Synthesis of a Purely Inorganic Three-Dimensional PorousFramework Based on Polyoxometalates and 4d-4f Heterometals

Haijun Pang, Chunjing Zhang, Dongmei Shi, and Yaguang Chen*

Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, NortheastNormal UniVersity, 5268 Ren Min Street, Changchun, 130024, P. R. China

ReceiVed April 21, 2008; ReVised Manuscript ReceiVed June 11, 2008

ABSTRACT: A new polyoxometalate-based 4d-4f heterometallic compound, [{Ag3(H2O)2}{Ce2(H2O)12}H5⊂{H2W11Ce-(H2O)4O39}2] ·8H2O (1), has been synthesized and characterized by routine methods. In 1, the dimers constructed by twomonosubstituted R-metatungstate [H2W11{Ce(H2O)4}O39]7- (simplified as {W11Ce}7-) units are linked by CeIII and AgI to formtwo-dimensional (2D) networks, and these networks are further connected by AgI-AgI ({Ag2}2+) bonds to obtain a purely inorganicthree-dimensional (3D) framework with two kinds of channels along the [1 0 0] and [0 1 0] directions. Furthermore, 1 exhibitsreversible water sorption capability.

Introduction

The design and synthesis of d-f heterometallic compoundshave attracted increasing interest because of their potentialapplications in catalysis,1 magnetism,2 and photochemicalsensors,3 as well as their intriguing architectures and topologies.4

In the vast amount of reported work, a typical strategy toconstruct d-f compounds is the assembly from mixed metal ionsand organic-ligands containing O- or N-donor atoms, such ascarbonyl,5 cyanide,6 pyridine-carboxylate ligands.2b,7 However,purely inorganic d-f heterometallic compound based on poly-oxometalates (POMs) has been reported only once up to now,8

which offers us great interest and opportunities. Because POMswith unique properties possess a large number of coordinationoxygen atoms and versatile coordination modes, compared withthe organic ligands, they have been viewed as ideal inorganicligands for the construction of multidimensional hybridmaterials.9,10 Hence, the use of POM clusters as connectorsprovides an appealing route to design novel purely inorganicd-f heterometallic compounds based on POMs.

In recent years, the compounds with porous structures haveattracted great attention, and many works have proved that thecrucial step in the assembly of functional porous frameworksis the use of organic linkers.11 However, the approach is limitedby the reduced framework stability intrinsic to metal-organicframework materials. In contrast, the synthesis of purelyinorganic frameworks based on POMs offers high potential forthe formation of a new type of porous materials stable andinsoluble in common organic solvents.12

On the basis of aforementioned points, we choose R-meta-tungstate13 clusters [H2W12O40]6- ({W12}6-), Ag+ and Ce3+

cations as connectors to construct purely inorganic porous high-dimensional 4d-4f heterometallic POM-based compounds basedon the following considerations. (i) Compared with the otherwell-known POMs, the 36 surface oxygen atoms of the {W12}6-

POM possess higher charge density to increase the coordinationtendency with metals. A known three-dimensional (3D) porousframework of [Ag(CH3CN)4]⊂{[Ag(CH3CN)2]4[H3W12O40]}has been reported by Cronin et al., in which each [H3W12O40]5-

cluster coordinated with eight {Ag2}2+ bridges.14 The exampleconfirms that {W12}6- units as inorganic ligands may be a better

choice to construct high-dimensional heterometallic compounds.(ii) Silver atoms with diverse coordination modes make covalentlinks to POMs with ease. Besides, because of their argentophi-licity,15 AgI cations may form {Ag-Ag}2+ dimers, whichprovide smart opportunities to extend the structure.16 Therefore,the silver(I) atom can be a good candidate as a 4d metal source.(iii) In our previous work, we have obtained a series oflanthanide compounds based on {W12}6- clusters because oftheir high charge density,17 which indicates that there shouldbe a significant affinity between the {W12}6- clusters and thelanthanide cations.

Fortunately, we obtained such a POM-based 4d-4f hete-rometallic purely inorganic 3D compound, [{Ag3(H2O)2}-{Ce2(H2O)12}H5⊂{H2W11Ce(H2O)4O39}2] ·8H2O (1), which ex-hibits two kinds of channels along the [1 0 0] and [0 1 0]directions. Note that a few examples based on the monosub-stituted R-metatungstate [H2W11ZO39]n- ions (Z ) MnIV, CrIII,VIV, VV, CoII, GaIII) have been obtained,18 but no crystalstructural characterization was provided. Therefore, 1 alsorepresents the first example of the compound characterized byX-ray diffraction analysis which contains [H2W11CeO39]7-

fragments.

Experimental Section

Materials. All reagents for the syntheses were purchased fromcommercial sources and used as received. (NH4)6[H2W12O40] ·3H2O wassynthesized according to the literature19 and characterized by IRspectroscopy and thermogravimetry (TG) analyses.

Physical Methods. Ce, Ag, and W were analyzed on a PLASMA-SPEC(I) ICP atomic emission spectrometer. IR spectra were obtainedon Alpha Centaurt FT/IR spectrometer with KBr pellets in the400-4000 cm-1 region. The TG analyses were performed on aPerkin-Elmer TGA7 instrument in flowing N2 with a heating rate of10 °C ·min-1. The powder X-ray diffraction (PXRD) patterns wereobtained with a Rigaku D/max 2500V PC diffractometer with Cu KRradiation, the scanning rate was 4°/s, 2θ ranging from 4-40°.

Synthesis of [{Ag3(H2O)2}{Ce2(H2O)12}H5⊂{H2W11Ce(H2O)4O39}2] ·8H2O (1). In a typical experiment, (NH4)6[H2W12O40] ·3H2O (0.752 g,0.25 mmol) was dissolved in water (8 mL). The pH value of the solutionwas carefully adjusted with a dilute NaOH solution to 5.06 and thenstirred for 50 min. A solution (10 mL) of Ce(NO3)3 ·6H2O (0.2171 g,0.50 mmol) and AgNO3 (0.8494 g, 0.50 mmol) was added. The pHvalue of the resulting solution was adjusted with a dilute NaOH solution(1 mol.L-1) to 3.26 and then stirred for 10 min at 60 °C. The resultingsolution was filtered and allowed to evaporate in air at room temper-ature. Two days later, some white precipitation was formed. The

* To whom correspondence should be addressed. E-mail: chenyg146@nenu.edu.cn. Phone: +86-431-85099667. Fax: +86-431-85098768.

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10.1021/cg800410x CCC: $40.75 2008 American Chemical SocietyPublished on Web 11/01/2008

precipitate was removed by filtration, and the resulting clear solutionwas evaporated in air at room temperature. After 3 weeks, yellow blockcrystals of compound 1 were obtained. Yield 0.52 g (32%, based onCe). Elemental analysis: Ag3Ce4H69O108W22 (1) (6726.27). Anal. Calcdfor 1: W, 60.13; Ag, 4.78; Ce, 8.33. Found: W, 60.28; Ag, 4.86; Ce,8.21 (%).

X-ray Crystallographic Study. The data were collected by a BrukerSmart Apex CCD diffractometer with Mo KR (λ ) 0.71073 Å) at 273K using the ω-scan technique. Empirical absorption correction wasapplied. The structure of 1 was solved by the direct method and refinedby the full-matrix least-squares on F2 using the SHELXTL-97 soft-ware.20 All of the non-hydrogen atoms were refined anisotropically.The hydrogen atoms attached to water molecules were located in idealposition. Further details of the X-ray structural analysis are given inTable 1.

Results and Discussion

Synthesis. 1 was synthesized by the reaction of(NH4)6[H2W12O40] ·3H2O, Ce(NO3)3 ·6H2O, and AgNO3 in theratio of 1:2:2 in aqueous solution (see the Experimental Section).Note that there are three features to facilitate the formation ofthe title compound. First, by controlling the higher pH value(5.06 initially), the monovacant isopolyanion [H2W11O39]10-

({W11}10-) may be formed. To our knowledge, {W11}10- hasnot been reported to date, and its formation mechanism is stillnot well understood. Herein, we speculate that its formingmechanism is similar to the degradation process of the Kegginheteropolyoxometalate with the increasing of pH value.21

Furthermore, the formation of {W11Ce}7- subunits stabilize the{W11}10- anions (Scheme 1); second, the unique characteristicof the {W11Ce}7- anions favors the high-connectivity for thefollowing reasons: (1) rare-earth-metal-substituted POMs pos-sess higher negative charge, which activates the surface oxygenatoms and makes the connection to cations (Ag+, Ce3+) withease; (2) Ce3+ cations do not fully insert the lacunary sites of{W11}10- subunits, and each Ce3+ cation has five additionalcoordination sites available, which can be used as linkers toconnect one or more other fragments. Finally, Ag+ cations as4d metals can be introduced into the {W11Ce}7- systemsuccessfully at the lower pH value (3.26 finally) because thefree Ag+ cations readily transformed to black Ag2O at a higherpH value.

Description of the Crystal Structure. Single-crystal X-raydiffraction analysis reveals that 1 shows a complicated 3D

porous structure, which is constructed by {W11Ce}7- subunits,Ag+ and Ce3+ cations, and water molecules. The valence sumcalculations22 show that all W and Ce atoms are in the +VIand +III oxidation states, respectively. Since 1 was isolatedfrom acidic aqueous solution, five protons were attached toPOMs to compensate for charge balance, which is similar tothe case of [Ag2(3atrz)2]2[(HPMoVI

10MoV2O40)];10d then 1 is

formulated as [{Ag3(H2O)2}{Ce2(H2O)12}H5⊂{H2W11Ce-(H2O)4O39}2] · 8H2O. The {W11Ce}7- subunit consists of themonovacant {W11}10- anion and the Ce(1)3+ cation fills thevacancy. It is interesting that each of the {W11Ce}7- subunitslinks eight metal atoms (four Ce and four Ag), which representsits highest connected number to date (Figure 1a). The Ce(1)3+

cation adopts a distorted square antiprism geometry coordinatedby nine oxygen atoms. The four oxygen atoms come from thevacant site of the {W11}10- anion, and the four other oxygenatoms are from the four coordinated water molecules. Theremaining one terminal oxygen atom [O(19)] is from theneighboring {W11Ce}7- subunit. So by Ce(1)3+ cation, twoneighboring {W11Ce}7- subunits link each other forming a

Table 1. Selected Crystallographic Data for 1a,b,c

compound 1formula Ag3Ce4H69O108W22

Mr 6726.27crystal system triclinicspace group P1ja/Å 12.3396(4)b/Å 13.2484(4)c/Å 18.1100(6)R/deg 94.6920(10)�/deg 91.8810(10)γ/deg 105.3390(10)V/Å3 2841.02(16)T/k 293(2)Z 1Dc/Mg cm-3 3.926µ(Mo KR)/mm-1 24.337total reflections 15247indep. reflections 10695Rint 0.0271R1, wR2 0.0533, 0.1466R1,wR2 (all data) 0.0727, 0.1558

a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) ∑[w(Fo2 - Fc

2)2]/∑[w(Fo2)2]1/2.

c Cambridge Crystallographic Data Centre (CSD) no.: 1, 418317.

Scheme 1. Process of Formation of {W11Ce}7- Subunit in 1

Figure 1. (a) Combined polyhedral/ball/stick representation of thecoordination mode of {W11Ce}7- subunit; (b) combined polyhedral/ball/stick representation of the dimer; (c) combined polyhedral/ball/stick representation of the connected mode of two dimers.

Inorganic 3D Porous d-f Heterometallic Frameworks Crystal Growth & Design, Vol. 8, No. 12, 2008 4477

dimer (Figure 1b). Furthermore, two adjacent dimers are linkedtogether by Ce(2)3+, Ag(1)+, and Ag(2)+ cations to achieve a1D structure (Figure 1c). Among these cations, the Ce(2)3+

cation is also nine-coordinated, and its coordination environmentis completed by six water molecules and three terminal oxygenatoms [O(4), O(5), and O(28)] from three dimers of the different1D chains. In other words, Ce(2)3+ cations link 1D structuresinto a 2D layer along the [1 0 0] direction (Figure 2a). Theaverage of Ce-O bond lengths [Ce-O ) 2.537 Å] are withinthe normal ranges for a 9-coordinate cerium system.9e The Ag(1)cation is coordinated by six oxygen atoms from two adjacentdimers (Figure 1c). [Ag(1)-O(21) ) 2.524(13), Ag(1)-O(22)) 2.362(13), and Ag(1)-O(35) ) 2.722(14) Å]. The Ag(2)+

cation is coordinated by one water molecule [Ag(2)-O(14W)) 2.475(27) Å], three terminal oxygen atoms from three{W11Ce}7- clusters [Ag(2)-O(17) ) 2.686(19), Ag(2)-O(33)) 2.472(13), and Ag(2)-O(33′) ) 2.770(13) Å], and one silveratom [Ag(2)-Ag(2′) ) 2.365(13) Å] (Supporting Information,Figure S1). Because of the τ ) 0.5185 value, the coordinationmode of Ag(2) is within the structural continuum betweentrigonal bipyramidal and rectangular pyramidal.23 Herein, itshould also be pointed out that the two adjacent silver atoms’silver-silver distance (2.365 Å) is much shorter than the vander Waals contact distance (3.44 Å) of silver.24 These signifi-cantly shorter bond lengths may be ascribed to the effect of theconstraints from oxygen atoms [O(33) and O(33′)] (SupportingInformation, Figure S1b, Table 2).

Another fascinating structural feature for 1 is that the 2Dnetworks are further connected by Ag(2)-Ag(2′) interactionsto obtain a purely inorganic 3D framework (Figure 2b andSupporting Information, Figure S2), which contains two kindsof channels with about 6.871 × 7.565 Å2 and 8.704 × 7.084Å2 along the [1 0 0] and [0 1 0] directions, respectively (Figure2c,d). Taking the van der Waals radii into account, however,the channels are only about 3.151 × 4.525 Å and 5.664 Å ×4.044 Å along the [1 0 0] and [0 1 0] directions, respectively.Calculations by PLATON reveal that the van der Waals freespace per unit cell (after the solvent-water molecules have beenremoved) is approximately 2841.6 Å3, corresponding to 29.2%of the crystal volume, and water molecules fill the channelsand participate in extensive hydrogen-bonding interactions withPOMs.

IR and TG Analysis. The IR curve for 1 exhibits thecharacteristic vibration patterns of the Keggin-type structure(Figure S3). In comparison with the IR spectrum of precursor(NH4)6[H2W12O40] ·3H2O (Supporting Information, Figure S4),the ν(W-Ot) (931.69), ν(W-Ob-W) (824.34 and 870.38), andν(W-Oc-W) (777.71) vibration frequencies have a blue shiftof 4-15.12 cm-1; the ν(W-Ob-W) vibration is split into twopeaks. The major reasons for this may be that the substitutionof cerium decreases the symmetry of the Keggin polyanion, andthat cerium-silver heterometal cations have strong interactionsto POMs.

In the TG curve of 1 the weight loss is divided into threestages (Supporting Information, Figure S5). The first weight lossis 12.05% (calcd 11.7%) in the temperature range 22-480 °C,and corresponds to the release of absorption water, crystal water,and coordination water. The successive weight loss of the secondstage is 1.52% (calcd 1.51%) in the temperature range 490 to626 °C, corresponding to the release of water from the

Figure 2. Polyhedral/ball/stick diagrams of 1 along [1 0 0] (a) and [0 1 0] (b) directions. Space-filling diagrams with the 1D channels along[1 0 0] (c) and [0 1 0] (d) directions. Hydrogen atoms, guest molecules are omitted for clarity.

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) forCompound 1a

Ce(1)-O(15W) 2.502(15) Ce(2)-O(5)#1 2.455(13)Ce(1)-O(19)#3 2.491(12) Ce(2)-O(28) 2.472(13)Ce(1)-O(7) 2.511(13) Ce(2)-O(12W) 2.502(16)Ce(1)-O(23) 2.522(15) Ce(2)-O(11W) 2.516(19)Ce(1)-O(3) 2.531(14) Ce(2)-O(4)#2 2.516(14)Ce(1)-O(5W) 2.533(19) Ce(2)-O(7W) 2.541(17)Ce(1)-O(2W) 2.514(19) Ce(2)-O(16W) 2.58(2)Ce(1)-O(4W) 2.550(14) Ce(2)-O(3W) 2.591(15)Ce(1)-O(11) 2.569(14) Ce(2)-O(1W) 2.604(19)Ag(1)-O(22)#1 2.363(13) Ag(2)-O(14W) 2.47(3)Ag(1)-O(21)#1 2.524(13) Ag(2)-O(33) 2.472(12)Ag(1)-O(21) 2.524(13) Ag(2)-Ag(2)#4 2.365(5)Ag(1)-O(22) 2.363(13) Ag(2)-O(17) 2.686(192)

O(15W)-Ce(1)-O(19)#3 134.9(5) O(5)#1-Ce(2)-O(12W) 68.7(5)O(15W)-Ce(1)-O(7) 68.8(5) O(28)-Ce(2)-O(12W) 85.7(6)O(19)#3-Ce(1)-O(7) 143.1(4) O(5)#1-Ce(2)-O(11W) 133.1(8)O(15W)-Ce(1)-O(23) 137.8(5) O(28)-Ce(2)-O(11W) 138.4(6)O(7)-Ce(1)-O(23) 69.9(5) O(5)#1-Ce(2)-O(4)#2 126.9(5)O(22)#1-Ag(1)-O(21)#1 66.2(5) O(33)-Ag(2)-O(14W) 117.9(7)O(22)-Ag(1)-O(21)#1 113.8(5) O(21)-Ag(1)-O(21)#1 180.0(2)

a Symmetry transformations used to generate equivalent atoms: #1-x, -y + 2, -z; #2 -x, -y + 1, -z; #3 -x, -y + 2, -z + 1; #4 -x+ 1, -y + 2, -z.

4478 Crystal Growth & Design, Vol. 8, No. 12, 2008 Pang et al.

decomposition of POMs; the weight loss of the third stage is0.29% (calcd 0.38%) from 626 to 776 °C and is ascribed to therelease of O2 from the decomposition of Ag2O. No weight lossoccurs above 776 °C. The total weight loss is about 13.86%,consistent with the calculated value of 13.59%.

De-/Rehydration Behavior. As shown in Figure 3a,b, thepeak positions of the simulated and as-synthesized 1 pattern atroom temperature are in agreement with each other, whichindicates the good phase purity of the compound 1. Thedifferences in intensity may be due to the preferred orientationand crystal morphology of the crystalline powder samples. Toinvestigate if 1 is robust to guest removal or exchange, itsdehydration and rehydration were studied by the PXRD shownin Figure 3. The as-synthesized 1 was heated at 125 °C in vacuofor 6 h, leading to a weight loss of 6.9%. According to TGanalysis, this corresponds to the release of absorption, nonco-ordinated and partial coordinated water molecules (SupportingInformation, Figure S5). The dehydrated phase has an almostfeatureless PXRD pattern (Figure 3c), which indicates that it isvery low crystalline or amorphous, resulting from the loss ofcoordination water and probable collapse of the porous frame-works.25 However, when the dehydrated phase was exposed towater vapor at room temperature for 1 day, the sample gainedits original weight, and recovered the original structure, asindicated by comparing its PXRD pattern to the as-synthesizedone (Figure 3d). Furthermore, this procedure was repeatedseveral times to demonstrate that the dehydration and rehydra-tion are reversible for 1 (Supporting Information, Figure S6).This is similar to previously reported porous frameworks.26

Conclusion

In summary, we have obtained a novel 3D porousframework with the cerium-substituted monolacunary R-meta-tungstate subunits, in which the {W11Ce}7- subunits as octa-dentate connectors, instead of organic linkers, link 4d-4fheterometallic atoms. Therefore, we name this frameworkas the first inorganic open frameworks based on POMs andd-f heterometals. The successful isolation of 1 not onlyprovides an intriguing example of a porous compound butalso confirms the significant potential of constructing purelyinorganic high-dimensional frameworks based on POMs andd-f heterometals. With hindsight, we can imagine that morenew other compounds could be prepared by replacement ofd-f heterometal segments and/or by appropriate choices ofdifferent POMs in the near future.

Acknowledgment. This work was supported by Analysis andTesting Foundation of the Northeast Normal University andZhong-Min Su group for Single-crystal X-ray diffraction.

Supporting Information Available: Crystallographic informationfile (CIF) and Figures S1-S6. This material is available free of chargevia the Internet at http://pubs.acs.org.

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