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  Since the discovery of slow magnetization relaxa-tion for [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄] in 1993, single molecule magnets (SMMs) have received great attention in the field of coordination chemistry because of their unique and intriguing properties and their potential applications in high-density information storage, quantum computing and molecular spintronics ^[1-2]. Up to date, polynuclear 3d transition metal complexes (especially polynuclear Mn clusters) ^[3-4], 3d-4f mixed-metal complexes ^[5-6], polynuclear pure lanthanide clusters ^[7] and mononuclear f-based (including acitinide) or 3d transition metal compounds ^[8-10] have been repo-rted by scientists all over the world. Nonetheless, we consider that the polynuclear 3d clusters, especially polynuclear Mn clusters, should not be ignored because it is important for better understanding of magnetic interactions between paramagnetic ions, the quantum tunneling of the magnetization and the mechanisms for slow magnetic relaxation.

  Choosing appropriate multidentate chelating ligands is vitally important for obtaining the above types of SMMs. At early stage, carboxylate ligands have been widely used in the construction of 3d clusters. However, in recently years, non-carboxylate ligands have been received more attention. These non-carbo-xylate ligands mainly include ^[11]: (ⅰ) alcohol/alkoxide-based chelating ligands, such as 2-(hydroxymethyl) pyridine (hmpH) ^[12], 2, 6-pyridinedimethanol (pdmH₂) ^[13], the gem-diolate form of di-2-pyridyl ketone (dpkd^2-) ^[14], 1, 1, 1-tris (hydroxymethyl) ethane (thmeH₃) ^[15]; (ⅱ) poly-dentate Schiff-base ligands, such as N, N′-2, 2-dimeth-ylpropylenedi (3-methoxysalicylideneiminato) ^[16], (E)-2, 2′-(2-hydroxy-3-((2-hydroxyphenylimino) methyl)-5-methylbenzylazanediyl)-diethanol ^[17]; (ⅲ) other N-and O-based chelating ligands, such as methyl-2-pyridyl ketone (mpkoH) ^[18], 2, 6-diacetylpyridine dioxime (dapdoH₂) ^[19], salicylaldoxime and its derivatives (H₂salox and R-H₂salox) ^[20]. By detailed investigation on these multidentate chelating ligands, we found that their symmetry are different, for example, pdmH₂ and thmeH₃ possess C₂ and C₃ symmetry axes, respectively, while hmpH, mpkoH and others have no symmetry axes. Additionally, the coordinate modes of these multidentate chelating ligands are different from each other. Therefore, novel structural complexes with distinctive magnetic properties compared with complexes containing single multidentate chelating ligand may be obtained by mixing two or more kinds of ligands possessing different symmetry in a solution containing magnetic spin carrier salts. Actually, we have obtained [Mn₄], [Mn₁₃] and [Mn₁₆] clusters containing mixed chelating ligands ^[21-22]. We have also found that the core structures of the tetranuclear Ni clusters can be transformed from a defect dicubane-like core to a cubane-like core by adding of auxiliary multidentate chelating ligand of 2-(hydroxymethyl) pyridine ligands ^[23]. As a part of our continuing studies on the synthesis and magnetic properties of SMMs clusters containing mixed multidentate chelating ligands, we report herein a heptanuclear Mn cluster, namely [Mn₃^ⅡMn₄^Ⅲ(Cl)₆(hmp)₆(thme)₂]·H₂O·3CH₃CN (1·H₂O·3CH₃CN). Magnetic studies reveal that overall ferromagnetic coupling between Mn^Ⅲ and Mn^Ⅱ or Mn^Ⅲ ions within complex 1·H₂O are present and weak frequency dependence of the ac-susceptibility was found.

  1   Experimental

  1.1   Materials and measurements

  All the starting materials for synthesis were commercially available and used as received. Elemental analyses for C, H and N were carried out using an Elementar Vario Perkin-Elmer 240C analyzer. IR spectra were obtained at room temperature using KBr pellets in the range of 4 000~400 cm^-1 on a VECTOR 22 spectrometer. Magnetic measurements on crystalline samples were performed on a Quantum Design MPMS-XL7 superconducting quantum interfe-rence device (SQUID) magnetometer. The direct current (dc) measurements were collected at an applied field of 2 kOe and from 1.8 to 300 K, and the alternating-current (ac) measurements were carried out in a 5.0 Oe ac field oscillating at various frequencies from 1 to 1 500 Hz and without dc field. The diama-gnetic corrections for the compounds were estimated using Pascal′s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder.

  1.2   Preparation of [Mn₃^ⅡMn₄^Ⅲ(Cl)₆(hmp)₆(thme)₂]·H₂O·3CH₃CN (1·H₂O·3CH₃CN)

  A mixture of MnCl₂·4H₂O (0.082 5 g, 0.4 mmol), 1, 1, 1-tris (hydroxymethyl) ethane (0.049 4 g, 0.4 mmol), 2-pyridinemethanol (0.045 5 g, 0.4 mmol), triethyla-mine (0.126 g, 1.2 mmol) in a molar ratio of 2: 1: 2: 2: 6 in CH₃CN was stirred at room temperature for half an hour, forming a red-orange solution from which brown crystals of the compounds 1·H₂O·3CH₃CN were formed after several days. Yield: 0.032 5 g (38% based on Mn). It should be noted that, for complex 1·H₂O·3CH₃CN, vacuum-drying has resulted in three MeCN solvent molecules free. Anal. Calcd. for 1·H₂O (C₄₆H₅₆ Cl₆Mn₇N₆O₁₃, %): C, 36.88; H, 3.77; N, 5.61. Found (%): C, 36.78; H, 3.68; N, 5.57. IR (KBr, cm^-1): 3 422 (s), 2 872 (m), 1 605 (m), 1 522 (m), 1 460 (m), 1 384 (m), 1 122 (w), 1 047 (s), 915 (w), 763 (w), 582 (m).

  1.3   X-ray crystallography

  For compound 1·H₂O·3CH₃CN, the framework of single crystal samples was collapsed in the air a few minutes later, so the suitable single crystal of 1·H₂O·3CH₃CN was located in a silica tube to collect crystallographic data. Diffraction data were collected on a Bruker Smart CCD area-detector diffractometer with Mo Kα radiation (λ=0.071 073 nm) by ω-scan mode operating at room temperature. The collected data were reduced with SAINT ^[24], and semi-empirical absorption correction was applied to the intensity data using the SADABS program ^[25]. The structure was solved by direct methods, and all non hydrogen atoms were refined anisotropically by least squares on F² using the SHELXTL program ^[26]. Hydrogen atoms were placed in calculated positions and refined isotropically using the riding model. Unit cell data and structure refinement details are listed in Table 1.

  Table 1.  Details of the data collection and refinement parameters for 1·H₂O·3CH₃CN

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  Table 1.  Details of the data collection and refinement parameters for 1·H₂O·3CH₃CN

  CCDC: 1045766.

  2   Results and discussion

  2.1   Crystal structure description

  Single-crystal X-ray diffraction analysis revealed that complex 1·H₂O·3CH₃CN crystallizes in mono-clinic space group I2/a. The crystal structure of complex 1·H₂O·3CH₃CN is shown in Fig. 1. Selected bond lengths and angles are given in Table 2. As can be seen from Fig. 1, complex 1·H₂O·3CH₃CN has seven manganese atoms that are roughly in the same plane. It consists of a central Mn^Ⅲ ion which is encircled by six Mn ions that form a roughly dislike heptanuclear Mn clusters. The outer six Mn atoms are connected with the central Mn through six μ₃-O atoms (O7, O8, O9 and O10, O11, O12) which are from thme^3- ligands located above and below the molecular plane, respectively. As can be seen from Fig. 1, 1·H₂O·3CH₃CN contains six hmp^- ligands, two thme^3- ligands and six Cl-terminal ligands. Each of the hmp^- ligands simultaneously binds two Mn atoms in a μ₂-η¹: η² fashion. Each hydroxyl O from thme^3- coordinates with three Mn ions, and the thme^3- ligand binds seven Mn ions in a η₃: η³: η³, μ₇-fashion. Oxidation-state deter-minations based on charge considerations and crystall-ographic evidences for Jahn-Teller elongated axes. It is concluded that Mn1, Mn3, Mn5, Mn7 are Mn^Ⅲ, Mn2, Mn₄, Mn₆ are Mn^Ⅱ. All the Mn atoms are six-coordinated with distorted octahedral geometry. For four Mn^Ⅲ ions, each of them clearly possesses a Jahn-Teller distortion in the form of axis elongation along N₂-Mn1-O7, N3-Mn3-O10, N₅-Mn5-O12 and O8-Mn7-O11 (black lines in Fig. 2), in which Jahn-Teller axes for Mn1, Mn3 Mn7 are roughly parallel while these are vertical to that of Mn5. Besides, the O-Mn7-O is almost linear, with O11-Mn7-O8, O12-Mn7-O7 and O9-Mn7-O10 being 175.780°, 177.41° and 176.32°, respectively. For Mn2, Mn₄ and Mn₆ ions, the bond lengths of Mn-O and Mn-Cl are in the range of 0.208 3~0.287 7 nm (Mn₄-O12 0.267 4 nm, Mn₆-O9 0.287 7 nm) and 0.238 4~0.243 9 nm, respectively, which meet the feature of Mn^Ⅱ ions ^[27].

  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1·H₂O·3CH₃CN

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1·H₂O·3CH₃CN
  Figure 1.  Molecular structure of complex 1·H₂O·3CH₃CN

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  Figure 2.  Black lines showing Jahn-Teller axes of Mn^Ⅲ ions in the complex 1·H₂O·3CH₃CN

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  2.2   Magnetic properties

  The direct-current (dc) magnetic susceptibility measurements for polycrystalline samples of 1·H₂O were performed between 1.8 and 300 K under an applied dc field of 2 000 Oe. The χ_MT versus T plot for 1·H₂O was shown in Fig. 3. At room temperature, the χ_MT value is 26.07 cm³·K·mol^-1, which is slightly higher than the spin-only values of 25.13 cm³·K·mol^-1 expected for three Mn^Ⅱ and four Mn^Ⅲ non-interacting ions. As the temperature is reduced to 30 K, the χ_MT product steadily increases to 32.46 cm³·K·mol^-1 and then drops to a value of 16.12 cm³·K·mol^-1 at 1.8 K. The χ_M values above 50 K obey the Curie-Weiss law (χ_M=C/(T-θ)) with C=24.99 cm³·K·mol^-1 and θ=13.65 K. The positive θ value and the increase in χ_MT on lowering the temperature show that the overall ferromagnetic coupling interactions between Mn ions within the cluster are present.

  Figure 3.  Temperature dependence of χ_MT for 1·H₂O

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  To obtain the sign and magnitude of the magnetic exchange interactions between Mn ions within the molecule of 1·H₂O, fitting of the magnetic suscepti-bility data was carried out. Due to no symmetry in the complex, each distance between Mn^Ⅱ/Mn^Ⅲ and Mn^Ⅲ is different, so a precise fitting of the magnetic data require too many exchange constants. However, from the crystal structure, the alternative Mn^Ⅱ and Mn^Ⅲ atoms in the Mn₆ ring are linked by one hydroxyl group O atom of an hmp^- and one O atom of a thme^3-, and each of Mn^Ⅱ or Mn^Ⅲ in the Mn₆ ring is connected to the central Mn7 by two O atoms of two different thme^3- ligands. Therefore, it is instructive to employ a simplified three-J model (Fig. 4), which leads to the following Heisenberg Hamiltonian:

  Figure 4.  Model for fitting the magnetic data of 1·H₂O

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  It should be noted that the above Heisenberg Hamiltonian based on the magnetic isotropic parame-ters, due to the dimension of full energy matrix being 135 000 for [Mn₃^ⅡMn₄^Ⅲ] if the magnetic anisotropic parameters were employed, which goes beyond the operating limit of our computer. The magnetic data above 30 K have been fitted by MAGPACK program ^[28], with the best parameters being: J₁=-0.6 cm^-1, J₂=4.1 cm^-1, J₃=0.75 cm^-1, g=2.0 and R=4.15×10^-4 (R=∑[(χ_MT)_obs-(χ_MT)_calc]²/∑(χ_MT)_obs²). The signs of the coupling constants show that the antiferromagnetically coupling interactions are present in the Mn₆ ring, in which Mn^Ⅱ and Mn^Ⅲ are further antiferromagnetically and ferromagnetically coupled to the central Mn^Ⅲ ion, respectively.

  To obtain the ground-state S, g and the magnitude of the zero-field splitting (ZFS) parameter (D), magnet-ization data were collected in the range of 1~7 T and 1.8~5.0 K. The plot of the reduced magnetization M versus H/T shows that the curves are not superposed (Fig. 5), giving an indication of the presence of magnetic anisotropy and/or the low-lying excited states. The magnetization data were fitted using the program ANISOFIT 2.0 ^[29], by matrix diagonalization to a model with axial ZFS (DS_z²) and isotropic Zeeman interactions, assuming only the ground state is populated. However, an acceptable fit could not be obtained using the data collected over the whole field range. This problem widely exits in high-nuclear clusters containing Mn²⁺ ions ^[30], which is caused by low-lying excited states, especially if some have an S value greater than that of the ground state (in other words, these excited states are populated even at low temperatures and at low magnetic fields). This conclusion is supported by the M versus H plot (Fig. 6), in which the magnetization steadily increases upon increasing magnetic field (H) and does not show saturation.

  Figure 5.  Plots of reduced magnetization M vs H/T for 1·H₂O

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  Figure 6.  Plots of magnetization M vs H for 1·H₂O

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  To probe the magnetization relaxation dynamics of 1·H₂O, alternating current (ac) magnetic susce-ptibility data were collected in a zero-applied dc field with a 5.0 Oe ac field oscillating at frequencies in the range of 1~1488 Hz and in the temperature range of 1.8~10 K. The in-phase (χ_M′, plotted as χ_M′T) and out-of-phase (χ_M″) ac susceptibility signals are shown in Fig. 7. The rapid decrease of χ_M′T with temperatures decreasing also supports a population of low-lying excited states with larger S compared to the ground-state S. Extrapolation of χ_M′T data down to 0 K gives~22.5 cm³·K·mol^{-1 [31]}, obtaining a ground state of S≈6.5 (Fig. 8). At lower temperatures (below 2.8 K), the frequency-dependent χ_M″ signals appear (Fig. 7), indi-cating slow relaxation of the SMMs behavior. However, the peak maxima may lie at temperature below 1.8 K, which goes beyond the operating limit of our instrument. Therefore, we cannot calculate the effective energy barrier (U_eff) by the Arrhenius law.

  Figure 7.  Plot of the temperature dependence of the out-of-phase (χ_M″) ac susceptibility signals for 1·H₂O at the indicated frequencies

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  Figure 8.  χ_M′T vs T plot for 1·H₂O

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  2.3   Compared with other [Mn7] clusters

  It should be noted that, in the literature, there are 14 reported dislike heptanuclear Mn clusters ^[32], which can be classified into four categories according to their oxidation states and their distribution of the Mn ions. The first class is Mn₄^ⅡMn₃^Ⅲ, in which alter-nating Mn^Ⅱ and Mn^Ⅲ ions consist of a six-member ring (Mn₆ ring) that encircles the central Mn^Ⅱ ions. The distribution of Mn ions of the second series Mn₇^Ⅱ and the third class Mn₆^ⅢMn^Ⅱ (only one compound was reported), are the same as the above class, except all Mn^Ⅲ for the former series replaced by Mn^Ⅱ ions and Mn^Ⅱ ions on the Mn₆ ring for the later class replaced by Mn^Ⅲ ion, respectively. However, for the last category Mn₃^ⅡMn₄^Ⅲ, the distribution of Mn ions is entirely different from the above three class, in this category, three Mn^Ⅱ ions consist of a linear, with its either side located by two Mn^Ⅲ ions. For 1·H₂O·3CH₃CN, alternating Mn^Ⅱ and Mn^Ⅲ ions form a Mn₆ ring that encircles the central Mn^Ⅲ, which is similar to that of the first class mentioned above, except the different oxidation state of the central Mn. Therefore, complex 1·H₂O·3CH₃CN represents an unprecedented oxidation state configuration, which has previously not been seen for this topology. For 1·H₂O·3CH₃CN, the central Mn is coordinated to six O atoms from two thme^3- ligands, so the change of central Mn compared to the first class may be due to the charge considera-tions and constraints imposed by multidentate chela-ting ligand thme^3-. Therefore, this paper provides a chance for changing structural configuration and/or oxidation state of polynuclear magnetic clusters by mixing two or more multidentate chelating ligands with different symmetry. Finally, it also should be noted that only a few of dislike heptanuclear Mn clusters show SMMs behaviors and 1·H₂O·3CH₃CN showing this magnetic behavior may be due to the very subtle changes of oxidation state of the central Mn.

  3   Conclusions

  In summary, a complex containing mixed multidentate chelating ligands with different symmetry, i.e. [Mn₃^ⅡMn₄^Ⅲ(Cl)₆(hmp)₆(thme)₂]·H₂O·3CH₃CN (1·H₂O·3CH₃CN), has been synthesized by the reactions of MnCl₂·4H₂O, hmpH and thmeH₃ in MeCN. For 1·H₂O·3CH₃CN, alternating Mn^Ⅱ and Mn^Ⅲ ions form a Mn₆ ring that encircles the central Mn^Ⅲ, and this structural topology has previously not been seen. This structural change may be due to the charge considerations and constraints imposed by multidentate chelating ligand thme^3-. Magnetic studies reveal that the overall ferromagnetic interactions between neigh-boring Mn atoms within 1·H₂O are present, and weak frequency dependence of the ac-susceptibility was found, which represents a few of examples with SMMs behavior in dislike Mn7 clusters. Finally, this work provides a chance for changing structural configura-tion and/or oxidation state of polynuclear magnetic clusters by mixing two or more multidentate chelating ligands with different symmetry.

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• 

  Figure 1  Molecular structure of complex 1·H₂O·3CH₃CN

  Hydrogen atoms and solvated molecules have been omitted for clarity

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  Figure 2  Black lines showing Jahn-Teller axes of Mn^Ⅲ ions in the complex 1·H₂O·3CH₃CN

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  Figure 3  Temperature dependence of χ_MT for 1·H₂O

  Solid lines represent the fitted results by Curie-Weiss law or by MAGPACK program^[28]

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  Figure 4  Model for fitting the magnetic data of 1·H₂O

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  Figure 5  Plots of reduced magnetization M vs H/T for 1·H₂O

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  Figure 6  Plots of magnetization M vs H for 1·H₂O

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  Figure 7  Plot of the temperature dependence of the out-of-phase (χ_M″) ac susceptibility signals for 1·H₂O at the indicated frequencies

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  Figure 8  χ_M′T vs T plot for 1·H₂O

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  Table 1.  Details of the data collection and refinement parameters for 1·H₂O·3CH₃CN

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  Table 2.  Selected bond lengths (nm) and bond angles (°) for 1·H₂O·3CH₃CN

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