Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
A Series of Banana-Shaped 3d-4f Heterometallic Cluster Substituted Polyoxometalates: Syntheses, Crystal Structures, and Magnetic Properties Ya-Nan Gu,† Yi Chen,† Yan-Lan Wu,† Shou-Tian Zheng,†,‡ and Xin-Xiong Li*,†,‡ †
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350108 China S Supporting Information *
ABSTRACT: Four seven-nuclearity 3d-4f heterometallic cluster {Fe6LnO28} substituted polyoxometalates (HPz)11K4Fe6Ln(μ3-O)2(B-α-GeW9O34)2(GeW6O26)·xH2O (1-Ln, Pz = piperazine, Ln = Tb, Dy, Ho, Er for x = 27, 25, 25, 24, respectively) have been hydrothermally prepared and structurally characterized by single-crystal X-ray diffraction, powder X-ray diffraction, infrared spectrometry, thermogravimetric analyses, elemental analyses, and electrospray ionization mass spectrometry. Single-crystal X-ray diffraction analyses revealed that 1-Ln contain an unprecedented banana-shaped polyanion constructed from an iron-lanthanide heterometallic {Fe6LnO28} cluster, two trilacunary {B-α-GeW9O34} units, and one hexalacunary {GeW6O26} fragment. The magnetic susceptibility surveying proved the presence of antiferromagnetic coupling in 1-Ln.
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INTRODUCTION Polyoxometalates (POMs), a large family of anionic metal oxide clusters of d-block transition metals in their high oxidation states (WVI, MoV,VI, VIV,V), have attracted persistent interest due to their versatile structural configurations and potential applications in catalysis,1 magnetism,2 medicine,3 and material sciences.4 It is well-known that lacunary polyoxoanions derived from saturated POMs can act as multidentate inorganic ligands. These lacunary polyoxoanions prefer to induce paramagnetic transition-metal or lanthanide ions to form various aggregates, generating a rapidly growing class of metal-substituted polyoxometalates (MSPs) with unique magnetic properties. 5 MSPs are ideal models for the investigation of magnetic coupling in metal clusters, because they are a type of well-insulated clusters with definite nuclearity and specific models.6 So far, most investigations in this area focus on the syntheses of transition-metal substituted polyoxometalates (TMSPs), and result in a tremendous amount of TMSPs with unique structures and interesting properties.7,8 Recently, the studies of lanthanide-metal-substituted polyoxometalates (LMSPs) have also been developed rapidly. A series of LMSPs containing lanthanide clusters with nuclearity ranging from 2 to 27 have been prepared.9−11 Compared with TMSPs and LMSPs, POMs incorporating 3d-4f heterometallic clusters have been far less explored and the number of which is rather limited.12 From the experimental point of view, this is mainly because the syntheses of 3d-4f © XXXX American Chemical Society
heterometallic cluster substituted POMs is still not an easy process and is usually subject to difficulties such as the variable and versatile coordination number/geometries of different metal ions, the different radius/oxophilicity induced competitive reactions between transition-metal and lanthanide-metal centers with the same lacunary polyanion, and usually only one type of metal ion was crystallized in the final products. Notably, some fascinating POMs containing both transition-metal ions and lanthanide ions on the polyanions have been also made; however, the transition-metal ions and lanthanide cations in these POMs are spatially separated and magnetically isolated from each other by lacunary POM fragments/organic ligands instead of being connected together to form heterometallic clusters.13 Therefore, the exploration of POMs containing 3d-4f heterometallic clusters remains an ongoing challenge that is of considerable scientific interest in POM chemistry. Previously, Kögerler and co-workers have created two POMs which incorporate 3d-4f heterometallic clusters [CeIV3MnIV2O6(OH)2]6+ and [CeIV3MnIV2O6(OH)2]6+ respectively through the reaction of Dawson-type lacuanry polyanions with a preformed 3d-4f heterometallic cluster [CeIV Mn IV 6 O 9 (O2CCH3)9(NO3)(H2O)2].12a,b Subsequently, Sécheresse successfully inserted a series of 3d-4f heterometallic cubane clusters {LnCu3(OH)3O} (Ln = La, Gd, Eu) to monovacant Received: October 24, 2017
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DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. X-ray Crystallographic Data for 1-Lna 1-Tb empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z F(000) ρcalcd (mg/m3) temperature (K) μ (mm−1) refl. collected independent refl. parameters GOF on F2 final R indices (I = 2σ(I)) R indices (all data)
C44H175N22K4Fe6Tb1Ge3W24O123 8261.4033 orthorhombic Cmcm 22.1335(11) 29.0201(11) 22.2019(8) 90 90 90 14260.6(10) 4 11748 3.175 150 21.155 64330 10811 330 0.978 R1 = 0.0520, wR2 = 0.1418 R1 = 0.0819, wR2 = 0.1502
1-Dy
1-Ho
C44H171N22K4Fe6Dy1Ge3W24O121 8228.9474 orthorhombic Cmcm 22.062(3) 29.0913(17) 22.2120(15) 90 90 90 14256(2) 4 11747 3.178 150 21.190 32979 8677 314 0.919 R1 = 0.0682, wR2= 0.1733 R1 = 0.1102, wR2= 0.1925
C44H171N22K4Fe6Ho1Ge3W24O121 8231.3777 orthorhombic Cmcm 22.1734(17) 29.277(2) 22.2094(17) 90 90 90 14417.9(19) 4 11756 3.143 150 20.982 35154 6770 312 1.101 R1 = 0.0398, wR2 = 0.1151 R1 = 0.0476, wR2 = 0.1190
1-Er C44H169N22K4Fe6Er1Ge3W24O120 8215.6911 orthorhombic Cmcm 22.066(3) 29.338(3) 22.238(3)) 90 90 90 14396(3) 4 11760 3.149 150 21.047 33169 6723 330 1.055 R1 = 0.0594, wR2 = 0.1777 R1 = 0.0721, wR2 = 0.1905
a R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2; w = 1/[σ2(Fo2) + (xP)2 + yP], P = (Fo2 + 2Fc2)/3, where x = 0.0821, y = 0 for 1Tb; where x = 0.1198, y = 0 for 1-Dy; where x = 0.0738, y = 144.8740 for 1-Ho; where x = 0.1402, y = 29.0393 for 1-Er.
Keggin-type polyanions under hydrothermal conditions.12c Recently, Reinoso et. al reported two 3d-4f heterometallic cluster sandwiched POMs [{Ce III (H 2 O) 2 } 2 Mn III 2 (B-αGeW9O34)2]8−/[{CeIV(OAc)}CuII3-(H2O)(B-αGeW9O34)2]11− by introducing Ce4+ ions to substitute transition-metal ions on the Weakley-type TMSPs [Mn II 4 (H 2 O) 2 - (B-α-GeW 9 O 34 ) 2 ] 12− /[Cu II 4 (H 2 O) 2 (B-αGeW9-O34)2]12−, separately.12g,h Nevertheless, the nuclearity of 3d-4f heterometallic clusters incorporated in the above POMs fall in the range of only four to seven. Additionally, the majority of lanthanide centers (Ce4+) in the first and the third cases are diamagnetic, and they have no magnetic exchange interactions with transition metal clusters. In this work, we report the syntheses, structures, and properties of a series of seven-nuclearity 3d-4f heterometallic cluster {Fe6LnO28} substituted POMs, namely, (HPz)11K4Fe6Ln(μ3-O)2(B-α-GeW9O34)2(GeW6O26)·xH2O (1-Ln, Pz = piperazine, Ln = Tb, Dy, Ho, Er for x = 27, 25, 25, 24, respectively). Single-crystal X-ray diffraction analyses revealed that 1-Ln contain banana-shaped polyanions consisting of an uncommon iron-lanthanide heterometallic cluster {Fe6LnO28}, two trilacunary {B-α-GeW9O34} units, and one hexalacunary {GeW6O26} fragment. So far, some banana-shaped polyanions based on trilacunary {XW9O34} units and hexalacunary {XW6O26} units (X = P, Co, As, V, Ge) have been reported,14 including [Co7(H2O)2(OH)2P2W25O94]16−,14a [Ni6As3W24O94(H2O)2]17−,14b [((MnOH2)Mn2PW9O34)2-(PW6O26)]17−,14c [Fe6 Ge3W24 O94-(H 2O) 2]14−,14d [(Co(OH 2)Co2VW 9O34) 2(VW 6 O 26 )] 17− , 14e [Mn 6 Ge 3 W 24 O 94 (H 2 O) 2 ] 18− , 14f and [M 6 (PW 6 O 26 )(P 2 W 15 O 56 ) 2 (H 2 O) 2 ] 23− (M = Co 2+ or Mn2+).14g However, lanthanide ions have not been incorporated into banana-shaped POMs. To the best of our knowledge, 1-Ln are not only the first examples of banana-shaped POMs incorporating 3d-4f heterometallic clusters, but also a series of
rare cases containing iron-lanthanide heterometallic clusters with second highest-nuclearity among reported iron-lanthanide heterometallic cluster substituted POMs.15
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EXPERIMENTAL SECTION
Materials and General Methods. K8Na2[GeW11O39]·nH2O was synthesized according to a literature method and confirmed by IR spectra.16 All other starting materials were of analytical grade and obtained from commercial sources without further purification. Elemental analyses of C, H, and N were carried out with a Vario EL III elemental analyzer. Infrared (IR) spectra (KBr pellet) were recorded on an Opus Vertex 70 FT-IR infrared spectrophotometer in the range of 450−4000 cm−1. Powder X-ray diffraction (PXRD) patterns were measured using a Rigaku DMAX 2500 diffractometer with CuKα radiation (λ = 1.54056 Å). Thermogravimetric analyses were performed on a Mettler Toledo TGA/SDTA 851e analyzer under an air-flow atmosphere with a heating rate of 10 °C/min in the temperature of 30−1000 °C. UV−vis adsorption spectra were collected on a PerkinElmer Lambda 35 spectrophotometer. The high-resolution electrospray ionization mass spectra (ESI-MS) were recorded on Thermo Scientific Exactive Plus mass spectrometer (German) and processed on Bruker Data Analysis (Version 4.0) software, and the simulation was performed on a Bruker Isotope Pattern software. Before the electrospray ionization mass spectrometry (ESI-MS) measurement, the pure crystals of 1-Ln were dissolved in 80 °C water with continuous magnetic stirring. Variable-temperature susceptibility measurements were performed in the temperature range of 2−300 K at a magnetic field of 0.1 T on polycrystalline samples with a Quantum Design PPMS-9T magnetometer. The experimental susceptibilities were corrected for Pascal’s constants. Synthesis of (HPz)11K4Fe6Er(μ3-O)2(B-α-GeW9O34)2(GeW6O26)· 24H2O (1-Er). A mixture of K8Na2[GeW11O39]·nH2O (0.320 g, about 0.100 mmol), FeSO4·7H2O (0.085 g, 0.300 mmol), Er(NO3)3· 6H2O (0.050 g, 0.100 mmol), piperazine (0.120 g, 1.390 mmol), and H2O (5.00 mL, 277 mmol) was stirred in a 35 mL stainless steel reactor with a Teflon liner for 1 h (pHs = 9.8) and heated at 140 °C for 3 days, and then cooled down to room temperature (pHe = 8.2). The brownish black fusiformis crystals were isolated. Yield: 36 mg B
DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (8.76% based on FeSO4·7H2O). Elemental analysis calcd (%) for C44H169N22K4Fe6O120Er1Ge3W24: C 5.84, H 1.78, N 3.36, W 53.70; Ge 2.65; Fe 4.08; K 1.90. Found (%): C 6.43, H 2.07, N 3.75, W 52.11; Ge 2.61; Fe 4.65; K 2.37. IR (KBr, cm−1): 3430(s), 3020(s), 1617(m), 1454(m), 945(s), 876(s),765(s), 710(s), 509(w), 465(w). Synthesis of (HPz)11K4Fe6Ho(μ3-O)2(B-α-GeW9O34)2(GeW6O26)· 25H2O (1-Ho). The reaction process of 1-Ho is similar to 1-Er except that Ho(NO3)3·5H2O (0.050 g, 0.110 mmol) was used to replace Er(NO3)3·6H2O (pHs = 9.8, pHe = 8.5). Yield: 40 mg (9.72% based on FeSO4·7H2O). Elemental analysis calcd (%) for C44H171N22K4Fe6O121Ho1Ge3W24: C 6.41, H 2.00, N 3.66; W 53.60; Ge 2.65; Fe 4.07; K 1.90, found (%): C 6.42, H 2.09, N 3.74; W 54.93; Ge 2.52; Fe 4.69; K 1.77. IR (KBr,cm−1): 3433(s), 3022(s), 1590(m), 1453(m), 942(s), 868(s), 761(s), 710(s), 514(w), 463(w). Synthesis of (HPz)11K4Fe6Dy(μ3-O)2(B-α-GeW9O34)2(GeW6O26)· 25H2O (1-Dy). The reaction process of 1-Dy is similar to 1-Er except that Dy(NO3)3·6H2O (0.050 g, 0.100 mmol) was used to replace Er(NO3)3·6H2O (pHs = 9.8, pHe = 8.3). Yield: 20 mg (4.86% based on FeSO4·7H2O). Elemental analysis calcd (%) for C44H171N22K4Fe6O121Ho1Ge3W24: C 6.31, H 1.99, N 3.70; W 53.60; Ge 2.65; Fe 4.07; K 1.90, found (%): C 6.42, H 2.09, N 3.74; W 54.58; Ge 2.33; Fe 4.63; K 2.08. IR (KBr, cm−1): 3420(s), 3004(s), 1579(m), 1446(m), 940(s), 866(s), 761(s), 707(s), 502(w), 463(w). Synthesis of (HPz)11K4Fe6Tb(μ3-O)2(B-α-GeW9O34)2(GeW6O26)· 27H2O (1-Tb). The reaction process of 1-Tb is similar to 1-Er except that Tb(NO3)3·6H2O (0.050 g, 0.100 mmol) was used to replace Er(NO3)3·6H2O (pHs = 9.8, pHe = 8.5). Yield: 17 mg (4.1% based on FeSO4·7H2O). Elemental analysis calcd (%) for C44H175N22K4Fe6O123Ho1Ge3W24: C 6.40, H 1.97, N 3.67; W 53.36; Ge 2.64; Fe 4.05; K 1.89, found (%): C 6.39, H 2.13, N 3.73; W 52.07; Ge 2.44; Fe 4.36; K 2.34. IR (KBr, cm−1): 3428(s), 3004(s), 1627(m), 1453(m), 941(s), 866(s), 761(s), 707(s), 510(w), 459(w). Single-Crystal Structure Analysis. Single-crystal X-ray diffraction data for 1-Ln were collected on a Bruker APEX II diffractometer at 150 K equipped with a fine focus, 2.0 kW sealed tube X-ray source (MoK radiation, λ = 0.71073 Å) operating at 50 kV and 30 mA. The program SADABS was used for the absorption correction. The structures were solved by the direct method and refined on F2 by fullmatrix least-squares methods using the SHELXL-2013 program package.17 The residual electron density that could not sensibly be modeled as protonated piperazine cations or guest water molecules are removed via program of the SQUEEZE function in PLATON.18 The final formulas of 1-Ln were determined by combining the single-crystal X-ray analyses with the results of the elemental analyses, ESI-MS results, and thermogravimetric analyses (Figure S1). Crystallographic data and structure refinements for 1-Ln were summarized in Table 1. CCDC 1578598 (1-Dy), 1578599 (1-Er), 1578600 (1-Ho), 1578601 (1-Tb) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.
Figure 1. (a) View of the [Fe 6 Er(μ 3 -O) 2 (B-α-GeW 9 O 34 ) 2 (GeW6O26)]15− polyanion with thermal ellipsoids at the 50% level; (b) polyhedral and stick representation of [Fe6Er(μ3-O)2(B-αGeW9O34)2(GeW6O26)]15− polyanion; (c) ball and stick representation of the 3d-4f heterometallic cluster {Fe6ErO28}; (d) view of trilacunary {B-α-GeW9O34} unit; (e) view of hexalacunary {GeW6O26} fragment; (f) structure of {Fe3O13} cluster; (g) view of the coordination environment of Er1 ion in 1-Er. Symmetry codes: a: 1 − x, y, 2.5 − z; b: x, y, 2.5 − z; c: 1 − x, y, z; d: 1 − x, y, 2.5 − z. Color codes: WO6, olive; GeO4, purple.
cluster can be described as two vertex-sharing trimeric ironoxo cluster {Fe3O13} (Figure 1f) are linked via sharing two vertexes, and further capped by one ErO8 polyhedraon (Figure 1g) on its square window (Figure S2). To the best of our knowledge, such a 3d-4f heterometallic cluster has not been reported up to now. In 1-Er-a, there are two crystallographic independent iron ions (Fe1, Fe2) and one unique erbium cation (Er1). All iron ions adopt a slightly distorted octahedral geometry. Fe1 is coordinated by one μ3-O (O28), two μ4-O atoms of GeO4 tetrahedra, two terminal O atoms of WO6 octahedra from a {B-α-GeW9O34} unit, and one terminal O atom of a WO6 octahedron from a {GeW6O26} fragment (Figure 1b). Fe2 is defined by two terminal O atoms of a WO6 octahedron from {B-α-GeW9O34} unit, two terminal O atoms of WO6 octahedra from a {GeW6O26} fragment, one μ4-O of a GeO4 tetrahedron and one bridging μ3-O of a {B-α-GeW9O34} unit. The erbium cation is eight-coordinated with a square antiprismatic geometry, and it is finished by two μ3-O atoms, two μ4-O of GeO4 tetrahedra, and four terminal O atoms of WO6 octahedra from two {B-α-GeW9O34} units (Figure 1b,g). The Fe−O and Er−O bond distances fall in the range of 1.891(6)−2.215(16) and 2.304 (15)−2.574 (13), which is comparable with that in reported iron−erbium heterometallic cluster substituted POMs.13a Bond valence sum calculations19 of 1-Er show that the oxidation states of Fe1, Fe2, and Er1 are +3, + 3, +3, respectively (Table S1). Since the divalent iron salt
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RESULTS AND DISCUSSION Crystal Structural Description of 1-Ln. Single-crystal Xray diffraction studies revealed that 1-Ln are isostructural; therefore, 1-Er was selected as a representative example for structural description. 1-Er crystallizes in the orthorhombic crystal system Cmcm space group, and its molecular structure consists of one banana-shaped iron−erbium heterometallic cluster substituted polyanion [Fe6Er(μ3-O)2(B-α-GeW9O34)2(GeW6O26)]15‑ (1-Er-a, Figure 1a,b), 4 K+ cations, 11 monoprotonated piperazine cations and 27 lattice water molecules. The whole structure of 1-Er-a can be viewed as a C-shaped iron−erbium heterometallic cluster {Fe6ErO28} (Figure 1c) capped by two trilacunary {B-α-GeW9O34} units on the both ends (Figure 1d) and one hexalacunary {GeW6O26} fragment (Figure 1e) on the equatorial position, respectively. The iron−erbium heterometallic {Fe6ErO28} C
DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry was used as starting material, it is obviously that those Fe3+ ions are from in situ oxidation of the Fe2+ ions.20 It is noteworthy that the structure of 1-Er-a is quite different from those reported banana-shaped polyanions based on {XW9O34} units and {XW6O26} units (X = P, Co, As, V, Ge)14 (Figure 2a,b). In other words, 1-Er-a cannot be simply
not be completely located and are distributed in the interspace between polyanions, as commonly observed in POMs.21 The presence of protonated piperazine species can also be clearly confirmed by the high-resolution ESI-MS spectrum. The peaks at m/z = 87.08 are assigned as monoprotonated piperazine cations HPz, which is in good agreement with the simulated peak based on HPz (Figure S4). Syntheses, PXRD Patterns, UV−vis, IR Spectra. So far, lacunary POM precursors have been widely used to react with transition metal ions or/and lanthanide cations for making novel MSPs.7,8 However, lacunary POM precursors often transformed into saturated species or smaller fragments during the reaction process due to their good reactivity and unstability.22 In this work, although {GeW11O39} polyanion was used as the starting material, {B-α-GeW9O34} and {GeW6O26} units were obtained in the final products, indicating that the {GeW11O39} precursor experienced the transformations from the monolacunary species to trilacunary and hexalacunary species. So far, trilacunary [A-α-XW9O34], multilacunary cyclic precursor [P8W48O154]40− transformed into {B-α-XW9O34} and {XW6O26} (X = P, Ge) units have been reported under hydrothermal conditions.22a,b Nevertheless, the in situ structural transformation from monolacunary {GeW11O39} to {B-α-GeW9O34} and {GeW6O26} units is scarce. Additionally, piperazine molecules are indispensable for the syntheses of 1-Ln. The introduction of piperazine molecules to the reaction system can not only adjust the pH value of the reaction system but also provide abundant potential charge compensation cations. During our exploration, lighter lanthanide ions such as La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+ were also introduced to the reaction system for making 1-Ln. However, our efforts were fruitless, which indicates that 1-Ln show good selectivity toward lanthanide ions during the crystallization process. For 1-Ln, the good accordance between the experimental PXRD patterns and the simulated ones based on single-crystal X-ray results indicates the phase purities of 1-Ln (Figure S5). In the UV−vis spectra of 1-Ln, two absorption bands peaking at ca. 193 and 252 nm can be observed (Figure S8). The first energy absorption band can be ascribed to the pπ−dπ charge transfer transitions of the Ot → W (Ot are terminal oxygen atoms) bonds. The second energy absorption band is attributed to the pπ−dπ charge transfer transitions of Ob → W (Ob are bridging oxygen atoms) bonds.23 When 1-Ln are dissolved in water, the pH values of the solutions are 8.3, 8.2, 8.4, 8.2 for 1-Tb, 1-Dy, 1-Ho, 1-Er, respectively. Generally, the configurations of polyanions are sensitive to the pH values of medium. In order to evaluate the stability of polyanions of 1-Ln in aqueous solution, 1-Dy was selected as an example, and the UV−vis spectra of 1-Dy in dilute aqueous solution at different pH were explored in the range of 190−800 nm. As shown in Figure S9, when the pH value gradually decreases to 3.2, there is no obvious change of the UV−vis spectra of 1-Ln. On the contrary, when the pH value is higher than 10.2, the two absorption bands progressively become weaker until they disappear. In other words, the polyanions of 1-Dy are stable in the aqueous solution in the pH range of ca. 3.2−10.2. In the IR spectra of 1Ln (Figure S6), four characteristic vibration peaks generated from the polyanion skeletons are observed at 945 cm−1, 874 cm−1, 765 cm−1, 706 cm−1, which correspond to ν(W−Ot), ν(Ge−Oa), ν(W−Ob), ν(W−Oc), separately.24 The stretching bands of C−H and N−H are observed at 2828−3433 cm−1. The peaks at 1451 and 1628 cm−1 are assigned to the bending
Figure 2. (a−b) Polyhedral structure of conventional banana-shaped polyanion based on {XW9O34} units and {XW6O26} units (X = P, Co, As, V); (c−d) polyhedral structure of 1-Er-a polyanion. Color codes: WO6, olive; XO4, purple.
considered as inserting one eight-coordinated Er3+ ion to the middle vacant of reported banana-shaped polyanion. Except for lanthanide ion, the main differences between 1-Er-a and reported banana-shaped polyanions are evident in the following aspects: (1) in reported banana-shaped polyanions, each transition metal triad is separated from each by the central {XW6O26} fragments (Figure 2b), while the {Fe3O13} triads in 1-Er-a are self-assembled into a {Fe6O24} cluster by sharing two μ4-O atoms (Figure 2c,d); (2) the binding mode of {Fe3O13} triads with {GeW6O26} units is different with that in reported banana-shaped polyanions (Figure S3); (3) the transition metal clusters in reported banana-shaped polyanions are stabilized not only by {XW9O34} units and {XW6O26} unit, but also terminal water ligands (Figure 2b), while the heterometallic cluster in 1Er-a was stabilized without water ligands; (4) the {GeW9O34} unit in 1-Er-a is rotate by about 60 °C along the vertical axis of the {Fe3O13} triad, compared with {XW9O34} units in reported banana-shaped polyanions (Figure 2). These differences further indicate that 1-Er-a is a quite novel polyanion. 1-Er-a is further charge balanced by potassium cations and monoprotonated piperazine ions. These monoprotonated piperazine ions could D
DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Field-dependent isothermal magnetization M(H) of 1-Ln at 2 K exhibits a magnetization increase from 0 to 90 kOe. As shown in Figure S7, the M values increase quickly at a very low field, reaching 10.00/9.15/10.83/10.10 Nβ at 10/10/12/10 kOe, and then increases smoothly to maximum values of 16.41/ 15.21/17.02/17.07 Nβ at 90 kOe. The maximum values are much less than the theoretical values of 36/35/34/33 Nβ for six Fe3+ and one Tb3+/Dy3+/Ho3+/Er3+ ion. The discrepancy between the observed values and the expected magnetization saturation values indicates the presence of magnetic anisotropy and/or low lying excited states.29 In order to investigate whether the magnetic interactions in 1-Ln can lead to the single-molecule magnet behavior, alternating-current magnetic susceptibilities for 1-Ln with a frequency between 111 and 1511 Hz were tested under an applied magnetic field of 3 Oe (Figure 4). The in-phase (χ′) and out-of-phase (χ″) signals at
vibration of C−H and N−H bonds, respectively. The presence of these characteristic signals also confirms the existence of organic species in 1-Ln. Magnetic Properties. The magnetic susceptibilities of 1Ln were measured in the temperature of 2−300 K with an external magnetic field of 1 kOe. The plots of χmT versus T of 1-Ln is shown in Figure 3. The experimental χmT value at room
Figure 3. Temperature dependence of the product of the molar magnetic susceptibility with temperature χmT (black) and temperature dependence of 1/χm (blue) for 1-Ln between 2 and 300 K.
temperature is 39.34/43.32/39.54/37.75 cm3 K mol−1 per formula unit for 1-Tb/1-Dy/1-Ho/1-Er, being agreement with the theoretical values of 38.07/40.42/40.32/37.73 cm3 K mol−1 for six uncoupled high spin Fe3+ (S = 5/2 and g = 2.0) and one Tb3+(S = 3, L = 3, g = 3/2)/Dy3+(S = 5/2, L = 5, g = 4/3)/ Ho3+(S = 4/2, L = 6, g = 5/4)/Er3+(S = 3/2, L = 6, g = 6/5) ion.25 Upon cooling, the χmT values of 1-Tb/1-Dy increase smoothly to the maximum values of 39.80/45.30 cm3 K mol−1 at 76/126 K (Figure 3), indicating the existence of weak ferromagnetic interactions. This profile may manifest that the SFe = 5/2 or STb = 3 or SDy = 5/2 local spin to some extent are liable to align along the same direction.26 Such phenomenon have also been reported.27 The χmT values of 1-Tb/1-Dy further undergo a sudden drop to the minimum values of 28.87/22.36 cm3 K mol−1 as the temperature is lowered from 76/126 to 2 K, which can be ascribed to the intracluster antiferromagnetic coupling.28 What is more, the temperature dependence of the reciprocal susceptibility (1/χm) obeys the Curie−Weiss law above 2/90 K for 1-Tb/1-Dy, respectively. The Curie constants and Weiss constants for 1-Tb/1-Dy are 38.62/43.48 cm3 K mol−1 and 2.86/4.52 K, which to some extent prove the existence of weak ferromagnetic interactions resulting from the parallel of local spins. For 1-Ho/1-Er, the χmT values drops smoothly to 35.77/30.60 cm3 K mol−1 at 47/ 50 K and then decrease abruptly to the minimum of 22.58/ 19.19 cm3 K mol−1 at 2 K (Figure 3). Such behaviors suggested the presence of overall antiferromagnetic interactions with the cluster in 1-Ho/1-Er. In addition, the temperature dependence of the reciprocal susceptibility (1/χm) obeys the Curie−Weiss law above 2/35 K. The Curie constants C = 39.84/39.54 cm3 K mol−1 for 1-Ho/1-Er, which are reasonable for six Fe3+ ions and one Ho3+/Er3+ ion per formula unit. The Weiss constants for 1-Ho/1-Er are −4.86/−16.68 K, respectively, which further supports the existence of antiferromagnetic coupling between the metal ions in the clusters.
Figure 4. Frequency-dependent behavior of χm′ and χm″ for 1-Ln in zero static field at 2−20 K.
low temperatures of 1-Ln do not show obvious frequency dependence with an increase of the frequency, distinctly suggesting the lack of slow magnetization relaxation in 1-Ln, which may originate from fast quantum tunnelling and/or the absence of low population excited levels.30
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CONCLUSIONS A series of 3d-4f heterometallic cluster substituted POMs 1-Ln have been successfully prepared under hydrothermal conditions. Structural analysis indicates that 1-Ln contain an unprecedented banana-shaped polyanion [Fe6Ln(μ3-O)2(B-αGeW9O34)2(GeW6O26)]15− (1-Ln-a) constructed from one seven-nuclearity heterometallic {Fe6LnO28} cluster, two {B-αGeW9O34} units, and one {GeW6O26} fragment. The structure of 1-Ln-a is much different from those reported banana-shaped polyanions in TMSPs. In addition, the deliberate introduction of an organic piperazine molecule into the reaction system is crucial for making 1-Ln. Finally, 1-Ln show antiferromagnetic behaviors and do not exhibit slow magnetization relaxation, which are probably due to fast quantum tunnelling and/or the absence of low population excited levels. These findings will not only enrich the structural diversity of 3d-4f heterometallic cluster substituted POMs, but also provide a new mind for exploring novel MSP materials. Further work in our research group is in progress. E
DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02728. Additional structural figures, additional characterizations such as TGA curves, IR spectra, PXRD patterns, the bond valence sum calculations for 1-Ln (PDF) Accession Codes
CCDC 1578598−1578601 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shou-Tian Zheng: 0000-0002-3365-9747 Xin-Xiong Li: 0000-0002-9903-2699 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundations of China (No. 21303018, 21371033, and 21401195), the Natural Science Foundation For Young Scholars of Fujian Province (No. 2015J05041), and Projects from State Key Laboratory of Structural Chemistry of China (No. 20150001, 20160020).
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DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.7b02728 Inorg. Chem. XXXX, XXX, XXX−XXX
