Single-Molecule Magnet Behavior of 1D Coordination Polymers

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Single-Molecule Magnet Behavior of 1D Coordination Polymers Based on DyZn2(salen)2 Units and Pyridin‑N‑Oxide-4-Carboxylate: Structural Divergence and Magnetic Regulation Cai-Ming Liu,*,† De-Qing Zhang,† Jing-Bu Su,‡ Yi-Quan Zhang,*,‡ and Dao-Ben Zhu†

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†

Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry, CAS Research/Education Center for Excellence in Molecular Science, Chinese Academy of Sciences, Beijing 100190, China ‡ Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, China S Supporting Information *

ABSTRACT: Two 1D coordination polymers composed of DyZn2(salen)2 units and pyridin-N-oxide-4-carboxylate have been prepared by solvothermal reactions. Complex 1 with the formula {[DyZn 2 (L a ) 2 (POC)](OH)(ClO 4 )}·H 2 O·MeOH [H2La] = N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane, POC− = pyridin-N-oxide-4-carboxylate] is composed of a zigzag chain cation [DyZn2(La)2(POC)]n2n+ as well as isolated hydroxide and perchlorate anions. Complex 2 with the formula {[Dy3Zn7(Lb)6(POC)6](OH)3(ClO4)2}·9H2O [H2Lb] = N,N′bis(3-methoxysalicylidene)-1,2-diaminoethane] is also an ionic compound containing isolated hydroxide and perchlorate anions, but its cation shows a novel nanowire structure, in which sixcoordinate zinc(II) ions with C3 symmetry act as the axis and [DyZn2(Lb)2(POC)]2+ structural units as spiral leaves. Complex 1 shows good single-molecule magnet performance with an Ueff/k value of 235.3(3.1) K (Hdc = 0 Oe), one of the largest values for coordination polymers. A butterfly-shaped magnetic hysteresis loop can be monitored at as high as 3.8 K for 1, while a dc field is necessary for complex 2 to display slow magnet relaxation owing to the quantum-tunneling effect, with a much smaller Ueff/k value of 14.6(0.2) K (Hdc = 1000 Oe). The difference of magnetic properties has been explained using detailed ab initio calculations.

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INTRODUCTION

An alternative strategy is to assemble 3d-4f polynuclear SMMs using one lanthanide(III) ion and one or more diamagnetic 3d metal ions.6 Among such 3d-4f SMMs, the performance of DyZn2(salen)2-type SMMs is impressive, because they can both exhibit large Ueff values and have good TB.7 In this type of SMMs, the diamagnetic zinc(II) ions do not participate in the magnetic exchange, which help to ensure the magnetic anisotropy of the Dy(III) ion. Moreover, the zinc(II) Schiff base complex fragments sometime may show an antenna effect and help the lanthanide ions to exhibit fluorescent properties.8 From the viewpoint of crystal engineering, two Zn atoms in a DyZn2(salen)2 SMM unit may be further linked by bridging ligands, generating molecular aggregates or extended structures. For example, {[Zn(Me2valpn)]2Dy} [H2Me2valpn = N,N′-2,2dimethylpropylenebis(3-methoxysalicylideneimine)] structural units have been successfully bridged by [Cr(CN)6]3− and [Co(CN)6]3− inorganic anions, forming two cyclic molecular aggregates {[Zn(Me2valpn)]2Dy(H2O)Co(CN)6}2·15H2O·

Single-molecule magnets (SMMs) are a special type of molecular magnets with magnetic bistability and magnetic relaxation characteristics, which have attracted wide attention due to their potential applications in high-density information storage, quantum information processing, and other fields.1,2 Naturally, the SMM properties often occur in nanoscale multinuclear cluster complexes, in which large spin ground states are easily obtained. However, the significant weakness is that their magnetic anisotropy is not easy to control because the magnetic moments from different paramagnetic ions possibly diminish each other. To overcome this obstacle, mononuclear lanthanide(III) complexes are generally used to study the SMM properties. The advantage is that the magnetic anisotropy can be ensured while the lanthanide(III) ion itself has large magnetic moment.3 Quite recently several mononuclear Dy(III) SMMs have been explored that have outstanding effective thermal barrier (Ueff) and high blocking temperature (TB).4 Notably, magnetic hysteresis may be observed at as high as 60 K for a dysprosocenium complex though it is air-sensitive.5 © XXXX American Chemical Society

Received: June 15, 2018

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DOI: 10.1021/acs.inorgchem.8b01653 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 2DMF·5CH3CN and {[Zn(Me2-valpn)]2Dy(H2O)Cr(CN)6}2· 7H2O·4DMF,9 which show SMM properties in the 0 and 2000 Oe dc fields with the energy barrier of 85.9 and 100.9 K, respectively. However, no coordination polymer based on DyZn2(salen)2 structural units has been explored. In recent years, we are very interested in the coordination polymers behaving as SMMs,10 because their nodes can be viewed as highly ordered SMMs in the dimension extension space, and the orientation and arrangement of such nodes may have an obvious effect on SMM properties;10f moreover, the guest molecules may be used to adjust SMM properties for microporous coordination polymers.11 We hope to link DyZn2(salen)2 SMM units with organic bridging ligands for construction of coordination polymers maintaining SMM properties. We chose pyridin-N-oxide-4-carboxylate as the organic bridging ligand to connect with DyZn2(salen)2 SMM units, and obtained two novel 1D ionic coordination polymers {[DyZn2(La)2(POC)](OH)(ClO4)}·H2O·MeOH [1, H2La = N,N′-bis(3-methoxysalicylidene)-1,3-diaminopropane, POC− = pyridin-N-oxide-4-carboxylate] and {[Dy3Zn7(Lb)6(POC)6](OH)3(ClO4)2}·9H2O [2, H2Lb = N,N′-bis(3-methoxysalicylidene)-1,2-diaminoethane]. Remarkably, the length of the imine carbon chain on the Schiff base ligands has a great influence on the structures, causing obvious differences in the SMM properties. Complex 1 has good SMM performance, while complex 2 shows a beautiful C3 symmetrical spiral-chain structure.

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630(w), 670(w), 477(w), 429(w). XPS: Cl 2p 207.45, C 1s 284.91, N 1s 399.15, O 1s 531.91, Zn 2p 1021.69, Dy 3d 1297.02 eV (Figure S1). Preparation of {[Dy3Zn7(Lb)6(POC)6](OH)3(ClO4)2}·9H2O (2). The procedure was the same as that for 1 but using Zn(Lb)(H2O) instead of Zn(La)(H2O). Yellowish needle crystals of 2 were obtained, washed with methanol, and then dried at ambient temperature. Yield: 65% based on Dy. Analysis calcd for C144H153Cl2Dy3N18O62Zn7 (2): C, 41.73; H, 3.72; N, 6.08. Found: C, 41.65; H, 3.76; N, 6.03. IR (KBr, cm−1): 3636(w), 3442(br, s), 3109(w), 3071(w), 3027(w), 2929(w), 2849(w), 2834(w), 1658(s), 1642(vs), 1602(w), 1555(m), 1472(s), 1450(s), 1415(s), 1336(w), 1286(s), 1248(s), 1172(w), 1152(w), 1107(s), 1087(s), 1073(s), 1038(w), 969(s), 957(w), 916(w), 872(w), 851(w), 785(m), 746(m), 738(m), 685(w), 656(m), 641(w), 623(m), 593(w), 559(w), 511(w), 451(w), 436(w). XPS: Cl 2p 207.40, C 1s 285.29, N 1s 399.01, O 1s 531.79, Zn 2p 1021.63, Dy 3d 1297.09 eV (Figure S2). X-ray Crystallography. X-ray diffraction was measured on a Rigaku ST-Saturn724+ CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 173 K. Two structures were solved by direct methods and refined by the full-matrix least-squares technique on F2 using the SHELXL97 program. All non-hydrogen atoms were refined anisotropically; all hydrogen atoms except those in hydroxide anions and solvent molecules were allowed for as riding atoms, because it is impossible to distinguish between the solvent water molecule and the free hydroxide anion. Crystallographic data are summarized in Table 1.

Table 1. Crystal Data and Structural Refinement Parameters for 1 and 2 1

2

C45H51ClDyN5O18Zn2 1278.68 monoclinic P21/c 14.7773(2) 25.0582(5) 14.8003(2) 104.5810(17) 5303.93(16) 4 1.592 2.415 173(2) 0.71073 29366 9357 7810

C144H153Cl2Dy3N18O62Zn7 4144.11 trigonal P3̅c1 23.4707(2) 23.4707(2) 17.9949(2) 90 8584.86(17) 2 1.595 2.369 173(2) 0.71073 87306 9241 6925

653 1.056 0.0697 0.1938

363 1.047 0.0675 0.1787

EXPERIMENTAL PROCEDURES formula FW crystal system space group a [Å] b [Å] c [Å] β [°] V [Å3] Z ρcalc [g·cm−3] μ [mm−1] T [K] λ(Mo Kα) [Å] reflections collected unique reflections observed reflections parameters GoF R1 wR2

Materials and Methods. All chemicals and solvents were obtained from commercial sources and used without further purification. The zinc(II) Schiff base mononuclear precursors Zn(La)(H2O)12a and Zn(Lb)(H2O)12b were synthesized as previously described.12 Caution! Perchlorate salts or perchlorate complexes are potentially explosive and should be handled with care and used only in small amounts. Physical Measurements. The elemental analyses were carried out on a Heraeus Chn-Rapid elemental analyzer. The infrared spectra were measured on a Pekin-Elmer 2000 spectrophotometer with pressed KBr pellets. The X-ray powder diffraction (XRD) pattern was recorded on a Rigaku D/max 2500 diffractometer with Cu Kα (λ = 1.5418 Å) radiation. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation. The 500 μm X-ray spot was used for SAXPS analysis. The base pressure in the analysis chamber was about 3 × 10−9 mbar. Typically the hydrocarbon C 1s line at 284.8 eV from adventitious carbon is used for energy referencing. Variabletemperature magnetic susceptibility, ac magnetic susceptibility, and field dependence of magnetization were performed on a Quantum Design MPMS-XL5 (SQUID) magnetometer. Diamagnetic corrections were estimated from Pascal’s constants for all constituent atoms. Preparation of {[DyZn2(La)2(POC)](OH)(ClO4)}·H2O·MeOH (1). Zn(La)(H2O) (0.2 mmol), Dy(ClO4)3·6H2O (0.1 mmol), pyridinN-oxide-4-carboxylic acid (0.2 mmol), and 10 mL of methanol were stirred at room temperature for 10 min, this mixture was then transferred into 25 mL Teflon-lined stainless-steel vessel and maintained at 100 °C for 3 days under autogenous pressure. After the autoclave had cooled to room temperature at rate of 0.15 °C/min, light brown plate crystals of 1 were harvested, washed with methanol, and then dried at ambient temperature. Yield: 35% based on Dy. Analysis calcd for C45H51ClDyN5O18Zn2 (1): C, 42.27; H, 4.02; N, 5.48. Found: C, 42.31; H, 4.05; N, 5.44. IR (KBr, cm−1): 3420(br, s), 3085(w), 2933(w), 2857(w), 2838(w), 1626(vs), 1597(s), 1573(s), 1475(w), 1463(m), 1410(m), 1369(s), 1315(w), 1291(w), 1275(w), 1223(m), 1192(w), 1160(w), 1087(w), 1076(w), 1020(w), 978(w), 954(w), 913(w), 852(w), 777(m), 741(w), 721(w), 677(w), 643(w),

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RESULT AND DISCUSSION Syntheses. The two 1D Dy−Zn heterometallic complexes were prepared using similar solvothermal procedures. The zinc(II) Schiff base mononuclear precursors Zn(La)(H2O) and Zn(Lb)(H2O) were utilized to treat with Dy(ClO4)3·6H2O and pyridin-N-oxide-4-carboxylic acid in methanol at 100 °C for 3 days, yielding 1 and 2, respectively. The phase purity of both compounds was checked by XRD spectra (Figures S3 and S4). The DyZn2(salen)2 trinuclear units were formed automatically during solvothermal reactions, and isolated B


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Inorganic Chemistry

phenoxide O atoms from the [La]2− Schiff base ligand, while the apical position is occupied by one carboxylate O atom of the POC− bridging ligand for the Zn1 atom and one pyridineN-oxide O atom of the POC− bridging ligand for the Zn2 atom. Notably, the Dy atom in 1 is eight-coordinate and not nine-coordinate as in other DyZn2(salen)2-type SMMs,7,9 whose coordination geometry is completed by one Ocarboxylate atom from the POC− bridging ligand, four O atoms from one [La]2− Schiff base ligand, and three O atoms from the other [La]2− Schiff base ligand in which one methoxy O atom is not coordinated (the distance between the central Dy atom and the O atom of the uncoordinated methoxy group is 3.010 Å). The SHAPE software14 analysis indicates that the coordination geometry of the Dy1 atom is closest to the biaugmented trigonal prism with a relatively large deviation value of 1.555 from the ideal C2v symmetry (Table S1), suggesting a low geometrical symmetry. The Dy−O distance of 2.275(6)− 2.524(6) Å is in the normal range, with the average value of 2.380(6) Å (Table S2). The [DyZn2(La)2]3+ trinuclear structural units are connected with each other though the POC− bridging ligands on the same side, forming a 1D zigzag cation chain [DyZn2(La)2(POC)]n2n+ along the b-axis (Figure 1b,c). The POC− anion is a tridentate bridging ligand, with its two carboxylate oxygen atoms connecting the Zn1 and Dy1 atoms of the [DyZn2(La)2]3+ structural unit, and using its pyridine-N-oxide O atom to connect the Zn2 atom of an adjacent [DyZn2(La)2]3+ structural unit. Complex 2 is also an ionic compound composed of 1D chain cation as well as hydroxide and perchlorate anions. However, its chain cation is quite different. As shown in Figure 2, a C3 symmetrical zinc(II) pyridin-N-oxide-4-carboxylate complex fragment, [Zn(POC)6]4−, exists in 2, which acts as a 6-interface bridge, connecting six [DyZn2(Lb)2]3+ building blocks. The Zn atom in the [Zn(POC)6]4− complex fragment is coordinated by six pyridine-N-oxide O atoms from six POC− anions, exhibiting a distorted octahedron configuration with the Zn−O distance of 2.102(5) Å (Table S2). The [DyZn2(Lb)2]3+ trinuclear structural unit in 2 is formed by using two opposing [Zn(Lb)] moieties to coordinate the Dy atom in a center of symmetry, with the Zn−Dy−Zn angle of 168.5°, a little smaller than that in 1 (174.5°). Each [Zn(Lb)] moiety provides two phenoxide O atoms and one methoxy O atom to coordinate to the Dy atom, while the other methoxy O atom is free (the distance between the central Dy atom and the O atom of the uncoordinated methoxy group is 2.987 Å). Moreover, the Dy atom is further coordinated by two carboxylate O atoms from two [Zn(POC)6]4− bridging ligands. Therefore, the Dy atom is also eight-coordinate, but showing a square antiprism geometry with a deviation value of 2.295 from the ideal D4d symmetry (Table S3). The mean Dy−O distance of 2.411(4) Å is a little larger than that of 1 (2.380(6) Å) (Table S2). Similar to 1, two zinc atoms in the [DyZn2(Lb)2]3+ building block of 2 also exhibit a classical square pyramidal geometry: Its base is composed of two imine N atoms and two phenoxide O atoms from the [Lb]2− Schiff base ligand, but its apical position is occupied by one Ocarboxylate atom from the [Zn(POC)6]4− bridging ligand. Each [Zn(POC)6]4− bridging ligand is connected to six [DyZn2(Lb)2]3+ structural units as a C3-symmetrical axis, which in turn also act as another bridging ligand to attach the [Zn(POC)6]4− ligands. Consequently, a novel C3-symmetrical cationic nanowire with the [Zn(POC)6]4− anion as the axis and spokes as well as the [DyZn2(Lb)2]3+ cation as the helical leaf is created along the c-

hydroxide and perchlorate anions exist in both complexes as counterions. Although uncommon, the lanthanide complexes containing uncoordinated hydroxide anions were also reported by other groups.13 Surprisingly, we noticed that a sixcoordinated [Zn(POC)6]4− complex fragment was formed in complex 2, in which the Zn(II) atom is coordinated by six pyridine-N-oxide O atoms from six POC− ligands, acting as an axis of C3 symmetry; therefore, the DyZn2(salen)2 trinuclear units in 2 are bridged by the [Zn(POC)6]4− anion rather than the POC− anion itself that is observed in 1. Two bridging modes lead to large differences in the structure of both complexes. Crystal Structures. Complexes 1 and 2 are crystallized in the P21/c and P3̅c1 space groups, respectively. As shown in Figure 1, complex 1 is a 1D zigzag chain ionic compound,

Figure 1. Structural fragment of the cation chain in 1 (a), symmetry code: #, x, 0.5 − y, −0.5 + z; 1D zigzag cation chain in 1 (b); view of the packing arrangement of 1 down the a-axis (c). All lattice solvent molecules and hydroxide anions are omitted for clarity.

whose cation is composed of POC − ligand bridging [DyZn2(La)2]3+ structural units, while hydroxide and perchlorate anions act as counterions. The central Dy ion is stuck in two almost vertical [Zn(La)] moieties, generating the [DyZn2(La)2]3+ trinuclear structural unit with the Zn−Dy−Zn angle of 174.5°, similar to the [DyZn2(Me2valpn)2(H2O)]3+ structural units in {[Zn(Me2valpn)]2Dy(H2O)Co(CN)6}2· 15H2O·2DMF·5CH3CN and {[Zn(Me2-valpn)]2Dy(H2O)Cr(CN)6}2·7H2O·4DMF.9 Both zinc atoms are the linking point of the POC− bridging ligand. Each zinc atom exhibits a classical square pyramidal configuration: the bottom of the square pyramid consists of two imine N atoms and two C


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Inorganic Chemistry

Figure 2. Structural fragment of the cation chain in 2 (a), symmetry codes:$, x − y, −y, 1.5 − z and &, −y, y − x, 1.5 − z; top view of the C3 symmetry spiral chain in 2 (b); side view of the C3 symmetry spiral chain in 2 (c), all lattice solvent molecules and hydroxide anions are omitted for clarity; schematic representation of the cation chain topology of 2, the link represents the POC− ligand (d).

the values of 4.98 Nβ for 1 and 15.76 Nβ for 2 at 5 T are obviously lower than the theoretical saturation values of 10 Nβ for one Dy3+ ion and 30 Nβ for three Dy3+ ions owing to magnetic anisotropy (Figure S5). The magnetic anisotropy of the single Dy3+ ion in both complexes was confirmed by the M versus H/T curves at different temperatures (2−6 K), which do not coincide (Figures S6 and S7). In order to explore the magnetic dynamics and magnetic relaxation of 1 and 2, their temperature-dependent alternating current (ac) susceptibility and frequency-dependent ac susceptibility were measured in details. As shown in Figure 4a, the out-of-phase component (χ′′) of ac susceptibility of 1 is obviously dependent on the frequency under zero dc magnetic field. Peaks appear between 16 and 19.5 K, the shift of peak temperature (Tp) of χ″ is usually determined using the parameter Φ = (ΔTp/Tp)/Δ(log ω). The Φ value of 1 is 0.16, closer to the normal value of superparamagnet (≥0.1) but much larger than that of spin glass (about 0.01), indicating slow magnetic relaxation, typical SMM behavior.15 The ln(τ) versus 1/T plot is well in line with the Arrhenius law, τ = τ0 exp(Ueff/kT), the best fitting gives the Ueff/k value of 235.3(3.1) K and the τ0 value of 4.3(1) × 10−11 s (Figure S8). The results indicate that complex 1 has a relatively high effective thermal barrier, which is the second highest Ueff/k value for DyZn2(salen)2-type SMMs under zero dc field7,8d and much larger than that of {[Zn(Me2valpn)]2Dy(H2O)Co(CN)6}2·15H2O·2DMF·5CH3CN (85.9 K),9 and is also one of the highest Ueff/k values for coordination polymers;16 additionally, its τ0 value of 4.3(1) × 10−11 s is a normal value for a SMM or single-ion magnet. Furthermore, the frequencydependent ac susceptibility of 1 indicates that the magnetic relaxation is also temperature-dependent at 2−18 K (Figure S9). Semicircular curves can be clearly observed in the Cole− Cole plots for each temperature, suggesting a single relaxation process (Figure 4b). The α values could be obtained by fitting the Cole−Cole plots with a generalized Debye model,17 giving α parameter in the range of 0.15−0.32 (Table S4), indicating that the distribution of relaxation times is narrow. It is necessary to point out that the observed increase of χ′′ of 1 in the low temperature zone in Figure 4a reveals the quantum tunneling process, which generally occurs between

axis (Figure 2b,c). The axial Zn−Zn spacing is 8.997 Å, and the Dy−Znaxis−Dy distance is 19.359 Å. In order to understand this structure more intuitively, we consider the [DyZn2(Lb)2]3+ unit as a point that is connected to the central zinc ion (another point) through the simplified linear ligand POC− to form the illustrated topology, as shown in Figure 2d, whose Schälfli symbol is (42)(42)(42)(46). Magnetic Properties. The direct current (dc) magnetic susceptibility measurements show that the room temperature χT values of 14.18 cm3 K mol−1 for 1 and 42.65 cm3 K mol−1 for 2 are in accordance with the theoretical values of 14.17 cm3 K mol−1 and 42.51 cm3 K mol−1 for one Dy3+ ion and three noninteracting Dy3+ ions (6H15/2, S = 5/2, L = 5, g = 4/3), respectively. As shown in Figure 3, the χT value of two compounds slowly decreases as the temperature decreases, most likely caused by the thermal depopulation of Mj levels of the Dy3+ ion. From 0 to 1 T, the field dependence of the magnetization at 2 K for both complexes reveals a rapid increase of the magnetization, which then increases slowly, and

Figure 3. Plots of χT versus T of 1 and 2 measured under 1000 Oe field. The lines are the simulation from ab initio calculation. D


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Inorganic Chemistry

Figure 5. Temperature dependence of χ′′ for 1 under 1000 Oe dc field (a); plot of ln(τ) versus 1/T for 1 (Hdc = 1000 Oe). The solid lines represent the best fitting with the Arrhenius law (blue) and Raman plus Orbach (cyan) (b).

Figure 4. Temperature dependence of χ′′ for 1 under zero dc field (a); Cole−Cole plots at 2−18 K for 1 (Hdc = 0 Oe and Hac = 2.5 Oe), the solid lines represent the best fitting (b).

degenerated ground states and may play a dominant role at low temperatures. Therefore, ac behavior under an applied field of 1000 Oe of 1 was also measured in order to carry out the analysis in a wider range of peak temperatures. As shown in Figure 5a, under a 1000 Oe dc field, peaks of χ′′ versus T curves appear between 9.5 and 24 K. This temperature range is greater than the temperature range in the 0 dc field (16−19.5 K), and a peak can occur at frequencies as low as 1 Hz. The fit of the ln(τ) versus 1/T plot to the Arrhenius law gives the Ueff/ k value of 358.6(7) K and the τ0 value of 3.7(0.3) × 10−11 s (Figure 5b). Because the ln(τ) versus 1/T plot shows nonlinear at low temperatures, the data can also fitted by Orbach and Raman processes: τ−1 = CTn + τ0−1 exp(−Ueff/kT) (Figure 5b), giving Ueff/k = 372.4(9) K, C = 3.7(0.1) × 10−3, n = 4.56(0.8), and τ0 = 2.39 × 10−11 s. The n value is reasonable (1 < n < 6).7b As expected, after the application of the dc field, the thermal barrier of 1 increases, and its τ0 value decreases.7b For 2, no obvious response signal of χ′′ could be observed under zero dc field owing to a fast relaxation process via the quantum tunneling effect (Figure S10). However, a small static field can induce peaks to appear in χ′′ and make it frequencydependent, because the quantum tunneling effect may be restrained by a suitable dc magnetic field. The optimum dc field was determined to be 1000 Oe through 997 Hz fielddependent ac susceptibilities at 3 K (Figure S11). As shown in Figure 6a, well-shaped frequency-dependent peaks could be observed in the frequency range of 250−1442 Hz under this optimum dc field. The Φ value of 2 is 0.44, also confirming its SMM behavior. Interestingly, two-step relaxation processes were clearly observed from temperature-dependent ac

Figure 6. Temperature dependence of χ′′ for 2 under 1000 Oe dc field (a). Cole−Cole plots at 1.9−2.9 K for 2 (Hdc = 1000 Oe and Hac = 2.5 Oe); the solid lines represent the best fitting (b).

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Inorganic Chemistry susceptibility at 1.9−2.9 K (Figure S12). The corresponding Cole−Cole plots display six incomplete double-semicircle shapes (Figure 6b), verifying the two-step relaxation process. The left incomplete semicircle corresponds to the fast relaxation (FR) phase and the right incomplete semicircle corresponds to the slow relaxation (SR) phase. The effective energy barriers were estimated with the Arrhenius law (Figure S13), giving Ueff/k = 7.7(0.2) K and τ0 = 1.2(0.1) × 10−5 s for the FR as well as Ueff/k = 14.6(0.2) K and τ0 = 7.2(0.7) × 10−7 s for the SR. The τ0 values of 2 are also normal, but its Ueff/k values are much smaller than that of 1. The Cole−Cole plots were fit by the sum of two modified Debye functions,18 giving α1 values in the range of 0.09−0.13 and α2 values in the range of 0.31−0.64 (Table S5), indicating the FR phase has a relatively narrower distribution of relaxation times than the SR phase. Owing to existence of only one type of Dy atom in 2, the two-step relaxation process should be attributed to field induction.19 Magnetic hysteresis, another important feature of SMMs, was measured with the normal sweep rate (100−300 Oe min−1). For complex 1, a butterfly-shaped hysteresis loop is observed at 1.9 K (Figure 7), which is characteristic of Dy(III)

ions; VTZ for close N; VDZ for distant atoms. The calculations employed the second-order Douglas−Kroll−Hess Hamiltonian, where scalar relativistic contractions were taken into account in the basis set and the spin−orbit couplings were handled separately in the restricted active space state interaction (RASSI-SO) procedure. For individual Dy3+ fragments, active electrons in 7 active spaces include all f electrons [CAS(9 in 7)] in the CASSCF calculation. To exclude all the doubts, we calculated all the roots in the active space. We have mixed the maximum number of spin-free state which was possible with our hardware (all from 21 sextets, 128 from 224 quadruplets, 130 from 490 doublets). Single_Aniso23 program was used to obtain the g tensors, energy levels, magnetic axes, and so on based on the above CASSCF/RASSI calculations. The calculated χT versus T plots of complexes 1 and 2 are shown in Figure 3. The lowest eight Kramers doublets (KDs) and the corresponding g tensors of individual Dy3+ fragment of complexes 1 and 2 are shown in Table 2. Considering the Table 2. Calculated Energy Levels (cm−1), g (gx, gy, gz) Tensors and mJ Values of the Lowest Eight Kramers Doublets (KDs) of Individual Dy3+ Fragment of Complexes 1 and 2 1 KDs

E (cm−1)

1

0.0

2

183.4

3

205.2

4

272.5

5

339.3

6

396.8

7

506.0

8

688.8

Figure 7. Hysteresis loops for 1 measured at different temperature.

single-ion magnets. The loop is narrowed as the temperature increases, but it can still be probed at 3.8 K (Figure 7), suggesting the blocking temperature of 1 is 3.8 K. Notably, although such butterfly-shaped hysteresis loops are common in multinuclear and mononuclear complexes,20 this phenomenon is seldom observed in coordination polymers, though recent researches indicated that a magnetic-diluted sample of the 1D coordination polymer [Dy(L)2(phen)(μ2-OH)(μ2-H2O)]n (HL= 4-nitrobenzoic acid, phen = 1,10-phenanthroline)16f and a magnetic-diluted sample of the 3D coordination polymer {(H3O)[Dy(NA)2]·H2O} n (H2NA = 5-hydroxynicotinic acid)21 also show butterfly-shaped hysteresis loops. As a contrast, no hysteresis loop can be detected for 2 at 1.9 K as expected (Figure S14). Theoretical Calculations. Complexes 1 and 2 have one type of Dy3+ ion; thus, only one individual Dy3+ fragment for them was calculated, respectively. Complete-active-space selfconsistent field (CASSCF) calculations on individual Dy3+ fragments of the model structures (see Figure S15 for the calculated model structure of complex 1) extracted from the compounds on the basis of single-crystal X-ray determined geometry have been carried out with MOLCAS 8.2 program package.22 The basis sets for all atoms are atomic natural orbitals from the MOLCAS ANO-RCC library: ANO-RCC-VTZP for Dy3+

g 0.001 0.001 19.658 0.425 1.294 15.899 0.544 0.975 15.605 1.356 2.498 12.733 2.255 4.766 9.835 2.255 4.766 9.835 0.546 0.802 16.354 0.046 0.111 19.106

2 mJ

E (cm−1)

±15/2

0.0

±13/2

24.3

±9/2

69.7

±11/2

133.6

±7/2

148.1

±5/2

266.2

±1/2

336.5

±3/2

482.3

g 0.332 0.740 18.706 0.148 0.446 17.714 0.847 1.101 13.181 1.352 1.566 13.685 9.850 7.293 1.988 2.249 3.072 9.708 1.525 4.816 11.607 0.122 0.353 18.515

mJ ±15/2

±13/2

±11/2

±7/2

±1/2

±5/2

±3/2

±9/2

large Dy−Dy distances in complexes 1 and 2, the Dy3+−Dy3+ interactions can be omitted, and their magnetic anisotropies are mainly from individual Dy3+ fragment. Table 2 shows the energy gaps between the lowest two KDs of individual Dy3+ fragment of complexes 1 and 2 are 183.4 and 24.3 cm−1, respectively, both of which are close to the experimental data. The above result also confirms that the magnetic anisotropies of 1 and 2 are mainly decided by individual Dy3+ fragment, as expected. Each state in Table 2 indicated by the predominant mJ value is not the pure one, which are composed by several mJ states. The mJ components for the lowest two doublets of F


Article

Inorganic Chemistry individual Dy3+ fragment for 1 and 2 are shown in Table 3, where the ground state of individual Dy3+ fragment of complex Table 3. Wave Functions with Definite Projection of the Total Moment |mJ⟩ for the Lowest Two Kramers Doublets (KDs) of Individual Dy3+ Fragment for Complexes 1 and 2 E (cm−1) 1 2

0.0 183.4 0.0 24.3

wave functions 97% 73% 85% 68%

|±15/2⟩ |±13/2⟩ + 9% |±11/2⟩ + 6% |±9/2⟩ + 8% |±7/2⟩ |±15/2⟩ + 7% |±11/2⟩ + 4% |±9/2⟩ |±13/2⟩ + 30% |±11/2⟩

1 is mostly composed by mJ = 15/2 state, but that of complex 2 is mixed by mJ = 15/2 and 13/2 states (the mJ = 15/2 state is predominant). The first excited state of complexes 1 and 2 are composed by several mJ states (mJ = 13/2, 11/2, 9/2, and 7/2 for 1, mJ = 13/2 and 11/2 for 2). The smaller gx,y and the purer ground state of 1 lead to the smaller tunneling gap in the ground state (Figure 8). The main magnetic axes on Dy3+ ions of complexes 1 and 2 are indicated in Figure 9, where the included angle of the magnetic axis and the vector connecting two Zn2+ ions for 1 is much smaller than that of 2. The much smaller included angle of 1 shows the axial symmetry of the

Figure 9. Calculated orientations of the local main magnetic axes of the ground Kramers doublet on the individual Dy3+ fragment of complexes 1 (a) and 2 (b).

individual Dy3+ fragment of 1 is much higher than that of 2, which is the reason why the energy gap between the lowest two KDs of individual Dy3+ fragment of 1 is much larger than that of 2.

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CONCLUSIONS In summary, two novel 1D heterometallic coordination polymers based on DyZn2(salen)2 units and pyridin-N-oxide4-carboxylate were prepared and characterized. The alkyl chain length of the Schiff base diimine has great influences on not only the 1D structure but also the SMM behavior. Both are ionic compounds: a zigzag cation chain [DyZn2(La)2(POC)]n2n+ is observed in 1, while a C3symmetrical spiral chain [Dy3Zn7(Lb)6(POC)6]n5n+ is formed in 2. Complex 1 shows good SMM performance under zero dc field, with a large effective energy barrier of 235.3(3.1) K and a butterfly-shaped magnetic hysteresis loop at 3.8 K, while complex 2 displays slow magnet relaxation under a 1000 Oe dc field, with a much smaller effective energy barrier of 14.6(0.2) K. Because the slow magnetic relaxation in such polymeric lanthanide systems comes from the single ion behavior of lanthanide ions, the difference in the Dy3+ ion coordination environment between the two complexes may play an

Figure 8. Magnetization blocking barriers for individual Dy3+ fragment in complexes 1 (a) and 2 (b). The thick black lines represent the Kramers doublets as a function of their magnetic moment along the magnetic axis. The green lines correspond to diagonal quantum tunneling of magnetization (QTM); the blue line represents off-diagonal relaxation process. The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of transition magnetic moment. G


Article

Inorganic Chemistry

Chen, Y.-C.; Tong, M.-L. Symmetry strategies for high performance lanthanide-based single-molecule magnets. Chem. Soc. Rev. 2018, 47, 2431. (4) (a) Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; Chen, X.-M.; Tong, M.-L. A Stable Pentagonal Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization Reversal Barrier over 1000 K. J. Am. Chem. Soc. 2016, 138, 5441. (b) Gupta, S. K.; Rajeshkumar, T.; Rajaraman, G.; Murugavel, R. An air-stable Dy(III) single-ion magnet with high anisotropy barrier and blocking temperature. Chem. Sci. 2016, 7, 5181. (c) Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z. On Approaching the Limit of Molecular Magnetic Anisotropy: A Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2016, 55, 16071. (d) Meng, Y.-S.; Xu, L.; Xiong, J.; Yuan, Q.; Liu, T.; Wang, B.-W.; Gao, S. Low-Coordinate Single-Ion Magnets by Intercalation of Lanthanides into a Phenol Matrix. Angew. Chem., Int. Ed. 2018, 57, 4673. (5) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular Magnetic Hysteresis at 60 K in Dysprosocenium. Nature 2017, 548, 439. (6) (a) AlDamen, M. A.; Clemente-Juan, J. M.; Coronado, E.; MartíGastaldo, C.; Gaita-Ariño, A. Mononuclear Lanthanide SingleMolecule Magnets Based on Polyoxometalates. J. Am. Chem. Soc. 2008, 130, 8874. (b) Yamashita, A.; Watanabe, A.; Akine, S.; Nabeshima, T.; Nakano, M.; Yamamura, T.; Kajiwara, T. WheelShaped ErIIIZnII3 Single-Molecule Magnet: A Macrocyclic Approach to Designing Magnetic Anisotropy. Angew. Chem., Int. Ed. 2011, 50, 4016. (c) Feltham, H. L. C.; Lan, Y.; Klöwer, F.; Ungur, L.; Chibotaru, L. F.; Powell, A. K.; Brooker, S. Non-sandwiched Macrocyclic Monolanthanide Single-Molecule Magnet:The Key Role of Axiality. Chem. - Eur. J. 2011, 17, 4362. (d) Upadhyay, A.; Singh, S. K.; Das, C.; Mondol, R.; Langley, S. K.; Murray, K. S.; Rajaraman, G.; Shanmugam, M. Enhancing the effective energy barrier of a Dy(III) SMM using a bridged diamagnetic Zn(II) ion. Chem. Commun. 2014, 50, 8838. (e) Liu, C.-M.; Zhang, D.-Q.; Hao, X.; Zhu, D.-B. Trinuclear [CoIII2−LnIII] (Ln = Tb, Dy) Single-Ion Magnets with Mixed 6-Chloro-2-Hydroxypyridine and Schiff Base Ligands. Chem. - Asian J. 2014, 9, 1847. (7) (a) Sun, W.-B.; Yan, P.-F.; Jiang, S.-D.; Wang, B.-W.; Zhang, Y.Q.; Li, H.-F.; Chen, P.; Wang, Z.-M.; Gao, S. High symmetry or low symmetry, that is the question-high performance Dy(III) single-ion magnets by electrostatic potential design. Chem. Sci. 2016, 7, 684. (b) Costes, J. P.; Titos-Padilla, S.; Oyarzabal, I.; Gupta, T.; Duhayon, C.; Rajaraman, G.; Colacio, E. Analysis of the Role of Peripheral Ligands Coordinated to ZnII in Enhancing the Energy Barrier in Luminescent Linear Trinuclear Zn-Dy-Zn Single-Molecule Magnets. Chem. - Eur. J. 2015, 21, 15785. (8) (a) Costes, J. P.; Titos-Padilla, S.; Oyarzabal, I.; Gupta, T.; Duhayon, C.; Rajaraman, G.; Colacio, E. Effect of Ligand Substitution around the DyIII on the SMM Properties of Dual-Luminescent Zn-Dy and Zn-Dy-Zn Complexes with Large Anisotropy Energy Barriers: A Combined Theoretical and Experimental Magnetostructural Study. Inorg. Chem. 2016, 55, 4428. (b) Burrow, C. E.; Burchell, T. J.; Lin, P.-H.; Habib, F.; Wernsdorfer, W.; Clérac, R.; Murugesu, M. SalenBased [Zn2Ln3] Complexes with Fluorescence and Single-MoleculeMagnet Properties. Inorg. Chem. 2009, 48, 8051. (c) Wen, H.-R.; Dong, P.-P.; Liu, S.-J.; Liao, J.-S.; Liang, F.-Y.; Liu, C.-M. 3d-4f heterometallic trinuclear complexes derived from amine-phenol tripodal ligands exhibiting magnetic and luminescent properties. Dalton Trans 2017, 46, 1153. (d) Boulkedid, A.-L.; Long, J.; Beghidja, C.; Guari, Y.; Beghidja, A.; Larionova, J. A luminescent Schiff-base heterotrinuclear Zn2Dy single-molecule magnet with an axial crystal field. Dalton Trans 2018, 47, 1402. (f) Maity, M.; Majee, M. C.; Kundu, S.; Samanta, S. K.; Sañudo, E. C.; Ghosh, S.; Chaudhury, M. Pentanuclear 3d-4f Heterometal Complexes of MII3LnIII2 (M = Ni, Cu, Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism, and Photoluminescence Properties. Inorg. Chem. 2015, 54, 9715.

important role in the behavior of the SMM, and the ab initio calculations rationalize the large difference of the effective energy barrier in the two complexes. This work demonstrates that the structure and the SMM behavior of DyZn2(salen)2containing coordination polymers can be adjusted by small changes in the alkyl chain length of the Schiff base diimine. Moreover, our study reveals that the use of DyZn2(salen)2 as the structural unit allows obtaining coordination polymers with good SMM performance, which opens up a new approach to high-performance molecular nanomagnets.

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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.inorgchem.8b01653. More structural and magnetic pictures and tables (PDF) Accession Codes

CCDC 1848139−1848140 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.

■

AUTHOR INFORMATION

Corresponding Authors

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

Cai-Ming Liu: 0000-0001-7184-6693 De-Qing Zhang: 0000-0002-5709-6088 Yi-Quan Zhang: 0000-0003-1818-0612 Dao-Ben Zhu: 0000-0002-6354-940X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471154, 91022014 and 11774178), the National Key Basic Research Program of China (2013CB933403) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010103).

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J


Single-Molecule Magnet Behavior of 1D Coordination Polymers

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