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Two Strandberg-type organophosphomolybdates: synthesis, crystal structures and catalytic properties† Jian-Ping Wang, Hong-Xin Ma, Lan-Cui Zhang,* Wan-Sheng You* and Zai-Ming Zhu* Two novel Strandberg-type organophosphomolybdate hybrid compounds [(Cu(H2O))2(μ-bipy)2(C6H5PO3)2Mo5O15]n (1) and [(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15]n (2) (bipy = 4,4’-bipyridyl) were prepared under mild hydrothermal conditions and structurally characterized by physico-chemical and spectroscopic methods. Single crystal X-ray diffraction analysis reveals that compounds 1 and 2 are polyoxometalate-based Cu-coordination polymers with a three-dimensional framework. In 1, the Cu2+ ions not only link [(C6H5PO3)2Mo5O15]4− (abbreviated as {(C6H5P)2Mo5}) polyanions, but also act as connectors of bipy ligands to produce two symmetrical 1-D chains, all 1-D chains are further held together by polyanions to
Received 24th August 2014, Accepted 18th September 2014
generate a 3-D network. In 2, each {(C6H5P)2Mo5} polyanion acting as a hexadentate ligand links four Cu(II)-bipy/H2O units, forming 2-D plane structures, which are further bridged by Cu(II)-bipy-Cu(II) frag-
DOI: 10.1039/c4dt02571g
ments to generate a 3-D network. Their fluorescence properties and catalytic properties for the synthesis
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of cyclohexanone ethylene ketal were also investigated.
Introduction The rational design and synthesis of novel polyoxometalates (POMs) and their substituted derivatives have received great interest because of their functional properties and desirable merits in a wide range of applications, including medicine,1 catalysis,2 and other materials science.3 In recent years, the Strandberg-type POMs (e.g. [P2Mo5O23]6−, abbreviated as {P2Mo5}), as the typical species, have become an important branch in the POM family.4 Firstly, the relatively small size of these POMs increases the electron density of the whole clusters, which can result in the presence of more counterions, and thus potentially results in the formation of more diverse architectures and stable clusters; secondly, the protrudent {PO4} fragments are also coordinated to metal–organic segments except for MoO6 octahedra; thirdly, this kind of POM cluster can be easily assembled through the reaction of simple starting materials under hydrothermal conditions. The phosphorus heteroatom in a Strandberg polyanion can be inorganic phosphorus
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China. E-mail: [email protected], [email protected], [email protected]; Fax: +86 411 82158559; Tel: +86 411 82156550 † Electronic supplementary information (ESI) available: ORTEP views and packing views of 1 and 2; IR, XRPD, TG-DTA and emission spectra. The optimal catalytic curves. CCDC 1012272 and 1012273. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02571g
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or organophosphine. So far, many Strandberg-type POMs based on organophosphine {RPO3H2} (R = –C6H4CO2H, –(CH2)nPO3H2 (n = 2–6, 9), –(CH2)nCO2H (n = 1, 2), –C6H5, –CH2CH3, –CH3) have been reported.5,6 Zubieta’s group has made important contributions to this field; they synthesized a series of organophosphonate hybrid materials using hydrothermal synthesis by employing organodiphosphonic acids as the key starting material.5 They also pointed out that variations in stoichiometry, reaction pH, and temperature are well-known to produce a variety of products. Such novel organic–inorganic hybrid materials composed of [(RPO3)2Mo5O15]4− (abbreviated as {(RP)2Mo5}) and metal-complex cations can enhance the performance of the two components and may result in new features. As far as we know, only magnetic properties and luminescence properties have been investigated for these Strandberg-type POMs, but their catalytic abilities for organic synthesis have not been reported. Our previous work reported {P2Mo5} POMs constructed from Strandberg-type inorganic anions [P2Mo5O23]6− and Zn(II)–H2biim/H2O subunits having good catalytic performances in an acid-catalyzed organic reaction.7 How about {(RP)2Mo5} POMs? In order to construct new inorganic–organic hybrid materials containing Strandberg-type organophosphomolybdate clusters, and to explore their catalytic activities in organic synthesis, we chose phenylphosphonic acid (C6H5PO3H2) as a P source, and used Na2MoO4·2H2O as an inorganic precursor, successfully, two organophosphomolybdate Cu-coordination polymers with a three-dimensional (3-D)
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framework, namely [(Cu(H2O))2(μ-bipy)2(C6H5PO3)2Mo5O15]n (1) and [(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15]n (2), were constructed under hydrothermal conditions. Their fluorescence properties and catalytic properties for the protection of carbonyl compounds with glycol were also reported.
Results and discussion Synthesis It is difficult to synthesise and isolate POMs containing organophosphomolybdates. We attempt to prepare the transition metal-complex decorated organophosphomolybdates by the use of Na2MoO4 and phenylphosphonic acid as the inorganic and organic precursors, respectively. Compounds 1 (yield 43%) and 2 (yield 5%) were initially obtained at the same time in one autoclave with the molar ratio of Mo/P/Cu/bipy = 2.5 : 1.0 : 1.0 : 1.0 under mild hydrothermal conditions (Scheme 1). In order to separate the two compounds and improve the yield of compound 2, we performed a large number of experiments. Parallel experiments showed that crystals of 2 can be obtained by the best mixed amount: Na2MoO4·2H2O (0.48 g, 2.0 mmol), CuCl2·2H2O (0.22 g, 1.3 mmol) and C6H5PO3H2 (0.13 g, 0.8 mmol), and 4,4′-bipyridyl (0.08 g, 0.5 mmol), i.e. the molar ratio of Mo/P/Cu/bipy = 4.0 : 1.6 : 2.6 : 1.0, and the order of addition was not the same as 1, kept at 160 °C for 3 days. Yield: 36% based on Mo of 2. Furthermore, experimental results indicate that reasonable yields of crystalline products can be obtained at pH = 3–4, so the pH value also plays a key role in the synthesis of these compounds. This indicates that many factors, such as the pH value, reaction time, temperature and molar ratio in the process of hydrothermal synthesis, can affect the formation and crystal growth of products. Structural analysis Single crystal X-ray diffraction analysis reveals that the 3-D Cucoordination polymer structure of 1 constructed by {(C6H5P)2Mo5} polyanions, [(Cu(H2O))2(μ-bipy)2]4+ cations through coordination bonding and hydrogen bonding interactions (Fig. 1a). As shown in Fig. 1b and S1a,† the adjacent {(C6H5P)2Mo5} polyanions are connected by two symmetrical Cu2+ (Cu1/Cu1′) ions forming infinite 1-D chains along the
Scheme 1 Schematic representation of pathways and experimental conditions for the formation of 1 and 2.
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Fig. 1 (a) Polyhedral and ball-and-stick representation of a 3-D structure of 1; (b) the 1-D chain of {(C6H5P)2Mo5} polyanions linked by Cu(II)-bipy/H2O fragments along the c axis in 1; (c) two symmetrical 1-D chains of [(Cu(H2O))2(bipy)2]4+ (the hydrogen atoms have been omitted for clarity).
c axis. In the 1-D chains (Fig. S2†), each Cu2+ ion links two polyanions, each {(C6H5P)2Mo5} polyanion acts as a tetradentate ligand, coordinates four Cu2+ ions through four O atoms belonging to three MoO6 octahedra. The Mo–O distances are in the normal range (1.686(2)–2.376(2) Å). Cu1/Cu1′ is five-coordinated by two N atoms (N1, N2) from two 4,4′-bipyridyl ligands, and two terminal O atoms (O2′, O6) from two POM polyanions, and one terminal water ligand (O1W) forming a square-pyramid. The bond lengths of Cu1–N1, Cu1–N2, Cu1– O2′, Cu1–O6 and Cu1–O1W are 2.013(3), 2.015(3), 1.955(3), 2.381(3) and 1.944(3) Å (Table S1†), respectively. The hydrogen bonds of O1W–H⋯O1/O9 are 2.609(4)/2.741(4) Å (Table S2†). Obviously, the Cu2+ ions not only link {(C6H5P)2Mo5} polyanions, but also act as connectors of 4,4′-bipyridyl ligands to produce two symmetrical 1-D chains along the b axis (Fig. 1c and S3†). As shown in Fig. S3,† all alternating vertical and horizontal 1-D chains are further held together by polyanions to generate a 3-D network. Compared to 1, 2 also has a 3-D Cu-coordination polymer structure built by [(Cu(H2O)2)2(μ-bipy)]4+ cations and {(C6H5P)2Mo5} polyanions (Fig. 2a and S4†). Each Cu2+ ion also links two polyanions, but the structural features of 2 are somewhat different from those of 1: the adjacent {(C6H5P)2Mo5} polyanions in 2 are connected by one Cu2+ ion, there is no infinite 1-D polyanion chain. As shown in Fig. 2b
Fig. 2 (a) Polyhedral and ball-and-stick representation of a 3-D structure of 2; (b) the 2-D plane of {(C6H5P)2Mo5} polyanions linked by Cu(II) units in 2; (c) the arrangement of Cu(II)-bipy-Cu(II) units (the hydrogen atoms have been omitted for clarity).
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and S5,† each {(C6H5P)2Mo5} polyanion acting as a hexadentate ligand links four Cu(II)-bipy/H2O units through six O atoms from four MoO6 octahedra, forming a 2-D plane structure. Each Cu2+ ion coordinates with one N atom (Cu1–N1 1.992(4) Å) from one 4,4′-bipyridyl molecule, and two terminal O atoms (Cu1–O10 2.617(3), Cu1–O5′ 1.927(3) Å) and one bridge O atom (Cu1–O8 1.956(3) Å) from two adjacent {(C6H5P)2Mo5} clusters, and two terminal water ligands (Cu1– O1W 1.962(3), Cu1–O2W 2.511(3) Å) (Table S3, Fig. S1b†), showing a strongly distorted octahedral geometry. The hydrogen bonds of O1W–H⋯O3/O4 are 2.800(4)/2.833(5) Å (Table S4†). As shown in Fig. 2c and S6,† each 4,4′-bipyridyl bridges two Cu2+ ions forming Cu(II)-bipy-Cu(II) fragments, which is further supported by polyanions. Moreover, there are two longer bonds (Mo3–O7 2.597(3) and Mo3–O8 2.519(3) Å) in the Mo3 subunit, while other Mo–O bond lengths are in the normal range (1.677(3)–2.379(3) Å), such obvious structural distortions can make its structure unstable. For instance, the thermal analysis shows that the polyanion skeleton of 2 completely collapses below 400 °C, indicating that its thermal stability is lower than that of 1 (see Fig. S9†). Characterization FT-IR spectroscopy. IR spectra of 1 and 2 (Fig. S7†) exhibiting bands at 3460–2868 cm−1 are characteristic vibrations of O–H, N–H and C–H. The bands in the region of 1618–1225 cm−1 are associated with the pyridine ring of a 4,4′bipyridyl molecule. The bands at 1140–1040 cm−1 are attributed to v(P–O) of the organophosphonate ligands and bands at 972–914, 907–824 and 700–673 cm−1 belong to v(Mo–Oterminal ) and v(Mo–Obridging) of polyoxoanion.6c X-ray power diffraction. X-ray power diffraction (XRPD) was checked at room temperature in contrast to the patterned data curve. As shown in Fig. S8 (ESI†), all major peaks of the simulated and experimental XRPD pattern are in agreement with each other, indicating their reasonable crystalline phase purity for compounds 1 and 2. The differences in intensity may be due to the preferred orientation of the crystalline powder samples. Thermal analysis. The curves corresponding to TG and DTA analysis of 1 and 2 are given in Fig. S9 (ESI†), in the temperature range of 30–700 °C; the total weight loss of 33.44% for 1 is consistent with the calculated value (calc. 33.35%). The weight loss is due to the removal of water, 4,4′-bipyridyl molecules and the phenyl group. For 2, in the range of 30–173 °C, the first weight loss of 5.20% corresponds to the loss of four coordinated water molecules. The obvious weight loss above 290 °C may be caused by combustion of organic components and the loss of phosphorus oxides. The total weight loss in the range of 290–550 °C (42.28%) is higher than the theoretical value (37.56%), indicating that the polyanion skeleton of 2 has completely collapsed, which is mainly related to its structural distortion and different environments of Cu(II). Thus, the thermal stability of 2 is lower than that of 1. Fluorescence properties. The photoluminescence properties of 1, 2 and the parent (NH4)4[(C6H5PO3)2Mo5O15] in the solid
17174 | Dalton Trans., 2014, 43, 17172–17176
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Fig. 3 Solid state emission spectra of the parent {(C6H5P)2Mo5}, 1 and 2 at room temperature.
state at room temperature are depicted in Fig. 3. As shown in Fig. 3, the parent {(C6H5P)2Mo5} cluster displays luminescence with two main emission peaks at 400 and 470 nm with λex = 300 nm, which should be assigned to LMCT (O → Mo).6c As shown in Fig. S10,† the emission peaks at about 293 and 363 nm with λex = 300 nm for phenylphosphonic acid and 4,4′bipy, respectively, should be assigned to π*→π transition. Upon excitation at 300 nm, 1 and 2 show emission peaks at 415 and 470 nm, respectively (Fig. 3), indicating that they have a similar structure. Compared with the emission of the parent {(C6H5P)2Mo5}, emissions of 1 and 2 are mainly caused by the {(C6H5P)2Mo5} cluster, but the peaks shift about 15 nm toward the longer wavelength due to changes in energy levels resulting from the coordination of {(C6H5P)2Mo5} to Cu2+ ions. On the other hand, the emission intensities of 1 and 2 have changed, that is, the coordination of {(C6H5P)2Mo5} to Cu2+ ions quenches partially LMCT (O → Mo). This indicates that 1 and 2 may be excellent candidates for photoactive materials. Catalytic activities of two compounds In multi-step organic synthesis, the acid-catalytic protection of the carbonyl group is one of the important reactions. In the presence of an acid catalyst, cyclohexanone reacts with glycol to form cyclohexanone ethylene ketal (Scheme S1†).8,9 We selected the synthesis of cyclohexanone ethylene ketal as a model reaction, to evaluate the catalytic activities of compounds 1, 2 and their parents. Taking 1 as an example, the reaction time, the molar ratio of the starting material, as well as the amount of catalyst, were systemically explored (Fig. S11– S13†). As shown in Fig. S11 and S12,† the yields of ketals increased quickly with increasing time within 2.5 hours, and the cyclohexanone/glycol molar ratio of 1 : 1.4 is a suitable substrate molar one. The amount of catalyst is also one of the important affective factors: when the catalyst is 1/200 of cyclohexanone, the maximum yield of cyclohexanone ethylene ketal is about 94% (Fig. S13†). According to the above results, the optimum conditions for the synthesis of cyclohexanone ethyl-
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Table 1 Catalytic performance of catalysts 1, 2, the parents {P2Mo5} and {(C6H5P)2Mo5}, and CuCl2 for the synthesis of cyclohexanone ethylene ketala
Entry
Catalyst
Solubility
Time (h)
Yield (%)
1 2 3 4 5 6
— {P2Mo5} {(C6H5P)2Mo5} CuCl2 Compound 1 Compound 2
— Insoluble Insoluble Soluble Insoluble Insoluble
2.5 2.5 2.5 2.5 2.5 2.5
7 51 76 ∼100 94 91
a Reaction conditions: the molar ratio of the catalyst (based on Mo) to cyclohexanone was 1 : 200, and the molar ratio of cyclohexanone to glycol was 1 : 1.4 (0.1 mol of cyclohexanone); water-carrying agent: 10 mL of cyclohexane; reaction temperature: 95–100 °C
ene ketal are as follows: the cyclohexanone/glycol molar ratio is 1 : 1.4; the molar ratio of the catalyst to cyclohexanone is 1 : 200; reaction temperature 95–100 °C, and reaction time 2.5 h. The acid-catalytic activities for compound 2, and their organo- and inorgano-parents under the same conditions are listed in Table 1. As shown in Table 1, the yields of ketal were 91, 76, 51 and 100% for compound 2, the parent {(C6H5P)2Mo5}, {P2Mo5} and CuCl2, respectively, while the yield of ketal was 7% without the catalysts, which shows that the title compounds have better acid-catalytic activity than their parent POMs. The Cu2+ and Mo6+ moieties, as the catalytically active centers, play important roles in the catalytic reaction. Moreover, we found that these POM catalysts maintain the catalytic activities during the later cycles. As shown in Fig. S14a,† when the catalysts were recovered by simple filtration after the reaction and reused, it was found that the catalytic activities decreased obviously during the second run compared with the fresh compounds. This may be attributed to the adsorption of the reactant (e.g. ethylene glycol or poly ethylene glycol) on the catalyst surface. If the catalysts were washed with ethyl ether and water, they could maintain higher activities after four cycles (Fig. S14b†). The above results suggest that compounds 1 and 2 are stable and efficient heterogeneous catalysts.
Experimental Materials and methods All chemicals were of reagent grade as received from commercial sources and used without further purification. C, H and N elemental analyses were performed on a Vario Elcube elemental analyzer, and P, Cu and Mo were analyzed on a Prodigy XP emission spectrometer. The infrared spectra were recorded on KBr pellets with a Bruker AXS TENSOR-27 FTIR spectrometer in the range of 4000–400 cm−1. The photoluminescence property was determined on a RILI F-7000 fluorescence spectrophotometer in the solid state at room temperature. Single crystal X-ray diffraction data were collected on a Bruker Smart APEX II X-diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). X-ray powder diffraction data were collected on a Bruker AXS D8 Advance diffractometer
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using Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 5–60° with a step size of 0.02°. TG analyses were performed on a Pyris Diamond TG-DTA thermal analyzer in air at a heating rate of 10 °C min−1. The yield of cyclohexanone ethylene ketal was confirmed on a JK-GC112A Gas Chromatograph. Syntheses of compounds Compound 1. Na2MoO4·2H2O (0.15 g, 0.63 mmol) and CuCl2·2H2O (0.04 g, 0.25 mmol) were dissolved in water (20 mL), and the mixture was stirred for 30 min. Then bipy (0.04 g, 0.25 mmol) was added with vigorous stirring. The pH value of the mixture was adjusted to 3–4 with 4 mol L−1 HCl under continuous stirring. Then phenylphosphonic acid (0.04 g, 0.25 mmol) was added. The resulting mixture was then transferred to a 30 mL Teflon-lined autoclave and kept at 150 °C for 3 days, dark blue block crystals of 1 were obtained (yield: ca. 43% based on Mo). Elemental analysis, Calcd for C32H30O23N4Cu2P2Mo5: C 25.5, H 2.0, N 3.7, P 4.1, Cu 8.4, Mo 31.8%; Found: C 25.4, H 2.05, N 3.8, P 4.1, Cu 8.45, Mo 31.7%. FT IR (KBr pellet), cm−1: 3454(w), 3146(m), 1607(m), 1535(w), 1497(w), 1410(m), 1225(w), 1126(s), 1040(m), 972(m), 907(s), 673(s), 557(m), 509(m). Compound 2. The mixture of Na2MoO4·2H2O (0.48 g, 2.0 mmol), CuCl2·2H2O (0.22 g, 1.3 mmol) and H2O (20 mL) was stirred for 30 min, and the pH value was also adjusted to 3–4, then phenylphosphonic acid (0.13 g, 0.8 mmol) and bipy (0.08 g, 0.50 mmol) were added in order with vigorous stirring. The resulting mixture was then transferred to a 30 mL Teflonlined autoclave and kept at 160 °C for 3 days (see Scheme 1). Light blue block crystals of 2 were obtained (yield: ca. 36% based on Mo). Elemental analysis, Calcd for C22H26O25N2Cu2P2Mo5: C 19.05, H 1.9, N 2.0, P 4.5, Cu 9.2, Mo 34.9%; Found: C 19.1, H 1.85, N 2.2, P 4.5, Cu 9.15, Mo 34.7%. FT IR (KBr pellet), cm−1: 3460(m), 3128(m), 2930(w), 2868(w), 1618(m), 1398(s), 1126(s), 1140(w), 1087(m), 914(m), 824(w), 700(m), 609(w), 545(w). Single-crystal X-ray diffraction The structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 using SHELXTL-97.10,11 An empirical absorption correction was applied using the SADABS program. Crystal data and structure refinement parameters of compounds 1 and 2 are listed in Table 2. Hydrogen atoms on C atoms were added in calculated positions. Bond lengths and angles are listed in Tables S1–S4.† CCDC reference numbers: 1012272 and 1012273. Catalytic experiment Protection for the carbonyl group. Acid-catalyzed synthesis of cyclohexanone ethylene ketal was used as a model reaction to evaluate the catalytic performances of 1 and 2. A typical procedure of the catalytic activity test is as follows: the catalyst (e.g. compound 1) was added to a mixture of cyclohexanone, glycol and cyclohexane (10 mL) in a 50 mL three-necked round-bottom flask fitted with a Dean–Stark apparatus to remove the water continuously from the reaction mixture. After
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Table 2
Dalton Transactions Crystal and refinement data for compounds 1 and 2
Formula Formula weight T/K Wavelength/Å Crystal system Space group a/Å b/Å c/Å α/° β/° γ /° V/Å3, Z Dc/g cm−3, F000 GOF Reflections collected Unique data, Rint θ range (°) R1(I > 2σ(I))a wR2 (all data)a a
1
2
C32H30O23N4Cu2P2Mo5 1507.34 296(2) 0.71073 Orthorhombic Fdd2 18.8251(2) 42.656(2) 10.6435(5) 90.00 90.00 90.00 8546.8(7), 8 2.343, 5854 1.037 10597
C22H26O25N2Cu2P2Mo5 1387.17 296(2) 0.71073 Monoclinic C2/c 17.3158(12) 9.8869(7) 21.7619(16) 90 102.907(1) 90 3631.5(4), 4 2.537, 2680 1.036 8868
3492, 0.0222 1.91 to 24.99 0.0189 0.0442
3198, 0.0292 1.92 to 24.99 0.0267 0.0614
R1 = ∑||F0 |− |FC||/∑|F0|; wR2 = ∑[w(F02 − FC2)2]/∑[w(F02)2]1/2.
completion of the reaction, the heterogeneous catalyst remained at the bottom of the reaction vessel and was easily separated from the organic phase containing product by decantation. The recovered catalyst was reused in a new reaction under identical experimental conditions. Each of the procedures was repeated for three cycles. The products obtained were characterized by gas chromatography. The above experiment was repeated under the same conditions except that compound 1 was replaced by compound 2, CuCl2, the parent polyoxometalate {P2Mo5} and {(C6H5P)2Mo5}, respectively.
Conclusions In summary, two unprecedented Strandberg-type POMs containing {(C6H5P)2Mo5} clusters and Cu(II)-(bipy)/H2O units have been successfully assembled by hydrothermal methods. These organophosphomolybdates were firstly used as acid-catalysts in organic reaction, and exhibited better catalytic performances than those of their parents {(C6H5P)2Mo5} and {P2Mo5}, which shows that the synergistic effect of polyanions and transition metal complexes in inorganic–organic hybrids lead to enhanced catalytic properties. The solid-state luminescent studies show that 1 and 2 exhibit photoluminescence properties. Further research on other organophosphomolybdates is underway in our group.
Acknowledgements This work was financially supported by the Natural Science Foundation of Liaoning Province (no. 2013020128) and the
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Foundation of Education Department of Liaoning Province (no. L2013414).
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Two Strandberg-type organophosphomolybdates: synthesis, crystal structures and catalytic properties† Jian-Ping Wang, Hong-Xin Ma, Lan-Cui Zhang,* Wan-Sheng You* and Zai-Ming Zhu* Two novel Strandberg-type organophosphomolybdate hybrid compounds [(Cu(H2O))2(μ-bipy)2(C6H5PO3)2Mo5O15]n (1) and [(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15]n (2) (bipy = 4,4’-bipyridyl) were prepared under mild hydrothermal conditions and structurally characterized by physico-chemical and spectroscopic methods. Single crystal X-ray diffraction analysis reveals that compounds 1 and 2 are polyoxometalate-based Cu-coordination polymers with a three-dimensional framework. In 1, the Cu2+ ions not only link [(C6H5PO3)2Mo5O15]4− (abbreviated as {(C6H5P)2Mo5}) polyanions, but also act as connectors of bipy ligands to produce two symmetrical 1-D chains, all 1-D chains are further held together by polyanions to
Received 24th August 2014, Accepted 18th September 2014
generate a 3-D network. In 2, each {(C6H5P)2Mo5} polyanion acting as a hexadentate ligand links four Cu(II)-bipy/H2O units, forming 2-D plane structures, which are further bridged by Cu(II)-bipy-Cu(II) frag-
DOI: 10.1039/c4dt02571g
ments to generate a 3-D network. Their fluorescence properties and catalytic properties for the synthesis
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of cyclohexanone ethylene ketal were also investigated.
Introduction The rational design and synthesis of novel polyoxometalates (POMs) and their substituted derivatives have received great interest because of their functional properties and desirable merits in a wide range of applications, including medicine,1 catalysis,2 and other materials science.3 In recent years, the Strandberg-type POMs (e.g. [P2Mo5O23]6−, abbreviated as {P2Mo5}), as the typical species, have become an important branch in the POM family.4 Firstly, the relatively small size of these POMs increases the electron density of the whole clusters, which can result in the presence of more counterions, and thus potentially results in the formation of more diverse architectures and stable clusters; secondly, the protrudent {PO4} fragments are also coordinated to metal–organic segments except for MoO6 octahedra; thirdly, this kind of POM cluster can be easily assembled through the reaction of simple starting materials under hydrothermal conditions. The phosphorus heteroatom in a Strandberg polyanion can be inorganic phosphorus
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China. E-mail: [email protected], [email protected], [email protected]; Fax: +86 411 82158559; Tel: +86 411 82156550 † Electronic supplementary information (ESI) available: ORTEP views and packing views of 1 and 2; IR, XRPD, TG-DTA and emission spectra. The optimal catalytic curves. CCDC 1012272 and 1012273. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02571g
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or organophosphine. So far, many Strandberg-type POMs based on organophosphine {RPO3H2} (R = –C6H4CO2H, –(CH2)nPO3H2 (n = 2–6, 9), –(CH2)nCO2H (n = 1, 2), –C6H5, –CH2CH3, –CH3) have been reported.5,6 Zubieta’s group has made important contributions to this field; they synthesized a series of organophosphonate hybrid materials using hydrothermal synthesis by employing organodiphosphonic acids as the key starting material.5 They also pointed out that variations in stoichiometry, reaction pH, and temperature are well-known to produce a variety of products. Such novel organic–inorganic hybrid materials composed of [(RPO3)2Mo5O15]4− (abbreviated as {(RP)2Mo5}) and metal-complex cations can enhance the performance of the two components and may result in new features. As far as we know, only magnetic properties and luminescence properties have been investigated for these Strandberg-type POMs, but their catalytic abilities for organic synthesis have not been reported. Our previous work reported {P2Mo5} POMs constructed from Strandberg-type inorganic anions [P2Mo5O23]6− and Zn(II)–H2biim/H2O subunits having good catalytic performances in an acid-catalyzed organic reaction.7 How about {(RP)2Mo5} POMs? In order to construct new inorganic–organic hybrid materials containing Strandberg-type organophosphomolybdate clusters, and to explore their catalytic activities in organic synthesis, we chose phenylphosphonic acid (C6H5PO3H2) as a P source, and used Na2MoO4·2H2O as an inorganic precursor, successfully, two organophosphomolybdate Cu-coordination polymers with a three-dimensional (3-D)
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framework, namely [(Cu(H2O))2(μ-bipy)2(C6H5PO3)2Mo5O15]n (1) and [(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15]n (2), were constructed under hydrothermal conditions. Their fluorescence properties and catalytic properties for the protection of carbonyl compounds with glycol were also reported.
Results and discussion Synthesis It is difficult to synthesise and isolate POMs containing organophosphomolybdates. We attempt to prepare the transition metal-complex decorated organophosphomolybdates by the use of Na2MoO4 and phenylphosphonic acid as the inorganic and organic precursors, respectively. Compounds 1 (yield 43%) and 2 (yield 5%) were initially obtained at the same time in one autoclave with the molar ratio of Mo/P/Cu/bipy = 2.5 : 1.0 : 1.0 : 1.0 under mild hydrothermal conditions (Scheme 1). In order to separate the two compounds and improve the yield of compound 2, we performed a large number of experiments. Parallel experiments showed that crystals of 2 can be obtained by the best mixed amount: Na2MoO4·2H2O (0.48 g, 2.0 mmol), CuCl2·2H2O (0.22 g, 1.3 mmol) and C6H5PO3H2 (0.13 g, 0.8 mmol), and 4,4′-bipyridyl (0.08 g, 0.5 mmol), i.e. the molar ratio of Mo/P/Cu/bipy = 4.0 : 1.6 : 2.6 : 1.0, and the order of addition was not the same as 1, kept at 160 °C for 3 days. Yield: 36% based on Mo of 2. Furthermore, experimental results indicate that reasonable yields of crystalline products can be obtained at pH = 3–4, so the pH value also plays a key role in the synthesis of these compounds. This indicates that many factors, such as the pH value, reaction time, temperature and molar ratio in the process of hydrothermal synthesis, can affect the formation and crystal growth of products. Structural analysis Single crystal X-ray diffraction analysis reveals that the 3-D Cucoordination polymer structure of 1 constructed by {(C6H5P)2Mo5} polyanions, [(Cu(H2O))2(μ-bipy)2]4+ cations through coordination bonding and hydrogen bonding interactions (Fig. 1a). As shown in Fig. 1b and S1a,† the adjacent {(C6H5P)2Mo5} polyanions are connected by two symmetrical Cu2+ (Cu1/Cu1′) ions forming infinite 1-D chains along the
Scheme 1 Schematic representation of pathways and experimental conditions for the formation of 1 and 2.
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Fig. 1 (a) Polyhedral and ball-and-stick representation of a 3-D structure of 1; (b) the 1-D chain of {(C6H5P)2Mo5} polyanions linked by Cu(II)-bipy/H2O fragments along the c axis in 1; (c) two symmetrical 1-D chains of [(Cu(H2O))2(bipy)2]4+ (the hydrogen atoms have been omitted for clarity).
c axis. In the 1-D chains (Fig. S2†), each Cu2+ ion links two polyanions, each {(C6H5P)2Mo5} polyanion acts as a tetradentate ligand, coordinates four Cu2+ ions through four O atoms belonging to three MoO6 octahedra. The Mo–O distances are in the normal range (1.686(2)–2.376(2) Å). Cu1/Cu1′ is five-coordinated by two N atoms (N1, N2) from two 4,4′-bipyridyl ligands, and two terminal O atoms (O2′, O6) from two POM polyanions, and one terminal water ligand (O1W) forming a square-pyramid. The bond lengths of Cu1–N1, Cu1–N2, Cu1– O2′, Cu1–O6 and Cu1–O1W are 2.013(3), 2.015(3), 1.955(3), 2.381(3) and 1.944(3) Å (Table S1†), respectively. The hydrogen bonds of O1W–H⋯O1/O9 are 2.609(4)/2.741(4) Å (Table S2†). Obviously, the Cu2+ ions not only link {(C6H5P)2Mo5} polyanions, but also act as connectors of 4,4′-bipyridyl ligands to produce two symmetrical 1-D chains along the b axis (Fig. 1c and S3†). As shown in Fig. S3,† all alternating vertical and horizontal 1-D chains are further held together by polyanions to generate a 3-D network. Compared to 1, 2 also has a 3-D Cu-coordination polymer structure built by [(Cu(H2O)2)2(μ-bipy)]4+ cations and {(C6H5P)2Mo5} polyanions (Fig. 2a and S4†). Each Cu2+ ion also links two polyanions, but the structural features of 2 are somewhat different from those of 1: the adjacent {(C6H5P)2Mo5} polyanions in 2 are connected by one Cu2+ ion, there is no infinite 1-D polyanion chain. As shown in Fig. 2b
Fig. 2 (a) Polyhedral and ball-and-stick representation of a 3-D structure of 2; (b) the 2-D plane of {(C6H5P)2Mo5} polyanions linked by Cu(II) units in 2; (c) the arrangement of Cu(II)-bipy-Cu(II) units (the hydrogen atoms have been omitted for clarity).
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and S5,† each {(C6H5P)2Mo5} polyanion acting as a hexadentate ligand links four Cu(II)-bipy/H2O units through six O atoms from four MoO6 octahedra, forming a 2-D plane structure. Each Cu2+ ion coordinates with one N atom (Cu1–N1 1.992(4) Å) from one 4,4′-bipyridyl molecule, and two terminal O atoms (Cu1–O10 2.617(3), Cu1–O5′ 1.927(3) Å) and one bridge O atom (Cu1–O8 1.956(3) Å) from two adjacent {(C6H5P)2Mo5} clusters, and two terminal water ligands (Cu1– O1W 1.962(3), Cu1–O2W 2.511(3) Å) (Table S3, Fig. S1b†), showing a strongly distorted octahedral geometry. The hydrogen bonds of O1W–H⋯O3/O4 are 2.800(4)/2.833(5) Å (Table S4†). As shown in Fig. 2c and S6,† each 4,4′-bipyridyl bridges two Cu2+ ions forming Cu(II)-bipy-Cu(II) fragments, which is further supported by polyanions. Moreover, there are two longer bonds (Mo3–O7 2.597(3) and Mo3–O8 2.519(3) Å) in the Mo3 subunit, while other Mo–O bond lengths are in the normal range (1.677(3)–2.379(3) Å), such obvious structural distortions can make its structure unstable. For instance, the thermal analysis shows that the polyanion skeleton of 2 completely collapses below 400 °C, indicating that its thermal stability is lower than that of 1 (see Fig. S9†). Characterization FT-IR spectroscopy. IR spectra of 1 and 2 (Fig. S7†) exhibiting bands at 3460–2868 cm−1 are characteristic vibrations of O–H, N–H and C–H. The bands in the region of 1618–1225 cm−1 are associated with the pyridine ring of a 4,4′bipyridyl molecule. The bands at 1140–1040 cm−1 are attributed to v(P–O) of the organophosphonate ligands and bands at 972–914, 907–824 and 700–673 cm−1 belong to v(Mo–Oterminal ) and v(Mo–Obridging) of polyoxoanion.6c X-ray power diffraction. X-ray power diffraction (XRPD) was checked at room temperature in contrast to the patterned data curve. As shown in Fig. S8 (ESI†), all major peaks of the simulated and experimental XRPD pattern are in agreement with each other, indicating their reasonable crystalline phase purity for compounds 1 and 2. The differences in intensity may be due to the preferred orientation of the crystalline powder samples. Thermal analysis. The curves corresponding to TG and DTA analysis of 1 and 2 are given in Fig. S9 (ESI†), in the temperature range of 30–700 °C; the total weight loss of 33.44% for 1 is consistent with the calculated value (calc. 33.35%). The weight loss is due to the removal of water, 4,4′-bipyridyl molecules and the phenyl group. For 2, in the range of 30–173 °C, the first weight loss of 5.20% corresponds to the loss of four coordinated water molecules. The obvious weight loss above 290 °C may be caused by combustion of organic components and the loss of phosphorus oxides. The total weight loss in the range of 290–550 °C (42.28%) is higher than the theoretical value (37.56%), indicating that the polyanion skeleton of 2 has completely collapsed, which is mainly related to its structural distortion and different environments of Cu(II). Thus, the thermal stability of 2 is lower than that of 1. Fluorescence properties. The photoluminescence properties of 1, 2 and the parent (NH4)4[(C6H5PO3)2Mo5O15] in the solid
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Fig. 3 Solid state emission spectra of the parent {(C6H5P)2Mo5}, 1 and 2 at room temperature.
state at room temperature are depicted in Fig. 3. As shown in Fig. 3, the parent {(C6H5P)2Mo5} cluster displays luminescence with two main emission peaks at 400 and 470 nm with λex = 300 nm, which should be assigned to LMCT (O → Mo).6c As shown in Fig. S10,† the emission peaks at about 293 and 363 nm with λex = 300 nm for phenylphosphonic acid and 4,4′bipy, respectively, should be assigned to π*→π transition. Upon excitation at 300 nm, 1 and 2 show emission peaks at 415 and 470 nm, respectively (Fig. 3), indicating that they have a similar structure. Compared with the emission of the parent {(C6H5P)2Mo5}, emissions of 1 and 2 are mainly caused by the {(C6H5P)2Mo5} cluster, but the peaks shift about 15 nm toward the longer wavelength due to changes in energy levels resulting from the coordination of {(C6H5P)2Mo5} to Cu2+ ions. On the other hand, the emission intensities of 1 and 2 have changed, that is, the coordination of {(C6H5P)2Mo5} to Cu2+ ions quenches partially LMCT (O → Mo). This indicates that 1 and 2 may be excellent candidates for photoactive materials. Catalytic activities of two compounds In multi-step organic synthesis, the acid-catalytic protection of the carbonyl group is one of the important reactions. In the presence of an acid catalyst, cyclohexanone reacts with glycol to form cyclohexanone ethylene ketal (Scheme S1†).8,9 We selected the synthesis of cyclohexanone ethylene ketal as a model reaction, to evaluate the catalytic activities of compounds 1, 2 and their parents. Taking 1 as an example, the reaction time, the molar ratio of the starting material, as well as the amount of catalyst, were systemically explored (Fig. S11– S13†). As shown in Fig. S11 and S12,† the yields of ketals increased quickly with increasing time within 2.5 hours, and the cyclohexanone/glycol molar ratio of 1 : 1.4 is a suitable substrate molar one. The amount of catalyst is also one of the important affective factors: when the catalyst is 1/200 of cyclohexanone, the maximum yield of cyclohexanone ethylene ketal is about 94% (Fig. S13†). According to the above results, the optimum conditions for the synthesis of cyclohexanone ethyl-
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Table 1 Catalytic performance of catalysts 1, 2, the parents {P2Mo5} and {(C6H5P)2Mo5}, and CuCl2 for the synthesis of cyclohexanone ethylene ketala
Entry
Catalyst
Solubility
Time (h)
Yield (%)
1 2 3 4 5 6
— {P2Mo5} {(C6H5P)2Mo5} CuCl2 Compound 1 Compound 2
— Insoluble Insoluble Soluble Insoluble Insoluble
2.5 2.5 2.5 2.5 2.5 2.5
7 51 76 ∼100 94 91
a Reaction conditions: the molar ratio of the catalyst (based on Mo) to cyclohexanone was 1 : 200, and the molar ratio of cyclohexanone to glycol was 1 : 1.4 (0.1 mol of cyclohexanone); water-carrying agent: 10 mL of cyclohexane; reaction temperature: 95–100 °C
ene ketal are as follows: the cyclohexanone/glycol molar ratio is 1 : 1.4; the molar ratio of the catalyst to cyclohexanone is 1 : 200; reaction temperature 95–100 °C, and reaction time 2.5 h. The acid-catalytic activities for compound 2, and their organo- and inorgano-parents under the same conditions are listed in Table 1. As shown in Table 1, the yields of ketal were 91, 76, 51 and 100% for compound 2, the parent {(C6H5P)2Mo5}, {P2Mo5} and CuCl2, respectively, while the yield of ketal was 7% without the catalysts, which shows that the title compounds have better acid-catalytic activity than their parent POMs. The Cu2+ and Mo6+ moieties, as the catalytically active centers, play important roles in the catalytic reaction. Moreover, we found that these POM catalysts maintain the catalytic activities during the later cycles. As shown in Fig. S14a,† when the catalysts were recovered by simple filtration after the reaction and reused, it was found that the catalytic activities decreased obviously during the second run compared with the fresh compounds. This may be attributed to the adsorption of the reactant (e.g. ethylene glycol or poly ethylene glycol) on the catalyst surface. If the catalysts were washed with ethyl ether and water, they could maintain higher activities after four cycles (Fig. S14b†). The above results suggest that compounds 1 and 2 are stable and efficient heterogeneous catalysts.
Experimental Materials and methods All chemicals were of reagent grade as received from commercial sources and used without further purification. C, H and N elemental analyses were performed on a Vario Elcube elemental analyzer, and P, Cu and Mo were analyzed on a Prodigy XP emission spectrometer. The infrared spectra were recorded on KBr pellets with a Bruker AXS TENSOR-27 FTIR spectrometer in the range of 4000–400 cm−1. The photoluminescence property was determined on a RILI F-7000 fluorescence spectrophotometer in the solid state at room temperature. Single crystal X-ray diffraction data were collected on a Bruker Smart APEX II X-diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). X-ray powder diffraction data were collected on a Bruker AXS D8 Advance diffractometer
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using Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 5–60° with a step size of 0.02°. TG analyses were performed on a Pyris Diamond TG-DTA thermal analyzer in air at a heating rate of 10 °C min−1. The yield of cyclohexanone ethylene ketal was confirmed on a JK-GC112A Gas Chromatograph. Syntheses of compounds Compound 1. Na2MoO4·2H2O (0.15 g, 0.63 mmol) and CuCl2·2H2O (0.04 g, 0.25 mmol) were dissolved in water (20 mL), and the mixture was stirred for 30 min. Then bipy (0.04 g, 0.25 mmol) was added with vigorous stirring. The pH value of the mixture was adjusted to 3–4 with 4 mol L−1 HCl under continuous stirring. Then phenylphosphonic acid (0.04 g, 0.25 mmol) was added. The resulting mixture was then transferred to a 30 mL Teflon-lined autoclave and kept at 150 °C for 3 days, dark blue block crystals of 1 were obtained (yield: ca. 43% based on Mo). Elemental analysis, Calcd for C32H30O23N4Cu2P2Mo5: C 25.5, H 2.0, N 3.7, P 4.1, Cu 8.4, Mo 31.8%; Found: C 25.4, H 2.05, N 3.8, P 4.1, Cu 8.45, Mo 31.7%. FT IR (KBr pellet), cm−1: 3454(w), 3146(m), 1607(m), 1535(w), 1497(w), 1410(m), 1225(w), 1126(s), 1040(m), 972(m), 907(s), 673(s), 557(m), 509(m). Compound 2. The mixture of Na2MoO4·2H2O (0.48 g, 2.0 mmol), CuCl2·2H2O (0.22 g, 1.3 mmol) and H2O (20 mL) was stirred for 30 min, and the pH value was also adjusted to 3–4, then phenylphosphonic acid (0.13 g, 0.8 mmol) and bipy (0.08 g, 0.50 mmol) were added in order with vigorous stirring. The resulting mixture was then transferred to a 30 mL Teflonlined autoclave and kept at 160 °C for 3 days (see Scheme 1). Light blue block crystals of 2 were obtained (yield: ca. 36% based on Mo). Elemental analysis, Calcd for C22H26O25N2Cu2P2Mo5: C 19.05, H 1.9, N 2.0, P 4.5, Cu 9.2, Mo 34.9%; Found: C 19.1, H 1.85, N 2.2, P 4.5, Cu 9.15, Mo 34.7%. FT IR (KBr pellet), cm−1: 3460(m), 3128(m), 2930(w), 2868(w), 1618(m), 1398(s), 1126(s), 1140(w), 1087(m), 914(m), 824(w), 700(m), 609(w), 545(w). Single-crystal X-ray diffraction The structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 using SHELXTL-97.10,11 An empirical absorption correction was applied using the SADABS program. Crystal data and structure refinement parameters of compounds 1 and 2 are listed in Table 2. Hydrogen atoms on C atoms were added in calculated positions. Bond lengths and angles are listed in Tables S1–S4.† CCDC reference numbers: 1012272 and 1012273. Catalytic experiment Protection for the carbonyl group. Acid-catalyzed synthesis of cyclohexanone ethylene ketal was used as a model reaction to evaluate the catalytic performances of 1 and 2. A typical procedure of the catalytic activity test is as follows: the catalyst (e.g. compound 1) was added to a mixture of cyclohexanone, glycol and cyclohexane (10 mL) in a 50 mL three-necked round-bottom flask fitted with a Dean–Stark apparatus to remove the water continuously from the reaction mixture. After
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Table 2
Dalton Transactions Crystal and refinement data for compounds 1 and 2
Formula Formula weight T/K Wavelength/Å Crystal system Space group a/Å b/Å c/Å α/° β/° γ /° V/Å3, Z Dc/g cm−3, F000 GOF Reflections collected Unique data, Rint θ range (°) R1(I > 2σ(I))a wR2 (all data)a a
1
2
C32H30O23N4Cu2P2Mo5 1507.34 296(2) 0.71073 Orthorhombic Fdd2 18.8251(2) 42.656(2) 10.6435(5) 90.00 90.00 90.00 8546.8(7), 8 2.343, 5854 1.037 10597
C22H26O25N2Cu2P2Mo5 1387.17 296(2) 0.71073 Monoclinic C2/c 17.3158(12) 9.8869(7) 21.7619(16) 90 102.907(1) 90 3631.5(4), 4 2.537, 2680 1.036 8868
3492, 0.0222 1.91 to 24.99 0.0189 0.0442
3198, 0.0292 1.92 to 24.99 0.0267 0.0614
R1 = ∑||F0 |− |FC||/∑|F0|; wR2 = ∑[w(F02 − FC2)2]/∑[w(F02)2]1/2.
completion of the reaction, the heterogeneous catalyst remained at the bottom of the reaction vessel and was easily separated from the organic phase containing product by decantation. The recovered catalyst was reused in a new reaction under identical experimental conditions. Each of the procedures was repeated for three cycles. The products obtained were characterized by gas chromatography. The above experiment was repeated under the same conditions except that compound 1 was replaced by compound 2, CuCl2, the parent polyoxometalate {P2Mo5} and {(C6H5P)2Mo5}, respectively.
Conclusions In summary, two unprecedented Strandberg-type POMs containing {(C6H5P)2Mo5} clusters and Cu(II)-(bipy)/H2O units have been successfully assembled by hydrothermal methods. These organophosphomolybdates were firstly used as acid-catalysts in organic reaction, and exhibited better catalytic performances than those of their parents {(C6H5P)2Mo5} and {P2Mo5}, which shows that the synergistic effect of polyanions and transition metal complexes in inorganic–organic hybrids lead to enhanced catalytic properties. The solid-state luminescent studies show that 1 and 2 exhibit photoluminescence properties. Further research on other organophosphomolybdates is underway in our group.
Acknowledgements This work was financially supported by the Natural Science Foundation of Liaoning Province (no. 2013020128) and the
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Foundation of Education Department of Liaoning Province (no. L2013414).
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