Synthesis and Structure of a Novel Type of Polyoxomolybdate LanthanideComplex: [(Ln(H2O)6)2(TeMo6O24)] (Ln � Ho, Yb)

Daniel Drewes and Bernt Krebs*

Münster, Institut für Anorganische und Analytische Chemie der Westfälische Wilhelms-Universität

Received March 2nd, 2005.

Dedicated to Professor Herbert W. Roesky on the Occasion of his 70th Birthday

Abstract. The new polyoxomolybdates [(Ho(H2O)6)2(TeMo6O24)] ·10 H2O (1) and [(Yb(H2O)6)2(TeMo6O24)] · 10 H2O (2) were prepa-red from aqueous solution by reaction of telluric acid, potassiummolybdate and holmium chloride and ytterbium chloride, respec-tively. They were characterized by X-ray structure analysis, energydispersive X-ray fluorescence analysis and IR spectroscopy. Bothcompounds contain the Anderson type anion (TeMo6O24)6� andtrivalent lanthanide atoms which achieve eightfold coordination by

Introduction

Polyoxomolybdates, -tungstates and -vanadates have at-tracted extensive interest in different fields of chemistry in-cluding catalysis, photochemistry, magnetism and medicine[1�3]. Polyoxometalates are formed in aqueous acidic solu-tion through condensation reactions and can be consideredas molecular blocks or fragments of metals oxides. Theyconsist of MO6 octahedra sharing corners, edges and faces.The remarkable size and challenging geometry of severalnano-structured polyoxomolybdates with well above onehundred molybdenum atoms synthesized by the group ofMüller indicates the fascination of polyoxometalate chemis-try [4]. In addition, some giant tungstate structures areknown in which polyoxoanion building blocks are connec-ted via lanthanide cations [5�8]. However, the mechanismsof formation are still far from being understood and com-monly described as self-assembley. This fact makes it diffi-cult to develop a straightforward reaction route for theaimed syntheses of polyoxometalates.

The incorporation of rare earth cations is of interest insupramolecular chemistry because of their magnetic andluminescent properties [3]. Lanthanide containing po-lyoxoanions differ fundamentally from transition metalsubstituted anions and exhibit unique structures because oftheir preferred coordination numbers eight and nine. In or-der to be able to predict the linkage of the polyoxometalateunits via lanthanide atoms, systematic studies on the role ofthe radii and nature of the rare earth cations are necessary.

* Prof. Dr. B. KrebsInst. f. Anorg. u. Analyt. Chemie d. UniversitätCorrensstraße 36D-48149 MünsterFax: �49-251-8338366E-mail: krebs@uni-muenster.de

Z. Anorg. Allg. Chem. 2005, 631, 2591�2594 DOI: 10.1002/zaac.200500089 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2591

two terminal oxygen atoms of the (TeMo6O24)6� anion and six cry-stal water molecules. While all known Ln/(TeMo6O24)6� com-pounds contain infinite one-dimensional chains in which the(TeMo6O24)6� anions are linked by rare earth cations, the neutralmolecular [(Ln(H2O)6)2(TeMo6O24)] complexes in 1 and 2 are onlyconnected via hydrogen bonds.Keywords: Anderson Type Anions; Lanthanides; Molybdenum; Po-lyoxometalates; Tellurium

Only a few studies presented in the last years are dealingwith this topic. For some examples of polyoxometalatestructures it was shown that these anions are formed by awide range of lanthanides, i. g. anions of the series[Ln4(MoO4)(H2O)16(Mo7O24)4]14� [9] or [Ln(W5O18)2]9�

[10]. The trivalent lanthanides are coordinated identicallyin all examples of these structure types. In contrast to that,the linkage of the monovacant (SiW11O39)8� Keggin ion bylanthanides depends on the nature of the rare earth cation[11]. The lanthanide atoms in the anions with the generalformula [Ln(H2O)x(SiW11O39)]5� exhibit coordinationnumbers of nine, eight or seven due to their different radii.

In this paper we present the crystal structure of two lan-thanide polyoxometalates containing the Anderson-typeanion (TeMo6O24)6� which exhibit the novel [(Ln(H2O)6)2-(TeMo6O24)] complexes. The crystal structures are comparedwith known members of the Ln/(TeMo6O24)6� series andthe role of rare earth cations on the aggregation of the po-lyoxomolybdate building units is discussed.

Results and Discussion

The new compounds [(Ho(H2O)6)2(TeMo6O24)] · 10 H2O(1) and [(Yb(H2O)6)2(TeMo6O24)] · 10 H2O (2) could besynthesized in a two stage reaction. At first, the(TeMo6O24)6� anion was formed by reaction of potassiummolybdate and telluric acid at pH 5.5. Aftwards, a solutionof holmium chloride and ytterbium chloride was added,respectively, to give a light pink or white precipitate. 1 and2 crystallize in the triclinic crystal system, space group P1,containing one formula unit per unit cell. The telluriumatom is situated at an inversion center (Wyckoff position1h). Details of data collection and refinement are summar-ized in Table 1. Figure 1 shows the [(Yb(H2O)6)2(Te-Mo6O24)] complex in 2 in an ellipsoidal representation withthe atom numbering scheme.

D. Drewes, B. Krebs

Table 1 Crystallographic data for [(Ho(H2O)6)2(TeMo6O24)] · 10H2O (1) and [(Yb(H2O)6)2(TeMo6O24)] · 10 H2O (2).

1 2

Formula H44 Ho2 Mo6 O46 Te H44 Mo6 O46 Te Yb2

Formula weight, g mol�1 1813.45 1829.67Crystal size, mm3 0.3 x 0.1 x 0.1 0.4 x 0.2 x 0.1Crystal color pink colorlessCrystal system triclinic triclinicSpace group P1 P1a, A 9.219(2) 9.208(2)b, A 10.016(2) 9.977(2)c, A 11.963(2) 11.915(2)�, ° 94.15(3) 94.30(3)β, ° 99.78(3) 99.53(3)γ, ° 116.91(3) 116.94(3)V, A3 956.6(3) 948.2(3)Z 1 1Index range � 11 � h � 11 � 11 � h � 11

� 12 � k � 12 � 12 � k � 12� 14 � l � 14 � 13 � l � 14

Density (calc), g cm�3 3.148 3.204µ, mm�1 6.863 7.683T, K 213(2) 213(2)2θ range 8.74° < 2θ < 52.28° 8.76° < 2θ < 52.14°Reflections collected 6895 6741Independent reflections 3479 [R(int) � 0.0901] 3445 [R(int) � 0.0825]Parameters 250 250R (I > 2σ (I)) R1

a) � 0.0427 R1a) � 0.0377

wR2b) � 0.1165 wR2

b) � 0.1062R (all data) R1

a � 0.0465 R1a � 0.0409

wR2b � 0.1197 wR2

b � 0.1105Goodness-of-fit on F2 1.064c) 1.081c)

Weighting schemeb) a � 0.0584 a � 0.0559b � 6.0026 b � 6.3715

Largest diff. peak and 2.372 / �2.945 2.242 / �3.071hole, e� A�3

a) R1 � Σ�Fobs� � �Fcalc� / Σ �Fobs�. b) wR2 � {Σ [w(Fobs2 � Fcalc

2)2] / Σ[w(Fobs

2)2]}1/2. w � 1/[σ2(Fobs2) � (aP)2 � bP]; with P � (max (Fobs

2, 0) �2 · Fcalc

2) / 3. c) goof � [Σw(Fobs2-Fcalc

2)2/(n-p)]1/2, n � number of reflections,p � parameter used.

Fig. 1 Ellipsoidal representation of the [(Yb(H2O)6)2(TeMo6O24)]complex in 2, with the ellipsoids drawn at 75 % probability level.

The new polyoxomolybdates 1 and 2 contain the(TeMo6O24)6� anion which was characterized for the firsttime in 1948 [12], but its structure was already predictedin 1937 [13]. The Anderson structure consists of a planararrangement of six edge-sharing MoO6-polyhedra aroundthe central tellurium atom. The six molybdenum atomsform a regular hexagon. Thus, all Mo···Mo and Te···Modistances within the (TeMo6O24)6� anions are almost ident-ical (see Table 2 for details). The central TeVI atom is sur-rounded octahedrally by six oxygen atoms. The Te-O bond

2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 2591�25942592

lengths and O-Te-O bond angles differ only slightly in bothcompounds and are in good agreement with those bondlengths and angles found in other compounds containingthe (TeMo6O24)6� anion [14, 16, 17].

The oxygen atoms of the anion can be divided into threegroups: terminal oxygen atoms with short Mo-O-bondlenghts, molybdenum-bridging oxygen atoms with middleMo-O bond lengths and oxygen atoms bonded to one tel-lurium and two molybdenum atoms bearing long Mo-Obond lengths. As expected, the terminal oxygen atoms showthe shortest Mo-O bond lengths (between 1.69(1) and1.75(1) A). The intermediate Mo-O bonds exhibit Mo-Odistances between 1.90(1) A and 1.96(1) A. The two longMo-O bonds are located trans to the short Mo-O bondsand show Mo-O distances between 2.24(1) A and 2.30(1) A.The distortion of the MoO6-octahedra is confirmed by theO-Mo-O bond angles (Table 2).

Table 2 Selected bond lengths/A and angles/deg with standard devi-ations in parentheses of the polyoxometalates 1 and 2.

1 2

Te···Mo 3.265(2)�3.275(1) 3.254(1)�3.270(2)Mo···Mo 3.264(2)�3.279(2) 3.252(2)�3.278(2)Te-O 1.923(5)�1.938(6) 1.918(5)�1.934(5)Mo-Ot

a) 1.695(7)�1.752(5) 1.690(7)�1.748(5)Mo-OMo2

b) 1.903(6)�1.959(5) 1.920(6)�1.953(5)Mo-OTeMo2

b) 2.249(5)�2.302(6) 2.237(5)�2.298(6)Ln-O 2.343(7)�2.382(6) 2.286(6)�2.369(7)O-Te-Ocis 85.3(3)�95.7(3) 85.3(2)�94.7(2)O-Te-Otrans 180.0 180.0O-Mo-Ocis 70.5(2)�107.0(3) 70.3(2)�107.5(3)O-Mo-Otrans 151.2(2)�163.7(2) 151.2(2)�163.5(3)O-Ln-Ocis 72.1(2)�78.9(2) 72.4(2)�79.6(2)O-Ln-Otrans 114.7(3)�145.0(2) 114.9(2)�145.1(2)

a) terminal. b) The subscripts indicate the atoms bound to the oxygen atom.

Table 3 Ln-O bond lengths/A with standard deviations in paren-theses of the polyoxometalates 1 and 2.

1 2

Ln-O(8) 2.375(7)a) 2.349(6)Ln-O(10) 2.366(5)a) 2.329(5)Ln-OW(1) 2.343(7) 2.286(6)Ln-OW(2) 2.347(6) 2.309(6)Ln-OW(3) 2.347(7) 2.318(6)Ln-OW(4) 2.368(6) 2.331(6)Ln-OW(5) 2.368(6) 2.332(5)Ln-OW(6) 2.382(6) 2.369(7)

a) Ln-O(8a), (10a)

The HoIII atoms in 1 and the YbIII atoms in 2 are coordi-nated by eight oxygen atoms. Two different molybdenumatoms each provide a terminal oxygen atom for the coordi-nation of the trivalent rare earth cations. The coordinationsphere is completed by six crystal water molecules (Figure2). As each (TeMo6O24)6� anion coordinates two LnIII

atoms, the anions can be regarded as a tetradentate ligandfor the LnIII atoms. The square-antiprismatic geometry ofthe LnO8-polyhedra is confirmed by applying the criteriafor eight-coordinate structure-type determination outlinedby Haigh: a gap of more than 20° between the 16th and 17th

lowest O-Ln-O angles denotes square antiprismatic coordi-

A Novel Type of Substituted Lanthanide Polyoxomolybdate Complex

nation [15]. In 1 and 2, a gap of 35.8° and 35.5°, respec-tively, can be observed which clearly affirms the square-antiprismatic coordination sphere (Figure 2). The resulting[(Ln(H2O)6)2(TeMo6O24)] complexes represent a novel typeof polyoxometalate complex with no known analogousstructures in the literature.

Fig. 2 Coordination spheres of Ho(1) in 1 and Yb(1) in 2.

Recently, we reported the crystal structures of some mol-ybdates containing the Anderson type anion (TeMo6O24)6�

and trivalent rare earth cations [16, 17]. Depending on thesize of the lanthanide cations, three different one-dimen-sional chains can be found: In the lanthanum compound,the (TeMo6O24)6� anion acts as a hexadentate ligand forthe LaIII atoms which achieve ninefold coordinationthrough three oxygen atoms of two (TeMo6O24)6� anionsand six water molecules. Each (TeMo6O24)6� anions isbound to four LaIII atoms, thus one-dimensional[(Ln(H2O)6)2(TeMo6O24)]n chains can be observed. If the(TeMo6O24)6� anions are linked via CeIII, PrIII, NdIII, EuIII

or GdIII, they act as a tetradentate ligand and coordinatefour rare earth cations. The coordination sphere of the LnIII

atoms is completed by seven water molecules, leading to[(Ln(H2O)7)2(TeMo6O24)]n chains. For TbIII, DyIII, HoIII

and ErIII, the crystal structures show that these cations arecoordinated by eight oxygen atoms, forming a square anti-prism. The (TeMo6O24)6� anion acts as a tridentate ligand,which leads to one-dimensional [Ln(H2O)5(TeMo6O24)]n3n�

chains. In short, in the one-dimensional chains the Ander-son-type anions are linked by two LnIII atoms if their coor-dination number is nine, and the anions are linked by oneLnIII atoms if the coordination number of the lanthanidesis eight.

Table 4 O···O distances/A indicating possible hydrogen bonds in 2. Ow � oxygen atom of a water molecule.

O(1)···Ow(2a) 2.664(9) O(2)···Ow(4f) 2.737(9) O(3)···Ow(7h) 2.813(9)O(4)···Ow(10) 2.916(9) O(5)···Ow(7) 2.754(8) O(6)···Ow(3a) 2.681(9)O(7)···Ow(8) 2.974(9) O(8)···Ow(5) 2.891(8) O(8)···Ow(6) 2.933(8)O(9)···Ow(8b) 2.808(9) O(10)···Ow(1) 2.850(8) O(10)···Ow(3) 2.803(10)O(10)···Ow(6) 2.837(8) O(11)···Ow(2e) 2.882(9) O(11)···Ow(3g) 2.894(8)O(12)···Ow(5e) 2.854(8) O(12)···Ow(9) 2.790(9) Ow(1)···Ow(11) 2.688(9)Ow(1)···Ow(9i) 2.742(9) Ow(1)···Ow(4) 2.772(9) Ow(1)···Ow(6) 2.789(9)Ow(1)···Ow(3) 2.845(10) Ow(2)···Ow(3) 2.800(10) Ow(2)···O(11c) 2.882(9)Ow(2)···Ow(4) 2.932(9) Ow(2)···Ow(5) 2.958(9) Ow(3)···O(6a) 2.681(9)Ow(3)···O(11g) 2.894(8) Ow(3)···Ow(4) 2.976(8) Ow(4)···O(2i) 2.737(9)Ow(4)···Ow(5) 2.773(9) Ow(4)···Ow(10i) 2.738(8) Ow(5)···O(12c) 2.854(8)Ow(5)···Ow(6) 2.776(8) Ow(5)···Ow(7c) 2.726(9) Ow(6)···Ow(9b) 2.700(9)Ow(6)···Ow(10b) 2.849(9) Ow(7)···Ow(5e) 2.726(9) Ow(7)···O(3d) 2.813(9)Ow(7)···Ow(11) 2.819(9) Ow(8)···Ow(9c) 2.796(9) Ow(8)···O(9b) 2.808(9)Ow(8)···Ow(11h) 2.883(10) Ow(9)···Ow(6b) 2.700(9) Ow(9)···Ow(1f) 2.743(9)Ow(9)···Ow(8e) 2.796(9) Ow(10)···Ow(4f) 2.738(8) Ow(10)···Ow(11b) 2.848(10)Ow(10)···Ow(6b) 2.849(9) Ow(11)···Ow(10b) 2.848(10) Ow(11)···Ow(8d) 2.883(10)Ow(11)···Ow(11j) 2.943(14)

Symmetry codes for generated atoms: a: �x, �y, �z�1; b: �x�1, �y�1, �z�2; c: x, y�1, z; d: x�1, y, z; e: x, y�1, z; f: x�1, y�1, z; g:�x, �y�1, �z�1;h: x�1, y, z; i: x�1, y�1, z; j: �x, �y�1, �z�2.

Z. Anorg. Allg. Chem. 2005, 631, 2591�2594 zaac.wiley-vch.de 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2593

A comparison of the crystal structures of 1 and 2 withthe polyoxomolybdates [(Ln(H2O)7)2(TeMo6O24)]n · 8nH2O (Ln � Pr, Nd) reported earlier [17] emphasizes thestructure-determining role of the trivalent rare earth cat-ions. Although the formulas of these four compounds areidentical with the exception of the lanthanide atoms, thestructures differ fundamentally as a result of the differentradii of the lanthanide cations. As the PrIII and NdIII atomsare coordinated by nine oxygen atoms forming a tricappedtrigonal antiprism, the HoIII and YbIII atoms are coordi-nated by only eight oxygen atoms. As a result of this, eitherone dimensional infinite [(Ln(H2O)7)2(TeMo6O24)]n-chains(Ln � Pr, Nd) or neutral [(Ln(H2O)6)2(TeMo6O24)] com-plexes (Ln � Ho, Yb) are present in the solid state. In thecase of Pr and Nd, the (TeMo6O24)6� anions are linked vialanthanides, whereas they are only connected via hydrogenbonds in 1 and 2, forming a three-dimensional network. Asno hydrogen atoms could be located during the structuredetermination, the possible hydrogen bonds have been de-termined by O···O distances [18]. Typical O···O distancesare in the range between 2.6 and 3.0 A , indicating a H···Obond valence between ca. 0.25 and 0.1. Figure 3 shows thelinkage of the polyoxometalate structure in 2 by hydrogenbonds along [010]. Table 4 summarizes all possible hydro-

Fig 3 Linkage of two [(Yb(H2O)6)2(TeMo6O24)] complexes in 2 along[010]. Mo light grey octahedra, Te dark grey octahedra, Yb grey, O white.

D. Drewes, B. Krebs

gen bonds in the solid structure of 2. The IR spectra of 1and 2 show beyond the O-H absorption bands three charac-teristic vibration peaks resulting from the (TeMo6O24)6�

anion. They can be attributed to νas(Mo-Oterminal) (944 in1, 927 cm�1 in 2), νas(Mo-Oedge sharing) (892, 895 cm�1) andνas(Te-O) (692, 666 cm�1).

Experimental Section

Syntheses

All starting materials were used without further purification.

Preparation of [(Ho(H2O)6)2(TeMo6O24)] · 10H2O (1)

1.0 g (4.2 mmol) of K2MoO4 was dissolved in 150 mL of water, towhich 0.160 g (0.7 mmol) of telluric acid, dissolved in 10 mL ofwater, was added. The pH was adjusted to 5.5 by adding 4M hydro-chloric acid. The colorless solution was heated to 80 °C and stirredfor 30 min. 0.532 g (1.4 mmol) of HoCl3 · 6 H2O were dissolved in10 mL of water and added slowly to the hot solution while stirring.� Yield 858 mg (68 %). IR: ν (cm�1) � 3378 (vs, br); 1640 (m);944 (s); 892 (s); 692 (s); 607 (s); 454 (s). EDX: calcd. ratio Ho : Te: Mo � 2.00 : 1.00 : 6.00; found 1.88 : 1.05 : 6.00.

Preparation of [(Yb(H2O)6)2(TeMo6O24)] · 10H2O (2)

Compound 2 was prepared similarly to 1, using 0.543 g (1.4 mmol)of YbCl3 · 6 H2O instead of HoCl3 · 6H2O. � Yield 621 mg (48 %).IR: ν (cm�1) � 3462 (vs, br); 1630 (w); 1594 (m); 927 (s); 895 (s);666 (s); 616 (s); 462 (s). EDX: calcd. ratio Yb : Te : Mo � 2.00 :1.00 : 6.00, found 1.82 : 0.93 : 6.00.

Instrumentation and Analytical Procedures

IR spectra were measured on a Perkin Elmer 683 spectrometer asKBr pellets. The atomic ratios of the heavy elements were deter-mined by energy dispersive X-ray fluorescence analysis (EDX). Dif-fraction experiments were performed on a STOE IPDS imagingplate system using Mo-Kα radiation (λ � 0.71073 A). Structureswere solved by direct methods with the SHELXS-97 and refinedwith the SHELXL-97 program [19]. No hydrogen atoms were in-cluded during the refinement. All atoms were refined aniso-tropically by full-matrix least-squares calculations based on F2. Theprogram DECAY was applied as absorption correction [20].Further details on the crystal structure investigations may be ob-tained from the Fachinformationszentrum Karlsruhe, D-76344Eggenstein-Leopoldhafen, Germany (fax: (�49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository num-bers CSD-415192 (1) and CSD-415193 (2).

Acknowledgement. Support by the Deutsche Forschungsgemein-schaft, the Fonds der Chemischen Industrie, and the Bundesministe-rium für Bildung und Forschung is gratefully acknowledged. Figs.1�3 were generated by Diamond Version 2.1c (copyright CrystalImpact GbR).

2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2005, 631, 2591�25942594

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