English

• 1. INTRODUCTION

  Design and synthesis of molecule-based magnetic materials (MMMs) are currently attracting considerable attention due to their impressive structural diversities, potential applications in quantum computation and information storage, as well as their significance in the interpretation of the fundamental magneto-structural correlation^[1-3]. Undoubtedly, a synthetic challenge involves developing simple and efficient methods to aggregate two or more spin carriers in a small, single molecular entity or in a high-dimensional framework by a short extended mediator^[4-6]. In this area, a subtle choice of five-membered heterocyclic 1, 2, 4-triazole has been proved to be an effective strategy for designing MMMs. In addition to adopting diverse bridging modes that aggregate spin carriers into a magnetic motif and mediating different magnetic couplings by -NN-/-NCN- moieties, 1, 2, 4-triazole can also create antisymmetric magnetic exchange, as well as spin-competition in metal-triazolate magnetic lattices, which can produce significant spin-canted antiferromagnetism, metamagnetism and spin frustration^[7-10]. Previous investigations have found that aromatic multicarboxylate co-ligands can influence the structure of metal-triazolate lattices and regulate magnetic behaviors by changing the number, positon and deprotonation of the carboxylate groups, as well as replace non-carboxylate substituents for aromatic multicarboxylates^[11-16]. Among these different carboxylate co-ligands, aromatic multicarboxylates with sulfo group can significantly disrupt the metal-triazolate magnetic lattices due to its different bridging groups with flexible connection modes^[15-17]. Moreover, the weak coordination bond between metal ion and sulfo group may induce single-crystal to single-crystal transformation^[17]. Up to date, several Cu^Ⅱ/Co^Ⅱ-trz-based complexes with 5-sulfoisophthalate or 2-sulfoterephthalate have been successively reported^[17-19], exhibiting ferrimagnetism, spin competition and magnetic switch from weak ferromagnetism to antiferromagnetism. More interesting, some of them even exhibit diverse structures and magnetism but contain the same components^[19]. Encouraged by those interesting results, 4, 8-disulfonyl-2, 6-naphthalenedicarboxylic acid (H₄L) was selected as a co-ligand to react with 3-amino-1, 2, 4-triazole (Hatz) and Co^Ⅱ salts. As well as we know, it is the first time that aromatic multicarboxylate with two sulfo groups was introduced in metal-triazolate system. A novel three-dimensional (3D) pillared-layer framework, [Co₄(H₂O)₄(μ₃-OH)₂(atz)₂(L)]_n, containing triazole extended Co₄^Ⅱ layers was isolated under solvothermal reaction. Magnetically, complex 1 displays strong spin-frustrated antiferromagnetism due to the triangular magnetic lattice and cooperative antiferromagnetic couplings mediated by quadruple heterobridges.

  2. EXPERIMENTAL

  2.1 Reagents and instruments

  Hatz, cobalt acetate tetrahydrate and trimethylamine were commercially purchased and used as received without further purification. H₄L was prepared according to the literature^[20]. Doubly deionized water was used for the conventional synthesis. Elemental analyses for C, H and N were carried out with a CE-440 (Leeman-Labs) analyzer. FT-IR spectrum (KBr pellet) was taken on an Avatar-370 spectrometer (Nicolet) in the range of 4000~400 cm^-1. Powder X-ray diffraction (PXRD) patterns were obtained from a Bruker D8 ADVANCE diffractometer at 40 kV and 40 mA for CuKα radiation (λ = 1.5406 Å), with a scan speed of 0.1 sec/step and a step size of 0.01º in the 2θ of 2~50°. TG experiment was carried out on a Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800 ℃ under a N₂ atmosphere at a heating rate of 10 ℃·min^–1. Variable-temperature magnetic susceptibility measurement of 1 was carried out at an applied DC field of 1 kOe from 2 to 300 K on a Quantum Design (SQUID) magnetometer MPMS-XL-7. Diamagnetic corrections were calculated by using Pascal′s constants, and an experimental correction for the sample holder was applied.

  2.2 Synthesis of [Co₄(H₂O)₄(μ₃-OH)₂(atz)₂(L)]_n (1)

  Co(OAc)₂·4H₂O (49.8 mg, 0.1 mmol), H₄L (42.4 mg, 0.1 mmol) and Hatz (16.8 mg, 0.2 mmol) were dissolved in mixed CH₃OH-H₂O medium (10.0 mL, V: V = 6:4) and the initial pH was adjusted to 6.0 by trimethylamine. The resulting mixture was then transferred into a Teflon-lined stainless-steel vessel (23.0 mL) and heated to 170 ℃ for 96 h under autogenous pressure. After the mixture was cooled to room temperature at a rate of 2.1 ℃·h^‒1, red block-shaped crystals of 1 were obtained in 66% yield (based on Co^Ⅱ salt). Elemental analysis (%) calcd. for C₁₆H₂₀Co₄N₈O₁₆S₂: C, 21.83; H, 2.29; N, 12.73. Found (%): C, 21.89; H, 2.34; N, 12.81. FT-IR (cm^-1): 3436(s), 3349(s), 1620(s), 1557(s), 1394(s), 1359(m), 1279(w), 1205(s), 1173(s), 1110(w), 1033(ms), 936(w), 872(w), 812(w), 768(w), 664(m), 615(m), 535(w), 473(w).

  2.3 Structure determination

  A suitable single crystal with dimensions of 0.25mm × 0.22mm × 0.20mm was carefully selected and glued on a thin glass fiber. X-ray single-crystal diffraction data for 1 were collected on an Agilent SuperNova, Dual, Cu at zero, AtlasS2 diffractometer equipped with mirror-monochromated Cu-Kα radiation (λ = 1.54184 Å) at 150 K. There was no evidence of crystal decay during data collection. Semi-empirical multiscan absorption corrections were applied by SCALE3 ABSPACK and the programs CrysAlisPro were used for the integration of diffraction profiles^{[21, 22]}. The structure was solved by direct methods and refined with the full-matrix least-squares technique using the ShelXT and ShelXL programs^{[23, 24]}. Anisotropic thermal parameters were assigned to all non-H atoms. The organic hydrogen atoms were geometrically generated. H atoms attached to water molecules were located from difference Fourier maps and refined with isotropic temperature factors. These data can be obtained, upon request, from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, U.K. For 1, a total of 5526 reflections with 2208 unique ones (R_int = 0.0434) were collected in the range of 2.60≤θ≤25.01^o, of which 2208 were observed with I > 2σ(I). The final R = 0.0385 and wR = 0.0874 (w = 1/[σ²(F_o²) + (0.0579P)² + 14.2637P], where P = (F_o² + 2F_c²)/3), S = 1.064, (Δρ)_max = 0.87 and (Δρ)_min = –0.67 e·Å^–3.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure

  Complex 1 crystallizes in the monoclinic P2₁/c space group with Z = 2 in each asymmetric unit (Table 1), exhibiting a 3D pillared-layer framework with atz^‒ extended [Co₄(μ₃-OH)₂]⁶⁺ layers supported by rigid L^4‒ ligands. There are two independent Co^Ⅱ ions, two coordinated water molecules, one atz^‒ anion, half a fully deprotonated L^4‒ ligand, and one μ₃-OH^‒ group in the asymmetric unit. As shown in Fig. 1, both unique Co^Ⅱ ions are six-coordinated in octahedral coordination geometries with different distortion. Co(1) is coordinated by four O atoms from one coordinated water molecule, one μ₃-OH⁻ group, one carboxylate group of L^4‒, one sulfo group of L^4‒ as well as two N atoms from two separate atz^‒ ligands. Co(2) is surrounded by five O donors from one coordinated water molecule, two μ₃-OH⁻ groups, one carboxylate group of L^4‒, one sulfo group of L^4‒ as well as one N atom form one atz^‒ ligand. The bond lengths of Co‒O and Co‒N fall in the range of 2.018(3)~2.302(3) Å (Table 1), compared to the previously reported Co^Ⅱ-based complexes with mixed carboxylate and polyazole ligands^[6-10]. The centrosymmetric L^4‒ ligand in 1 adopts an octahedral dentate coordination mode with two μ₂-k²: O3, O4-COO^‒ and two μ₂-k²: O6, O8-SO₃^‒, in which the carboxylate group exhibits syn, syn-configuration.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–N(3)A	2.125(3)		Co(1)–O(1)(W)	2.105(3)		Co(2)–O(4)	2.109(3)
  Co(1)–O(3)	2.134(3)		Co(1)–N(1)	2.092(4)		Co(2)–O(5)(W)	2.158(3)
  Co(1)–O(2)	2.018(3)		Co(2)–O(2)C	2.107(3)		Co(2)–O(8)D	2.302(3)
  Co(1)–O(6)B	2.244(3)		Co(2)–O(2)	2.018(3)		Co(2)–N(2)C	2.084(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Co(1)–N(3)A	98.47(14)		O(1)(W)–Co(1)–O(6)B	90.70(12)		O(2)–Co(2)–O(2)C	79.65(13)
  O(1)(W)–Co(1)–N(1)	96.15(13)		O(2)–Co(1)–O(3)	90.34(12)		O(2)–Co(2)–O(8)D	82.00(12)
  O(2)–Co(1)–O(6)B	88.39(12)		O(2)–Co(1)–N(3)A	92.21(13)		O(2)C–Co(2)–N(2)C	85.70(13)
  O(3)–Co(1)–O(6)B	83.79(11)		O(3)–Co(1)–N(3)A	90.02(13)		O(4)–Co(2)–O(8)D	87.31(12)
  O(6)B–Co(1)–N(1)	87.74(13)		O(4)–Co(2)–O(5)(W)	86.70(12)		O(5)–Co(2)–O(2)C	85.44(12)
  O(1)(W)–Co(1)–O(3)	86.06(12)		O(2)–Co(2)–O(4)	96.10(12)		O(2)–Co(2)–O(5)(W)	98.01(12)
  O(1)(W)–Co(1)–N(3)A	88.32(13)		O(4)–Co(2)–N(2)C	100.03(13)		O(2)C–Co(2)–O(8)D	100.45(12)
  O(2)–Co(1)–N(1)	87.33(13)		O(5)(W)–Co(2)–N(2)C	92.18(13)		O(8)D–Co(2)–N(2)C	89.48(13)
  Symmetry transformation: A: x, 1/2 – y, –1/2 + z; B: 1 – x, 1 – y, 1 – z; C: –x, 1 – y, 1 – z; D: –1 + x, + y, z

  Figure 1

  

  Figure 1.  Local coordination environments of Co^Ⅱ ions in 1 (Hydrogen atoms are omitted for clarity; symmetry codes: A = x, 1/2 – y, – 1/2 + z; B: 1 – x, 1 – y, 1 – z; C: –x, 1 – y, 1 – z; D: –1 + x, y, z)

  DownLoad: Full-Size Img PowerPoint

  As shown in Fig. 2, a pair of symmetry-related μ₃-OH⁻ groups hold four separate Co^Ⅱ ions together, generating a centrosymmetric tetranuclear [Co₄(μ₃-OH)₂]⁶⁺ cluster. The Co^Ⅱ···Co^Ⅱ separations and Co‒O‒Co bond angles by two μ₃-OH^‒ groups range from 3.1687(2) to 3.5614(2) Å and 100.345(3) to 115.268(3)°. As shown in Fig. 2, adjacent Co₄^Ⅱ units are extended by μ₃-N1, N2, N4-trz^‒, generating a wavy layer with the inter-subunit distance of 6.1787(3) Å. Furthermore, the Co₄^Ⅱ cluster-based layers are periodically supported by L^4- ligands through coordination bonds between Co^Ⅱ ions and syn, syn-COO^‒/μ₂-SO₃^‒, generating a 3D pillaredlayer framework with the adjacent interlayer Co^Ⅱ···Co^Ⅱ separation of 10.4454(7) Å (Fig. 3). In addition, 3D framework of 1 is consolidated through abundant hydrogen-bonding interactions between μ₃-OH^‒ group/coordinated water molecules/amino group of atz^‒ anion and carboxylate/sulfo groups (Table S1).

  Figure 2

  

  Figure 2.  2D wavy layer with Co₄^Ⅱ units extended by triazole bridges and its triangular magnetic pathways (Green lines represent the magnetic coupling pathways)

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  Figure 3

  

  Figure 3.  3D Pillared-layer framework of 1

  DownLoad: Full-Size Img PowerPoint

  From the viewpoint of magneto-structural correlations, the magnetic interactions are significantly mediated by the quadruple heterobridges (μ₃-OH^‒, μ₃-atz^‒, syn, syn-COO^‒ and μ₂-SO₃^‒) within the 2D layer, because the nearest Co^Ⅱ···Co^Ⅱ separations across the L^4- linker are far from those between adjacent 2D layers. More interestingly, such 2D magnetic layer exhibits vertexand edge-sharing triangle lattice (Fig. 2) when the magnetic pathways are treated as linkers. Obviously, this type of spin arrangement can geometrically induce a spin-frustration, when all magnetic pathways transfer antiferromagnetic couplings^{[7, 25]}.

  3.2 PXRD, TGA and FT-IR spectra

  The structural consistency and phase purity of the bulk products of 1 have been evidenced by comparing the experimental and computer-simulated PXRD patterns (Fig. S1). TGA analysis was carried out to explore the thermal stability of 1. The first weight-loss process appeared at 250 ℃ and ended at 330 ℃ (Fig. S2), which should be ascribed to the removal of coordinated water molecules (calcd. 8.2%, obs. 8.3%). Then the mixed ligands are rapidly removed when the temperature is above 330 ℃. No complete decomposition was observed until 800 ℃.

  In the IR spectrum of 1, strong and sharp adsorptions at ca. 3436 and 3349 cm^-1 are the characteristic vibration of O‒H and N‒H, suggesting the presence of water molecule and amino group. The disappearance of a characteristic band at ca. 1710 cm^-1 is indicative of full deprotonation of the carboxylic group. And the asymmetric (ν_as) and symmetric vibrations (ν_s) of carboxylate group are found at 1620, 1557, 1394 and 1359 cm^-1, respectively. The value of Δν between ν_as and ν_s (Δν = ν_as − ν_s) is 163 and 261 cm^-1, indicating the presence of bidentate bridging coordination modes of the carboxylate groups^[26]. The typical asymmetric or symmetric stretching peaks of -SO₃^‒ group are around 1205 and 1033 cm^‒1, respectively^[20]. Thus, the IR results are in good agreement with the crystallographic data.

  3.3 Magnetic properties

  Variable-temperature (2~300 K) magnetic susceptibility was measured on the polycrystalline samples of 1 under an applied direct-current (dc) field of 1 kOe. As shown in Fig. 4, the χ_MT value for each Co₄^Ⅱ subunit of 1 is 11.50 cm³·K·mol^–1 at room temperature, which is comparable with the spin-only value (7.50 cm³·K·mol^–1) of tetranuclear Co^Ⅱ cluster with S = 3/2 and g = 2.0. Upon cooling, the χ_MT monotonously decreases to 1.34 cm³·K·mol⁻¹ at 2.0 K. Above 40.0 K, the plot of χ_M⁻¹ vs. T obeys the Curie-Weiss law well with C = 15.13 cm³·K·mol⁻¹ and θ = −95.2 K (Fig. 4 inset). Apparently, the large and negative Weiss constant θ suggests strong antiferromagnetic coupling between the neighboring Co^Ⅱ ions. According to the established magneto-structural relationships, the μ₃-OH⁻ group generally transfers antiferromagnetic couplings due to large Co^Ⅱ‒O‒Co^Ⅱ bond angles (> 97°)^[27]. Moreover, μ₃-atz^‒, syn, syn-COO^‒ and μ₂-SO₃^‒ bridges also prefer to transmit antiferromagnetic couplings in Co^Ⅱ system^{[10, 27]}. To quantitatively describe the magnetic couplings within the layer of 1, a 2D magnetic model with different magnetic bridges should be reasonably needed. However, considering the strong anisotropy of Co^Ⅱ ions, no appropriate model could be currently used to evaluate the magnetic couplings within/between the Co₄^Ⅱ cluster. Thus, a model reported by Rueff et al. was used to roughly separate the spin-orbit coupling and antiferromagnetic exchange interactions^[28]. The magnetic fitting was performed by using equation (χ_MT = Aexp(‒E₁/kT) + Bexp(‒E₂/kT)) above 10 K. Here, A + B equals to the Curie constant, and E₁ and E₂ represent the activation energies corresponding to the spin-orbit coupling and the antiferromagnetic exchange interaction. The best-fit parameters are A + B = 14.9 cm³·K·mol⁻¹, E₁ = 91.68 K and E₂ = 7.76 K. The value for C = A + B agrees with that obtained from the Curie-Weiss law in the high temperature range, and the value for E₁/k is consistent with those given in the literature for both the effects of spin-orbit coupling and the site distortion (E₁/k of the order of 100 K). The magnetic coupling constant between Co^Ⅱ ions mediating by the quadruple bridges is about ‒15.5 K based on the relationship of χ_MT ∝ exp(J/2kT). The field dependence of magnetization was measured at 2 K (Fig. S3). At 70 kOe, the magnetization is 2.77 Nβ which is far from the saturation (2~3 Nβ for one Co^Ⅱ ion). All these results confirm that 1 exhibits strong antiferromagnetic behavior. Notably, the absence of a peak in the χ_M vs. T curve indicates that the temperature of antiferromagnetic ordering (T_N) is below 2 K, which is obviously due to the triangular alignment of spin carriers within the antiferromagnetic layer. Spin frustration parameter f is larger than 47.6 evaluated by the equation f = |θ|/T_N^[27], indicating strong spin frustration. Thus, the triangular spin arrangement and cooperative antiferromagnetic couplings mediated by the quadruple heterobridges are responsible for strong spin frustration of 1.

  Figure 4

  

  Figure 4.  Plots of χ_MT and χ_M vs. T for 1 (Solid lines correspond to the best least-square fits indicated in the text; Inset: plot of χ_M^-1 vs. T)

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In conclusion, a new pillared-layer framework with triazolate extended Co₄^Ⅱ layers was solvothermally obtained, which displays strong spin-frustrated antiferromagnetism due to the triangular magnetic lattice and cooperative antiferromagnetic couplings.

  
  
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• 

  Figure 1  Local coordination environments of Co^Ⅱ ions in 1 (Hydrogen atoms are omitted for clarity; symmetry codes: A = x, 1/2 – y, – 1/2 + z; B: 1 – x, 1 – y, 1 – z; C: –x, 1 – y, 1 – z; D: –1 + x, y, z)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  2D wavy layer with Co₄^Ⅱ units extended by triazole bridges and its triangular magnetic pathways (Green lines represent the magnetic coupling pathways)

  下载: 全尺寸图片 幻灯片
  

  Figure 3  3D Pillared-layer framework of 1

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Plots of χ_MT and χ_M vs. T for 1 (Solid lines correspond to the best least-square fits indicated in the text; Inset: plot of χ_M^-1 vs. T)

  下载: 全尺寸图片 幻灯片

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–N(3)A	2.125(3)		Co(1)–O(1)(W)	2.105(3)		Co(2)–O(4)	2.109(3)
  Co(1)–O(3)	2.134(3)		Co(1)–N(1)	2.092(4)		Co(2)–O(5)(W)	2.158(3)
  Co(1)–O(2)	2.018(3)		Co(2)–O(2)C	2.107(3)		Co(2)–O(8)D	2.302(3)
  Co(1)–O(6)B	2.244(3)		Co(2)–O(2)	2.018(3)		Co(2)–N(2)C	2.084(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Co(1)–N(3)A	98.47(14)		O(1)(W)–Co(1)–O(6)B	90.70(12)		O(2)–Co(2)–O(2)C	79.65(13)
  O(1)(W)–Co(1)–N(1)	96.15(13)		O(2)–Co(1)–O(3)	90.34(12)		O(2)–Co(2)–O(8)D	82.00(12)
  O(2)–Co(1)–O(6)B	88.39(12)		O(2)–Co(1)–N(3)A	92.21(13)		O(2)C–Co(2)–N(2)C	85.70(13)
  O(3)–Co(1)–O(6)B	83.79(11)		O(3)–Co(1)–N(3)A	90.02(13)		O(4)–Co(2)–O(8)D	87.31(12)
  O(6)B–Co(1)–N(1)	87.74(13)		O(4)–Co(2)–O(5)(W)	86.70(12)		O(5)–Co(2)–O(2)C	85.44(12)
  O(1)(W)–Co(1)–O(3)	86.06(12)		O(2)–Co(2)–O(4)	96.10(12)		O(2)–Co(2)–O(5)(W)	98.01(12)
  O(1)(W)–Co(1)–N(3)A	88.32(13)		O(4)–Co(2)–N(2)C	100.03(13)		O(2)C–Co(2)–O(8)D	100.45(12)
  O(2)–Co(1)–N(1)	87.33(13)		O(5)(W)–Co(2)–N(2)C	92.18(13)		O(8)D–Co(2)–N(2)C	89.48(13)
  Symmetry transformation: A: x, 1/2 – y, –1/2 + z; B: 1 – x, 1 – y, 1 – z; C: –x, 1 – y, 1 – z; D: –1 + x, + y, z

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