From Monomeric Species to One-Dimensional Chain: Enhancing Slow

Article pubs.acs.org/crystal

From Monomeric Species to One-Dimensional Chain: Enhancing Slow Magnetic Relaxation through Coupling Mononuclear Fragments in Ln-rad System Cun Li, Juan Sun, Meng Yang, Guifang Sun, Jianni Guo, Yue Ma, and Licun Li* Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry and Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: By reacting nitronyl nirtroxide radical NIT-Ph2OEt (NIT-Ph2OEt = 2-(2′-ethoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) with Ln(hfac)3(hfac = hexafluoroacetylacetonate), four new Ln−nitronyl nitroxide complexes, namely, mononuclear species [Ln(hfac)3(NIT-Ph2OEt)2](LnIII = Gd 1a, Tb 1b) and one-dimensional chains [Ln(hfac)3(NITPh2OEt)]n (LnIII = Gd 2a, Tb 2b), have been successfully obtained through controlling the reaction temperature and the ratio of radical ligand and Ln(hfac)3. DC magnetic susceptibilities indicate that the ferromagnetic couplings occur between the coordination radicals and the Ln(III) ions for four complexes. No nonzero out-of-phase signals are observed for mononuclear segment 1b, whereas the corresponding 1D chain 2b exhibits frequency-dependent out-of-phase signals indicating single-chain magnet behavior, which implies that the intrachain next-nearest-neighbor (NNN) LnLn magnetic interactions play a crucial role for enhancing slow magnetic relaxation in the chain. Moreover, Tb chain exhibits rare three-step field-induced metamagnetic behavior.

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INTRODUCTION

NO groups, which can be coordinated to two different metal ions. Moreover, strong magnetic coupling can be achieved in such a system due to the direct coordination of NO groups to metal ions.23,24 Accordingly, nitronyl nitoxide-anisotropic metal one-dimensional chains are appealing candidates for SCMs. SCM behavior has been observed in a few nitronyl nitroxideCoII/LnIII one-dimensional compounds25−35 including the first reported single-chain magnet, a Co(II)-nitronyl nitoxide chain.4 In such systems, a complete analysis of intrachain magnetic exchange couplings is essential to establish magneto-structural correlation, thus developing a rational strategy to increase blocking temperature (TB) of the system. For Ln-nitronyl nitoxide chains, the quantitative analysis of the magnetic exchange couplings is an arduous task because of the complexity of intrachain magnetic couplings with regard to the nearest-neighbor and next-nearest-neighbor magnetic

In the area of molecule-based magnetic materials, the research on one-dimensional compounds has been going through oscillations. In the early going, much attention was paid to the high-dimensional compounds so that high Tc molecular magnets could be obtained,1 albeit the first molecular magnets, [Fe(Me 5 C 5 ) 2 ] + [TCNE] − MeCN and [MnCu(pbaOH)(H2O)3], were based on one-dimensional supramolecular systems.2,3 Recently, one-dimensional systems have been the focus of molecular magnetic materials because slow magnetic relaxation behavior is observed in the Ising type 1D chain compounds.4−19 Such one-dimensional compounds have been named single-chain magnets (SCMs) owing to the observed magnetic hysteresis arising from slow dynamics of the magnetization of a pure one-dimensional character rather than 3D magnetic ordering. SCMs promise novel applications such as high-density information storage and quantum spintronic devices.20−22 Nitronyl nitroxides are excellent bridging ligands to construct one-dimensional systems because they possess two equivalent © XXXX American Chemical Society

Received: September 16, 2016 Revised: October 19, 2016 Published: November 14, 2016 A

DOI: 10.1021/acs.cgd.6b01369 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Summary of Crystal Data and Structure Refinements for Compounds 1 and 2 1a Gd Formula Formula weight Crystal system Space group a /Å b /Å c /Å α, deg β, deg γ, deg V /Å3 Z Dcalcd /g cm−3 F(000) θmin, θmax deg Reflections collected Unique reflns/ Rint GOF (F2) R1/wR2 [I > 2σ(I)] R1/wR2 (all data)

1b Tb

2a Gd

C45H45F18LnN4O12 1333.10 1134.78 Trigonal Trigonal P3121 P3121 18.065(6) 18.076(2) 18.065(6) 18.076(2) 15.748(3) 15.594(2) 90 90 90 90 120 120 4451(4) 4412.6(13) 3 3 1.492 1.507 1995 1998 1.30, 27.87 3.44, 27.55 38924 46881 7092/0.0486 6810/0.0363 1.007 1.006 0.0500/0.1336 0.0495/0.1421 0.0557/0.1442 0.0525/0.1444

2b Tb

C30H24LnF18N2O9 1055.76 1057.44 Monoclinic Monoclinic P21/c P21/c 22.297(2) 22.275(5) 16.6711(2) 16.7028(2) 23.000(2) 23.047(2) 90 90 116.697(2) 116.6010(1) 90 90 7638.0(12) 7667.1(12) 8 8 1.836 1.832 4128 4136 3.01, 27.52 3.01,27.54 75546 77383 35147/0.0317 35328/0.0291 1.028 0.972 0.0621/0.1565 0.0621/0.1689 0.0689/0.1612 0.0627/0.1694

Table 2. Selected Important Bond Lengths [Å] and Angles [°] for Complexes 1−2 Ln-O(rad) Ln-O(hfac) Ln-O-N O(rad)-Ln-O(rad)

1a Gd

1b Tb

2a Gd

2b Tb

2.324(4) 2.365(4)−2.388(4) 143.9(4) 140.6(2)

2.317(4) 2.359(4)−2.376(4) 144.4(4) 140.6(2)

2.312(6)−2.381(6) 2.319(7)−2.389(6) 134.4(5)−153.5(6) 138.4(2)−141.7(3)

2.334(5)−2.389(8) 2.328(6)−2.396(7) 134.8(5)−154.2(5) 138.0(2)−142.2(2)

KBr pellets. Magnetic measurements were performed on a Quantum Design SQUID VSM magnetometer by using the powder samples mixed with n-eicosane. The magnetic susceptibility data were corrected for all the constituent atoms using Pascal’s tables. Preparation of [Ln(hfac)3(NIT-Ph2OEt)2] (LnIII = Gd 1a, Tb 1b). Gd(hfac)3·2H2O or Tb(hfac)3·2H2O (0.1 mmol) was dissolved in heptane (25 mL) by heating, and then the resulted solution was refluxed for 3 h. The obtained solution was cooled down to 70 °C, and a solution of CH2Cl2 (5 mL) of NIT-Ph2OEt radical (0.2 mmol) was added. The resulting solution was kept at 70 °C for 10 min and then cooled it to ambient temperature. Deep red crystals were obtained after a week by slow evaporation of the filtrate at room temperature. Complex 1a: Yield: 61%. Anal. Calc. for C45H45F18GdN4O12 (%): C 40.54; H 3.40; N 4.20. Found: C 40.42, H 3.51, N 4.17. IR (KBr, cm−1): 2928 (m), 1657 (s), 1554 (m), 1457 (s), 1355 (s), 1290 (s), 1251 (s), 1208 (s), 1191 (m), 792 (m), 755 (s), 661 (s), 585 (s). Complex 1b: Yield: 51%. Anal. Calc. for C45H45F18TbN4O12 (%): C 40.50; H 3.40; N 4.20. Found: C 40.71, H 3.33, N 4.21. IR (KBr, cm−1): 2927 (m), 1657 (s), 1554 (m), 1456 (s), 1355 (s), 1291 (s), 1250 (s), 1207 (s), 1190 (m), 794 (m), 756 (s), 660 (s), 585 (s). Preparation of [Ln(hfac)3(NIT-Ph2OEt)]n (LnIII = Gd 2a, Tb 2b). Gd(hfac)3·2H2O or Tb(hfac)3·2H2O (0.2 mmol) in 30 mL heptane was heated to reflux for 3 h. When this solution was cooled to 90 °C, NIT-Ph2OEt radical (0.2 mmol) dissolved in CH2Cl2 (5 mL) was added. The resulting solution was mantained at 90 °C for 10 min and then cooled to room temperature. Three days later, slow evaporation of the filtrate gave single crystals as red blocks. Complex 2a: Yield: 47%. Anal. Calc. for C30H24GdF18N2O9(%): C 34.13; H 2.30; N 2.65. Found: C 34.28, H 2.05, N 2.84. IR (KBr, cm−1): 2923 (m), 1651 (s), 1530 (m), 1396 (s), 1344 (s), 1297 (s), 1253 (m), 1220 (m), 1134 (m), 797 (s), 754 (s), 661 (s), 586 (s). Complex 2b: Yield: 44%. Anal. Calc. for C30H24TbF18N2O9 (%): C 34.08; H 2.29; N 2.65. Found: C 34.13, H 2.41, N 2.67. IR (KBr, cm−1): 2922 (m), 1650 (s), 1532 (m), 1395 (s), 1346 (s), 1295 (s), 1254 (m), 1221 (m), 1135 (m), 796 (s), 758 (s), 663 (s), 587 (s).

interactions. Mononuclear fragments [Ln(hfac)3(NIT-R)2] can be regarded as the building units of those Ln-radical chains, which provide a unique opportunity for in-depth understanding of the effect of magnetic coupling on spin dynamics of magnetic chains.36 A combined study of Ln-radical chain and the corresponding building block will pave the way for the rational design of Ln-radical based SCMs. Herein, we prepared two series of interesting lanthanide-nitronyl nitroxide radical compounds by means of NIT-Ph2OEt radical. One is trispin mononuclear fragment [Ln(hfac)3(NIT-Ph2OEt)2](LnIII = Gd 1a, Tb 1b; hfac = hexafluoroacetylacetonate; NIT-Ph2OEt = 2(2′-ethoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3oxide), in which two radical ligands are coordinated to the Ln center as terminal ligands. Changing the ratio of radical ligand and Ln(hfac)3 and reaction temperature, the other series of Lnrad one-dimensional compounds, namely, [Ln(hfac)3(NITPh2OEt)]n (LnIII = Gd 2a, 2b), were obtained. Magnetic investigations show that no slow magnetic relaxation occurs in mononuclear fragment 1b, whereas the corresponding onedimensional chain 2b exhibits frequency-dependent out-ofphase signals, indicating the significance of intrachain nextnearest-neighbor LnLn magnetic couplings in enhancing the spin-reversal energy barrier of the 1D Ln-rad chains.

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EXPERIMENTAL SECTION

Materials and Measurements. The radical ligand NIT-Ph2OEt was synthesized according to previous reports in the literature.37 All starting chemicals and solvents used in the synthesis were obtained commercially and used as received without further purification. The elemental analyses (C, H, and N) were carried out on a Perkin−Elmer 240 elemental analyzer. Infrared spectra were measured in the 400− 4000 cm−1 region on a Bruker Tensor 27 Spectrophotometer using B



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Figure 1. (left) Crystal structure of complex 1b (H and F atoms are omitted for clarity) and (right) the coordination polyhedron of Tb(III) ion in 1b. X-ray Crystallography. Crystallographic data collections for compounds 1−2 were carried out on a Rigaku Saturn CCD diffractometer at 113 K by means of graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The absorption correction program SADABS38 was applied to correct the data. The structures of four compounds were obtained by direct methods and all non-H atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs.39,40 The hydrogen atoms were placed at calculated positions with fixed isotropic thermal parameters. Crystallographic data of four compounds are given in Table 1. Selected important bond lengths and angles are listed in Table 2.

Table 3. SHAPE Analysis for the Ln Coordination Spheres

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RESULTS AND DISCUSSION Synthesis. Mononuclear complexes [Ln(hfac) 3(NITPh2OEt)2] and one-dimensional chains [Ln(hfac)3(NITPh2OEt)]n have been successfully isolated by varying the experimental conditions. If the reaction temperature is not high (about 70 °C), mononuclear species are to be obtained in heptane. When the ratio of nitronyl ntroxide ligand/Ln(hfac)3 is 1:1 and the reaction solution of heptane is heated to 90 °C or higher, one-dimensional Ln-rad chains can be achieved. It is worth noting that even if the ratio of Ln(hfac)3/nitronyl ntroxide ligand is 1:1, only the monomer was isolated when the reaction temperature was low. This suggests that mononuclear fragments are thermodynamically favored over to the 1D Lnchains. The obtained mononuclear Tb complex and the corresponding 1D chain gave us the opportunity to study how the intrachain magnetic couplings affect slow magnetic relaxation of 1D Ln-chains. Description of the Crystal Structures. Single-crystal Xray diffraction analyses indicate that complexes 1a and 1b consist of discrete molecules while complexes 2a and 2b exhibit one-dimensional chain structure. Complexes 1a and 1b are isostructural and crystallize in the trigonal space group P3121. Two complexes possess mononuclear structure in which two NIT-Ph2OEt radical molecules are ligated to the lanthanide ion as monodenate ligands through the NO groups (Figures 1 and S1). The Ln center is surrounded by eight O atoms with six O atoms from three bidentate hfac ligands (Ln-O: 2.365(4)− 2.388(4) Å for 1a and 2.359(4)−2.376(4) Å for 1b) and two O atoms from two NO groups. Systematic analysis of the central ions using SHAPE software41,42 reveals that all of the eightcoordinated LnIII ions may be mostly taken as D2d triangular dodecahedron (Table 3). The Ln-Orad distance is 2.324(4) for 1a and 2.317(4) for 1b, which are consistent with those reported in related systems.28−35,43−46 The Ln−O−N−C torsion angle is 87.44° for 1a and 86.04° for 1b, which imply that the magnetic coupling between the Gd ion and the radical

compound

SAPR-8

TDD-8

JBTPR-8

BTPR-8

JSD-8

1a Gd 1b Tb 2a Gd1 2a Gd2 2a Gd3 2a Gd4 2b Tb1 2b Tb2 2b Tb3 2b Tb4

1.512 1.555 1.185 2.850 2.110 1.616 1.160 2.814 2.082 1.586

0.304 0.272 1.041 0.114 0.206 0.251 1.026 0.116 0.200 0.254

2.496 2.547 1.851 3.005 2.423 2.593 1.874 3.027 2.446 2.641

1.984 2.008 1.327 2.405 1.874 1.965 1.330 2.397 1.909 1.968

2.713 2.751 3.540 2.787 2.855 3.077 3.546 2.815 2.851 3.111

could be ferromagnetic.47,48 Packing arrangements of complexes 1a and 1b are shown in Figure S2−S3. The shortest Ln··· Ln contact is 10.943 Å for 1a and 10.926 Å for 1b, the closest uncoordinated NO···ON distance is 6.502 Å for 1a and 6.473 Å for 1b, respectively. Compounds 2a and 2b crystallize in the P21/c space group of the monoclinic system (Figures 2 and S4). The asymmetry unit contains four crystallographically independent Ln(hfac)3(NITPh2OEt) moieties which produce four different Ln-rad 1D chains in the crystal lattice. In the chain, each NIT-Ph2OEt radical acts as bridging ligand to connect two Ln(hfac)3 units via its two NO groups, leading to infinite one-dimensional chain. Each Ln ion is coordinated by two O atoms from two radical liagnds and six O atoms from three hfac ligands. Coordination geometries around the lanthanide ions were analyzed using SHAPE software. Shape analysis of the coordination sphere of the LnIII center indicates that Ln2, Ln3, and Ln4 are located in the distorted dodecahedral geometry (D2d), while Gd1 and Tb1 may be considered intermediate between square antiprism (D4d) and dodecahedron geomertry(Table 3, Figures S4 and S5). The Ln−Ohfac bond lengths are in the range of 2.319(7)−2.389(6) Å for 2a and 2.328(6)−2.396(7) Å for 2b, Ln−Orad bond lengths vary from 2.312(6) Å to 2.381(6) Å and 2.334(5) Å to 2.389(8) Å for 2a and 2b, respectively. The Ln−O−N−C torsion angles are in the range of 69.85−86.87° for 2a and 69.76−86.19° for 2b. The view of the chain compounds packing in 2a and 2b are given in Figures S6 and S7, respectively. The shortest intrachain Ln···Ln distances are 8.347 Å for 2a and 8.357 Å for 2b. The nearest interchain Ln···Ln contacts are found to be 11.405 and 11.344 Å for 2a and 2b, respectively. Magnetic Properties. Variable-temperature magnetic susceptibilities of four complexes have been studied with 1000 Oe applied magnetic field from 2.0 to 300 K. The χMT vs C



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Figure 2. (left) Chain structure of 2b and H and F atoms are not shown for the sake of clarity. (right) Core in 2b.

Figure 3. Plots of χMT versus T for complexes 1a (left) and 1b (right).

T plots of four compounds are shown in Figure 3 for complexes 1a and 1b and Figure 5 for complexes 2a and 2b. At room temperature, the χMT products are 8.95 cm3 K mol−1 for 1a and 12.07 cm3 K mol−1 for 1b, which are close to the expected values (8.63 cm3 K mol−1 for 1a, 12.57 cm3 K mol−1 for 1b) for one isolated LnIII ion (GdIII: 8S7/2, S = 7/2, L = 0, g = 2, C = 7.88 cm3 K mol−1, TbIII: 7F6, S = 3, L = 3, g = 3/2, C = 11.82 cm3 K mol−1) plus two S = 1/2 radicals. For complex 1a, with decreasing temperature, the χMT value increases slowly in a wide temperature range of 50−300 K. Afterward it increases more quickly to reach a maximum value of 10.96 cm3 mol−1 K at 3.0 K, then reduces to a value of 10.66 cm3 K mol−1 at 2.0 K. This behavior indicates that ferromagnetic interactions are present between the Gd(III) ion and the coordinated NO group. On the basis of the crystal structure, two kinds of magnetic exchange interactions may be expected, namely, the magnetic exchange coupling between the directly bound NO group and the GdIII ion (J1) and the magnetic interaction between two radical ligands through Gd(III) ion (J2) (Scheme 1). The magnetic data were fitted with the susceptibility eq 1 based on the spin Hamiltonian

χM =

Ng 2β 2 × 4kT ⎡ ⎢ 165 + 84 exp ⎢ ⎢ 5 + 4 exp ⎣

) + 35 exp( ) ⎤⎥ ⎥ ) + 3 exp( ) ⎥⎦

−9J1

−7J1 − 2J2

kT

kT

( ) + 84 exp( ( ) + 4 exp( −9J1

−7J1 − 2J2

kT

kT

−16J1 kT

−16J1 kT

(1)

χtotal = χM /[1 − (zJ ′χM /Ng 2β 2)]

The weak intermolecular magnetic interactions are considered by using molecular field approximation (zJ′). The best-fit parameters obtained are g = 2.03, J1 = 0.95 cm−1, J2 = −2.41 cm−1, zJ′ = −0.017 cm−1 with an agreement factor R = 1.47 × 10−4 (R = Σ[(χM)obs − (χM)calc]2/Σ[(χM)obs]2). The small positive J1 value indicates that there is weak ferromagnetic coupling between the GdIII ion and the NIT-Ph2OEt radical. The observed ferromagnetic Gd-NO interaction is in agreement with the expected result due to the large Gd−O−N−C torsion angle (87.44°).47,48 This ferromagnetic interaction can be ascribed to the results of electron transfer of unpaired electrons of the radical ligand into the empty 5d or 6s orbitals of the Gd(III) ion, which stabilizes the ground state with the higher spin multiplicity based on Hund’s rule.49,50 The negative J2 value indicates that important antiferromagnetic coupling exists between the two intramolecular radicals, which is comparable with the previously similar GdIII−radical complexes.51,52 The antiferromagnetic interaction between two terminal nitronyl nitroxides via the Gd ion can be explained based on the superexchange of two radicals through the empty 6s and 5d orbitals of Gd(III) ion.53 For 1b, upon cooling, the χMT stays practically constant until 100 K, and then rapidly drops to reach 11.35 cm3 K mol−1 at 10 K, mainly originating from the thermal depopulation of excited Stark sublevels of the Tb(III) ion. Below 10 K, the χMT value upturns and reaches a

̂ SRad1 ̂ ̂ SRad2 ̂ ) − 2J SRad1 ̂ SRad2 ̂ Ĥ = −2J1(SGd + SGd 2

Scheme 1. Magnetic Exchange Pathways in Complex 1a

D



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value of 11.4 cm3 K mol−1 at 6 K, and then drops dramatically to the minimum of 10.93 cm3 K mol−1 at 2 K. The increase in χMT at low temperature may be ascribed to the ferromagnetic interaction between the Tb(III) ion and the coordinated NO group of the organic radical, while the decrease of χMT at low temperature mainly arises from the intermolecular antiferromagnetic interaction. Furthermore, the field dependency of magnetization in the 0−70 kOe field range for complexes 1a and 1b have been determined at 2.0 K (Figures 4 and S8). The experimental

Figure 5. Plots of χMT versus T for complexes 2a and 2b.

neighbor (NNN) Gd−Gd and radical−radical magnetic interactions in present magnetic chain, in which NNN antiferromagnetic interactions dominate.53−55 For 2b, when the temperature decreases, the χMT value is almost a constant until 70 K, then the value of χMT rapidly decreases to reach a minimum value of 5.59 cm3 K mol−1 at 5.5 K. Upon further cooling, the χMT value increases to 6.14 cm3 K mol−1 at 2 K (Figure 5). The M vs H curves of 2a and 2b are shown in Figures S9 and 6, respectively. Complex 2a shows an increase in magnetization

Figure 4. Field dependence (H/kOe) of magnetization (M/Nβ) of 1a at 2.0 K. The red line represents the theoretical cure for the sum of the Brillouin function for noncoupled two radicals (S = 1/2) and one GdIII (S = 7/2) with g = 2 and the blue line represents the Brillouin function for S = 9/2 with g = 2.0.

magnetization of 1a increases rapidly in low field and reaches 9.2 Nβ at 7T, which is in line with the expected saturation value of 9.0 Nβ. As seen in Figure 4, for the low field, the calculated magnetization of 1a with the classical Brillouin function for uncoupled one S = 7/2 and two S = 1/2 spin centers is significantly lower than the experimental one, which is further suggestive of the ferromagnetic exchange interaction between the GdIII ion and the coordinated NIT-Ph2OEt radical. Moreover, the magnetization curve correlates well with the Brillouin function for S = 9/2 and g = 2.0, which indicates that complex 1a possesses an S = 9/2 spin ground state, resulting in the Gd−radical ferromagnetic interaction. Magnetization of complex 1b shows a sharp increase in low magnetic field upon increasing the magnetic field to reach a value of 7.77 Nβ at 70 kOe, which is less than the expected saturation value. This may be attributed to the presence of magnetic anisotropy in the system. The room-temperature χMT values of 8.46 and 12.35 cm3 K mol−1 for 2a and 2b, respectively, are in agreement with the expected values of 8.26 and 12.20 cm3 K mol−1 for noninteracting one LnIII (GdIII: 8S7/2, S = 7/2, L = 0, g = 2, C = 7.88 cm3 K mol−1, TbIII: 7F6, S = 3, L = 3, g = 3/2, C = 11.82 cm3 K mol−1) and one S = 1/2 radical. For 2a, while decreasing the temperature, the χMT value keeps almost constant until 95 K, below which it continuously decreases to reach a minimum of 3.00 cm3 K mol−1 at 2 K (Figure 5). The nearest-neighbor Gd−radical magnetic interaction is expected to be ferromagnetic owing to the big Gd−O−N−C torsion angles.47,48 The decrease of χMT value can be ascribed to the competition of ferromagnetic nearest-neighbor (NN) Gd− radical magnetic couplings and antiferromagnetic next-nearest-

Figure 6. M versus H plot at 2 K for complex 2b.

to 7.17 Nβ at 70 kOe with rising field, which is lower than the theoretical saturation value. However, this value is in line with the reported results.29 For 2b, the M vs H curve displays a characteristic S-shape, indicating metamagnetic property. The dM/dH curve demonstrates three-step field induced transition. The first critical field (H1) is at about 440 Oe, which might relate to overcoming the weak antiferromagnetic interactions between chains. The third inflection at around 20.3 kOe (H3) may be attributed to the spin−flip transition from intrachain antiferromagnetic to ferromagnetic interactions, which results in the spin toward the field direction.56 The second step at ca. H2 = 12.7 kOe is tentatively assigned to an intermediate state of spin reorientation in the chains for aligning with the field.57,58 M value of 2b reaches 5.90 Nβ at 70 kOe and this value is consistent with the expected value of 6 Nβ for a ferromagnetic rad-Tb(III) system assuming terbium ion with an effective spin of S = 1/2 with g// = 10 and g⊥= 0.59,60 Moreover, a small E



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Figure 7. Temperature-dependent ac magnetic susceptibility of 2b with an oscillation of 3 Oe in zero dc field.

hysteresis loop at 2.0 K has been observed with small coercive field of ca. 500 Oe (Figure S10). In order to investigate spin dynamic magnetic behaviors of 1b and 2b, ac magnetic susceptibility measurements were carried out in the temperature range of 2.0−10.0 K under a zero applied dc field with different frequencies. For complex 1b, unfortunately, no obvious frequency dependent out-of-phase signals are observed (Figure S11). For complex 2b, as shown in Figure 7, strong frequencydependent in-phase χ′ and out-of phase χ″ signals are observed. Moreover, upon increasing temperature, the peaks of the χ″ data shift to higher frequencies, thus clearly confirming slow relaxation of the magnetization. The values of the peak temperatures (Tf) of the in-phase signal (χM′) may be measured by employing a parameter φ = (ΔTf/Tf)/Δ(log f) = 0.26, which rules out the possibility of a spin-glass behavior (0.01 < φ < 0.08).61 The characteristic dynamic parameters obey the Arrhenius equation of τ = τ0 exp(Δτ/kT), in which τ0 is a pre-exponential factor and Δτ/kB is the effective anisotropy energy barrier for the relaxation process. Obtained best fitting values are τ0 = 9.69 × 10−10 s and Δτ/kB = 56.06 K (Figure 8). These values are fall well in the range for the previously reported Tb-radical chains of SCMs,28−35 indicating SCM behavior(Table 4). Furthermore, ac susceptibilities as a function of ac frequency were measured at the temperature of 3.0, 3.5, 4.0, and 4.3 K in a zero applied magnetic field. The obtained χ″ and χ′ data (Figure 9) were fitted to the generalized Debye model to

Table 4. Structural and Magnetic Data for Tb-Nitronyl Nitroxide Radical 1D Chain Compounds compound [Tb(hfac)3NITPhOPh]n [Tb(hfac)3NIT3BrPhOMe]n [Tb(hfac)3NIT-2Thien] [Tb(hfac)3(NIT3Brthien)]n [Tb(hfac)3(NITI)]n [Tb(hfac)3(NITPh2OEt)]n

energy barrier Δ [K]

relaxation time τ0 [S] −9

ref

Δ = 45 Δ = 58.75

τ0 = 9.6 × 10 τ0 = 2.25 × 10−7

29 33

Δ = 77.2 Δ = 56.8

τ0 = 1.9 × 10−9 τ0 = 1.1 × 10−8

31 35

Δ = 70.2 Δ = 56.06

τ0 = 7.09 × 10−10 τ0 = 9.69 × 10−10

34 this work

Figure 9. Cole−Cole plots of 2b. The red solid line represents the best fitting result obtained with the Debye model.

produce α values of 0.26 for 3.0 K, 0.48 for 3.5 K, 0.44 for 4.0 K, and 0.39 for 4.3 K, which are in the range for reported SCMs14−19 and suggest a distribution of relaxation times. The slightly larger α value might be attributed to the presence of four different Tb-rad chains in the structure. For the ferromagnetic Ising-like 1D chain, its susceptibility obeys the equation χT = Cexp(Δξ/kT); thus, the plot of ln(χT) versus T−1 will afford a straight line, and the slope directly corresponds to the exchange energy barrier. The plot of ln(χT) vs 1/T of complex 2b is shown in Figure 10 and a linear region is found between 3.0 and 5.0 K confirming the one-dimensional Ising-like character of 2b. The linear fitting of the data yields Δξ = 2.58 K. Noticeably, the Δξ value is significantly lower than Δτ obtained from ac measurements. This is not surprising because

Figure 8. Plot of ln(τ) versus T−1 fitting to the Arrhenius law for complex 2b. F



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behavior whereas the corresponding Tb mononuclear species does not show any visible out-of-phase component of the ac susceptibility owing to the missing NNN Tb−Tb interaction. Furthermore, Tb chain exhibits interesting three-step metamagnetism. This work represents an efficient approach to understanding how the magnetic exchange couplings influence the relaxation dynamics of the Ln-radical magnetic chains, which will provide the valuable information for the rational construction of SCMs based on lanthanides and radicals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01369. Tables of selected bond lengths and angles, crystal structures and packing diagrams, M versus H plots at 2.0K for 1b and 2a, hysteresis loop of 2b, ac magnetic data of 1b, and the plot of ln(χT) vs 1/T of 2a (PDF)

Figure 10. ln(χT) vs 1/T plot for 2b (the red solid line representing the linear fit).

the real system cannot be described by the simple Glauber model. The additional contribution associated with the relaxation of the single ion should be considered. For complex 2b, the magnetic relaxations are observed above 3.0 K, which is located in the “infinite-chain” regime. In this regime, the activation barrier should be given as Δτ = 2Δξ + ΔA,62,63 where ΔA is the anisotropy barrier arising from the single-ion anisotropy, leading to ΔA= 48.90 K. As seen, the spin dynamic behaviors of two Tb complexes are completely different: no slow relaxation of magnetization was founded in monomer, whereas 1D Tb chain displays clearly frequency-dependent out-of-phase signals. Structurally, the coordination spheres of Tb centers are almost identical in two complexes. However, the situation of magnetic exchange couplings is quite different. For mononuclear species, there exist the nearest-neighbor (NN) Tb−rad magnetic coupling and the next-nearest-neighbor (NNN) rad−rad magnetic interaction via Tb ion. DC magnetic data indicate that the nearest-neighbor (NN) Tb-rad exchange coupling is ferromagnetic, while the next-nearest-neighbor (NNN) rad−rad interaction is expected to be antiferromagnetic based on the above magnetic analysis of Gd-based mononuclear fragment. For the magnetic chain, the magnetic exchange interactions are more complicated. Except for the NN Tb−rad and NNN rad− rad interactions, there is another next-nearest-neighbor (NNN) interaction: Tb−Tb magnetic coupling through NIT moiety, which is antiferromagnetic in nature29 and dominates in the magnetic chain.56 Therefore, the enhancing energy barrier of Tb chain can be mainly attributed to the presence of the significant Tb−Tb interactions along the chain. It should be noted that the magnetic relaxation of lanthanide ion is very sensitive to the coordination environment. Based on the crystal structures, there exist slightly different structural parameters for Tb(III) ions in two compounds; however, the plot of ln(χT) versus T−1 indicates that Tb chain is an Ising-like 1D chain. Moreover, the obtained energy barrier and value of τ0 for the Tb chain compare well with those previously reported Tbradical chains.29,31,33−35 Thus, the magnetic relaxation behavior of the Tb chain should originate from the Ising-like 1D chain nature, not single ion.

Accession Codes

CCDC 1501249−1501252 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (No. 21471083) and MOE Innovation Team (IRT13022). REFERENCES

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CONCLUSION A new family of Ln-nitronyl nitroxide compounds including two Ln-rad magnetic chains and their corresponding monomers has been reported. Complex 2b exhibits the remarkable SCM G



Article

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