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  0    Introduction

  Molecular magnetic materials have attracted scientists attention in the past two decades due to their potential applications in high-density magnetic memories, quantum computing devices and molecular spintronics^[1-6].Among the different approaches to prepare molecular magnetic materials the metal-radical strategy that consists of matching paramagnetic organic molecules with transition metal complex gives rise to a variety of compounds with different structural and magnetic dimensionalities.Up to now various organic paramagnetic molecules such as verdazyl, semiquinone and nitronyl nitroxide (NIT) radicals have been widely studied in the field of molecular magnetism^[7-14].Among them, nitronyl nitroxide radicals have received noteworthy attention because this type of radicals can act as bidentate ligands with identical N-O coordination groups.Besides, nitronyl nitroxide family of radicals are relatively stable and easy to obtain derivatives with substituents containing donor atoms.However, NIT radicals are poorly donating ligands, thus utilization of strong electron withdrawing coligands such as hexafluoroacetylacetonate (hfac) ^[15-17] and trifluoroacetylacetonate (tfac) ^[18-20] in the metal to improve weak coordination ability is necessary.The steric demand of hfac or tfac restrict the dimensionality of the resulting metal-radical complex.Thus numbers of zero- and one-dimensional complexes were prepared by this strategy.Recently lanthanide ions based molecular magnetic materials have been extensively studied because lanthanide ions such as terbium (III) and dysprosium (III) are good candidates for the construction of SMMs due to their significant magnetic anisotropy arising from the large, unquenched orbital angular momentum^[21-24].

  To further study the magnetic properties of NIT radical-lanthanide compounds, in this paper we report a nitronyl nitroxide radical (Scheme 1) and its corres-ponding Ln-nitronyl nitroxide compounds [Ln (hfac) ₃ (NIT -Ph-4-Br) ₂] (Ln=Gd (1) , Tb (2) , Dy (3) , hfac=hexafluo-roacetylacetonate, NIT-Ph-4-Br=2- (4′-bromine phenyl) -4, 4, 5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide) , their crystal structures and magnetic properties were described in detail.

  

  图Scheme 1 Molecular structure of NIT-Ph-4-Br

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  1    Experimental

  1.1    Materials and measurements

  All reagents and solvents were purchased from Aladdin and used without further purification.The radical ligand NIT-Ph-4-Br was synthesized according to literature ^[25].Elemental analyses (C, H and N) were performed on a Perkin-Elmer 240C elemental analyzer.IR spectra were recorded on a Nicolet IS10 IR spectrometer using KBr pellets in the range of 4 000~500 cm^-1.The magnetic measurements were carried out with MPMSXL-7 SQUID magnetometer.Diamagnetic corrections were made with Pascals constants for all the constituent atoms.

  1.2    Synthesis of complex 1

  A suspension of Gd (hfac) ₃·2H₂O (0.05 mmol) in 15 mL dry boiling heptane was heated to reflux for about 1 h.Then the solution was cooled to 65 ℃, a solution of NIT-Ph-4-Br 0.1 mmol) in 2 mL of CHCl₃ was added.The resulting solution was stirred for about 2 min and then cooled to room temperature.The filtrate was allowed to stand at room temperature for slow evaporation.Slow evaporation of the final solution for about four days yielded dark-blue block crystals suitable for single-crystal X-ray analysis.Yield: 43.5% based on rare-earth.Elemental analysis calculated for C₄₁H₃₅Br₂GdF₁₈N₄O₁₀ (%) : C: 35.10； H: 2.51； N: 3.99.Found (%) : C: 33.88； H: 2.79； N: 4.12.FTIR (KBr, cm^-1) : 1 656 (s) , 1527 (w) , 1385 (w) , 1350 (w) , 1255 (s) , 1198 (s) , 1096 (s) , 796 (w) , 660 (w) .

  1.3    Synthesis of Complex 2

  Complex 2 was synthesized using the same procedure for complex 1 with Tb (hfac) ₃·2H₂O instead of Gd (hfac) ₃·2H₂O.Yield: 47.9%.Elemental analysis calculated for C₄₁H₃₅Br₂GdF₁₈N₄O₁₀ (%) : C: 35.06； H: 2.51； N: 3.99.Found (%) : C: 35.69； H: 2.57； N: 4.11.FTIR (KBr, cm^-1) : 1 655 (s) , 1599 (w) , 1528 (w) , 1399 (w) , 1352 (w) , 1255 (s) , 1199 (s) , 1096 (w) , 797 (w) , 660 (w) .

  1.4    Synthesis of Complex 3

  Complex 3 was synthesized using the same procedure for complex 1 with Dy (hfac) ₃·2H₂O instead of Gd (hfac) ₃·2H₂O.Yield: 42.9%.Elemental analysis calculated for C₄₁H₃₅Br₂GdF₁₈N₄O₁₀ (%) : C: 34.97； H: 2.51； N: 3.98.Found (%) : C: 34.91； H: 2.77； N: 3.87 (%) .FTIR (KBr, cm^-1) : 1 655 (s) , 1600 (w) , 1527 (w) , 1399 (w) , 1352 (w) , 1255 (s) , 1199 (s) , 1096 (w) , 798 (w) , 662 (w) .

  1.5    X-ray Crystallographic Study

  表1 Crystal data and structure refinement for 1, 2 and 3

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  表1 Crystal data and structure refinement for 1, 2 and 3
  

  The crystal structure data of complexes 1, 2 and 3 were collected using a Rigaku Saturn CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) .Crystal size of complexes 1, 2 and 3 are 0.15 mm×0.12 mm× 0.11 mm, 0.17 mm×0.11 mm×0.10 mm, 0.12 mm×0.11 mm×0.10 mm respectively.The structures were solved by the direct methods with SHELXS-97^[26] and refined by full-matrix least-squares methods on F ² with SHELXL-97 program package^[27].Anisotropic thermal parameters were assigned to all non-hydrogen atoms.The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter.The details of the crystal parameters, data collection, and refinements for these complexes were listed in Table 1 and 2.

  CCDC: 1496095, 1； 1496096, 2； 1496097, 3.

  表2 Selected bond lengths (nm) and angles (°) for 1 , 2 and 3

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  表2 Selected bond lengths (nm) and angles (°) for 1 , 2 and 3
  

  2    Results and discussion

  2.1    Crystal structures of complexes 1~3

  As is shown in Fig.1, Compound 1 is a mononuclear coordination compound crystallizing in the monoclinic space group P2_1/c with Z=4.The central Gd (III) ion is eight coordinated in slightly distorted triangular dodecahedral GdO₈ geometry completed by two non-bridged NO groups from two separate organic radicals and three bischelate hfac- anions.The distances of Gd-O bonds range from 0.235 6 (4) to 0.240 7 (4) nm.The coordinated N (3) -O (2) and N (2) -O (3) bond lengths of nitronyl nitroxide radicals are 0.130 7 (5) nm and 0.129 9 (6) nm respec-tively and the uncoordinated N (4) -O (1) and N (1) -O (4) bond lengths are 0.126 4 (6) nm and 0.126 0 (7) nm respectively, which are comparable to those of reported tri-spin radical-Ln (III) -radical complexes^[28-33].The nearest Gd…Gd distance between adjacent molecules is 1.071 6 (5) nm (Fig.1) .

  

  图1 Molecular structure (left) with thermal ellipsoids drawn at 30% probability and crystal packing diagram (right) of complex 1

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  Compound 3 is also isostructural to compound 1 and the bond lengths of Dy-O are in the range of 0.233 0 (2) ~0.238 6 (2) nm, which are a little bit shorter than the bond lengths of Gd-O and Tb-O.The nearest Dy…Dy distance between adjacent molecules is 1.060 8 (3) nm.

  Compound 2 is isostructural to compound 1 and the bond lengths of Tb-O are in the range of 0.233 8 (5) ~0.239 4 (5) nm, which are a little bit shorter than the bond lengths of Gd-O.The nearest Tb…Tb distance between adjacent molecules is 1.081 2 (4) nm.

  2.2    Magnetic property of complex 1

  

  图2 Temperature dependence of χ_MT (left) and field dependence of magnetization at 2.0 K (right) for complex 1

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  The temperature dependence of the magnetic susceptibilities 1, 2, and 3 were measured from 300 to 2.0 K in an applied field of 1 kOe, and the magnetic behaviors of complex 1 are shown in Fig.2.A_t 300 K, the χ_MT value is 8.61 cm³·K·mol^-1.The values are in good agreement with the theoretical value of 8.63 cm³·K·mol^-1 (uncoupled one Gd (III) ion ( f ⁷ electron confi-guration, χ_MT=7.88 cm³·K·mol^-1) plus two organic radicals (S=1/2, χ_MT=0.375 cm³·K·mol^-1) ) .Upon cooling, the χ_MT value of complex 1 increase steadily to a maximum of 10.03 cm³·K·mol^-1 at 13.9 K, afterward decreases to 9.13 cm³·K·mol^-1 at 2.0 K.

  Furthermore, the field dependence of magnetization of complex 1 has been determined at 2 K in the range of 0~70 kOe (Fig.2) .Upon increasing in the applied field, M increases up to 8.87Nβ at 70 kOe, which corresponds well to the value expected for a ground state with a spin multiplicity of S=9/2 in the case of one Gd (III) and two radical ferromagnetically coupled.A_t the lower fields the value is smaller than the magnetization calculated with the Brillouin function for noncoupled S=7/2 and two S=1/2 spin centers (g=2.0, T=2 K) , which suggests the dominant antiferromagnetic interaction between the Gd (III) ion and the coordinated NIT radical.

  The magnetic interactions between Gd (III) and the radicals can be described by isotropic exchange interaction.Therefore the experimental data for complex 1 can be analyzed with an expression derived from a spin Hamiltonian.Considering the g value range of the radical and Gd (III) ion, we assume that the radical and Gd (III) ion have the same g value.Thus, the variable-temperature magnetic susceptibility data for complex 1 can be analyzed by a theoretical expression (Eq. (2) ) deduced from a spin Hamiltonian (Eq. (1) ) .The J_Rad-Gd represent the magnetic coupling for the Gd-radical and J_Rad-Rad for radical-radical, and the zJ′ in Eq. (2) representing the intermolecular interactions.

  The best fitting results give coupling parameters g=1.99, J_Rad-Gd=2.81 cm^-1, J_Rad-Rad=-10.85 cm^-1, zJ′=-0.02 cm^-1, R=1.64×10^-5, where R is defined as R= ( χ_{M, obs}- χ_{M, calc}) ²/ ( χ_{M, obs}) ² for complex 1.The positive value of J_Rad-Gd indicates that there is a weak ferromagnetic interaction between the Gd (III) and the radicals in the molecule.The negative J_Rad-Rad indicates the antiferromagnetic interaction between the two intramolecular radicals.It is worth noting that the value of zJ′is much smaller than that of J_Rad-Rad.The obtained J value is comparable with the previously reported Gd (III) -radicals compounds^[28-34].

  There are two kinds of magnetic interactions in this radical-Gd (III) -radical complex at the same time.The first one is Gd (III) -radical interaction and the second one is radical-radical interaction.

  2.3    Magnetic properties of complex 2 andcomplex 3

  While for complex 2 (Fig.3) , at 300 K, the χ_MT value is 12.46 cm³·K·mol^-1, and the values are in good agreement with to the theoretical value 12.57 cm³·K·mol^-1 in uncoupled system of one Tb (III) ion ( f 9 electron configuration, χ_MT=11.82 cm³·K· mol^-1) and two organic radical (S=1/2, χ_MT=0.375 cm³·K·mol^-1) .Upon cooling, the χ_MT values of complex 2 maintain a constant behavior down to about 50 K then the value decrease gradually and reach a minimum of 10.37 cm³·K·mol^-1.

  

  图4 Field dependence of magnetization of 2 and 3 at 2.0 K

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  In the expression, Δ is the zero-field-splitting parameter, g is the Lande factor, k is the Boltzmann constant, β is the Bohr magneton constant and N is Avogadros constant. The zJ′ parameter based on the molecular field approximation in Eq. (7) is introduced to simulate the magnetic interactions between all the paramagnetic species in the system.Thus the magnetic data can be analyzed by the following approximate treatment of Eq. (4) ~ (7) ^{[29, 31]}.Giving the best fitting parameters for complex 2 are g=1.49, =3.89×10^-2 cm^-1, zJ′=1.63×10^-2 cm^-1, R=1.12×10^-5 and for complex 3 are g=1.36, Δ=3.17×10^-2 cm^-1 , zJ′=2.14×10^-2 cm^-1, R=1.92×10^-5, where R is defined as R= ( χM, obs-χM, calc) 2/ ( χM, obs) 2.The very small positive zJ′ values are indicative of very weak ferromagnetic interaction between Ln (III) ions and the coordinated nitronyl nitroxide, which is consistent with the reported heavy lanthanide-nitronyl nitroxide complexes.

  

  图3 Temperature dependence of χ_MT for complex 2 (left) and complex 3 (right)

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  Complex 3 shows similar magnetic properties with complex 2 (Fig.3) .A_t 300 K, the χ_MT value is 14.31 cm³·K·mol^-1, close to the theoretical value of 14.92 cm³·K·mol^-1 (one Dy (III) ion ( f 9 electron configuration, χ_MT=14.17 cm³·K·mol^-1) plus two organic radicals (S=1/2, χ_MT=0.375 cm³·K·mol^-1) ) .Upon cooling, the χ_MT values of complex 3 maintain a constant behavior down to about 50 K then the value decrease gradually and reach a minimum of 8.21 cm³·K·mol^-1.

  There is no available expression to determine the magnetic susceptibilities of Ln (III) systems with large anisotropy.To obtain a rough quantitative estimation of the magnetic interaction between Ln (III) and radicals, the magnetic susceptibility χtotal of the complex can be assumed as the sum of χLn of the isolated Dy (III) or Tb (III) ion and χRad of the radical (Eq. (4) ) .The χTb and χDy can be described as Eq. (5) and (6) , respectively.

  The field dependences of magnetizations for complexes 2 and 3 have been determined at 2 K in the range of 0~70 kOe (Fig.4) .Upon increasing in the applied field, M increases up to 5.87Nβ and 7.73Nβ at 70 kOe for 2 and 3, respectively, which is much lower than the saturation value of 11.0Nβ (one Tb (III) ion with g=3/2 and J=6 (9.0Nβ) plus two radicals with g=2.0 and S=1/2) and 12.0Nβ (one Dy (III) ion with g=4/3 and J=15/2 (10.0Nβ) plus two radicals with g=2.0 and S=1/2) .Considering the strong spin-orbit coupling in Ln (III) ions, the large gaps between experimental data and theoretical saturation values for compounds 2 and 3 can be ascribed to the presence of a magnetic anisotropy and/or low-lying excited states in the system.

  2.4    Dynamic magnetic properties for 2 and 3

  Alternating current (ac) susceptibility measurements for 2 and 3 were carried out in the low temperature region under a zero dc field with frequency of 111 and 511Hz.The result (Fig.5) shows that there are no obvious frequency dependent in-phase ( χ′) and out-of-phase ( χ″) signals for both complex 2 and 3, they do not express SMMs behavior at low temperature.

  

  图5 Temperature dependence of the in-phase and out-of-phase components of ac susceptibility for 2 (left) and 3 (right) in zero dc field with an oscillation of 3.5 Oe

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  3    Conclusions

  In conclusion, we report three new complexes based on nitronyl nitroxide radicals and lanthanide ions.These three compounds have similar structures, in which two radical ligands are coordinated to the Ln (III) ions via the oxygen atoms of the nitroxide to form the three spin system.The magnetic studies reveal that ferromagnetic interactions (between the intramolecular Ln and radical) and antiferromagnetic interactions (between the intramolecular radicals) coexist in complex 1.Complexes 2 and 3 show very weak ferromagnetic interaction between Ln (III) ions and the coordinated nitronyl nitroxide.Both complex 2 and 3 do not have SMMs behavior at low temperature, this may due to the small energy barrier which could not prevent the inversion of spin.

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• 

  Scheme 1  Molecular structure of NIT-Ph-4-Br

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  图 1  Molecular structure (left) with thermal ellipsoids drawn at 30% probability and crystal packing diagram (right) of complex 1

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  图 2  Temperature dependence of χ_MT (left) and field dependence of magnetization at 2.0 K (right) for complex 1

  Left: The solid lines represent the theoretical values based on the corresponding equations； Right: the solid line represents the theoretical curve for the sum of the Brillouin function for one Gd (III) (S=7/2) and two radical (S=1/2) with g=2

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  图 3  Temperature dependence of χ_MT for complex 2 (left) and complex 3 (right)

  The solid lines represent the theoretical values based on the corresponding equations

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  图 4  Field dependence of magnetization of 2 and 3 at 2.0 K

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  图 5  Temperature dependence of the in-phase and out-of-phase components of ac susceptibility for 2 (left) and 3 (right) in zero dc field with an oscillation of 3.5 Oe

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  表 1  Crystal data and structure refinement for 1, 2 and 3

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  表 2  Selected bond lengths (nm) and angles (°) for 1 , 2 and 3

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•