One-dimensional Gd^III–M^II (M = Mn, Co) acetate chains exhibiting a large cryogenic magnetocaloric effect at ΔH = 3 T

Abstract

Two one-dimensional acetate chains, Gd^III–Mn^II (1) and Gd^III–Co^II (2), have been prepared. Magnetic investigations indicate that the magnetic entropy changes (−ΔS_m) in 1 have maximum values of 38.70 J kg⁻¹ K⁻¹ at ΔH = 7 T and 31.08 J kg⁻¹ K⁻¹ at ΔH = 3 T, while these in 2 have maximum values of 35.18 J kg⁻¹ K⁻¹ at ΔH = 7 T and 28.67 J kg⁻¹ K⁻¹ at ΔH = 3 T.

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The magnetocaloric effect (MCE) is a magneto-thermodynamic phenomenon in which a change in temperature of a suitable material is caused by exposing the material to a changing magnetic field.¹ Magnetic refrigeration is a cooling technology based on the MCE.² Owing to its environmental friendliness and energy efficiency, magnetic refrigeration is regarded as the most promising technology to replace traditional refrigeration, especially for acquiring and maintaining low temperatures.³ Although the factors influencing the MCE of magnetic materials are known now, large-scale applications using magnetic refrigeration remain a great challenge. One of the obstacles to applying magnetic refrigeration is that the magnetic entropy of the magnetic materials synthesized so far is not large enough at a relatively low magnetic field.⁴

Coordination polymers, because of their structural diversity and controllability, have recently attracted great attention in the field of magnetic refrigeration.^5–8 On one hand, their structural diversity and controllability facilitate the adjustment of the magnetic interaction between the metal ions, leading to a significantly larger MCE than that of lanthanide alloys and magnetic nanoparticles.^9,10 On the other hand, when compared with metal cluster compounds, coordination polymers often possess a relatively higher density, and long-range magnetic ordering,^5–8 resulting in an MCE larger than that of metal cluster compounds.^4,11

The isotropic Gd^III (f⁷) and Mn^II (d⁵) ions, which possess the largest magnetic entropy of the cations in the f and d blocks, respectively, have been widely chosen to make excellent molecular cryocoolers.^4b,7b,12 Owing to their paramagnetic or weakly ferromagnetic interactions, their low-lying excited spin states are of key importance for obtaining a large MCE at a low magnetic field.^7b All Mn^II ions should be alternately separated by Gd^III ions, so as to avoid the relatively strong Mn^II⋯Mn^II magnetic coupling, when using Gd^III and Mn^II to synthesize magnetic materials. Although some Mn–Gd species, with the Mn^II ions alternately separated by the Gd^III ions such as Mn^II₄Gd^III₆,^4b Mn^II₉Gd^III₉^4b and Mn^II₄Gd^III₄ ¹² clusters, have been synthesized, Mn–Gd coordination polymers, with the Mn^II ions alternately separated by the Gd^III ions, remain rare.^7b Herein, we report the structures and MCEs of two one-dimensional (1D) Gd^III–M^II acetate chains, formulated as [GdMn_0.5(OAc)₄(H₂O)₂]·3H₂O (1) and [GdCo_0.5(OAc)₄(H₂O)₂]·3H₂O (2). Single-crystal structural analysis reveals that the M^II ions in 1 and 2 are alternately separated by Gd^III ions. Magnetic investigations indicate that the magnetic entropy is up to 31.08 J kg⁻¹ K⁻¹ for 1 and 28.67 J kg⁻¹ K⁻¹ for 2 even at ΔH = 3 T.

Complex 1 was prepared by the reaction of Gd(OAc)₃·6H₂O (0.388 g, 1 mmol) and Mn(OAc)₂·4H₂O (0.245 g, 1 mmol) in a mixed solution of deionized water (10 mL) and anhydrous ethanol (5 mL). Complex 2 was prepared in a similar way as described for 1. As complexes 1 and 2 are isomorphous, only the structure of 1 is described in detail. Single-crystal structure analysis revealed that complex 1 crystallized in the triclinic P1̄ space group. Each asymmetric unit in 1 consists of one Gd³⁺ ion, 0.5 Co²⁺ ions, four acetates, two coordinated water molecules and three guest water molecules. As shown in Fig. 1, each Gd^III ion is located in a capped square-antiprism geometry and coordinated by two water molecules, two acetates in a chelate mode and three acetates in a monodentate mode. Each Mn(II) ion is located in an octahedron geometry and coordinated by six acetates in a monodentate mode. The two adjacent Gd^III ions bridged by a pair of μ–η²:η¹ acetates, and the adjacent Gd^III and Mn^II ions bridged by one μ–η²:η¹ acetate and two syn–syn acetates, generate a 1D chain structure with each Mn^II ion alternately separated by two Gd^III ions. This 1D chain structure is very similar to that of NdCo_0.5 ¹³ as reported previously. The bond lengths of Gd–O are 2.340(3)–2.731(3) Å, and the bond lengths of Mn–O are 2.173(3)–2.192(3) Å, which are comparable to the corresponding values in previously reported 3D Gd–Mn magnetic materials.^7b,14 The Gd⋯Gd distance is 4.277(2) Å, and the Gd–O8–Gd angle is 114.77(1)°, which are comparable to the corresponding values in previously reported 1D Gd³⁺ acetate chains.^5b The Gd⋯Mn distance is 4.030(6) Å, this value is smaller than that previously reported 3D Gd–Mn magnetic materials.^7b,14 The Gd–O2–Mn angle is 122.42(1)°. The Gd–Mn–Gd and Gd–Gd–Mn angles are 180° and 132.56(2)°, respectively. There exist intramolecular hydrogen bonds between one coordinated water molecule and two acetate ligands (O10⋯O1 = 2.882(4) Å, H10A⋯O1 = 2.08 Å, ∠O10–H10A⋯O1 = 157.3°, O10⋯O6 = 2.827(4) Å, H10B⋯O6 = 2.00 Å, ∠O10–H10B⋯O6 = 163.6°).

Fig. 1 (a) The 1D chain structure of 1. (b) The 2D structure of 1. Cyan: Gd, purple: Mn, red: O, gray: C. The H atoms in the 2D network are omitted for clarity.

The adjacent 1D chains extend into a 2D layer structure via water tetramers, each formed by two guest water molecules, O12 and O13 (O13⋯O12 = 2.775(6) Å, H13B⋯O12 = 1.88(9) Å, ∠O13–H13B⋯O12 = 154(6)°), which are hydrogen-bonded to the carboxylate groups of the adjacent chains (O12⋯O3 = 2.840(4) Å, H12B⋯O3 = 1.99 Å, ∠O12–H12B⋯O3 = 174.9°). The extension of the adjacent 2D layer structures via the water tetramers hydrogen-bonded to the coordinated water molecules (O13⋯O9 = 2.676(5) Å, H13A⋯O9 = 1.97 Å, ∠O13–H13A⋯O9 = 139.8°) from two adjacent 2D layer structures, generates the 3D structure of 1 (ESI Fig. S1†).

The bond lengths of Gd–O in 2 are 2.330(2)–2.732(2) Å, compared to these in 1, and the bond lengths of Co–O are 2.083(2)–2.112(2) Å. The values are comparable to the previously reported ones in a NdCo_0.5 1D chain structure.¹³ The Gd⋯Gd distance is 4.277(9) Å, and the Gd–O–Gd angle is 114.88(8)°, comparable to those in 1. The Gd⋯Co distance and the Gd–O–Co, Gd–Co–Gd and Gd–Gd–Co angles are 4.007(5) Å, 124.01(9)°, 180° and 132.20(2)°, respectively.

The temperature dependence of the magnetic susceptibilities of 1 and 2 was measured from 2 to 300 K with an applied direct-current (dc) magnetic field of 1000 Oe. As shown in Fig. 2a and b, the χ_mT value of 9.84 cm³ K mol⁻¹ for 1 at 300 K is close to the expected spin-only value of 10.07 cm³ K mol⁻¹ for one isolated Gd^III ion (S = 7/2, g = 2) and 0.5 isolated Mn^II ions (S = 5/2, g = 2). With the temperature decreasing, the χ_mT values remained essentially constant until 100 K, after which they decreased gradually as T decreased to 10 K, and then decreased rapidly to 8.92 cm³ K mol⁻¹ at 2 K, which is probably due to the antiferromagnetic interactions of the intra-chain Gd–Gd–Mn. The χ_mT value of 8.82 cm³ K mol⁻¹ for 2 at 300 K equals the expected spin-only value of 8.82 cm³ K mol⁻¹ for one isolated Gd^III ion (S = 7/2, g = 2) and 0.5 isolated Co^II ions (S = 3/2). With the temperature decreasing, the χ_mT values stayed essentially constant until 100 K, then, they decreased gradually as T decreased to 10 K, after which they dropped rapidly to 8.22 cm³ K mol⁻¹ at 2 K, which can probably be ascribed to the antiferromagnetic interactions of the intra-chain Gd–Gd–Co and the zero-field splitting of the ground states.¹⁵ The data obey the Curie–Weiss law with C = 9.804 cm³ K mol⁻¹, θ = −0.34 K for 1 and C = 9.901 cm³ K mol⁻¹, θ = −2.32 K for 2. This result indicates that 1 and 2 have weak antiferromagnetic interactions. The stronger antiferromagnetic interaction in 2 is attributed to the anisotropy of the Co^II ions.¹⁶

Fig. 2 Temperature dependence of χ_mT and the inverse molecular susceptibility plots for 1 (a) and 2 (b).

The field dependence of the magnetization of 1 and 2 was also investigated at T = 2 K. As shown in Fig. 3a and b, the magnetization M reaches a value of 9.37 Nμ_B for 1 and 8.42 Nμ_B for 2. The former is close to the saturated value of 9.5 Nμ_B calculated for one isolated Gd^III ion (S = 7/2) and 0.5 isolated Mn^II ions (S = 5/2) in 1, while the latter is higher than that of 8.2 Nμ_B calculated for one isolated Gd^III ion (S = 7/2) and 0.5 isolated Co^II ions (S = 1/2) in 2. The higher experimental value of the magnetization M for 2, as compared to the calculated one, is related to the anisotropy of the Co^II ions.¹⁶ For comparison, the field dependence of the magnetization for 1 and 2 at T = 2 K was calculated using the Brillouin function. As shown in Fig. S2 and S3 (ESI†), the field dependence of the magnetization calculated for 1 is slightly higher than that obtained from the experimental data, while the field dependence of the magnetization calculated for 2 is consistent with that obtained from the experimental data.

Fig. 3 Magnetization versus applied field of 1 (a) and 2 (b) at T = 2–10 K and H = 0–7 T. Experimental −ΔS_m obtained from the magnetization data of 1 (c) and 2 (d) at various fields and temperatures.

The magnetic entropy changes, −ΔS_m of 1 and 2 (Fig. 3c and d) for evaluating the MCEs can be calculated using the equation¹⁷ ΔS_m(T, ΔH) = ∫[∂M(T,H)/∂T]_HdH from the experimental magnetization data in Fig. 3a and b. The maximum value of −ΔS_m for 1 is 38.70 J kg⁻¹ K⁻¹ (78.68 mJ cm⁻³ K⁻¹) at 2.5 K and ΔH = 7 T. This is smaller than the value 48.40 J kg⁻¹ K⁻¹ calculated using the equation¹⁷ −ΔS_m = nR ln(2S + 1) with one isolated Gd^III (S = 7/2) and 0.5 Mn^II (S = 5/2) ions. The maximum value of −ΔS_m for 2 is 35.18 J kg⁻¹ K⁻¹ (72.89 mJ cm⁻³ K⁻¹) at 2.5 K for ΔH = 7 T, which is also smaller than the calculated value of 44.93 J kg⁻¹ K⁻¹ with one isolated Gd^III (S = 7/2) and 0.5 Co^II (S = 3/2) ions. The experimental maximum values of −ΔS_m for 1 and 2 are smaller than the theoretical values probably because of the weak antiferromagnetic interactions in 1 and 2.^4c,d Surprisingly, the MCEs can also get satisfying values of 31.08 J kg⁻¹ K⁻¹ (63.19 mJ cm⁻³ K⁻¹) for 1 and 28.67 J kg⁻¹ K⁻¹ (59.40 mJ cm⁻³ K⁻¹) for 2 at ΔH = 3 T. These values are even higher than that for the commercial magnetic refrigerant GGG (−ΔS_m ≈ 24 J kg⁻¹ K⁻¹, at ΔH = 3 T).¹⁸ Complexes 1 and 2 exhibit a higher MCE than that of GGG at ΔH = 3 T, which may be attributed to the weak magnetic interactions, since the magnetic interaction in GGG is paramagnetic,¹⁹ and the use of paramagnetic materials as low-temperature refrigerants often requires relatively large fields to provide a relevant cooling power.²⁰

In summary, two 3d–4f acetate chains, 1 and 2, with high MCEs were reported. The MCE for complex 1 (−ΔS_m = 38.70 J kg⁻¹ K⁻¹) is larger than that of 2 (−ΔS_m = 35.18 J kg⁻¹ K⁻¹) at ΔH = 7 T. More importantly, the −ΔS_m values for 1 and 2 at a lower and applicable field of ΔH = 3 T are up to 31.08 J kg⁻¹ K⁻¹ and 28.67 J kg⁻¹ K⁻¹, respectively. The large MCEs for 1 and 2 are attributed to the small acetate ligands, the increased dimension and the alternating M–Gd–M structure in the complexes. The preparation of other multidimensional 3d–4f structures with other small-molecule ligands and alternating arrangements is in progress.

Experimental section

Synthesis of 1

Gd(OAc)₃·6H₂O (0.388 g, 1 mmol) and Mn(OAc)₂·4H₂O (0.245 g, 1 mmol) were dissolved in a mixture of deionized water (10 mL) and anhydrous ethanol (5 mL). Then acetic acid glacial (500 μl, 8.75 mmol) was added under stirring, and a freshly prepared aqueous solution of NaOH (3 mL, 1.0 M) was added dropwise. The mixture was heated to 70 °C and refluxed for 1 day. Evaporation of the filtrate over one month under ambient conditions afforded 0.16 g of colourless cubic-shaped crystals (yield 32% based on Gd).

Synthesis of 2

Gd(OAc)₃·6H₂O (0.194 g, 0.5 mmol) and Co(OAc)₂·4H₂O (0.125 g, 0.5 mmol) were dissolved in methanol (40.0 mL), then triethylamine (0.145 mL, 1.0 mmol) was added to it. The mixture was heated to 70 °C and refluxed for 1 day. Evaporation of the filtrate over one month under ambient conditions afforded 0.22 g of pink cubic-shaped crystals (yield 43% based on Gd).

Crystal data for 1

GdMn_0.5C₈O₁₃H₂₂, M = 510.98, triclinic, a = 8.2014(6) Å, b = 10.2761(6) Å, c = 11.3982(10) Å, α = 111.426(7)°, β = 108.079(7)°, γ = 93.090(6)°, V = 834.76(11) ų, T = 173 K, space group P1̄, Z = 2. The density was 2.033 g cm⁻³. 3274 reflections were measured, 3079 independent reflections (R_int = 0.0314). The final R₁ value was 0.0283 (I > 2σ(I)). The final wR(F²) value was 0.0693 (I > 2σ(I)). The goodness of fit on F² was 1.038. Anal. Calcd for 1: C, 18.79; H, 4.31; N, 0. Found: C, 18.73; H, 4.47; N, 0.091. IR (KBr pellet): 3405(b, s), 1552(s), 1432(s), 1384(w), 1022(w), 950(w), 676(m), 616(w).

Crystal data for 2

GdCo_0.5C₈O₁₃H₂₂, M = 512.97, triclinic, a = 8.1656(5) Å, b = 10.2359(7) Å, c = 11.3405(6) Å, α = 111.699(6)°, β = 108.094(5)°, γ = 92.947(5)°, V = 822.17(8) ų, T = 173 K, space group P1̄, Z = 2. The density was 2.072 g cm⁻³. 3218 reflections were measured, 3096 independent reflections (R_int = 0.0245). The final R₁ value was 0.0203 (I > 2σ(I)). The final wR(F²) value was 0.0516 (I > 2σ(I)). The goodness of fit on F² was 1.030. Anal. Calcd for 2: C, 18.71; H, 4.29; N, 0. Found: C, 18.67; H, 4.32; N, 0.800. IR (KBr pellet): 3392(b, s), 1547(s), 1428(s), 1384(w), 1029(w), 952(w), 680(m), 619(w).

Acknowledgements

We thank the 973 project from MSTC (grant no. 2012CB821704 and 2014CB845601), the NNSFC (grant no. 21390391) and the Doctoral Fund of the Ministry of Education of China (20120121110012) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The field dependence of magnetization for 1 and 2 at T = 2 K calculated using Brillouin function, PXRD patterns, TGA diagram and CIF files. CCDC 1006167 and 1006168. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00078a

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