Tuning the Origin of Magnetic Relaxation by Substituting the 3d or

Oct 16, 2015 - The origin of the magnetic relaxation behavior was tuned by substituting the 3d or rare-earth ions into three isostructural cyano-bridg...
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Tuning the Origin of Magnetic Relaxation by Substituting the 3d or Rare-Earth Ions into Three Isostructural Cyano-Bridged 3d−4f Heterodinuclear Compounds Yan Zhang,† Zhen Guo,† Shuang Xie,† Hui-Li Li,† Wen-Hua Zhu,*,† Li Liu,† Xun-Qing Dong,† Wei-Xun He,† Jin-Chao Ren,† Ling-Zhi Liu,*,‡ and Annie K. Powell§ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, P. R. China ‡ College of Science, Huazhong Agricultural University, Wuhan 430070, P. R. China § Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, Karlsruhe 76131, Germany S Supporting Information *

ABSTRACT: Three isostructural cyano-bridged 3d−4f compounds, [YFe(CN)6(hep)2(H2O)4] (1), [DyFe(CN)6(hep)2(H2O)4] (2), and [DyCo(CN)6(hep)2(H2O)4] (3), were successfully assembled by site-targeted substitution of the 3d or rare-earth ions. All compounds have been structurally characterized to display slightly distorted pentagonal-bipyramidal local coordination geometry around the rare-earth ions. Magnetic analyses revealed negligible magnetic coupling in compound 1, antiferromagnetic intradimer interaction in 2, and weak ferromagnetic coupling through dipolar−dipolar interaction in 3. Under an applied direct-current (dc) field, 1 (Hdc = 2.5 kOe, τ0 = 1.3 × 10−7 s, and Ueff/kB = 23 K) and 3 (Hdc = 2.0 kOe, τ0 = 7.1 × 10−11 s, and Ueff/kB = 63 K) respectively indicated magnetic relaxation behavior based on a single [FeIII]LS ion and a DyIII ion; nevertheless, 2 (Hdc = 2.0 kOe, τ0 = 9.7 × 10−8 s, and Ueff/kB = 23 K) appeared to be a single-molecule magnet based on a cyano-bridged DyFe dimer. Compound 1, which can be regarded as a single-ion magnet of the [FeIII]LS ion linked to a diamagnetic YIII ion in a cyano-bridged heterodimer, represents one of the rarely investigated examples based on a single FeIII ion explored in magnetic relaxation behavior. It demonstrated that the introduction of intradimer magnetic interaction of 2 through a cyano bridge between DyIII and [FeIII]LS ions negatively affects the energy barrier and χ″(T) peak temperature compared to 3.

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INTRODUCTION With the rapid growth of the demand of information storage behind the background of information explosion, new materials have always been sought by chemists, physicists, and material scientists. Single-molecule magnets (SMMs) have been pursued as potential materials for high-density information storage, quantum computing, and molecular spintronics in the last 3 decades.1 SMMs exhibit magnetic bistability at one uniform molecule level in homogeneous solids and interesting magnetic phenomenon such as magnetic hysteresis, slow magnetic relaxation, and quantum tunneling of magnetization (QTM).2 The development track along SMMs and single-chain magnets (SCMs) containing transition-metal ions,3 single-ion magnets (SIMs) involving single 4f or 3d ions,4 to 3d−4f SMMs incorporating 3d and 4f ions simultaneously5 illustrates that there is an urgent need to acquire SMMs with high blocking temperature, as well as a high-spin ground state (S) and negative uniaxial anisotropy (D), to achieve a high energy barrier (Ueff/kB). To utilize the high magnetic moment and large intrinsic single-ion anisotropy of 4f ions and to elucidate the effect of magnetic interaction between 3d and 4f ions on the SMM © 2015 American Chemical Society

properties, most efforts have been focused on metal−oxo 3d− 4f clusters.5 Because of the specific affinities of 4f ions to oxygen donor atoms, several successful synthetic strategies toward the combination of 3d and 4f ions into heterometallic 3d−4f SMMs have been widely adopted involving the design of a variety of organic ligands, such as Schiff-base ligands, oximes, pyridonates, amino acids, and alkylolamines, with different kinds of pockets for the coordination of both 3d and 4f ions, or utilizing the coligands to assist in the self-assembly process.5b On the other hand, for the last 2 decades, the incorporation of 3d and 4f ions in cyano-bridged systems has been pursued for high-dimensional high-TC molecule magnets,6 low-dimensional compounds for the study of magnetostructural correlations,7 and heterotrimetallic systems.8,9b−d,g In contrast, the association of 3d and 4f ions in cyano-bridged lowdimensional systems for the construction of SMMs, SCMs, or SIMs has been less investigated.9 The design of lowdimensional 3d−4f cyano-bridged systems involves the choice of building blocks [M(CN)6]3− (M = Fe, Co, Cr, Mn, etc.), the Received: July 24, 2015 Published: October 16, 2015 10316

DOI: 10.1021/acs.inorgchem.5b01763 Inorg. Chem. 2015, 54, 10316−10322

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performed by the SADABS method.13 The structures were solved by direct methods of the SHELXS-97 program and refined by the fullmatrix least-squares techniques based on F2 using the SHELXL-97 program.14a,b All of the ordered non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were introduced in calculated positions and refined with isotropic thermal parameters and a fixed geometry riding on their parent atoms, while the coordinates of hydroxyl hydrogen atoms were located from the difference Fourier map and then constrained to ride on their parent atom (e.g., O−H 0.96 Å).14c,d The crystallographic data and structural refinement details of 1−3 are given in Table 1. The selected bond lengths and angles are listed in Tables S1, S3, and S5. CCDC 1414271−1414273 are given in the Supporting Information.

preparation of partially blocked precursor [M(L)(CN)x](x+1−m)− (L = chelate site-blocking ligand), and the change of the secondary blocking ligand. It is easy to implement tailoring on the 3d metallocyanate precursor, to control the steric hindrance of the ancillary ligands, and to tune the magnetic properties by site-targeted substitution of the 3d or 4f ions. Therefore, one needs to design cyano-bridged 3d−4f model compounds to study the SMM properties and to determine the effect of magnetic interactions via a cyano bridge on the magnetic relaxation behavior, despite the possible weakening of the SMM properties by the strong ligand field provided by cyanide ligands. In this Article, three isostructural cyano-bridged 3d−4f heterodinuclear RE−M (RE = rare earth ions; M = 3d metal ions) compounds, YFe (1), DyFe (2), and DyCo (3), were successfully assembled under ambient conditions by the intentional substitution of 3d or rare-earth ions, in turn, for an exploration of the influence on the magnetic relaxation properties. The diverse origin of the magnetic relaxation behavior was discovered in three compounds under an external field. The relaxation parameters of DyFe (2) and DyCo (3) were compared to elaborate the effect of the intradimer DyFe magnetic interaction on the relaxation properties. It is worth noting that the YFe compound 1, according to an extensive literature survey of the SIMs based on transition-metal ions such as MnIII, FeI, FeII, FeIII, CoII, and NiI,10 represents one of the extremely rarely investigated examples of the SIMs based on a FeIII ion [including the different spin states of low spin (LS), isotropic high spin (HS), and intermediate spin].10f,g,11

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Table 1. Crystallographic Data and Structural Refinements Parameters for 1−3 formula Mr T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) μ(Mo Kα) (mm−1) F(000) no. of reflns collected no. of uniq reflns Rint no. of param R1 [I ≥ 2σ(I)] wR2 (all data) GOF

EXPERIMENTAL SECTION

General Procedures. Unless otherwise stated, all chemicals and solvents were of analytical reagent grade and were used as purchased without further purification. All reactions were carried out under aerobic conditions. Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario Micro Cube elemental analyzer (Elementar Aanlysensysteme GmbH, Germany). IR spectra (4000− 400 cm−1) on powered samples were recorded on a PerkinElmer Spectrum One spectrophotometer using KBr pellets. Powder X-ray diffraction (PXRD) data for the as-prepared samples were collected on a D8 ADVANCE (Bruker AXS, Germany) diffractometer at room temperature using Cu Kα radiation. Caution! Cyanides are potentially poisonous complexes. Suitable precautions should be taken when handling them. It is of the utmost importance that all preparations be performed and stored in well-ventilated areas. Static magnetic measurements including temperature-dependent magnetic susceptibility in the range 2−300 K at 1000 Oe, fielddependent magnetization of 1−3, and hysteresis loop of 2 at 2 K were carried out on a Quantum Design MPMS-XL SQUID magnetometer. Alternating-current (ac) susceptibilities of 1 and 2 were measured using a Quantum Design PPMS magnetometer with an oscillating field of 3 Oe and ac frequencies in the range of 100−10000 Hz. The ac susceptibility data of 3 were collected with the use of a Quantum Design MPMS-XL SQUID magnetometer with an oscillating field of 3 Oe and ac frequencies ranging from 100 to 1500 Hz. All of the magnetic measurements were performed on polycrystalline samples, tightly packed, and sealed with a capsule to avoid the anisotropic orientation. Diamagnetic corrections were made with Pascal’s constants for all of the constituent atoms.12 X-ray Crystallography. Determination of the unit cell and data collection for complexes 1−3 at 298 K were performed on a Bruker Smart APEX II CCD area detector diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). All of the diffraction data were collected at room temperature and corrected for Lorentz and polarization effects. Adsorption corrections were

1

2

3

C18H30N8O8YFe 631.26 298 monoclinic C2/c

C18H30N8O8DyFe 704.84 298 monoclinic C2/c

C18H30N8O8DyCo 707.92 298 monoclinic C2/c

14.117(6) 12.865(6) 15.180(6) 90 109.588(7) 90 2597.4(2) 4 1.614

14.201(1) 12.949(1) 15.294(1) 90 109.561(1) 90 2650.0(4) 4 1.767

14.174(1) 12.908(1) 15.262(1) 90 109.400(2) 90 2633.8(2) 4 1.785

2.839

3.401

3.501

1292 8113

1400 9822

1404 7844

3119

3322

2608

0.0263 187

0.0223 187

0.0263 187

0.0290

0.0181

0.0288

0.0754

0.0469

0.0598

1.037

0.917

1.092

Syntheses of 1−3. A similar procedure was employed to prepare compounds 1−3. Synthesis of YFe (1). To a solution of Y(NO3)3·6H2O (0.0958 g, 0.25 mmol) and 1-(2-hydroxyethyl)-2-pyrrolidinone (hep; 0.0646 g, 0.5 mmol) in water (5 mL) was added dropwise a solution of K3[Fe(CN)6] (0.0823 g, 0.25 mmol) in water (5 mL). The solution was left undisturbed in air, and yellow crystals suited for X-ray diffraction were obtained after 2 weeks. The yield (0.0635 g) was about 40.2% based on the Y III ion. Elem anal. Calcd for C18H30N8O8YFe: C, 34.25; H, 4.79; N, 17.75. Found: C, 33.92; H, 4.66; N, 17.64. IR peaks (KBr, cm−1): 3507 (s), 3163 (s), 2894 (s), 2378 (w), 2133 (vs, νCN), 2119 (vs, νCN), 1650 (vs), 1508 (s), 1475 (m), 1434 (m), 1421 (m), 1386 (m), 1358 (m), 1324 (m), 1315 (m), 1294 (m), 1274 (m), 1249 (w), 1226 (w), 1154 (vw), 1093 (m), 1059 (s), 1031 (m), 998 (w), 930 (vw), 864 (m), 815 (s), 755 (s), 666 (s), 579 (m), 531 (w). Synthesis of DyFe (2). Compound 2 was synthesized in the same manner as that for 1 but using Dy(NO3)3·6H2O (0.1142 g, 0.25 10317

DOI: 10.1021/acs.inorgchem.5b01763 Inorg. Chem. 2015, 54, 10316−10322

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Inorganic Chemistry mmol) instead of Y(NO3)3·6H2O. After 3 days, yellow block crystals formed from the resulting solution. The yield (0.0705 g) was about 40.0% based on the DyIII ion. Elem anal. Calcd for C18H30N8O8DyFe: C, 30.77; H, 4.30; N, 15.95. Found: C, 30.77; H, 4.29; N, 15.94. IR peaks (KBr, cm−1): 3639 (vw), 3514 (vw), 2991 (s), 2890 (s), 2831 (s), 2396 (w), 2131 (vs, νCN), 2119 (vs, νCN), 1653 (vs), 1506 (vs), 1475 (m), 1434 (m), 1421 (m), 1382 (m), 1358 (m), 1323 (m), 1315 (m), 1294 (m), 1274 (m), 1249 (w), 1226 (w), 1153 (vw), 1093 (w), 1076 (m), 1059 (m), 1031 (w), 998 (w), 930 (vw), 863 (s), 810 (s), 754 (s), 665 (s), 582 (m), 522 (w). Synthesis of DyCo (3). Compound 3 was obtained using the same procedure as that for 1 except that K3[Fe(CN)6] (0.0823 g, 0.25 mmol) and Y(NO3)3·6H2O were substituted by K3[Co(CN)6] (0.0831 g, 0.25 mmol) and Dy(NO3)3·6H2O (0.1142 g, 0.25 mmol), respectively. After 3 days, colorless block crystals were collected from the solution. The yield (0.1098 g) was about 62.0% based on the DyIII ion. Elem anal. Calcd for C18H30N8O8DyCo: C, 30.54; H, 4.27; N, 15.83. Found: C, 30.53; H, 4.22; N, 15.78. IR peaks (KBr, cm−1): 3509 (s), 3153 (br), 2895 (s), 2396 (w), 2143 (vs, νCN), 2129 (vs, νCN), 1634 (vs), 1508 (s), 1475 (m), 1434 (m), 1422 (m), 1385 (m), 1358 (m), 1324 (m), 1315 (m), 1294 (m), 1274 (m), 1249 (w), 1227 (w), 1154 (vw), 1093 (m), 1060 (s), 1031 (m), 998 (w), 930 (vw), 864 (m), 815 (s), 755 (s), 664 (s), 581 (m), 532 (w).

O2#1 = 172.14(9)° for 2 and ∠O2−Dy1−O2#1 = 171.78(16)° for 3), as listed in Tables S1, S3, and S5. The coordination geometry around the rare-earth ions was further analyzed using SHAPE 2.1,15 confirming only slight distortion from the pentagonal-bipyramidal geometry (D5h, with minimum continuous shaped measures (CShM) values of 0.225, 0.222, and 0.253 respectively for 1−3) with a N1O6 donor set (see Table S7 for the complete results of systematic geometric analysis). The RE−O bond distances range from 2.225(2) to 2.310(2) Å (1), from 2.2503(16) to 2.3410(16) Å (2), and from 2.256(3) to 2.346(3) Å (3). The RE−N bond lengths of 1−3 are 2.434(3), 2.454(3), and 2.471(5) Å, which are in agreement with the reported cyano-bridged lanthanide complexes.6−9 The M−C bond lengths vary in the ranges 1.915(3)−1.931(3) Å (1), 1.923(3)−1.943(3) Å (2), and 1.884(5)−1.900(4) Å (3), which results in distortion of the ideal octahedral geometry around the 3d metal ion center. The bond angles N1−C1−M1, N4−C4−M1, C1−N1−RE1 (M = Fe, Fe, Co; RE = Y, Dy, and Dy for 1−3) are all 180.0°, and the M1−C1−N1−RE1 linkages are in very strict linearity, which may be significant for magnetic research. To the best our knowledge, such strict linearity is very rare in 3d−4f cyano-bridged systems, a majority of which deviate from linearity;16 for instance, Fe1−C19−N4 and C19− N4−Dy1 angles are 173.8(2)° and 165.1(2)° for a reported one-dimensional [DyIIIFeIII] chain compound.16c The 3d ions are in a slightly distorted octahedral surrounding. Especially for 1, the hardly any distortion from the octahedral polyhedron resulting from deviation of C2−Fe1−C2#1 [178.52(12)°] and C3−Fe1−C3#1 [175.58(13)°] from 180.0°, together with the axial elongation of Fe1−C1 due to coordination, is the main origin of the anisotropy of the LS FeIII ion, which is related to the magnetic properties discussed below. The values of the intradimer distances Fe1···Y1, Fe1···Dy1, and Co1···Dy1 for compounds 1−3, respectively, are quasiidentical [5.4970(24) Å (1), 5.5338(5) Å (2), and 5.5124(3) Å (3)]. The packing pattern is displayed in Figure S1, with the shortest interdimer M···M (Figure S2, left) and RE···RE (Figure S2, right) metal separations being respectively 8.4564(26) Å (1), 8.5167(6) Å (2), and 8.5169(3) Å (3) and 8.6503(25) Å (1), 8.7106(6) Å (2), and 8.7052(3) Å (3) along the a + c direction. As listed in Tables S2, S4, and S6, there are extensive hydrogen bonds between the oxygen atoms of the coordination water molecules [O2−H21···N3#2, O2− H22···N2#3, O3−H32···N4#5 (#2, −x − 1/2, y − 1/2, −z + 3/2; #3, x, −y + 2, z + 1/2; #5, x, y − 1, z)], the oxygen atoms of the hydroxyl group from the hep ligand [O4−H4···N2#6 (#6, −x, −y + 2, −z + 1)] with the nitrogen atoms of the [M(CN)6]3− entity from the neighboring dimer, and the oxygen atoms of the coordination water with the oxygen atoms of the hydroxyl group from the hep ligand [O3−H31···O4#4 (#4, −x − 1/2, −y + 3/2, −z + 1)]. However, as far as the shortest interdimer M··· M (Figure S2, left) and RE···RE (Figure S2, right) metal separations are concerned, the metal ions between the dimers are relatively well separated in spite of the hydrogen-bonding network. Also, the magnetic interactions between the dimers may mainly come from dipolar−dipolar interactions. Magnetic Properties. The phase purities of the asprepared samples were indicated by the good agreement of the PXRD patterns with the corresponding ones simulated by single-crystal structure data (Figures S3−S5). The magnetic susceptibility data for compounds 1−3 were measured in the range of 2−300 K under a 1 kOe external magnetic field, as shown in Figure 2. The magnetic susceptibility, χM, stands for

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RESULTS AND DISCUSSION Crystal Structures. Single-crystal X-ray diffraction indicates that compounds 1−3 (Figure 1, left) are isostructural and

Figure 1. View of the heterodinuclear structure of compound 1 (left) and the coordination geometry around the YIII ion of compound 1 (right).

crystallize in the monoclinic space group C2/c, as listed in Table 1. Therefore, the overall structure of compound 1 will be described as a representative, with the other two compounds as comparisons for the structural parameters. Selected bond lengths and angles are given in Tables S1, S3, and S5. As depicted in Figure 1 (left), the [Fe(CN)6]3− entity acts as a bridging ligand toward a YIII ion, affording a heterodinuclear structure. The asymmetric unit for 1 contains 0.5 crystallographically independent YIII ion, 0.5 [Fe(CN)6]3−, one hep ligand, and two coordinated water molecules. The coordination environment of the YIII ion consists of one nitrogen atom from a [Fe(CN)6]3−, two oxygen atoms from the carbonyl group of the organic ligands (hep), and four oxygen atoms from water molecules, leading to a slightly distorted seven-coordinated pentagonal-bipyramidal geometry (Figure 1, right). The nearly coplanar coordination atoms O1, O1#1 (#1, −x, y, −z + 3/2), O3, O3#1, and N1 lie in the equatorial plane, with the dihedral angle of 4.10° between the planes Y1−O1−O1#1−N1 and Y1−O3−O3#1 (4.24° for 2 and 4.81° for 3), and O2 and O2#1 reside on the apical positions of the pentagonal bipyramid, with the bond angle O2−Y1−O2#1 being 172.39(9)° (∠O2−Dy1− 10318

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diamagnetic nature of the CoIII ion in the structure and the high moments of the DyIII ions with the shortest interdimer Dy···Dy (Figure S2, right) separation of 8.7052(3) Å.19,20 The linear fit of χM−1 versus T with the Curie−Weiss law above 20 K educes C = 13.51 cm3 mol−1 K and θ = −2.43 K (Figure S9). The negative θ value and decrease of the χMT value with decreasing temperature may be ascribed to the large magnetic anisotropy and progressive thermal depopulation of the excited Stark sublevels of the 6H15/2 state of the DyIII ion. To examine the magnetic nature of the intradimer DyIII and FeIII ions in 2, a typical empirical approach was attempted involving a comparison with two other isomorphous compounds, 1 and 3, with one spin carrier substituted by a diamagnetic analogue to provide comparable ligand-field effects.21 In the high-temperature range, the ΔχMT = χMT(2) − χMT(3) curve basically follows the χMT(1) curve. Below approximately 90 K, ΔχMT deviates from the χMT(1) curve and decreases rapidly to less than zero, which indicates dominant antiferromagnetic interactions in 2. According to the above magnetic analysis, however, because of the high moments of the DyIII ions and not so large interdimer separations, nonnegligible interdimer ferromagnetic interactions occur in 3. In view of the similar ligand fields between 2 and 3, we can determine the magnetic nature of the intradimer DyIII and FeIII ions in 2 to be antiferromagnetic. The field dependence of magnetization for compounds 1−3 has been determined at 2.0 K in the field range of 0−50 kOe (see Figure 3). For 1, the magnetization increases slowly

Figure 2. Temperature dependence of χMT (for 1−3) in the range of 2−300 K at 1 kOe dc field. Inset: Enlarged view of the upturn of χMT (3) in the low-temperature range.

one [RE−M] unit. At room temperature, the χMT value for 1 is 0.43 cm3 K mol−1, a value lower than one magnetically noninteracting LS Fe III ion with a significant orbital contribution ([FeIII]LS and S = 1/2 with spin−orbit coupling of the 2T2g ground term).16b The χMT value remains roughly constant upon cooling to 50 K, then decreases slowly, and achieves 0.39 cm3 K mol−1 at 2 K. The subtle decrease of χMT in low temperature may be attributed to the orbital contribution of the [FeIII]LS ion. As shown in Figure S6, the best Curie−Weiss law fitting of χM−1 versus T at 300−30 K gives the Curie constant C = 0.43 cm3 mol−1 K and the small Weiss constant θ = −0.48 K, supporting the nearly paramagnetic characteristics of compound 1 considering the diamagnetic nature of the YIII ion in the structure and the LS-state FeIII ions (S = 1/2) with the shortest interdimer Fe···Fe (Figure S2, left) separation of 8.4564(26) Å.11 The obtained χMT value of 14.19 cm3 K mol−1 at 300 K for 2 is slightly lower than the expected values (14.71−14.85 cm3 K mol−1) for one [FeIII]LS (values in the range of 0.54−0.68 cm3 K mol−1, S = 1/2, and g = 2.1)16b and one noninteracting DyIII ion (14.17 cm3 K mol−1, S = 5/2, L = 5, 6H15/2, J = 15/2, and g = 4 /3). For compound 2, the χMT value decreases slowly from 300 to 100 K and then falls tremendously to a value of 9.20 cm3 K mol−1 at 2 K. The continued decrease of χMT is mainly due to the comprehensive effect of the orbital contribution of the [FeIII]LS ion, the progressive thermal depopulation of the excited Stark sublevels of the 6H15/2 state of the DyIII ion, and the possible intradimer antiferromagnetic interaction between neighboring DyIII and FeIII ions through a cyano bridge. A fit of χM−1 versus T by the Curie−Weiss law between 30 and 300 K gives rise to the fitting constants of θ = −7.52 K (C = 14.56 cm3 K mol−1) for 2 (Figure S7). It is not appropriate to infer the magnetic nature between intradimer DyIII and FeIII ions from the negative θ value because of the strong spin−orbit coupling effects in the DyIII and [FeIII]LS ions. For 3, the χMT value is 13.39 cm3 K mol−1 at 300 K, slightly lower than the theoretical value for one isolated DyIII ion. The χMT value decreases gradually with decreasing temperature above 60 K and then decreases quickly upon further cooling to a minimum value of 10.88 cm3 K mol−1 at 5 K. Below 5 K, the χMT value upturns upon cooling and reaches 11.08 cm3 K mol−1 at 2 K (see Figure 2, inset). A similar increase of the χMT value in the low-temperature range has been seen in some other dysprosium-containing compounds.17,18 These behaviors indicate the occurrence of a weak ferromagnetic coupling through dipolar−dipolar interaction between DyIII ions considering the

Figure 3. Field dependence of magnetization (for 1−3) and ∑M [=M(1) + M(3)] at 2.0 K in the range of 0−50 kOe dc field.

withincreasing field, and the value of 0.92 Nβ at 50 kOe is lower than the theoretical saturation value ([FeIII]LS and g × S = 2.1 × 1/2 = 1.05 Nβ), indicating the presence of magnetic anisotropy. For 2 and 3, the magnetization rises abruptly at low fields. The values of 5.82 and 5.84 Nβ at 50 kOe do not reach the theoretical saturation value of 11.05 Nβ for two uncorrelated DyIII and FeIII ions ([FeIII]LS, g × S = 2.1 × 1/2 = 1.05 Nβ; DyIII, gJ × J = 4/3 × 15/2 = 10 Nβ) for 2 and 10.00 Nβ for one magnetically isolated DyIII ion for 3. As shown in Figure 3, the magnetization of 2 is lower than the sum of the magnetization curves [∑M = M(1) + M(3)] of 1 and 3 in the measured field range. This tentative comparison further revealed the antiferromagnetic interaction in compound 2. The hysteresis loop experiment of 2 was performed at 2.0 K in the range of −20 to +20 kOe (Figure S8). No hysteresis effect was detected at 2.0 K with the sweep rates used in the magnetometer. 10319

DOI: 10.1021/acs.inorgchem.5b01763 Inorg. Chem. 2015, 54, 10316−10322

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For compound 2, temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility signals under 2 kOe external field (Figure 5) exhibit frequency dependence below

We aimed at tuning the origin of the magnetic relaxation properties in the isostructural cyano-bridged 3d−4f systems by substituting the 3d or rare-earth ions. To probe the magnetic relaxation behavior of 1−3, ac susceptibility measurements have been performed under zero direct-current (dc) field and 3 Oe ac field (Figures S10−S12). The χ′ (in-phase) component of ac susceptibilities increases constantly with decreasing temperature, while the χ″ (out-of-phase) component remains close to zero in the measured temperature range with only a small frequency dependence arising in compound 3, suggesting almost no magnetic relaxation observed for 1−3 under zero external field. The fast relaxation in 1−3 is mainly attributed to the QTM effect. When a 2.5 kOe dc field was applied, however, both χ′ and χ″ components of the ac susceptibility for 1 showed significant frequency dependence within the measured temperature 2−6 K (see Figure 4). For both χ′ and χ″ components, broad peaks

Figure 5. Temperature dependence of the in-phase (top) and out-ofphase (bottom) ac susceptibility components for compound 2 under a 2 kOe dc field at the indicated ac frequency.

10 K, with evident maximum values observed at ac fields with relatively high frequencies for the χ″ signals, indicating the existence of slow magnetic relaxation behavior. The data of χ″ vs χ′ at 2.0, 2.4, 2.8, 3.2, 3.6, and 4.0 K were selected to draw the Cole−Cole diagram (Figure S14), the best fitting of which by the generalized Debye model presented the distribution coefficient α values of 0.15−0.16. The small α values indicate the narrow distribution of relaxation time and are in correspondence with the residual quantum tunneling relaxation (Figure 5). The relaxation dynamics of compound 2 can be associated with the magnetic interaction of intradimer DyIII and [FeIII]LS ions.20 As can be seen from Figure 7, within the measured temperature range, the relaxation time is significantly decreased for 2 under 2 kOe relative to 1 under 2.5 kOe. The best fitting of the plot of lnτ versus T−1 gives the parameters τ0 of 9.7 × 10−8 s and Ueff/kB of 23 K, with the τ0 lower than and the energy barrier Ueff/kB approximately equivalent to the values of 1 under 2.5 kOe. In comparison, the χ″(T) peak temperature inferred from the curve of χ″−T with f = 1000 Hz appeared to be 3.0 K, lower than the corresponding value of 4.2 K for 1 under 2.5 kOe. Comparatively, ac susceptibility measurements of compound 3 with a diamagnetic CoIII ion substituted for the [FeIII]LS ion in compound 2 were also conducted under a 2 kOe dc field to tune the origin of the magnetic relaxation behavior and to evaluate the contribution of the magnetic interaction between intradimer DyIII and [FeIII]LS ions in 2 (Figure 6). Fitting the Cole−Cole plots at 3.0, 3.4, 3.8, 4.2, 4.6, and 5.0 K (Figure S15) to the generalized Debye model gives a constant small α value of 0.15, suggesting the almost eliminated QTM effect, which is further demonstrated by the near-linear plot of ln τ versus T−1. The best fitting provides the parameters τ0 of 7.1 × 10−11 s and Ueff/kB of 63 K. In view of the diamagnetic feature of the CoIII ion and the long interdimer Dy···Dy (Figure S2, right) distance of 8.7052(3) Å in 3, the magnetic relaxation can be attributed to stemming from the single anisotropic DyIII ion, which represents a new way to design lanthanide SIMs through a diamagnetic transition metallocyanate ligand. The larger energy barrier Ueff/kB (23 K for 2 and 63 K for 3) and the higher χ″(T) peak temperature at f = 1000 Hz (3.0 K for 2 and 4.8 K for 3) of compound 3 than those of 2 manifest that the

Figure 4. Temperature dependence of the in-phase (top) and out-ofphase (bottom) ac susceptibility components for compound 1 under 2.5 kOe dc field at indicated ac frequency.

appeared at the ac fields with all applied frequencies. Owing to the diamagnetic characteristics of the YIII ion and the long interdimer Fe···Fe (Figure S2, left) distance of 8.4564(26) Å in 1, the magnetic relaxation behavior can be ascribed to originate from the single anisotropic [FeIII]LS ion, which, to the best of our knowledge, has not been revealed as SIMs consisting of transition-metal ions in the literature.10f,g,11 At temperatures of 4, 5, and 6 K (Figure S13), the Cole−Cole plots displayed nearly semicircular shape and were best fitted by the generalized Debye model, giving the distribution coefficient α values 0.15, 0.14, and 0.15. The relatively small but nonzero values of the α parameter imply small distributions of relaxation times and correlate with the not completely quenched quantum tunneling relaxation (see Figure 4). In the range of 373−7197 Hz, the preexponential factor τ0 and anisotropic energy barrier Ueff/kB to reverse the magnetization can be calculated from analysis of the frequency dependence of the χ″ peak temperature, Tp, on the basis of the Arrhenius law [τ = τ0 exp(Ueff/kBTp), where τ = 1/2πf ]. As shown in Figure 7, the best fitting of the plot of ln τ versus T−1 affords the magnetic relaxation parameters τ0 of 1.3 × 10−7 s and Ueff/kB of 23 K (∼16 cm−1), which are comparable to those of transition-metalion-based SIMs (τ0 in the range of ∼10−5−10−11 s and Ueff/kB within the range 5.6−226 cm−1).4f According to the curve of χ″−T with f = 1000 Hz (f is the oscillating frequency of the ac field) under a 2.5 kOe dc field, the maximum χ″(T) value appeared at 4.2 K. 10320

DOI: 10.1021/acs.inorgchem.5b01763 Inorg. Chem. 2015, 54, 10316−10322

Inorganic Chemistry

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01763. Crystallographic information for 1 (CCDC 1414273) in CIF format (CIF) Crystallographic information for 2 (CCDC 1414272) in CIF format (CIF) Crystallographic information for 3 (CCDC 1414271) in CIF format (CIF) Selected bond lengths and angles for 1−3, hydrogenbonding geometry for 1−3, CShM values for 1−3, illustrated crystal packing of 1, view of the shortest interdimer Fe···Fe and Y···Y metal separations for 1, PXRD patterns for 1−3, plots of the temperature dependence of χM−1 for 1−3, plot of the hysteresis loop for 2, temperature dependence of the ac susceptibility components under a 0 Oe dc field for 1− 3, and Cole−Cole plots for 1−3 (PDF)

Figure 6. Temperature dependence of the in-phase (top) and out-ofphase (bottom) ac susceptibility components for compound 3 under a 2 kOe dc field at the indicated ac frequency.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Figure 7. Natural logarithm of magnetization relaxation time versus reciprocal temperature, ln τ versus T−1, plot for compound 1 at 2.5 kOe (□), 2 at 2 kOe (○), and 3 at 2 kOe (△) dc field. The solid line is fitted with the Arrhenius law.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Yan Peng for helpful discussions on magnetism. This work was supported by the National Natural Science Foundation of China (Grants 21201061 and 21205043), the State Scholarship Fund of China (Grant 201308420653), and the Scientific Research Foundation of Education Commission of Hubei Province (Grant Q20111008).

intradimer DyIII and [FeIII]LS antiferromagnetic interaction plays a negative role in the magnetic relaxation behavior. In conclusion, three isomorphous cyano-bridged 3d−4f compounds were prepared by substituting the 3d or rareearth ions at the intradimer metal sites. The dc magnetic measurements showed distinct magnetic interactions propagated in the three compounds, negligible magnetic coupling between interdimer [FeIII]LS ions for compound 1, antiferromagnetic coupling between the intradimer DyIII and FeIII ions for 2, and very weak ferromagnetic coupling through interdimer dipolar−dipolar interaction for 3. Under an external field, the three compounds show different ac magnetic relaxation dynamics, which originate from a single anisotropic [FeIII]LS ion for 1, intradimer antiferromagnetic interaction for 2, and a single anisotropic DyIII ion for 3, respectively. As can be inferred from a comparison of the relaxation parameters Ueff/kB and χ″(T) peak temperature between 2 and 3, the magnetic coupling between intradimer DyIII and [FeIII]LS ions through a cyano bridge seems to have a negative effect on the manifestation of the SMM behavior. Nevertheless, the magnetic coupling through dipolar−dipolar interaction in 2 and 3 may affect the magnetic relaxation dynamics to some extent. In order to differentiate more explicitly the factors influencing the magnetic relaxation dynamics, we are on our way to applying ab initio calculations to determine the 3d−4f exchange interactions in such systems and to synthesize more isolated cyanobridged 3d−4f dimers to eliminate essentially the dipolar− dipolar interaction.

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REFERENCES

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