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Two novel trivacant Keggin-type polytungstates supported manganesecarbonyl derivatives synthesized by degradation of metastable[γ-XW10O36]

8− (X = GeIV, SiIV)†

Juan Zhao, Junsheng Wang, Junwei Zhao, Pengtao Ma, Jingping Wang* and Jingyang Niu*

Received 6th December 2011, Accepted 23rd January 2012DOI: 10.1039/c2dt12130a

Two novel trivacant Keggin-type polyoxometallate-based manganese carbonyl derivatives K8[(OC)3Mn-(A-α-H2GeW9O34)]·10H2O (1) and K8[(OC)3Mn(A-α-H2SiW9O34)]·11H2O (2) have been synthesized bydegradation of the metastable [γ-XW10O36]

8− (X = GeIV, SiIV) in CH3CN–H2O solvent (1 : 2, vol.) andstructurally characterized by elemental analyses, IR spectra, thermogravimetric analysis, and single-crystalX-ray diffraction. X-ray diffraction analyses indicate that 1 and 2 are isomorphic and consist of a[(OC)3Mn]+ group and a trivacant [A-α-H2XW9O34]

8− fragment, which exhibit the first examples oftrivacant Keggin-type metal carbonyl derivatives. Additionally, the electrocatalytic properties of both forNO2

− oxidation have been investigated.

Introduction

The continuous interest in the search for and synthesis of poly-oxometallate (POM)-based organometallic derivatives is domi-nantly driven by their intrinsic properties and widespreadapplications in catalysis, electro- and photo-chromism, magnet-ism and medicine.1,2 Since the first POM-incorporated organo-titanium derivative α-[(Cp)Ti(PW11O39)]

4− was synthesized byHo and Klemperer in 1978,3 considerable efforts have beendirected toward the synthesis and characterization of the organo-metallic derivatives. Up to now, the research into POM-basedorganogermanium2,4–6 and organotin2,4,5a,7–9 derivatives hasmade great achievements and the noble organometallic (organo-ruthenium,10 organorhodium11) derivatives of POMs also havebeen paid more attention in recent years. However, the organicmoieties on the reported organometallic compounds mainlyinclude cyclopentadienyl, phenyl and alkyl groups. On the otherhand, the development of transition metal carbonyl derivativesof POMs is very slow. Furthermore, most of the reported com-pounds are dominated by Lindqvist-type derivatives.12,13

However, reports on Keggin-type-POM-based transition metalcarbonyl derivatives are rare, such as [(Ph3P)3Rh(CO)]n[XM12O40] (X = P, M = Mo or W, n = 3; X = Si, M = Moor W, n = 4) and [α-SiW11O39Ru

II(CO)]6−.14a,b,g Hitherto, noKeggin-type POM-supported transition metal carbonyl derivative

has been discovered, although some germanium and stannumcarbonyl adducts joined via metal–metal bonds are known.14c,d

Additionally, from the viewpoint of coordination chemistry, d6

transition metal carbonyl cations in the form of fac-[(OC)3Mn-(NCCH3)3]

+ (M = Mn, Re)15a–c should be predisposed to reactwith polyoxoanions. Some documents have confirmed that theycan react readily with several different two-electron donorligands L to afford kinetically stable d6 low-spin octahedral 18-electron moieties fac-[(OC)3ML3]

+ (M = Mn, Re).12c So it ismost likely that trigonal adjacent surface oxygens in KegginPOMs as two-electron L ligands coordinate with [(OC)3M]+ fea-turing the fac-[(OC)3MO3]

+ moiety, which is similar to those inFig. 1.12–14e,f The background provides us with a good opportu-nity to exploit this system containing Keggin POMs and metalcarbonyl components. Our interest in this field stemmed fromthe study of the reactions of Keggin polyoxoanions with Mn-(CO)5Br. We choose the metastable divacant [γ-XW10] (X =GeIV, SiIV) precursors to react with Mn(CO)5Br based on thefollowing considerations: (1) if conditions are appropriate,[γ-XW10] can retain their original γ-frameworks and the lacunary

Fig. 1 Illustrations of some representative POM-supported metal car-bonyl derivatives previously reported (M = Mn, Re; N = Nb, Ta, W).

†Electronic supplementary information (ESI) available: Related figureson the IR, XPS, TG, Uv-vis. CCDC 848377 and 848378 for 1 and 2,respectively. For ESI and crystallographic data in CIF or other electronicformat see DOI: 10.1039/c2dt12130a

Institute of Molecular and Crystal Engineering, College of Chemistryand Chemical Engineering, Henan University, Kaifeng, Henan 475004,P. R. China. E-mail: jpwang@henu.edu.cn, jyniu@henu.edu.cn; Fax:(+86) 378-3886876

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sites may act as the structure-directing agent (SDA) to inducethe formation of POM-supported/-incorporated clusters.15d (2)The metastable [γ-XW10] can easily transform to other POMfragments, such as plenary,16a monolacunary,16b trilacunary,17a–c

and even tetralacunary16c,18 Keggin POM units (Fig. 2), whichprovides the possibility for the structural variety of desired pro-ducts. By adjusting the volume ratio of the CH3CN–H2O mixedsolvent to tune the subtle reactions between electrodonors(POMs) and electrophiles (Mn(CO)5Br), K8[(OC)3Mn-(A-α-H2GeW9O34)]·10H2O (1) and K8[(OC)3Mn(A-α-H2Si-W9O34)]·11H2O (2) have been obtained in the CH3CN–H2Omixed solvent with the volume ratio 1 : 2 and structurally charac-terized by IR spectroscopy (Fig. S1†), thermogravimetric (TG)analyses (Fig. S2†), elemental analyses, UV spectra and single-crystal X-ray diffraction. Their electrocatalytic properties forNO2

− have been investigated.

Experimental section

Materials and methods

The precursors K8[γ-GeW10O36]·6H2O17d and K8[γ-

SiW10O36]·12H2O17e were synthesized as previously described.

All other chemicals were used as purchased without purification.IR spectra were obtained from a solid sample pelletized withKBr on a Nicolet 170 SXFT-IR spectrometer in the range400–4000 cm−1. Elemental analyses (C, H, and N) were per-formed on a Perkin-Elmer 240C elemental analyzer. Inductivelycoupled plasma (ICP) spectra were obtained on a Perkin-ElmerOptima 2000 ICP-OES spectrometer. UV spectra were obtainedwith a U-4100 spectrometer at room temperature. All electroche-mical measurements were performed at room temperature in astandard three-electrode cell connected to a LK98 microcom-puter-based electrochemical system (LANLIKE, Tianjin, China).A freshly cleaned glassy carbon disk electrode (3 mm diameter)was used as a working electrode, a platinum wire served as thecounter electrode and an Ag/AgCl electrode as the referenceelectrode. Magnetic susceptibility measurements were carriedout with a Quantum Design MPMS-XL-7 magnetometer in thetemperature range of 2.7–300 K. The susceptibility data werecorrected from the diamagnetic contributions as deduced byusing Pascal’s constant tables.

Synthesis of K8[(OC)3Mn(A-α-H2GeW9O34)]·10H2O (1)

Mn(CO)5Br (0.137 g, 0.498 mmol) in 7.0 mL CH3CN wasrefluxed in the dark for 20 min and then cooled to room tempera-ture, then dropped into a solution of K8[γ-GeW10O36]·6H2O(1.500 g, 0.516 mmol) in 15 mL distilled water and the pHvalue was 7.04. The resulting suspension was stirred at 80 °C for30 min and red clear solution was observed. Then the solutionwas cooled to room temperature and filtered. The filtrate wasallowed to stand in the dark for slow evaporation. Yellow blockcrystals of 1 were isolated after some days. (Yield: ca. 10%based on Mn(CO)5Br). Elemental analysis (%) calcd for 1: C,1.24; H, 0.76 C; K, 10.77; Mn, 1.89; Ge, 2.50; W, 56.95; found:C, 1.30; H, 0.65; K, 10.89; Mn, 2.06; Ge, 2.31; W, 60.17. IR(KBr pellet): 2020(s), 1896(s), 1636(s), 966(m), 912(s), 838(s),680(w), 597(m), 552(m) cm−1.

Synthesis of K8[(OC)3Mn(A-α-H2SiW9O34)]·11H2O (2)

The synthetic process of 1 was used withK8[γ-SiW10O36]·12H2O (1.600 g, 0.531 mmol) replacingK8[γ-GeW10O36]·6H2O. The pH value of the resulting suspen-sion was 7.16. Yellow block crystals of 2 were obtained after aweek. (Yield: ca. 16% based on Mn(CO)5Br). Elemental analysis(%) calcd for 2: C, 1.26; H, 0.84; K, 10.93; Mn, 1.92; Si, 0.98;W, 57.80; found: C, 1.27; H, 0.86; K, 11.20; Mn, 1.73; Si, 1.25;W, 60.01. IR (KBr pellet): 2020(s), 1903(s), 1635(s), 986(m),912(w), 847(s), 699(w), 644(w), 533(s) cm−1.

Crystallographic data collection and refinement

A crystal with dimensions, 0.42 × 0.31 × 0.28 mm3 for 1, 0.37 ×0.15 × 0.12 mm3 for 2 was stuck on a glass fiber and intensitydata were collected on a Bruker Apex-II CCD detector usinggraphite monochromatized Mo Kα radiation (λ = 0.71073 Å) atroom temperature. The structures were solved by direct methodsand refined using full-matrix least squares on F2. All calculationswere performed using the SHELXTL-97 program package.Routine Lorentz and polarization corrections were applied. Theabsorption correction was based on multiple and symmetry-equivalent reflections in the data set by using the SADABSprogram. All hydrogen atoms were refined isotropically as ariding mode using the default SHELXTL parameters. No hydro-gen atoms associated with the molecules were located from thedifference Fourier map. All non-hydrogen atoms were refinedanisotropically except for some water molecules. A summary ofcrystal data and structure refinements for 1 and 2 is listed inTable 1. CCDC reference no. 848377and 848378 for 1 and 2,respectively.

Results and discussion

Synthesis

It should be pointed out that the structural transformationsbetween [γ-XW10O36]

8− (X = GeIV, SiIV) and other POM unitsare very common. In our experiments, the [γ-XW10] (X = GeIV,SiIV) unit as a starting material was transformed to the [A-α-XW9] unit in CH3CN–H2O mixed solvent with pH = 7.04 for

Fig. 2 Transformations between γ-XW10O36 and different POM units.

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1 and pH = 7.16 for 2 at 80 °C. Such degradation phenomenahave previously been observed at 95 °C in pH 8.2 aqueous solu-tion.19a Up to now, some reactions in which the [γ-XW10] wasretained occurred in pH = 3.5–4.8 HAc–KAc buffer solutions(Table S5†). In 2007, Kortz et al. isolated the [{β-Fe2Ge-W10O37(OH)2}2]

12− by the [γ-GeW10] precursor in pH = 4.8HAc–NaAc/HAc–KAc buffer solution at 45 °C.19d Interestingly,when the temperature increased to 50 °C, [Cu3(H2O)-(B-β-GeW9O33(OH))(B-β-GeW8O30(OH))]

12− was obtained,19e

in which [γ-GeW10] was degraded to [γ-GeW9] and [γ-GeW8].Recently, our group addressed the [Co(H2O)6]2·[Co(H2O)3-(α-GeW11CoO38)3]

6− by reaction of [γ-GeW10] with Co2+ inaqueous solution at room temperature.18b However, the Hillgroup reported [{Fe2(OH)3(H2O)2}3(γ-SiW10O36)3]

15− in pH =4.3 aqueous solution at room temperature, in which the architec-ture of [γ-SiW10] was retained.

19f Meanwhile, we also attemptedto make the [γ-XW10]-supported or incorporated manganese car-bonyl derivatives in HAc–KAc buffer solutions at the differenttemperatures. However, we failed. As shown in Table S5,† thechanges of pH value, temperature, molar ratio of the TM cation/the precursor influence the transformations of [γ-XW10] to otherPOM units. Therefore, it is very difficult to summarize a lawdetermining under what conditions the [γ-XW10] can retain theiroriginal architecture or transform to other POM fragments,which may be related to the instability of the metastable[γ-XW10]. In addition, parallel experiments indicate that the[γ-XW10] unit plays an important role in the formation of 1 and2. When [A-α-XW9] replaced [γ-XW10], 1 and 2 cannot be iso-lated. Moreover, the ratio of CH3CN–H2O is a crucial factor inthe formation and crystal growth of the product phases. Goodcrystals of 1 and 2 can only be obtained in the range of CH3CN–H2O = 1 : 2–1 : 5 with the optimum ratio of 1 : 2. On increasingor decreasing the ratio, only yellow power was obtained.

Structure description

The experimental XRPD patterns for 1 and 2 are in good agree-ment with the simulated XRPD patterns from the single-crystalX-ray diffraction, indicating the good phase purity of 1 and 2(Fig. S3†). Single-crystal X-ray diffraction analyses reveal that 1and 2 are isomorphic and crystallize in the triclinic space groupP1̄, hence only the structure of 1 is well described herein. On thebasis of bond valence sum calculations,20 the oxidation states ofall W and Ge atoms in 1 are +6 and +4 (Table S3†), respectively.Furthermore, in order to determine whether the oxidation statesof Mn ions in 1–2 are +1 or not, X-ray photoelectron spec-troscopy (XPS) and magnetic susceptibility of 1 have beenmeasured. The Mn(2p3/2) binding energy is 639.5 eV for 1(Fig. S4a†), being consistent with the Mn(2p3/2) binding energyof 639.6 eV for [Mn(CO)5]Br (Fig. S4b†), which indicates thatthe oxidation state of the Mn ion in 1 is +1. The result is furtherconfirmed by the result of magnetic susceptibility measurement(Fig. S5†). In addition, considering the charge balance of twocompounds, two protons have been added in 1 and 2 (in ESI†),respectively. As shown in Fig. 3a and b, the [(OC)3Mn-(A-α-H2GeW9O34)]

8− unit is constructed by a (OC)3Mn+ clusterand a trivacant [A-α-H2GeW9O34]

8− fragment. The [A-α-H2GeW9O34]

8− fragment is derived from the well-known tri-vacant Keggin ion by removal of a corner-shared W3O6 triadfrom a saturated Keggin [α-XW12O40] unit with the Ot, μ2-O,μ3-O and μ4-O in the range of 1.733(9)–1.767(9) Å, 1.800(8)–2.184(8) Å, 2.029(9)–2.272(9) Å and 2.290(8)–2.310(8) Å,respectively. The [(OC)3Mn]+ cluster is attached to the “W3O9”

cap in [A-α-GeW9O34]10− via three Mn–O–W bonds (Mn–O:

2.044(8)–2.057(9) Å), affording a distorted six-coordinated octa-hedron fac-configuration defined by two oxygen atoms from the[A-α-GeW9O34]

10− fragment and two carbonyl groups standingon the basal plane and another oxygen atom from [A-α-GeW9O34]

10− fragment and carbonyl group occupying theaxial position [Mn–O: 2.044(8)–2.057(9) Å, Mn–C: 1.772(14)–

Table 1 Crystallographic data and structural refinements for 1 and 2

1 2

Empirical formula C3H22GeK8MnO47W9 C3H24SiK8MnO47W9Formula weight 2905.22 2862.56Crystal system Triclinic TriclinicSpace group P1̄ P1̄a/Å 12.3018(6) 12.265(2)b/Å 14.6505(7) 14.692(3)c/Å 16.4725(8) 16.497(3)α/° 71.5410(10) 71.580(3)β/° 76.2200(10) 76.382(3)γ/° 81.5910(10) 81.643(4)V/Å3 2726.9(2) 2733.1(8)Z 2 2Dc/g cm−3 3.543 3.497μ/mm−1 20.373 19.812T/K 296(2) 296(2)Limiting indices −14 ≤ h ≤ 14 −14 ≤ h ≤ 14

−17 ≤ k ≤ 17 −8 ≤ k ≤ 17−15 ≤ l ≤ 19 −19 ≤ l ≤ 19

Rint 0.0434 0.0448Goodness-of-fit on F2 1.025 1.005Final R indices[I > 2σ(I)]

R1 = 0.0468 R1 = 0.0569wR2 = 0.1249 wR2 = 0.1376

R indices (all data) R1 = 0.0526 R1 = 0.0726wR2 = 0.1299 wR2 = 0.1445

Fig. 3 (a) Ball-and-stick representation of 1. (b) A polyhedral/ball-and-stick representation of 1. (c) and (d) The packing arrangements of 1along the a-axis. The hydrogen atoms, K cations and lattice water mol-ecules are omitted for clarity.

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1.789(16) Å]. This supporting model is very similar to those inFig. 1, in which electrophilic groups [(OC)3M]+ (M = Mn, Re)are attached to the κ3-O sites of surface oxygen atoms ofPOMs.2 Notably, the [(OC)3Mn]+ cluster is supported on thebridging oxygen atoms of the W3O9 triad, in which three oxygenatoms represent a triangle of adjacent surface oxygen atoms,forming kinetically stable d6 low-spin octahedral 18-electroncomplex fac-[(OC)3MnO3]

+, rather than connect to two terminaloxygen atoms in equatorial positions forming a fac-[(OC)3MO2(OH2)]

+ coordination, which is similar to [Re-(CO)3(H2O){Mo5O13(OMe)4·(NO)}]

2−.21a Although Kortz’sgroup reported [(RuC6H6)2XW9·O34]

6− (X = Si, Ge),21b inwhich one RuC6H6 group is supported on the “W3O9” cap in[A-α-XW9O34]

10− and the other is attached to the [XW9O34]fragment via two terminal oxygen atoms and one μ3-oxygenatom. However, there are a few cases where the [(OC)3Mn]+ unitis attached to the POM framework by either one or two oxygenatoms, or two terminal and one μ3-oxygen atoms simul-taneously.2 These confirm that this supporting model of[(OC)3Mn]+ may be to their stability. Polyhedral and ball–stickarrangements along the a-axis are shown in Fig. 3c and d.

IR spectra

The IR spectra of 1 and 2 (Fig. S1†) show the similar character-istic stretching vibration fashion to other [α-GeW9O34]

10− and[α-SiW9O34]

10− containing species.19,22a Four characteristicbands assigned to the ν(W–Ot), ν(X–Oa) (X = Ge–Si), ν(W–Ob)and ν(W–Oc) vibrations appear at 966, 838, 912 and 680 cm−1

for 1, 986, 912, 847 and 699 cm−1 for 2, respectively. Inaddition, a pair of absorption peaks of CvO groups in 1 and 2are observed at 2020 and 1896–1903 cm−1, which arise from theνasym(C–O) and νsym(C–O) stretching vibrations, respectively.Similar bands are observed for a number of C3ν metal tricarbonylcomplexes in the carbonyl stretching region.12b Furthermore, theresonance at 3430 cm−1 is attributed to water molecules. Com-parison of spectra of 1 and 2 with the spectra of [A-α-GeW9O34]

10− and [A-α-SiW9O34]10− in the metal–oxygen

stretching regions also yields structural information. In the680–1000 cm−1 region, compared to [A-α-GeW9O34]

10− and[A-α-SiW9O34]

10−, respectively, both spectra of 1 and 2 have notremarkable shifts. On the whole, the comparison suggests that 1and 2 may be isomorphic. This deduction is in good agreementwith the results of X-ray diffraction structural analysis.

Cyclic voltammetry

In order to explore the electrochemical behaviors of 1 and 2,cyclic voltammetric measurements of both were carried out inthe pH 6.7 mixed solvent of CH3CN–Na2SO4 (0.4 mol L−1)(1 : 2, vol.) where 1 and 2 were stable by measuring their pHstable range (Fig. S6†). The electrochemical properties of twoproducts were investigated in the potential range of +1.5 to −1.5V, which indicates that both show similar electrochemical beha-viors at the positive potential region because of the presence ofMn(I). As shown in Fig. 4a, at the positive potential region of 1,an irreversible (I′, Epc = 0.314 V) and a quasi-reversible (II–II′,Epc = 0.498, Epa = 0.575 V) oxidation peak might be assigned to

the oxidation of Mn centers,19b which agree with those observedin {[H2W8O30]–Mn(CO)3]2}

8−.14f The reduction of W centers of1 occurred through three separated steps with the E1/2 peakpotentials at −1.011 (III–III′), −1.287 (IV–IV′) and −1.441 V(V–V′), respectively.22 The peak potential differences of threecouples of redox waves are 77, 70, 88 mV, respectively, indicat-ing that they are one-electron charge-transfer processes. Thecathodic peak currents are proportional to the scan rate in therange of 50–200 mV s−1, demonstrating that the redox processesof 1 is surface-controlled (Fig. S7a†). As for 2 (Fig. 4b), theshape of the oxidation Mn(I) waves of 2 is the same to those of 1except that the waves of 2 are shifted to the positive potentialdirection (I′, Epc = 0.379 V; II–II′, Epc = 0.928 V, Epa = 0.703V). At the positive potential region, only one reversible one-elec-tron charge-transfer process of W-centered redox couple with theE1/2 peak potential at −1.133 V was observed. Compared withthe reduction W wave of [A-α-SiW9],

22a,23 the shapes aresimilar but the reduction W wave of 2 is shifted to the negativepotential direction. The redox process of 2 is also surface-con-trolled in the range of 20–100 mV s−1 (Fig. S7b†). In the poten-tial domain explored in Fig. 4, the CV patterns of 1 and 2 arestill reproducible from run to run, which proves the remarkable

Fig. 4 (a) Cyclic voltammograms of 1: at scan rates of 50, 80, 100,120, 150, and 180 mV s−1; (b) cyclic voltammograms of 2: at scan ratesof 20, 30, 50 and 80 mV s−1; (c) and (e) cyclic voltammograms of 1 and2 with different concentrations of NaNO2: 0, 0.5, 1.5, 2.5, 3.5, 4.5, and5.0 mM, respectively (from top to bottom) at a scan rate of 100 mV s−1;(d) and (f ) cyclic voltammograms of Na10GeW9O34 and Na10SiW9O34

solutions with different concentrations the same as (c) and (e). All con-centrations above were 1 × 10−3 M in CH3CN–Na2SO4 (0.4 mol L−1)(1 : 2, vol.). The working electrode was glassy carbon (3 mm) and thereference electrode was Ag/AgCl.

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stabilization of the [α-XW9] (X = SiIV or GeIV) fragment21b–d bythe presence of one [(OC)3Mn]+. The electrocatalytic propertiesof 1 and 2 for NO2

− oxidation were investigated under the sameconditions. With stepwise addition of nitrite, the catalytic cur-rents increase dramatically with the oxidation peak of NO2

−

appearing at Epc = 0.850–0.900 V. Comparing 1 (Fig. 4c) withNa10[A-α-GeW9O34] for NO2

− electrocatalytic oxidation(Fig. 4d), it is not difficult to find that the catalytic current of 1 islarger than that of Na10[A-α-GeW9O34]. This phenomenon canalso be observed between 2 and Na10[A-α-SiW9O34] (Fig. 4eand f). In fact, the results of our experiments show that Mn-(CO)5Br has no electrocatalytic activity for NO2

− in the samesystem (Fig. S8†). However, the incorporation of the [(OC)3Mn]unit into [A-α-XW9] makes the electrocatalytic activity increase,and hence the presence of the [(OC)3Mn] unit in 1 and 2 resultsin the oxidation proceeding faster than with the POM ligand[A-α-XW9].

Thermogravimetric (TG) analysis

The TG curves of 1 and 2 can be viewed as two stages of weightloss (Fig. S2†), giving a total loss of 9.60% (calcd 9.09%) for 1and 10.51% (calcd 9.85%) for 2 in the temperature range of25–800 °C. For 1, the first stage from 25 to 180 °C is attributedto the loss of ten lattice water molecules, and the observedweight loss 6.80% is consistent with the calculated value 6.20%.The second stage with the weight loss of 2.80% occurs between180 and 800 °C, which may be assigned to the removal of threecarbonyl groups (calcd 2.89%). As for 2, the first weight loss is7.65% between 25 and 200 °C due to the removal of elevencrystal water molecules (calcd 6.92%). The second weight loss is2.86% in the range of 220–800 °C because of the release ofthree carbonyl molecules (calcd 2.93%).

Conclusions

In summary, two Keggin POM-supported tricarbonyl metalderivatives have been harvested by the conventional method,which exhibit the first examples of trivacant Keggin-type metalcarbonyl derivatives. The electrochemical behavior of 1 exhibitsgood electrocatalytic activity toward the oxidation of NO2

−. Suc-cessful preparations of 1 and 2 enrich the structural diversity ofmetal carbonyl compounds and provide us an enlightening syn-thetic method for the construction of POM-supported metal car-bonyl derivatives. Although there are a limited number of POMscontaining sufficient charge density at their surface oxygenatoms to covalently bond to metal carbonyl groups, we believethat it is possible to overcome these difficulties. Further work toprepare much more novel manganese carbonyl derivatives is inprogress. Some interesting results will be obtained in due course.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China, the Special Research Fund for the DoctoralProgram of Higher Education, Innovation Scientists and Tech-nicians Troop Construction Projects of Henan Province, the

Foundation of Education Department of Henan Province, andthe Natural Science Foundation of Henan Province.

Notes and references

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2 P. Gouzerh and A. Proust, Chem. Rev., 1998, 98, 101.3 R. K. C. Ho and W. G. Klemperer, J. Am. Chem. Soc., 1978, 100, 6772.4 (a) W. H. Knoth, J. Am. Chem. Soc., 1979, 101, 2211; (b) W. H. Knoth,J. Am. Chem. Soc., 1979, 101, 759.

5 (a) F. Zonnevijlle and M. T. Pope, J. Am. Chem. Soc., 1979, 101, 2731;(b) G. Sazani and M. T. Pope, Dalton Trans., 2004, 1989.

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5836 | Dalton Trans., 2012, 41, 5832–5837 This journal is © The Royal Society of Chemistry 2012

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This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 5832–5837 | 5837

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