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Synthesis, crystal structure and magnetism of a new mixed germanium–polyoxovanadate cluster
Inorganica Chimica Acta 363 (2010) 4399–4404
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Synthesis, crystal structure and magnetism of a new mixed germanium–polyoxovanadate cluster Jing Wang, Christian Näther, Paul Kögerler, Wolfgang Bensch * Institut für Anorganische Chemie der Christian-Albrechts-Universität, Kiel, Germany Institut 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: Germanium Vanadium Solvothermal synthesis Crystal structure Polyoxovanadate Magnetic properties
a b s t r a c t A new germanium–polyoxovanadate, (H3aep)4[V14Ge8O50]2(aep)13H2O (1), has been synthesized under solvothermal conditions applying GeO2, NH4VO3, Cu(NO3)23H2O and an aqueous solution of 1-(2-aminoethyl)-piperazine (aep, C6H18N3) in the temperature range from 110 to 150 °C. The compound crystallizes in 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 [VIV14GeIV8O50]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 corner-sharing of two [GeO4]4 tetrahedra. At temperatures above 150 °C the compound (H2pip)4(Hpip)4[VIV14GeIV8O50(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 molecules in the structure of 1 are emitted at low temperatures. The magnetic properties are dominated by strong intra-cluster antiferromagnetic coupling and the strongest exchange between edge- and corner-sharing VO5 square pyramids results in an eight-membered spin ring to which two three-membered spin bridges are joined. The magnetic susceptibility data suggest that even at the low temperature of 2 K several multiplet states are still significantly populated. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the last decade magnetic polyoxometalate (POM)-based systems have been under intense investigation by several groups. The systems may be divided into several classes according to the identity of the spin centers: whereas in polyoxomolybdates and polyoxotungstates magnetic functionality is introduced by integration of heterometallic spin centers (usually 3d transition metal cations embedded into the otherwise diamagnetic polyanion frameworks), in polyoxovanadate(IV) clusters the VO2+ vanadyl groups themselves define quantum spin (S = 1/2) centers [1]. The most prominent example of the latter group of compounds is [VIV15AsIII6O42(H2O)]6 [2–5], an archetypal example of molecular geometric spin frustration, featuring an equilateral spin triangle characterized by an S = 1/2 ground state. [V15As6O42(H2O)]6 serves as a model for the study of the fundamental properties of such magnetic systems [6,7]. Besides the [V15As6O42(H2O)]6 cluster anion, several mixed-valent polyoxovanadate compounds are known, for example (NHEt3)4[V12As8O40(H2O)]H2O containing VIV and VV centers in an 8:4 ratio [8]; furthermore, numerous chemi-
* Corresponding author. Address: Institut für Anorganische Chemie der Universität Kiel, Max-Eyth-Str. 2, D-24118 Kiel, Germany. Fax: +49 (0) 431/880 1520. E-mail address: [email protected] (W. Bensch). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.06.065
cally modified VIV-containing compounds like (trenH3)2[V15Sb6O42]0.33trennH2O (tren: tris(2-aminoethyl)amine) [9], (C6H17N3)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 characterized derivatives of the [V18O42] archetype structure, the spin centers exhibit strong antiferromagnetic exchange interactions, dominated by exchange via the l-bridging O atoms. The magnetic ground states are determined by the number of unpaired spins, the geometry of the cluster anions and the relative strength of the exchange interactions. Interestingly, the magnetic properties of Ge-containing polyoxovanadates were not investigated despite the differing arrangement of the VO5 pyramids within the cluster anion compared to arsenato- and antimonato-polyoxovanadate systems. The additional substitution of VO2+ units in chemically modified polyoxovanadates by transition metal cations offers a path towards new cluster architectures exhibiting e.g. interesting magnetic properties such as spin-glass behavior in larger spin clusters. In the past, some experiments were done in this field by investigating the 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 the TM cation was incorporated within the cluster-shell like in [NiV13As8O41], [Cd2V12As8O40], [ZnV13As8O41], [Zn2V12As8O40], and [CdV13As8O41] [13d,16e,16f,17d], or TM complexes with organic
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molecules were formed acting as counterions or as connecting ligands [13–18]. Until now no TM-containing polyoxovanadates were reported with heteroelements other than As and Sb. We started to investigate solvothermal reactions employing simultaneously Ge, Cu, V compounds and amines as solvents and structure-directing molecules or templates. During the series of experiments we obtained for the first time the compound (H3aep)4[V14Ge8O50]2(aep)13H2O (1). The syntheses were originally performed to incorporate copper into mixed germanato-polyoxovanadates but in all cases elemental Cu was obtained as by-product. In this paper, we present the synthesis, crystal structure and properties of this new mixed polyoxovanadate.
Table 1 Details of the data collections and selected refinement results for 1. 1 Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z F(0 0 0) Reflections collection/independent Goodness of fit (GOF) on F2 R1a [I > 2r(I)] wR2b Absolute structure parameter Largest residuals (e Å 3)
2. Experimental 2.1. Syntheses Compound 1 was prepared under solvothermal conditions by reacting 210 mg (2 mmol) GeO2, 351 mg (3 mmol) NH4VO3, and 243 mg (1 mmol) Cu(NO3)23H2O in a solution of 1-(2-aminoethyl)-piperazine (75% in water, 6 mL). The experiments to synthesize 1 were performed in a temperature range from 110 to 180 °C. In typical experiments the mixtures were heated in PTFE-lined steel autoclaves for 9 days. The final pH value of all reaction mixtures was around 11.5. Dark brown polyhedral crystals of 1 were obtained just below 150 °C, and black octahedrons of the known compound (pipH2)4(pipH)4[V14Ge8O50(H2O)] (2) [12] were formed above 150 °C. We note that the synthesis of compound 1 is also successful without Cu(NO3)23H2O in the reaction mixture. The yield 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%.
The crystallographic data for compounds 1 has been deposited with the Cambridge Crystallographic Data Center as publication No. CCDC 781153 (1). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1 EZ, UK (mail: [email protected]). a R1 = R ||Fo| |Fc||/R |Fo|. b wR2 = |R w(|Fo|2 |Fc|2)|/R|w(Fo2)2|1/2.
Table 2 Atomic coordinates (104) and equivalent isotropic displacement parameters Ueqa (Å2 103) for 1.
2.2. Crystal structure determination The single crystal X-ray intensities were collected at room temperature with a STOE-1 Imaging Plate Diffraction System (IPDS-1) with Mo Ka radiation (k = 0.71073 Å). Selected crystal data and details of the structure determination are summarized in Table 1. The intensities were corrected for Lorentz and polarization effects. The structures were solved with SHELXS-97 [19] and refined against F2 with SHELXL-97 [20]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The C–H hydrogen atoms were positioned with idealized geometry and were refined using a riding model. After structure refinement of 1 small electron density maxima were detected in the difference Fourier map indicating the presence of water molecules in the structure. The maxima are located in the large channels of the structure and all attempts to refine these molecules failed due to a very strong disorder. Hence, the intensity data were corrected for disordered solvent molecules using the SQUEEZE option in PLATON [21]. Atomic coordinates and selected interatomic distances and angles are listed in Tables 2 and 3 (1). a
2.3. Elemental analysis The C, H, and N contents were determined by combustion analysis on a CHNS-Rapid-Element-Analyzer (Heraeus GmbH) using sulfanilamide as standard. 2.4. Thermal analysis The DTA–TG investigations were carried out in a nitrogen atmosphere (purity: 5.0; heating rate 4 K/min; flow rate: 75 mL/min; Al2O3 crucibles) using a Netzsch STA-409CD instrument.
C36H128Ge8N18O63V14 3115.3961 tetragonal P-421c 17.1932(10) 17.1932(10) 16.5014(11) 90 90 90 4877.9(5) 2 2532 49352/5873 1.049 0.0298 0.0751 0.014(10) 0.573/ 0.472
Atom
x/a
y/b
z/c
Ueq (Å2)
Ge(1) Ge(2) V(1) V(2) V(3) V(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) N(1) C(1) C(2) N(2) C(3) C(4) C(5) C(6) N(3)
6855(1) 6165(1) 5000 5276(1) 5382(1) 6865(1) 7071(2) 7056(2) 7337(2) 5840(1) 5000 5402(2) 6073(2) 5502(2) 6162(2) 7633(2) 6998(2) 5925(2) 7005(2) 6139(2) 6021(3) 6541(3) 6393(2) 6423(4) 5904(3) 5763(3) 6268(3) 7043(2)
4439(1) 3440(1) 5000 3375(1) 2712(1) 3678(1) 4505(2) 5357(2) 3706(2) 4294(2) 5000 2687(2) 3232(1) 1784(2) 3191(2) 3145(2) 4574(1) 4439(1) 3218(2) 2745(2) 2192(3) 2409(3) 3213(2) 3776(3) 3524(3) 2502(3) 1909(3) 2200(2)
6855(1) 3149(1) 7321(1) 6519(1) 4938(1) 5002(1) 5821(1) 7271(2) 7347(2) 6952(2) 8284(2) 7163(2) 5700(1) 4974(2) 4186(1) 5023(2) 4302(1) 3073(2) 2650(2) 10144(2) 9482(2) 8772(2) 8500(2) 9168(3) 9873(3) 10900(3) 11322(3) 11457(2)
20(1) 21(1) 19(1) 19(1) 19(1) 19(1) 23(1) 25(1) 35(1) 25(1) 40(1) 37(1) 22(1) 34(1) 22(1) 33(1) 22(1) 26(1) 36(1) 34(1) 40(1) 42(1) 41(1) 53(1) 49(1) 47(1) 47(1) 47(1)
Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
2.5. Magnetic measurements Low-field magnetic susceptibility data for 1 were recorded between 2.0 and 290 K at an external field of B0 = 0.1 Tesla using a Quantum Design MPMS-5XL SQUID magnetometer and PTFE sample holders. Field-dependent magnetization measurements were performed at 2.0 K (B0 = 0.1–5.0 Tesla). The susceptibility data were corrected for diamagnetic contributions for counter ions and crystal solvent molecules. Note that polyoxovanadate cluster
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J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404 Table 3 Selected interatomic distances (Å) and angles (°) for 1. Estimated standard deviations are given in parentheses. Ge(1)–O(3) Ge(1)–O(1) Ge(1)–O(2) Ge(1)–O(4) V(1)–O(5) V(1)–O(12B) V(1)–O(12A) V(1)–O(4C) V(1)–O(4) V(2)–O(6) V(2)–O(11A) V(2)–O(7) V(2)–O(4) V(2)–O(12A) V(2)–V(3) V(3)–O(8) V(3)–O(11A) V(3)–O(7) V(3)–O(1°) V(3)–O(9) N(1)–C(1) N(1)–C(5) N(1)–C(4) C(1)–C(2) O(3)–Ge(1)–O(1) O(3)–Ge(1)–O(2) O(1)–Ge(1)–O(2) O(13)–Ge(2)–O(9) O(13)–Ge(2)–O(2A) O(9)–Ge(2)–O(2A) O(13)–Ge(2)–O(12) O(9)–Ge(2)–O(12) O(2A)–Ge(2)–O(12) O(5)–V(1)–O(12B) O(5)–V(1)–O(12A) O(12B)–V(1)–O(12A) O(5)–V(1)–O(4C) O(12B)–V(1)–O(4C) O(12A)–V(1)–O(4C) O(5)–V(1)–O(4) O(12B)–V(1)–O(4) O(12A)–V(1)–O(4) O(4C)–V(1)–O(4) O(6)–V(2)–O(11A) O(6)–V(2)–O(7) O(11A)–V(2)–O(7) O(6)–V(2)–O(4) O(11A)–V(2)–O(4) O(7)–V(2)–O(4) O(6)–V(2)–O(12A) O(11A)–V(2)–O(12A) O(7)–V(2)–O(12A) O(4)–V(2)–O(12A) O(6)–V(2)–V(3) O(11A)–V(2)–V(3) O(7)–V(2)–V(3) O(4)–V(2)–V(3) O(12A)–V(2)–V(3) O(8)–V(3)–O(11A) O(8)–V(3)–O(7) O(11A)–V(3)–O(7) O(8)–V(3)–O(1A) O(11A)–V(3)–O(1A) O(7)–V(3)–O(1A) O(8)–V(3)–O(9) O(11A)–V(3)–O(9) O(7)–V(3)–O(9) O(1A)–V(3)–O(9) O(8)–V(3)–V(2) O(11A)–V(3)–V(2) O(7)–V(3)–V(2) O(1A)–V(3)–V(2) O(9)–V(3)–V(2) O(8)–V(3)–V(4A) O(11A)–V(3)–V(4A)
1.712(3) 1.751(2) 1.756(3) 1.770(3) 1.590(3) 1.971(3) 1.971(3) 1.983(2) 1.983(2) 1.605(3) 1.924(2) 1.940(3) 1.988(3) 1.993(3) 2.8530(8) 1.610(3) 1.938(2) 1.948(2) 1.995(2) 2.005(3) 1.463(5) 1.464(5) 1.470(6) 1.521(6) 114.10(14) 112.41(14) 106.28(12) 114.65(14) 111.57(13) 105.09(12) 112.33(14) 107.64(12) 104.86(12) 109.25(8) 109.25(8) 141.49(16) 107.88(8) 76.19(10) 92.07(11) 107.88(8) 92.07(11) 76.19(10) 144.24(16) 107.79(13) 105.81(14) 84.88(10) 106.40(14) 145.49(11) 90.33(11) 108.59(14) 89.19(11) 145.26(11) 75.59(10) 107.60(12) 42.55(7) 42.89(7) 127.95(8) 126.61(8) 108.87(14) 110.68(13) 84.30(10) 107.79(13) 79.39(10) 141.28(11) 110.14(14) 140.80(11) 78.66(10) 92.32(10) 111.74(12) 42.19(7) 42.68(8) 116.78(7) 116.42(7) 111.20(11) 38.83(7)
Ge(2)–O(13) Ge(2)–O(9) Ge(2)–O(2A) Ge(2)–O(12) V(3)–V(4A) V(3)–V(4) V(4)–O(10) V(4)–O(11) V(4)–O(7) V(4)–O(1) V(4)–O(9) V(4)–V(3B) O(1)–V(3B) O(2)–Ge(2B) O(11)–V(2B) O(11)–V(3B) O(12)–V(1A) O(12)–V(2B) O(3)–Ge(1)–O(4) O(1)–Ge(1)–O(4) O(2)–Ge(1)–O(4) O(10)–V(4)–O(11) O(10)–V(4)–O(7) O(11A)–V(3)–V(4) O(7)–V(3)–V(4) O(1A)–V(3)–V(4) O(9)–V(3)–V(4) O(11)–V(4)–O(7) O(10)–V(4)–O(1) O(11)–V(4)–O(1) O(7)–V(4)–O(1) O(10)–V(4)–O(9) O(11)–V(4)–O(9) O(7)–V(4)–O(9) O(1)–V(4)–O(9) O(10)–V(4)–V(3B) O(11)–V(4)–V(3B) O(7)–V(4)–V(3B) O(1)–V(4)–V(3B) O(9)–V(4)–V(3B) O(10)–V(4)–V(3) O(11)–V(4)–V(3) O(7)–V(4)–V(3) O(1)–V(4)–V(3) O(9)–V(4)–V(3) V(3B)–V(4)–V(3) Ge(1)–O(1)–V(4) Ge(1)–O(1)–V(3B) V(4)–O(1)–V(3B) Ge(1)–O(2)–Ge(2B) Ge(1)–O(4)–V(1) Ge(1)–O(4)–V(2) V(1)–O(4)–V(2) V(2)–O(7)–V(4) V(2)–O(7)–V(3) V(4)–O(7)–V(3) Ge(2)–O(9)–V(4) Ge(2)–O(9)–V(3) V(4)–O(9)–V(3) V(2B)–O(11)–V(3B) V(2B)–O(11)–V(4) V(3B)–O(11)–V(4) Ge(2)–O(12)–V(1A) Ge(2)–O(12)–V(2B) V(1A)–O(12)–V(2B)
1.705(3) 1.763(2) 1.771(3) 1.771(3) 3.0206(8) 3.0452(8) 1.607(3) 1.939(2) 1.942(3) 1.993(3) 1.995(2) 3.0206(8) 1.995(2) 1.771(3) 1.924(2) 1.938(2) 1.971(3) 1.993(3) 109.31(14) 107.80(12) 106.57(12) 111.64(13) 109.80(13) 115.93(8) 38.40(7) 123.55(8) 40.29(7) 138.56(11) 104.26(14) 79.40(10) 90.26(10) 105.83(13) 90.11(10) 79.05(10) 149.91(11) 110.74(11) 38.80(7) 122.19(8) 40.78(7) 125.12(8) 112.21(11) 120.72(8) 38.54(7) 124.18(8) 40.54(7) 137.05(3) 125.20(13) 134.50(14) 98.49(10) 117.28(13) 131.31(14) 124.13(14) 103.95(11) 149.17(14) 94.43(11) 103.06(11) 123.51(14) 134.61(14) 99.17(10) 95.26(11) 151.50(15) 102.37(11) 133.32(14) 122.40(13) 104.21(11)
Table 3 (continued) O(7)–V(3)–V(4A) O(1A)–V(3)–V(4A) O(9)–V(3)–V(4A) V(2)–V(3)–V(4A) O(8)–V(3)–V(4) V(2)–V(3)–V(4) V(4A)–V(3)–V(4)
117.37(8) 40.73(7) 124.79(8) 79.15(2) 115.61(11) 78.68(2) 132.82(3)
Symmetry transformations used to generate equivalent atoms: A: y, B: y + 1, x, z + 1; C: x + 1, y + 1, z.
x + 1,
z + 1;
anions exhibit strong TIP contributions which thus preclude the immediate calculation of their combined diamagnetic/TIP susceptibilities.
3. Results and discussion 3.1. Synthetic aspects The original aim of the syntheses was the interconnection of the germanato–polyoxovanadato clusters by Cu2+ complexes. However, all attempts to incorporate Cu2+ cations into POM architectures failed. Interestingly, the Cu2+ was reduced to metallic Cu during the syntheses as evidenced by X-ray powder diffractometry. Additional experiments have demonstrated that compound 1 could be obtained without a copper source. Compound 2 crystallized only applying a Cu2+ salt when 1-(2-aminoethyl)-piperazine (Scheme 1) was used as solvent, and syntheses without Cu2+ afforded crystallization of compound 1. The structure of 2 contains two types of piperazinium cations (Scheme 1), i.e., the 1-(2-aminoethyl)-piperazine molecule was fragmented under in situ conditions. Solvothermal syntheses performed with piperazine and without a Cu2+ salt afforded the formation of compound 2 [12]. We note that the structure 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 disordered piperazine molecules [12]. We solved and refined the structure of 2 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 both compounds coexist and the results demonstrate that small
Fig. 1. The [V14Ge8O50(H2O)]12 cluster anion in compound 1. Only selected atoms are labeled. Note that the central H2O molecule is not shown.
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amounts of 2 are present in the reaction product of the synthesis performed at 150 °C in the presence of a Cu2+ salt. The role of the Cu2+ salt in the formation of compound 2 is not clear and further
experiments are underway, especially in situ X-ray scattering investigations, to clarify the influence of Cu2+ on the crystallization. 3.2. Crystal structures
Fig. 2. The hydrogen bonding interactions between the protonated amine molecules and the [V14Ge8O50(H2O)]12 cluster anion in 1 indicated by dashed lines.
The new compound 1 crystallizes in space group P-421c with two formula units per unit cell as brown crystals with irregular shape. Inside the [V14Ge8O50]12 cluster several electron density maxima were found which were assumed to be oxygen of a H2O molecule. But due to the strong disorder no satisfactory model could be found. The structure of compound 1 features an isolated spherical [V14Ge8O50(H2O)]12 cluster anion (Fig. 1) as main structural motif and protonated amine molecules as countercations. The spherical [V14Ge8O50(H2O)]12 cluster is related to the [V18O42] archetype cluster, and the structure of both germanium–polyoxovanadate cluster anions can be derived from this fundamental POM unit when four VO5 groups are replaced by four Ge2O7 dumbbells. The Ge2O7 groups are inserted in the shell of the [V18O42] unit by sharing corners with six VO5 pyramids. The substitution of the four VO5 pyramids leads to the formation of a central ring consisting of eight edge-sharing VO5 units. The remaining six VO5 pyramids are grouped in two caps which are rotated by 90° to each other and are condensed to the central ring through common edges. In 1 charge compensation is achieved by four crystallographically independent triple protonated 1-(2-aminoethyl)-piperazine
Fig. 3. Arrangement of cluster anions and organic cations in 1 (dashed lines: hydrogen bonds).
4403
578
635
672 741 776
986
1200
1100
1000
900
800
ν / cm
700
600
500
400
–1
Fig. 5. IR spectrum of compound 1.
Scheme 1. Organic bases in compounds 1 and 2.
Thermal analysis shows that compound 1 decomposes at comparably low temperatures (Fig. 4) and according to simultaneous mass spectrometry investigations the emission of water starts at about 50 °C. The thermal degradation occurs in a more or less continuous fashion without pronounced decomposition steps, and the simultaneously recorded DTA curve shows no significant thermal event. Even at about 550 °C the mass loss is ongoing. In the IR spectrum of 1 (Fig. 5) the strong V–O stretching vibration is located at 986 cm 1 in accordance with the energetic position of the vanadyl band. 3.3. Magnetism
100 95 90 Δm / %
1098
(H3aep, C6H18N33+) molecules. The Ge atoms are surrounded by four oxygen atoms in a slightly distorted tetrahedral coordination with Ge–O bond lengths ranging from 1.712(3) to 1.771(3) Å, and O–Ge–O angles close to the ideal tetrahedral value (104.9– 114.7°). The VO5 pyramids have typical geometric parameters with basal and apical bond lengths from 1.924(2) to 2.005(3) Å and 1.590(3) to 1.610(3) Å respectively. The O–V–O angles vary between 75.6° and 149.9°. These parameters closely match those reported for other mixed germanato-polyoxovanadates [12,22]. The oxidation states of the V and Ge atoms in 1 were calculated with the bond valence sum method (BVS) [23]. The resulting values of 4.1 for V and 4.0 for Ge justify the assignment of the valence states V4+ and Ge4+. The potential free solvent area was calculated with the PLATON program suite [21] yielding 1517 Å3 being about 31% of the unit cell volume. Between the clusters and the organic ammonium molecules of 1, several hydrogen bonds are found. Each [V14Ge8O50]12 cluster is 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 a three-dimensional network. Along the c-axis, layers of [V14Ge8O50(H2O)]12 units alternate in an ABAB sequence (Fig. 3, top), and each cluster anion is surrounded by six other anions. The anions in every A layer are rotated by 90° with respect to the clusters in the B layers. Within the layers which are oriented parallel to the (0 0 1) plane, the cluster anions form a rectangular net with channels running along [0 0 1] (Fig. 3, bottom). The diameter of the channels is about 6.4 6.4 Å (measured from coordinate-to-coordinate). It can be assumed that the co-crystallized water molecules are located within the channels. The geometric parameters of 2 are comparable with those obtained for 1, i.e. bond lengths and angles are very similar for the two compounds. The bond valence sum for all V and Ge atoms in 2 indicates that the average oxidation states are close to +4. In contrast 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 be observed. Every [V14Ge8O50(H2O)]12 cluster in 2 is surrounded by 12 amine molecules through such H-bonding interactions leading to a three-dimensional H bonded network. Obviously, the differing supramolecular arrangements of anions and cations lead to the crystallization in different space groups with significantly differing lattice parameters. The densities of the two compounds are different with 2.121 g/cm3 for 1 and 2.238 g/cm3 for 2. The more dense packing of the constituents in 2 is reflected by the lower potential solvent area of 247.5 Å3, equivalent to about 1.5% of the unit cell volume.
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85 80 75 100
200
300
400
500
T / °C Fig. 4. The TG curve of the thermal decomposition of compound 1.
600
The fourteen S = 1/2 vanadyl centers in the polyanion cluster in 1 experience strong intra-antiferromagnetic coupling as evidenced in the low-field susceptibility (Fig. 6). At room temperature, vmT only 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 temperatures vmT reaches a plateau between ca. 20 and 200 K of 2.2 cm3 K/mol, matching the value for six uncoupled vanadyl centers (2.205 cm3 K/mol). The strongest exchange, caused by nearlinear V–l–O–V and angled V(–l–O–)2V exchange pathways between edge- and corner-sharing VO5 square pyramids, results in an eight-membered spin ring to which two three-membered spin bridges are joined. Considering only the V(–l–O–)2V contacts of neighboring edge-sharing VO5 pyramids, the resulting ‘skeletal’ spin array is not geometrically frustrated. However, the competing linear V–l–O–V contacts render the spin structure as consisting of
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Acknowledgements Financial support by the State of Schleswig-Holstein is grateful acknowledged. References
Fig. 6. Temperature dependence of vmT for compound 1 at 0.1 Tesla. Inset: fielddependent magnetization of 1 at 2.0 K.
four pairs of edge-joined spin triangles interconnected by four additional spin centers. Moreover, multiple additional (weaker) exchange pathways involving the germanate groups add more interactions, complicating a full magnetochemical analysis. At the lowest temperature of our preliminary susceptibility measurements (2.0 K), the field-dependent magnetization does not follow a scaled Brillouin function, indicating that several multiplet states are 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 at elevated temperatures in the presence of a Cu2+ source. The results of the syntheses demonstrate the complexity of the solvothermal approach where different reaction parameters determine the formation of a distinct compound. At the moment one can only speculate about the role of Cu2+ for the crystallization of compound 2. Here, the decomposition of aep by elimination of the aminoethyl group appears to be induced by Cu2+. After formation of pip the reduction of VV in the metavanadate salt to VIV in the cluster anion occurs. This scenario seems to be likely because 2 is formed in 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 dominated by strong antiferromagnetic exchange interactions mediated by V–l–O–V and bent V(–l–O–)2V bridges. A detailed study of the magnetic properties is underway as well as further synthetic efforts for the generation of new germanato–polyoxovanadato cluster architectures.
[1] (a) A. Müller, F. Peters, M.T. Pope, D. Gatteschi, Chem. Rev. 98 (1998) 239; (b) A. Müller, P. Kögerler, A.W.M. Dress, Coord. Chem. Rev. 222 (2001) 193; (c) P. Kögerler, B. Tsukerblat, A. Müller, Dalton Trans. 39 (2010) 21. [2] (a) A. Müller, J. Döring, Angew. Chem. 100 (1988) 1789; (b) D. Gatteschi, L. Pardi, A.L. Barra, A. Müller, J. Döring, Nature 354 (1991) 463. [3] (a) I. Chiorescu, W. Wernsdorfer, A. Müller, H. Bögge, B. Barbara, Phys. Rev. Lett. 84 (2000) 3454; (b) K. Kajiyoshi, T. Kambe, M. Mino, H. Nojiri, P. Kögerler, M. Luban, J. Magn. Magn. Mater. 310 (2007) 1203; (c) D. Procissi, A. Lascialfari, E. Micotti, M. Bertassi, P. Carretta, Y. Furukawa, P. Kögerler, Phys. Rev. B 73 (2006) 184417. [4] S. Bertaina, S. Gambarelli, T. Mitra, B. Tsukerblat, A. Müller, B. Barbara, Nature 453 (2008) 203. [5] R.E.P. Winpenny, Angew. Chem., Int. Ed. 47 (2008) 7992. [6] Y. Furukawa, Y. Nishisaka, K. Kumagai, P. Kögerler, J. Magn. Magn. Mater. 310 (2007) 1429. [7] (a) A. Tarantul, B. Tsukerblat, A. Müller, Inorg. Chem. 46 (2007) 161; (b) B. Tsukerblat, A. Tarantul, A. Müller, J. Chem. Phys. 125 (2006) 054714. [8] R. Basler, G. Chaboussant, A. Sieber, H. Andres, M. Murrie, P. Kögerler, H. Bögge, D.C. Crans, E. Krickemeyer, S. Janssen, H. Mutka, A. Müller, H.-U. Güdel, Inorg. Chem. 41 (2002) 5675. [9] R. Kiebach, C. Näther, P. Kögerler, W. Bensch, Dalton Trans. (2007) 3221. [10] R. Kiebach, C. Näther, W. Bensch, Solid State Sci. 8 (2006) 964. [11] A. Wutkowski, C. Näther, P. Kögerler, W. Bensch, Inorg. Chem. 47 (2008) 1916. [12] T. Whitfield, X. Wang, A.J. Jacobson, Inorg. Chem. 42 (2003) 3728. [13] (a) X.B. Cui, Y.Q. Sun, G.Y. Yang, Inorg. Chem. Commun. 6 (2003) 259; (b) X.B. Cui, J.Q. Xu, Y.H. Sun, Y. Li, L. Ye, G.Y. Yang, Inorg. Chem. Commun. 7 (2004) 58; (c) X.B. Cui, J.Q. Xu, Y. Li, Y.H. Sun, G.Y. Yang, Eur. J. Inorg. Chem. (2004) 1051; (d) X.B. Cui, J.Q. Xu, H. Meng, S.T. Zheng, G.Y. Yang, Inorg. Chem. 43 (2004) 8005. [14] B.X. Dong, J. Peng, C.J. Gómez-García, S. Benmansour, H.Q. Jia, N.H. Hu, Inorg. Chem. 46 (2007) 5933. [15] G.P. Zhou, Y. Xu, C.Y. Guo, X.F. Zheng, Inorg. Chem. Commun. 10 (2007) 849. [16] (a) S.T. Zheng, J. Zhang, G.Y. Yang, Eur. J. Inorg. Chem. (2004) 2004; (b) S.T. Zheng, J. Zhang, J.Q. Xu, G.Y. Yang, J. Solid State Chem. 178 (2005) 3740; (c) S.T. Zheng, J. Zhang, G.Y. Yang, J. Mol. Struct. 752 (2005) 25; (d) S.T. Zheng, Y.M. Chen, J. Zhang, G.Y. Yang, Z. Anorg. Allg. Chem. 632 (2006) 155; (e) S.T. Zheng, J. Zhang, G.Y. Yang, Inorg. Chem. 44 (2005) 2426; (f) S.T. Zheng, M.H. Wang, G.Y. Yang, Inorg. Chem. 46 (2007) 9503. [17] (a) Y.F. Qi, Y.G. Li, C. Qin, E.B. Wang, H. Jin, D.R. Xiao, X.L. Wang, S. Chang, Inorg. Chem. 46 (2007) 3217; (b) Y.F. Qi, Y.G. Li, E.B. Wang, H. Jin, Z.M. Zhang, X.L. Wang, S. Chang, Inorg. Chim. Acta 360 (2007) 1841; (c) Y.F. Qi, Y.G. Li, E.B. Wang, H. Jin, Z.M. Zhang, X.L. Wang, S. Chang, J. Solid State Chem. 180 (2007) 382; (d) Y.F. Qi, Y.G. Li, E.B. Wang, Z.M. Zhang, S. Chang, Dalton Trans. (2008) 2335. [18] L.J. Zhang, X.L. Zhao, J.Q. Xu, T.G. Wang, J. Chem. Soc., Dalton Trans. (2002) 3275. [19] G.M. Sheldrick, SHELXS 97, Program for the Solution of Crystal Structures, University of Göttingen, Germany, 1997. [20] G.M. Sheldrick, SHELXL 97, Program for the Refinement of Crystal Structures, University of Göttingen, Germany, 1997. [21] A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, The Netherlands, 2000. [22] D. Pitzschke, J. Wang, D. Hoffmann, R. Pöttgen, W. Bensch, Angew. Chem., Int. Ed. 45 (2006) 1305. [23] (a) I.D. Brown, K.K. Wu, Acta Crystallogr., Sect. B 32 (1976) 1957; (b) I.D. Brown, D. Altermatt, Acta Crystallogr., Sect. B 41 (1985) 244.
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Synthesis, crystal structure and magnetism of a new mixed germanium–polyoxovanadate cluster Jing Wang, Christian Näther, Paul Kögerler, Wolfgang Bensch * Institut für Anorganische Chemie der Christian-Albrechts-Universität, Kiel, Germany Institut 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: Germanium Vanadium Solvothermal synthesis Crystal structure Polyoxovanadate Magnetic properties
a b s t r a c t A new germanium–polyoxovanadate, (H3aep)4[V14Ge8O50]2(aep)13H2O (1), has been synthesized under solvothermal conditions applying GeO2, NH4VO3, Cu(NO3)23H2O and an aqueous solution of 1-(2-aminoethyl)-piperazine (aep, C6H18N3) in the temperature range from 110 to 150 °C. The compound crystallizes in 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 [VIV14GeIV8O50]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 corner-sharing of two [GeO4]4 tetrahedra. At temperatures above 150 °C the compound (H2pip)4(Hpip)4[VIV14GeIV8O50(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 molecules in the structure of 1 are emitted at low temperatures. The magnetic properties are dominated by strong intra-cluster antiferromagnetic coupling and the strongest exchange between edge- and corner-sharing VO5 square pyramids results in an eight-membered spin ring to which two three-membered spin bridges are joined. The magnetic susceptibility data suggest that even at the low temperature of 2 K several multiplet states are still significantly populated. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the last decade magnetic polyoxometalate (POM)-based systems have been under intense investigation by several groups. The systems may be divided into several classes according to the identity of the spin centers: whereas in polyoxomolybdates and polyoxotungstates magnetic functionality is introduced by integration of heterometallic spin centers (usually 3d transition metal cations embedded into the otherwise diamagnetic polyanion frameworks), in polyoxovanadate(IV) clusters the VO2+ vanadyl groups themselves define quantum spin (S = 1/2) centers [1]. The most prominent example of the latter group of compounds is [VIV15AsIII6O42(H2O)]6 [2–5], an archetypal example of molecular geometric spin frustration, featuring an equilateral spin triangle characterized by an S = 1/2 ground state. [V15As6O42(H2O)]6 serves as a model for the study of the fundamental properties of such magnetic systems [6,7]. Besides the [V15As6O42(H2O)]6 cluster anion, several mixed-valent polyoxovanadate compounds are known, for example (NHEt3)4[V12As8O40(H2O)]H2O containing VIV and VV centers in an 8:4 ratio [8]; furthermore, numerous chemi-
* Corresponding author. Address: Institut für Anorganische Chemie der Universität Kiel, Max-Eyth-Str. 2, D-24118 Kiel, Germany. Fax: +49 (0) 431/880 1520. E-mail address: [email protected] (W. Bensch). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.06.065
cally modified VIV-containing compounds like (trenH3)2[V15Sb6O42]0.33trennH2O (tren: tris(2-aminoethyl)amine) [9], (C6H17N3)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 characterized derivatives of the [V18O42] archetype structure, the spin centers exhibit strong antiferromagnetic exchange interactions, dominated by exchange via the l-bridging O atoms. The magnetic ground states are determined by the number of unpaired spins, the geometry of the cluster anions and the relative strength of the exchange interactions. Interestingly, the magnetic properties of Ge-containing polyoxovanadates were not investigated despite the differing arrangement of the VO5 pyramids within the cluster anion compared to arsenato- and antimonato-polyoxovanadate systems. The additional substitution of VO2+ units in chemically modified polyoxovanadates by transition metal cations offers a path towards new cluster architectures exhibiting e.g. interesting magnetic properties such as spin-glass behavior in larger spin clusters. In the past, some experiments were done in this field by investigating the 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 the TM cation was incorporated within the cluster-shell like in [NiV13As8O41], [Cd2V12As8O40], [ZnV13As8O41], [Zn2V12As8O40], and [CdV13As8O41] [13d,16e,16f,17d], or TM complexes with organic
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molecules were formed acting as counterions or as connecting ligands [13–18]. Until now no TM-containing polyoxovanadates were reported with heteroelements other than As and Sb. We started to investigate solvothermal reactions employing simultaneously Ge, Cu, V compounds and amines as solvents and structure-directing molecules or templates. During the series of experiments we obtained for the first time the compound (H3aep)4[V14Ge8O50]2(aep)13H2O (1). The syntheses were originally performed to incorporate copper into mixed germanato-polyoxovanadates but in all cases elemental Cu was obtained as by-product. In this paper, we present the synthesis, crystal structure and properties of this new mixed polyoxovanadate.
Table 1 Details of the data collections and selected refinement results for 1. 1 Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z F(0 0 0) Reflections collection/independent Goodness of fit (GOF) on F2 R1a [I > 2r(I)] wR2b Absolute structure parameter Largest residuals (e Å 3)
2. Experimental 2.1. Syntheses Compound 1 was prepared under solvothermal conditions by reacting 210 mg (2 mmol) GeO2, 351 mg (3 mmol) NH4VO3, and 243 mg (1 mmol) Cu(NO3)23H2O in a solution of 1-(2-aminoethyl)-piperazine (75% in water, 6 mL). The experiments to synthesize 1 were performed in a temperature range from 110 to 180 °C. In typical experiments the mixtures were heated in PTFE-lined steel autoclaves for 9 days. The final pH value of all reaction mixtures was around 11.5. Dark brown polyhedral crystals of 1 were obtained just below 150 °C, and black octahedrons of the known compound (pipH2)4(pipH)4[V14Ge8O50(H2O)] (2) [12] were formed above 150 °C. We note that the synthesis of compound 1 is also successful without Cu(NO3)23H2O in the reaction mixture. The yield 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%.
The crystallographic data for compounds 1 has been deposited with the Cambridge Crystallographic Data Center as publication No. CCDC 781153 (1). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1 EZ, UK (mail: [email protected]). a R1 = R ||Fo| |Fc||/R |Fo|. b wR2 = |R w(|Fo|2 |Fc|2)|/R|w(Fo2)2|1/2.
Table 2 Atomic coordinates (104) and equivalent isotropic displacement parameters Ueqa (Å2 103) for 1.
2.2. Crystal structure determination The single crystal X-ray intensities were collected at room temperature with a STOE-1 Imaging Plate Diffraction System (IPDS-1) with Mo Ka radiation (k = 0.71073 Å). Selected crystal data and details of the structure determination are summarized in Table 1. The intensities were corrected for Lorentz and polarization effects. The structures were solved with SHELXS-97 [19] and refined against F2 with SHELXL-97 [20]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The C–H hydrogen atoms were positioned with idealized geometry and were refined using a riding model. After structure refinement of 1 small electron density maxima were detected in the difference Fourier map indicating the presence of water molecules in the structure. The maxima are located in the large channels of the structure and all attempts to refine these molecules failed due to a very strong disorder. Hence, the intensity data were corrected for disordered solvent molecules using the SQUEEZE option in PLATON [21]. Atomic coordinates and selected interatomic distances and angles are listed in Tables 2 and 3 (1). a
2.3. Elemental analysis The C, H, and N contents were determined by combustion analysis on a CHNS-Rapid-Element-Analyzer (Heraeus GmbH) using sulfanilamide as standard. 2.4. Thermal analysis The DTA–TG investigations were carried out in a nitrogen atmosphere (purity: 5.0; heating rate 4 K/min; flow rate: 75 mL/min; Al2O3 crucibles) using a Netzsch STA-409CD instrument.
C36H128Ge8N18O63V14 3115.3961 tetragonal P-421c 17.1932(10) 17.1932(10) 16.5014(11) 90 90 90 4877.9(5) 2 2532 49352/5873 1.049 0.0298 0.0751 0.014(10) 0.573/ 0.472
Atom
x/a
y/b
z/c
Ueq (Å2)
Ge(1) Ge(2) V(1) V(2) V(3) V(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) N(1) C(1) C(2) N(2) C(3) C(4) C(5) C(6) N(3)
6855(1) 6165(1) 5000 5276(1) 5382(1) 6865(1) 7071(2) 7056(2) 7337(2) 5840(1) 5000 5402(2) 6073(2) 5502(2) 6162(2) 7633(2) 6998(2) 5925(2) 7005(2) 6139(2) 6021(3) 6541(3) 6393(2) 6423(4) 5904(3) 5763(3) 6268(3) 7043(2)
4439(1) 3440(1) 5000 3375(1) 2712(1) 3678(1) 4505(2) 5357(2) 3706(2) 4294(2) 5000 2687(2) 3232(1) 1784(2) 3191(2) 3145(2) 4574(1) 4439(1) 3218(2) 2745(2) 2192(3) 2409(3) 3213(2) 3776(3) 3524(3) 2502(3) 1909(3) 2200(2)
6855(1) 3149(1) 7321(1) 6519(1) 4938(1) 5002(1) 5821(1) 7271(2) 7347(2) 6952(2) 8284(2) 7163(2) 5700(1) 4974(2) 4186(1) 5023(2) 4302(1) 3073(2) 2650(2) 10144(2) 9482(2) 8772(2) 8500(2) 9168(3) 9873(3) 10900(3) 11322(3) 11457(2)
20(1) 21(1) 19(1) 19(1) 19(1) 19(1) 23(1) 25(1) 35(1) 25(1) 40(1) 37(1) 22(1) 34(1) 22(1) 33(1) 22(1) 26(1) 36(1) 34(1) 40(1) 42(1) 41(1) 53(1) 49(1) 47(1) 47(1) 47(1)
Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
2.5. Magnetic measurements Low-field magnetic susceptibility data for 1 were recorded between 2.0 and 290 K at an external field of B0 = 0.1 Tesla using a Quantum Design MPMS-5XL SQUID magnetometer and PTFE sample holders. Field-dependent magnetization measurements were performed at 2.0 K (B0 = 0.1–5.0 Tesla). The susceptibility data were corrected for diamagnetic contributions for counter ions and crystal solvent molecules. Note that polyoxovanadate cluster
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J. Wang et al. / Inorganica Chimica Acta 363 (2010) 4399–4404 Table 3 Selected interatomic distances (Å) and angles (°) for 1. Estimated standard deviations are given in parentheses. Ge(1)–O(3) Ge(1)–O(1) Ge(1)–O(2) Ge(1)–O(4) V(1)–O(5) V(1)–O(12B) V(1)–O(12A) V(1)–O(4C) V(1)–O(4) V(2)–O(6) V(2)–O(11A) V(2)–O(7) V(2)–O(4) V(2)–O(12A) V(2)–V(3) V(3)–O(8) V(3)–O(11A) V(3)–O(7) V(3)–O(1°) V(3)–O(9) N(1)–C(1) N(1)–C(5) N(1)–C(4) C(1)–C(2) O(3)–Ge(1)–O(1) O(3)–Ge(1)–O(2) O(1)–Ge(1)–O(2) O(13)–Ge(2)–O(9) O(13)–Ge(2)–O(2A) O(9)–Ge(2)–O(2A) O(13)–Ge(2)–O(12) O(9)–Ge(2)–O(12) O(2A)–Ge(2)–O(12) O(5)–V(1)–O(12B) O(5)–V(1)–O(12A) O(12B)–V(1)–O(12A) O(5)–V(1)–O(4C) O(12B)–V(1)–O(4C) O(12A)–V(1)–O(4C) O(5)–V(1)–O(4) O(12B)–V(1)–O(4) O(12A)–V(1)–O(4) O(4C)–V(1)–O(4) O(6)–V(2)–O(11A) O(6)–V(2)–O(7) O(11A)–V(2)–O(7) O(6)–V(2)–O(4) O(11A)–V(2)–O(4) O(7)–V(2)–O(4) O(6)–V(2)–O(12A) O(11A)–V(2)–O(12A) O(7)–V(2)–O(12A) O(4)–V(2)–O(12A) O(6)–V(2)–V(3) O(11A)–V(2)–V(3) O(7)–V(2)–V(3) O(4)–V(2)–V(3) O(12A)–V(2)–V(3) O(8)–V(3)–O(11A) O(8)–V(3)–O(7) O(11A)–V(3)–O(7) O(8)–V(3)–O(1A) O(11A)–V(3)–O(1A) O(7)–V(3)–O(1A) O(8)–V(3)–O(9) O(11A)–V(3)–O(9) O(7)–V(3)–O(9) O(1A)–V(3)–O(9) O(8)–V(3)–V(2) O(11A)–V(3)–V(2) O(7)–V(3)–V(2) O(1A)–V(3)–V(2) O(9)–V(3)–V(2) O(8)–V(3)–V(4A) O(11A)–V(3)–V(4A)
1.712(3) 1.751(2) 1.756(3) 1.770(3) 1.590(3) 1.971(3) 1.971(3) 1.983(2) 1.983(2) 1.605(3) 1.924(2) 1.940(3) 1.988(3) 1.993(3) 2.8530(8) 1.610(3) 1.938(2) 1.948(2) 1.995(2) 2.005(3) 1.463(5) 1.464(5) 1.470(6) 1.521(6) 114.10(14) 112.41(14) 106.28(12) 114.65(14) 111.57(13) 105.09(12) 112.33(14) 107.64(12) 104.86(12) 109.25(8) 109.25(8) 141.49(16) 107.88(8) 76.19(10) 92.07(11) 107.88(8) 92.07(11) 76.19(10) 144.24(16) 107.79(13) 105.81(14) 84.88(10) 106.40(14) 145.49(11) 90.33(11) 108.59(14) 89.19(11) 145.26(11) 75.59(10) 107.60(12) 42.55(7) 42.89(7) 127.95(8) 126.61(8) 108.87(14) 110.68(13) 84.30(10) 107.79(13) 79.39(10) 141.28(11) 110.14(14) 140.80(11) 78.66(10) 92.32(10) 111.74(12) 42.19(7) 42.68(8) 116.78(7) 116.42(7) 111.20(11) 38.83(7)
Ge(2)–O(13) Ge(2)–O(9) Ge(2)–O(2A) Ge(2)–O(12) V(3)–V(4A) V(3)–V(4) V(4)–O(10) V(4)–O(11) V(4)–O(7) V(4)–O(1) V(4)–O(9) V(4)–V(3B) O(1)–V(3B) O(2)–Ge(2B) O(11)–V(2B) O(11)–V(3B) O(12)–V(1A) O(12)–V(2B) O(3)–Ge(1)–O(4) O(1)–Ge(1)–O(4) O(2)–Ge(1)–O(4) O(10)–V(4)–O(11) O(10)–V(4)–O(7) O(11A)–V(3)–V(4) O(7)–V(3)–V(4) O(1A)–V(3)–V(4) O(9)–V(3)–V(4) O(11)–V(4)–O(7) O(10)–V(4)–O(1) O(11)–V(4)–O(1) O(7)–V(4)–O(1) O(10)–V(4)–O(9) O(11)–V(4)–O(9) O(7)–V(4)–O(9) O(1)–V(4)–O(9) O(10)–V(4)–V(3B) O(11)–V(4)–V(3B) O(7)–V(4)–V(3B) O(1)–V(4)–V(3B) O(9)–V(4)–V(3B) O(10)–V(4)–V(3) O(11)–V(4)–V(3) O(7)–V(4)–V(3) O(1)–V(4)–V(3) O(9)–V(4)–V(3) V(3B)–V(4)–V(3) Ge(1)–O(1)–V(4) Ge(1)–O(1)–V(3B) V(4)–O(1)–V(3B) Ge(1)–O(2)–Ge(2B) Ge(1)–O(4)–V(1) Ge(1)–O(4)–V(2) V(1)–O(4)–V(2) V(2)–O(7)–V(4) V(2)–O(7)–V(3) V(4)–O(7)–V(3) Ge(2)–O(9)–V(4) Ge(2)–O(9)–V(3) V(4)–O(9)–V(3) V(2B)–O(11)–V(3B) V(2B)–O(11)–V(4) V(3B)–O(11)–V(4) Ge(2)–O(12)–V(1A) Ge(2)–O(12)–V(2B) V(1A)–O(12)–V(2B)
1.705(3) 1.763(2) 1.771(3) 1.771(3) 3.0206(8) 3.0452(8) 1.607(3) 1.939(2) 1.942(3) 1.993(3) 1.995(2) 3.0206(8) 1.995(2) 1.771(3) 1.924(2) 1.938(2) 1.971(3) 1.993(3) 109.31(14) 107.80(12) 106.57(12) 111.64(13) 109.80(13) 115.93(8) 38.40(7) 123.55(8) 40.29(7) 138.56(11) 104.26(14) 79.40(10) 90.26(10) 105.83(13) 90.11(10) 79.05(10) 149.91(11) 110.74(11) 38.80(7) 122.19(8) 40.78(7) 125.12(8) 112.21(11) 120.72(8) 38.54(7) 124.18(8) 40.54(7) 137.05(3) 125.20(13) 134.50(14) 98.49(10) 117.28(13) 131.31(14) 124.13(14) 103.95(11) 149.17(14) 94.43(11) 103.06(11) 123.51(14) 134.61(14) 99.17(10) 95.26(11) 151.50(15) 102.37(11) 133.32(14) 122.40(13) 104.21(11)
Table 3 (continued) O(7)–V(3)–V(4A) O(1A)–V(3)–V(4A) O(9)–V(3)–V(4A) V(2)–V(3)–V(4A) O(8)–V(3)–V(4) V(2)–V(3)–V(4) V(4A)–V(3)–V(4)
117.37(8) 40.73(7) 124.79(8) 79.15(2) 115.61(11) 78.68(2) 132.82(3)
Symmetry transformations used to generate equivalent atoms: A: y, B: y + 1, x, z + 1; C: x + 1, y + 1, z.
x + 1,
z + 1;
anions exhibit strong TIP contributions which thus preclude the immediate calculation of their combined diamagnetic/TIP susceptibilities.
3. Results and discussion 3.1. Synthetic aspects The original aim of the syntheses was the interconnection of the germanato–polyoxovanadato clusters by Cu2+ complexes. However, all attempts to incorporate Cu2+ cations into POM architectures failed. Interestingly, the Cu2+ was reduced to metallic Cu during the syntheses as evidenced by X-ray powder diffractometry. Additional experiments have demonstrated that compound 1 could be obtained without a copper source. Compound 2 crystallized only applying a Cu2+ salt when 1-(2-aminoethyl)-piperazine (Scheme 1) was used as solvent, and syntheses without Cu2+ afforded crystallization of compound 1. The structure of 2 contains two types of piperazinium cations (Scheme 1), i.e., the 1-(2-aminoethyl)-piperazine molecule was fragmented under in situ conditions. Solvothermal syntheses performed with piperazine and without a Cu2+ salt afforded the formation of compound 2 [12]. We note that the structure 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 disordered piperazine molecules [12]. We solved and refined the structure of 2 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 both compounds coexist and the results demonstrate that small
Fig. 1. The [V14Ge8O50(H2O)]12 cluster anion in compound 1. Only selected atoms are labeled. Note that the central H2O molecule is not shown.
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amounts of 2 are present in the reaction product of the synthesis performed at 150 °C in the presence of a Cu2+ salt. The role of the Cu2+ salt in the formation of compound 2 is not clear and further
experiments are underway, especially in situ X-ray scattering investigations, to clarify the influence of Cu2+ on the crystallization. 3.2. Crystal structures
Fig. 2. The hydrogen bonding interactions between the protonated amine molecules and the [V14Ge8O50(H2O)]12 cluster anion in 1 indicated by dashed lines.
The new compound 1 crystallizes in space group P-421c with two formula units per unit cell as brown crystals with irregular shape. Inside the [V14Ge8O50]12 cluster several electron density maxima were found which were assumed to be oxygen of a H2O molecule. But due to the strong disorder no satisfactory model could be found. The structure of compound 1 features an isolated spherical [V14Ge8O50(H2O)]12 cluster anion (Fig. 1) as main structural motif and protonated amine molecules as countercations. The spherical [V14Ge8O50(H2O)]12 cluster is related to the [V18O42] archetype cluster, and the structure of both germanium–polyoxovanadate cluster anions can be derived from this fundamental POM unit when four VO5 groups are replaced by four Ge2O7 dumbbells. The Ge2O7 groups are inserted in the shell of the [V18O42] unit by sharing corners with six VO5 pyramids. The substitution of the four VO5 pyramids leads to the formation of a central ring consisting of eight edge-sharing VO5 units. The remaining six VO5 pyramids are grouped in two caps which are rotated by 90° to each other and are condensed to the central ring through common edges. In 1 charge compensation is achieved by four crystallographically independent triple protonated 1-(2-aminoethyl)-piperazine
Fig. 3. Arrangement of cluster anions and organic cations in 1 (dashed lines: hydrogen bonds).
4403
578
635
672 741 776
986
1200
1100
1000
900
800
ν / cm
700
600
500
400
–1
Fig. 5. IR spectrum of compound 1.
Scheme 1. Organic bases in compounds 1 and 2.
Thermal analysis shows that compound 1 decomposes at comparably low temperatures (Fig. 4) and according to simultaneous mass spectrometry investigations the emission of water starts at about 50 °C. The thermal degradation occurs in a more or less continuous fashion without pronounced decomposition steps, and the simultaneously recorded DTA curve shows no significant thermal event. Even at about 550 °C the mass loss is ongoing. In the IR spectrum of 1 (Fig. 5) the strong V–O stretching vibration is located at 986 cm 1 in accordance with the energetic position of the vanadyl band. 3.3. Magnetism
100 95 90 Δm / %
1098
(H3aep, C6H18N33+) molecules. The Ge atoms are surrounded by four oxygen atoms in a slightly distorted tetrahedral coordination with Ge–O bond lengths ranging from 1.712(3) to 1.771(3) Å, and O–Ge–O angles close to the ideal tetrahedral value (104.9– 114.7°). The VO5 pyramids have typical geometric parameters with basal and apical bond lengths from 1.924(2) to 2.005(3) Å and 1.590(3) to 1.610(3) Å respectively. The O–V–O angles vary between 75.6° and 149.9°. These parameters closely match those reported for other mixed germanato-polyoxovanadates [12,22]. The oxidation states of the V and Ge atoms in 1 were calculated with the bond valence sum method (BVS) [23]. The resulting values of 4.1 for V and 4.0 for Ge justify the assignment of the valence states V4+ and Ge4+. The potential free solvent area was calculated with the PLATON program suite [21] yielding 1517 Å3 being about 31% of the unit cell volume. Between the clusters and the organic ammonium molecules of 1, several hydrogen bonds are found. Each [V14Ge8O50]12 cluster is 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 a three-dimensional network. Along the c-axis, layers of [V14Ge8O50(H2O)]12 units alternate in an ABAB sequence (Fig. 3, top), and each cluster anion is surrounded by six other anions. The anions in every A layer are rotated by 90° with respect to the clusters in the B layers. Within the layers which are oriented parallel to the (0 0 1) plane, the cluster anions form a rectangular net with channels running along [0 0 1] (Fig. 3, bottom). The diameter of the channels is about 6.4 6.4 Å (measured from coordinate-to-coordinate). It can be assumed that the co-crystallized water molecules are located within the channels. The geometric parameters of 2 are comparable with those obtained for 1, i.e. bond lengths and angles are very similar for the two compounds. The bond valence sum for all V and Ge atoms in 2 indicates that the average oxidation states are close to +4. In contrast 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 be observed. Every [V14Ge8O50(H2O)]12 cluster in 2 is surrounded by 12 amine molecules through such H-bonding interactions leading to a three-dimensional H bonded network. Obviously, the differing supramolecular arrangements of anions and cations lead to the crystallization in different space groups with significantly differing lattice parameters. The densities of the two compounds are different with 2.121 g/cm3 for 1 and 2.238 g/cm3 for 2. The more dense packing of the constituents in 2 is reflected by the lower potential solvent area of 247.5 Å3, equivalent to about 1.5% of the unit cell volume.
transmission
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85 80 75 100
200
300
400
500
T / °C Fig. 4. The TG curve of the thermal decomposition of compound 1.
600
The fourteen S = 1/2 vanadyl centers in the polyanion cluster in 1 experience strong intra-antiferromagnetic coupling as evidenced in the low-field susceptibility (Fig. 6). At room temperature, vmT only 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 temperatures vmT reaches a plateau between ca. 20 and 200 K of 2.2 cm3 K/mol, matching the value for six uncoupled vanadyl centers (2.205 cm3 K/mol). The strongest exchange, caused by nearlinear V–l–O–V and angled V(–l–O–)2V exchange pathways between edge- and corner-sharing VO5 square pyramids, results in an eight-membered spin ring to which two three-membered spin bridges are joined. Considering only the V(–l–O–)2V contacts of neighboring edge-sharing VO5 pyramids, the resulting ‘skeletal’ spin array is not geometrically frustrated. However, the competing linear V–l–O–V contacts render the spin structure as consisting of
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Acknowledgements Financial support by the State of Schleswig-Holstein is grateful acknowledged. References
Fig. 6. Temperature dependence of vmT for compound 1 at 0.1 Tesla. Inset: fielddependent magnetization of 1 at 2.0 K.
four pairs of edge-joined spin triangles interconnected by four additional spin centers. Moreover, multiple additional (weaker) exchange pathways involving the germanate groups add more interactions, complicating a full magnetochemical analysis. At the lowest temperature of our preliminary susceptibility measurements (2.0 K), the field-dependent magnetization does not follow a scaled Brillouin function, indicating that several multiplet states are 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 at elevated temperatures in the presence of a Cu2+ source. The results of the syntheses demonstrate the complexity of the solvothermal approach where different reaction parameters determine the formation of a distinct compound. At the moment one can only speculate about the role of Cu2+ for the crystallization of compound 2. Here, the decomposition of aep by elimination of the aminoethyl group appears to be induced by Cu2+. After formation of pip the reduction of VV in the metavanadate salt to VIV in the cluster anion occurs. This scenario seems to be likely because 2 is formed in 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 dominated by strong antiferromagnetic exchange interactions mediated by V–l–O–V and bent V(–l–O–)2V bridges. A detailed study of the magnetic properties is underway as well as further synthetic efforts for the generation of new germanato–polyoxovanadato cluster architectures.
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