography system equipped with a thermos conductivity detector with N2 as a carrier gas. Syntheses and Characterizations. 4,4'-(1,4-phenylene)bis-Pyridine (PBPY) was prepared according to the literature methods32 and characterized by 1HNMR.(400 MHz, CDCl3): δ= 8.71 (d, J = 6.0 Hz, 4H), 7.77 (s, 4H), 7.56 (d, J = 6.0 Hz, 4H). Synthesis of Co-POM: A mixture of K6[P2W18O62]·14H2O (90mg, 0.02mmol), PBPY (10 mg, 0.05mmol), Co(NO3)2·6H2O (50 mg, 0.17mmol), and oxalic acid (OX) (25 mg, 0.28mmol) were dissolved in 8mL mixed solvents of H2O in as crew-capped vial. the pH of the mixture was adjusted to about 3.1 with 1.0 mol/L NaOH, the suspension was put into a Teflon-lined autoclave and kept under autogenous pressure at 160°C for 3 days. After cooling the autoclave to room temperature, red block single crystals were separated, washed with water and air-dried. Yield: 50 % (based on the crystal dried in vacuum). Elemental analyses Calcd for C156H120N18O92P2 W18Co9 (7620.03): C, 24.56; H, 1.57; N, 3.31; P, 0.81;W, 43.46; Co, 6.96. Found: C, 24.45; H, 1.59; N, 3.39; P, 0.85; W,43.50; Co, 6.99. Synthesis of Zn-POM: The synthesis process of Co-POM was similar to that of Co-POM, except that Zn(NO3)2·H2O (50mg, 0.17mmol) instead of Co(NO3)2·6H2O. Yield: 50 % (based on the crystal dried in vacuum). Elemental analyses Calcd for C156H120N18O92P2W18Zn9 (7678.17): C, 24.38; H, 1.56; N, 3.28; P, 0.81; W, 43.13; Zn, 7.61. Found: C, 24.41; H, 1.59; N, 3.31; P, 0.86; W, 43.19; Zn, 7.59.


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Typical procedure for Aerobic Photooxidative Mannich Reaction. Photoinduced Mannich Reaction C-C bond form was made in a 15mL flask. Varying kinds of the Co/Zn-POM catalyst and N-Ph-1,2,3,4-tetrahydroisoquinoline derivative in 3:1 Me2CO/H2O were added to obtain a total volume of 5.5mL under air atmosphere. After that, the samples were irradiated by a 300W Xenon Lamp, the reaction temperature was 293K by using a water filter to absorb heat. The yield was calculated by integration of the characteristic GC peaks area of substrates and products. Typical Procedure of Photoreductive Hydrogen Production. Photoinduced hydrogen evolution was made in a 20mL flask. Varying kinds of the Co/Zn-POM catalyst and TEA in 3:1 Me2CO/H2O were added to obtain a total volume of 5.5mL. The flask was sealed with a septum, degassed by bubbling nitrogen for 20 min under atmospheric pressure at room temperature. After that, the samples were irradiated by a 300 W Xenon Lamp, the reaction temperature was 293 K by using a water filter to absorb heat. The generated photoproduct of H2 was characterized by GC 7890T instrument analysis using a 5Å molecular sieve column (0.6m×3mm), thermal conductivity detector, and nitrogen used as carrier gas. The amount of hydrogen generated was determined by the external standard method. Hydrogen in the resulting solution was not measured and the slight effect of the hydrogen gas generated on the pressure of the flask was neglected for calculation of the volume of hydrogen gas. Typical Procedure of Photocatalytic Oxidation and Reduction Coupling. Photoinduced hydrogen and 3,4-Dihydroisoquinoline evolution were made in a 20mL flask. Varying kinds of the Co/Zn-POM catalyst and 1,2,3,4-Tetrahydroisoquinoline in Me2CO or H2O/Me2CO (1:3 in volume) were added to obtain a total volume of 5.5mL. The flask was sealed with a septum, degassed by bubbling nitrogen for 20 min under atmospheric pressure at room temperature. After


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that, the samples were irradiated by a 300 W Xenon Lamp, the reaction temperature was 293 K by using a water filter to absorb heat. Table 1.Crystal data and structure refinements. Compound Formula

Co-POM C156H120N18O92P2W18Co9

Zn-POM C156H120N18O92P2W18Zn9

Formula weight

7620.03

7678.17

Crystal system

trigonal

trigonal

P6/m

P6/m

a/Å

18.794

18.834

c/Å

15.729

15.722

V/Å3

4811.2(8)

4830(2)

1

1

Dcalcd/g cm-3

2.626

2.636

T/K

293(2)

293(2)

µ/mm-1

11.575

11.875

Refl. Measured

25291

30949

Refl. Unique

3189

3188

Rint

0.1765

0.0576

GoF on F2

1.078

0.976

0.0644/0.2663

0.0666/0.2108

Space group

Z

R1/wR2 [I≥2σ(I)]

R1= ∑║Fo│─│Fc║/∑│Fo│. wR2= ∑[w(Fo2─Fc2)2]/∑[w(Fo2)2]1/2


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Crystallography. X-Ray intensity data were measured on a Bruker SMART APEX CCD-based diffractometer (Mo–Kα radiation, λ = 0.71073Å) using the SMART and SAINT programs. All the structures were solved by Direct Method of SHELXS-97 and refined by full-matrix leastsquares techniques using the SHELXL-97 program within WINGX. The hydrogen atoms of PBPY ligands were generated geometrically, while the hydrogen atoms of water molecules can not be found from the residual peaks and were directly included in the final molecular formula. The detailed crystallographic data and structure refinement parameters are summarized in Table 1. Crystallographic data for the structure reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC Number: 1587738 and 1587331. RESULTS AND DISCUSSION Synthesis and Characterization of Zn-POM and Co-POM. Heating a mixture of 4,4’(1,4-phenylene)bipyridine (PBPY), OX, cobalt nitrate/zinc nitrate and K6[P2W18O62]·14H2O afforded the POM-based coordinated polymers Co-POM and Zn-POM in an average yield of 50%. Single-crystal structural analysis revealed that Co-POM and Zn-POM are isomorphic, and both crystallized in the space group P6/m. The cobalt ions were coordinated in an octahedral geometry, where Co(1) is connected to four oxygen atoms from two OX molecules and two nitrogen donors from two PBPY ligands, Co(2) is coordinated by four oxygen atoms from a water molecule and two OX molecules and two nitrogen donors from two PBPY ligands (Figure 1a). These two types of cobalt ions are alternatively connected via corner sharing oxygen atoms from OX anions to form a two dimension sheet with Kagomé structure (Figure 1b). The adjacent sheets are further connected by rigid PBPY ligands via Co-N bonds to form a three-dimensional cationic pore (Figure1e and S4). Along the (0,1,0) axis, the 1D nanotubes have a diameter of 12.3 Å, and each of the isolated voids of the 1D nanotubes is occupied by the Dawson anions


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(Figure 1c and 1d), like coaches of a train. The redox active metal centers and photosensitizing [P2W18O62]6- centers are imbedded in close proximity, such that the potential photoinduced electron transfer from the photosensitizer to the redox active center were efficiently and directly, enable the new polymers possibly split tetrahydroisoquinoline into hydrogen and 3,4dihydroisoquinoline under without the presence of any other additives. In our multifunction coordinated polymers, the [P2W18O62]6- ions acted as photosensitive and photooxidation centers,21,33 and the Co-POM was designed with CoII as the metal node and pyridine-based PBPY-oxalate linkers to balance the charged of the [P2W18O62]6- anions (Figure 1a). The special coordination geometry of the Co(II) centers with open axial sites to allow H2O coordination provides redox activity and proton activation sites for the electrochemical hydrogen evolution from the water.34 The two dimensional Kagomé structure within the polymers consist of triangular and hexagonal structures (Figure S4), allowing the electronic delocalization and migration among the a and b direction of the cell through the entire sheets via the Co-O coordination bonds, as cobalt oxide layers have been highlighted in the interest of superconductivity.35-37 The strong magnetic exchanges with the short metal-metal distances and favorable angles (Figure 1b) confirmed the strong electronic coupling of the cobalt centers.38-41 The bipyridine ligands and cobalt metals connected via Co-N coordination bonds with d-π interactions leading the pores can also provide electron transport on the c direction (Figure 1e).34,42 and the PLATON program was used to calculate the void volume approximately 210Å3. We envisioned that the polyoxometalate embedding polymers could exhibit efficient surface electron transport along the axial and lateral directions to facilitate multi electron injection into the coordinated polymers, benefiting the photocatalytic hydrogen production and oxidation reactions on the surface or inside the pores of the new material (Figures 1e and 1f).


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Figure 1. (a) Diagram of the coordination environment of the [P2W18O62]6- cluster and Co2+, showing the coordination geometry of the two types of cobalt ions. (b) view of the twodimensional layer structure. (c) Side view of the one-dimensional pore structure (nanotube) from (e) formed by the ligands with embedded [P2W18O62]6- units in the interior. (d) Size perspective of the assembled structure, showing the constitutes and fragments, i.e., [P2W18O62]6-, OX, CoII, and PBPY, of the coordinated polymers. (e) and (f) Face view of the three dimensional structure, showing the embedded [P2W18O62]6- units within the pores along the different direction.


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Figure 2. (a) Fluorescence quenching spectra of Co-POM upon the addition of N-phenyl-tetrahydroisoquinoline and corresponding simulated Stern-Volmer curve. (b) Solid-state cyclic voltammograms of Co-POM (red) and Zn-POM (black) with a scan rate of 70 mVs−1 in the scan range of 0.4 to -0.7V. (c) X-ray photoelectron spectroscopy spectra of W4f in polyoxometalate before irradiation (black line) and hetero-polyblue after irradiation (red line). (d) Absorption curve of heteropolyblue under UV irradiation; irradiation times: 0, 3, 5, 7, and 10 min, respectively.

Physical Characterization Data. Solid-state UV-vis absorption spectra of both Zn-POM and Co-POM exhibited a polyoxometalate based absorption band centered at 350 nm and an additional visible metal-to-ligand charge transfer absorption band at 500 nm (Figure S7). Both Zn-POM and Co-POM showed an emission band at approximately 409 nm (Figure S8). In this case, the free energy change (E0−0) between the ground state and the vibrationally related excited


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state of ca. 3.10eV (Figure S9), such a high excitation energy was able to create an active excited state for the photooxidation of substrates or electron donors. Solid-state electrochemical measurements of Co-POM and Zn-POM exhibited four quasi-reversible redox peaks belong to [P2W18O62]6-; the first two redox couples (I and II) correspond to one-electron redox processes that can induce oxidative reactions, and the last two redox couples (III and IV) correspond to two-electrons redox processes (Figure 2b).29 Meanwhile, the Co-POM containing exhibited another broad peak at approximately −1.02 V with a shoulder at approximately −0.98 V, corresponding to the CoII/CoI and CoIII/CoII couples.43 (Figure S12). These potentials possess well range of the potential for proton reduction in aqueous media, indicating that Co-POM in its reduced state is able to reduce protons under electrochemical conditions. Table 2. Photocatalytic Oxidative Mannich Reaction of Co-POM and Zn-POM under light irradiation.[a]

[a] Reaction conditions: Catalyst (1.5 µmol, 0.5 mol% based on the molecule cage unit), Lproline (0.03 mmol, 10 mol%), H2O/acetone solution (1:3), [b] Conversions were determined by GC areas, with that of the Zn-POM listed in the brackets, under irradiation 300W xenon lamp for 6 hours.


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Photooxidative Mannich Reaction. The luminescence of both two polymers was considerably quenched by N-phenyl-tetrahydroisoquinoline in MeCN in an Ar atmosphere (Figures 2a and S10), suggesting electron transfer from the substrate to the excited state of embedded [P2W18O62]6-. Therefore, these Co/Zn-POM are promising candidates for the photocatalytic transformation of N-phenyl-tetrahydroisoquinoline, i.e., the Mannich reaction that involves the oxidation of α-amino C−H bonds to generate reactive iminium ions and subsequent C−C bond formation between the iminium ions and a carbon nucleophile.44-48 The mannich reaction was initially examined using N-phenyl-tetrahydroisoquinoline and acetone as coupling partners, along with Co/Zn-POM under λ =350-780nm irradiation (Table 2). Clearly, an excellent yield of 90-99% (entry 1-5, turnover frequency ca 3300 umol g-1 h-1) was achieved after 6 h of irradiation. Kinetics curve of photocatalytic the mannich reactions has been followed in the Figures S13. Control experiments for the C-C coupling reaction of the substrate with acetone were investigated and are summarized in Table S1. Nearly no conversion was observed when the reaction was conducted in the dark or under light with a wavelength λ = 420-780 nm, the photooxidative reaction is attributed to the ultra visible light irradiation. The successful C-C coupling catalyzed by Co-POM only under Uv irradiation suggested that P2W18O626- functioned as a photooxidation catalyst. The excited photocatalytic P2W18O626- unit first oxidizes the tertiary amine into a radical cation species. Then, the reduced [P2W18O62]6- unit promotes the reduction of O2 to O2•−,45 and the radical cation species is deprotonated by O2•− to form a highly reactive iminium ion,45 which enters the organic catalytic cycle and reacts with enamine nucleophile generated by L-proline activated acetone to give the desired cross-coupling product. To better verify the surface catalyst structure, scanning electron microscopy (SEM) measurements were performed. The SEM images of the initial Co-POM displayed particle sizes


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of approximately 0.15-0.17 mm, which showed a catalytic yield of 72% after 6 hours. Fine crystalline materials with sizes of 0.19~0.37µm, achieved by grinding the Co-POM crystals, displayed a catalytic yield of 98% after 6 hours (Figure S17). These results demonstrated that the surface area is a crucial factor for high yield. The Zn-POM catalyst exhibited similar catalytic activities and gave the same products with conversions greater than 90% under the same experimental conditions. Photoreductive Hydrogen Production. As a proof-of-concept study, we next investigated the photochemical proton reduction reaction, as the CoII/CoI and CoIII/CoII redox processes have potentials that allow for the electrochemical reduction of protons to hydrogen.43 The hydrogen evolution experiments were designed using the Co-POM as the photocatalyst to utilize its redox catalytic sites. For a H2O/ Me2CO (1:3 by volume) solution containing Co-POM (0.3µmol) and triethylamine (NEt3, TEA) (9%v:v), light irradiation lead to considerable hydrogen evolution after 21h. Control experiments revealed that changing any of these individual components in the reaction resulted in little or no hydrogen production (Table S2). When Zn-POM or K6P2W18O62 was used as a photocatalyst, no hydrogen was detected under the same experimental conditions. A mixture of K6P2W18O62 and Co(NO3)2 at the same concentration in the solution produced only a very low quantum yield of hydrogen compared to the aforementioned cobalt-based system. Most importantly, light irradiation transformed the clear reaction solution into a blue suspension, and a visible absorption band at 650 nm increased with time under UV irradiation (Figure 2d). The WVI centers on the heterpolyoxometalate anions are postulated to transform into WV, forming a heteropolyblue species33 (Figures 2c, S19 and S20). Co-POM can also produce hydrogen under visible light after UV excitation, and further experiments revealed that hydrogen evolution with Co-POM under light with a wavelength in the 350-780 nm region (turnover


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number = 700) was more efficient than under the 350-400nm region (turnover number = little) (Table S2). In addition, the hetero-polyblue visible absorption reduced under O2 exposure, and the blue suspension solution converted back to a clear solution (Figure S21). Zn-POM catalyst cannot be used for light-driven hydrogen, as this material does not contain the proton reduction catalyst. These experiments and results reveal that the POM excited state first receives electrons from the NEt3 electron donor to form the heteropolyblue species. The latter absorbs photons to reach its excited state and undergoes photoinduced electron transfer from the new excited state to the cobalt centers. Water molecule coordination to the cobalt centers and the formation of lowvalence cobalt species further reduce protons to yield hydrogen, whereas the heteropolyblue species converts back to heterpolyoxometalate to complete the overall catalytic cycle. The loading about 3mg catalyst was able to produce 9 mL of hydrogen gas after irradiation for 65 h (turnover frequency ca 2000 umol g-1 h-1), the calculated turnover number of the entire reaction thus was approximately 1300 per mole of Co-POM catalyst (Figure 3a). While the turnover number is not as large as those of the normal homogeneous systems that contain redox active sites and a large excess of photosensitizer, the use of the cobalt-based heterogeneous catalyst as both the photosensitizer and redox active sites is significant, as this material allows two photon excitation to complete the catalytic cycle: a UV photon to excite the heteroPOM to start the photooxidation reaction with the electron donor, and a visible photon for to excite the heteropolyblue to reduce the cobalt ions and the directly recover the heteroPOM. Photocatalytic Oxidation and Reduction Coupling. The above photooxidative Mannich reaction and photoinduced hydrogen production reveal that [P2W18O62]6- can act as a promising photosensitizer and oxidation catalyst and that the cobalt centers exhibit suitable redox potentials


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Figure 3. (a) Time-dependent evolved hydrogen volume using Co-POM (0.15umol or 0.3umol) with TEA as a sacrificial electron donor. (b) Calculated turnover number of Co-POM in the catalytic dehydrogenation of tetrahydroisoquinoline in different H2O/Me2CO (1:3 by volume) or acetone solvent. Time dependence curve of hydrogen evolution in the photocatalytic system at different concentrations of tetrahydroisoquinoline (c), the symbols used to denote are: 3mmol (light blue ■), 2mmol (pink ●), 1mmol (blue▲) under the catalyst 0.1umol Co-POM and (green ★) for 0.3umol Zn-POM, and with different concentration of catalysts (d), the symbols used to denote are: Co-POM of 0.3umol (light blue ■), 0.2umol (pink●), 0.1umol (blue▲), Zn-POM o.3umol (green ★) under the 3mmol tetrahydroisoquinoline. Condition: H2O/Me2CO (1:3 by volume 5.5ml) at 25°C, 300W Xenon Lamp. Conversions were determined by GC areas.


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for electrochemical proton reduction in these polymers. Co-POM is a promising candidate for photocatalytic oxidation and reduction coupling. Fortunately, we successfully achieved the MOF-based heterogeneous catalyst capable of accelerating photocatalytic dehydrogenation involving tetrahydroisoquinoline oxidation and reduction coupling under mild condition. For a solution containing Co-POM (1mg, 0.1 µmol based on a single molecular cage unit) and tetrahydroisoquinoline (3 mmol) in H2O/Me2CO (1:3 by volume) at 25°C, 3,4-dihydroisoquinoline 1 (turnover number ca 640 and turnover frequency ca 920 umol g-1 h-1) and hydrogen gas (turnover number ca 630 and turnover frequency ca 920 umol g-1 h-1) were obtained under 68 hours light irradiation without the presence of any other additives (Figure 3b). The almost same turnover number of the two products demonstrated the two products come from the same origins with high selectivity of transformation. We compare with the relative studies29,30,45. These similar MOF photocatalytic properties only research the half of oxidation or reduction reaction. However, we designed a multifunctional molecular material could photocatalytic oxidation and reduction Coupling come true. Although several examples of photocatalysts and noble metal catalysts have been reported to promote dehydrogenation under homogeneous conditions.14,15 Co-POM represents the first example of a heterogeneous bifunctional catalyst that merges a cobalt redox active catalyst for hydrogen evolution and polyoxometallate catalyst for the substrate oxidation within a single material. Control experimental results suggested that water is the necessary unit for the photocatalytic transformation, as the absence of water lead to the significant decrease of the conversion. Control experiments also revealed that nearly no conversion was observed when the reaction was performed with Zn-POM (Table S3 and Figure 3c, 3d). The almost linear increase of the hydrogen evolution with the concentration of the CoPOM and the substrate in the time dependence curves of hydrogen evolution in the


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photocatalytic system suggested that the photocatalytic systems were quite stable. As the dehydrogenation of tetrahydroisoquinoline is important organic catalytic transformation, and its combination with hydrogen production avoids the use of stoichiometric oxidants and displays potential application for hydrogen storage. CONCLUSIONS In summary, we designed a simple and effective strategy to prepare a multifunctional molecular material via embedding a polyoxometalate anions into a redox active coordinated polymers. The new material allows the Uv excitation for the substrate oxidation and visible light excitation for the electron donating. The promising photocatalytic behavior combined with the presence of water coordinated redox active metal centers enable Co-POM to catalyze a coupled cross dehydrogenative and hydrogen production reaction successfully without the presence of any other additives. The modular nature of this synthetic approach should allow for further fine tuning and optimization leading to highly active heterogeneous catalysts in solar energy utilization.


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxx. 1

H-NMR, GC-MS, GC and catalysis details (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions W. L. Sun and C. Y. Duan conceived and designed the experiments. C. Y. Duan contributed materials and analysis tools. W. L. Sun and C. Y. Duan co-wrote the paper. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (21025102 and 21273027).


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REFERENCES (1) Zhang, G. T.; Liu, C.; Yi, H.; Meng, Q. Y.; Bian, C. L.; Chen, H.; Jian, J. X.; Wu, L. Z.; Lei, A. W. External Oxidant-Free Oxidative Cross-Coupling: A Photoredox Cobalt-Catalyzed Aromatic C−H Thiolation for Constructing C−S Bonds. J. Am. Chem. Soc. 2015, 137, 92739280 (2) Maier, A. F. G.; Tussing, S.; Schneider, T.; Flçrke, U.; Qu, Z. W.; Grimme, S.; Paradies, J. Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles. Angew. Chem. Int. Ed. 2016, 55, 12219-12223 (3) Zhu, Q. L.; Xu, Q. Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage. Energy Environ. Sci. 2015, 8, 478-512. (4) Yi, H.; Niu, L. B.; Song, C. L.; Li, Y. Y.; Dou, B. W.; Singh, A. K.; Lei A. W. Photocatalytic Dehydrogenative Cross-Coupling of Alkenes with Alcohols or Azoles without External Oxidant. Angew. Chem. Int. Ed. 2017, 56,1120-1124. (5) Gao, X. W.; Meng, Q. Y.; Li, J. X.; Zhong, J. J.; Lei, T. Li, X. B.; Tung, C. H.; Wu, L. Z. Visible Light Catalysis Assisted Site-Specific Functionalization of Amino Acid Derivatives by C−H Bond Activation without Oxidant: Cross-Coupling Hydrogen Evolution Reaction. ACS Catal. 2015, 5, 2391-2396. (6) Meng, Q. Y.; Zhong, J. J.; Liu, Q.; Gao, X. W.; Zhang, H. H.; Lei, T.; Li, Z. J.; Feng, K.; Chen, B.; Tung, C. H.; Wu, L. Z. A Cascade Cross-Coupling Hydrogen Evolution Reaction by Visible Light Catalysis. J. Am. Chem. Soc. 2013, 135, 19052-19055. (7) Moromi, S. K.; Siddiki, S. M. A. H.; K. Kon, Toyao, T.; Shimizu, K. I. Acceptorless dehydrogenation of N-heterocycles by supported Pt catalysts. Catal. Today, 2017, 281, 507-


21


Page 22 of 28

511. (8) Wu, J. J.; Talwar, D.; Johnston, S.; Yan, M.; Xiao, J. L. Acceptorless Dehydrogenation of Nitrogen Heterocycles with a Versatile Iridium Catalyst. Angew. Chem. Int. Ed. 2013, 52, 6983-6987. (9) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N‑Heterocycles. J. Am. Chem. Soc. 2014, 136, 8564-8567. (10) Xu, R. B.; Chakraborty, S.; Yuan, H. M.; Jones, W. D. Acceptorless, Reversible Dehydrogenation and Hydrogenation of N‑Heterocycles with a Cobalt Pincer Catalyst. ACS Catal. 2015, 5, 6350-6354. (11) Manas, M. G.; Sharninghausen, L. S.; Lin, E.; Crabtree R. H. Iridium catalyzed reversible dehydrogenation-Hydrogenation of quinoline derivatives under mild conditions. Journal of Organomet. Chem. 2015, 792, 184-189. (12) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. I. Homogeneous Catalytic System for Reversible Dehydrogenation Hydrogenation Reactions of Nitrogen Heterocycles with Reversible Interconversion of Catalytic Species. J. Am. Chem. Soc. 2009, 131, 8410-8412. (13) Dobereiner, G. E.; Nova, A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree, R. H. Iridium-Catalyzed Hydrogenation of N-Heterocyclic Compounds under Mild Conditions by an Outer-Sphere Pathway. J. Am. Chem. Soc. 2011, 133, 7547-7562. (14) Qin, Y.; Oestreich, M. Photocatalysis Enabling Acceptorless Dehydrogenation of Benzofused Saturated Rings at Room Temperature. Angew. Chem. Int. Ed. 2017, 56, 77167718.


22

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) He, K. H.; Tan, F. F.; Zhou, C. Z.; Zhou, G. J.; Yang, X. L.; Li, Y. Acceptorless Dehydrogenation of N-Heterocycles by Merging Visible-Light Photoredox Catalysis and Cobalt Catalysis. Angew. Chem. Int. Ed. 2017, 56, 3080-3084. (16) Dobereiner, G. E.; Nova, A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree, R. H. Iridium-Catalyzed Hydrogenation of N-Heterocyclic Compounds under Mild Conditions by an Outer-Sphere Pathway. J. Am. Chem. Soc. 2011, 133, 7547-7562. (17) Wang, D. W.; Wang, X. B.; Wang, D. S.; Lu, S. M.; Zhou, Y. G.; Li, Y. X. Highly Enantioselective

Iridium-Catalyzed

Hydrogenation

of

2-Benzylquinolines

and

2-

Functionalized and 2,3-Disubstituted Quinolines. J. Org. Chem. 2009, 74, 2780-2787. (18) Wu, J. J.; Barnard, J. H.; Zhang, Y.; Talwar, D.; Robertson, C. M.; Xiao, J. L. Robust cyclometallated Ir(III) catalysts for the homogeneous hydrogenation of N-heterocycles under mild conditions. Chem. Commun. 2013, 49, 7052-7054. (19) Fujita, K. I.; Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. Homogeneous Perdehydrogenation and Perhydrogenation of Fused Bicyclic N‑Heterocycles Catalyzed by Iridium Complexes Bearing a Functional Bipyridonate Ligand. J. Am. Chem. Soc. 2014, 136, 4829-4832. (20) Kojima,

M.;

Kanai

M.

Tris(pentafluorophenyl)borane-Catalyzed

Acceptorless

Dehydrogenation of N-Heterocycles. Angew. Chem. Int. Ed. 2016, 128, 12412-12415. (21) Wang, S. S.; Yang, G. Y. Recent Advances in Polyoxometalate-Catalyzed Reactions. Chem. Rev. 2015, 115, 4893-4962. (22) Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science, 2010, 328, 342-345.


23


Page 24 of 28

(23) Han, X. B.; Zhang, Z. M.; Zhang, T.; Li, Y. G.; Lin, W. B.; You, W. S.; Su, Z. M.; Wang, E. B. Polyoxometalate-Based Cobalt−Phosphate Molecular Catalysts for Visible Light-Driven Water Oxidation. J. Am. Chem. Soc. 2014, 136, 5359-5366. (24) Zhang, Z. Y.; Lin, Q. P.; Kurunthu, D.; Wu, T.; Zuo, F.; Zheng, S. T.; Bardeen, C. J.; Bu, X. H.; Feng, P. Y. Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K10[Nb2O2(H2O)2][SiNb12O40].12H2O. J. Am. Chem. Soc. 2011, 133, 69346937. (25) Li, S. J.; Liu, S. M.; Liu, S. X.; Liu, Y. W.; Tang, Q.; Shi, Z.; Ouyang, S. X.; Ye, J. H. {Ta12}/{Ta16} Cluster-Containing Polytantalotungstates with Remarkable Photocatalytic H2 Evolution Activity. J. Am. Chem. Soc. 2012, 134, 19716-19721. (26) 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. (27) Song, F. Y.; Ding, Y.; Ma, B. C.; Wang, C. M.; Wang, Q.; Du, X. Q.; Fu, S.; Song, J. K7[CoIIICoII(H2O)W11O39]: a molecular mixed-valence Keggin polyoxometalate catalyst of high stability and efficiency for visible light-driven water oxidation. Energy Environ. Sci. 2013, 6, 1170-1184. (28) Santoni, M. P.; Ganga, G. L.; Nardo, V. M.; Natali, M.; Puntoriero, F.; Scandola, F.; Campagna, S. The Use of a Vanadium Species As a Catalyst in Photoinduced Water Oxidation. J. Am. Chem. Soc. 2014, 136, 8189-8192. (29) Zhang, Z. M.; Zhang, T.; Wang, C.; Lin, Z. K.; Long, L. S.; Lin, W. B. Photosensitizing


24

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Metal−Organic Framework Enabling Visible-Light-Driven Proton Reduction by a Wells−Dawson-Type Polyoxometalate. J. Am. Chem. Soc. 2015, 137, 3197-3200. (30) Kong, X. J.; Lin, Z. K.; Zhang, Z. M.; Zhang, T.; Lin, W. B. Hierarchical Integration of Photosensitizing Metal–Organic Frameworks and Nickel-Containing Polyoxometalates for Efficient Visible-Light-Driven Hydrogen Evolution. Angew. Chem. Int. Ed. 2016, 55, 64116416. (31) Tian, J.; Xu, Z. Y.; Zhang, D. W.; Wang, H.; Xie, S. H.; Xu, D. W.; Ren, Y. H.; Wang, H.; Liu, Y.; Li, Z. T. Supramolecular metal-organic frameworks that display high homogeneous and heterogeneous photocatalytic activity for H2 production. Nat. Commun. 2016, 7, 1158011588. (32) Gu, Z. G.; Grosjean, S.; Bräse, S.; Wöll, C.; Heinke, L. Enantioselective adsorption in homochiral metal–organic frameworks: the pore size influence. Chem. Commun. 2015, 51, 8998-9001. (33) Fu, N.; Lu, G. X. Graft of lacunary Wells–Dawson heteropoly blue on the surface of TiO2 and its photocatalytic activity under visible light. Chem. Commun. 2009, 3591-3593. (34) Artero, V.; Chavarot-Kerlidou, M.; Fontecave, M. Splitting Water with Cobalt. Angew. Chem. Int. Ed. 2011, 50, 7238-7266. (35) Takada, K.; Sakurai, H.; Takayama-Muromachi, E.; Izumi, F.; Dilanian, R.; Sasaki, T. Superconductivity in two-dimensional CoO2 layers. Nature 2003, 422, 53-55. (36) Badding, J. V. Superconducting materials: Cobalt oxide layers. Nat. Mater. 2003, 2, 208210. (37) Schaak, R. E.; Klimczuk, T.; Foo, F. L.; Cava, R. J. Superconductivity phase diagram of NaxCoO2·1.3H2O. Nature 2003, 424, 527-529.


25


Page 26 of 28

(38) Zhang, X. M.; Zhang, X. H.; Wu, H. S.; Tong, M. L.; Ng, S. W. Hybrid Cobalt Hydroxyoxalate Material Containing 3D Co-O-Co Connectivity and Showing Ferrimagnetic Ordering. Inorg. Chem. 2008, 47, 7462-7464. (39) Coronado, E.; Martí-Gastaldo, C.; Galán-Mascarós, J. R.; Cavallini, M. Polymetallic Oxalate-Based 2D Magnets: Soluble Molecular Precursors for the Nanostructuration of Magnetic Oxides. J. Am. Chem. Soc. 2010, 132, 5456-5468. (40) Salah, M. B.; Vilminot, S.; Andre, G.; Richard-Plouet, M.; Mhiri, T.; Takagi, S.; Kurmoo, M. Nuclear and Magnetic Structures and Magnetic Properties of the Layered Cobalt Hydroxysulfate Co5(OH)6(SO4)2(H2O)4 and Its Deuterated Analogue, Co5(OD)6(SO4)2 (D2O)4. J. Am. Chem. Soc. 2006, 128, 7972-7981. (41) Rujiwatra, A.; Kepert, C. J.; Claridge, J. B.; Rosseinsky, M. J.; Kumagai, H.; Kurmoo, M. Layered Cobalt Hydroxysulfates with Both Rigid and Flexible Organic Pillars: Synthesis, Structure, Porosity, and Cooperative Magnetism. J. Am. Chem. Soc. 2001, 123, 10584-10594. (42) Sun, L.; Campbell, M. G.; Dinca, M. Electrically Conductive Porous Metal–Organic Frameworks. Angew. Chem. Int. Ed. 2016, 55, 3566-3579. (43) Jing, X.; He, C.; Yang, Y.; Duan, C. Y. A Metal−Organic Tetrahedron as a Redox Vehicle to Encapsulate Organic Dyes for Photocatalytic Proton Reduction. J. Am. Chem. Soc. 2015, 137, 3967-3974. (44) Boess, E.; Schmitz, C.; Klussmann, M. A Comparative Mechanistic Study of Cu-Catalyzed Oxidative Coupling Reactions with N-Phenyltetrahydroisoquinoline. J. Am. Chem. Soc. 2012, 134, 5317-5325. (45) Johnson, J. A.; Luo, J.; Zhang, X.; Chen, Y. S.; Morton, M. D.; Echeverría, E.; Torres, F. E.; Zhang, J. Porphyrin-Metalation-Mediated Tuning of Photoredox Catalytic Properties in


26

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Metal−Organic Frameworks. ACS Catal. 2015, 5, 5283-5291. (46) Zhang, G.; Ma, Y. X.; Wang, S. L.; Zhang, Y. H.; Wang, R. Enantioselective Metal/OrganoCatalyzed Aerobic Oxidative sp3 C−H Olefination of Tertiary Amines Using Molecular Oxygen as the Sole Oxidant. J. Am. Chem. Soc. 2012, 134, 12334-12337. (47) Ratnikov, M. O.; Xu, X. F.; Doyle, M. P. Simple and Sustainable Iron-Catalyzed Aerobic C−H Functionalization of N,N‑Dialkylanilines. J. Am. Chem. Soc. 2013, 135, 9475-9479. (48) Li, Z. P.; Li, C. J. CuBr-Catalyzed Direct Indolation of Tetrahydroisoquinolines via CrossDehydrogenative Coupling between sp3 C-H and sp2 C-H Bonds. J. Am. Chem. Soc. 2005, 127, 6968-6969.


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In the designed Co-POM polymer, Uv light excitation gives the excited state of the Dawson-type polyoxometalate firstly to oxidize electron donors or substrates; the reduced form formed (i.e., heteropolyblue) adsorbs visible light to achieve a new excited state, which electron transfer to the cobalt redox sites and facilitates hydrogen evolution reaction. The photosensitizer recovered to the ground-state, completing the catalytic cycle.


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Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light

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