Strong antiferromagnetic interaction in a 3D copper–organic framework and spin-glass-like behaviour in a 1D nickel compound

Abstract

The in situ hydrothermal reaction of 5-(4-carboxy-1H-1,2,3-triazol-1-yl)isophthalic acid (H₃ctia) with M(NO₃)₂·nH₂O (M = Cu, Ni) afforded two new coordination polymers, {[Cu₄(tia)₄(H₂O)₃]·H₂O}_n (1) and {Ni(H₂O)₆·[Ni₂(ctia)₂(H₂O)₆]·2H₂O}_n (2) (tia²⁻ = 5-(1H-1,2,3-triazol-1-yl)isophthalate). X-ray structural analysis reveals complex 1 is an unusual 3D framework containing D_4h paddle-wheel copper units and two kinds of mononuclear copper units, and its resulting structure can be rationalized as a new topology with the Schläfli symbol of {4.8²}2{4².8⁵.10⁶.12²}{8².10}2{8³}2. Compound 2 displays a 1D zigzag chain structure. Strong antiferromagnetic interactions are observed among dinuclear units in complex 1. Interestingly, complex 2 exhibits spin-glass-like behaviour with the spin glass freezing temperature at 15.8 K. In addition, the thermal stability of these compounds was also studied.

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Introduction

The construction of coordination networks containing metal ions with magnetic anisotropy is particularly attractive owing to their aesthetic structure and potential application in the field of molecule-based magnetic materials.¹ However, the notion of effective design and synthesis of such materials has received less attention. When aiming to achieve such materials, the choice of appropriate bridging ligand and metal ions is of great importance.

So far, the majority of magnetic frameworks are first row transition metals (Co, Ni and Cu) complexes.² That is because they have an adjustable spin quantum number³ and magnetic anisotropy.⁴

As far as the ligand is concerned, heterocyclic polycarboxylics such as pyridine-2,4-dicarboxylic acid,⁵ pyrazine-2,3-dicarboxylic acid,⁶ 1H-benzimidazole-5,6-dicarboxylic acid,⁷ isonicotinic,⁸ etc. were verified to be candidates due to their versatile coordination conformations and strong coordination ability. It has been demonstrated that the mixed multiple heterobridges constructed by polydentate polyazole⁹ and conformation-dependent carboxylate groups can efficiently mediate the different magnetic couplings with variable strength and nature.¹⁰ H₃ctia is a new phenyl heterocyclic polycarboxylic ligand explored by our group and Guo¹¹ (see Scheme 1). We selected phenyl heterocyclic polycarboxylic acid H₃ctia (see Scheme 1) as the starting material to assemble different paramagnetic ions to form metal-frameworks in consideration of the following points. (i) Two carboxyl groups and one triazol ring with one carboxyl group lie in meta-position, which may help construct the multi-dimensional structure. (ii) Short bridging such carboxyl and triazol bridging may be responsible for the formation of a metal cluster. (iii) Long bridging across two aromatic rings may block the magnetic interaction among the clusters. Previously, a series of 3D Co–Ln heterometallic, 2D and 1D transition metal coordination polymers based on this ligand were reported.¹¹ Herein, we reported two new complexes, one 3D framework {[Cu₄(tia)₄(H₂O)₃]·H₂O}_n (1) with strong antiferromagnetic interaction and one 1D chain {Ni(H₂O)₆·[Ni₂(ctia)₂(H₂O)₆]·2H₂O}_n (2) with spin glass behaviour.

Scheme 1 Conversion of H₃ctia in construction of 1–2.

Experimental

1 Materials and physical measurements

The reagents and solvents were obtained from commercial sources and used as received. Elemental analyses were determined on a Perkin-Elmer PE 2400 CHNS/O analyzer. IR spectra (KBr pellets) were recorded on a Perkin-Elmer spectrometer in the range 4000–400 cm⁻¹. Temperature- and field-dependent magnetic measurements were carried out on a SQUID-MPMS-XL magnetometer. Diamagnetic corrections were made with Pascal's constants. X-ray powder diffraction (XRPD) intensities were measured on Rigaku D/max-IIIA diffractometer (Cu kα, λ = 1.54056 Å). The single crystalline powder samples were prepared by crushing the crystals and scanned from 3° to 6° with a step of 0.1° s⁻¹. Thermogravimetric analysis were performed on a NETZSCH TG 209 instrument with a heating rate of 10 °C min⁻¹ in the flowing air atmosphere.

2 General synthesis procedure

Synthesis of H₃ctia. H₃ctia was prepared according the literature¹² (yield: 70% based on 5-azidoisophthalic acid). ¹HNMR (300 Hz, DMSO-d₆, ppm): 8.02 (s, 1H), 8.65 (s, 2H), 9.65 (s, 2H). IR (KBr pellets, cm⁻¹): 3129.6, 1723.9, 1545.2, 1475.0, 1414.0, 1331.0, 1255.4, 1188.2, 757.9, 712.1, 669.7 (see Fig. S1 and S2 in ESI†).

Synthesis of {[Cu₄(tia)₄(H₂O)₃]·H₂O}_n (1). Cu(NO₃)₂·3H₂O (48.2 mg, 0.2 mmol) and a solution of H₃ctia (16 mL, 0.02 mmol) whose pH value was adjusted to 3.5 by sodium hydrate was placed in a Teflon-lined autoclave and heated to 180 °C for three days, then cooled down to room temperature at a rate of 2.5 °C h⁻¹. Blue block-shaped crystals suitable for X-ray analysis were directly obtained, collected, washed with water and dried in the air. Yield about 30% (based on Cu(NO₃)₂·3H₂O). Anal. calcd (%) for C₄₀H₂₈Cu₄N₁₂O₂₀ (1250.90): C, 38.41%; H, 2.26%; N: 13.44%. Found: C, 38.50%; H, 2.2%; N, 13.44%. IR (KBr pellets, cm⁻¹): 3385, 1620, 1561, 1425, 1320, 1078, 787, 716.

Synthesis of {Ni(H₂O)₆·[Ni₂(ctia)₂(H₂O)₆]·2H₂O}_n (2). Ni(NO₃)₂·6H₂O (0.058 g, 0.02 mmol) and a solution of H₃ctia (16 mL, 0.02 mmol) whose pH value was adjusted to 7.5 by sodium hydrate was placed in a Teflon-lined autoclave and heated to 160 °C for three days, then cooled down to room temperature at a rate of 2.0 °C h⁻¹. Green block-like crystals suitable for X-ray analysis were obtained, washed with distilled water and dried in the air. Yield: 80% based on the Ni(NO₃)₂·6H₂O. Anal. calcd (%) for C₂₂H₃₆N₆Ni₃O₂₆ (976.70): C, 27.06%; H, 3.72%; N: 8.61%. Found: C, 27.31%; H, 3.68%; N, 8.62%. IR (KBr pellets, cm⁻¹): 3448, 1633, 1586, 1428, 1373, 1248, 1077, 772, 724 cm⁻¹.

3 X-ray structure determination of complexes 1–2

Diffraction data for complexes 1–2 were collected with a Bruker SMART APEX CCD instrument with graphite monochromatic Mo-Ka radiation (λ = 0.71073 Å). The data were collected at 293(2) K. The absorption corrections were made by multi-scan methods. The structure was solved by charge flipping methods with the program Olex2 and refined by full-matrix least-squares methods on all F² data with Olex2. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms of water molecules were located in a difference Fourier map and refined isotropically in the final refinement cycles. Other hydrogen atoms were placed in calculated positions and refined by using a riding model. Crystallographic data for 1–2 are given in Table 1. Selected bond lengths and angles are given in Tables S1–S2.†

Table 1 Crystal data and structure refinement for 1 and 2

Identification	Compound 1	Compound 2
Formula	C40H28Cu4N12O20	C22H36N6Ni3O26
Mwt (g mol^−1)	1250.90	976.70
T (K)	273(2)	293(2)
Crystal system	Triclinic	Triclinic
Space group	P1̄	P1̄
a (Å)	10.3314(15)	6.958(2)
b (Å)	12.3196(18)	10.909(4)
c (Å)	19.042(3)	11.877(4)
α (deg)	106.785(3)	82.523(10)
β (deg)	93.694(3)	82.286(10)
γ (deg)	94.284(3)	71.561(9)
Z	2	1
ρ (g cm^−3)	1.803	1.922
μ (mm^−1)	1.917	1.771
F (000)	1256	502
2θ scan range (°)	3.09 to 27.49	3.10 to 25.01
Rint	0.0485	0.0482
Reflns collected	16 474	5590
Indep reflns	10 233	2951
Parameters	691	318
R1, ωR2 [I > 2σ(I)]	0.0679, 0.1774	0.0550, 0.1225
R1, ωR2 [all data]	0.0946, 0.1952	0.0701, 0.1298
GOF on F^2	1.115	1.116
Largest residuals (e Å^−3)	1.409, −1.217	0.468, −0.568

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Results and discussion

1 Conversion of H₃ctia

Both target compounds were directly isolated by using 5-(4-carboxy-1H-1,2,3-triazol-1-yl)isophthalic acid (H₃ctia) as the starting materials. During the course of constructing 1, H₃ctia underwent decarboxylation and doubly deprotonation to tia²⁻ (5-(1H-1,2,3-triazol-1-yl)isophthalate) (see Scheme 1). While in compound 2, H₃ctia triply deprotonated to ctia³⁻. Except for the reaction temperature and the pH of the solution, the other reaction conditions are the same, so these two factors may be responsible for the decarboxylation of H₃ctia in construction complex 1.

2 Structure descriptions

{[Cu₄(tia)₄(H₂O)₃]·H₂O}_n (1). Complex 1 is a 3 D metal–organic framework based on the D_4h paddle-wheel units [Cu₂(O₂CR)₄N₂] (see Fig. 3(a)) which were connected by two kinds five coordinated monoclear Cu(II) units as linkers (Cu3 and Cu4, see Fig. 1). Two kinds of μ₄-bridging ligand tia²⁻ (as designated infra with L^A and L^B, respectively, see Scheme 2) adopt four different modes to bridge Cu(II) centers (as designated infra with L^A, L^B1, L^B2, and L^B3, respectively, see Fig. 2). In 2, each [Cu₂(O₂CR)₄N₂] dimer contains two antisymmetric Cu1 and Cu2 centers. Both of them take square-pyramid geometries, surrounded by four carboxylate O and one N donors from five distinct tia²⁻ ligands (one L^A, One L^B1, two L^B2 and one L^B3 for Cu1; Two L^A, One L^B1, one L^B2 and one L^B3 for Cu2) (see Fig. 1 and 2). The bond lengths and angles around Cu1 and Cu2 are similar. The bond length of Cu1–O and Cu2–O ranges from 1.941(4) to 2.019(4) Å and 1.929(4) to 1.980(4) Å, respectively. The bond length of Cu1–N and Cu2–N is 2.147(4) Å and 2.168(4) Å, respectively. The cis bond angles around Cu1 and the cis bond angles around Cu2 range from 87.80(17) to 101.69(17)° and 86.09(18) to 99.24(17)°, respectively. However, Cu3 and Cu4 adopt distorted trigonal bipyramid geometries. Cu3 is coordinated by O12 from the ligand L^B2, O16 from the ligand L^B3, N9 from the ligand L^B3 and two water molecules (see Fig. 1). The bond lengths and cis angles around Cu3 range from 1.906(4) to 2.158(5) Å and from 86.99(18) to 127.3(3)°. Cu4 is coordinated by O3 and O4 from the ligand L^A, O8 and N6 from two ligands L^B1 and one water molecule with the bond lengths and cis angles ranging from 1.910(5) to 2.010(5) Å and from 86.4(2) to 94.5(2)°. In 1, the four carboxyl groups from the ligands L^A, L^B1, L^B2 and L^B3 bridge Cu1 and Cu2 to afford D_4h paddle-wheel units [Cu₂(O₂CR)₄N₂]¹⁴ with a separation of 2.6339 Å (see Fig. 3). These dinuclear units [Cu₂(O₂CR)₄N₂], Cu3 and Cu4 ions are linked by L^A, L^B1, L^B2 and L^B3 to form 3D framework (Fig. 3(b)). Compounds of the type M₂(O₂CR)₄ with D_4h paddle-wheel molecule structure are known for a variety of transition metals¹³ such as Cu, Mo, Ru etc.

Fig. 1 Local coordination environments of Cu(II) in 1.

Scheme 2 Coordination modes of tia²⁻ in 1 (a) μ₄-bridging: L^A; (b) μ₄-bridging: L^B.

Fig. 2 Bridging Cu(II) modes of the ligands tia²⁻. (a) L^A: μ₄-bridging; (b) L^B1: μ₄-bridging; (c) L^B2: μ₄-bridging; (d) L^B3: μ₄-bridging.

Fig. 3 (a) Paddle-wheel Cu(II) units; (b) Cu3 and Cu4 linked by the ligand tia²⁻ to form 3D framework. (c) Topologically representation for the 3D structure of 1.

Better insight into this framework can be achieved by topology analysis. In 1, the binuclear Cu(II) can be viewed as 6-connected nodes which are connected to two L^A, two L^B2, one L^B1 and one L^B3 ligands. While the Cu3, Cu4, the ligands L^A and L^B can be seen as 3-connected nodes (see Fig. 3(c)). Therefore, this structure can be simplified as an unordinary 4-nodal (3,3,3,6)-connected topological network with the Schläfli symbol of {4.8²}2{4².8⁵.10⁶.12²}{8².10}2{8³}2 {representing the ligand L^A and L^B2, binuclear Cu(II), L^B1 and L^B3, mononuclear}.

{Ni(H₂O)₆·[Ni₂(ctia)₂(H₂O)₆]·2H₂O}_n (2). Compound 2 has a 1D polymeric zigzag chain structure, which is similar to one Co(II) coordination polymer reported by Y. Li.¹⁵ The asymmetric unit of 2 consists of two Ni(II) centers, one Ni(H₂O)₆²⁺ cation and one independent ctia³⁻ (Fig. 4(a)). The Ni1 exhibits an octahedral coordination environment, where four oxygen atoms from the coordinated water molecules lie in the equatorial position and two O1 atoms from the symmetry-related ligands ctia³⁻ with bond length of Ni1–O1, 2.026(3) Å. The Ni2 takes a distorted octahedral geometry with two O6 and two N3 atoms from the two symmetry ligands ctia³⁻ on the equatorial position, while the apical position are occupied by two coordinated water molecules. As illustrated in Fig. 4(b), the adjacent Ni ions are connected by the ligands ctia³⁻ with a separation of 8.5701(22) Å, which is larger than the distance between them and their adjacent counter Ni(H₂O)₆²⁺, to form a polymeric chain.

Fig. 4 (a) ORTEP view of 2 with the thermal ellipsoids at 30% probability showing the coordination environments of the Ni(II) atoms; (b) the 1D Ni(II) coordination chain.

3 XRPD results

X-ray powder diffraction measurements studies verify the as-obtained bulk substance of complexes 1 and 2 are homogeneous phase (see Fig. S3 in ESI†).

4 Magnetic properties

The magnetic properties of 1 and 2 were investigated in the 2.0–300 K range at 1000 Oe. The magnetic susceptibility of 1 versus temperature is shown in Fig. 5. The χ_MT value is 0.94 cm³ K mol⁻¹ at 300 K, which is significantly smaller than the spin-only value of 1.5 cm³ K mol⁻¹ calculated for four Cu(II) ions (S = 1/2, g = 2). This suggests antiferromagnetic couplings exist between the paramagnetic ions even at room temperature. Upon lowering the temperature, the χ_MT decreases rapidly to 0.86 cm³ K mol⁻¹ from 300 K to 75 K, then keep constant until 15 K. when temperature continues to decrease, the χ_MT value slowly increases. The per mole magnetic susceptibility of 1 can viewed as sum of the contributions of one dinuclear Cu(II) unit and two free Cu(II) ions. The magnetic susceptibility of the dinuclear Cu(II) unit can be expressed by the Bleaney–Bowers equation. The χ_M of 1 is adequately represented by the eqn (1).

Fig. 5 Temperature dependence of the χ_MT and χ_M⁻¹ curve for 1. The solid line represents the best fit.

The least-squares fit to the data (60–300 K) leads to g = 2.17, 2J = −173.4 cm⁻¹, R = ∑(χ_obsd − χ^′_cacld)²/∑(χ_obsd)² = 3.03 × 10⁻⁴. Below 60 K, the data deviate substantially from the dimer model, presumably because of the presence of paramagnetic impurities.

From the view point of the relationship of magnetism and structure, the strong antimagnetic coupling of 1 mainly comes from paddle-wheel binuclear Cu(II) ions¹⁶ bridged by tetra-carboxyl groups with short distance of Cu1–Cu2. The drop in χ_MT to a value of 0.86 cm³ K mol⁻¹ at low temperature, which is very close to 0.75 cm³ K mol⁻¹ for two isolated Cu(II) with S = 1/2, g = 2, indicates the binuclear Cu(II) ions in the paddle-wheel units are strongly antiferromagnetically coupled to each other to a diamagnetic S = 0 ground state. When the temperature drops to 10 K, the χ_MT begins to increase slightly. That may be ascribed to the result of a minor paramagnetic impurity phase.¹⁷

As shown in Fig. 6, complex 2 has a χ_MT value of 3.67 cm³ K mol⁻¹, which is close to the theoretical value of 3.3 cm³ K mol⁻¹ for S = 1 with g = 2.2.¹⁸ Upon cooling from room temperature, the χ_MT value decrease from 3.67 to 3.48 cm³ K mol⁻¹ at ca. 40 K, which indicates a dominant antiferromagnetic interaction¹⁹ between the Ni(II) ions. The magnetic susceptibility above 40 K obeys the Curie–Weiss law with Weiss constant, θ = −4.23 K, and Curie constant, C = 1.141 cm³ K mol⁻¹. Then the χ_MT value increases rapidly to a maximum of 3.80 cm³ K mol⁻¹ at 26 K and finally drops to 2.05 cm³ K mol⁻¹.

Fig. 6 Temperature dependence of the χ_MT and χ_M⁻¹ curve for 2. The solid line represents the Curie Weiss fit.

The zero-field alternative-current (ac) magnetic susceptibility was performed under H_ac = 3.5 Oe and a frequency of 10–800 Hz (see Fig. S4 in the ESI†). Both of the in-phase and out-of-phase signals, χ^′_M and χ^′′_M display a very small frequency-dependent behaviour (see Fig. S4 in the ESI and Fig. S4′†). The shift of the peak temperature (T_p) of χ^′_M is measured by a parameter φ = (ΔT_p/T_p)/Δ(log f) = 0.01, which is in the range of a spin-glass.²¹ Unfortunately, the peaks of the out-of-phase are too small and close to the noise of the instrument, so the relaxation time (τ) and Δ/k_B can't be calculated from the Arrhenius law.²²

To further investigate the magnetic behaviour of 2 at low temperature, the field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements were performed at 50 Oe in the 2–30 K range (see Fig. 7). The temperature correspondent to the ZFC maximum (15.8 K) is the spin glass freezing temperature.²⁰

Fig. 7 Field-cooled (FC) and zero-field-cooled (ZFC) magnetization plots of 2.

In addition, the field-dependent isothermal magnetization M(H) was performed at 2 K, a value of 4.8 Nμ_B at 40 kOe is far below the saturation value of 6.6 Nμ_B expected for the Ni₃ unit because of the spin glass component (see Fig. S5 in the ESI†). Magnetization curve versus applied field measured at 2.0 K of complex 2 did not exhibit an obvious hysteresis effect under our experimental conditions (Fig. S5 in the ESI: inset†). Thus, it is difficult for us to further confirm the concrete magnetic behavior of 2. But we can confirm complex 2 should be a spin glass.

5 Thermal stability properties

The TGA curve of 2 exhibits an initial weight loss from 100 °C to 300 °C, with the observed weight loss of 25.3% corresponding to the release of lattice and coordinated-water molecules (calcd 25.8%) (see Fig. 8). After the water molecules were released, the skeleton collapse begins at about 300 °C, which is lower than those of coordinated polymers reported previously.²³ That may be because complex 2 contains too much water molecules. The TGA diagram of complex 1 displays the loss of lattice water molecules at 170 °C, which is higher than complex 2. This can be ascribed to the fact that the three dimension framework can hinder the escape of the water molecules in some degree. Above 500 °C, there is almost no weight loss in complexes 1 and 2. The remnants of complexes 1 and 2 are 23.8% and 26.5%, respectively.

Fig. 8 The TGA curves for complexes 1 and 2.

Conclusions

In this contribution, we have presented two new metal–organic frameworks with the starting materials H₃ctia. Complex 1 is a 3D metal–organic framework based on the D_4h paddle-wheel units [Cu₂(O₂CR)₄N₂] which were connected by two kinds five coordinated mononuclear Cu(II) units as linkers. Among the paddle-wheel units, the strong antiferromagnetic interaction can be observed between the Cu atoms. Compound 2 displays 1D zigzag chain structure. Interestingly, complex 2 exhibits spin-glass-behaviour with the spin glass freezing temperature at 15.8 K. Furthermore, the thermal stability of these compounds show the framework structure can affect the release of lattice water molecules.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22371103 and no. 20771062) and the Tianjin Natural Science Foundation (no. 08JCZDJC21100).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Selected bonds and angles for 1 and 2, powder X-ray diffraction patterns of 1 and 2, Temperature dependence of ac susceptibility at various frequencies of 2 and Field dependence of magnetization and the hysteresis loop at 2 K for 2. CCDC 901576 and 901577. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02942a

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