A New Family of Heterometallic Ln III [ 12-MC FeIIIN ( shi )-4 ] Complexes : Syntheses , Structures and Magnetic Properties

A new family of LnIII [12-Metallacrown-4] compounds of formulas (C5H6N) [LnFe4(shi)4(C6H5COO)4(Py)4]·3.5Py [Ln = EuIII (1); GdIII (2); TbIII (3); DyIII (4); and, H3shi = salicylhydroxamic acid] were obtained through one-pot reactions with H3shi, Fe(NO3)3·9H2O, and, Ln(NO3)3·6H2O as reagents. Single-crystal X-ray analyses show that they are isostructural and have the similar [12-MCFe N(shi)-4] core, with four benzoate molecules bridging the central LnIII ion to the ring FeIII ions. The negative charge of the 12-MC-4 metallacrown is balanced by one pyridinium cation, which forms the hydrogen bond with an adjacent solvent pyridine molecule. Magnetic measurements demonstrate antiferromagnetic coupling interactions and field-induced slow magnetic relaxation in complex 4.


Introduction
With the rapid development of information technology, it is essential to produce information storage materials with higher storage density and faster response speed.Single molecule magnet (SMM), acting as a separate magnetic domain, behaves potential applications in information storage and quantum computation [1][2][3][4].The first SMM was reported in 1993 [5], then, such cases have attracted considerable attention from chemists and physicists due to their unique magnetic properties [6][7][8][9].As we all know, the involved metallic ions for the studies of SMMs mainly consist of paramagnetic three-dimensional (3d) ions, heterometallic 3d-4f ions and homometallic 4f ions.With the high magnetic anisotropy of 3d ions and large spin-orbital coupling of 4f ions, the heterometallic 3d-4f complexes have represented extreme properties in magnetic investigations.So far, the most studied 3d-4f complexes include heterometallic Mn-Ln, Cu-Ln [10,11], Zn-Ln [12], and Co-Ln [13,14], SMMs, and a few Fe-Ln SMMs.Furthermore, the survey of heterometallic Fe-Ln complexes only shows Fe 2 Ln 2 and Fe 3 LnO 2 butterfly core [15,16], Fe 2 Ln triangular system [17], and Fe 4 Dy 2 S-shape [18] structural frameworks.Few cyclic Fe-Ln compounds have been documented [19,20].Therefore, it is interesting to investigate the heterometallic Fe-Ln complexes with cyclic structures and to explore their magnetic properties.Metallacrowns (MCs), which are a type of metallic macrocyclic polynuclear complexes, are usually regarded as metal ions and nitrogen atoms instead of methylene carbons of organic crowns [21,22].The first MC with the formula represented by {[VO (shi) (MeOH)] 3 (9-MC-3) shi = salicylhydroxamic acid} was reported in 1989 [23], since then, a great deal of metallacrowns with different structural types from 9-MC-3 to 60-MC-20 have been explored [24][25][26][27][28].The ring metal ions for these MCs contain  ].The purity of the single-crystal samples was determined by the powder X-ray diffraction analyses (Figure S1).The complex 3 was obtained by the same way for 1 with Tb(NO 3 ) 3 •6H 2 O (0.05 mmol) instead of Eu(NO 3 )   .The purity of the single-crystal samples was determined by the powder X-ray diffraction analyses (Figure S1).

X-ray Crystallography
Single-crystal X-ray diffraction data for compounds 1-4 were collected on a Bruker Smart CCD area-detector diffractometer (Bruker AXS Inc., Madison, WI, USA, MoKα, λ = 0.71073 Å) by ω-scan mode operating at 298 K.The program SAINT (version 2014/7) was used for the integration of the diffraction profiles and semiempirical absorption corrections were applied using SADABS (version 2.03).All of the structures were solved by direct methods using the SHELXS (version 2014/7) program of the SHELXTL (version 2014/7) package, and were refined by full-matrix least-squares methods with SHELXL [39].Further details for crystallography are listed in Table 1.

Description of Crystal Structures
Single-crystal X-ray structural analyses indicate that 1-4 are isostructural heterometallic compounds.The molecular structure of complexes 1-4 are shown in Figure 1a-d.The complex 4 is described as a representative example in detail.It crystallizes in the monoclinic space group P2 1 /n.The asymmetrical unit includes a representative 12-MC Fe III shi -4 structural framework, which is composed of four Fe III ions, four deprotonated shi 3-ligands, four benzoate ligands, four dative pyridine molecules, and one Dy III ion.The oxidation states of four Fe ions were determined through the bond lengths, charge balance, and the BVS calculations with the values, as shown in Table S1.The Fe 2p XP spectras of a monolayer of complexes 1-4 further prove the the oxydation state of Fe ions [40] (Figure S3, Table S5).Each Fe III ion is six-coordinated with N 2 O 4 atoms from carbonyl oxygen, oxime oxygen, hydroxyl oxygen, oxime nitrogen, pyridyl nitrogen, and benzoate oxygen, respectively.Further, each fully deprotonated shi 3-ligand links two adjacent Fe III ions through their oxime oxygen and oxime nitrogen to form the N-O bridgings between these Fe III ions.Thus, four Fe III ions and four shi 3-ligands are held together to form a 12-MC-4 MC core with the ring presenting Fe-N-O repeat unit.For these four shi 3-ligands, an obvious difference is that three ligands are located in the MC plane, and the fourth ligand is nearly perpendicular to the MC plane.It may be attributed to the steric hindrance to make the fourth ligand distortion.The dihedral angle between the twisted ligand and the MC plane is 81.8(9) • .
Complexes 1-3 have the similar structural configuration with 4 and the difference is discussed.The deviation distances of Ln III ions to the oxime oxygen mean plane (OoxMP) and to the Fe III mean plane (FeMP) are shown in Table 2. From the data, we can see that, as the radius of Ln III ions It is worthily noted that the 12-MC-4 MC unit displays the negative valence with its charge balanced by one pyridinium cation, where the pyridine nitrogen is protonated.The angle of C-N-C is 138.748(254)• , which is larger than that of the parent pyridine complex, which results in the occurrence of the protonation on this site [41][42][43].Meanwhile, the hydrogen bond interaction is formed between the pyridinium cation and an adjacent pyridine molecule, which is also reported in [12-MC Ga III N(shi) -4] [44].Complexes 1-3 have the similar structural configuration with 4 and the difference is discussed.The deviation distances of Ln III ions to the oxime oxygen mean plane (O ox MP) and to the Fe III mean plane (FeMP) are shown in Table 2. From the data, we can see that, as the radius of Ln III ions decrease, the Ln III are approach the plane much more.The similar tendency was also observed in the reported 12-MC Mn III (N)shi -4 [45].The distances between Ln-O and the distortion angles of benzoate for these four complexes are slightly different, further details are shown in Tables S3 and S4.

Magnetic Properties
The variable temperature magnetic susceptibilities of the complexes 1-4 were determined in the temperature range of 1.8-300 K and an applied field of 0.1 T. The χMT versus T plots are shown in Figure 2. The values of χMT for complexes 1-4 are 16.20 (1), 21.27 (2), 26.01 (3), and 29.22 (4) cm 3 mol −1 K at 300 K, respectively, which are lower than expected values of non-interacting four Fe ІІІ ions (d 5 , S = 5/2, g = 2) and one Ln ІІІ [Eu III , 7 F0; Gd III , 8 S7/2, g = 2; Tb III , 7 F6, g = 3/2; Dy III , 6 H15/2, g = 4/3] ion of 19.00 (1), 25.21 (2), 29.15 (3), and 31.50 (4) cm 3 mol −1 K.With the temperature reducing, the χMT values decrease gradually to 0.18 (1), 7.90 (2), 8.62 (3), and 11.01 (4) cm 3 mol −1 K at 1.8 K, respectively (Table 3)   Similar to other reported Fe-Ln complexes, the magnetic behavior of this series of compounds is also related to the Fe III -Fe III , Fe III -Ln III , and the intrinsic magnetic properties of the Ln III ions.For complex 1, including the Eu III ion, we may try to explore the magnetic interaction mode between metal ions, while for other complexes it is difficult to define.The ground state of Eu III ion is 7 F0 and the configuration is 4 f6 ( 7 F0, S = 3, L = 3, J = 0).At a low temperature, only the infinitesimal excited  Similar to other reported Fe-Ln complexes, the magnetic behavior of this series of compounds is also related to the Fe III -Fe III , Fe III -Ln III , and the intrinsic magnetic properties of the Ln III ions.For complex 1, including the Eu III ion, we may try to explore the magnetic interaction mode between metal ions, while for other complexes it is difficult to define.The ground state of Eu III ion is 7 F 0 and the configuration is 4 f 6 ( 7 F 0 , S = 3, L = 3, J = 0).At a low temperature, only the infinitesimal excited states mixing into 7 F 0 [46] occupied the nonmagnetic ground level.Thus, the magnetic properties of complex 1 at a low temperature are mainly caused by the exchange interaction between Fe III ions.This indicates that the Eu III ion can be deemed to be the diamagnetic ion at low temperature.The extrapolation of χ M T value to 0 K is approaching zero, suggesting that the ground state spin of 1 can be recognized to be S = 0. Therefore, we can come to a conclusion that the Fe III -Fe III interaction mainly lead to the antiferromagnetic behavior of 1. Complexes 2-4 also present antiferromagnetic behavior, but the nonzero ground-state spins may attributed to uncanceled spins between the Fe 4 unit and Ln III ion.However, owing to the complexity magnetic coupling interactions between Fe III -Ln III (Gd III , Tb III , Dy III ) and the intrinsic magnetism of Ln III ions, it is very difficult to obtain the appropriate coupling constants for complexes 2-4.The magnetization of the complexes 1-4 was measured in the 1-7 T magnetic fields and 1.8-8 K temperature range.As shown in Figures S11-S14, the magnetization increases rapidly in the low magnetic field, and then a linear increase without clear saturation at 7 T, with values of 2.52 µ B for 1, 6.42 µ B for 2, 5.96 µ B for 3, and 6.92 µ B for 4 at 1.8 K.The reduced magnetization (M/Nµ B −H/T) curves show the non-superposition, suggesting the magnetic anisotropy of metal ions in the molecules and the lack of a well-defined ground state.
In order to further study the magnetic relaxation dynamics of 1-4, the ac susceptibilities were carried out at frequencies in the range of 1-999 Hz and in the temperature range of 1.8-15 K under zero-applied dc field and 2000 Oe dc field for complexes 1-3 and 1000 Oe dc field for complex 4, with a 2.0 Oe ac field oscillating.Complexes 1-4 exhibit similar curves for the in-phase (χ M ) and out-of-phase (χ" M ) under zero-applied dc field, showing the absence of SMM behavior (Figures S4,  S6, S8, and S10).When a 2000 Oe dc field was applied for 1-3 and a 1000 Oe dc field was used for 4, the out-of-phase (χ" M ) signals of complexes 1-3 represent absence of frequency-dependent (Figures S5, S7 and S9), however, complex 4 demonstrates obvious frequency-dependent, revealing the field-induced slow magnetic relaxation (Figure 3).Owing to the absence of maximum value of χ" M for 4, the energy barrier (∆E eff ) and preexponential factor (τ 0 ) can only be calculated by the Debye equation: ln (χ"/χ ) = ln (ωτ 0 ) + ∆E eff /k B T [16,47] (Figure 4).The perfect fitting data are shown in Table 4.The characteristic times is 10 −6 s for complex 4, values that are in agreement with the observed preexponential factors and effective energy barriers for Ln III -containing SMMs [34].In our Fe 4 Ln analogues, however, only the Fe 4 Dy complex represented the magnetic dependence upon the frequencies at 1000 Oe dc field.May be the intrinsic properties of trivalent Ln III ions can account for the phenomenon.In most of the coordination environment, Dy III , as the Kramers ion, could always keep the doubly degenerate ground state under the magnetic field.Nevertheless, the non-Kramers ion, Tb III , needs strict axial crystal-field symmetry.Furthermore, the Eu III ion has a ground state of J = 0, while the Gd III ion is isotropic.
mainly lead to the antiferromagnetic behavior of 1. Complexes 2-4 also present antiferromagnetic behavior, but the nonzero ground-state spins may attributed to uncanceled spins between the Fe4 unit and Ln III ion.However, owing to the complexity magnetic coupling interactions between Fe ІІІ -Ln ІІІ (Gd III , Tb III , Dy III ) and the intrinsic magnetism of Ln III ions, it is very difficult to obtain the appropriate coupling constants for complexes 2-4.
The magnetization of the complexes 1-4 was measured in the 1-7 T magnetic fields and 1.8-8 K temperature range.As shown in Figures S11-S14, the magnetization increases rapidly in the low magnetic field, and then a linear increase without clear saturation at 7 T, with values of 2.52 μB for 1, 6.42 μB for 2, 5.96 μB for 3, and 6.92 μB for 4 at 1.8 K.The reduced magnetization (M/NμB−H/T) curves show the non-superposition, suggesting the magnetic anisotropy of metal ions in the molecules and the lack of a well-defined ground state.
In order to further study the magnetic relaxation dynamics of 1-4, the ac susceptibilities were carried out at frequencies in the range of 1-999 Hz and in the temperature range of 1.8-15 K under zero-applied dc field and 2000 Oe dc field for complexes 1-3 and 1000 Oe dc field for complex 4, with a 2.0 Oe ac field oscillating.Complexes 1-4 exhibit similar curves for the in-phase (χ′M) and out-of-phase (χ″M) under zero-applied dc field, showing the absence of SMM behavior (Figures S4, S6, S8, and S10).When a 2000 Oe dc field was applied for 1-3 and a 1000 Oe dc field was used for 4, the out-of-phase (χ″M) signals of complexes 1-3 represent absence of frequency-dependent (Figures S5, S7 and S9), however, complex 4 demonstrates obvious frequency-dependent, revealing the field-induced slow magnetic relaxation (Figure 3).Owing to the absence of maximum value of χ″M for 4, the energy barrier (∆Eeff) and preexponential factor (τ0) can only be calculated by the Debye equation: ln (χ″/χ′) = ln (ωτ0) + ∆Eeff/kB T [16,47] (.The perfect fitting data are shown in Table 4.The characteristic times is 10 −6 s for complex 4, values that are in agreement with the observed preexponential factors and effective energy barriers for Ln III -containing SMMs [34].In our Fe4Ln analogues, however, only the Fe4Dy complex represented the magnetic dependence upon the frequencies at 1000 Oe dc field.May be the intrinsic properties of trivalent Ln III ions can account for the phenomenon.In most of the coordination environment, Dy III , as the Kramers ion, could always keep the doubly degenerate ground state under the magnetic field.Nevertheless, the non-Kramers ion, Tb III , needs strict axial crystal-field symmetry.Furthermore, the Eu III ion has a ground state of J = 0, while the Gd III ion is isotropic.

Conclusions
We prepared a new family of heterometallic Ln ІІІ [12-MCFe III N(shi)-4] (Ln = Eu III , Gd III , Tb III , Dy III ) MCs through the one-pot reactions of H3Shi ligand with the corresponding iron and lanthanide metal salts.The 12-MC-4 structural unit exhibits a monovalent negative ion with the charge being balanced by one pyridinium cation.The arched structure of the 12-MCFe III N(shi)-4 is related to the radius of Ln III ions, as the Ln III ions' radius decrease, the complex has a less domed structure.The magnetic behavior of the family of compounds was discussed in detail, including the Fe III -Fe III and Fe III -Ln III interactions.Fe III -Fe III interaction within all of the compounds may be antiferromagnetic.The nonzero ground-state spins may attributed to uncanceled spins between the Ln III and Fe III ions.All of the compounds reveal antiferromagnetic behavior and the Fe4Dy analogue with high anisotropy and large spin shows slow magnetization relaxation at a dc field of 1000 Oe.From the experiment, we can draw a conclusion that the choice of Ln III is important for the SMM properties.

Supplementary Materials:
The following are available online at www.mdpi.com/link, Figure S1: The experimental XRD pattern of samples and the simulated XRD pattern of single crystal X-ray diffraction data for complexes 1-4, Figure S2

Conclusions
We prepared a new family of heterometallic Ln III [12-MC Fe III N(shi) -4] (Ln = Eu III , Gd III , Tb III , Dy III ) MCs through the one-pot reactions of H 3 Shi ligand with the corresponding iron and lanthanide metal salts.The 12-MC-4 structural unit exhibits a monovalent negative ion with the charge being balanced by one pyridinium cation.The arched structure of the 12-MC Fe III N(shi) -4 is related to the radius of Ln III ions, as the Ln III ions' radius decrease, the complex has a less domed structure.The magnetic behavior of the family of compounds was discussed in detail, including the Fe III -Fe III and Fe III -Ln III interactions.Fe III -Fe III interaction within all of the compounds may be antiferromagnetic.The nonzero ground-state spins may attributed to uncanceled spins between the Ln III and Fe III ions.All of the compounds reveal antiferromagnetic behavior and the Fe 4 Dy analogue with high anisotropy and large spin shows slow magnetization relaxation at a dc field of 1000 Oe.From the experiment, we can draw a conclusion that the choice of Ln III is important for the SMM properties.H for complex 1 at 1.8-8 K (left).Plots of magnetization M vs. H/T for complex 1 at 1-7 T (right), Figure S12: Plots of isothermal magnetization M vs. H for complex 2 at 1.8-8 K (left).Plots of magnetization M vs. H/T for complex 2 at 1-7 T (right), Figure S13: Plots of isothermal magnetization M vs. H for complex 3 at 1.8-8 K (left).Plots of
), and the angles are shown in Table S2.The encapsulated Dy III ion and ring Fe III ions are further bridged through two oxygen atoms of -Obz groups with the bond lengths of Fe-O and Dy-O in the ranges 2.361(6)-2.475(5)and 1.901(6)-2.041(6)Å , and the angles of Fe−O−Dy in the range 119.2(2)-123.2(2)°,respectively.
, manifesting the antiferromagnetic coupling in the complexes.The fitting of the Curie-Weiss law for the high-temperature χMT values resulted in different θ values, with −131.96K, −59.83 K, −43.72 K, and −40.00K for complexes 1-4, respectively.

Figure 2 .
Figure 2. χ M T vs. T plots for complexes 1-4 in an applied 1000 Oe dc field.

Figure 3 .
Figure 3. (a) Temperature dependence of the in-phase (χ M ) of ac susceptibility signals for complex 4 measured under a 1000 Oe dc field.(b) Temperature dependence of the out-of phase (χ M ) of ac susceptibility signals for complex 4 measured under a 1000 Oe dc field.

Figure 3 .
Figure 3. (a) Temperature dependence of the in-phase (χ′M) of ac susceptibility signals for complex 4 measured under a 1000 Oe dc field.(b) Temperature dependence of the out-of phase (χ′′M) of ac susceptibility signals for complex 4 measured under a 1000 Oe dc field.

:
Distorted square-antiprismatic geometries around Dy1(a), Eu1(b), Gd1(c), Tb1(d), Figure S3: Fe 2p XP spectras of complexes 1 (a), 2 (b), 3 (c), 4 (d) in monolayers, Figure S4: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 1 measured under 2.0 Oe field with a 0 dc field, Figure S5: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 1 measured under 2.0 Oe field with a 2000 Oe dc field, Figure S6: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 2 measured under 2.0 Oe field with a 0 dc field, Figure S7: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 2 measured under 2.0 Oe field with a 2000 Oe dc field, Figure S8: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 3 measured under 2.0 Oe field with a 0 Oe dc field, Figure S9: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 3 measured under 2.0 Oe field with a 2000 Oe

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4352/8/5/229/s1,FigureS1: The experimental XRD pattern of samples and the simulated XRD pattern of single crystal X-ray diffraction data for complexes 1-4, FigureS2: Distorted square-antiprismatic geometries around Dy1(a), Eu1(b), Gd1(c), Tb1(d), Figure S3: Fe 2p XP spectras of complexes 1 (a), 2 (b), 3 (c), 4 (d) in monolayers, Figure S4: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 1 measured under 2.0 Oe field with a 0 dc field, Figure S5: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 1 measured under 2.0 Oe field with a 2000 Oe dc field, Figure S6: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 2 measured under 2.0 Oe field with a 0 dc field, Figure S7: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 2 measured under 2.0 Oe field with a 2000 Oe dc field, Figure S8: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 3 measured under 2.0 Oe field with a 0 Oe dc field, Figure S9: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 3 measured under 2.0 Oe field with a 2000 Oe dc field, Figure S10: Temperature dependence of the in-phase (χ'M) and out-of phase (χ''M) ac susceptibility signals of complex 4 measured under 2.0 Oe field with a 0 Oe dc field, Figure S11: Plots of isothermal magnetization M vs.

Table 1 .
Crystal data and structure refinement for complexes 1

Table 2 .
The deviation distance from Ln III ions to O ox MP and FeMP.

Table 3 .
Expected and measured χ M T values for 1-4.

Table 4 .
Measured ∆E eff /k B and τ 0 values for complex 4.