A Monomeric Tricobalt(II)-Substituted Dawson-Type Polyoxometalate

Aug 16, 2017 - synthesis of monomeric Dawson-type cobalt-substituted POMs ... previously reported Dawson-type [P2W15Nb3O62]9− polyoxoan-...
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A Monomeric Tricobalt(II)-Substituted Dawson-Type Polyoxometalate Decorated by a Metal Carbonyl Group: [P2W15O56Co3(H2O)3(OH)3Mn(CO)3]8− Jiage Jia, Yanjun Niu, Panpan Zhang, Dongdi Zhang, Pengtao Ma, Chao Zhang, Jingyang Niu,* and Jingping Wang* Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, Henan, China S Supporting Information *

synthesis of monomeric Dawson-type cobalt-substituted POMs is still challenging. The [Mn(CO)3]+ fragment is generally attached to a triangle of three contiguous bridging O atoms of a metal−oxo cluster, forming the kinetically stable d6 low-spin octahedral 18-electron moieties fac-[(OC)3ML3]+ (L = ligand).15,16 POMs, as a kind of multidentate ligand, can coordinate with the [M(CO)3]+ fragment, forming a new branch of POMs, namely, POMsupported metal carbonyl derivatives (PMCDs). Recently, some PMCDs have already been catalogued in the literature. Nevertheless, reports about Dawson-type PMCDs remain relatively rare (Table S1).17−21 Herein, a metal carbonyl compound was involved in the reaction system of TMSPs, which has become an intriguing result. A POM-supported [Mn(CO)3]+ complex containing the mixed-metal cubane [Na(H 2 O) 5 ](NH 4 ) 7 [P 2 W 15 O 56 Co 3 (H2O)3(OH)3Mn(CO)3]·19H2O (1) has been successfully synthesized by a conventional aqueous solution method, which is the first example of a monomeric tricobalt(II)-substituted Dawson-type PMCD. Orange crystals of 1 were prepared in the simple one-pot reaction of [α-P2W15O56]12− with Co(OAc)2· 4H2O and Mn(CO)5Br in a slightly acidic (pH 6.0) solution. It is worth noting that the category of the cation is important for the formation of suitable diffraction-quality crystals. The experimental XRD pattern of sample 1 is basically consistent with the simulated XRD pattern, which comes from the data of singlecrystal X-ray diffraction (XRD), indicating the phase purity of the product (Figure S1). Single-crystal XRD analysis indicates that 1 crystallizes in the monoclinic I2/a space group. The detailed crystallographic data and structure refinement parameters are listed in Table S2. Complex 1 contains the discrete cluster anion [P2 W15O 56 Co 3(H 2O) 3(OH) 3 Mn(CO) 3 ]8− (1a), Na/NH 4 + mixed cations, and lattice water molecules. This new polyanion 1a can be described as grafting the carbonyl metal group {Mn(CO)3} onto the tricobalt-substituted POM framework {P2W15Co3O62} (Figure 1), which is similar to that of the previously reported Dawson-type [P2W15Nb3O62]9− polyoxoanion-supported Re(CO) 3 + complex, [P 2 W 15 Nb 3 O 62 Re(CO)3]8−.17 The {P2W15Co3O62} subclass is constructed of a {Co3O6} cap embedded in the defect site of the [α-P2W15O56]12−

ABSTRACT: A monomeric tricobalt(II)-substituted phosphotungstate polyanion, [H9P2W15O62Co3]9−, is stabilized by the attachement of an organometallic group, {Mn(CO)3}, for the first time. The resulting polyoxometalate-supported [Mn(CO)3]+ complex (1) can be used as an efficient catalyst in the cycloaddition of CO2 with epoxides under mild reaction conditions with pyrrolidinium bromide as a cocatalyst. Besides, magnetic measurements show that the compound exhibits weaker ferromagnetic interactions at low temperature.

T

ransition-metal-substituted polyoxometalates (TMSPs) possess intriguing structure diversity and special properties applicable to diverse fields such as materials science,1 magnetism,2,3 and catalysis,4,5 which have been focused on recently.6 The trivacant Dawson fragment [(α-P2W15O56)]12− (P2W15), an important precursor, has been utilized to make some neotype TMSPs.7 However, the d-electron-containing metal centers will seriously affect the structures of TMSPs. Generally, high-valent metals (VV, NbV, TaV, and SnIV) produce the trisubstituted parent species [M3P2W15O62]n−, while low-valent metals (FeIII, MnII, NiII, and CoII) give rise to the dimeric sandwich-type species, which are presumably related to the size of the transition metals, the overall charge of the polyoxometalates (POMs), and other thermodynamic factors.8 It is worth noting that preservation of the monomeric P2W15 skeleton has come true in spite of using the higher d-electron-containing transition metal in recent years. In that context, [(FeX)2Fe(OH2)(P2W15O59)]11− (X = Cl/ OH) is the first example of a monomeric Dawson structure trisubstituted with low-valent transition metals, in the synthesis of which a high ionic strength (4 M NaCl) is necessary,8 while the monomeric Mn-substituted Dawson POM [Mn4O3(Ac)3(αP2W15O56)]8− is prepared by a fragment of the prototypal {Mn12} complex in a CH3COOH/H2O solution.9 The decorated cubane-type [MnIII3MnIVO4] is the key fragment to immobilize the P2W15 framework. By contrast, the reports on monomeric Nisubstituted Dawson POMs are relatively more,9−14 but most complexes were made under hydrothermal conditions, where organic ligands play a tunable role for the transition-metal Ni cations further capping the P2W15 fragment.9−12 However, the © 2017 American Chemical Society

Received: May 18, 2017 Published: August 16, 2017 10131

DOI: 10.1021/acs.inorgchem.7b01231 Inorg. Chem. 2017, 56, 10131−10134

Communication

Inorganic Chemistry

The magnetic behavior of {MnCo3O4}-containing metal−oxo cluster 1 is displayed in Figure 2 as a plot of the product χMT

Figure 1. Ball-and-stick and polyhedral representations of polyanion 1a [side view (a and b) and top view (c and d)], polyhedral representations of the cubane {MnCo3O4} motif (e), and coordination configurations of Co (f) and Mn (g). Color code: P, yellow balls; W, azure balls; Co, pink balls; O, red balls; C, black balls; Mn, green balls; WO6 octahedra, azure; PO4 tetrahedra, yellow; CoO6 octahedra, pink. Water molecules and cations are omitted for clarity. Figure 2. Plots of χMT versus T for 1. Inset: Field dependence of magnetization of 1 at different temperatures.

ligand through six Co−O−W bridged linkages [Co−O, 2.041− 2.064 Å; W−O, 1.743−1.777 Å] and one Co−O−P bond [Co− O, 2.186−2.240 Å; P−O, 1.544 Å], forming a P2W18-like saturated Well−Dawson structure. Then the [Mn(CO)3]+ fragment is bonded to a triangle of three contiguous surface O atoms of the Co3O6 “crown” in {P2W15O62Co3} via three Mn− O−Co bonds. The Co3O6 “crown” is aggregated together by three edge-sharing CoO6 octahedra. The terminal O atom on each of the CoO6 octahedra is from a coordinated water molecule. The Mn atom of the {Mn(CO)3} fragment affords a distorted six-coordinated octahedral fac configuration defined by three O atoms from the {P2W15Co3O62} subunit and three C atoms from CO (Figure 1g). The {Mn(CO)3} unit and Co3O6 “crown” are connected by three bridged O atoms, notably, forming a mixed-metal cubane motif, {MnCo3O4} (Figure 1e). The {MnCo3O4} cubane is similar to that of the {MnM3O4} subunit of the {H2M8O30(Mn(CO)3)2} (M = Mo/W) polyanion reported by our group.22,23 The special structure is related to the kinetically stable d6 low-spin octahedral 18-electron moieties fac[(OC)3ML3]+ (M = Mn/Re), containing metal tricarbonyl units bonded to a trigonal adjacent O atoms on the surface of POMs. It is worth mentioning that, like the capping WO6 in the Nicontaining Dawson-type analogue [Ni3(OH)3(H2O)3P2W16O59]9−, reported by Hill’s group in 2016,14 the Mn(CO)3 fragment also plays a role of the stabilizing the Dawson-like {P2W15Co3O62} unit, which is the key factor of the constitution of monomeric low-valent transition-metal-substituted POMs with the well-known [P2W15O56]12− ligand. Besides, two neighboring 1a were linked by two [Na(H2O)5]+ bridges to form a dimeric structure (Figure S5b). The Na ion is coordinated by five O atoms from water molecules and one O atom from the WO6 (W11) octahedron at the equator of one polyanion and one O atom from the WO6 (W8) octahedron of the other polyanion. Two Na and four W atoms are almost in the same plane, forming a hexagonal geometry (Figure S5c). Bondvalence-sum (BVS) calculations (Tables S3) reveal that the range of bond valences for the three terminal O atoms on Co sites as 0.284−0.311 Å, confirming that all of them are diprotonated, while three μ3-O atoms (O57, O58, and O59) linking three adjacent Co2+ ions are monoprotonated, as indicated by the BVS calculations (1.186, 1.200, and 1.209 Å).

versus T measured between 1.8 and 300 K in a magnetic field of 1000 Oe. The χMT value at room temperature (300 K) of 9.74 emu K mol−1 is much higher than the expected value of 5.63 emu K mol−1 for three spin-only CoII ions (S = 3/2 and g = 2), which is associated with the strong orbital contribution of the high-spin octahedral Co2+.24 The χMT versus T curve exhibits a continuous decrease smoothly from 300 K to a minimum of 7.57 emu K mol−1 at 18 K. Below 18 K, the χMT value increases rapidly to reach a maximum of 7.96 emu K mol−1 at about 5 K and then falls sharply to 7.59 emu K mol−1 at 1.8 K. The decrease of χMT from room temperature is due to the characteristic spin−orbit coupling of the single CoII ions, while the sharp upturn of χMT at the low temperature is indicative of the ferromagnetic Co···Co interactions within the trinuclear spin cluster. Finally, the χMT value decreases below 5 K, which is related to magnetic anisotropy. Besides, the field dependences of magnetization for 1 at low temperatures were studied. The result shows that the magnetization increases abruptly to 3.38 μB at 25 kOe. Above 25 kOe, it increases in a relatively linear manner and finally reaches 4.03 μB at 70 kOe, which is still not saturated (Figure 2, inset). The increase of magnetization is in accordance with the presence of ferromagnetic interactions, while the lack of saturation is further indicative of magnetic anisotropy. What is more, the M versus H/T curves at different temperatures show that the three lines do not overlap, which further supports the above conclusion. The experimental magnetization of 4.03 μB at 70 kOe suggests a g factor of 2.6 and is in line with three high-spin CoII ions with an effective spin of 1/2 for each CoII center. This result corresponds to an average g factor of 2.4, which obtained from the experimental (χMT)max of 7.96 emu K mol−1 at 5 K, further manifesting the presence of magnetic anisotropy of the {Co3} cluster.25−27 In short, in the special mixed-metal cubane motif {MnCo3O4}, CoII ions represent d7 high-spin centers (S = 3 /2), whereas the MnI ion is d6 low-spin (S = 0); therefore, unpaired electrons will be on the CoII centers.28 The measurements of magnetic susceptibility prove the presence of ferromagnetic interaction within the basal Co3 triangle at low temperature. 10132

DOI: 10.1021/acs.inorgchem.7b01231 Inorg. Chem. 2017, 56, 10131−10134

Communication

Inorganic Chemistry On the basis of previous reports,29 the catalytic activity of complex 1 in the cycloaddition of CO2 was studied (Table 1).

substituted POMs [(n-C7H15)4N]x[α-M(H2O)SiW11O39] (M = Co2+, Mn2+, Ni2+, Fe3+, and Cu2+; x = 5 and 6) are used to catalyze the reaction of CO2 with 1,2-epoxypropane, and the deduced mechanism is proposed by Sakakura et al.34 Herein, we speculated that the reaction mechanism is analogous to that supposed by Sakakura et al: First, CO2 is activated via coordination to the Co center. Simultaneously, Br−, dissociated from pyrrolidinium bromide, attacks the C atom of the epoxide. Then the O atom of CO2 attacks the C atom of the activated epoxide ring, followed by a ring-opening reaction step, resulting in the formation of the nucleophilic alkoxide that eventually reacts with the electrophilic CO2 to give the cyclic carbonates. In conclusion, a novel Dawson-type PMCD has been successfully isolated by assembling a [(α-P2W15O56)]12− building block with a cubane-like core, {MnICoII3O4}. The enhanced stability of the tricobalt species is most likely attributed to the unique coordination environment of the tricarbonyl metal group [Mn(CO)3]+. Compound 1 is the first example of monomeric tricobalt(II)-substituted Dawson-type PMCDs. Furthermore, 1 shows high catalytic activity in the cycloaddition of CO2 with epoxides under mild reaction conditions with pyrrolidinium bromide as a cocatalyst. Most importantly, the strategy of the introduction of a metal carbonyl compound into the reaction system of TMSP chemistry has further shown promising results. More Dawson-type PMCDs containing other transition metals will be prepared in the future.

Table 1. Cycloaddition of CO2 Catalyzed by Catalyst 1 with Ionic Liquid 2a

a

Reaction conditions: catalyst 1 (0.23 mol %), epoxides (5 mmol), ionic liquid 2 (8 mol %, 80 mg). bCO2 was charged into an autoclave. c Determined by gas chromatography using an internal standard technique; the selectivities were over 99% in all cases. dSecond run. e Third run.

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The experimental results show that compound 1 is a high active catalyst for the coupling epoxide and CO2 to afford the cyclic organic carbonate (COC) in 91% yield after 1.5 h at 70 °C under 1.5 MPa with the ionic liquid pyrrolidinium bromide (2; entry 3). The conversion of epoxide A (glycidyl phenyl ether) was evaluated by a combination of complex 1 (0.23 mol %) with ionic liquid (8 mol %), providing a quantitative yield for the COC product B (3-phenoxy-1,2-propylene carbonate). Complex 1 itself proved to be unproductive, whereas the ionic liquid alone gave a 38% yield of B (entries 7 and 1). In order to seek optimal conditions, three factors were investigated in the reaction: (1) The formation of COC is strongly affected by the reaction temperature. Raising the reaction temperature from 65 to 70 °C resulted in a rapid increase of the yield from 60% to 91% (entry 2). (2) The reaction time also influences the yield of COC. When the time was prolonged to 1.5 h from 1 h, the yield enhanced 27% (entry 5). Nevertheless, the yield would come up to 97% when the reaction time was 2 h (entry 6). (3) Compared with the reaction temperature and time, the CO2 pressure has relatively little influence for the yield of COC. The yield just increased by 8%, when the CO2 pressure was added to 1.5 MPa from 1.0 MPa (entry 4). Besides, it appears that, under the same optimal conditions, the raw material, like P2W15, Mn(CO)5Br, and Co(OAc)2·4H2O, showed somewhat low catalytic activity compared with catalyzer 1 (Table S6). The cyclic performance of catalyst 1 in the reaction was also investigated. 1 was used to catalyze the cycloaddition of CO2 for three cycles (entries 8 and 9). After three cycles, the carbonyl groups fall from the skeleton of POMs, which was shown by a comparison of the IR spectra of the used and fresh catalysts (Figure S8), indicating that the catalyst was not relatively stable in this system. Other epoxides (glycidyl methacrylate/chloromethyloxirane) were then also examined (entries 10 and 11). The results display that compound 1 was also activated for these epoxides with the ionic liquid as a cocatalyst. The mechanism of the cycloaddition of CO2 and epoxides has been investigated extensively.30−33 In 2005, the transition-metal-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01231. Synthesis, BVS, XRD, IR, EDX, TGA, UV−vis, CV, and catalytic properties of 1 and the structures of stacking and {Na2W4} clusters (PDF) Accession Codes

CCDC 1542974 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.N.). *E-mail: [email protected] (J.W.). ORCID

Jingyang Niu: 0000-0001-6526-7767 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (Grants 20771034 and 21401042), Postdoctoral Science Foundation of Henan Province (Grant 2015031), and 2015 Young Backbone Teachers Foundation from Henan Province (Grant 2015GGJS-017). 10133

DOI: 10.1021/acs.inorgchem.7b01231 Inorg. Chem. 2017, 56, 10131−10134

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01231 Inorg. Chem. 2017, 56, 10131−10134