Two Lanthanide–nitronyl nitroxide radicals compounds with slow magnetic relaxation behavior

Journal of Molecular Structure 1081 (2015) 348–354

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Two Lanthanide–nitronyl nitroxide radicals compounds with slow magnetic relaxation behavior Chen-Xi Zhang a,⇑, Xue-Mei Qiao b, Yu-Kun Kong b, Bingwu Wang c, Yu-Ying Zhang d, Qing-Lun Wang d,⇑ a

College of Science, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China College of Material Science and Chemical Engineering, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China c College of Chemistry and Molecular Engineering, Beijing 100871, People’s Republic of China d Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Synthesize two mononuclear tri-spin

Two Lanthanide-radical compounds were synthesized. Complex 1 is dodecahedral geometry; while complex 2 is square antiprism geometry. Two complexes show intramolecular antiferromagnetic interactions and exhibit slow magnetic relaxation behavior. 6 6

111Hz 311Hz 511Hz 911Hz 1111Hz 1311Hz 1511Hz 1911Hz 2111Hz 2311Hz

-1

5 4

3

χ'M / cm Kmol

complexes based on lanthanide ions and nitronyl nitroxide radicals.  Complex 1 is dodecahedral (DD) geometry; while in complex 2, the geometry is square antiprism (SAPR).  Complexes 1 and 2 exhibit slow magnetic relaxation, suggesting single-molecule magnet behavior.

3

111Hz 511Hz 1111Hz 1511Hz 2111Hz 2311Hz

5 4 3 2

2

1

1 0 0.60

0 1.0

-1

0.30

Hdc= 2 KOe

Hdc= 0 Oe

0.6

3

χ''Μ / cm Kmol

0.8 0.45

0.4

0.15

0.2 0.0

0.00

-0.2 0

3

6

9

12

T/K

a r t i c l e

i n f o

Article history: Received 22 August 2014 Received in revised form 21 October 2014 Accepted 22 October 2014 Available online 31 October 2014 Keywords: Nitronyl nitroxide Lanthanides Crystal structure Magnetic properties

15

18

21

3

6

9

12

15

18

21

T/K

a b s t r a c t Two Lanthanide compounds with nitronyl nitroxide radicals [Dy(hfac)3(NITPh-p-Cl)20.5CH3(CH2)5CH3] (1) (hfac = hexafluoroacetylacetonate; NITPh-p-Cl = 40 -chlorphenyl-4,4,5,5-tetramethylimida-zoline1-oxyl-3-oxide, CH3(CH2)5CH3 is heptane as solvent molecule) and [Tb(hfac)3(NITPh-p-Cl)2] (2) were synthesized and structurally characterized. The X-ray crystallographic analyses show that the structures of the two compounds are similar and all consist of isolated molecules, in which central Ln(III) ions are coordinated to six oxygen atoms from three hexafluoroacetylacetonate ligands and two oxygen atoms from nitronyl nitroxide radicals. Ac magnetic susceptibility studies show complexes 1 and 2 exhibit slow magnetic relaxation, suggesting single-molecule magnet behavior. Ó 2014 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding authors. Tel.: +86 022 60600808; fax: +86 022 23502082 (C.-X. Zhang). E-mail addresses: [email protected] (C.-X. Zhang), [email protected] (B. Wang), [email protected] (Q.-L. Wang). http://dx.doi.org/10.1016/j.molstruc.2014.10.051 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

The study of single molecular magnets (SMMs) and single chain magnets (SCMs) is currently one of the special hot topics in the field of molecular magnetism and magnetic materials [1–3]. SMMs and SCMs exhibit slow magnetic relaxation, which arises from the combination of a large ground state spin S and a strong Ising-type

C.-X. Zhang et al. / Journal of Molecular Structure 1081 (2015) 348–354 Table 1 Crystal data and structure refinements for 1 and 2. Compound

1

2

Formula Fw T(K) Crystal syst Space group a (Å) b (Å) c (Å) a (deg) b (deg) c (deg) V (Å3) Z q [g cm3] l [mm1] h (deg) Index ranges

C44.50H43Cl2DyF18N4O10 1369.23 113(2) Triclinic  P1

C41H35Cl2TbF18N4O10 1315.55 113(2) Triclinic  P1

11.426(4) 12.764(4) 19.224(7) 91.263(6) 105.548(8) 95.243(7) 2686(16) 2 1.693 1.610 1.60–27.92 15 6 h 6 14 16 6 k 6 16 25 6 l 6 25 34511

11.450(13) 12.775(14) 19.256(18) 91.320(13) 105.277(7) 95.251(11) 2702.5(5) 2 1.617 8.406 2.38–72.39 14 6 h 6 13 15 6 k 6 15 23 6 l 6 23 30332

12789 12789/465/903 1.010 0.0388, 0.0736 0.0468, 0.0763

10086 10086/0/694 1.055 0.0511, 0.1325 0.0537, 0.1357

Reflns collected independent data/ restraints

2

GOF on F R1, wR2 [I > 2r(I)] R1, wR2 (all data)

anisotropy [4,5]. The strong magnetic anisotropy provides an energy barrier for the reversal of magnetization. Therefore, SMMs

349

and SCMs can be used as potential candidates in high-density magnetic memories, quantum computing devices and molecular spintronics [6]. Nitronyl nitroxides radicals have used as widespread ligands to synthesize molecular magnetic materials [7–9]. A large number of metal-radical complexes with various structures have been synthesized and magnetically characterized [10–12]. At the same time, lanthanide metal ions, especially heavy lanthanide ions, such as terbium(III) and dysprosium(III), have large magnetic anisotropies, which arise from the large, unquenched orbital angular momentum [13]. Therefore, lanthanide ions become good candidates for synthesizing SMMs and SCMs [14–16]. Especially since the first 4f–2p SMM [Dy(hfac)3NITpPy] [17] and SCM [Co(hfac)2 (NITPhOMe)] [18] was discovered, the ‘‘metal-radical’’ strategy of combining organic radicals with lanthanide ions has been particularly successful employed to construct SCMs and SMMs [19]. However, nitronyl nitroxide radicals are poorly donating ligands, the use of strongly electron-withdrawing coligands at the metal, like hexafluoroacetylacetonate(hfac) [20,21], is often required, such as Dy(hfac)3(NIT-3Brthien) [22], Ln(hfac)3(NITNapOMe)2 [23] and so on. To many lanthanide-nitronyl nitroxide radicals complexes, the slightly different ligand field can drastically affect the magnetic relaxation of the magnetization [24,25]. To develop the new magnetic coupling systems of 4f-radicals and better understand the nature of 4f–2p magnetic interaction, we use nitronyl nitroxide radicals and lanthanide ions to construct novel 4f–2p SMMs. Herein we report two new 4f–2p complexes: [Dy(hfac)3(NITPh-p-Cl)20.5CH3(CH2)5CH3] (1), (hfac = hexafluoroacetylacetonate; NITPh-p-Cl = 40 -chlorphenyl-4,4,5,5-tetramethylimida-zoline-1-oxyl-3-oxide) and [Tb(hfac)3(NITPh-p-Cl)2] (2).

Fig. 1. (a) The molecular structure of 2. Fluorine and hydrogen atoms are not shown for the sake of clarity. Thermal ellipsoids are scaled to enclose 50% probability. (b) The coordination geometry of Tb(III) ion in compound 2.

Fig. 2. (a) The molecular structure of 1. Fluorine and hydrogen atoms are not shown for the sake of clarity. Thermal ellipsoids are scaled to enclose 50% probability. (b) The core in compound 1.

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C.-X. Zhang et al. / Journal of Molecular Structure 1081 (2015) 348–354

Complexes 1 and 2 exhibit the presence of frequency-dependent signals of out-of phase of susceptibility at low temperature, which suggest single-molecular magnets behaviors. Experimental All reagents and solvents were purchased from commercial sources and used without purification. The reaction of Tb(hfac)3 2H2O, Dy(hfac)32H2O [26] with NITPh-p-Cl [9] in dichloromethane and n-heptane gave single crystal of the two complexes. Infrared spectra were recorded on a BRUKER FT-IR EQUINOX 55 using KBr pellets in the region 4000–400 cm1. The C, H, and N elemental analyses were carried out with a Perkin–Elmer 240C elemental analyzer. The magnetic data were recorded on a Quantum Design MPMS-7 SQUID magnetometer. The Variable-temperature magnetic susceptibilities of crystallite samples were measured in applied magnetic field of 1000 Oe to avoid a strong torque on the crystal. Diamagnetic corrections were made with Pascal’s constants for all the constituent atoms [27]. Two complexes were synthesized by the similar method. Therefore, the synthesis of compound 1 is detailed herein. 41 mg (0.05 mmol) of Dy(hfac)32H2O was dissolved in 15 ml boiling heptanes for azeotropically removing hydration water of molecules. Then the solution was cooled to 65 °C, and a solution of NITPh-pCl (30 mg, 0.1 mmol) in 4 ml CH2Cl2 was added. The resulting green solution was stirred for 5 min and cooled down to room temperature. The filtrate was allowed to stand for 2 days, and dark green crystals were collected. Complex 2 was prepared in a similar way to 1, using Tb(hfac)32H2O instead of Dy(hfac)32H2O. Anal. Calcd (1) for C44.50H43Cl2DyF18N4O10 (Yield: 0.026 g, 43%): C, 39.04%, H, 3.17%, N, 4.09%. Found: C, 39.51%; H, 3.07%, N, 4.08%. IR (KBr) v/ cm1: 1655 (vs), 1528 (w), 1387 (w), 1352 (w), 1255 (vs), 1199 (vs), 1095 (w), 795 (w), 661 (w). Anal. Calcd (2) for C41H35Cl2TbF18N4O10 (Yield: 0.03 g, 41%): C, 37.43%, H, 2.69%, N, 4.26%. Found: C, 37.86%; H, 2.73%, N, 4.18%. IR (KBr) v/cm1: 1654 (vs), 1598 (w), 1529 (w), 1398 (w), 1352 (w), 1255 (vs), 1198 (vs), 1096 (w), 795 (w), 661 (w). Crystal structure determination Crystals of 1 and 2 were mounted on glass fibers. Determination of the unit cell and data collection were performed with Mo Ka radiation (k = 0.71073 Å) on a Bruker SMART 1000 diffractometer equipped with a CCD camera. The x–u scan technique was employed. The structures were solved primarily by direct method and second by Fourier difference techniques and refined by the full-matrix least-squares method. The computations were

performed with the SHELXL-97 program [28,29]. Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter. A summary of the crystallographic data and structure refinement is given in Table 1, and selected bond distances and angles for 1 and 2 are listed in Supplementary material (Tables S1). Further details of the crystal structure determination have been deposited to the Cambridge Crystallographic Data Centre as supplementary publication. CCDC deposition numbers 957787 for 1 and 957788 for 2 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Results and discussion Crystal structures The crystal structures of complexes 1 and 2 are almost isomorphous and isostructural, except half heptanes as solve molecule in complex 1. Therefore, only the crystal structure of complex 2 is  space described here. Complex 2 crystallizes in the Triclinic P1 group. The structure of complex 2 is shown in Fig. 1. The Tb(III) ions are eight-coordinated in slightly distorted square antiprism geometry. Two oxygen atoms of the N–O group from nitronyl nitroxide radicals and six oxygen atoms from three different hfac anions are coordinated to the metal ions. The bond lengths of Tb(1)–O (radical) are 2.351(3) Å for Tb(1)–O(1) and 2.340(3) Å Tb(1)–O(3), while the bond lengths of Tb(1)–O (hfac) are in the range of 2.336(3)–2.398(3) Å. These bond lengths are comparable to the reported metal–nitronyl nitroxide complexes [30]. The angles between the N–O groups from two radical ligands and Tb(III) ion are: N(1)–O(1)–Tb(1) 139.080(15)°, N(3)–O(3)–Tb(1) 140.433(15)°, O(1)–Tb(1)–O(3) 137.08(9)°. The nitronyl nitroxide moiety O1–N1–C1–N2–O2 makes a dihedral angle of 31.353° with the plane of benzene ring. Complex 1 is isomorphic to complex 2 except for the substitution of Tb(III) with the Dy(III) ion, which makes the bond distances and angles vary a little (Fig. 2). The eight-coordinated geometry is mostly taken as the D2d-dodecahedron (DD), C2v-bicapped trigonal prism (TP) and D4d-square antiprism (SAP). Therefore, the semi-quantitative method of polytopal analysis is applied for complexes 1 and 2 [31,32]. The relevant dihedral angles calculated for two complexes were compared with the values for the ideal polyhedral (Table S2 and S3 in the Supporting Information). In complex 1, the minimum of the S is 3.43, thus, the most reasonable geometry of the Dy(III) ions in 1 is dodecahedral (DD). However, in complex 2, minimum

16 6

12

12 2

10

χM [cm3 mol -1]

4

χMT [cm3 K mol-1]

χM [cm3 mol -1]

14

10 3

8

8

0

0 6 0

100

200

300

6 0

100

200

T/K

T/K

(a)

(b)

Fig. 3. Experimental and calculated variations of vM (s) and vMT (r) vs. T plots for 1 (a) and 2 (b).

300

3 -1 χMT [cm K mol ]

6

351

C.-X. Zhang et al. / Journal of Molecular Structure 1081 (2015) 348–354 Table 2 Selected magnetic parameters for Ln(III) and nitroxide radical compounds. Molecular formula

Bond Lengths of the Ln–O (oxygen atoms from nitronyl nitroxide) (Å)

zJ0 (cm1)

Dy(hfac)3(NITNapOMe)2 Tb(hfac)3(NITNapOMe)2 Dy(hfac)3(NITPh-3-Br-4-OMe)2 Tb(hfac)3(NITPh-3-Br-4-OMe)2 Dy(hfac)3(NITPh-p-Cl)20.5CH3(CH2)5CH3 Tb(hfac)3(NITPh-p-Cl)2

2.322(4) 2.342(2) 2.308(2) 2.308(2) 2.318(2) 2.340(3)

0.04 0.26 0.09 0.07 0.019 0.016

of the S is 12.68, the most reasonable geometry of Tb(III) ions in 2 is square antiprism (SAPR). Static magnetic properties of 1 and 2 The temperature dependence of the magnetic susceptibilities of complexes 1 and 2 were measured for polycrystalline sample in the range 2–300 K range in an applied magnetic field of 1 kOe and the magnetic behaviors are shown in Fig. 3. At room temperature the value of vMT for complexes 1 and 2 are 14.85, 12.48 cm3 K mol1, respectively. These values are close to the expected values (1: 14.92; 2: 12.56 cm3 K mol1) for an uncoupled system for one Ln(III) ions (Dy(III): 6H15/2, S = 5/2, L = 5, g = 4/3; Tb(III): 7F6, S = 3, L = 3, g = 3/2) and organic radicals (S = 1/2). For complex 1, upon cooling, the vMT values maintain a constant behavior down to about 50 K, and then the value of vMT decrease gradually and reach a minimum of 10.99 cm3 K mol1. For complex 2, upon cooling, the values of vMT remain practically constant from room temperature to 50 K, and then the value of vMT rapidly increases to a maximum of 12.14 cm3 K mol1 at 9.5 K, and then the value of vMT decrease and reach a minimum of 11.68 cm3 K mol1 at 2 K. These could arise from a selective depopulation of the excited crystal field state and/or antiferromagnetic interaction between Ln(III) ion and radical. 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

References [9] [23] Our work

between Ln(III) and radicals, the magnetic susceptibility vtotal of the complex can be assumed as the sum of vLn of the isolated Dy(III) or Tb(III) ion and vRad of the radical (Eq. (1)). The vDy and vTb can be described as Eqs. (3) and (4), respectively. In the expression, D is the zero-field-splitting parameter, g is the Lande factor, k is the Boltzmann constant, b is the Bohr magneton constant, and N is Avogadro’s number. The zJ0 parameter based on the molecular field approximation in Eq. (5) 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 Eqs. (1)–(5) [33,34]. Giving the best fitting parameters for Dy(III) complex 1 are g = 1.33, D = 2.69  102 cm1, zJ0 = 1.89  102cm1, R = 3.27  105, and for Tb(III) complex 2, g = 1.46, D = 4.21  102 cm1, zJ0 = 1.63  102 cm1, 2 P R = 1.75  105 where R is defined as R ¼ ðvM Þobs  ðvM Þcalc = P 2 ðvM Þobs . In complex 2, The very small positive zJ0 values are indicative of very weak ferromagnetic interaction between Tb(III) ions and the coordinated nitronyl nitroxide, which is consistent with the reported heavy lanthanide–nitronyl nitroxide complexes [35,36].

vðtotalÞ ¼ vLn þ 2vRad vRad ¼

  Ng 2Rad b2 1 1 ðg Rad ¼ 2Þ þ1 3KT 2 2

     D  D  D  D  D  D D þ 169exp 169 þ 121exp 121 þ 81exp 81 þ 49exp 49 þ 25exp 25 þ 9exp 9 þ exp 4kT 4kT 4kT 4kT 4kT  4kT 4kT 225D 169D 121         D D D D D þ exp 49 þ exp 25 þ exp 9 þ exp 4kT exp 4kT þ exp 4kT þ exp 4kT D þ exp 81 4kT 4kT 4kT 4kT

ð1Þ

ð2Þ

225D

vDy ¼

Ng 2 b2 225exp 4kT

vTb ¼

 D  D  D  D  D   þ 25 exp 25 þ 16 exp 16 þ 9 exp 9 þ 4 exp 4 þ exp kTD 2Ng 2 b2 36 exp 36 36DkT 25DkT 16D kT 9D kT 4D kT D kT 2 exp kT þ 2 exp kT þ 2 exp kT þ 2 exp kT þ 2 exp kT þ 2 exp kT þ 1

4kT

1

ð3Þ

ð4Þ

2

Fig. 4. Spin density distribution maps of complexes 1 and 2 in the low-spin states (blue and red regions indicate positive and negative spin populations, respectively; the isodensity surface represented corresponds to a value of 0.002 e bohr3). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

352

vM ¼

C.-X. Zhang et al. / Journal of Molecular Structure 1081 (2015) 348–354

v

total  1  2ZJ0 =Ng 2 b2 vtotal

ð5Þ

Compared with other Ln(III)-radical complexes (Ln = Dy,Tb) in Table 2, all the Ln-O bond lengths of Ln(III) ion and radical are similar and value of zJ0 are small, which indicate very weak magnetic interaction between the lanthanide ions and the coordinated nitronyl nitroxide. In our work, the nitronyl nitroxide substituent group is halogen atom Cl with electron-withdrawing effect, however, in other work (Table 2), the nitronyl nitroxide substituent group is methoxyl group with electron-donating effect, the

111Hz 311Hz 511Hz 911Hz 1111Hz 1311Hz 1511Hz 1911Hz 2111Hz 2311Hz

8

χ' M /cm3 Kmol -1

strength of the ligand field around metal ion increases, which may have a strong effect on the magnetization [25], thus, The values of zJ0 are larger than that in our work. To obtain the isotropic exchange coupling constant J more accurately, Orca 3.0 calculations [37] were performed with the popular hybrid functional B3LYP proposed by Becke [38,39] and Lee [40]. Triple-f with one polarization function TZVP [41,42] basis set was used for all atoms, and the scalar relativistic treatment (ZORA) was used in all calculations. The large integration grid (grid = 6) was applied to Dy(III) and Tb(III) for ZORA calculations. Tight convergence criteria were selected to ensure that the results are well

6

4

111Hz 511Hz 1111Hz 1511Hz 2111Hz 2311Hz

5 4 3 2

2

1

0

0

χΜ'' / cm3 K mol-1

0.3 0.04

H dc =2 k Oe

Hdc =0 Oe

0.2 0.02 0.1

0.0

0.00 0

3

6

9

12

15

18

21 0

3

6

9

T/K

12

15

18

21

T/K

Fig. 5. Temperature dependence of the in-phase and out-of-phase components of the ac magnetic susceptibility for 1 in 0 (left) and 2 kOe (right) dc fields with an oscillation of 3 Oe.

6 111Hz 311Hz 511Hz 911Hz 1111Hz 1311Hz 1511Hz 1911Hz 2111Hz 2311Hz

6

χ'M / cm3 Kmol -1

5 4 3

111Hz 511Hz 1111Hz 1511Hz 2111Hz 2311Hz

5 4 3 2

2

1

1

0 1.0

0 0.60

χ''Μ / cm3Kmol-1

0.8 0.45

Hdc = 2 KOe

Hdc = 0 Oe

0.6

0.30

0.4 0.2

0.15

0.0

0.00

0

3

6

9

12

T/K

15

18

21

3

6

9

12

15

18

-0.2 21

T/K

Fig. 6. Temperature dependence of the in-phase and out-of-phase components of the ac magnetic susceptibility for 2 in 0 (left) and 2 kOe (right) dc fields with an oscillation of 3 Oe.

C.-X. Zhang et al. / Journal of Molecular Structure 1081 (2015) 348–354 8.0 7.5 7.0

ln (ι-1)

6.5 6.0 5.5 5.0 4.5 0.40

0.42

0.44

0.46 -1

T /K 1

0.48

0.50

-1

1

Fig. 7. Plot of ln(s ) vs. (T ) for complex 2 (h). The solid line is a least-squares fitting to the Arrhenius equation. The solid line is a least-squares fitting to the Arrhenius equation.

converged with respect to technical parameters. For each complex, we calculated the energies of two spin states: the high-spin state (SHS = Sradical + SLn + Sradical), and the low-spin state (in Fig. 4, flip the spins of two radicals, SLS = –Sradical + SLn–Sradical), we can obtain the LnIII-radical coupling constants J according to the spin Hamiltonian H = 2J(SradicalSLn + SLnSradical) in the absence of spin–orbital coupling. The equations to obtain J are as follows:

J¼

ELS  EHS 12

for 1

ð6Þ

J¼

ELS  EHS 14

for 2

ð7Þ

From Eqs. (6) and (7), small negative J values are obtained: 1.81 cm1 for compound 1 and 1.43 cm1 for compound 2. Compared the approximate treatment result of Eqs. (1)–(5), the calculations result are much larger, which may be the inevitable result of different treatment model in these lanthanide–radical system. However, both result of magnetic exchange coupling constant are small, which indicated very weak antiferromagnetic interaction between the paramagnetic species (Ln(III) and radicals) in these two 4f–2p tri-spin systems. The small exchange coupling constant is reasonable due to the completed 5s and 5p subshells shielding 4f electrons. Dynamic magnetic properties for 1 and 2 Thermal ac susceptibility measurements on complexes 1 and 2 were performed with an ac field of 3 Oe to probe the presence of slow relaxation of magnetization. Complexes 1 and 2 all show the SMM behavior under zero dc fields, as indicated by the temperature dependence of the out-of-phase ac susceptibility. However, no maximum in out-of-phase component (v00 ) of ac susceptibility until 1.8 K (Figs. 5 and 6), which may be attributed to the fast quantum tunneling relaxation induced by the large transverse anisotropy at local Dy(III) or Tb(III) sites. To reduce quantum tunneling of magnetization (QTM), the dc fields of 2000 Oe were applied in compound 1 and 2. The application of dc field has a substantial influence on their dynamic behavior of magnetization, as seen in ac susceptibility signals. Compound 1 and 2 shows the clearly frequency-dependent signals; all the maximum of in-phase component and some outof-phase component (v00 ) of ac susceptibility in complexes 2 were observed at low temperature, indicating the field-induced slow magnetic relaxation behavior. The relaxation time s data derived

353

from the analysis of the frequency dependence of the v00M peaks through Arrhenius law (s = s0exp(Ueff/kBT)) permits estimation of magnetization-relaxation parameters,[43–46] and best fitting (Fig. 7) affords the parameter values: the pre-exponential factor s0 = 8.46  1010 s and the energy barriers for the relaxation of the magnetization Ueff/kB = 32.38 K (R = 0.99) for compound 2. The obtained small s0 values are consistent with those reported for similar SMMs [24,25]. The origin of the mechanism of SMM behavior in lanthanide-containing complexes is different from that of the 3d metal clusters [47]. The SMM behavior for complexes 1 and 2 may be mainly ascribed to its less symmetrical ligand field splitting pattern [24]. For these two complexes, different magnetic behaviors are observed: Complex 2 showed clear frequency-dependence and peaks in v0M vs. T and v00M vs. T plots at static field, while 1 did not show peaks in v00M vs. T plots. The coordination geometry of the lanthanide ions may be the vital factor of the different magnetic behaviors. In complex 1, the most reasonable geometry is dodecahedral (DD); while in complex 2, the geometry is square antiprism (SAPR). In fact, the most common polyhedron for lanthanide mononuclear SMMs is D4d-square antiprism geometry [24,48– 51]. In these mononuclear SMMs, the magnetic anisotropy required for observing slow relaxation of the magnetization arises from the interaction between a single lanthanide ion and its ligand field (LF) which creates a strong preferential orientation of the magnetic moment. Conclusion In conclusion, we report two new complexes based on rareearth radicals and lanthanide ions. The results show that these complexes 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 temperature dependencies of magnetic susceptibilities for two complexes are investigated. Furthermore, Complex 2 showed clear frequency-dependence and peaks in v0M vs. T and v00M vs. T plots at static field, while 1 did not show peaks in v00M vs. T plots. The frequency dependence of the ac susceptibility justifies that complex 2 exhibits SMM behavior. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20771081, 21071085, 21371104, 90922032) and Tianjin Natural Science Foundation (No. 11JCYBJC03500). Appendix A. Supplementary material Tables S1–S3. CCDC ID: 957787 and 957788 contain the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.molstruc.2014.10.051. References [1] D.N. Woodruff, R.E.P. Winpenny, R.A. Layfield, Chem. Rev. 113 (2013) 5110. [2] R. Vincent, S. Klyatskaya, M. Ruben, W. Wernsdorfer, F. Balestro, Nature 488 (2012) 357. [3] D.C.M. Yamanouchi, F. Matsukura, H. Ohno, Nature 428 (2004) 539. [4] E. Coronado, D. Chiba, A. Recuenco, A. Tarazón, F.M. Romero, A. Camón, F. Luis, Inorg. Chem. 50 (2011) 7370. [5] M.N. Leuenberger, D. Loss, Nature 410 (2001) 789. [6] P. Hu, M. Zhu, X.L. Mei, H.X. Tian, Y. Ma, L.C. Li, D.Z. Liao, Dalton Trans. 41 (2012) 14651. [7] H. Nagashima, H. Inoue, N. Yoshioka, J. Phys. Chem. 108 (2004) 6144.

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