Magnetic Interaction Affecting the Zero-Field Single-Molecule Magnet

Sep 6, 2017 - (2) Nevertheless, it must be stressed that fast quantum tunneling of magnetization (QTM) under zero-dc field occurs in most lanthanide S...
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Magnetic Interaction Affecting the Zero-Field Single-Molecule Magnet Behaviors in Isomorphic {NiII2DyIII2} and {CoII2DyIII2} Tetranuclear Complexes Haipeng Wu,†,⊥ Min Li,†,⊥ Sheng Zhang,†,‡ Hongshan Ke,† Yiquan Zhang,*,§ Guilin Zhuang,*,∥ Wenyuan Wang,† Qing Wei,† Gang Xie,† and Sanping Chen*,† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710127, China ‡ College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210097, China ∥ College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China S Supporting Information *

ABSTRACT: Great interest is being shown in investigating magnetic interactions that efficiently influence lanthanide singlemolecule magnet behavior. A series of heterometallic complexes [M2Ln2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (M = NiII, Ln = DyIII (1), GdIII (2), and YIII (3); M = CoII, Ln = DyIII (4), GdIII (5), and YIII (6)) have been prepared with a compartmental Schiff-base ligand, 1-(2-hydroxy-3-methoxybenzylidene)-semicarbazide (H2hms), featuring a zigzag-shaped MIILnIII-LnIII-MII metallic core arrangement. In complexes 1−6, a unique monophenoxo/diacetate asymmetric bridging connects MII ion with LnIII ion, and four acetates bridge two LnIII ions where acetates play essential roles as coligand in generating the tetranuclear units. Magnetic studies reveal the presence of predominant ferromagnetic coupling in DyIII and GdIII derivatives, and slow relaxation of magnetization is observed for {Ni2IIDy2III} and {CoII2DyIII2} with an energy barrier of 16.0 K for {Ni2IIDy2III} and 6.7 K for {CoII2DyIII2} under zero static field. Compared with the analogue {CoII2DyIII2}, the {Ni2IIDy2III} shows longer relaxation time and an absence of the quantum tunnelling of the magnetization (QTM) at low temperatures. Ab initio calculations suggest that the zero-field QTM of {Ni2IIDy2III} is effectively interrupted thanks to the ferromagnetic exchange coupling generated between NiII and DyIII ions. The presence of ferromagnetic exchange between NiII and DyIII ions is more conducive to zero-field single-molecule magnet behaviors than in isomorphic {CoII2DyIII2} where the exchange is antiferromagnetic.

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INTRODUCTION

strategies to suppress QTM: one is dedicated to constructing an axial symmetry crystal-field environment for lanthanide ion;2d,4 the other is to obtain strong exchange coupling between metal ions.5 Although the former method has achieved remarkable fruits in isolating mononuclear SMMs, the latter still plays an important role in accelerating evolution of multinuclear systems. To some extent, the exchange interaction should be efficient so that the fast zero-field QTM effect of lanthanide ions is suppressed by avoiding mixing of low-lying excited states. For the mixed transition metal and lanthanidebased complexes, the magnetic exchange interaction can be observed easily, which is markedly stronger than that in the pure lanthanide complexes due to the shielding effect of the 5s

In the past few years, single-molecule magnets (SMMs) incorporating lanthanide ions have undergone explosive growth in the field of molecule-based magnetic materials, resulting from fascinating slow relaxation of the magnetization and potential applications in high-density information storage and quantum computing.1 At present, most efforts have been devoted to deep understanding of the mechanism of magnetism and to exploring the factors influencing effective energy barriers (Ueff) and magnetic blocking temperature (TB).2 Nevertheless, it must be stressed that fast quantum tunneling of magnetization (QTM) under zero-dc field occurs in most lanthanide SMMs, which often reduce the barrier height and limiting relaxation times.3 This has become one of the biggest obstacles to acquiring high-performance lanthanide SMMs. Recently, experimental and theoretical studies have verified two main © XXXX American Chemical Society

Received: July 19, 2017

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DOI: 10.1021/acs.inorgchem.7b01840 Inorg. Chem. XXXX, XXX, XXX−XXX

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(w). 1H NMR (DMSO-d6): δ10.22 (s, 1H), 9.29 (s, 1H), 8.17 (s, 1H), 7.39 (d, 1H), 6.92 (d, 1H), 6.77 (t, 1H), 6.42 (s, 2H), 3.81 (s, 3H) (Figure S1). Synthesis of Complexes 1−6. All the complexes were prepared according to the same general procedure. The H2hms (0.1 mmol) and M(CH3COO)2·4H2O (0.4 mmol) (MII = NiII, CoII) were dissolved in 5 mL methanol for 1h at room temperature. A solution of containing Ln(NO3)3·6H2O (0.1 mmol) (LnIII = DyIII, GdIII, YIII) in dichloromethane (15 mL) was added and the mixture was stirred for 1h. The resultant solution was filtered and crystals suitable for X-ray studies were obtained by slow evaporation of the volatiles after 2−3 weeks at room temperature. The characterization data of the products obtained are given below. [Ni2Dy2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (1). Yield 62% based on Dy. Anal. Elemental analysis (%): Calcd for C32H50Ni2Dy2N8O28: H, 3.48; C, 26.73; N, 7.79. Found: H, 3.73; C, 26.51; N, 7.87. IR (KBr, cm−1): 3434 (s), 3299 (s), 3244 (s), 1653 (s), 1584 (s), 1416 (s), 1286 (s), 1228 (s), 1143 (m), 1010 (m), 951 (m), 762 (m), 723 (m), 635 (w), 481 (w). [Ni2Gd2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (2). Yield 65% based on Gd. Anal. Elemental analysis (%): Calcd for C32H50Ni2Gd2N8O28: H, 3.50; C, 26.93; N, 7.85. Found: H, 3.71; C, 26.64; N, 7.92. IR (KBr, cm−1): 3433 (s), 3340 (s), 3298 (s), 3242 (s), 3077 (m), 2383 (w), 1668 (s), 1565 (s), 1481 (s), 1415 (s), 1226 (s), 1155 (m), 1089 (m), 1018 (m), 956 (m), 824 (w), 787 (w), 741 (m), 669 (m), 614 (w), 493 (w). [Ni2Y2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (3). Yield 64% based on Y. Anal. Elemental analysis (%): Calcd for C32H50Ni2Y2N8O28: H, 3.87; C, 29.78; N, 8.68. Found: H, 3.84; C, 29.94; N, 8.46. IR (KBr, cm−1): 3343 (s), 3341 (s), 3298 (s), 3244 (s), 3077 (m), 2357 (w), 1687 (s), 1563 (s), 1445 (s), 1467 (s), 1296 (s), 1155 (m), 1099 (m), 1018 (s), 857 (w), 825 (w), 787 (w), 741 (m), 669 (m), 615 (m), 492 (w). [Co2Dy2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (4). Yield 72% based on Dy. Anal. Elemental analysis (%): Calcd for C32H50Co2Dy2N8O28: H, 3.47; C, 26.72; N, 7.79. Found: H, 3.69; C, 26.54; N, 7.86. IR (KBr, cm−1): 3433 (s), 3296 (s), 3243 (s), 3075 (m), 1627 (s), 1562 (s), 1416 (s), 1319 (s), 1263 (m), 1171 (m), 1016 (m), 739 (m), 623 (m), 485 (w). [Co2Gd2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (5). Yield 74% based on Gd. Anal. Elemental analysis (%): Calcd for C32H50Co2Gd2N8O28: H, 3.50; C, 26.92; N, 7.85. Found: H, 3.69; C, 26.79; N, 7.94. IR (KBr, cm−1): 3433 (s), 3298 (s), 3241 (s), 3075 (m), 1670 (s), 1562(s), 1416 (s), 1319 (s), 1279 (s), 1013 (m), 739 (m), 613 (m), 487 (w). [Co2Y2(Hhms)2(CH3COO)6(CH3OH)2(H2O)2]·(NO3)2 (6). Yield 68% based on Y. Anal. Elemental analysis (%): Calcd for C32H50Co2Y2N8O28: H, 3.87; C, 29.77; N, 8.68. Found: H, 3.97; C, 29.54; N, 8.76. IR (KBr, cm−1): 3432 (s), 3296 (s), 3245 (s), 3075 (m), 1562 (s), 1417 (s), 1318 (s), 1268 (m), 1175 (m), 1015 (m), 744 (m), 614 (m), 487 (w). X-ray Single-Crystal Diffraction Analysis. The single-crystal Xray data were collected on a Bruker Smart Apex II CCD diffractometer equipped with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan modes at room temperature. The data integration and reduction were processed with SAINT software. Absorption corrections were applied using the SADABS program.12 The single-crystal structures were solved by direct methods using SHELXTL and refined by means of full-matrix least-squares procedures on F2 using SHELXS-97 programs.13 All non-hydrogen atoms were placed using subsequent Fourier-difference methods and refined anisotropically. The hydrogen atoms were located in geometrically calculated positions. The crystal data are summarized in Table S1. All crystallographic data for 1−6 can be found in the CCDC with the numbers 1506841(1), 1506842(2), 1506844(3), 1506837(4), 1506838(5), 1506840(6). Ab Initio Calculations for Complexes 1 and 4. Completeactive-space self-consistent field (CASSCF) calculations on the individual DyIII and transition metal fragments of {Ni2IIDy2III} and {CoII2DyIII2} on the basis of X-ray determined geometry have been

and 5p orbitals for lanthanide ions.6 Additionally, the splitting of the 2J + 1 degenerate ground state of lanthanide ions is highly sensitive to the environment, and subtle changes can drastically influence the magnetic properties.7 The crystal field of lanthanide ions can be optimized through selecting the transitional metal ions to obtain more efficient exchange coupling.8 As expected, 3d-4f heterometallic SMMs have been extensively investigated.3a,5e,9 Among these complexes, butterfly type {CrII2DyIII2} and {CoII2DyIII2} metal clusters have been studied by Murray et al. and Song et al.,5a,b respectively, indicating that the magnetic coupling between 3d and 4f ions can significantly reduce the QTM effect and benefit for producing considerably longer relaxation times. However, there are only a few heterometallic complexes exampling the magnetic exchange interaction to effectively reduce the QTM process.5a−d,10 In order to better understand the relationship between the SMM behavior and the exchange interactions, it is very necessary to purposely design and synthesize heteronuclear SMMs. In 3d-4f systems, a ligand with a relatively constant coordination pocket containing both O- and N-donors and a step-by-step synthesis strategy should be emphatically considered. Herein, on the one hand, metal acetates were introduced into the systems to assemble 3d-4f complexes due to the enriching bridging modes of acetate, possibly promoting exchange interaction. On the other hand, the anisotropic metals NiII/CoII and DyIII ions were selected. Besides, anisotropic DyIII ion was replaced by the diamagnetic YIII and the isotropic GdIII to check the properties of the 3d metal ions. Based on the considerations above, six heterometallic coordination complexes, [M 2 Ln 2 (Hhms) 2 (CH 3 COO) 6 (CH3OH)2(H2O)2]·(NO3)2 (M = NiII, Ln = DyIII (1), GdIII (2), and YIII (3); M = CoII, Ln = DyIII (4), GdIII (5), and YIII (6)), have been isolated. Fortunately, the {NiII2LnIII2} and {CoII2LnIII2} series have an isomorphous structure, which makes it possible to further understand the magnetic coupling between the metal centers. Detailed magnetic behaviors and ab initio calculations for the complexes were carried out.

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EXPERIMENTAL SECTION

General Procedures and Materials. Commercially available reagents and solvents were purchased without further purification. The phase purity of the bulk samples was verified by powder X-ray diffraction (PXRD) measurements using a Bruker D8 ADVANCE Xray powder diffractometer (Cu Kα, 1.5418 Å), with a step size of 0.04° in 2θ and a scan speed of 5° min−1. The Fourier transform infrared (FTIR) spectra were performed with a EQUINOX 55 FT/IR spectrophotometer in the 400−4000 cm−1 region. The 1H NMR spectra were recorded using an AVANCE III Bruker 400 MHz instrument in a dimethyl sulfoxide (DMSO)-d6 solution at room temperature. Elemental analyses for C, H and N were carried out on an Elementar Vario EL III analyzer. Magnetic susceptibility measurements were performed in the temperature range 2−300 K, using a Quantum Design MPMS XL-7 SQUID magnetometer equipped with a 7T magnet. Alternating-current (ac) measurements were performed in a 3.0 Oe oscillating ac field. The magnetization was measured in the field range 0−7 T. The diamagnetic corrections were estimated using Pascal’s tables and magnetic data were corrected for diamagnetic contributions of the sample holder. Synthesis of (2-Hydroxy-3-methoxybenzylidene)-semicarbazide (H2hms). The ligand was synthesized through a condensation reaction, following previously reported procedure.11 IR (KBr pellet, cm−1): 3425 (m), 3071 (m), 2653 (m), 2537 (m), 1930 (w), 1687 (s), 1593 (s), 1509 (m), 1421 (m), 1318 (s), 1278 (s), 1174 (s), 1113 (m), 1012 (w), 944 (w), 851 (m), 775 (m), 704 (w), 657 (w), 529 B

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Figure 1. (a) Molecular structures of {NiII2DyIII2}. (b) Coordination mode of ligand. (c) Connection modes of the metal cores. (d) Three specific coordinate modes of acetate groups: μ3-η1:η2, μ2-η1:η1, μ2-η1:η2. (e) Coordination polyhedra of distorted octahedral geometry around the NiII ion and monocapped square antiprism geometry around the DyIII ion in 1. Hydrogen atoms are omitted for clarity. carried out with MOLCAS 8.0 program package.14 In the calculation of DyIII fragment, we replaced two CoII or NiII ions with diamagnetic ZnII ions, while in the calculation of CoII or NiII fragment, we replaced the other transition metal and DyIII ions with the diamagnetic ZnII and LuIII ions, respectively. For CASSCF calculations, the basis sets for all atoms are atomic natural orbitals from the MOLCAS ANO-RCC library: ANO-RCC-VTZP for DyIII, CoII or NiII; VTZ for close O and 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 coupling was handled separately in the restricted active space state interaction (RASSI-SO) procedure. In the calculation of DyIII fragment, the active electrons in 7 active spaces include all f electrons (CAS (9 in 7)) for {Ni2IIDy2III} and {CoII2DyIII2}, while in the calculation of transition metal fragment, the active electrons in 5 active spaces include all d electrons (CAS (7 in 5) for CoII and CAS (8 in 5) for NiII) 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 and 130 from 490 doublets for DyIII fragments; all from 10 quadruplets, all from 40 doublets for CoII fragment; all from 10 triplets, all from 15 singlets for NiII fragment).

with the simulated XRD patterns based on the single-crystal solution, confirming the phase purity of the bulk complexes (Figure S2). Crystal Structures. The crystal structures of 1−6 were determined by single-crystal X-ray crystallography analyses. All {MII2LnIII2} complexes crystallized in the same monoclinic space group P21/c and occupied similar cell parameters. The six complexes are isomorphous with a general formula [M 2 Ln 2 (Hhms) 2 (CH 3 COO) 6 (CH 3 OH) 2 (H 2 O) 2 ]·(NO 3 ) 2 (MII = NiII, CoII and LnIII = GdIII, DyIII, YIII). Two nitrate anions as counterions reside in the crystal lattice, neutralizing the excess cationic charge on the coordination spheres of 1−6. Due to their analogous structures, as a representative, only the crystal structure of {Ni2IIDy2III} is presented herein. In {Ni2IIDy2III}, the tetranuclear cation core represents a zigzag Ni−Dy−Dy−Ni arrangement with Ni−Dy−Dy angles of 107.5°. The periphery of the tetranuclear core is wrapped by two compartmental ligands via the tridentate ONOO pocket coordination to the NiII ion and the o-vanillin to the DyIII ion (Figure 1(a), 1(b)). The part of the NiII and the DyIII metallic centers are held together via six acetate ligands which consolidate this tetranuclear {Ni2IIDy2III} cluster (Figure 1(c)). Therein, the central DyIII ions are bridged by four acetates with two different coordination modes coexisting in the crystal structure: two acetates bridge in a μ2-η1:η2 fashion, whereas the other two bridge in a syn-syn μ3-η1:η2 fashion. The DyIII ion and the NiII ion are connected by the bridging coordination of a phenolate oxygen atom, an acetate oxygen atom, as well as one acetate ligand in a syn-syn μ2-η1:η1 mode with a Dy···Ni separation of 3.415(1) Å (Figure 1(c), 1(d)). The hinge angle of the Ni−O2−Dy bridging fragment (the dihedral angle between the O−Ni−O and O−Dy−O planes in the bridging fragment) is of 12.11° and two different Ni−O− Dy bridging angles of 105.26° and 95.45° in {Ni2IIDy2III}. Besides, a water molecule coordinates to the terminal NiII ion completing the six-coordinate octahedral geometry. The Ni− O/N distances lie in the range 2.004(9)−2.168(7) Å. For the

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RESULTS AND DISCUSSION Synthetic Aspects. The compartmental H2hms ligand was chosen as competent ligand to assemble heterometallic clusters, because hard O-donors favor to coordinate with LnIII ions whereas soft N-donors have higher affinity with the 3d MII ions. A series of tetranuclear {MII2LnIII2} complexes were obtained by using a step-by-step synthesis strategy to construct heteronuclear complexes at room temperature. It is worth noting that acetate anions as the auxiliary bridge play a crucial role in forming tetranuclear structure and tuning magnetic properties. In addition, enormous attempts were made to synthesize its isomorphic diamagnetic analogue. Unfortunately, trinuclear {ZnIILnIIIZnII} structures were isolated instead of the tetranuclear {ZnII2LnIII2}. The powder X-ray diffraction patterns of the bulk crystalline material are in good agreement C

DOI: 10.1021/acs.inorgchem.7b01840 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Temperature dependence of the χMT values in 1000 Oe for 1−3 (a) and 4−6 (b). The solid lines represent the best simulation of the corresponding complexes.

central DyIII ion, there is a methanol molecule bounded to the DyIII ion, making it nine-coordinate with a distorted monocapped square antiprism geometry (calculated by the SHAPE 2.1, Table S2) (Figure 1(e)).15 The Dy−O bond length falls in the range of 2.270(6)−2.566(7) Å, and the Dy··· Dy distance is 3.930 Å for {Ni2IIDy2III}. The selected bond lengths and angles are listed in Table S3. Relative to the reported {CoII2LnIII2} and {NiII2LnIII2} derivatives with the tetranuclear clusters, the present zigzagshaped complexes are relatively more rare than those in the arrangements of cubane-like,16 defect-dicubane, or butterfly type5a,9d,17 metallic cores. The resulting tetranuclear metallic core is strongly dependent on the coordinate modes of acetate groups. Different from the reported {MII2LnIII2} complexes bridged by acetate groups with the usual μ2-η1:η1 or μ2-η1:η2 mode which only make a bridge between Ln···Ln,10,18 an additional μ3-η1:η2 bridging in complexes 1−6 narrows the distance between Ni1 and Dy1A (d = 5.931(1) Å), causing a zigzag-shaped metallic core arrangement. As a result, the special bridging of acetates extends the scope of the interaction of magnetic centers and induces two asymmetric Ni−O−Dy bridging angles: of the two bridging oxygen atoms one is derived from the phenoxo of the deprotonated ligand, leading to a larger angle of 105.26°, whereas the other is derived from the acetate ligand, producing a smaller angle of 95.45°. These differences of bridging angles would exert influence on the magnetism of complexes. Magnetic Properties. The temperature dependent magnetic susceptibility measurements of complexes 1−6 were performed on polycrystalline samples in the temperature range 2−300 K under an applied direct field of 1000 Oe. The plots of χMT vs. T for the analogous nickel complexes 1−3 and the cobalt complexes 4−6 are presented in Figure 2a and 2b, respectively. At 300 K, the χMT product per {NiII2YIII2} unit is 2.5 cm3 K mol−1, which is close to the expected ones for two noninteracting octahedral NiII S = 1 ions with g = 2.23. Upon lowering the temperature, the χMT value of {NiII2YIII2} is nearly constant down to 25 K and then sharply decreases to the minimum value of 1.48 cm3 K mol−1 at 2 K. The constant χMT value is consistent with the large distance between the two nickel ions (8.852 Å), which suggests that there is no interaction between the two nickel ions through the diamagnetic YIII centers. The rapid decrease below 25 K is likely due to weak intercluster interaction and/or depopulation

of the Zeeman split mS states of NiII ions. The room temperature χMT value of {NiII2GdIII2} is about 18.86 cm3 K mol−1, slightly larger than the spin-only value (17.76 cm3 K mol−1) for two noninteracting NiII (S = 1) and two GdIII (J = 7/2, L = 0, S = 7/2, 8S7/2) ions. With decreasing temperature, the χMT values increase nearly constantly to a value of 19.22 cm3 K mol−1 at 50 K and then further rise to a maximum value of 25.14 cm3 K mol−1 at 2 K, resulting from ferromagnetic interactions within the molecules. For the {CoII2YIII2} unit, the χMT value 4.37 cm3 K mol−1 at 300 K is more than the calculated value of 3.75 cm3 K mol−1 for two spin-only CoII ions (S = 3/2, g = 2.0). The χMT product is almost constant upon lowering the temperature until 100 K before rapidly decreasing at lower temperature. The decrease at low temperatures arises from the orbital contribution of the CoII ions. The χMT value for {CoII2GdIII2} is about 21.31 cm3 K mol−1 at room temperature which is practically constant down to the minimum value of 20.40 cm3 K mol−1 at 20 K and then rapidly increases to 21.69 cm3 K mol−1 at 2 K, indicating the presence of intramolecular ferromagnetic interactions between the spin carriers. Moreover, in order to identify the coupling mechanism of yttrium and gadolinium complexes, magnetic fitting was conducted by use of MAGPACK19 programs. For {NiII2YIII2}, J and D were used to describe the magnetic coupling between NiII ions and single-ion anisotropy of NiII ions, respectively. For {NiII2GdIII2}, J1, J2, and J3 were employed to describe magnetic propagation in the Gd−Ni by the two μ2-O bridges (dNi−Gd = 3.414 Å), the Gd−Ni by the syn-anti acetate bridge (dNi•••Gd = 5.936 Å), and the Gd−Gd by the syn-syn (μ3-η1:η2) and chelating bidentate acetate bridges (dGd−Gd = 3.934 Å), respectively, while DNi was used to describe single-ion anisotropy of NiII ions (see the connection Sketch in Figure S3). Both Hamilton operators (H1 for Ni2Y2, H2 for Ni2Gd2) were presented in eq 1. H1 = −2JS Ni1S Ni2 H2 = −2[J1(SGd 2S Ni1 + SGd1S Ni2) + J2 (SGd1S Ni1 + SGd 2S Ni2) + J3SGd1SGd2]

(1)

Least-squares fitting of the experimental data with eq 1 yield best fit parameters: J = 0.01 cm−1, DNi = −1.63, g = 2.25, ESD = 2.66 × 10−2 cm3 K mol−1, and R = 1.73 × 10−4 for {NiII2YIII2}; D

DOI: 10.1021/acs.inorgchem.7b01840 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Temperature-dependent (a) and frequency-dependent (b) in-phase χ′ (top) and out-of-phase χ″ (bottom) ac susceptibility signals for {Ni2IIDy2III} under zero dc field.

J1 = 0.15 cm−1, J2 = 0.40 cm−1, J3 = −0.045 cm−1, DNi = −1.12 cm−1, g = 2.05, ESD = 8.30 × 10−2 cm3 K mol−1, and R = 3.80 × 10−5 for {NiII2GdIII2} (R = ∑[(χMT)obs − (χMT)calcd]2/ ∑[(χMT)obs]2). According to obtained results, we can conclude the following aspects: (1) Magnetic coupling (J = 0.01 cm−1) of Ni···Ni (d = 8.852 Å) in {NiII2YIII2} is nearly ignored, shedding light on its magnetic property being dominated by the ZFS phenomenon of NiII ions and intercluster interaction. (2) Different coupling values of two types of Gd−Ni, consistent with experimental reports previously,20 demonstrate that connection modes play an important role in magnetic coupling. (3) Octahedral-symmetry NiII ions in {NiII2Y III2 } and {NiII2GdIII2} featuring largely different single-ion anisotropy, implicitly associate with the effects of lanthanide ions. Such a phenomenon is reconciled with the results of {GdNi6} and {LaNi6}.20a (4) Weak ferromagnetic interaction between GdIII ions is attributed to integrated results of two connections (syn− syn and chelating bidentate), coinciding with previously computational results.21 Furthermore, the theoretical magnetization curve at 2 K derived from current magnetic parameters closely matches with experimental ones (Figure S4a). However, an attempt to fit the experiment χMT vs T curve of {CoII2LnIII2} failed by the fitting program, because six-coordinate CoII ions usually feature obvious first-order orbital momentum, and the spin-Hamiltonian must include the orbital-dependent exchange interactions and spin−orbit coupling effects.22 For the dysprosium complexes {Ni 2 I I Dy 2 I II } and {Co2IIDy2III}, the χMT values at 300 K of 30.15 cm3 K mol−1 for {NiII2DyIII2} and 34.40 cm3 K mol−1 for {CoII2DyIII2}, closely to the theoretical value 30.3 cm3 K mol−1 for two uncoupled NiII (S = 1) and two DyIII ions (J = 15/2, L = 5, S = 5/2, 6H15/2) but larger than the expected value for the two uncoupled CoII ions (S = 3/2) and the two uncoupled DyIII ions (J = 15/2, L = 5, S = 5/2, 6H15/2) due to the orbital contribution of the high-spin CoII ions. Upon cooling, the χMT curve is slowly decreasing and reaches a minimum value at 25− 30 K around 27.13 cm3 K mol−1 for {Ni2IIDy2III} and 27.30 cm3 K mol−1 for {Co2IIDy2III}, before increasing at lower temperatures up to 38.89 cm3 K mol−1 and 37.81 cm3 K mol−1 at 2 K,

respectively. For {Ni2IIDy2III}, the slight decline of χMT products from room temperature is due to the depopulation of the mJ states of the DyIII ions, whereas for {Co2IIDy2III}, the preliminary decline from room temperature should be explained by the total contribution of the additional orbital contribution of the CoII ion and thermal depopulation of the mJ states of the DyIII ions. The rapid growth at low temperature suggests ferromagnetic interactions dominating between the metal ions.23 The field dependences of the magnetization for complexes 1−6 recorded at 2.0 K are shown in Figure S4. M vs H plots of each complexes show an abrupt rise at low fields, and magnetizations reach 14.68, 18.99, and 4.13 Nβ for 1−3 and 14.65, 18.82 and 5.19Nβ for 4−6 at 70 kOe, respectively, without achieving complete saturation. The observed magnetization values at 70 kOe are much lower than expected values, indicating the presence of a significant magnetic anisotropy and/or low-lying excited states. Furthermore, the M vs HT−1 data for {Ni2IIDy2III} and {CoII2DyIII2} at 2.0−5.0 K were measured, showing nonsuperimposed magnetization curves that suggest the presence of a significant anisotropy and/or low-lying excited states (Figure S5).24 The alternating-current (ac) susceptibility experiments for complexes 1−6 were examined in a 3.0 Oe field oscillating at various frequencies from 1 to 1500 Hz in the range temperature of 2.0−15 K with a zero dc field. Complexes 2, 3, 5, and 6 do not show out-of-phase signals. For {Ni2IIDy2III}, in-phase (χ′) and out-of-phase (χ″) susceptibilities exhibit strong temperature dependence and good peaks are observed below 3.8 K (Figure 3(a)). Due to the 2.0 K temperature limit of the instrument, a maximum in the χ″ signal was not observed below 100 Hz. This characteristic clearly indicates that the {NiII2DyIII2} complex possesses slow magnetic relaxation, typical of SMM behavior. Analysis of the temperature dependence of the χ″ peaks through the Arrhenius law τ = τ0 exp(Ueff/kBT) acquires estimation of the pre-exponential factor and the effective barrier τo = 6.5 × 10−7 s; Ueff = 16.9 K (Figure S6). In order to further probe the dynamics of the magnetic relaxation, the frequency dependencies of the ac susceptibility E

DOI: 10.1021/acs.inorgchem.7b01840 Inorg. Chem. XXXX, XXX, XXX−XXX

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4, inset). The data of Cole−Cole has been fitted to the generalized Debye model (Figure S7),25 and the values of the α parameters were obtained in the range 0.107−0.186 (between 1.8 and 5.5 K), indicating a narrow distribution of relaxation times at these temperatures.5f As shown by this result, in order to further confirm that relaxation process is dominated by thermal relaxation (Arrhenius-like behavior) and does not cross over to a quantum tunneling mechanism, an external magnetic field of 400 Oe field was selected (Figure S8). Compared with the zero-field ac measurement, no significant shift in the χ″ vs v plot was observed under the external dc field (Figure S9−S10). The value of the effective energy barrier gives the same result (Figure S11), as expected, indicating zero field SMM behavior without the observation of QTM for the {NiII2DyIII2} complex. For {CoII2DyIII2}, the out-of-phase (χ″) magnetic susceptibility also shows temperature-dependent and frequencydependent behavior under zero dc field (Figure 5(a−b)), indicating that {CoII2DyIII2} possesses SMM behavior. The relaxation time τ data derived from the out-of-phase (χ″) peaks can be fitted by the Arrhenius law. The best fitting results yield the pre-exponential factor τ0 = 6.4 × 10−6 s and the effective energy barrier Ueff = 6.7 K, representing a small effective energy barrier (Figure 6). Semicircular Cole−Cole plots of χ′ vs χ″were obtained within the temperature range 1.8−5.5 K (Figure 6, inset) and were fitted by a generalized Debye model giving α values in the range 0.089−0.155 (Figure S12),25 which also points to the existence of a narrow distribution of slow relaxation. The ac susceptibility measurements for {CoII2DyIII2} were also performed under small dc field of 800 Oe to further study the magnetic dynamics (the 800 Oe field was selected because the slowest relaxation rate occurs close to this field, as shown in Figure S13). The variable frequency signals exhibit a significant shift in the maxima of χ″ toward lower frequencies, indicating that the QTM is fully or partly suppressed (Figure S14). The fit of relaxation times to the Arrhenius law in the range 2.0−5.5 K gave a small increase of τ0 and Ueff (τ0 = 3.2 × 10−6 s and Ueff =

were measured under zero dc field (Figure 3(b)). The peaks of the out-of-phase ac susceptibility (χ″) in {Ni2IIDy2III} gradually shift from middle frequency to high frequency with increasing temperature. Obviously, {NiII2DyIII2} reveals slow magnetic relaxation, associated with the presence of frequency-dependent out-of-phase maxima. At this point, the magnetization relaxation time (τ) has also been estimated from the frequency dependencies of the ac susceptibility. It is found that the plots of ln(τ) versus 1/T are linear over the entire temperature range (Figure 4), which suggests that it relaxes predominantly

Figure 4. Plots of ln(τ) vs T−1 constructed from the ac magnetization relaxation dynamics under zero dc field for {Ni2IIDy2III}. The red solid lines represent the fitting by the Arrhenius law for all the data. Inset: The Cole−Cole plots at 1.8−5.5 K of {Ni2IIDy2III} under zero dc field.

through an Orbach process in the investigated temperature and frequency range. Depending on the Arrhenius law, the preexponential factor (τ0) and the effective barrier (Ueff) were obtained as 8.1 × 10−7 s and 16.0 K for {Ni2IIDy2III}. The values are consistent with the front values extracted from the temperature-dependent data. The Cole−Cole plots in the form of χ′ versus χ″ display semicircular profiles and indicate just one single relaxation process exists in {Ni2IIDy2III} (Figure

Figure 5. Temperature-dependent (a) and frequency-dependent (b) in-phase χ′ (top) and out-of-phase χ″ (bottom) ac susceptibility signals for {CoII2DyIII2} at the indicated frequencies under zero dc field. F

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smaller Dy−O−Ni bond angles (95.44°). In the case of the {CoII2DyIII2}, although the ferromagnetic coupling and singlemolecule magnet behavior are observed, contrary to the isomorphous nickel complex, zero field QTM has not been effectively reduced. Theoretical Analysis. To elucidate the effect of 3d−4f magnetic interactions on the difference between {Ni2IIDy2III} and {CoII2DyIII2} and to gain further insight into the magnetization blocking of both complexes, complete-activespace self-consistent field (CASSCF) calculations on one type of individual DyIII and transition metal fragments for {Ni2IIDy2III} and {CoII2DyIII2} on the basis of X-ray determined geometries have been carried out with MOLCAS 8.014 and SINGLE_ANISO27 programs (see Supporting Information for details). The calculated eight lowest calculated Kramers doublets (KDs) and the g tensors of the individual magnetic center (DyIII ion) in {Ni2IIDy2III} and {CoII2DyIII2} using CASSCF/RASSI are shown in Table S6, and the calculated several lowest energy levels and g tensors in the ground state of individual CoII and NiII in {Ni2IIDy2III} and {CoII2DyIII2} using CASSCF/RASSI are shown in Table S7. From Table S6, the calculated ground gz values of the DyIII fragments are both close to 20 (19.390 for {Ni2IIDy2III} and 19.342 for {CoII2DyIII2}), suggesting a large contribution from the mJ = ± 15/2 state of the 6H15/2 multiplet of DyIII. The g tensors of 3d ions in two complexes are much different, where the NiII fragments of {Ni2IIDy2III} are almost isotropic, while the CoII fragments of {CoII2DyIII2} are anisotropic and the g values of CoII show larger gx, gy components, indicating transverse anisotropy (Table S7). The orientations of the local anisotropy axes in the ground doublets for {Ni2IIDy2III} and {CoII2DyIII2} are shown in Figure S17, where the magnetic axes on two DyIII centers have the same directions owing to the centrosymmetric structure of complexes and both are along the Dy−Ophenoxo direction. Given the corresponding magnetic properties of the mononuclear fragments for {Ni2IIDy2III} and {CoII2DyIII2}, the exchange interaction between the magnetic centers is considered within the Lines model,28 while the account of the dipole−dipole magnetic coupling is treated exactly. The results obtained for the exchange parameters from Table 1 (see

Figure 6. Plots of ln(τ) vs T−1 constructed from the ac magnetization relaxation dynamics under zero dc field for {CoII2DyIII2}. The red solid lines represent the fitting by the Arrhenius law for all the data. Inset: The Cole−Cole plots at 1.8−5.5 K of {CoII2DyIII2} under zero dc field.

13.8 K, as shown in Figure S15). Application of an external dc field removes the degeneracy of the energy levels and acquires the longer relaxation time at a specified temperature. Consequently, the quantum tunneling mechanism can be reduced by the extra dc field for {CoII2DyIII2}. Single-molecule magnet behavior is observed for both the {CoII2DyIII2} and {NiII2DyIII2} systems. In fact, the isomorphous nickel and cobalt heterometallic complexes with slow relaxation of magnetization under zero applied dc field are still uncommon for containing DyIII heteronuclear complexes. When anisotropic DyIII is replaced by the diamagnetic YIII and the isotropic GdIII for complexes 2, 3, 5, and 6, it has been certified that the slow relaxation of the magnetization for {Ni2IIDy2III} and {CoII2DyIII2} is dominated by the local anisotropic DyIII and not from anisotropic NiII and CoII. In spite of the large interest in NiII−DyIII type complexes, it is surprising that only a few examples exhibit SMM behavior under zero field with maxima in the ac out-of-phase peaks.9e,10,16a,26 Among them, only one case, the {NiII2DyIII2} complex reported by Shanmugam et al.,10 displays similar magnetic relaxation where QTM is significantly reduced and/or suppressed in the temperature range studied under zero field, associated with the relatively strong ferromagnetic interaction between the NiII and the DyIII ion. The larger the planarity of the Ni−O2−Dy bridging core of 8.89°, the wider the Dy−O− Ni bond angles of 105.76(13)° and 107.64(12)° observed in this case than those for other ferromagnetically coupled {NiII2LnIII2} complexes. In {Ni2IIDy2III}, in spite of the smaller Dy−O−Ni bond angles (107.92° and 95.44°) and larger Ni− O2−Dy dihedral angle (12.11°) compared to the reported {NiII2DyIII2} compound, ferromagnetic coupling between metal ions still operates and quenches the QTM without an applied dc field. The phenomenon is closely related to the bridging modes in {Ni2IIDy2III}. The {Ni2IIDy2III} presents a monophenoxo/diacetate triply bridged fragment between DyIII ion and NiII ion which is different from other reported {Ni2IIDy2III} bridged by diphenoxo and/or the acetate in μ2-η1:η1/μ2-η1:η2 fashion. And more specifically, one of the bridging acetates of NiII and DyIII ions employs a μ3-η1:η2 fashion connecting to another DyIII (Dy1A) ion, causing a zigzag-shaped metallic core arrangement which shortens the distance between Ni1 and Dy1A thereby extending the scope of the magnetic coupling. As a result, Dy1A and Ni1 can also mediate a magnetic exchange which might compensate for the exchange coupling induced by

Table 1. Exchange Coupling Parameters (cm−1) Obtained from Simulation and Qualitative Analysis between Magnetic Ions in {Ni2IIDy2III} and {CoII2DyIII2} Dy1−Dy1A

Dy1−M1

Dy1A−M1

Jdip Jexch Jtotal Jdip Jexch Jtotal Jdip Jexch Jtotal

1 (M = Ni)

4 (M = Co)

2.91 1.00 3.91 0.39 1.30 1.69 0.18 0.40 0.58

2.99 1.00 3.99 0.32 −1.65 −1.33 0.17 −0.30 −0.13

the Supporting Information for more details) were employed to fit the magnetic susceptibilities of {Ni 2 II Dy 2 III } and {CoII2DyIII2} using the POLY_ANISO27 program. All parameters given in Table 1 were calculated with the pseudospin s̃ = 1/2 of the DyIII and CoII. Due to the weak magnetic anisotropy of NiII ions, the spin of NiII was regarded as S = 1 during the G

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Figure 7. Left: the magnetization blocking barriers for individual DyIII fragments in {NiII2DyIII2} (a) and {CoII2DyIII2} (b). Right: the magnetization blocking barriers for the whole molecules of {NiII2DyIII2} (a) and {CoII2DyIII2} (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 the off-diagonal relaxation process. The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of the transition magnetic moment.

fitting. For two complexes, the total coupling parameters J (Jtotal), which include dipolar (Jdip) and exchange (Jexch) interactions, were included to fit the magnetic susceptibility. The intermolecular interactions zJ of {Ni2IIDy2III} and {CoII2DyIII2} were both fitted to −0.01 cm−1. The calculated and experimental magnetic susceptibilities of {Ni2IIDy2III} and {CoII2DyIII2} are shown in Figure 2, where the fit for {Ni2IIDy2III} is close to the experimental data.5b But, there is some deviation of the fit for {CoII2DyIII2} since the CoII ions cannot be completely regarded as Ising type. From Table 1, the Dy−Dy interactions in {Ni2IIDy2III} and {CoII2DyIII2} both reveal relatively large ferromagnetic interactions and are dominant in the two cases. Besides, the calculated dipolar interactions (Jdip) of Dy−Ni and Dy−Co are ferromagnetic. But interestingly a considerable difference is found that the extracted exchange coupling (Jexch) of Dy−Co in {CoII2DyIII2} is antiferromagnetic while Dy−Ni exchange in {Ni2IIDy2III} reverses to ferromagnetic. As a result of the stronger exchange interaction than the dipolar interaction between NiII/CoII and DyIII, the total magnetic interaction is antiferromagnetic for Dy−Co whereas it is ferromagnetic for Dy−Ni. The remarkable difference in the magnitude and particularly the type of magnetic exchange couplings between MII and DyIII within {Ni2IIDy2III} and {CoII2DyIII2} mainly arise

from different anisotropy of 3d metal ions and slightly different bridging parameters. The individual CoII fragments in {CoII2DyIII2} reveal the larger transverse components (gx, gy) of the lowest Kramers doublets (KDs) which may arise from the antiferromagnetic term. Moreover, the {Ni2IIDy2III} shows a smaller hinge angle of 12.11° as well as larger MII−O−DyIII angles of 105.26° and 95.45° than the hinge angle of 12.5° as well as angles of 104.28° and 95.05° in {CoII2DyIII2}, respectively. The smaller the hinge angle of the M−O2−Ln angle and the bigger the M−O−Ln angles, the easier the ferromagnetic dipole−dipole interaction becomes.29 Despite their very similar structures, the subtle structural variations change the sign of the exchange interaction, as a result, affecting the QTM and the zero field SMM behavior.26a,30 On the other hand, as expected, the ferromagnetic exchange is also present in the acetate bridged Dy1A−Ni1 (in a μ3-η1:η2 fashion, Jex = 0.4 cm−1) which is complementary to the ferromagnetic coupling mediated by monophenoxo/diacetate triply bridged Dy1−Ni1 and jointly exerts an influence on reducing QTM. Thus, this should be the reason that ferromagnetic coupling mediated by the smaller Dy−O−Ni bond angles can also significantly suppress zero-field QTM for the {NiII2DyIII2}. The exchange coupled levels and main gz values of {Ni2IIDy2III}and {CoII2DyIII2} were also calculated theoretically H

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Inorganic Chemistry in Table S8, where the gz values of the ground exchange state for {Ni2IIDy2III} and {CoII2DyIII2} are 41.214 and 42.985, respectively, which confirms that the Dy−Dy couplings are both ferromagnetic. And the exchange spectrum and corresponding tunneling gaps are shown in Figure 7. According to a recent proposal by Ungur and co-workers,31 the blocking barrier can be defined by the shortest relaxation paths related to the tunneling gaps, where these quantities are the largest. According to the relaxation path indicated in Figure 7, we deduced that the blocking barriers of {Ni2IIDy2III} and {CoII2DyIII2} corresponding to the spin-phonon transitions from −9 to +10 and −7 to +8 are 9.4 and 3.1 cm−1, respectively, which are both very close to the corresponding experimental fitting values and are in the same sequence of the experimental ones. The effective energy barrier (11.1 cm−1 for {Ni2IIDy2III} and 4.7 cm−1 for {CoII2DyIII2}) is much smaller than the first excited Kramers doublet of individual DyIII ions (99.0 cm−1 for {Ni2IIDy2III} and 109.3 cm−1 for {CoII2DyIII2}), which indicates the relaxation of magnetization through the exchange states rather than the excited KDs of the DyIII ions. Additionally, in contrast to the calculated tunnelling splitting, the ground state based on individual DyIII ions without regard to magnetic interaction reveals much larger tunnelling splitting (an order of 10−1 cm−1). When the magnetic exchange coupling interaction is taken into account in the system, a smaller tunneling gap is found in the ground state (an order of 10−6 cm−1), indicating that QTM is suppressed largely by magnetic exchange between the 3d and 4f ions. Compared to other similar 3d-4f SMMs,5a,9d,e,31 the energy barriers of {Ni2IIDy2III} and {CoII2DyIII2} are both very small for the reason that the energy separations between the exchange states split by the weak Dy−Dy, Dy−Co, and Dy−Ni exchange couplings are very small (the highest exchange states for {Ni2IIDy2III} and {CoII2DyIII2} are only 16.7 and 3.1 cm−1, respectively). The spin−phonon transition from −9 to +10 for {Ni2IIDy2III} and from −7 to +8 for {CoII2DyIII2} appears. Most importantly, it has been proved that slow magnetic relaxation of the magnetization in the 3d-4f system is mainly dominated by the local anisotropy of the lanthanide ion.32 It is found that the Dy−Dy magnetic couplings, especially ferromagnetic dipolar interactions, are much larger in {Ni2IIDy2III} and {CoII2DyIII2} than in other similar NiII/CoII−DyIII SMMs5b,9e associated with the unique μ3-η1:η2 bridging modes of acetate. It is likely that the weak interactions between the DyIII ions provide transverse components of anisotropy which reduce uniaxial anisotropy and allow underbarrier relaxation. Moreover, a relatively low symmetry of the axial crystal field may also result in a low magnetic anisotropy. The results demonstrate that the magnetic exchange interactions of 3d-4f ions are an effective method to suppress QTM, but it is still necessary to consider the high magnetic anisotropy of the lanthanide ions. In view of the development of lanthanide monomolecular magnets, improving the symmetry of the crystal field to increase the effective energy barrier has gradually approached the limitation.4a,b Therefore, the effective interactions of 3d-4f ions combined with high crystal field symmetry to suppress the quantum tunneling may become a new growth point to increase the effective energy barriers and magnetic blocking temperature.

structural arrangement and feature unique monophenoxo/ diacetate bridging groups between MII and LnIII ions. The special bridging modes induce predominant ferromagnetic interactions in DyIII and GdIII derivatives. The dynamic behaviors of {Ni2IIDy2III} and {CoII2DyIII2} exhibit discriminatively slow relaxation of the magnetization where QTM is significantly suppressed under zero field for {Ni2IIDy2III} but does not show in {CoII2DyIII2}. As explored by ab initio calculations, the different types of exchange couplings (Jex) generated by MII and DyIII ions, ferromagnetic for {Ni2IIDy2III} and antiferromagnetic for {CoII2DyIII2}, are mainly responsible for reducing the zero-field QTM. Howerer, although the QTM effect is suppressed by the 3d−4f exchange interaction, small energy barriers associated with multilevel exchange type have been observed for {Ni2IIDy2III} and {CoII2DyIII2}, which should be caused by low local anisotropy of DyIII ions. The present work highlights that a ferromagnetic exchange mediated by the specific linker along with paramagnetic 3d metal ions is more conducive to suppressing QTM and the uniaxial anisotropy of LnIII ions also needs to be ensured for constructing highperformance SMMs.

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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.7b01840. NMR and crystal data, PXRD curves, selected bond lengths and angles, ac susceptibility, and Arrhenius plots (PDF) Accession Codes

CCDC 1506837−1506838, 1506840−1506842, and 1506844 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 data_request@ccdc. cam.ac.uk, 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

*Dr. Yiquan Zhang E-mail: [email protected]. *Dr. Guilin Zhuang E-mail: [email protected]. *Dr. Sanping Chen E-mail: [email protected]. ORCID

Yiquan Zhang: 0000-0003-1818-0612 Wenyuan Wang: 0000-0003-0898-3278 Sanping Chen: 0000-0002-6851-7386 Author Contributions ⊥

H.W. and M.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We gratefully acknowledge financial support from the National Natural Science Foundation of China (grant no. 21673180, 21373162, 21473135, 21371141, 21671172, and 21703002), and Natural Science Foundation of Jiangsu Province of China (BK20151542).

CONCLUSION In summary, a family of isomorphic {MII2LnIII2} (M = NiII/ CoII) heterometallic complexes were synthesized. The structures of each complex exhibit unusual zigzag-shape I

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