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Spin-Canted Antiferromagnetic Ordering in Transition Metal-Organic Frameworks Based on Tetranuclear Clusters with Mixed V- and Y-Shaped Ligands Chen Cao, Sui-Jun Liu, Shu-Li Yao, Teng-Fei Zheng, Yong-Qiang Chen, Jing-Lin Chen, and He-Rui Wen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00682 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Crystal Growth & Design

Spin-Canted Antiferromagnetic Ordering in Transition MetalOrganic Frameworks Based on Tetranuclear Clusters with Mixed Vand Y-Shaped Ligands Chen Cao,† Sui-Jun Liu,*,† Shu-Li Yao,† Teng-Fei Zheng,† Yong-Qiang Chen,*,‡ Jing-Lin Chen,† and He-Rui Wen*,† †

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi Province, P.R. China

‡

College of Chemistry and Chemical Engineering, Jinzhong University, Jinzhong 030619, Shanxi Province, P.R. China

Supporting Information Placeholder ABSTRACT: Four transition metal-organic frameworks were solvothermally synthesized with V-shaped 1,3-bis(1imidazolyl)benzene (bib) and two Y-shaped carboxylate ligands (1,3,5-benzenetricarboxylic acid (H3btc) and 3,5pyridinedicarboxylic acid (H2pydc)), namely {[M2(µ3-OH)(bib)(btc)]·H2O}n (M = Co (1) and Fe (2)) and {[M′2(µ3-OH)(µ2-NO3) (bib)(pydc)]·solvents}n (M′ = Co (3) and Ni (4)). All of them are characterized by single-crystal X-ray diffraction, infrared spectra and powder X-ray diffraction. Both complexes 1 and 2 present a two-dimensional (3,8)-connected layer and the point symbol is (3.42)2(34.46.56.68.73.8), but Fe2 in 2 is weakly coordinated to O5 as a trigonal bipyramidal geometry compared with the tetrahedral Co2 in 1. While complexes 3 and 4 exhibit a three-dimensional ‘pillar-layer’ structure with (3,8)-connected tfz-d net. Interestingly, nitrate anions with rare µ2-η2 coordination mode take part in coordination and rectangular channels along the c axis exist in 3 and 4. Magnetic studies indicate 1 and 2 present antiferromagnetic behaviors, while 3 and 4 show the coexistence of spin canting and long-range magnetic ordering.

INTRODUCTION As an emerging multifunctional solid crystalline materials over the last two decades,1-3 metal-organic frameworks (MOFs) have been a hotspot not only the diversity of architectures and fascinating topologies but also potential applications in gas storage and separation,4-6 catalysis,7-8 magnetism,9-10 optics11 and chemical sensing.12-13 Among them, magnetic MOFs (MMOFs) have received much attention due to the interesting magnetic phenomena and great potential applications in high density information storage, quantum computing and magnetic refrigeration.14-20 To controllable synthesis of MOFs with definite structures and desired properties, many strategies have been developed based on the utilization of various organic ligands with different structures and coordination abilities.21-24 On account of several advantages, the mixed-ligand strategy has been well employed to prepare a large number of classical and excellent MOF materials.25-29 Generally, the ligands including multidentate O- and N-donors are helpful for the formation of cluster-based or chain-based MOFs.30 On the other hand, the use of organic ligands with distinct shapes is helpful for the control of pores in target MOFs and thus types of functional MOFs could be obtained.31-32 In this regard, the rigid organic ligands with different coordination sites and shapes are usually used to construct robust MOFs with permanent porosity.33-35 As such, for rigid N-donor ligands to prepare porous MOF materials, the acid-base system of mixed ligands is significant because of its special nature that could compensate charge balance, coordination deficiency, repulsive vacuum and weakly interaction all at once.25

Scheme 1. The ligands bib, H3btc and H2pydc used for the synthesis of 1–4.

A V-shaped imidazole-containing ligand, 1,3-bis(1imidazolyl)benzene (bib) (Scheme 1) with expanded rigid configuration, may afford MMOFs with attractive framework and topology. To our best knowledge, bib has rarely been used to synthesize MOFs because the µ2 coordinated mode limits the formation of complicated structures.36-39 To compensate the deficiency of bib ligand system, we take full advantage of the acid-base system of mixed ligands to prepare fascinating MMOFs. Therefore, two common and similar Y-shaped carboxylate ligands (1,3,5-benzenetricarboxylic acid (H3btc) and 3,5-pyridinedicarboxylic acid (H2pydc)) were utilized as co-ligands, taking into account the effect of different carboxylates on the structures in the acid-base system. Compared with H3btc, H2pydc has less coordination sites and thus more rigidity in configuration is presented. This work

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may provide certain proofs to investigate the effect of the rigidity of organic co-ligand on the structures of MMOFs in mixed-ligand system. To prepare desired MMOFs, the suitable choice of paramagnetic metal ions is another important factor. Transition metal ions with different valence states are good candidates because they usually have strong coordination ability, rich coordination configuration and various magnetic behaviors. Among them, FeII, CoII and NiII ions have been well used to construct transition MMOFs with fascinating structures and interesting magnetic phenomena. Herein, we have successfully synthesized and characterized four MMOFs, namely {[M2(µ3-OH)(bib)(btc)]·H2O}n (M = Co (1) and Fe (2)) and {[M′2(µ3-OH)(µ2-NO3) (bib)(pydc)]·solvents}n (M′ = Co (3) and Ni (4)). Structural characterization revealed that complexes 1 and 2 show twodimensional (2D) (3,8)-connected framework based on tetranuclear clusters with a slight difference. While complexes 3 and 4 present three-dimensional (3D) (3,8)-connected tfz-d net derived from tetranuclear clusters with rectangular channels along the c axis, showing a point symbol of (43)2(46.618.84). Magnetic investigations suggest that 1 and 2 display antiferromagnetism, while 3 and 4 show spin-canted antiferromagnetic ordering.

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~60%). Large block of crystalline bib was then obtained by recrystallization of the white powder in CH2Cl2/CH3OH. Bib crystallizes in the monoclinic P21/c group and its molecular structure is shown in Figure S1 (SI).

Scheme 2. The synthetic route to bib.

Synthesis of {[Co2(µ3-OH)(bib)(btc)]·H2O}n (1): A mixture of Co(NO3)2·6H2O (58 mg, 0.2 mmol), bib (21 mg, 0.1 mmol) and H3btc (21 mg, 0.1 mmol) in 5 mL of component solvent (VDMF:Vdeionized water = 4:1) was sealed in a 23 mL Teflon-lined autoclave and heated at 120 ℃ for 72 h. After the autoclave was cooled to RT in 24 h, purple block crystals were obtained in about 50% yield based on bib. Elemental analysis (%) calcd for C21H16Co2N4O8: C, 44.23; H, 2.83; N, 9.83. Found: C, 43.72; H, 2.95; N, 9.95. IR (KBr, cm1 ): 3535w, 3426w, 3123m, 1616s, 1566s, 1518s, 1438m, 1368s, 1195m, 1112w, 1070m, 998w, 936w, 854m, 773m, 708s, 653w, 550w, 458m. Synthesis of {[Fe2(µ3-OH)(bib)(btc)]·H2O}n (2): A mixture of Fe(SO4)2·7H2O (56 mg, 0.2 mmol), bib (21 mg, 0.1 mmol) and H3btc (21 mg, 0.1 mmol) in 8 mL of component solvent (VDMF:Vdeionized water:Vethanol = 4:2:2) was sealed in a 23 mL Teflon-lined autoclave and heated at 180 ℃ for 72 h. Then the autoclave was cooled to RT in 24 h, and yellow block crystals were obtained in ~25% yield based on bib. Elemental analysis (%) calcd for C21H16Fe2N4O8: C, 44.72; H, 2.86; N, 9.93. Found: C, 42.53; H, 2.29; N, 9.22. IR (KBr, cm-1): 3485w, 3124w, 1616s, 1566m, 1516m, 1442w, 1367s, 1282w, 1254w, 1113w, 1069w, 931w, 855w, 772w, 740m, 599w, 456w. Synthesis of {[Co2(µ3-OH)(µ2-NO3) (bib)(pydc)]·solvents}n (3): A mixture of Co(NO3)2·6H2O (58 mg, 0.2 mmol), bib (21 mg, 0.1 mmol) and H2pydc (17 mg, 0.1 mmol) in 5 mL of component solvent (VDMF:Vdeionized water = 4:1) was sealed in a 23 mL Teflon-lined autoclave and heated at 160 ℃ for 36 h. And then the autoclave was cooled to RT in 24 h, red block crystals were obtained in ~40% yield based on bib. IR (KBr, cm-1): 3430w, 3104w, 2366w, 1616s, 1517m, 1385s, 1248w, 1121w, 1063w, 935w, 827w, 765w, 655w, 586w, 443w. Synthesis of {[Ni2(µ3-OH)(µ2-NO3)(bib) (pydc)]·solvents}n (4): Complex 4 was synthesized by the similar procedure used for preparation of 3, except that Co(NO3)2·6H2O (58 mg, 0.2 mmol) was replaced by Ni(NO3)2·6H2O (58 mg, 0.2 mmol). After the autoclave was cooled to RT in 24 h, green block crystals were obtained in ~60% yield based on bib. IR (KBr, cm-1): 3448w, 3099w, 1620s, 1516m, 1386s, 1300w, 1247w, 1120w, 1062w, 1002w, 935w, 832w, 768w, 740w, 654w, 455w. X-ray Data Collection and Structure Determinations. All of crystallographic data were collected on Bruker D8 QUEST diffractometer with Mo-Kα radiation (λ = 0.71073 Å) by ω scan mode. Program SAINT was used for integration of the diffraction profiles.40 All structures were solved by direct method and refined by full-matrix least-squares methods through the SHELXTL program.41 The non-hydrogen atoms

EXPERIMENTAL SECTION Materials and Physical Measurements. All reagents were of analytical grade and used as purchased except the bib ligand. Elemental analyses (C, H and N) were carried out with a Perkin-Elmer 240C analyzer. Powder X-ray diffraction (PXRD) patterns were recorded on an Empyrean diffractometer (PANalytical B.V.) with a Cu-target tube and a graphite monochromator. The simulated PXRD spectra were from the single-crystal data and the Mercury (Hg) program obtained free from the website at http://www.iucr.org. The IR spectra were obtained on a Bruker ALPHA FT-IR spectrometer with KBr pellets. Thermogravimetric analyses (TGAs) were carried out on a NETZSCH STA2500 (TG/DTA) thermal analyser under nitrogen atmosphere at a heating rate of 10 ℃ min-1. Magnetic susceptibility measurements were performed on a Quantum Design MPMS superconducting quantum interference device (SQUID). All magnetic data were corrected for the diamagnetic susceptibility by using Pascal's constants and for contribution of the sample holder by experimental measurement. Synthesis of bib: The synthetic route to bib is shown in Scheme 2. A mixture of 1,3-dibromobenzene (2.36 g, 10 mmol), imidazole (1.63 g, 24 mmol), 1,10-phenanthroline monohydrate (0.48 g, 2.4 mmol), K2CO3 (5.53 g, 40 mmol) and CuI (0.34 g, 1.8 mmol) was dissolved in 100 mL N,Ndimethylformamide (DMF) with 250 mL flask and refluxed under N2 atmosphere for 72 h. After cooling down to room temperature (RT), the reaction mixture was extracted with CH2Cl2 for three times and the organic phases were combined. Then the product was further washed by distilled water for five times and dried with anhydrous MgSO4. Through the filtration and evaporation to dryness under reduced pressure to remove CH2Cl2, the reddish-brown crude was obtained. The crude was further purified with a flash chromatography with V(CH2Cl2)/V(CH3OH) = 20:1, yielding a white power (1.26 g,

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were identified by successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms of organic ligands were generated by the riding mode and refined isotropically with fixed thermal factors. The hydrogen atoms of water molecules in 1 and 2 were added by the difference Fourier maps and refined with

suitable constrains. The solvent molecules in the channels of 3 and 4 are highly disordered, and removed by SQUEEZE program in PLATON.42 A summary of the crystal data and structure refinements for bib and 1–4 is given in Table 1. The selected bond lengths and angles of 1–4 are provided in Table S1 (SI).

Table 1. Crystal data and structure refinements for bib and 1–4 Compound formula Mr T (K) crystal system space group a (Å) b (Å) c (Å) α (º) β (º) γ (º) V (Å3) Z F(000) Dcalc (g cm-3) µ (mm–1) reflections collected/unique Rint R1a/wR2b [I>2σ(I)] GOF on F2

bib C12H10N4 210.24 293(2) Monoclinic P21/c 8.883(3) 13.246(5) 11.332(3) 90 128.672(17) 90 1041.0(6) 4 440 1.341 0.086

1 C21H16Co2N4O8 570.24 299(2) Triclinic P 10.3297(9) 10.9548(10) 11.1672(10) 108.744(2) 90.697(2) 117.049(2) 1046.82 (16) 2 576 1.809 1.645

2 C21H16Fe2N4O8 564.08 293(2) Triclinic P 10.4480(9) 11.0387(9) 11.1649(9) 109.0524(17) 89.9812(17) 117.1885(12) 1065.56 (15) 2 572 1.758 1.421

3 C19H14Co2N6O8 572.22 293(2) Monoclinic C2/c 29.293(11) 11.127(4) 19.674(7) 90.00 95.699(8) 90.00 6381(4) 8 2304 1.191 1.081

4 C19H14Ni2N6O8 571.78 293(2) Monoclinic C2/c 28.978(4) 11.0040(15) 19.605(3) 90.00 95.042(3) 90.00 6227.4(15) 8 2320 1.220 1.252

12016/1813

11482/3659

21864/3760

15306/5219

48000/5476

0.0775 0.0623/0.1443 1.111

0.0220 0.0260/0.0634 1.054

0.0164 0.0267/0.0682 1.064

0.0951 0.1092/0.3101 1.069

0.0345 0.0815/0.2339 1.068

a

R1 = ∑||Fo|-|Fc||/∑|Fo|, bwR2 = [∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

shaped tetranuclear cluster [Co4(µ3-OH)2(CO2)4] (Figure 1b). The adjacent tetranuclear CoII clusters are further linked by btc3- and bib ligands to give the 2D structure. Interestingly, the role of bib ligands in this structure likes the arch bridge over the lake. The topology analysis of 1 indicates that a (3,8)connected net with a point symbol of (3.42)2(34.46.56.68.73.8) can be rationalized by TOPOS 4.044 as shown in Figure 1d, where Co4 clusters are considered as eight-connected nodes, and btc3- ligands act as three-connected nodes. In addition, the 3D packing structure (Figure S2a, SI) indicates the existence of the weak intermolecular C-H…O hydrogen bonds due to the short distances (Table S2, SI). The accessible volume for 1 calculated by PLATON42 is 69.5 Å3 (6.6%) per unit cell volume when the solvent molecules were removed. Complex 2 takes a similar structure with 1 except that Fe2 is weakly coordinated to O5B as a trigonal bipyramidal geometry compared with the tetrahedral Co2 in 1 (Figure S2b, SI). Crystal Structure of {[Co2(µ3-OH)(µ2-NO3) (bib)(pydc)]·solvents}n (3) and {[Ni2(µ3-OH)(µ2-NO3) (bib)(pydc)]·solvents}n (4). Complexes 3 and 4 are isostructural, and thus herein the structure of 3 was described in detail. Complex 3 belongs to the monoclinic C2/c space group. The asymmetry unit contains two crystallographically independent CoII ions, one bib ligand, one pydc2- ligand, one µ3-OH- and one NO3- anion. As shown in Figure 2a, each CoII ion presents six-coordinated irregular octahedral geometry. Co1 is surrounded by five O atoms (O1, O3C, O5, O5A and O6) from independent pydc2- ligands, NO3- and OH- anions, and one N atom (N1) from bib ligand [Co1–O = 2.074(8)–

RESULTS AND DISCUSSION Synthesis. By using mixed-ligand strategy, four tetranuclear clusters based MMOFs have been successfully produced and the ratio of MII salts, bib and corresponding carboxylate ligands is 2:1:1. Some differences of synthetic conditions indicate that the synthesis of FeII-based MOFs is more difficulty than that of other transition MOFs, which is resulted from iron ions exhibiting a strong tendency to undergo hydrolysis into a stable polymeric hydrated iron oxide.43 Crystal Structure of {[Co2(µ3-OH)(bib)(btc)]·H2O}n (1) and {[Fe2(µ3-OH)(bib)(btc)]·H2O}n (2). Complex 1 crystalizes in the triclinic P space group. The asymmetric unit of 1 consists of two crystallographically independent of CoII ions, one bib ligand, one btc3- ligand, one µ3-OH- and one lattice water molecule. As shown in Figure 1a, Co1 is sixcoordinated by three carboxylate O atoms (O1, O3A and O5B) from three different btc3- ligands, two O atoms (O7 and O7C) from distinct OH- anions, and one N atom (N1) from bib ligand, which generate the distorted [CoNO5] octahedral geometry [Co1–O = 2.0728(15)–2.1552(15) Å and Co1–N = 2.108(2) Å]. While Co2 is four-coordinated with irregular [CoNO3] tetrahedral geometry by two carboxylate O atoms (O2 and O4D) from two independent btc3- ligands, one O atom (O7) from the OH- group and one N atom (N4A) from bib ligand [Co2–O = 1.9414(17)–1.9718(15) Å and Co2—N = 2.074 Å]. The CoII centers are connected by µ3-OH- and synsyn-µ2-η1-η1 carboxylates of btc3- ligands to form butterfly-

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2.167(8) Å and Co1–N = 2.097(9) Å]. And Co2 is coordinated by four O atoms (O2, O4B, O5 and O6) from different pydc2ligands, NO3- and OH- anions, and two N atoms (N4D and N5C) from bib ligand and pydc2- ligand [Co2－O = 2.051(7) – 2.212(8) Å and Co2–N = 2.131(9)–2.213(8) Å]. To our best knowledge, the NO3- anion in with µ2-η1 coordination mode is rare in CoII and NiII complexes.45-48 The neighboring CoII ions are interlinked by four carboxylate groups with syn-syn-µ2-η1η1 bridging mode, two µ3-OH- and two NO3- anions, generating a tetranuclear CoII center with butterfly shape, namely [Co4(µ3-OH)2(µ2-NO3)2(CO2)4] (Figure 2b). Meanwhile, the combination of Co4 centers and pydc2- ligands form two-dimensional layer. The layers are further bridged by bib ligands to generate 3D frameworks, accompanying with one-dimensional rectangular channels (8.5× 13.2 Å in size) along the c axis. Therefore, the structure can be seen as ‘pillarlayer’ framework. After the solvent molecules were removed from the channels, the accessible volume for 3 calculated by PLATON42 is 2740.2 Å3 (42.9%) per unit cell volume. To gain a better visualization and understanding of the structures of 3,

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topological analysis was performed. If Co4 clusters were treated as eight-connected nodes and pydc2- ligands as threeconnected nodes, the topology of 3 is a 3D (3,8)-connected tfzd network with a point symbol of (43)2(46.618.84) (see Figure 2d and Figure S2c, SI). Structural Comparison. Although both 1 and 3 take tetranuclear clusters based high-dimensional structures with (3,8)-connected net, their structures are much different. Firstly, Co2 in 1 is four-coordinated, and while both Co1 and Co2 in 3 are six-coordinated, which lead to the complicated 3D structure of 3. Secondly, nitrate anions in 3 take part in coordination and exhibits rare µ2-η2 coordination mode. Last but not the least, the btc3− in 1 and pydc2- in 3 reveals µ5η1:η1:η1:η1:η1 bridging model (see Figure 3) and both of them are completely protonated, but one atom in btc3− is free. Notably, the btc3− in 2 exhibits µ6-η1:η1:η1:η1:η2 bridging model considering the existence of the weak FeO coordination bond. In addition, the rigidity of carboxylate ligands may favor the construction of higher dimensional framework.

Figure 1. Views of (a) the coordination environments and linkage modes of CoII in 1 (H atoms omitted for clarity); (b) the butterflyshaped Co4 cluster in 1; (c) the polyhedron representation 2D network of 1; (d) the (3,8)-connected topology of 1. Symmetry codes: A: 1x, 1-y, 1-z; B: -x, -y, 1-z; C: -x, 1-y, 1-z; D: -1+x, y, z.

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(e) Figure 2. Views of (a) the coordination environments and linkage modes of CoII in 3 (H atoms omitted for clarity); (b) the 2D network based on butterfly-shaped Co4 clusters; (c) the 3D pillar-layer framework of 3 showing open 1D channels along the c axis; (d) the 1D channel running along the c axis; (e) the (3,8)-connected tfz-d topology of 3. Symmetry codes: A: 1/2-x, 1/2-y, 1-z; B: x, 1-y, 1/2+z; C: 1/2x, 1/2+y, 1/2-z; D: 1/2+x, 1/2+y, z.

Figure 3. Views of the bridging model of deprotonated btc3- and pydc2- ligands with CoII ions (a, c) and FeII ions (b).

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PXRD patterns and TGA. To confirm the phase purities of these complexes, the PXRD patterns were performed on polycrystalline samples. The PXRD patterns of the assynthesized samples are well consistent with their corresponding simulated ones, indicating the phase purities of the bulk samples (Figure S3, SI). To study the thermal stabilities of these complexes, they were carried out for TGA experiments from 20 to 900 ℃ under nitrogen atmosphere. As shown in Figure S4 (SI), the results indicate that the lattice water of 1 and 2 were removed below 286 and 256 ℃, and their frameworks finally decomposed at about 380 and 353 ℃, respectively. And the framework of 3 and 4 keep stable until 340 and 390 ℃, respectively. All above suggest they have relatively good thermal stabilities. Magnetic Properties. The direct current (dc) magnetic susceptibilities of 1–4 have been performed on polycrystalline samples from 2 K to 300 K at an applied field of 1 kOe. In order to investigate the M4 unit (M = Co, Fe, Ni), the calculated molecular weights for magnetic data of 1–4 are 1140.48, 1128.16, 1144.44 and 1143.56, respectively. For 1 and 3, the χmT values at 300 K are 10.19 and 8.95 cm3 mol−1 K, which are significantly larger than that (7.50 cm3 mol−1 K) expected for four isolated high-spin CoII ions (S = 3/2, g = 2.0). This is a common phenomenon for CoII ions due to its strong spin–orbit coupling interactions. For 1, as the temperature decreases, the χmT value slowly decreases down to a minimum value of 0.33 cm3 mol−1 K at 2.0 K, indicating the exsitence of antiferromagnetic behavior. On cooling, the χmT

(a)

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value of 3 gradually decreases down to a minimum value of 3.40 cm3 mol-1 K at 16 K, which is much smaller than the theoretical values (7.50 cm3 mol-1 K) of four spin-only CoII ions and indicates antiferromagnetic coupling between CoII ions. Upon lowering the temperature to 10 K, the χmT abruptly increase to a maximum value (4.21 cm3 mol-1 K) and then the χmT has a rapid linear drop with a minimum value of 1.53 cm3 mol-1 K at 2.0 K. Above mentioned indicates canted antiferromagnetism exsits in 3.14 Curie–Weiss fitting in the high temperatur range leads to C = 13.18 cm3 mol−1 K and θ = −78.16 K for 1 and C = 10.49 cm3 mol−1 K and θ = −43.89 K for 3, respectively. The C values corresponds to g = 2.65 (1) and 2.37 (3), respectively, being normal for the significant spin– orbit coupling. The strength of the antiferromagnetic exchange interaction caused by spin–orbit coupling of CoII at high temperature was estimated based on eq (1).49

χ T = Aexp( − E1 /kT ) + Bexp( − E2 /kT )

(1)

In eq (1), A + B equals the curie constant, and E1 and E2 represent the “activation energies” corresponding to the spinorbit coupling and the antiferromagnetic exchange interaction. The best fitting of the experimental data gives that A + B = 12.40 cm3 mol-1 K, –E1/k = –59.92 K, –E2/k = –4.05 K for 1, and A + B = 10.32 cm3 mol-1 K, –E1/k = –46.43 K, –E2/k = – 6.93 K for 3 (Figures 4a and 4b). The negative values of –E2/k indicate that dominant antiferromagnetic interactions between CoII ions within the Co4 clusters exist in 1 and 3.

(b)

(c)

Figure 4. The χmT vs T plots (red lines reprensent for the best fitting) and of 1 (a), 3 (b) and 4 (c) under applied field of 1kOe.

For 2, the room-temperature χmT value is 15.46 cm3 mol−1 K (see Figure S5a, SI), which is somewhat larger than the expected vlaue (12 cm3 mol−1 K) for four isolated high-spin FeII ions (S = 2, g = 2.0). With the temperature decreasing, the χmT linearly decreases to a minimum value of 0.31 cm3 mol−1 K, indicating strong antiferromagnetic exchange interactions. As for 4, the χmT value at room temperature is 3.51 cm3 mol−1 K, being somewhat smaller than the expected vaule of 4 cm3 mol−1 K for four uncoupled NiII ions (S = 1, g = 2.0). On cooling the temperature to 24 K, the χmT value almost remain unchanged above 100 K and gradually decreases down to a minimum value of 2.75 cm3 mol-1 K at 24 K, indicating the antiferromagnetic interaction between NiII centers. Upon further lowering the temperature to 14 K, the χmT abruptly increase to a maximum value (3.65 cm3 mol-1 K) and then the χmT has a rapid linear drop with a minimum value of 0.47 cm3 mol-1 K at 2.0 K. Curie–Weiss fitting above 26 K gives C = 3.57

χM-1 vs

T plots (red lines reprensent for the Curie-Weiss fitting)

cm3 mol−1 K and θ = −4.38 K. The negative θ value further confirm the antiferromagnetic couplings between adjacent NiII ions. Therefore, complex 4 exhibits canted antiferromagnetic behavior on the whole.

Scheme 3. The schematic representation of magnetic mode in 4.

Taking into account the fact that the NiII-NiII interaction between Ni4 units is expected to be negligibly small, complex 4 can be simplified as a tetranuclear NiII complex from

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magnetic point of view (Scheme 3). A quantitative analysis for 4 has been performed on the basis of expression derived from the spin-only Hamlitonian, as shown in eq (2) and the expression is revealed in eq (3).50 Hˆ = −2J 1Sˆ 1Sˆ 1A − 2J 2(Sˆ 1Sˆ 2 + Sˆ 1Sˆ 2A + Sˆ 1ASˆ 2A + Sˆ 1ASˆ 2 ) (2) 180 + 84e 8b + 30e14b + 6e18b + 168e 4 a+ 4b + 60e 4 a+10b + 12e 4a+14b + 60e 6a+8b + 30e 8a+ 6b Ng β + 6e 8a+10b + 12e10 a+8b × 3kT 9 + 7e 8b + 5e14b + 3e18b + e 20b + 14e 4 a+ 4b + 10e 4a +10b + 6e 4a+14b + 10e 6 a+8b + 5e 8a +6b 2

χM =

2

based CoII MOFs, but only 3 exhibits spin canting behavior, which suggests that the anisotropy of CoII is not the key factor. Hence, in the present case, the existence of inversion centers between the adjacent spins in 1 forbids the occurrence of antisymmetric exchange interactions, and due to the fact that there are no symmetric centers between the neighboring tetranuclear units, the intercluster magnetic exchanges in 3 and 4 can be antisymmetric.

+ 3e8a +10b + e 8a +12b + 6e10 a+8b + e12a +8b

where a = –J1/kT and b = –J2/kT (3) Least-squares ﬁtting of the experimental data at 26-100 K leads to the following set of parameters: gNi = 2.00 (fixed), J1 = −0.14 cm-1 and J2 = −2.18 cm-1 (Figure 4c). The negative values of J1 and J2 further indicates that antiferromagnetic interaction exists among the neighboring NiII centers. The M vs. H plots (at 2 K) for 1–4 are shown in Figure S5b (SI). The magnetizations of 1–4 increase slowly and reach to 1.80, 3.2, 2.42 and 1.15 Nβ at 70 kOe, which are far from the saturation values, respectively. The results further suggest antiferromagnetic couplings between the adjacent metal centers. The canting angles of 3 and 4 at 2 K are about 4.2◦ and 3.7◦ estimated with the equation sin(γ) = MR/MS (MR obtained by extrapolating the high-field linear part of the magnetization curve at 2 K to zero field).51 To investigate the low-temperature behaviors of 3 and 4, the temperature dependencies of field-cooled (FC) and zero-fieldcooled (ZFC) magnetization were performed under a field of 20 Oe upon warming from 2 K (Figure 5). The FC and ZFC curves of 3 and 4 completely diverge below 11 K and 18.5 K, respectively, suggesting the onset of long-range ordering at low temperture. Moreover, no obvious frequency-dependent ac signals appear although anisotropic CoII centers exist in 3. Both the in-phase (χ') and out-of-phase (χ'') signals of 3 and 4 are consistent with the spin-canted antiferromagnetic ordering behaviors (Figure S6, SI). Moreover, the obvious peaks of χ'' curves in 3 is resulted from the strong anisotropy of CoII ions. As shown in Table 2, tetranuclear clusters based CoII and II Ni MOFs usually contain M4(µ3-OH)2 cores and exhibit antiferromagnetic interactions within the clusters. The reason may be ascribed to the existence of syn-syn-carboxylate and µ3-OH bridges. As is known, spin canting may be ascribed to two mechanisms: the single-ion magnetic anisotropy and the antisymmetric magnetic exchange.52-54 Although 1–4 all belong to centrosymmetric space group, only 3-4 present spincanting behaviors. Both 1 and 3 are tetranucluear clusters

(a)

(b) Figure 5. The FC/ZFC curves at low temperature for 3 (a) and 4 (b).

Table 2. Structures and magnetic properties of some selected tetranuclear clusters based CoII and NiII MOFs Compounds

[Co2(µ3-OH)(µ2-H2O)(pyrazine)(oba)(obaH)]n55 {[Co4(ina)5(µ3-OH)2(H2O)(EtOH)]·NO3·2EtOH·4H2O}n56 {[Ni4(ina)5(µ3-OH)2(H2O)(EtCOO)]·6EtOH·2H2O}n56 {[Ni4(µ3-OH)2(ina)2(dpda)2(H2O)3]·9H2O·3C2H6O}n54 [Me2NH2][Co2(bptc)(µ3-OH)(H2O)2]n57 [Me2NH2][Ni2(bptc)(µ3-OH)(H2O)2]n57

Cluster core

Network

intra-cluster magnetic interactions

Co4(µ3-OH)2(µ2-H2O)2

8-c

AF

Co4(µ3-OH)2

7-c vmr

AF

Ni4(µ3-OH)2

9-c bct-9-P21/c

AF

Ni4(µ3-OH)2

6-c

AF

Co4(µ3-OH)2 Ni4(µ3-OH)2

(4,8)-c scu (4,8)-c scu

AF F

7


Bulk Magnetic behaviour

spin canting slow relaxation spin canting spin glass spin canting spin glass spin canting spin glass antiferromagnet ferromagnet


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Co4(µ3-OH)2 Co4(µ3-OH)2(µ2-NO3)2 Ni4(µ3-OH)2(µ2-NO3)2

1 3 4

(3,8)-c (3,8)-c tfz-d (3,8)-c tfz-d

Page 8 of 10 AF AF AF

antiferromagnetism

Spin canting Spin canting

oba = 4,4′-oxybis(benzoate), ina = isonicotinate, dpda = 2,6-dimethyl-pyridine-3,5-dicarboxylate, bptc = 3,3′,4,4′-biphenyltetracarboxylate AF = antiferromagnetic and F = ferromagnetic.

CONCLUSION

REFERENCES

By the employment of mixed-ligand strategy, four new (3,8)-connected 2D/3D MMOFs have been successfully synthesized. The bib ligands serve as arch bridges and µ3-OH anions play a key role in the formation of tetranuclear butterfly-shaped clusters, while nitrate anions with rare µ2-η2 coordination mode take part in coordination in 3 and 4. Magnetic investigates indicate that 1 and 2 exhibit antiferromagnetic behaviors, while 3 and 4 show the coexistence of spin canting and long-range magnetic ordering. Further studies of cluster-based MMOFs with interesting structures and magnetic phenomena are underway in our group.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx. Supplementary tables, structural figures, PXRD patterns, TGA curves and magnetic data. Accession Codes CCDC 1548660 and 1548330-1548333 for bib and complexes 1– 4, respectively, 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 Author *E-mail: [email protected]. Tel: +86-797-8312204. *E-mail: [email protected]. Tel: +86-797-8312553. *E-mail: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (No. 21501077, 21561013 and 21501078), the China Postdoctoral Science Foundation (No. 2016M592107), the Jiangxi Provincial NSF of China (Nos. 20151BAB213003, 20161ACB21013, 20171BCB23066, 20143ACB21017 and 20171BAB203005), the Shanxi Provincial NSF of China (No. 2015011026), the Jiangxi Provincial Postdoctoral Preferred Project of China (No. 2015KY34), the Jiangxi Provincial Postdoctoral Daily Project of China (No. 2016RC31), the Project of Jiangxi Provincial Department of Education (No. GJJ150634), and the Program for Qingjiang Excellent Young Talents, JXUST.

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Spin-Canted Antiferromagnetic Ordering in Transition Metal-Organic Frameworks Based on Tetranuclear Clusters with Mixed V- and YShaped Ligands

Chen Cao, Sui-Jun Liu,* Shu-Li Yao, Teng-Fei Zheng, Yong-Qiang Chen,* Jing-Lin Chen, and He-Rui Wen*

Four CoII, FeII and NiII magnetic metal-organic frameworks derived from tetranuclear clusters with some differences have been synthesized by using mixed-ligand strategy. Complexes 3 and 4 show the coexistence of spin canting and long-range magnetic ordering.

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