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Solid State Sciences 12 (2010) 471–475

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Solid State Sciences

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[Cu(mim)4]2[a-Mo8O26] – A layer-type octamolybdate framework

Nure Alam, Claus Feldmann*

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, D-76131 Karlsruhe, Germany

a r t i c l e i n f o

Article history:Received 30 October 2009Received in revised form7 December 2009Accepted 7 December 2009Available online 16 December 2009

Keywords:Layer-type copper molybdateSynthesisCrystal structureCharacterization

* Corresponding author. Tel.: þ49 721 6082855; faxE-mail address: claus.feldmann@kit.edu (C. Feldm

1293-2558/$ – see front matter � 2009 Elsevier Masdoi:10.1016/j.solidstatesciences.2009.12.010

a b s t r a c t

By reaction of (NH4)6Mo7O24$4H2O, Cu(NO3)2$2.5H2O and 1-methylimidazole (mim) under hydrothermalconditions the novel copper molybdate [Cu(mim)4]2[a-Mo8O26] is obtained in the form of blue, rectan-gular-shaped crystals. The title compound crystallizes with monoclinic lattice symmetry in the spacegroup P21/n. The predominant structural feature of the title compound is a two-dimensional frameworkthat is constituted by [a-Mo8O26]4�octamolybdate units as framework nods and the copper complex[Cu(mim)4]2þ as a linker. In addition to single-crystal structure analysis [Cu(mim)4]2[a-Mo8O26] is char-acterized by powder diffraction as well as by FT-IR and UV–vis spectroscopy.

� 2009 Elsevier Masson SAS. All rights reserved.

1. Introduction

Polyoxomolybdates are of general interest due to theirfascinating structures, properties and potential applications [1–3].The latter includes properties as for instance magnetism, catalysisand photocatalysis, use as sorption material and pharmaceuticalapplications such as cancer therapy [4–12]. In particular, thestructural diversity is accompanied by large building units anda versatile chemical reactivity [13,14]. Amongst the manifoldof polyoxomolybdate structures, the octamolybdate [Mo8O26]4� isknown for its stability, and therefore observed quite often.[Mo8O26]4� has been described to occur with as much as eightisomeric forms, indicated as the a- to q-form [15,16]. The structuraldiversity of polyoxomolybdates is often based on characteristiclarge building units – e. g. the [Mo8O26]4� octamolybdate anion –that are interconnected by different linkers to allow for a widerange of novel structural motifs, ranging from closed cages tobasket-, bowl-, barrel-, and belt-shaped building units [17–19].Moreover, there has been growing interest in introducing ligandsthat allow for conformational flexibility [20–30]. The combinationof polyoxometalates as building blocks with such flexible linkers, ingeneral, has already given rise to a novel class of open frameworkstructures [31,32].

: þ49 721 6084892.ann).

son SAS. All rights reserved.

With the compound [Cu(mim)4]2[a-Mo8O26] we present a novellayer-type polyoxomolybdate framework that is constituted by theformal building units [Cu(mim)4]2þ and [a-Mo8O26]4�. In additionto single-crystal structure analysis, structure and composition ofthe title compound are validated by X-ray powder diffractionpattern, FT-IR as well as UV–vis spectroscopy.

2. Results and discussion

[Cu(mim)4]2[a-Mo8O26] is prepared by hydrothermal reaction ofCu(NO3)2$2.5H2O, (NH4)6Mo7O24$4H2O and 1-methylimidazole.The synthesis results in large quantities of phase-pure blue, rect-angular crystals. According to X-ray structure analysis based onsingle-crystals, the title compound crystallizes monoclinically withspace group P21/n. The unit cell displays a two-dimensionalframework that consists of [Cu(mim)4]2þ and [a-Mo8O26]4� as itsformal structural building units (Fig. 1).

The [a-Mo8O26]4� building unit exhibits a center of inversion,resulting in only four crystallographically independent Mo sites.In sum, the molybdate cage is constituted by six (MoO6)-octa-hedra and two (MoO4)-tetrahedra (Fig. 2) [33]. Whilst theoctahedra exhibit corner- as well as edge-sharing; the tetra-hedra are – as expected – only connected via joint corners, andexclusively to (MoO6)-octahedra. Altogether, the [a-Mo8O26]4�

cage is shaped like a distorted cube-octahedron. The six-valentmolybdenum atoms of the molybdate cage are bound, in sum, to10 terminal, 10 doubly bridging and six triply bridging oxygenatoms (Table 1). Four of the doubly bridging oxygen atoms are

Fig. 1. Unit cell of [Cu(mim)4]2[a-Mo8O26] with (MoO6)-units as blue octahedra and(Cu(Nmim)4O2)-octahedra as green octahedra.

Table 1Selected bond lengths (Å) in [Cu(mim)4]2[a-Mo8O26].

Mo1–O5 1.7748(3) Mo3–O3 1.8902(3)Mo1–O6 1.7748(4) Mo3–O4 1.8868(4)Mo1–O7 1.7732(4) Mo3–O5 2.4803(4)Mo1–O13 1.6872(3) Mo3–O7 2.4613(8)

Mo3–O9 1.6947(4)Mo2–O2 1.8938(7) Mo3–O12 1.6723(5)Mo2–O3 1.8928(5)Mo2–O6 2.3580(3) Cu–N1 2.0019(3)Mo2–O7 2.4076(4) Cu–N3 2.0053(5)Mo2–O8 1.6944(3) Cu–N5 1.9946(5)Mo2–O10 1.6900(2) Cu–N7 1.9971(3)

Cu–O8 2.4288(4)Mo3–O1 1.6802(5) Cu–O11 2.4569(4)Mo3–O2 1.8991(2)Mo3–O4 1.9114(3)Mo3–O5 2.3549(4)Mo3–O6 2.3928(7)Mo3–O11 1.7032(3)

N. Alam, C. Feldmann / Solid State Sciences 12 (2010) 471–475472

linked, on the one hand to molybdenum, and on the other handto copper. The (Mo–O) bond lengths inside the octamolybdateunit with 1.672–1.690 Å (Mo–m1O), 1.887–1.911 Å (Mo–m2O) and1.687–2.480 Å (Mo–m3O) indicate the distortion of the [a-Mo8O26]4� cage and are well in accordance to literature data(Mo–m1O: 1.69–1.71 Å; Mo–m2O: 1.90 Å; Mo–m3O: 1.78–2.43 Å)[16,34].

Cu2þ in the [Cu(mim)4]2þ complex is coordinated almost planarby four nitrogen atoms of four 1-methylimidazole ligands (Fig. 2).The (Cu–N) bonds with a mean distance of 1.999(4) Å are inagreement to the expectation (Table 1). Moreover, [Cu(mim)4]2þ

Fig. 2. The building units [Cu(mim)4]2þ and [a-Mo8O26]4� in [Cu(mim)4][a

interlinks two of the octamolybdate units so that Cu2þ is alsocoordinated by two oxygen atoms with a mean (Cu–O) distance of2.443(4) (Table 1). In sum, Cu2þ exhibits an elongated octahedralcoordination with four shorter bonds to nitrogen and two signifi-cantly enlengthened bonds to oxygen. This finding can be ascribedto Jahn–Teller distortion as it is typical for a d9 valence-electronconfiguration. Due to this elongation of the (Cu–m2O) bond, theadjacent (m2O–Mo) bond between [Cu(mim)4]2þ and the a-[Mo8O26]4� cage with a mean value of 1.697(3) Å is comparablyshort (Table 1).

With regard to the three-dimensional packing of the struc-tural building units, the octamolybdate cages are positionedsimilarly to the tungsten-type of structure with a-[Mo8O26]4�

located on tungsten positions (Fig. 3). Altogether four[Cu(mim)4]2þ complexes interlink each [a-Mo8O26]4� to fourneighboured octamolybdate units. Accordingly, a layer is formedthat is oriented along the [10,11] lattice plane of the unit cell

-Mo8O26] (thermal ellipsoids drawn with 50% probability of finding).

Fig. 3. Orientation of N2 [Cu(mim)4]2[a-Mo8O26] layers along [10,11] with a view

perpendicular to these layers: upper layer indicated by lucent polyhedra and lowerlayer indicated by compact polyhedra (blue: [a-Mo8O26] units; green: [Cu(Nmim)4O2]octahedra; for clarity the 1-methylimidazole ligands are only indicated by its coordi-nation around Cu2þ).

Fig. 4. Orientation of N2 [Cu(mim)4]2[a-Mo8O26] layers with a view in parallel to these

layers (blue: [a-Mo8O26] units; green: [Cu(Nmim)4O2] octahedra).

N. Alam, C. Feldmann / Solid State Sciences 12 (2010) 471–475 473

(Figs. 1 and 3). Structurally identical upper/lower layers areshifted in such a way that the octamolybdate unit is almostcentered above/below the void of four octamolybdates of themean layer. Note also that no covalent bonds exist between thelayers (Fig. 4). P-stacking of the mim ligand is also not observed.Hence, the layers are connected only by long-range Coulomb([Mo8O26]4�4 [Cu(mim)4]2þ) as well as by Van-der-Waals(between mim ligands) interactions.

In addition to single-crystal structure analysis, purity andcrystal structure of [Cu(mim)4]2[a-Mo8O26] are validated byX-ray powder diffraction analysis, too (Fig. 5). The similarity ofline-patterns calculated based on single-crystal data and theobserved powder diffraction pattern is striking. Moreover, the

Fig. 5. Measured diffraction pattern of [Cu(mim)4]2[a-Mo8O26] in comparison to

composition of the title compound is characterized by FT-IRspectroscopy (Fig. 6). Accordingly, the characteristic vibrationsof the 1-methylimidazole ligand (n(C–Haromatic): 3150�3100 cm�1, n(C–Halkyl): 2900� 2850 cm�1, fingerprint area:1600� 800 cm�1) as well as the additional strong IR-bands of[Mo8O26]4� (n(Mo]O) and n(Mo–O–Mo): 960� 550 cm�1) arevisible in the measured region [16,35]. For clear identification ofthe characteristic ligand vibrations, a spectrum of as-used andnon-dried 1-methylimidazole is pictured, too. Finally, UV–visspectra show a broad absorption starting at about 500 nm andreaching far into the infrared (>800 nm) (Fig. 7). This finding iswell in agreement with the deep blue color of [Cu(mim)4]2[a-Mo8O26] that originates from the Nmim / Cu2þ ligand-to-metalcharge transfer.

line-pattern calculated from the results of single-crystal structure analysis.

Fig. 6. FT-IR spectra of [Cu(mim)4]2[a-Mo8O26] and the as-used, non-dried 1-methyl-imidazole (mim) ligand.

Fig. 7. UV–vis spectrum of [Cu(mim)4]2[a-Mo8O26].

Table 2Crystallographic data of [Cu(mim)4]2[a-Mo8O26].

formula Mo8Cu2O26N16C32H48

Crystal system/spacegroup

Monoclinic/P21/n (no. 14)

Cell parameters a¼ 13.024(2) Åb¼ 16.152(3) Åc¼ 14.509(3) Åb¼ 104.0 (1)�

V¼ 2961.6 (1) Å3

Formula per unit cell Z¼ 4Calculated density rcalc¼ 2.21 g cm�3

Temperature T¼ 305 KMeasurement device Imaging plate diffraction system (IPDS II, STOE)Radiation Graphite-monochromated Mo-Ka (l¼ 0.71073 Å)Measurement limits 3.3� 2q� 52.1�

�15� h� 15, �19� k� 1 9, �17� l� 17Crystal size 0.16� 0.13� 0.09 mmAbsorption coefficient m¼ 2.4 mm�1

Absorption correction NumericalNo. of reflections 18,925 measured, 5578 independantData averaging Rint¼ 0.068(1)Structure refinement Full matrix least-squares on F0

2; anisotropicdisplacement parameters for all non-hydrogenatoms

No. of parameters 384Residual electron density 1.18 to �1.50e �3

Figures of merit R1¼ 0.037R1 (3834 F0

2> 4s(F02))¼ 0.046

wR2¼ 0.096GooF¼ 1.032

N. Alam, C. Feldmann / Solid State Sciences 12 (2010) 471–475474

3. Conclusions

The layer-type copper molybdate [Cu(mim)4]2[a-Mo8O26] isprepared by reaction of aqueous solutions of (NH4)6Mo7O24$4H2O,Cu(NO3)2$2.5H2O and 1-methylimidazole (mim). According tosingle-crystal structure analysis the two-dimensional frameworkstructure of the title compound is constituted of octamolybdateunits [a-Mo8O26]4� as a framework nod and the copper complex[Cu(mim)4]2þ as a linker. The resulting layers are connected to eachother by long-range Coulomb as well as by Van-der-Waals inter-actions. [a-Mo8O26]4� as the most spaceous building unit, inparticular, is arranged similar to the atoms of the tungsten-type ofstructure. In addition to crystal structure analysis, powder diffrac-tion pattern, FT-IR and UV–vis spectra confirms the compositionand purity of the title compound.

4. Experimental section

All chemicals employed in this study were of analytical grade,purchased from commercial sources, and used without a specialpurification. All experimental and analytical studies were per-formed in air.

The synthesis of [Cu(mim)4]2[a-Mo8O26] was performed asfollows: 274.3 mg (1.2 mmol) of Cu(NO3)2$2.5H2O, 231.0 mg(0.2 mmol) of (NH4)6Mo7O24$4H2O, 0.5 ml of 1-methylimidazole,0.2 ml of 32% HCl and 40 ml of H2O were stirred at room temper-ature. Accordingly, a light blue solution was obtained, which wasstirred for 24 h. This light blue solution was then transferred intoa teflon-lined stainless-steel autoclave (40 ml), kept at 80 �C for 5days, and then slowly cooled to room temperature. As a result ofthis treatment, the solution turned dark blue. After a few weekslarge quantities of rectangular blue crystals of the title compoundcrystallized from the dark blue solution. These phase-pure crystalswere separated from the mother liquer by filtration and twicewashed with small portions of water.

Single-crystal structure analysis was conducted with an IPDS IIdiffractometer (STOE, Darmstadt) using Mo-Ka1 radiation(l¼ 0.71073 Å, graphite monochromator). A suited crystal wasfixed onto a glass filament and begird in perfluorated oil (Kel-F).Structure solution and refinement were conducted based on theprogram package SHELX [36]. A numerical absorption correctionwas applied. All non-hydrogen atoms were refined with anisotropicparameters of displacement. Hydrogen atoms were geometricallyconstructed. All crystallographic illustrations were created withDIAMOND [37]. Additional information concerning data collection,structure solution and refinement are listed in Table 2. More detailsconcerning the crystallographic data can be requested from theCambridge Crystallographic Data Center [deposit@ccdc.cam.ac.uk]with the deposition number CCDC 753202.

Powder diffraction pattern were obtained with a STADI-P diffrac-tometer (Stoe, Darmstadt) using germanium monochromatized Cu-Ka1 radiation. The sample was filled in a glass capillary and measuredat ambient temperature. Fourier-transform infrared spectra (FT-IR)were recorded on a Bruker Vertex 70 FT-IR spectrometer (Bruker,Ettlingen) using KBr pellets. For this purpose 400 mg of dried KBr werecarefully pestled with 1 mg of the sample and pressed to a thin pellet.UV–vis spectra were recorded with a Varian Cary Scan 100 (Varian,Darmstadt) by measuring the diffuse reflection of powder samplesthat were filled in a specimen attached to an integrating sphere.

Acknowledgments

The authors are grateful to the Center of Functional Nano-structures (CFN) of the Deutsche Forschungsgemeinschaft (DFG) atthe Karlsruhe Institute of Technology (KIT) for financial support.

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