English

• 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 K₂Sb₂L₂ ligand 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 N₂ flow 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-Kα radiation (λ = 0.71073 Å). Powder X-ray diffraction patterns were gathered in the 2θ range 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) and rac-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 MoKα radiation (λ = 0.71073 Å). The structure of 1 was solved by direct methods and refined by full-matrix least-squares method on F² using 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 group P2₁/n with a = 16.8688(10), b = 9.4734(5), c = 22.5825(14) Å, V = 3599.8(3) ų, Z = 2, C₂₄H₅₅Cu₆MnNa₆O₆₅Sb₆, M_r = 2688.30, D_c = 2.480 g/cm³, F(000) = 2580, μ(MoKα) = 4.275 mm^-1, the final R = 0.0317 and wR = 0.0827(w = 1/[σ²(F_o²) + (0.0312P)² + 21.8403P], where P = (F_o² + 2F_c²)/3), S = 1.062. The selected bond lengths and bond angles are reported in Table 1.

  Table 1

  

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

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Mn(1)–O(19)	1.950(2)		Mn(1)–O(20)	2.002(4)		Mn(1)–O(21)	2.128(1)
  Cu(1)–O(15)	1.943(7)		Cu(1)–O(15)	1.961(6)		Cu(1)–O(20)	1.976(3)
  Cu(1)–O(7)	2.585(6)		Cu(1)–O(12)	2.005(6)		Cu(1)–O(19)	2.364(4)
  Cu(2)–O(3)	1.948(0)		Cu(2)–O(6)	1.948(2)		Cu(2)–O(13)	2.687(0)
  Cu(2)–O(18)	2.016(7)		Cu(2)–O(20)	2.408(1)		Cu(2)–O(21)	1.968(9)
  Cu(3)–O(1)	2.542(0)		Cu(3)–O(6)	2.041(0)		Cu(3)–O(9)	1.978(1)
  Cu(3)–O(12)	1.990(9)		Cu(3)–O(21)	2.278(0)		Cu(3)–O(19)	2.021(0)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(12)–Cu(1)–O(15)	89.329(1)		O(3)–Cu(2)–O(4)	142.76(7)		O(9)–Cu(3)–O(10)	143.25(6)
  O(15)–Cu(1)–O(16)	142.72(1)		O(6)–Cu(2)–O(21)	85.669(1)		O(1)–Cu(3)–O(9)	116.387(1)
  O(19)–Mn(1)–O(21)	85.749(1)		O(20)–Mn(1)–O(21)	86.239(1)		O(19)a–Mn(1)–O(20)	86.288(1)

  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)₃ scaffolds (Fig. 1a) sandwich a Cu₆^IIMn^III middle layer to form the cluster ion, a similar arrangement to what was found in the previously reported Cu₇^II cluster^{[19, 24]}. In the middle layer, all the seven metal ions lie in an almost regular hexagon (Fig. 1b), with Mn^III ion in the center and six Cu^II ions along the edges of the hexagon. The central Mn^III ion is hexacoordinated in a regular octahedral fashion with the Mn–O bond lengths ranging from 1.950 to 2.128 Å. This unique Mn^III ion is bridged to the six surrounding Cu^II ions by six μ₄-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 Cu₇^II clusters^[18], all of the Cu^II ions 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 Cu^II ions is completed by two μ₄-O and four other oxygen atoms from two tartrate acid ligands.

  Figure 1

  

  Figure 1.  (a) Sb₃(μ₃-O)(tartrate)₃ scaffold, (b) Cu₆^IIMn^III 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

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  Table 2

  

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

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  Atom	MnII	MnIII	Atom	CuI	CuII
  Mn1	2.76	3.01	Cu1	1.70	1.89
  			Cu2	1.68	1.87
  			Cu3	1.61	1.79
  aThe underlined value is the closest to the charge for which it was calculated. The oxidation state is the nearest whole number to the underlined value

  The Cu₆^IIMn^III layer is capped by the upper and lower {Sb₃(μ₃-O)} units. In these units, the μ₃-O atom lies in the center of a triangle formed by three Sb^III ions. All Sb^III cations display the typical one-sided coordination environment expected for lone-pair cations^[26]. Four Sb^III cations 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 Sb^III cations 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).

  Figure 2

  

  Figure 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

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  3.2 Infrared spectra of 1

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

  Figure 3

  

  Figure 3.  Fourier transforms infrared spectrum of compound 1

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  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.

  Figure 4

  

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

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  The thermogravimetric analysis of complex 1 under N₂ atmosphere 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.

  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 of χ_MT vs T for 1 is shown in Fig. 5. The room temperature χ_MT value 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⁻¹ for one Mn^III (S = 2) and six respective Cu^II ions (S = 1/2). Upon lowering the temperature, the χ_MT value decreases rapidly from room temperature to 1.76 cm³⋅K⋅mol⁻¹ at 25 K and then drops gradually to 1.56 cm³⋅K⋅mol⁻¹ at 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 gives g_Cu = 2.26, g_Mn = 1.98, J_Cu–Cu = 176.34 cm⁻¹ and J_Cu–Mn = –14.44 cm⁻¹ (S₁ = S₂ = S₃ = S₄ = S₅ = S₆ = S_Cu, S₇ = S_Mn). Two exchange pathways of Cu–Cu and Cu–Mn can be seen clearly in the asymmetric unit.

  $\text{H}=-2{{J}_{1}}\left( {{S}_{1}}{{S}_{2}}+{{S}_{2}}{{S}_{3}}+{{S}_{3}}{{S}_{4}}+{{S}_{4}}{{S}_{5}}+{{S}_{5}}{{S}_{6}} \right)-2{{J}_{2}}\left( {{S}_{1}}{{S}_{7}}+{{S}_{2}}{{S}_{7}}+{{S}_{3}}{{S}_{7}}+{{S}_{4}}{{S}_{7}}+{{S}_{5}}{{S}_{7}}+{{S}_{6}}{{S}_{7}} \right)$

  (1)

  Figure 5

  

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

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  4. CONCLUSION

  In conclusion, we have presented a new transition metal-antimony oxo-cluster constructed by a Cu₆^IIMn^III middle 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 with J_Cu–Cu = 176.34 and J_Cu–Mn = –14.44 cm⁻¹. The work to design and synthesize new transition metal-antimony oxo-cluster is in progress.

  
  
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• 

  Figure 1  (a) Sb₃(μ₃-O)(tartrate)₃ scaffold, (b) Cu₆^IIMn^III 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

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  Figure 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

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  Figure 3  Fourier transforms infrared spectrum of compound 1

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

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

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Mn(1)–O(19)	1.950(2)		Mn(1)–O(20)	2.002(4)		Mn(1)–O(21)	2.128(1)
  Cu(1)–O(15)	1.943(7)		Cu(1)–O(15)	1.961(6)		Cu(1)–O(20)	1.976(3)
  Cu(1)–O(7)	2.585(6)		Cu(1)–O(12)	2.005(6)		Cu(1)–O(19)	2.364(4)
  Cu(2)–O(3)	1.948(0)		Cu(2)–O(6)	1.948(2)		Cu(2)–O(13)	2.687(0)
  Cu(2)–O(18)	2.016(7)		Cu(2)–O(20)	2.408(1)		Cu(2)–O(21)	1.968(9)
  Cu(3)–O(1)	2.542(0)		Cu(3)–O(6)	2.041(0)		Cu(3)–O(9)	1.978(1)
  Cu(3)–O(12)	1.990(9)		Cu(3)–O(21)	2.278(0)		Cu(3)–O(19)	2.021(0)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(12)–Cu(1)–O(15)	89.329(1)		O(3)–Cu(2)–O(4)	142.76(7)		O(9)–Cu(3)–O(10)	143.25(6)
  O(15)–Cu(1)–O(16)	142.72(1)		O(6)–Cu(2)–O(21)	85.669(1)		O(1)–Cu(3)–O(9)	116.387(1)
  O(19)–Mn(1)–O(21)	85.749(1)		O(20)–Mn(1)–O(21)	86.239(1)		O(19)a–Mn(1)–O(20)	86.288(1)

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

  Atom	MnII	MnIII	Atom	CuI	CuII
  Mn1	2.76	3.01	Cu1	1.70	1.89
  			Cu2	1.68	1.87
  			Cu3	1.61	1.79
  aThe underlined value is the closest to the charge for which it was calculated. The oxidation state is the nearest whole number to the underlined value

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•