A New Heterometallic 3d-3d Transition Metal Oxo-cluster{CuII6MnIII}: Synthesis, Crystal Structure and Magnetic Property①

WANG Yi-Man PENG Zhi-Wei LIAO Jia-Min LI Ao LIU Yuan-Yan ZHANG Jing-Jing ZHOU Nian LI Xu-Dong LI Shu MENG Wei

(Department of Materials & Chemistry Engineering, Hunan City Uniνersity, Yiyang 413000, China)

ABSTRACT A new transition metal-antimony oxo-cluster based compound has been synthesized in water under room temperature. Its formula is Na6[Cu6MnSb6(μ3-OH)2(OH)(μ4-O)6(tartrate)6]·20H2O (1), where tartrate represents rac-tartaric acid. It was characterized by elemental analysis, infrared spectrum and X-ray single-crystal diffraction. The compound crystallizes in the monoclinic system, space group P21/n. Structural analyses revealed that two Sb3(μ3-O)(tartrate)3 scaffolds sandwich a CuII6MnIII middle layer to form the cluster. In the middle layer, all the seven metal ions lie in an almost regular hexagon, with MnIII ion in the center and six CuII ions along the edges of the hexagon. As a 4-connected node, each cluster is interlinked to its nearest four {Cu6Mn} neighbors through Na+, generating a 3D supramolecular framework. The temperature-dependent magnetic susceptibilities indicated dominating antiferromagnetic interactions in 1 with JCu-Cu = 176.34 and JCu-Mn = -14.44 cm-1.

Keywords: dipotassium bis(μ-tartrato)-diantimony(III) ligand, heterometallic transition metal oxo-cluster,crystal structure, magnetic study; DOI: 10.14102/j.cnki.0254-5861.2011-3229

1 INTRODUCTION

Heterometallic transition metal oxo-clusters have recently attracted great attention due to their intriguing geometrical characteristics and fascinating physical properties[1-7]. One of the driving forces for this is to explore the exchange interactions among multiple non-equivalent spin carrying centers in a single molecule[8-10]. Between the nearest nonequivalent neighboring spin carriers, the magnetic interactions may be ferromagnetic or antiferromagnetic[11,12].Especially when the metal ions are strongly anisotropic, the combination with various hetero-spin carriers can lead to a new generation of molecule-based magnetic materials[13,14].In addition, different metal ions to assemble these clusters can induce different functionality, such as the combination of magnetic, optical, chiral and biological activities with catalytic properties[15-18].

Previously, we have been interested in using the inorganic ligand pool, K2Sb2L2(H4L = tartaric acid), namely dipotassium bis(μ-tartrato)-diantimony(III), as a starting material for the synthesis of pure divalent late transition metal-oxo clusters[18,19]. The K2Sb2L2ligand was selected to construct high nuclearity oxo-clusers because of two reasons.On one hand, it can undergo decomposition and recombination to form two types of scaffolds in an aqueous medium.On the other hand, tartaric acid contains both alkoxide and carboxylate groups. In this research field, Jacobson and co-workers discovered a series of sandwich-type clusters by using the enantiopure forms of K2Sb2L2[20-22]. Meanwhile,Huang et al. also reported several transition metal-antimony oxo-cluster[2]. However, Cu(II)/Mn(III) ions sandwiched by{Sb3(μ3-O)} have not yet been reported. In this work, we present the synthesis, characterization and magnetism of an anionic cluster, Na6[Cu6MnSb6(μ3-OH)2(OH)(μ4-O)6(tartrate)6]·20H2O (1).

2 EXPERIMENTAL

2. 1 General materials and methods

All reagents and solvents were of AR grade and used without further purification. Infrared spectrum test was measured on a WQF-410 FTIR spectrometer with wave number of 500～4000 cm-1. Thermogravimetric analysis(TGA) measurements were carried out using a DSC/TG pan A1203 system in N2flow at a heating rate of 10 ℃/min.Elemental analyses were performed (C, H) by Thermo Scientific FLASH 2000 elemental analyzer; Mn, Cu and Na were analyzed on a Varian (720) ICP atomic emission spectrometer. Single-crystal X-ray analyses were carried out at room temperature on a Siemens SMART platform diffractometer outfitted with an Apex II area detector and monochromatized Mo-Kαradiation (λ= 0.71073 Å). Powder X-ray diffraction patterns were gathered in the 2θrange of 5～80° at room temperature on a Rigaku D/Max 2500 diffractometer. Magnetic susceptibility was measured on a MPMS RSO Instrument.

2. 2 Synthesis of Na6[Cu6MnSb6(μ3-OH)2(OH)(μ4-O)6(tartrate)6]·20H2O (1)

A mixture of Cu(OAc)2(0.62 mmol), Mn(OAc)2(0.31 mmol) andrac-K2Sb2(tartrate)2(0.62 mmol) was added to a sodium acetate/acetic acid buffer solution (pH 5.5, 0.5 M NaOAc/HOAc, 10 mL). The solution was stirred for 8 h and filtered, and the gray-green filtrate was left undisturbed to concentrate slowly by evaporation. Green crystals of 1 were obtained with the yield of 46% (based on Cu) after three weeks. Anal. Calcd. for C24H55Cu6MnNa6O65Sb6: C, 10.71; H,2.05; Cu, 14.18; Mn, 2.04; Na, 5.13; Sb, 27.16. Found: C,10.63; H, 2.01; Cu, 14.25; Mn, 2.11; Na, 5.05; Sb, 27.22. IR(KBr pellet, cm-1): 1613 (s), 1418 (s), 1366 (s), 1110 (s), 1069(m), 925 (w), 905 (w), 854 (m), 751 (w), 648 (s), 545 (w), 515(w).

2. 3 Crystal structure determination

A green needle single crystal of 1 with dimensions of 0.43mm × 0.40mm × 0.38mm was selected and mounted on a glass fiber. Data collection was performed at 294 K on a Smart Apex II CCD with graphite-monochromated MoKαradiation (λ= 0.71073 Å). The structure of 1 was solved by direct methods and refined by full-matrix least-squares method onF2using the SHELXTL-97 crystallographic software package[23]. More details on the crystallographic studies as well as atomic displacement parameters are given in the CIF files. All carbon-bonded hydrogen atoms were placed in geometrically calculated positions; hydrogen atoms in water molecules were not assigned or directly included in the molecular formula. Compound 1 crystallizes out in monoclinic, space groupP21/nwitha= 16.8688(10),b=9.4734(5),c= 22.5825(14) Å,V= 3599.8(3) Å3,Z= 2,C24H55Cu6MnNa6O65Sb6,Mr= 2688.30,Dc= 2.480 g/cm3,F(000) = 2580,μ(MoKα) = 4.275 mm-1, the finalR= 0.0317 andwR= 0.0827(w= 1/[σ2(Fo2) + (0.0312P)2+ 21.8403P],whereP= (Fo2+ 2Fc2)/3),S= 1.062. The selected bond lengths and bond angles are reported in Table 1.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Structure description for Na6[Cu6MnSb6(μ3-OH)2(OH)(μ4-O)6(tartrate)6]·20H2O (1)

Compound 1 contains the [Cu6MnSb6(μ3-OH)2(OH)(μ4-O)6(tartrate)6]6-cluster ion. In this cluster ion, two Sb3(μ3-O)(tartrate)3scaffolds (Fig. 1a) sandwich a CuII6MnIIImiddle layer to form the cluster ion, a similar arrangement to what was found in the previously reported CuII7cluster[19,24].In the middle layer, all the seven metal ions lie in an almost regular hexagon (Fig. 1b), with MnIIIion in the center and six CuIIions along the edges of the hexagon. The central MnIIIion is hexacoordinated in a regular octahedral fashion with the Mn-O bond lengths ranging from 1.950 to 2.128 Å.This unique MnIIIion is bridged to the six surrounding CuIIions by sixμ4-O atoms, similar to the arrangement of metal ions in an Anderson cluster[25]. The oxidation state of the Mn and Cu ions was determined by bond-valence sum (BVS)calculation (Table 2). Compared with the previously reported CuII7clusters[18], all of the CuIIions are six-coordinated, each with five “normal” Cu-O bonds (ca. 1.94～2.40 Å) and one long Cu-O bond (ca. 2.54～2.68 Å). The coordination sphere of six CuIIions is completed by twoμ4-O and four other oxygen atoms from two tartrate acid ligands.

Fig. 1. (a) Sb3(μ3-O)(tartrate)3 scaffold, (b) CuII6MnIII middle layer, (c) Top view and (d) side view of a ball-and-stick representation of the{Cu6Mn} cluster in 1. Color scheme: Cu, blue; Mn, yellow; Sb, green; Na, black; O, red; and C, gray. H atoms are omitted for clarity

Table 2. Bond-valence Sums for the Mn and Cu Atoms of Complexes 1a

Fig. 2. Ball-and-stick representation of 1. (a) Packing arrangement viewed along the a-axis, (b) Packing arrangement viewed along the c-axis.Color scheme: Cu, blue; Mn, yellow; Sb, green; Na, black; O, red; and C, gray. H atoms are omitted for clarity

The CuII6MnIIIlayer is capped by the upper and lower{Sb3(μ3-O)} units. In these units, theμ3-O atom lies in the center of a triangle formed by three SbIIIions. All SbIIIcations display the typical one-sided coordination environment expected for lone-pair cations[26]. Four SbIIIcations are coordinated with five oxygen atoms in a distorted tetragonal pyramidal arrangement with four strong Sb-O bonds(1.966～2.387 Å) and one weak Sb-O bond (2.807～2.808 Å), while another two SbIIIcations coordinate to four oxygen atoms with four strong Sb-O bonds (1.945～2.307 Å). As a 4-connected node, each cluster is interlinked to its nearest four {Cu6Mn} neighbors through the Na(1) and Na(3)cations, generating a three-dimensional supramolecular framework (Fig. 2).

3. 2 Infrared spectra of 1

Infrared spectrum of complex 1 demonstrates a very intense band at about 3400 cm-1ascribed to the characteristic absorption peaks of O-H stretching vibrations[27]. In addition,the peaks at 1613 and 1366 cm-1are attributed to asymmetrical and symmetrical stretching vibrations of-COOH from tartaric acid ligands, respectively[28].

Fig. 3. Fourier transforms infrared spectrum of compound 1

3. 3 Powder X-ray diffraction (PXRD) and thermogravimetric analysis of 1

In order to check phase purity of complex 1, the sample was characterized by PXRD at room temperature. As reported in Fig. 4a, the peak positions of the simulated and experimental PXRD patterns are consistent with each other,which were confirmed high phase purity of the as-synthesized samples. The slight difference in intensity for experimental and simulated powder diffraction data may be caused by the preferred orientation of the crystalline powder samples.

The thermogravimetric analysis of complex 1 under N2atmosphere from 30 to 800 ℃ at a heating rate of 10 ℃/min is shown in Fig. 4b. The initial weight loss process occurs from room temperature to 260 ℃, which can be assigned to the release of the free and lattice water molecules (obsd.:15.36%, calcd.: 13.41%). As the temperature is increased beyond 260 ℃, a sharp increase in the weight loss occurs,indicating the decomposition of the tartrate acid ligands.

Fig. 4. (a) Comparison of stimulated (red) and experimental (black) powder XRD patterns of complex 1, (b) Thermogravimetric curves for complex 1

3. 4 Magnetic property

Magnetic susceptibility of 1 was measured in a temperature range of 5～300 K with field of 1 kOe. Plot of the temperature dependence ofχMTνsTfor 1 is shown in Fig. 5. The room temperatureχMTvalue of 1 is 5.55 cm3·K·mol-1, which is almost the same as the expected spin-only (g= 2.0) value of 5.50 cm3·K·mol-1for one MnIII(S= 2) and six respective CuIIions (S= 1/2). Upon lowering the temperature, theχMTvalue decreases rapidly from room temperature to 1.76 cm3·K·mol-1at 25 K and then drops gradually to 1.56 cm3·K·mol-1at 5.0 K, indicating dominating antiferromagnetic interactions. The above data were fitted to the spin Hamiltonian in Eq. 1 using the program PHI[29], which givesgCu= 2.26,gMn= 1.98,JCu-Cu=176.34 cm-1andJCu-Mn= -14.44 cm-1(S1=S2=S3=S4=S5=S6=SCu,S7= SMn). Two exchange pathways of Cu-Cu and Cu-Mn can be seen clearly in the asymmetric unit.

Fig. 5. χMT (▪) vs T plots for 1. The red line represents the best fit using the spin-Hamiltonian

4 CONCLUSION

In conclusion, we have presented a new transition metal-antimony oxo-cluster constructed by a CuII6MnIIImiddle layer capping by two {Sb3(μ3-O)} scaffolds and six tartrate acid ligands from the aqueous medium under mild conditions. In addition, the magnetic property of 1 was investigated, indicating dominating antiferromagnetic couplings withJCu-Cu= 176.34 andJCu-Mn= -14.44 cm-1. The work to design and synthesize new transition metal-antimony oxo-cluster is in progress.