Subscriber access provided by Yale University Library
Communication
Cu(II)-MOF with Open Coordination Metal Sites for Low Temperature Thermochemical Water Oxidation Jian-Ping Ma, Shen-Qing Wang, Chao-Wei Zhao, Yang Yu, and Yu-Bin Dong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00754 • Publication Date (Web): 13 May 2015 Downloaded from http://pubs.acs.org on May 16, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 5
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
Chemistry of Materials
Cu(II)-MOF with Open Coordination Metal Sites for Low Temperature Thermochemical Water Oxidation Jian-Ping Ma,* Shen-Qing Wang, Chao-Wei Zhao, Yang Yu, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China. Water is omnipresent and essential for life. Owing to its very high formation enthalpy (-286 kJ/mol), water is very stable.1 Compared to water reduction, the oxidation of water process is much more difficult due to its implicit complexity which requires the loss of four electrons and four protons from two water molecules (2H2O → O2 + 4H+ + 4e-). Although a remarkable progress in artificial photoinduced water oxidation has been made recently,2-6 the thermochemical water oxidation, however, has received much less attention. As we know, the construction of the viable materials for more energy efficient water oxidation is very hard because of the inherent difficulty in the design and synthesis of the compounds with specific active-sites to trigger water molecule under relative mild reaction conditions. Porous metal-organic frameworks (MOFs), as a new class of functional materials, provide a variety of attractive features. The organic/inorganic hybrid composition, together with the porous structural feature endows the MOFs colorful physical and chemical properties.7-11 In principle, the MOFs with open metal coordination sites can be obtained, and they might be able to activate the small molecules such as water, subsequently trigger their chemical reactions under milder conditions. So the low temperature water oxidation based on such kind of porous materials is deserved to be explored although it is unprecedented. Herein, we report a novel porous 3D porous Cu(II)-MOF material with open Cu(II) coordination sites. It can effectively activate and oxidize water molecule at lowtemperature. The generated O2 was expediently detected by a molecular ferrocenyl-fulvene probe via isotopic approach. In addition, the oxygen-evolving mechanism is preliminary investigated. The design and synthesis of MOF-1 is shown in Scheme 1. Triflate Cu(II) salt is selected for construction of the Cu(II)MOF with open metal coordination sites because of its weak coordinative nature. The combination of the tetradentate pyridyl-capped ligand L (Supporting Information) with copper(II) triflate (molar ratio of 1:2) in water to quantitatively afford MOF-1 ([Cu(L)](SO3CF3)2·2(H2O)) as deep blue crystalline solids. The formula of MOF-1 was determined based on single-crystal X-ray diffraction studies (Supporting Information) and elemental microanalysis. The simulated and the measured XRPD patterns are identical (Figure 1), indicating that the bulk crystals was obtained in a pure phase and
Scheme 1. Synthesis of MOF-1 and its coordination sphere of Cu(II). The photograph of crystal MOF-1 is inserted.
Figure 1. Top: 2-fold interpenetrating 3D framework (view down c axis) and the overall network topology in the structure of MOF1. The two sets of 3D frameworks are shown in different colors; bottom left: The simulated and the measured XRPD patterns. The photograph of bulk sample MOF-1 is inserted; bottom right: XPS spectrum of MOF-1.
possesses the same structure as the single crystal. Singlecrystal analysis revealed that MOF-1 crystallizes in the tetragonal space group P4(2)22, and the Cu(II) center lies in a square planar {CuN4} coordination sphere (Scheme 1). The square is composed of four terminal pyridyl donors with Cu-N bond lengths of 2.016(4)-2.024(4) Å. In addition, two triflate counter ions are located on each side of the square plane with a very long Cu-O distance of 2.517(15) Å, indicating the ex-
ACS Paragon Plus Environment
Chemistry of Materials
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
istence of very weak interactions between the triflate oxygen atoms and Cu(II) center.12 The ligand in MOF-1 adopts a cross-like conformation. The two 4, 4-pyridyl arms rotate in opposite directions around the central C-C bond to form a torsional angle of ∼44.4° (Supporting Information), which is similar to the equilibrium angle found in biphenyl. An inspection of Figure 1 reveals that the octahedral Cu(II) centers are linked together by the tetradentate ligands into a novel 2-fold interpenetrating 3D network. Although the 2-fold interpenetration effectively reduces the void volume present in the structure, the overall array contains a 3D intersecting channels with nanoscale open windows, which is reflected in a large anion and solvent-accessible volume of 1498.0 Å3 (32.6% of the total unit-cell volume of 4589.9(3) Å3). For example, tubular channels with an approximate diameter of 13 Å (down c axis) was observed, and the triflate ions and guest water molecules (TGA analysis, Supporting Information) are located in the channel spaces (Figure 1). In addition, ellipse-like channels with an approximate effective cross section of ca. 15 × 10 Å along the crystallographic a and b axes are also found (Supporting Information). Such structural feature of MOF-1 definitely facilitates the small molecules to access to open metal coordination sites in more isotropic directions. The unusual topology of the network found in MOF-1 may be simplified by considering the ligands and metal ions as 4-connecting nodes. This generates a 3D binodal 4-connected network (Figure 1). MOF-1 is a very unusual network (Schläfli symbol {4^3.6^2.8}); we are unaware of a precedent despite the large number of 4-connected networks reported in the literature. The X-ray photoelectron spectroscopy (XPS) indicates that the copper cat ion in MOF-1 is bivalent,13-14 which is well in agreement with the singlecrystal analysis (Figure 1). As shown above, the Cu(II) in MOF-1 adopts a coordinatively unsaturated square planar sphere. Based on this, we wondered if MOF-1 is able to activate and oxidize water molecule under hydrothermal conditions. For excluding the interference of air and light, the reaction was carried out in an inert atmosphere of helium in dark.
Scheme 2. Synthesis of MOF-2 and its coordination sphere of Cu(I). The photograph of crystal MOF-2 is inserted.
When a mixture of MOF-1 (36.80 mg, 0.04 mmol) and water (2 mL, air-free) was sealed with helium in a glass tube and heated at 150°C for 72 h to afford yellow crystals (Yield, 74 %, Scheme 2). Single-crystal analysis revealed that MOF-1 transformed to MOF-2 under the reaction conditions. MOF-2 ([Cu3(L)2(H2O)](SO3CF3)3·2.1H2O) crystallizes in a triclinic space group P-1. As shown in Scheme 2, three kinds of crystallographic Cu(I) centers ({Cu(1)N3}, {Cu(2)N3}, and
Page 2 of 5
{Cu(3)N2O}) are linked together by L into a 2D sheet (Schläfli symbol {4^2.6^3.8}{4^2.6}). In MOF-2, the ligands act as the 4-connecting linkages, while the Cu(I) ions simply act as the 3-connecting nodes which is the typical coordination fashion of Cu(I) ion. The 2D sheets further stack together along the crystallographic a axis to generate two types of (~10 × 4 and ~9 × 8 Å) channels in which the SO3CF3- counter ions and guest water molecules are located (TGA, Supporting Information). The XRPD pattern of MOF-2 indicates that the bulk material was obtained in a pure phase (Figure 2).
Figure 2. Top: Crystal packing of MOF-2 (view down a axis) and the overall network topology in the structure of MOF-2; bottom left: The simulated and measured XRPD patterns, and the photograph of bulk sample MOF-2 is inserted; bottom right: XPS spectrum of MOF-2.
Besides the visual color change and X-ray single-crystal analysis, the transformation from Cu(II) in MOF-1 to Cu(I) in MOF-2 was further confirmed by XPS. As shown in Figures 1 and 2, the observation of Cu 2p3/2 peak at 931.49 eV in XPS of MOF-2 demonstrated the reduction from Cu(II) in MOF-1 (934.82 eV) to Cu(I) in MOF-2. The binding energy peak shift of 3.33 eV is well consistent with the reported difference of binding energy peak between Cu(II) and Cu(I).13-14 Notably, the bubble formed in the aqueous solution and tempestuously released from the reaction system when the sealed glass tube was opened. The obtained gas tested positive for the oxidizing gas using the potassium iodide-starch paper. Supposedly, the gas generated from hydrothermal reaction could be oxygen which came from the water molecules under reaction conditions. In order to demonstrate this hypothesis, the online gas chromatography was used to detect the generated gas species. The evolved O2 was confirmed and analyzed by gas chromatography (Figure 3a). The reaction gave 0.163 mL (7.28 µmol) of O2 (Yield, 72.3 %) based on MOF-1. So the reaction from MOF-1 to MOF-2 can be described as eq 1: 4Cu(II)-MOF (1) + 2H2O
150oC Hydrothermal
4Cu(I)-MOF (2) + O2 + 4H+ (eq1)
According to eq 1, the pH value of the reaction system should be decreased after the reaction. The pH value meas-
2
ACS Paragon Plus Environment
Page 3 of 5
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
Chemistry of Materials
urement indicated that the pH values of the reaction system changed from 6.76 to 4.78, which is well in accord with the above observation and discussion. In addition, the generated oxygen amount is enhanced with the increase of reaction temperature. For example, the evolved O2 can be detected at 120°C, and the yield of O2 is up to 55% at 135°C. No difference in yields for O2 and MOF-2 was observed above 150°C (Supporting Information). For further confirming this low-temperature thermochemical water oxidation process, the isotopic parallel experiments were carried out based on an O2 probe (Supporting Information). The molecular oxygen probe used herein was developed in our lab.15 As shown in Figure 3b, it is a ferrocenyl-fulvene molecule (3, C39H36N2Fe2) which contains a central hydropyridazino moiety. The remarkable feature of 3 is that it can be easily oxidized by O2 in solution at room temperature to generate a unique ferrocenyl-cyclopentenone species of 4 (C39H34N2OFe2) in high yield, meanwhile the solution changed from orange (3) to green (4).15 Therefore, it is an excellent visual molecular probe to detect O2 species. When H216O was combined with MOF-1 to perform the hydrothermal reaction under the reaction conditions, the generated O2 was trapped by 3 and the resulted deep green oxidized product of 4 was isolated. As shown in Figure 3c, cold-spray ionization mass spectroscopy (CSI-MS) provides substantial evidence for the formation of a 16O-attached ferrocenylcyclopentenone ([M + 1]+, m/z = 659.1461) (Supporting Information), while the corresponding 18O-attached product was almost no detectable. Notably, when the hydrothermal reaction was carried out in a mixture of H218O and H216O, both generated 18O2 and 16O2 were trapped by 3 (Supporting Information). As indicated in Figure 3c, two isotopic [M + 1]+ peaks of 4, corresponding to [C39H34N216OFe2 + H]+ and [C39H34N218OFe2 + H]+, were found at m/z = 659.1476 and m/z = 661.1609, respectively. This result unambiguously demonstrates that O2 was generated during this Cu(II)-MOF involved water oxidation hydrothermal reaction. To explore the mechanism of this thermochemical oxygen producing process, the ESR measurement was performed on the reaction solution at room temperature, in which DMPO (5, 5-dimethyl-1-pyrroline N-oxide) was used to trap the generated active species. As depicted in Figure 4a, four characteristic peaks of DMPO-·OH were obviously observed once the reaction solution was immediately added to an aqueous solution of DMPO (44 µL DMPO in H2O (2 mL)).16-18 In addition, DMPO-1O2 signals were also detected in the solution,16-18 which can be an additional direct evidence for the formation of molecular oxygen during the water oxidation process. Notably, when the reaction tube was opened and exposed to air for 1 min., the characteristic signals of the DMPO-·OH and DMPO1 O2 species were significantly weakened once the reaction solution was added to the aqueous solution of DMPO. After exposed to air for 30 min., no characteristic peaks related to the DMPO-·OH and DMPO-1O2 were observed, confirming that the ·OH and O2 species did generate from the water oxidation instead of the ambient environment (Supporting Information). In addition, the generated H2O2 was also detected by a highly selective probe of peroxyfluor-1 (PF1) which was previously reported by Chang group (Supporting Information).19 Upon treatment with reaction solution, hydro-
Figure 3. (a) Normalized gas chromatographic traces of air (black line) and the gas species (red line) generated from hydrothermal reaction (air was used as the reference). As time goes on, the evolved N2 amount is constant, while the evolved O2 amount is largely enhanced; (b) scheme representation of oxygen trapping process based on 3 (photographs of solid samples 3-4 and their CH2Cl2 solutions are inserted); (c) the corresponding CSI-MS spectra of 4 labelled by 16O and 18O, respectively.
Figure 4. (a) ESR spectrum of the reaction solution (DMPO-·OH and DMPO-1O2 signals were marked as “◊” and “∆”, respectively); (b) H2O2 detection based on PF1. Luminescent spectra (λex = 450 nm) of PF1 (CPF1 = 1 × 10-5 mol/L), PF1/H2O2 (standard sample, CH2O2 = 2 × 10-4 mol/L), and PF1/reaction solution (0.5 mL). The corresponding sample pictures are inserted. Collected emissions were integrated between 460-700 nm; (c) proposed thermochemical water oxidation mechanism based on MOF-1.
3
ACS Paragon Plus Environment
Chemistry of Materials
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
lytic deprotection of PF1 occurred and the characteristic emission of the generated fluorescein was clearly observed, which is the same as to the luminescence response of PF1 toward H2O2 (Figure 4b).19 Based on the results of mechanistic studies, the proposed mechanism of this thermochemical water oxidation mechanism is shown in Figure 4c. In the suggested mechanism, the water molecules might be activated by the open Cu(II) sites in MOF-1 via weak metal-water coordination,20 and then underwent a redox reaction to provide ·OH and Cu(I)-MOF under hydrothermal conditions. Meanwhile, the coupling of ·OH led to the formation of H2O2. Then the in situ formed H2O2 decomposed to O2 and H2O at 150°C in the presence of the copper ions.21 Therefore, MOF-1 herein with open Cu(II) coordination sites indeed facilitates this thermochemical water oxidation at low temperature. In summary, a novel porous Cu(II)-MOF material with new topology structure has been prepared. It contains open-metal sites which can effectively activate water molecules and furthermore facilitate water oxidation to release oxygen at low temperature. The structural feature and multi-pyridyl ligand system of the Cu(II)-MOF, as well as the ease of synthesis, are notable. Given the synthetic versatility of this porous MOF material, a door might be opened for the systematic investigation of a large number of related porous MOF materials for thermochemical water oxidation under milder conditions. In addition, we developed a novel isotopic method for O2 detection based on a molecular ferrocenyl-fulvene probe, which is a practical characterization approach for currently very active water oxidation and MOFs fields.
ASSOCIATED CONTENT
(4)
(5) (6)
(7) (8) (9) (10)
(11) (12)
(13)
(14)
(15)
(16)
Supporting Information. Synthesis and characterization of L, MOF-1 and MOF-2, experimental details for oxygen isotopic detection, H2O2 detection and parallel experiments of water oxidation at different temperatures, single-crystal measurement, TGA, ESR, CSI-MS spectra can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
(17)
(18)
(19)
AUTHOR INFORMATION Corresponding Author
(20)
[email protected],
[email protected].
Notes The authors declare no completing financial interest.
ACKNOWLEDGMENT We are grateful for financial support from NSFC (Grant Nos. 21475078, 21271120 and 21201112), 973 Program (Grant Nos. 2012CB821705 and 2013CB933800) and the Taishan scholar’s construction project.
REFERENCES (1) (2)
(3)
(21)
Page 4 of 5
Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. Rüttinger, W.; Dismukes, G. C. Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation. Chem. Rev. 1997, 97, 1-23. Yachandra, V. K.; Sauer, K.; Klein, M. P. Manganese Cluster in Photosynthesis: Where Plants Oxidize Water to Dioxygen. Chem. Rev. 1996, 96, 2927-2950. O’Keeffe, M.; Yaghi, O. M. Introduction to Metal–Organic Frameworks. Chem. Rev. 2012, 112, 675-702. Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal–Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869-932. Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Luminescent Functional Metal–Organic Frameworks. Chem. Rev. 2012, 112, 1126-1162. Wang. C.; Zhang, T.; Lin, W. Rational Synthesis of Noncentrosymmetric Metal–Organic Frameworks for Second-Order Nonlinear Optics. Chem. Rev. 2012, 112, 1084-1104. Yoon, M.; Srirambalaji, R.; Kim, K. Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis. Chem. Rev. 2012, 112, 1196-1231. Farrugia, L. J.; Middlemiss, D. S.; Sillanpää, R.; Seppälä, P. J. A Combined Experimental and Theoretical Charge Density Study of the Chemical Bonding and Magnetism in 3-Amino-propanolato Cu(II) Complexes Containing Weakly Coordinated Anions. J. Phys. Chem. A 2008, 112, 9050-9067. Balamurugan, B.; Mehta, B. R.; Shivaprasad, S. M. Surface-modified CuO Layer in Size-Stabilized Single-Phase Cu2O Nanoparticles. Appl. Phys. Lett. 2001, 79, 3176. Huang, Y. G.; Mu, B.; Schoenecker, P. M.; Carson, C. G.; Karra, J. R.; Cai, Y.; Walton, K. S. A Porous Flexible Homochiral SrSi2 Array of SingleStranded Helical Nanotubes Exhibiting Single-Crystal-to-Single-Crystal Oxidation Transformation. Angew. Chem., Int. Ed. 2011, 50, 436-440. Li, J.; Ma, J.-P.; Liu, F.; Wu, X.; Dong, Y.-B.; Huang, R.-Q. A New Approach for Ferrocenyl-Cyclopentenone and Ferrocenyl-Cyclopentenedione Compound Synthesis. Organometallics, 2008, 27, 5446-5452. Harbour, J. R.; Issler, S. L.; Hair, M. L. Singlet Oxygen and Spin Trapping with Nitrones. J. Am. Chem. Soc. 1980, 102, 7778-7779. Bilski, P.; Reszka, K.; Bilska, M.; Chignell, C. F. Oxidation of the Spin Trap 5, 5-Dimethyl-1-Pyrroline N-Oxide by Singlet Oxygen in Aqueous Solution. J. Am. Chem. Soc. 1996, 118, 1330-1338. Xu, Y.; Li, X.; Cheng, X.; Sun, D.; Wang, X. Degradation of Cationic Red GTL by Catalytic Wet Air Oxidation over Mo–Zn–Al–O Catalyst under Room Temperature and Atmospheric Pressure. Environ. Sci. Technol. 2012, 46, 2856-2863. Chang, M. C. Y.; Pralle, A.; Isocoff, E. Y.; Chang, C. J. A Selective, CellPermeable Optical Probe for Hydrogen Peroxide in Living Cells. J. Am. Chem. Soc. 2004, 126, 15392-15393. Owing to Jahn-teller effect, Cu(II) ion exhibits interesting coordination flexibility, Lewis acid property and reversible guest binding ability. (a) Chen, B.; Eddaoudi, M.; Reineke, T. M.; Kampf, J. W.; O’Keeffe, M.; Yaghi, O. M. Cu2(ATC)·6H2O: Design of Open Metal Sites in Porous Metal−Organic Crystals (ATC: 1,3,5,7-Adamantane Tetracarboxylate). J. Am. Chem. Soc. 2000, 122, 11559-11560. (b) Noro, S.-i.; Horike, S.; Tanaka, T.; Kitagawa, S.; Akutagawa, T.; Nakamura, T. Flexible and Shape-Selective Guest Binding at CuII Axial Sites in 1-Dimensional CuII−1,2-Bis(4-pyridyl)ethane Coordination Polymers. Inorg. Chem. 2006, 45, 9290-9300. Sigel, H.; Flierl, C.; Griesser, R. Metal Ions and Hydrogen Peroxide. XX. On the Kinetics and Mechanism of the Decomposition of Hydrogen Peroxide, Catalyzed by the Cu2+-2,2'-Bipyridyl Complex. J. Am. Chem. Soc. 1969, 91, 1061-1064.
Süss-Fink, G. Water Oxidation: A Robust All-Inorganic Catalyst. Angew. Chem., Int. Ed. 2008, 47, 5888-5890. Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863-12001. Yaghi, M.; Kaneko, M. Molecular Catalysts for Water Oxidation. Chem. Rev. 2001, 101, 21-35.
4
ACS Paragon Plus Environment
Page 5 of 5
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
Chemistry of Materials
Low-temperature water oxidation based on a porous 3D Cu(II)-MOF is reported.
5
ACS Paragon Plus Environment
