SHORT COMMUNICATION

DOI: 10.1002/zaac.200900383

The Chain-like Copper Molybdate [Cu(dien)]2[MoO4]2·H2O

Nure Alam[a] and Claus Feldmann*[a]

Dedicated to Professor Arndt Simon on the Occasion of His 70th Birthday

Keywords: Polyoxometalates; Hydrothermal synthesis; X-ray diffraction; Organic-inorganic hybrid composites; Copper; Molybdenum

Abstract. [Cu(dien)]2[MoO4]2·H2O is prepared by hydrothermal reac-tion of (NH4)6Mo7O24·4H2O and Cu(NO3)2·4H2O in the presence ofdiethylenetriamine. According to single-crystal structure analysis, thecompound crystallizes monoclinic (C2/c; a = 12.623(3); b = 7.813(2);c = 20.850(4) Å; β = 105.5(1) °; Z = 4) and is composed of (Cu(N)3O3)

IntroductionPolyoxomolybdates are well-known for their versatile struc-tural chemistry – especially including huge anionic buildingunits – as well as for their interesting features aiming at, e.g.,catalysis, nitrogen fixation or lithium-ion batteries [1–3]. Re-cently, copper molybdates attracted particular attention due todrastic phase-transition-induced alterations of, for instance,color, shape, electrical conductivity and magnetism as well asdue to their promising photocatalytic activity [4–6].In addition to CuMoO4 [7] and its modifications [8], several

organic-inorganic hybrids were reported as derivatives in thepseudo-quaternary (ligand)/Cu/Mo/O system. Herein, (MoO4)tetrahedra as well as (Mo3O10), (Mo4O13), (Mo8O27) and evenlarger molybdate units are connected to copper atoms [9–13].Copper is additionally coordinated by organic ligands that in-terlink the oxometalate units, typically, to form extended three-dimensional networks. Most often 4,4'-bipyridine was used asthe organic linker; in some cases other ligands such as 3,4'-dipyridylketone, 1,2-bis(4-pyridyl)ethene, terephthalate, 1,10-phenanthrolin were applied.In this study, [Cu(dien)]2[MoO4]2·H2O is presented as an or-ganic-inorganic hybrid composite with diethylenetriamine(dien: [H2NCH2CH2]2NH) as organic ligand. The title com-pound exhibits a novel infinite chain-like copper–molybdatestructure.

Experimental SectionSynthesis of [Cu(dien)]2[MoO4]2·H2OAll chemicals employed were of analytical grade (p.a.), purchasedfrom commercial sources and used without further purification. Syn-

* Prof. Dr. C. FeldmannE-Mail: claus.feldmann@kit.edu

[a] Institute of Inorganic ChemistryKarlsruhe Institute of Technology (KIT)Engesserstrasse 1576131 Karlsruhe, Germany

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octahedra and (MoO4) tetrahedra that are interlinked to form infinite1∞[{Cu(dien)O1/2O2/3}){(MoO2/1O1/2O1/3)}]2 chains. Between thesechains one molecule of water per formula unit is incorporated throughhydrogen bonding. FT-IR and UV/Vis spectroscopy as well as thermo-gravimetry confirm the composition and purity of the title compound.

thesis and characterization of the title compound were performed inair. The title compound was synthesized by dissolving Cu(NO3)2·5H2O(244 mg, 1 mmol) and (NH4)6Mo7O24·4H2O (188 mg, 0.15 mmol) indiethylenetriamine (dien) (0.2 mL) and H2O (35 mL). The resultinglight blue solution was first stirred at room temperature for 2–4 hours.Afterwards, the solution was transferred into a Teflon-lined stainless-steel autoclave, kept at 150 °C for 7 days and afterwards slowly cooledto room temperature. By this treatment the solution turned dark blue.After about two weeks, rectangular dark blue crystals formed with ayield of approximately 60 %.

X-ray Crystallography

Single-crystal structure analysis of the title compound was performedon an IPDS II diffractometer (STOE, Darmstadt) using graphite mono-chromatized Mo-Kα radiation (λ = 0.71073 Å). Suitable crystals wereisolated in inert oil and mounted on a glass capillary. Structure solutionand refinement were conducted based on the program package SHELX[14]. The results are listed in Table 1. A numerical absorption correc-tion was applied. Hydrogen atoms were geometrically constructed.All crystallographic illustrations were created with DIAMOND [15].More details concerning the crystallographic data can be requestedfrom the Cambridge Crystallographic Data Center [E-Mail:deposit@ccdc.cam.ac.uk] with the deposition number CCDC-744604.

Results and DiscussionThe unit cell of the monoclinic compound [Cu(dien)]2[MoO4]2·H2O is shown in Figure 1. According to single-crystal structureanalysis, copper is hexacoordinated by three nitrogen and threeoxygen atoms. Pairs of these octahedra are connected by edge-sharing through joint oxygen atoms. Moreover, these pairs of octa-hedra are interlinked by two (MoO4) tetrahedra to form infinite1∞[{Cu(dien)O1/2O2/3}){(MoO2/1O1/2O1/3)}]2 double chains (Fig-ure 1). Here, molybdenum-centered tetrahedra and copper-cen-tered octahedra exhibit joint corners of oxygen atoms. The cop-per–molybdate double chains are stacked parallel to the

N. Alam, C. FeldmannSHORT COMMUNICATION

Table 1. Crystallographic data of [Cu(dien)]2[MoO4]2·H2O.

Chemical formula: Mo2Cu2O9N6C8H28Crystal system / Space monoclinic C2/c (Nr. 15)group:Lattice parameter: a = 12.623(3) Å β = 105.5(1) °

b = 7.813(2) Åc = 20.850(4) Å

Z: 4Density (calculated): ρ = 2.25 g·cm–3Absorption coefficient: μ = 3.41 mm–1Crystal size: 0.1 × 0.2 × 0.03 mmMeasurement conditions: λ(Mo-Kα) = 71.073 pm T = 200 KLimiting indices: 2θmin = 2.29°

2θmax = 59.53°–16 < h < 16–10 < k < 9–28 < l < 28

Number of reflections: 9632 (2534 independent)Average determination: Laue class 2/m Rint = 0.058Total number of least 140squares parameters:Largest diff. peak and hole: +1.03 to –1.47 e·Å–3Final R indices and GooF: R1 = 0.036 (2358 Fo > 4σ(Fo))

R1 = 0.038 (all data)wR2 = 0.096 GooF = 1.038

crystallographic a axis. Finally, one molecule of water per formulaunit is located between these double chains and connected to themthrough hydrogen bonding.

Figure 1. View of the unit cell of the title compound[Cu(dien)]2[MoO4]2·H2O along the crystallographic b axis. The coordi-nation of copper (blue) and molybdenum (green) is indicated by poly-hedra; Hydrogen atoms of the dien ligand are omitted for clarity.

As given by the description {(MoO2/1O1/2O1/3)}, molybde-num is coordinated by two terminal oxygen atoms [Mo–μ1O:1.741(6) Å and 1.755(7) Å] and two bridging oxygen atoms[Mo–μ2O: 1.733(7) Å and Mo–μ3O: 1.794(3) Å]. The (O–Mo–O)angles [106.3(1)–108.4(1) °] are in accordance with the ex-pected tetrahedral coordination (Figure 2). Altogether, thesebond lengths and angles fit well with literature data [7, 16].

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Figure 2. View of the 1∞[{Cu(dien)O1/2O2/3}){(MoO2/1O1/2O1/3)}]2 dou-

ble-chain with the connectivity of the individual atoms (with displace-ment ellipsoids) as well as with selected bond lengths /Å.

Copper is surrounded by three joint oxygen atoms of (MoO4)tetrahedra and three nitrogen atoms stemming from the diethyl-enetriamine (dien) ligand (Figure 2). The latter is coordinatedto copper as a mer-form. The (Cu–O) distances range from1.943(7) Å (Cu–μ3O), 2.310(3) Å (Cu–μ3O), to 2.522(5) Å(Cu–μ2O) and indicate a significant Jahn–Teller distortion atthe copper atom. The (Cu–N) bond lengths range from1.996(1) Å (central nitrogen atom) to 2.005(4) Å and

Figure 3. FT-IR (top) and UV/Vis spectra (bottom) of[Cu(dien)]2[MoO4]2·H2O.

The Chain-like Copper Molybdate [Cu(dien)]2[MoO4]2·H2O

2.020(4) Å (terminal nitrogen atoms), which again correspondsvery well to literature data (2.03–2.04 Å) [17]. The (Cu–N)bond lengths as well as the almost tetrahedral (Cu–N–C) bondangles [108.2(1)–109.5(1) °] prove that dien is incorporated asa non-charged ligand. Based on this finding as well as on thecharge neutrality of the title compound, the oxidation state ofcopper and molybdenum can be deduced to the expected val-ues of +II and +VI, respectively. Finally, the double chains areinterlinked through edges of opposite (MoO4) tetrahedra andwater molecules in between (Figure 1). Distances of2.635(1) Å (OH2O–μ1OMo) are in accordance with typical hy-drogen bonding (2.3–3.0 Å).For a more detailed characterization of the title compound,FT-IR and UV/Vis spectroscopy as well as thermogravimetry(TG) were involved. According to FT-IR spectra (Figure 3),the presence of the dien ligand (ν(N–H): 3300–3100 cm–1;ν(C–H): 2950–2850 cm–1, fingerprint area: 1500–1000 cm–1),water (ν(O–H): 3500–3400 cm–1; δ(H–O–H): 1660–1640 cm–1) and molybdate (ν(Mo–O) and δ(Mo–O–Mo): 900–500 cm–1) is reliably confirmed. To identify the characteristicligand vibrations, a spectrum of the as-used diethylenetriamineligand is pictured, too. UV/Vis spectra show a broad absorp-tion starting at about λ = 500 nm and reaching up to λ =800 nm (Figure 3). This finding is well in agreement with thedeep blue color of [Cu(dien)]2[MoO4]2·H2O that originatesfrom the Ndien→Cu2+ ligand-to-metal charge transfer. Finally,TG elucidates the thermal decomposition of the title compound(Figure 4). Thus, decomposition occurs with three steps. Afirst step with 2.6 % (0.8 mg) is in accordance with the loss

Figure 4. Thermogravimetric plot of [Cu(dien)]2[MoO4]2·H2O.

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of one molecule of water per formula unit (expected: 0.8 mg).The second (130–230 °C: 8.6 %, 2.6 mg) and third (230–300 °C: 10.1 %, 3.0 mg) decomposition step surprisingly referto a loss of ethylenediamine (expected: 2.7 mg). Above300 °C, a continuous weight loss and decomposition is ob-served with CuMoO4 as solid residue. With regard to the lowtemperature of H2O loss, a structure analysis of the dehydratedcompound was aspired, too. However, dehydration till nowonly has led to turbid crystals of low quality.

AcknowledgementThe authors are grateful to the Center of Nanostructured Materials(CFN) of the Deutsche Forschungsgemeinschaft (DFG) for financialsupport.

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Received: August 11, 2009Published Online: January 29, 2010