A Rare Water and Hydroxyl-Extended One-Dimensional Dysprosium

Jul 31, 2017 - Synopsis. A novel water and hydroxyl-extended one-dimensional dysprosium(III) chain was hydrothermally generated, exhibiting effective ...
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A Rare Water and Hydroxyl-Extended One-Dimensional Dysprosium(III) Chain and Its Magnetic Dilution Effect Yan Li,† Pu Zhao,† Shan Zhang,† Rui Li,† Yi-Quan Zhang,*,§ En-Cui Yang,*,† and Xiao-Jun Zhao*,†,‡ †

College of Chemistry, Key Laboratory of Inorganic−Organic Hybrid Functional Material Chemistry, Ministry of Education, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Tianjin Normal University, Tianjin 300387, People’s Republic of China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, People’s Republic of China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, People’s Republic of China S Supporting Information *

ABSTRACT: A novel water and hydroxyl-extended onedimensional dysprosium(III) chain was hydrothermally obtained, which exhibits a relatively high spin-reversal energy barrier of 88.7 K and intrachain ferromagnetic interaction with the coupling constant Jexch = 3.04 cm−1 calculated by fitting magnetic susceptibilities using POLY_ANISO program based on ab initio calculations. To deeply understand the respective role of the single-ion anisotropy and intrachain exchange on the effective energy barrier, three crystallographically isostructural analogues containing isotropic Gd(III)-, diamagnetic Y(III)-, as well as Y(III)-doped Dy0.05Y0.95 were prepared and characterized structurally and magnetically. Due to the absence of significant intrachain exchange interaction, the effective energy barrier of the Dy0.05Y0.95 decreased by 9.9 K as compared with that of parent dysprosium(III) chain. Thus, it can be concluded that the intrachain ferromagnetic coupling and the magnetic anisotropy of the Dy(III) ion synergistically enhance the effective energy barrier of the dysprosium(III) chain, in which the single-ion anisotropy becomes more predominant.

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INTRODUCTION Lanthanide(III)-based single-chain magnets (Ln(III)-based SCMs) have recently drawn intense focus due to their deep understanding of the magnetostructural correlations and promising applications in high-density information storage, quantum computation, and molecular spintronics.1−5 Two crucial factors, locally large anisotropy of the lanthanide ion and favorable intrachain magnetic interactions (exchange coupling and dipolar interaction), can significantly enhance the effective energy barrier of the magnetization reversal (Ueff) and prompt the hysteresis loop with large coercive field or high remnant magnetization, which can also greatly advance the feasible applications of the SCMs.6,7 It is well-known that magnetic anisotropy of the Ln(III)-based SCMs is essentially dominated by fine-tuning the ligand field around the spin carrier, which can be successfully achieved by manipulating the coordination number, coordination polyhedron, crystallographically axial symmetry, electronic structure, and/or the electrostatic potential of the donor sites of the ligand field.8−12 On the other hand, small but important intrachain magnetic couplings are closely related to the bridging geometric parameters (such as the interspin distance, bridging angle, conformation, and so on) involving the different organic mediators.13−20 Therefore, one of important aspects is to design and synthesize a suitable © 2017 American Chemical Society

organic mediator to efficiently transfer magnetic interactions of the Ln(III)-based SCMs. Among the various mediators investigated up to the present date (including diverse nitronyl nitroxide radicals, various carboxylates, and different types of Schiff-base derivatives),13−20 short magnetic bridges (such as Cl−, OH−, H2O, and CH3OH) play essential roles in the improvement of the superexchange interaction due to the nearest interspin distance. In particular, the ferromagnetic interaction is the ultimate goal by carefully controlling the bridging geometric parameters, because the ferromagnetic coupling can efficiently suppress the quantum tunneling of magnetization (QTM) and significantly increase the energy barrier for spin reversal.21−23 However, it is quite challenging to simultaneously satisfy the large magnetic anisotropy of the lanthanide(III) ion and strong magnetic interactions in Ln(III)based SCMs because of the high coordination number and structural flexibility of the lanthanide(III) ion. Especially for Dy(III)-based SCMs, it is highly important to efficiently arrange strong axially ligand-field of oblate Dy(III) ions in an intense manner through short magnetic bridges. Herein, a new Dy(III)-based complex, [Dy(μ-H2O)(phen)(μ-OH)(nb)2]n (1, Received: April 26, 2017 Published: July 31, 2017 9594

DOI: 10.1021/acs.inorgchem.7b01058 Inorg. Chem. 2017, 56, 9594−9601

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Inorganic Chemistry Table 1. Crystal and Structure Refinement Data for 1−4a empirical formula Fw cryst size [mm] cryst syst space group a [Å] b [Å] c [Å] V [Å3] Z, Dc [g cm−3] h/k/l F(000) μ [mm−1] reflections collected/ unique Rint data/restraints/params R1a, wR2b [I > 2σ (I)] R1, wR2 [all data] GOFon F2 Δρmax, Δρmin [e·Å−3] a

1

2

3

4

C26H19DyN4O10 709.95 0.250 × 0.220 × 0.200 Orthorhombic Pnma 7.3475(3) 30.4358(13) 11.2763(5) 2521.69(19) 4, 1.870 −8, 8 / −36, 25 / −13, 12 1396 3.032 7750/2268

C26H19GdN4O10 704.70 0.220 × 0.210 × 0.180 Orthorhombic Pnma 7.4411(4) 30.5807(15) 11.3721(6) 2587.8(2) 4, 1.809 −9, 8 / −38, 38 / −7, 14 1388 2.630 14035/2733

C26H19Dy0.05Y0.95N4O10 640.04 0.220 × 0.210 × 0.180 Orthorhombic Pnma 7.4411(4) 30.5807(15) 11.3721(6) 2587.8(2) 4, 1.643 −9, 9 / −28, 38 / −14, 10 1293 2.352 8558/2725

C26H19YN4O10 636.36 0.220 × 0.210 × 0.180 Orthorhombic Pnma 7.3564(16) 30.648(6) 11.348(2) 2558.5(9) 4, 1.652 −9, 9 / −38, 18 / −13, 14 1288 2.347 14019/2699

0.0415 2268/18/190 0.0267, 0.0589 0.0306, 0.0619 1.106 0.622, −0.901

0.0397 2733/42/190 0.0286, 0.0707 0.0312, 0.0716 1.066 1.734, −1.078

0.0386 2725/0/190 0.0388, 0.0797 0.0530, 0.0847 1.048 0.438, −0.727

0.0185 2699/12/190 0.0264, 0.0652 0.0297, 0.0664 1.069 0.594, −0.433

R1 = Σ(∥F0| − |Fc∥)/Σ|F0|. bwR2 = [Σw(|F0|2 − |Fc|2)2/Σw(F02)2]1/2. 44.40, H, 2.68; N, 8.01%. FT-IR (KBr pellet, cm−1): 3424 (s), 1622 (m), 1583 (w), 1517 (m), 1424 (m), 1400 (m), 1346 (s), 1142 (w), 1104 (w), 854 (w), 798 (w), 724 (m), 664 (w), 520 (w). Synthesis of [Dy0.05Y0.95(μ-H2O)(phen)(μ-OH)(nb)2]n (3). The magnetic-site diluted sample was obtained with the same procedures as those of 1 except that DyCl3·6H2O was replaced by a mixture of DyCl3·6H2O and YCl3·6H2O in a molar ratio of 1:19. Yield: 53% based on Hnb. Calcd for C26H19Dy0.05Y0.95N4O10: C, 48.17; H, 2.91; N, 8.68%. Found: C, 48.65, H, 2.62; N, 8.53%. Dy(III) and Y(III) content in 3 was 4.96% and 95.04% measured by dispersive spectroscopy (EDS) of field-emission scanning electron microscope (Table S1). Synthesis of [Y(μ-H2O)(phen)(μ-OH)(nb)2]n (4). Complex 4 was synthesized by the same procedures as those of 1 except that DyCl3· 6H2O was replaced by YCl3·6H2O. Yield: 53% based on Hnb. Calcd for C26H19YN4O10: C, 49.07; H, 3.01; N, 8.80%. Found: C, 48.86, H, 2.92; N, 8.92%. FT-IR (KBr pellet, cm−1): 3431 (s), 1623 (m), 1583 (w), 1519 (m), 1424 (m), 1402 (m), 1346 (s), 1143 (w), 1104 (w), 854 (w), 798 (w), 724 (m), 694 (w), 521 (w).

nb = p-nitrobenzoate and phen = 1,10-phenanthroline), was hydrothermally constructed by mixed-ligand strategy. Complex 1 exhibits a bent chain with anisotropic Dy(III) ions interconnected by double μ-H2O and μ-OH− heterobridges. More interestingly, the unusual water and hydroxyl-bridged dysprosium(III) chain has a relatively high effective energy barrier of 88.7 K and intrachain ferromagnetic coupling up to Jexch = 3.04 cm−1 evaluated by ab initio calculations. To further determine the respective role of the single-ion anisotropy and the intrachain magnetic exchange, three isostructural complexes analogous to 1, [Gd(μ-H2O)(phen)(μ-OH)(nb)2]n (2), [Dy0.05Y0.95(μ-H2O)(phen)(μ-OH)(nb)2]n (3), and [Y(μH2O)(phen)(μ-OH)(nb)2]n (4), have also been synthesized and characterized structurally and magnetically.

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EXPERIMENTAL SECTION

All initial materials were commercially purchased from either Acros or Tianjin Chemical Reagent Factory and used as received without further purification. The apparatus for general physical characterizations, the measurement procedures and conditions, the X-ray crystallography, as well as the ab initio calculations (Figure S1) were almost similar to our previous investigations,24 which are described in more detail in the Supporting Information. Synthesis of [Dy(μ-H2O)(phen)(μ-OH)(nb)2]n (1). A mixture containing DyCl3·6H2O (72.8 mg, 0.2 mmol), Hnb (33.4 mg, 0.2 mmol), and phen (90.0 mg, 0.5 mmol) was dissolved in doubly deionized water (10.0 mL) with constant stirring. The resulting mixture was then sealed in a Parr Teflon-lined stainless steel vessel (23.0 mL) and heated at 140 °C for 96 h under autogenous pressure. After the mixture was cooled to 25 °C at a rate of 2.0 °C h−1, yellow block-shaped crystals suitable for single-crystal X-ray structural determination were grown directly, separated manually, and dried in air (yield: 50% based on Hnb). Calcd for C26H19DyN4O10: C, 43.98; H, 2.70; N, 7.89%. Found: C, 43.77; H, 2.52; N, 7.92%. IR (KBr, cm−1): 3433 (s), 2024 (w), 1579 (s), 1519 (m), 1403 (m), 1347 (s), 1101 (w), 849 (w), 797 (w), 724 (w), 517(w). Synthesis of [Gd(μ-H2O)(phen)(μ-OH)(nb)2]n (2). Complex 2 was synthesized by the same procedures as those of 1 except that DyCl3·6H2O was replaced by GdCl3·6H2O. Yield: 50% based on Hnb. Calcd for C26H19GdN4O10: C, 44.31; H, 2.72; N, 7.95%. Found: C,

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RESULTS AND DISCUSSION Syntheses and IR Spectra. Yellow block-shaped single crystals of 1, 2, and 4 with anisotropic Dy(III), isotropic Gd(III), as well as diamagnetic Y(III) ions were successfully prepared with a molar ratio of 5:2:2 (phen:Hnb:Ln(III)). The molar ratio of the three reactants, especially the excessive phen ligand, was a decisive factor for the successful preparation of the targeted complexes with the mixed ligands. Behaving as a hard acid, lanthanide ion preferred to coordinate with O-containing ligand, rather than N-based donors. Therefore, the excessive phen ligand increased the opportunity for co-coordination of the mixed ligands. The isostructural nature of 1 and 4 rendered the successful preparation of the magnetic diluted sample 3, which was obtained by tuning the molar ratio of Dy(III) and Y(III) salts. The final content of the Dy(III) ion was almost consistent with the value 5.0% employed in the synthesis. Acting as mixed ligands, the conjugated phen and Hnb were initially used to decrease steric hindrance upon the coordination process. Notably, the generations of these four

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Figure 1. (a) Local coordination environments and distorted triangular dodecahedron of Dy(III) ion in 1 (Hydrogen atoms were omitted for clarity, symmetry codes: A = x − 1/2, y, 1/2 − z; B = x, 1/2 − y, z). (b) Bent chain of 1 with Dy(III) ions extended by water and hydroxyl bridges.

2.855).17,27 Careful comparisons of these geometric parameters with the known values reveal that 1 has a pronounced shortest Dy−O bond (2.215(3) Å), and the difference between the longest and shortest bond lengths is 0.373 Å (Table S3). By contrast, for the previously reported triangular dodecahedra, the differences between the longest and shortest bonds around the Dy(III) ion vary only between 0.017 and 0.256 Å.14,28 The appearance of the shortest Dy−Ohydroxyl bond in 1 may significantly modulate the single-ion anisotropy of the oblate Dy(III) spin responsible for the slow magnetic relaxation of 1.24,29 Adjacent Dy(III) ions are periodically bridged by pairs of water and hydroxyl group mediators, generating an infinite Zshaped chain running along the crystallographic a-direction (Figure 1b). The intrachain Dy(III)···Dy(III) separation is 3.8766(2) Å and the bridging angles of ∠DyODy are 103.92(12)° and 119.08(15)° for water and hydroxyl bridges, respectively. Apparently, the Dy(III)···Dy(III) distance separated by water and hydroxyl heterobridges is shorter than those previously reported dysprosium(III) chains aggregated by nitronyl nitroxide radical and carboxylate linkages (3.998− 11.8813 Å).16,30 Empirically, the intrachain Dy(III)···Dy(III) distance and bridging ∠DyODy angles are expected to mediate favorable ferromagnetic exchange coupling.31 The individual chains of 1 are well isolated by weak interchain C−H··· Ocarboxylate hydrogen-bonding and π···π stacking interactions (Figure S3 and Table S4), leading to a noncovalent twodimensional layer with the shortest interchain Dy(III)···Dy(III) separation of 11.2763(5) Å. Obviously, these bent chains of 1 are well separated and the interchain exchange coupling is negligible. PXRD Patterns and Thermogravimetric Analysis. Structural consistency and phase purity of the bulk samples of 1−4 were further evidenced by good superposition of experimental and theoretical PXRD patterns (Figure S4). Additionally, complex 1 can keep its compositional stability before 166 °C. Upon further heating, complex 1 exhibits an obvious weight-loss process for the broken of the polymeric chain (Figure S5).

isostructural complexes were particularly sensitive to the position and electronic effect of the nitro group attached on the phenyl ring. The changes on both the position (ortho- or meta-position) and the type of the substituent group (such as −H, −CH3, −NH2, −OH, and −pseudohalogen/halogen) only led to amorphous powder or microcrystals that were structurally different from the target complexes. In the IR spectra (Figure S2), strong and broad absorption at 3430 ± 6 cm−1 resulted from the characteristic stretching vibration of O−H, confirming the presence of water molecules and/or hydroxyl groups in 1−4. The absence of a peak at 1694 cm−1 indicated the deprotonation of Hnb ligand. The asymmetric (νas) and symmetric (νs) vibrations for deprotonated carboxylate moiety were respectively observed at 1580 ± 3, 1519, 1400 ± 3, and 1346 ± 1 cm−1. The separation for the asymmetric and symmetric bands Δν (Δν = νas − νs) was ca. 180 cm−1, suggesting the terminally monodentate mode of carboxylate group in 1−4.25 Crystal Structures. Complexes 1−4 are crystallographically isostructural. They all crystallize from the orthorhombic Pnma space group with Z = 4 (Table 1), exhibiting a bent chain with octa-coordinate metal ions extended by scarcely observed μH2O and μ-OH− heterobridges. Due to their analogous motif, only the crystal structure of 1 was described herein as representative. The fundamental structural unit of 1 includes a centrosymmetric Dy(III) ion located at an inversion center, a bridging water molecule, a bridging μ-OH− group, a neutral phen molecule, as well as two deprotonated nb− anions. The water, μ-OH−, and phen in the unit cell are all mirrorsymmetric. As shown in Figure 1a, the centrosymmetric Dy(III) ion is surrounded by N2O6 donors coming from one bidentate chelating phen molecule, two monodentate carboxylate O atoms belonging to two mirror-symmetric nb− anions, two bridging water molecules, as well as two bridging hydroxyl groups. The coordination geometry of the Dy(III) ion can be described as a slightly distorted triangular dodecahedron (D2d) evaluated by a Continuous Shape Measure26 with the agreement factor of 0.703 (Table S2). The agreement factor for the triangular dodecahedral Dy(III) configuration of 1 is comparable with those of previously reported values (0.059− 9596

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of ground-state multiplets,34 the exchange coupling along the isotropic Gd(III)-chain cannot exactly describe the magnetic coupling in the anisotropic dysprosium(III) chain. Therefore, the CASSCF calculations were performed on the model structure of 1. The calculated main gz value for the Dy(III) ion in the ground state of 1 is 19.628, which is obviously bigger than those of gx and gy components (gx = 0.128 and gy = 0.260). The nearest neighbor Dy(III)···Dy(III) interaction in 1 can be regarded as the Ising type, and the Lines model implemented with the POLY_ANISO program was used to evaluate the coupling strength along the Ising-like Dy(III)-chain.35,36 The least-squares fitting of the experimental magnetic susceptibility to the exchange Hamiltonian operator based on ab initio calculations affords Jexch = 3.04 cm−1, Jdipolar = 1.71 cm−1, and Jtotal = 4.75 cm−1. Undoubtedly, the positive coupling constant Jexch further confirms the weak ferromagnetic exchange between the adjacent Dy(III) ions that is transmitted by μ-H2O and μOH− heterobridges. Up to the present date, weak ferromagnetic couplings have been found in six carboxylate/nitronyl nitroxide radical-extended dysprosium(III) chains.10,16,20,37−39 Furthermore, the magnetization for each Dy(III) ion along the chain is parallel with each other, as shown in Figure 2b. Additionally, the ln(χ′T) versus 1/T plot of 1 displays a linear region between 10 and 40 K (Figure 2a inset, χ′ is the in-phase ac susceptibility of 1 Hz measured at different temperature under zero dc field), also suggesting the Ising-type anisotropy of 1. Fitting the linear regime of these magnetic data to the expression χ′T = Ceff exp(Δξ/kBT) yields the correlation energy Δξ/kB of 4.89 K and the effective Curie constant Ceff of 16.44 cm3 K mol−1 per magnetic unit. The Δξ is the energy to create a domain wall in the chain and kB is the Boltzmann constant.40 The χMT product for the diluted sample 3 is 0.71 cm3 K mol−1 at 300 K, which is compatible to one free Dy(III) ion multiplied by the doping ratio (14.17 × 5.0% = 0.71 cm3 K mol−1). With the decreasing temperature, the χMT value only slightly decreases to 0.46 cm3 K mol−1 at 2.0 K, implying that the intrachain exchange coupling is seriously destroyed by the doping of the diamagnetic Y(III) matrix. Isothermal M vs H curves of 1−3 were measured at 2.0 K to investigate the effect of intrachain exchange on the magnetization in the presence of external magnetic field (Figure 3). The magnetization of 1 exhibits an initially rapid increase at low fields (H < 8 kOe) and then is followed by a steady increase to 7.11 Nβ at 70 kOe. The rapid rise of 1 at low field indicates weak intrachain ferromagnetic interaction mediated by μ-H2O

Static Magnetic Properties. Variable-temperature (300− 2.0 K) direct current (dc) magnetic susceptibilities were performed on the crushed polycrystalline sample of 1 under an external field of 1 kOe (Figure 2a). The observed χMT product

Figure 2. (a) Temperature dependence of χMT for 1−3 (Inset: plot of ln(χ′T) vs 1/T for 1 measured at an ac frequency of 1 Hz under Hdc = 0 Oe and Hac = 2.5 Oe. Solid lines represent the best fits indicated in the text). (b) Orientation of the magnetic axis for the ground Kramers doublet of 1 obtained by CASSCF calculation.

for each Dy(III) ion of 1 is 14.22 cm3 K mol−1 at 300 K, comparable to the theoretical value (14.17 cm3 K mol−1) for one free Dy(III) ion (6H15/2, S = 5/2, L = 5, g = 4/3, and C = 14.17 cm3 K mol−1). Arising from the progressive depopulation of the MJ sublevels of the anisotropic Dy(III) ion by the crystalfield effect,32 the χMT value of 1 decreases slightly with the decreasing temperature, approaching the minimum of 13.3 cm3 K mol−1 at 42.0 K. Then, it increases rapidly to a maximum of 24.01 cm3 K mol−1 at 2.0 K, indicating a dominant ferromagnetic exchange between the adjacent Dy(III) spins at low temperature. To quantitatively estimate the strength of the intrachain exchange, the dc susceptibilities of 2 with isotropic Gd(III) spin and 3 with partial diamagnetic Y(III) ion were recorded under the same measurement conditions as those of 1. The χMT product for each Gd(III) ion of 2 is 8.00 cm3 K mol−1 at room temperature (Figure 2a), which is close to the expected value of 7.88 cm3 K mol−1 for one free Gd(III) ion (8S7/2, S = 7/2, L = 0, g = 2, and C = 7.88 cm3 K mol−1). Upon cooling, the χMT remains almost constant above 100 K, then rapidly declines to 5.79 cm3 K mol−1 at 2.5 K. The magnetic data of 2 can be treated by an isotropic Heisenberg chain model with S = 7/2, and the interchain magnetic interaction (zJ′) is considered by molecule filed approximation (see SI).33 The resulting fitting parameters were g = 2.0, J = −0.031 cm−1, zJ′ = −0.001 cm−1, and R = 3.3 × 10−4 (R = ∑[(χMT)obsd − (χMT)calcd]2/[(χMT)obsd]2). Obviously, the weak antiferromagnetic interaction is mediated between the adjacent Gd(III) ions by double μ-OH− and μ-H2O heterobridges along the isotropic chain. Significantly resulting from the different electronic structures of the individual f-shells and the ligand field splitting

Figure 3. Field-dependent magnetizations for 1−3 measured at 2.0 K (the dashed line represents the Brillouin function for S = 7/2). 9597

DOI: 10.1021/acs.inorgchem.7b01058 Inorg. Chem. 2017, 56, 9594−9601

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Inorganic Chemistry and μ-OH− heterobridges. The isothermal magnetization of the isotropic Gd(III)-chain of 2 increases with the enhanced magnetic field and slowly reaches a saturation value of 7.0 Nβ at 70 kOe field. Moreover, the experimental M−H curve of 2 is below the theoretical Brillouin function for S = 7/2 and g = 2.0, further confirming weak intrachain antiferromagnetic interaction along the isotropic Gd(III)-chain. The isothermal magnetization of 3 only slightly increases with the increasing field, leading to 0.16 Nβ at 20 kOe. Especially, an obvious magnetic transition is clearly found for 3 below 5 kOe (Figure 3 inset), suggesting weaker magnetic interaction of 3 than that of 1 due to the replacement of the paramagnetic Dy(III) ion by diamagnetic Y(III) matrix. Upon further improving the magnetic field, the magnetization of 3 remains almost unchanged, hinting the absence of the intrachain magnetic interaction in the magnetically diluted sample. The magnetizations of 1 and 3 are far from the saturated values (10.0 and 0.5 Nβ) at the highest magnetic field determined, which imply the existence of magnetic anisotropy and/or low-lying excited states caused by the crystal-field effect.8 At a field sweep rate of 200 Oe s−1, 1 shows butterfly shaped hysteresis loops below 3.0 K with a negligible remanent magnetization at zero dc field (Figure 4 left). The loop shape of

pristine complex 1, the stepped character of the hysteresis loop of 3 becomes more apparent and a slight opening of the hysteresis loop is observed (110 Oe) at 2.0 K. These results indicate that the cutting of the ferromagnetic exchange interaction can evidently slow down the magnetic relaxation rate and increase the relaxation time. In fact, only three Dy(III)based SCMs with nitronyl nitroxide radicals and acetate linkages have ever exhibited hysteresis loops with slightly different openings by far.16,17,39 Magnetic Dynamics. To explore the effect of the intrachain exchange on the magnetic dynamics, frequencydependent alternating current (ac) magnetic susceptibility of 1 and 3 were measured under zero dc field with an oscillating field of 2.5 Oe. As illustrated in Figure 5 and Figure S6, obvious frequency- and temperature-dependent out-of-phase (χ″) signals are clearly detected, confirming slow magnetization relaxation.41 The peak maxima of the χ″ signal are found at 3.0 and 26.0 K for an oscillating field of 0.3 and 1000 Hz. Moreover, the peak maximum of the χ″ signal shifts toward lower frequency with the decreasing temperature. Notably, no apparent temperature-independent χ″ signals are detected below 3.0 K, suggesting that the QTM channel is efficiently suppressed by intrachain ferromagnetic coupling. Cole−Cole plots of χ″ versus χ′ of 1 show approximate semicircles (Figure 6), confirming the single relaxation process within the temperature range examined. Fitting the frequencydependent ac data between 3.0 and 26.0 K to the generalized Debye model gave temperature-dependent relaxation time (τ) and distribution of the relaxation times (α).41 These α values of 1 are between 0.03 (at 26.0 K) and 0.35 (at 3.0 K, Table S5), indicating a narrow-to-moderate distribution of the relaxation times. The higher the temperature is, the more narrow the distribution of the relaxation times becomes. The relaxation times extracted from the generalized Debye model fit could be used to analyze the relaxation mechanisms of 1 under zero dc field. A multiple relaxation model that including Orbach (τo−1exp(−Ueff/kBT)), Raman (CRamanTn), and quantum tunneling (τQTM) pathways (eq 1) under zero dc field can be used to fit the frequency-dependent data of 1.42

Figure 4. Magnetic hysteresis loops of 1 and 3 measured at a sweep rate of 200 Oe s−1.

1 becomes more and more like a butterfly with the decreasing temperature. For the magnetic diluted sample of 3, the influence of the temperature on the loop shape is further evidenced (Figure 4 right). Moreover, as compared with the

Figure 5. Frequency dependence of ac susceptibility measured under zero dc field and plot of ln τ vs 1/T for 1 and 3 (Inset: red, blue, and green solid lines represent the best fits to the sum of Orbach and Raman pathways, the individual Orbach and Raman relaxation processes, respectively). 9598

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Figure 6. Cole−Cole plots for 1 and 3 under zero dc field (solid lines represent the best fit to the generalized Debye model).

Table 2. Selected Magnetic Parameters for the Known Dy(III)-Based SCMs SCMa

Ueff/K

τo/s

loop temperature/K

exchange coupling

[Dy(hfac)3(NITMe)]n [Dy(hfac)3(NIT-3BrPhOMe)]n19 [Dy2(hfac)6(NITThienPh)2]n17

14.734 39.8 47.3 36.8 48.9 69 71 55.8

1.22 × 10−8 2.43 × 10−10 7.73 × 10−15 1.73 × 10−9 8.68 × 10−15 1.9 × 10−12 1.3 × 10−10 1.6 × 10−10

− − 3.7

AF AF AF

− 1.60 − −

AF F F AF

[Dy(hfac)3{NIT-Ph(OMe)2}]n7 [Dy(hfac)3{NIT(C6H4OPh)}]6 [Dy(ppmc)2·4H2O]·ppmc·H2O44 {[Dy(L)3(H2O)]·5H2O}n10 a

Abbreviations: hfac = hexafluoroacetylacetonate, NITMe = 2,4,4,5,5-pentamethylimidazolyl-1-oxyl-3-oxide, NIT-3BrPhOMe = 2-(3′-bromo-4′methoxyl)-4,4,5,5-tetramethyl-imidazolyl-1-oxyl-3-oxide, NITThienPh = 2-(5-phenyl-2-thienyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, NITPh(OMe) = 2-(2′,4′-dimethoxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide, NIT(C6H4OPh) = 2-(4′-C6H4OPh)-4,4,5,5tetramethylimidazoline-1-oxyl-3-oxide), Hppmc = 2-phenylpyrimidine-4-carboxylic acid, HL = D-(−)-quinicacid.

τ −1 = τQTM −1 + C RamanT n + τo−1exp( −Ueff /kBT )

Magnetic dynamics of the magnetic diluted sample 3 is drastically different from its concentrated counterpart 1 due to the absence of intrachain ferromagnetic couplings. As illustrated in Figure 5, the peak maxima of the frequency-dependent χ″ signals cannot be clearly located below 6.0 K, revealing that the magnetic relaxation process is very slow at low temperature due to the damage of intrachain ferromagnetic interaction. By contrast, the frequency-dependent peak maxima of the χ″ signal are recorded above 6.0 K, which occur at 6.0 and 26.0 K for an oscillating field of 0.15 and 355 Hz. As compared with the ac data of dysprosium(III) chain, the peak maxima of 3 at a given temperature shift toward low frequency, implying that the broken of the ferromagnetic interaction in the magnetic dilution sample slows down the magnetic relaxation rate and increases the relaxation time (Table S6). The Cole−Cole plots of 3 become more symmetric arcs than those of 1 with 0.03 (26.0 K) < α < 0.20 (6.0 K) (Figure 6 and Table S6), suggesting more narrow distribution of the relaxation times in the absence of intrachain ferromagnetic interaction. The best least-squares fit of ln τ vs 1/T curve of 3 to eq 1 gave rise to Ueff/kB = 78.8 ± 3.9 K, τo = (2.55 ± 0.15) × 10−5 s, CRaman= (2.90 ± 0.50) × 10−4 s−1 K−4.6, and n = 4.55 ± 1.26. The Ueff/ kB value of 3 decreased by 9.9 K than that of 1, strongly suggesting that the intrachain ferromagnetic coupling and the magnetic anisotropy of Dy(III) ion synergistically enhance the effective energy barrier of 1 and the single-ion anisotropy is more predominant for the enhancement of the energy barrier. Additionally, the relaxation time of the diluted sample 3 at a given low temperature is almost three times longer than that of the parent dysprosium(III) chain (Figure 5 inset), which is certainly resulting from the absence of intrachain ferromagnetic coupling. The delayed relaxation time and slowed relaxation

(1)

Since the QTM pathway was not observed above 3.0 K, the relaxation time from the QTM pathway is neglected during the fitting process of ln τ vs 1/T. As shown in Figure 4, the strong linear dependence of ln τ vs 1/T between 13.0 and 26.0 K is indicative of a dominant Orbach relaxation mechanism. Below 13.0 K, the plot of ln τ vs 1/T displays an apparent curvature, implying that more than one relaxation pathway coexist or compete with each other in 1 between 9.0 and 13.0 K. The obtained fitting parameters are Ueff/kB = 88.7 ± 1.7 K, τo = (5.58 ± 0.66) × 10−6 s, CRaman= 0.07 ± 0.01 s−1 K−2.9, and n = 2.89 ± 0.05. The obtained CRaman and n values are as expected for the Raman process for the Kramers ion.43 The Ueff/kB of 1 means that water and hydroxyl-aggregated dysprosium(III) chain exhibits relatively high effective energy barrier under zero dc field. To the best of our knowledge, only 24 Dy(III)-, Tb(III)-, and Ho(III)-chains with nitronyl nitroxide radicals, carboxylates, and Schiff-base derivatives as mediators have shown slow magnetic relaxation under zero dc field, although lots of 4f-derived one-dimensional complexes have been successfully obtained.37−39,44 Among these 4f-based chains with slow magnetic relaxations, only seven Dy(III)-based 1D systems exhibit effective energy barriers with 14 K < Ueff/kB < 71 K (Table 2). Therefore, complex 1 with water and hydroxyl mediators has become one of the high-performance dysprosium(III) chains. However, the τo value of 1 is obviously larger than those of experimental values (10−8−10−12 s) for typical SCMs, and fall in the normal range of a single molecule magnet.6 Thus, the slow relaxation of 1 can be interpreted as single-molecule magnet (SMM) behavior. Remarkably, the contribution of the anisotropy of Dy(III) ion to the effective energy barrier is more significant than that of intrachain ferromagnetic interaction (Δξ). 9599

DOI: 10.1021/acs.inorgchem.7b01058 Inorg. Chem. 2017, 56, 9594−9601

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(4) Wernsdorfer, W.; Aliaga-Alcalde, N.; Hendrickson, D. N.; Christou, G. Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets. Nature 2002, 416, 406−409. (5) Sun, H.-L.; Wang, Z.-M.; Gao, S. Strategies towards single-chain magnets. Coord. Chem. Rev. 2010, 254, 1081−1100. (6) Bernot, K.; Bogani, L.; Caneschi, A.; Gatteschi, D.; Sessoli, R. A Family of Rare-Earth-Based Single Chain Magnets: Playing with Anisotropy. J. Am. Chem. Soc. 2006, 128, 7947−7956. (7) Wang, X.-F.; Li, Y.-G.; Hu, P.; Wang, J.-J.; Li, L.-C. Slow magnetic relaxation and field-induced metamagnetism in nitronyl nitroxide− Dy(III) magnetic chains. Dalton Trans. 2015, 44, 4560−4567. (8) (a) Zhang, P.; Zhang, L.; Wang, C.; Xue, S.-F.; Lin, S.-Y.; Tang, J.-K. Equatorially Coordinated Lanthanide Single Ion Magnets. J. Am. Chem. Soc. 2014, 136, 4484−4487. (b) Guo, Y.-N.; Xu, G.-F.; Gamez, P.; Zhao, L.; Lin, S.-Y.; Deng, R.-P.; Tang, J.-K.; Zhang, H.-J. Two-Step Relaxation in a Linear Tetranuclear Dysprosium(III) Aggregate Showing Single-Molecule Magnet Behavior. J. Am. Chem. Soc. 2010, 132, 8538−8539. (c) Zhang, P.; Zhang, L.; Lin, S.-Y.; Xue, S.-F.; Tang, J.-K. Modulating Magnetic Dynamics of Dy2 System through the Coordination Geometry and Magnetic Interaction. Inorg. Chem. 2013, 52, 4587−4592. (d) Guo, Y.-N.; Xu, G.-F.; Wernsdorfer, W.; Ungur, L.; Guo, Y.; Tang, J.; Zhang, H.-J.; Chibotaru, L. F.; Powell, A. K. Strong Axiality and Ising Exchange Interaction Suppress Zero-Field Tunneling of Magnetization of an Asymmetric Dy2 Single-Molecule Magnet. J. Am. Chem. Soc. 2011, 133, 11948−11951. (e) Zhang, L.; Jung, J.; Zhang, P.; Guo, M.; Zhao, L.; Tang, J.; Le Guennic, B. SiteResolved Two-Step Relaxation Process in an Asymmetric Dy2 SingleMolecule Magnet. Chem. - Eur. J. 2016, 22, 1392−1398. (9) Na, B.; Zhang, X.-J.; Shi, W.; Zhang, Y.-Q.; Wang, B.-W.; Gao, C.; Gao, S.; Cheng, P. Six-Coordinate Lanthanide Complexes: Slow Relaxation of Magnetization in the Dysprosium(III) Complex. Chem. Eur. J. 2014, 20, 15975−15980. (10) Han, T.; Leng, J.-D.; Ding, Y.-S.; Wang, Y.-Y.; Zheng, Z.-P.; Zheng, Y.-Z. Field and dilution effects on the magnetic relaxation behaviors of a 1D dysprosium(III)-carboxylate chain built from chiral ligands. Dalton Trans. 2015, 44, 13480−13484. (11) Habib, F.; Brunet, G.; Vieru, V.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. Significant Enhancement of Energy Barriers in Dinuclear Dysprosium Single-Molecule Magnets through ElectronWithdrawing Effects. J. Am. Chem. Soc. 2013, 135, 13242−13245. (12) Ruamps, R.; Maurice, R.; de Graaf, C.; Guihéry, N. Interplay between Local Anisotropies Binuclear Complexes. Inorg. Chem. 2014, 53, 4508−4516. (13) (a) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. A N23−Radical-Bridged Terbium Complex Exhibiting Magnetic Hysteresis at 14 K. J. Am. Chem. Soc. 2011, 133, 14236−14239. (b) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Strong exchange and magnetic blocking in N23−-radical-bridged lanthanide complexes. Nat. Chem. 2011, 3, 538−542. (c) Demir, S.; Zadrozny, J. M.; Nippe, M.; Long, J. R. Exchange Coupling and Magnetic Blocking in Bipyrimidyl RadicalBridged Dilanthanide Complexes. J. Am. Chem. Soc. 2012, 134, 18546−18549. (14) Long, J.; Habib, F.; Lin, P.-H.; Korobkov, I.; Enright, G.; Ungur, L.; Wernsdorfer, W.; Chibotaru, L. F.; Murugesu, M. Single-Molecule Magnet Behavior for an Antiferromagnetically Superexchange-Coupled Dinuclear Dysprosium(III) Complex. J. Am. Chem. Soc. 2011, 133, 5319−5328. (15) (a) Sulway, S. A.; Layfield, R. A.; Tuna, F.; Wernsdorfer, W.; Winpenny, R. E. P. Single-molecule magnetism in cyclopentadienyldysprosium chlorides. Chem. Commun. 2012, 48, 1508−1510. (b) Zhou, Q.; Yang, F.; Liu, D.; Peng, Y.; Li, G.-H.; Shi, Z.; Feng, S.-H. Synthesis, Structures, and Magnetic Properties of Three Fluoride-Bridged Lanthanide Compounds: Effect of Bridging Fluoride Ions on Magnetic Behaviors. Inorg. Chem. 2012, 51, 7529−7536. (16) Bogani, L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Molecular Engineering for Single-Chain-Magnet Behavior in a One-Dimensional Dysprosium-Nitronyl Nitroxide Compound. Angew. Chem., Int. Ed. 2005, 44, 5817−5821.

rate of 3 also make the butterfly shaped hysteresis loop occur at the relatively higher temperature, as evidenced in Figure 3.

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CONCLUSIONS An unusual water and hydroxyl-derived one-dimensional dysprosium(III) chain with a relatively high spin-reversal energy barrier of 88.7 K and favorable ferromagnetic superexchange was hydrothermally synthesized. Comparisons with the magnetic data of isostructural Gd(III)- and Y(III)-doped analogues strongly suggest that the slow relaxation process of the dysprosium(III) chain significantly results from the cooperative effects of intrachain ferromagnetic coupling and the strong Ising-type anisotropy of Dy(III) ion, in which the latter is the main contribution to the effective energy barrier.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01058. Selected bond lengths and angles, PXRD patterns, TG curve and structural figure for 1, as well as fitting results for Cole−Cole plots for both 1 and 3 (PDF) Accession Codes

CCDC 1527249 and 1545680−1545682 contain 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 Authors

*E-mail: [email protected] (Y.-Q. Z.). *E-mail: [email protected] (E.-C. Y.). *E-mail: [email protected] (X.-J. Z.). ORCID

Yi-Quan Zhang: 0000-0003-1818-0612 Xiao-Jun Zhao: 0000-0002-6371-9528 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the 973 Program (2014CB845601), National Natural Science Foundation of China (21571140, 21531005, and 21371134), the Program for Innovative Research Team in University of Tianjin (TD12− 5038), the Doctoral Program Foundation of Tianjin Normal University (52XB1609), and the Natural Science Foundation of Jiangsu Province of China (BK20151542).

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