Inorganica Chimica Acta 363 (2010) 4399–4404

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Inorganica Chimica Acta

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Synthesis, crystal structure and magnetism of a new mixedgermanium–polyoxovanadate cluster

Jing Wang, Christian Näther, Paul Kögerler, Wolfgang Bensch *

Institut für Anorganische Chemie der Christian-Albrechts-Universität, Kiel, GermanyInstitut für Anorganische Chemie, RWTH Aachen University, Aachen, Germany

a r t i c l e i n f o

Article history:Available online 6 July 2010

Dedicated to Achim Muller

Keywords:GermaniumVanadiumSolvothermal synthesisCrystal structurePolyoxovanadateMagnetic properties

0020-1693/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.ica.2010.06.065

* Corresponding author. Address: Institut für Anorsität Kiel, Max-Eyth-Str. 2, D-24118 Kiel, Germany. Fa

E-mail address: wbensch@ac.uni-kiel.de (W. Bensc

a b s t r a c t

A new germanium–polyoxovanadate, (H3aep)4[V14Ge8O50]�2(aep)�13H2O (1), has been synthesized undersolvothermal conditions applying GeO2, NH4VO3, Cu(NO3)2�3H2O and an aqueous solution of 1-(2-amino-ethyl)-piperazine (aep, C6H18N3) in the temperature range from 110 to 150 �C. The compound crystallizesin the non-centrosymmetric tetragonal space group P-421c with a = 17.193(1) Å, c = 16.501(1) Å,V = 4877.9(5) Å3 and Z = 2. The structure consists of isolated spherical [VIV

14GeIV8O50]12� cluster anions

and protonated amine molecules as counterions. The cluster anion can be viewed as a derivative of the[V18O42] archetype by replacing four VO5 pyramids by four Ge2O7 units. The latter are formed by cor-ner-sharing of two [GeO4]4� tetrahedra. At temperatures above 150 �C the compound (H2pip)4(Hpip)4-[VIV

14GeIV8O50(H2O)] (2) (pip = piperazine, C4N2H10) is formed and during the reaction Cu2+ is reduced

to elemental copper. This redox reaction is essential for the formation of 2. The crystal water moleculesin the structure of 1 are emitted at low temperatures. The magnetic properties are dominated by strongintra-cluster antiferromagnetic coupling and the strongest exchange between edge- and corner-sharingVO5 square pyramids results in an eight-membered spin ring to which two three-membered spin bridgesare joined. The magnetic susceptibility data suggest that even at the low temperature of 2 K several mul-tiplet states are still significantly populated.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction cally modified VIV-containing compounds like (trenH3)2[V15Sb6-

Over the last decade magnetic polyoxometalate (POM)-basedsystems have been under intense investigation by several groups.The systems may be divided into several classes according to theidentity of the spin centers: whereas in polyoxomolybdates andpolyoxotungstates magnetic functionality is introduced by integra-tion of heterometallic spin centers (usually 3d transition metal cat-ions embedded into the otherwise diamagnetic polyanionframeworks), in polyoxovanadate(IV) clusters the VO2+ vanadylgroups themselves define quantum spin (S = 1/2) centers [1]. Themost prominent example of the latter group of compounds is[VIV

15AsIII6O42(H2O)]6� [2–5], an archetypal example of molecular

geometric spin frustration, featuring an equilateral spin trianglecharacterized by an S = 1/2 ground state. [V15As6O42(H2O)]6�

serves as a model for the study of the fundamental properties ofsuch magnetic systems [6,7]. Besides the [V15As6O42(H2O)]6� clus-ter anion, several mixed-valent polyoxovanadate compounds areknown, for example (NHEt3)4[V12As8O40(H2O)]�H2O containing VIV

and VV centers in an 8:4 ratio [8]; furthermore, numerous chemi-

ll rights reserved.

ganische Chemie der Univer-x: +49 (0) 431/880 1520.h).

O42]�0.33tren�nH2O (tren: tris(2-aminoethyl)amine) [9], (C6H17-N3)4[V16Sb4O42]�2H2O, (NH4)4[V14Sb8O42]�2H2O [10], [V16Sb4O42-(H2O){VO(C6H14N2)2}4] [11], and Cs8[V16Ge4O42(OH)4]�4.7H2O[12] were isolated and characterized. In all magnetically character-ized derivatives of the [V18O42] archetype structure, the spin centersexhibit strong antiferromagnetic exchange interactions, dominatedby exchange via the l-bridging O atoms. The magnetic groundstates are determined by the number of unpaired spins, the geome-try of the cluster anions and the relative strength of the exchangeinteractions. Interestingly, the magnetic properties of Ge-contain-ing polyoxovanadates were not investigated despite the differingarrangement of the VO5 pyramids within the cluster anion com-pared to arsenato- and antimonato-polyoxovanadate systems.

The additional substitution of VO2+ units in chemically modifiedpolyoxovanadates by transition metal cations offers a path towardsnew cluster architectures exhibiting e.g. interesting magneticproperties such as spin-glass behavior in larger spin clusters. Inthe past, some experiments were done in this field by investigatingthe reaction of TM (TM = Co, Ni, Cu, Zn and Cd) with [V14As8O42]4�,[V15As6O42]6� and [V14Sb8O42]4� clusters in amine solutions [13–18]. The resulting compounds comprise cluster anions where theTM cation was incorporated within the cluster-shell like in [Ni-V13As8O41], [Cd2V12As8O40], [ZnV13As8O41], [Zn2V12As8O40], and[CdV13As8O41] [13d,16e,16f,17d], or TM complexes with organic

Table 1Details of the data collections and selected refinement results for 1.

1

Formula C36H128Ge8N18O63V14

Formula weight 3115.3961Crystal system tetragonalSpace group P-421ca (Å) 17.1932(10)b (Å) 17.1932(10)c (Å) 16.5014(11)a (�) 90b (�) 90c (�) 90V (Å3) 4877.9(5)Z 2

4400 J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404

molecules were formed acting as counterions or as connecting li-gands [13–18].

Until now no TM-containing polyoxovanadates were reportedwith heteroelements other than As and Sb. We started to investi-gate solvothermal reactions employing simultaneously Ge, Cu, Vcompounds and amines as solvents and structure-directing mole-cules or templates. During the series of experiments we obtainedfor the first time the compound (H3aep)4[V14Ge8O50]�2(aep)�13H2O(1). The syntheses were originally performed to incorporate copperinto mixed germanato-polyoxovanadates but in all cases elementalCu was obtained as by-product. In this paper, we present the syn-thesis, crystal structure and properties of this new mixedpolyoxovanadate.

F(0 0 0) 2532Reflections collection/independent 49352/5873Goodness of fit (GOF) on F2 1.049R1

a [I > 2r(I)] 0.0298wR2

b 0.0751Absolute structure parameter 0.014(10)Largest residuals (e �3) 0.573/�0.472

The crystallographic data for compounds 1 has been deposited with the CambridgeCrystallographic Data Center as publication No. CCDC 781153 (1). Copies of the datacan be obtained, free of charge, on application to CCDC, 12 Union Road, CambridgeCB2 1 EZ, UK (mail: deposit@ccdc.ca.ac.uk).

a R1 = R ||Fo| � |Fc||/R |Fo|.b wR2 = |R w(|Fo|2 � |Fc|2)|/R|w(Fo

2)2|1/2.

Table 2Atomic coordinates (�104) and equivalent isotropic displacement parameters Ueq

a

(Å2 � 103) for 1.

Atom x/a y/b z/c Ueq (Å2)

Ge(1) 6855(1) 4439(1) 6855(1) 20(1)Ge(2) 6165(1) 3440(1) 3149(1) 21(1)V(1) 5000 5000 7321(1) 19(1)V(2) 5276(1) 3375(1) 6519(1) 19(1)V(3) 5382(1) 2712(1) 4938(1) 19(1)V(4) 6865(1) 3678(1) 5002(1) 19(1)O(1) 7071(2) 4505(2) 5821(1) 23(1)O(2) 7056(2) 5357(2) 7271(2) 25(1)O(3) 7337(2) 3706(2) 7347(2) 35(1)O(4) 5840(1) 4294(2) 6952(2) 25(1)O(5) 5000 5000 8284(2) 40(1)O(6) 5402(2) 2687(2) 7163(2) 37(1)O(7) 6073(2) 3232(1) 5700(1) 22(1)O(8) 5502(2) 1784(2) 4974(2) 34(1)O(9) 6162(2) 3191(2) 4186(1) 22(1)O(10) 7633(2) 3145(2) 5023(2) 33(1)O(11) 6998(2) 4574(1) 4302(1) 22(1)O(12) 5925(2) 4439(1) 3073(2) 26(1)O(13) 7005(2) 3218(2) 2650(2) 36(1)N(1) 6139(2) 2745(2) 10144(2) 34(1)C(1) 6021(3) 2192(3) 9482(2) 40(1)C(2) 6541(3) 2409(3) 8772(2) 42(1)N(2) 6393(2) 3213(2) 8500(2) 41(1)C(3) 6423(4) 3776(3) 9168(3) 53(1)C(4) 5904(3) 3524(3) 9873(3) 49(1)C(5) 5763(3) 2502(3) 10900(3) 47(1)C(6) 6268(3) 1909(3) 11322(3) 47(1)N(3) 7043(2) 2200(2) 11457(2) 47(1)

a Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

2. Experimental

2.1. Syntheses

Compound 1 was prepared under solvothermal conditions byreacting 210 mg (2 mmol) GeO2, 351 mg (3 mmol) NH4VO3, and243 mg (1 mmol) Cu(NO3)2�3H2O in a solution of 1-(2-amino-ethyl)-piperazine (75% in water, 6 mL). The experiments to synthe-size 1 were performed in a temperature range from 110 to 180 �C.In typical experiments the mixtures were heated in PTFE-linedsteel autoclaves for 9 days. The final pH value of all reaction mix-tures was around 11.5. Dark brown polyhedral crystals of 1 wereobtained just below 150 �C, and black octahedrons of the knowncompound (pipH2)4(pipH)4[V14Ge8O50(H2O)] (2) [12] were formedabove 150 �C. We note that the synthesis of compound 1 is alsosuccessful without Cu(NO3)2�3H2O in the reaction mixture. Theyield based on GeO2 was about 73% for 1.

CHN analysis, Anal. Calcd. (%) for 1: N, 8.1; C, 13.9; H, 4.1. Found:N, 7.8; C, 14; H, 3.8%.

2.2. Crystal structure determination

The single crystal X-ray intensities were collected at room tem-perature with a STOE-1 Imaging Plate Diffraction System (IPDS-1)with Mo Ka radiation (k = 0.71073 Å). Selected crystal data and de-tails of the structure determination are summarized in Table 1. Theintensities were corrected for Lorentz and polarization effects. Thestructures were solved with SHELXS-97 [19] and refined against F2

with SHELXL-97 [20]. All non-hydrogen atoms were refined withanisotropic displacement parameters. The C–H hydrogen atomswere positioned with idealized geometry and were refined using ariding model. After structure refinement of 1 small electron densitymaxima were detected in the difference Fourier map indicating thepresence of water molecules in the structure. The maxima are lo-cated in the large channels of the structure and all attempts to refinethese molecules failed due to a very strong disorder. Hence, theintensity data were corrected for disordered solvent molecules usingthe SQUEEZE option in PLATON [21]. Atomic coordinates and selectedinteratomic distances and angles are listed in Tables 2 and 3 (1).

2.3. Elemental analysis

The C, H, and N contents were determined by combustion anal-ysis on a CHNS-Rapid-Element-Analyzer (Heraeus GmbH) usingsulfanilamide as standard.

2.4. Thermal analysis

The DTA–TG investigations were carried out in a nitrogen atmo-sphere (purity: 5.0; heating rate 4 K/min; flow rate: 75 mL/min;Al2O3 crucibles) using a Netzsch STA-409CD instrument.

2.5. Magnetic measurements

Low-field magnetic susceptibility data for 1 were recorded be-tween 2.0 and 290 K at an external field of B0 = 0.1 Tesla using aQuantum Design MPMS-5XL SQUID magnetometer and PTFE sam-ple holders. Field-dependent magnetization measurements wereperformed at 2.0 K (B0 = 0.1–5.0 Tesla). The susceptibility datawere corrected for diamagnetic contributions for counter ionsand crystal solvent molecules. Note that polyoxovanadate cluster

Table 3Selected interatomic distances (Å) and angles (�) for 1. Estimated standard deviationsare given in parentheses.

Ge(1)–O(3) 1.712(3) Ge(2)–O(13) 1.705(3)Ge(1)–O(1) 1.751(2) Ge(2)–O(9) 1.763(2)Ge(1)–O(2) 1.756(3) Ge(2)–O(2A) 1.771(3)Ge(1)–O(4) 1.770(3) Ge(2)–O(12) 1.771(3)V(1)–O(5) 1.590(3) V(3)–V(4A) 3.0206(8)V(1)–O(12B) 1.971(3) V(3)–V(4) 3.0452(8)V(1)–O(12A) 1.971(3) V(4)–O(10) 1.607(3)V(1)–O(4C) 1.983(2) V(4)–O(11) 1.939(2)V(1)–O(4) 1.983(2) V(4)–O(7) 1.942(3)V(2)–O(6) 1.605(3) V(4)–O(1) 1.993(3)V(2)–O(11A) 1.924(2) V(4)–O(9) 1.995(2)V(2)–O(7) 1.940(3) V(4)–V(3B) 3.0206(8)V(2)–O(4) 1.988(3) O(1)–V(3B) 1.995(2)V(2)–O(12A) 1.993(3) O(2)–Ge(2B) 1.771(3)V(2)–V(3) 2.8530(8) O(11)–V(2B) 1.924(2)V(3)–O(8) 1.610(3) O(11)–V(3B) 1.938(2)V(3)–O(11A) 1.938(2) O(12)–V(1A) 1.971(3)V(3)–O(7) 1.948(2) O(12)–V(2B) 1.993(3)V(3)–O(1�) 1.995(2) O(3)–Ge(1)–O(4) 109.31(14)V(3)–O(9) 2.005(3) O(1)–Ge(1)–O(4) 107.80(12)N(1)–C(1) 1.463(5) O(2)–Ge(1)–O(4) 106.57(12)N(1)–C(5) 1.464(5) O(10)–V(4)–O(11) 111.64(13)N(1)–C(4) 1.470(6) O(10)–V(4)–O(7) 109.80(13)C(1)–C(2) 1.521(6) O(11A)–V(3)–V(4) 115.93(8)O(3)–Ge(1)–O(1) 114.10(14) O(7)–V(3)–V(4) 38.40(7)O(3)–Ge(1)–O(2) 112.41(14) O(1A)–V(3)–V(4) 123.55(8)O(1)–Ge(1)–O(2) 106.28(12) O(9)–V(3)–V(4) 40.29(7)O(13)–Ge(2)–O(9) 114.65(14) O(11)–V(4)–O(7) 138.56(11)O(13)–Ge(2)–O(2A) 111.57(13) O(10)–V(4)–O(1) 104.26(14)O(9)–Ge(2)–O(2A) 105.09(12) O(11)–V(4)–O(1) 79.40(10)O(13)–Ge(2)–O(12) 112.33(14) O(7)–V(4)–O(1) 90.26(10)O(9)–Ge(2)–O(12) 107.64(12) O(10)–V(4)–O(9) 105.83(13)O(2A)–Ge(2)–O(12) 104.86(12) O(11)–V(4)–O(9) 90.11(10)O(5)–V(1)–O(12B) 109.25(8) O(7)–V(4)–O(9) 79.05(10)O(5)–V(1)–O(12A) 109.25(8) O(1)–V(4)–O(9) 149.91(11)O(12B)–V(1)–O(12A) 141.49(16) O(10)–V(4)–V(3B) 110.74(11)O(5)–V(1)–O(4C) 107.88(8) O(11)–V(4)–V(3B) 38.80(7)O(12B)–V(1)–O(4C) 76.19(10) O(7)–V(4)–V(3B) 122.19(8)O(12A)–V(1)–O(4C) 92.07(11) O(1)–V(4)–V(3B) 40.78(7)O(5)–V(1)–O(4) 107.88(8) O(9)–V(4)–V(3B) 125.12(8)O(12B)–V(1)–O(4) 92.07(11) O(10)–V(4)–V(3) 112.21(11)O(12A)–V(1)–O(4) 76.19(10) O(11)–V(4)–V(3) 120.72(8)O(4C)–V(1)–O(4) 144.24(16) O(7)–V(4)–V(3) 38.54(7)O(6)–V(2)–O(11A) 107.79(13) O(1)–V(4)–V(3) 124.18(8)O(6)–V(2)–O(7) 105.81(14) O(9)–V(4)–V(3) 40.54(7)O(11A)–V(2)–O(7) 84.88(10) V(3B)–V(4)–V(3) 137.05(3)O(6)–V(2)–O(4) 106.40(14) Ge(1)–O(1)–V(4) 125.20(13)O(11A)–V(2)–O(4) 145.49(11) Ge(1)–O(1)–V(3B) 134.50(14)O(7)–V(2)–O(4) 90.33(11) V(4)–O(1)–V(3B) 98.49(10)O(6)–V(2)–O(12A) 108.59(14) Ge(1)–O(2)–Ge(2B) 117.28(13)O(11A)–V(2)–O(12A) 89.19(11) Ge(1)–O(4)–V(1) 131.31(14)O(7)–V(2)–O(12A) 145.26(11) Ge(1)–O(4)–V(2) 124.13(14)O(4)–V(2)–O(12A) 75.59(10) V(1)–O(4)–V(2) 103.95(11)O(6)–V(2)–V(3) 107.60(12) V(2)–O(7)–V(4) 149.17(14)O(11A)–V(2)–V(3) 42.55(7) V(2)–O(7)–V(3) 94.43(11)O(7)–V(2)–V(3) 42.89(7) V(4)–O(7)–V(3) 103.06(11)O(4)–V(2)–V(3) 127.95(8) Ge(2)–O(9)–V(4) 123.51(14)O(12A)–V(2)–V(3) 126.61(8) Ge(2)–O(9)–V(3) 134.61(14)O(8)–V(3)–O(11A) 108.87(14) V(4)–O(9)–V(3) 99.17(10)O(8)–V(3)–O(7) 110.68(13) V(2B)–O(11)–V(3B) 95.26(11)O(11A)–V(3)–O(7) 84.30(10) V(2B)–O(11)–V(4) 151.50(15)O(8)–V(3)–O(1A) 107.79(13) V(3B)–O(11)–V(4) 102.37(11)O(11A)–V(3)–O(1A) 79.39(10) Ge(2)–O(12)–V(1A) 133.32(14)O(7)–V(3)–O(1A) 141.28(11) Ge(2)–O(12)–V(2B) 122.40(13)O(8)–V(3)–O(9) 110.14(14) V(1A)–O(12)–V(2B) 104.21(11)O(11A)–V(3)–O(9) 140.80(11)O(7)–V(3)–O(9) 78.66(10)O(1A)–V(3)–O(9) 92.32(10)O(8)–V(3)–V(2) 111.74(12)O(11A)–V(3)–V(2) 42.19(7)O(7)–V(3)–V(2) 42.68(8)O(1A)–V(3)–V(2) 116.78(7)O(9)–V(3)–V(2) 116.42(7)O(8)–V(3)–V(4A) 111.20(11)O(11A)–V(3)–V(4A) 38.83(7)

Table 3 (continued)

O(7)–V(3)–V(4A) 117.37(8)O(1A)–V(3)–V(4A) 40.73(7)O(9)–V(3)–V(4A) 124.79(8)V(2)–V(3)–V(4A) 79.15(2)O(8)–V(3)–V(4) 115.61(11)V(2)–V(3)–V(4) 78.68(2)V(4A)–V(3)–V(4) 132.82(3)

Symmetry transformations used to generate equivalent atoms: A: y, �x + 1, �z + 1;B: �y + 1, x, �z + 1; C: �x + 1, �y + 1, z.

J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404 4401

anions exhibit strong TIP contributions which thus preclude theimmediate calculation of their combined diamagnetic/TIPsusceptibilities.

3. Results and discussion

3.1. Synthetic aspects

The original aim of the syntheses was the interconnection of thegermanato–polyoxovanadato clusters by Cu2+ complexes. However,all attempts to incorporate Cu2+ cations into POM architecturesfailed. Interestingly, the Cu2+ was reduced to metallic Cu duringthe syntheses as evidenced by X-ray powder diffractometry. Addi-tional experiments have demonstrated that compound 1 could beobtained without a copper source. Compound 2 crystallized onlyapplying a Cu2+ salt when 1-(2-aminoethyl)-piperazine (Scheme 1)was used as solvent, and syntheses without Cu2+ afforded crystalli-zation of compound 1. The structure of 2 contains two types of pip-erazinium cations (Scheme 1), i.e., the 1-(2-aminoethyl)-piperazinemolecule was fragmented under in situ conditions. Solvothermalsyntheses performed with piperazine and without a Cu2+ saltafforded the formation of compound 2 [12]. We note that the struc-ture of 2 was reported in space group P42/nnm (a = 14.9950(7),c = 18.408(1) Å, V = 4139.0(4) Å3) with slightly disorderedpiperazine molecules [12]. We solved and refined the structure of2 in space group I41/acd (a = 21.2564(8), c = 36.8017(18) Å, V =16628.3(12) Å3) with fully ordered organic cations.

Further syntheses were undertaken to probe whether bothcompounds coexist and the results demonstrate that small

Fig. 1. The [V14Ge8O50(H2O)]12� cluster anion in compound 1. Only selected atomsare labeled. Note that the central H2O molecule is not shown.

4402 J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404

amounts of 2 are present in the reaction product of the synthesisperformed at 150 �C in the presence of a Cu2+ salt. The role of theCu2+ salt in the formation of compound 2 is not clear and further

Fig. 3. Arrangement of cluster anions and organic

Fig. 2. The hydrogen bonding interactions between the protonated amine mole-cules and the [V14Ge8O50(H2O)]12� cluster anion in 1 indicated by dashed lines.

experiments are underway, especially in situ X-ray scatteringinvestigations, to clarify the influence of Cu2+ on the crystallization.

3.2. Crystal structures

The new compound 1 crystallizes in space group P-421c withtwo formula units per unit cell as brown crystals with irregularshape. Inside the [V14Ge8O50]12� cluster several electron densitymaxima were found which were assumed to be oxygen of a H2Omolecule. But due to the strong disorder no satisfactory modelcould be found.

The structure of compound 1 features an isolated spherical[V14Ge8O50(H2O)]12� cluster anion (Fig. 1) as main structural motifand protonated amine molecules as countercations. The spherical[V14Ge8O50(H2O)]12� cluster is related to the [V18O42] archetypecluster, and the structure of both germanium–polyoxovanadatecluster anions can be derived from this fundamental POM unitwhen four VO5 groups are replaced by four Ge2O7 dumbbells. TheGe2O7 groups are inserted in the shell of the [V18O42] unit by shar-ing corners with six VO5 pyramids. The substitution of the four VO5

pyramids leads to the formation of a central ring consisting of eightedge-sharing VO5 units. The remaining six VO5 pyramids aregrouped in two caps which are rotated by 90� to each other andare condensed to the central ring through common edges.

In 1 charge compensation is achieved by four crystallographi-cally independent triple protonated 1-(2-aminoethyl)-piperazine

cations in 1 (dashed lines: hydrogen bonds).

Scheme 1. Organic bases in compounds 1 and 2.

1200 1100 1000 900 800 700 600 500 400

trans

mis

sion

578

635

672

986

1098

741

776

ν / cm–1

Fig. 5. IR spectrum of compound 1.

J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404 4403

(H3aep, C6H18N33+) molecules. The Ge atoms are surrounded by

four oxygen atoms in a slightly distorted tetrahedral coordinationwith Ge–O bond lengths ranging from 1.712(3) to 1.771(3) Å, andO–Ge–O angles close to the ideal tetrahedral value (104.9–114.7�). The VO5 pyramids have typical geometric parameters withbasal and apical bond lengths from 1.924(2) to 2.005(3) Å and1.590(3) to 1.610(3) Å respectively. The O–V–O angles vary be-tween 75.6� and 149.9�. These parameters closely match those re-ported for other mixed germanato-polyoxovanadates [12,22]. Theoxidation states of the V and Ge atoms in 1 were calculated withthe bond valence sum method (BVS) [23]. The resulting values of4.1 for V and 4.0 for Ge justify the assignment of the valence statesV4+ and Ge4+. The potential free solvent area was calculated withthe PLATON program suite [21] yielding 1517 Å3 being about 31% ofthe unit cell volume.

Between the clusters and the organic ammonium molecules of1, several hydrogen bonds are found. Each [V14Ge8O50]12� clusteris in contact with eight H3aep molecules by strong l1–O� � �H–N(1.80–1.83 Å, angles: 156.9–162.1�) and a weak l1–O� � �H–N(2.34 Å; angle: 151.4�) H-bonding interactions (Fig. 2) forming athree-dimensional network.

Along the c-axis, layers of [V14Ge8O50(H2O)]12� units alternatein an ABAB sequence (Fig. 3, top), and each cluster anion is sur-rounded by six other anions. The anions in every A layer are ro-tated by 90� with respect to the clusters in the B layers. Withinthe layers which are oriented parallel to the (0 0 1) plane, the clus-ter anions form a rectangular net with channels running along[0 0 1] (Fig. 3, bottom). The diameter of the channels is about6.4 � 6.4 Å (measured from coordinate-to-coordinate). It can be as-sumed that the co-crystallized water molecules are located withinthe channels.

The geometric parameters of 2 are comparable with those ob-tained for 1, i.e. bond lengths and angles are very similar for thetwo compounds. The bond valence sum for all V and Ge atoms in2 indicates that the average oxidation states are close to +4. In con-trast to 1, in 2 only weak hydrogen bonds between N–H� � �l1–O(2.69–2.94 Å, corresponding N–H� � �O angles: 138–153�) could beobserved. Every [V14Ge8O50(H2O)]12� cluster in 2 is surroundedby 12 amine molecules through such H-bonding interactions lead-ing to a three-dimensional H bonded network. Obviously, the dif-fering supramolecular arrangements of anions and cations lead tothe crystallization in different space groups with significantly dif-fering lattice parameters. The densities of the two compoundsare different with 2.121 g/cm3 for 1 and 2.238 g/cm3 for 2. Themore dense packing of the constituents in 2 is reflected by the low-er potential solvent area of 247.5 Å3, equivalent to about 1.5% ofthe unit cell volume.

100 200 300 400 500 600

75

80

85

90

95

100

Δm /

%

T / °C

Fig. 4. The TG curve of the thermal decomposition of compound 1.

Thermal analysis shows that compound 1 decomposes at com-parably low temperatures (Fig. 4) and according to simultaneousmass spectrometry investigations the emission of water starts atabout 50 �C. The thermal degradation occurs in a more or less con-tinuous fashion without pronounced decomposition steps, and thesimultaneously recorded DTA curve shows no significant thermalevent. Even at about 550 �C the mass loss is ongoing.

In the IR spectrum of 1 (Fig. 5) the strong V–O stretching vibra-tion is located at 986 cm�1 in accordance with the energetic posi-tion of the vanadyl band.

3.3. Magnetism

The fourteen S = 1/2 vanadyl centers in the polyanion cluster in1 experience strong intra-antiferromagnetic coupling as evidencedin the low-field susceptibility (Fig. 6). At room temperature, vmTonly reaches 2.4 cm3 K/mol, far below the value of 5.15 cm3 K/mol for 14 uncoupled vanadyl groups (g = 1.98). With lower tem-peratures vmT reaches a plateau between ca. 20 and 200 K of2.2 cm3 K/mol, matching the value for six uncoupled vanadyl cen-ters (2.205 cm3 K/mol). The strongest exchange, caused by near-linear V–l–O–V and angled V(–l–O–)2V exchange pathways be-tween edge- and corner-sharing VO5 square pyramids, results inan eight-membered spin ring to which two three-membered spinbridges are joined. Considering only the V(–l–O–)2V contacts ofneighboring edge-sharing VO5 pyramids, the resulting ‘skeletal’spin array is not geometrically frustrated. However, the competinglinear V–l–O–V contacts render the spin structure as consisting of

Fig. 6. Temperature dependence of vmT for compound 1 at 0.1 Tesla. Inset: field-dependent magnetization of 1 at 2.0 K.

4404 J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404

four pairs of edge-joined spin triangles interconnected by fouradditional spin centers. Moreover, multiple additional (weaker) ex-change pathways involving the germanate groups add more inter-actions, complicating a full magnetochemical analysis. At thelowest temperature of our preliminary susceptibility measure-ments (2.0 K), the field-dependent magnetization does not followa scaled Brillouin function, indicating that several multiplet statesare still significantly populated.

4. Conclusions

A chemically modified VIV-containing cluster with composition[V14Ge8O50(H2O)]12� was obtained under solvothermal conditions.A second compound with the same cluster motif is formed only atelevated temperatures in the presence of a Cu2+ source. The resultsof the syntheses demonstrate the complexity of the solvothermalapproach where different reaction parameters determine the for-mation of a distinct compound. At the moment one can only spec-ulate about the role of Cu2+ for the crystallization of compound 2.Here, the decomposition of aep by elimination of the aminoethylgroup appears to be induced by Cu2+. After formation of pip thereduction of VV in the metavanadate salt to VIV in the clusteranion occurs. This scenario seems to be likely because 2 is formedin the presence of pip without addition of a Cu2+ source.Preliminary investigations of the magnetic properties of the[V14Ge8O50(H2O)]12� cluster show a complex behavior dominatedby strong antiferromagnetic exchange interactions mediated byV–l–O–V and bent V(–l–O–)2V bridges. A detailed study of themagnetic properties is underway as well as further syntheticefforts for the generation of new germanato–polyoxovanadatocluster architectures.

Acknowledgements

Financial support by the State of Schleswig-Holstein is gratefulacknowledged.

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