The effect of magnetic coupling on magneto-caloric behaviour in two 3D Gd(III)–glycolate coordination polymers

Abstract

Two different 3D coordination polymers, [Gd(glc)(Hglc)(H₂O)]_n·nH₂O (1) and [Gd(Hglc)₃]_n (2) (H₂glc = glycolic acid), have been prepared based on the reaction of Gd(III) ions with H₂glc in different pH environments. Due to the deprotonation of the α-OH groups from half of the H₂glc ligands in 1, [Gd₂] units with a Gd–O–Gd bridging model are observed and suggest a stronger magnetic coupling than that in 2. Magnetic and heat capacity studies reveal the significant impact of the strength of magnetic coupling on the magneto-caloric effect (MCE) in systems. Although a theoretical calculation suggests the −ΔS_m (50.5 J kg⁻¹ K⁻¹) of 1 is larger than 2 (45.2 J kg⁻¹ K⁻¹), the stronger antiferromagnetic coupling in 1 decreases the number of low-lying excited spin states upon the lowering of temperature, thus giving a smaller value of −ΔS_m than 2 at various fields (e.g. −ΔS_m,max = 24.8 J kg⁻¹ K⁻¹ and 41.1 J kg⁻¹ K⁻¹ for 1 and 2 respectively at ΔH = 3 T). This case reveals that the effect of magnetic coupling on MCE plays a dominant role for designing low-temperature 3D Gd(III)-based magnetic coolants.

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Introduction

The magnetocaloric effect (MCE), which was first observed in metallic iron in 1881 by Warburg, is dependent on the change of magnetic entropy upon application of a varying magnetic field.¹ This interesting phenomenon can be used in cooling applications via adiabatic demagnetization. In recent years, molecular magnetic refrigerators working at the low temperature region with a large MCE have attracted intense interest from researchers due to their obvious merits, e.g. energy-efficiency, economy, environmentally friendly nature, synthetic controllability and functional tunability, in contrast to traditional refrigerators.² Theoretically, the magnetic coolants will possess a large entropy change (−ΔS_m) when meeting certain conditions: (1) a large spin ground state S due to the magnetic entropy amount R ln(2S + 1);³ (2) a small magnetic anisotropy can hinder the polarization of net molecular spins along the easy axis;⁴ (3) a high magnetic density (or a large metal/ligand mass ratio), since the diamagnetic ligands have negative impact on MCE;⁵ (4) a weak magnetic coupling in the magnetic coolant will result in a lower working temperature and a maximum of −ΔS_m close to its paramagnetic limit.⁶

Therefore, the combination of light ligands and Gd(III), which has a large spin value (S = 7/2) and is isotropic, has been proven to be one of the effective strategies for obtaining magnetic coolers with a large MCE. To date, a large number of Gd(III) based molecular clusters and coordination polymers with impressive MCE have been reported.^5,7–9 Among them, the construction of high-dimensional Gd(III) based polymers, such as [Gd(HCOO)(OAc)₂(H₂O)₂]_n,^9a [Gd(HCOO)₃]_n,^9b [Gd(OH)CO₃]_n,^9c and [GdF₃]_n,^8f is beneficial to obtain materials with promising MCE, when considering the enhanced magnetic density due to the sharing of bridging ligands between magnetic centers and that the nonmagnetic guest or solvent molecules are more difficult to trap in such structures.

On the other hand, with increasing magnetic density in systems, the distance between the Gd(III) ions will be shorter, which may result in stronger magnetic coupling. Although the magnetic coupling between Gd(III) ions is weak due to the shielding effect of 5s and 5p orbitals, the strengthening of magnetic coupling, which may result in a higher ordering temperature,¹⁰ is unfavorable for magnetic coolers working at lower temperature, especially in the ultra-low temperature region (i.e., sub-millikelvin).

Herein, we report a family of 3D coordination polymers, [Gd(glc)(Hglc)(H₂O)]_n·nH₂O (1) and [Gd(Hglc)₃]_n (2) (H₂glc = glycolic acid). The structural analysis indicates the α-OH groups of half of the H₂glc ligands in 1 are deprotonated, giving di-deprotonated glc²⁻ anions, while only mono-deprotonated Hglc⁻ anions, as a result of deprotonation of the carboxyl groups of H₂glc, are observed in 2. Although antiferromagnetic (AF) couplings are found in both complexes, the different degree of deprotonation of H₂glc leads to different strengths of magnetic couplings in 1 and 2. For 1, each deprotonated hydroxyl oxygen atom bridges two Gd(III) ions into a [Gd₂] unit, leading to a stronger intra-dinuclear AF coupling than 2, in which the Gd(III) ions are more separated with a Gd–OCO–Gd connecting model (Scheme 1), thus giving a very weak AF magnetic coupling. As confirmed by the MCE measurements, although the theoretical maximum −ΔS_m of 1 (50.5 J kg⁻¹ K⁻¹) is larger than 2 (45.2 J kg⁻¹ K⁻¹), the relatively strong AF coupling in 1 decreases the number of low-lying excited spin states, resulting in a weaker MCE (−ΔS_{m, max} = 40.7 J kg⁻¹ K⁻¹, ΔT_{ad, max} = 16 K at ΔH = 7 T) than 2 (−ΔS_m,max = 43.7 J kg⁻¹ K⁻¹, ΔT_ad,max = 24.5 K at ΔH = 7 T). Additionally, at a lower temperature down to 0.5 K at ΔH = 7 T, the −ΔS_m of 2 still maintains a considerable value of 39.5 J kg⁻¹ K⁻¹, which is obviously higher than that of 1 (−ΔS_m,max = 17.1 J kg⁻¹ K⁻¹), confirming that the weak magnetic coupling in 2 makes it a promising magnetic refrigerator working at low temperature.

Scheme 1 The bridging model of di-deprotonated glc²⁻ (left) and mono-deprotonated Hglc⁻ ligand (right) with Gd(III) ions.

Experimental

Synthesis

All chemicals were obtained from commercial sources and used as received without further purification.

[Gd(glc)(Hglc)(H₂O)]_n·nH₂O (1). A mixture of Gd(acac)₃·3H₂O (51 mg, 0.1 mmol) and H₂glc (30 mg, 0.4 mmol) was dissolved in 10 ml H₂O/MeOH (v : v = 1 : 1) followed by the addition of [(CH₃)₄N]OH·5H₂O (0.009 g, 0.05 mmol). The resulting solution was sealed in a Teflon-lined, stainless steel vessel (23 mL) and heated at 120 °C for 3 days, and then cooled to room temperature at a rate of 5 °C h⁻¹. Colorless block crystals (yield: 81%) were obtained. Elemental analysis (%) calc for C₄H₉GdO₈: C 14.03, H 2.65; found: C 13.90, H 2.68. Infrared (KBr disc, cm⁻¹): 3599 (m), 3448 (m), 3276 (m), 3140 (m), 2889 (m), 2827 (m), 2640 (m), 2440 (w), 2214 (w), 2106 (w), 1599 (s), 1545 (s), 1410 (s), 1227 (w), 1105 (s), 1057 (s), 1005 (w), 937 (m), 716 (m), 575 (m), 530 (m), 467 (m), 277 (m).

[Gd(Hglc)₃]_n (2). A mixture of Gd(acac)₃·3H₂O (51 mg, 0.1 mmol) and H₂glc (30 mg, 0.4 mmol) in 10 ml solution of H₂O/MeOH (v : v = 1 : 1) was sealed in a Teflon-lined, stainless steel vessel (23 mL) and heated at 120 °C for 3 days, and then cooled to room temperature at a rate of 5 °C h⁻¹. Colorless block crystals (yield: 60%) were obtained. Elemental analysis (%) calcd for C₆H₉GdO₉: C, 18.84, H 2.37; found: C 18.61, H 2.49. Infrared (KBr disc, cm⁻¹): 3395 (br), 2948 (m), 2813 (w), 2659 (w), 1590 (vs), 1475(w), 1448 (w), 1403 (m), 1385 (s), 1333 (s), 1332 (w), 1080 (s), 1064 (s), 998 (w), 940 (m), 817 (w), 707 (m), 575 (w), 540 (m).

General characterization

Elemental analyses were performed on an Elementar Vario EL elemental analyzer. Powder X-ray diffraction measurements (Fig. S1, ESI†) were performed with a Bruker D8 X-Ray Diffractometer. The FTIR spectra were measured on a Thermo NICOLET AVATAR 330 FTIR spectrometer.

Crystal data

Crystal diffraction data were recorded at 150(2) K on a Rigaku R-AXIS SPIDER Image Plate Diffractometer with MoKα radiation. The structures were solved by direct methods and all nonhydrogen atoms were refined anisotropically by least squares on F² using the SHELXTL program.¹¹

For 1: C₄H₉GdO₈, M_r = 342.36, monoclinic, space group P2₁/c, a = 6.2789(3) Å, b = 9.0295(6) Å, c = 14.8336(9) Å, α = 90°, β = 92.585(2)°, γ = 90°, V = 840.14(9) ų, Z = 2, ρ = 2.707 g cm⁻³; R₁ = 0.0292 (I > 2σ(I)), wR₂ = 0.0729 (all data). Final GooF = 1.035.

For 2: C₆H₉GdO₉, M_r = 382.38, monoclinic, space group P2₁, a = 8.1242(11) Å, b = 8.0394(13) Å, c = 8.4324(12) Å, α = 90°, β = 117.490(4)°, γ = 90°, V = 488.57(13) ų, Z = 2, ρ = 2.599 g cm⁻³; R₁ = 0.0423 (I > 2σ(I)), wR₂ = 0.1001 (all data). Final GooF = 1.099.

CCDC 1426346 (1) and 1426347 (2) contain the supplementary crystallographic data for this paper.

Magnetic measurements

The magnetic measurements were performed by using a Quantum Design MPMS XL-7 SQUID magnetometer and a Quantum Design PPMS on polycrystalline samples with an empirical diamagnetic correction. The specific heat was studied in a Quantum Design PPMS with the ³He option adopting the standard relaxation technique.

Results and discussion

Both complexes are synthesized through a hydrothermal process between Gd³⁺ and H₂glc in H₂O/MeOH solution. The presence of tetramethylammonium hydroxide is essential for separating complexes 1 and 2, for which the α-OH group of H₂glc could be deprotonated in an alkaline environment, thus giving the di-deprotonated glc²⁻ ligand. Besides, the acetylacetonate (acac) may serve as an auxiliary ligand to prevent the hydrolysis of Gd³⁺ ions into other compounds.^7a

Single-crystal X-ray diffraction analysis reveals complex 1 crystallizes in the monoclinic P2₁/c space group with one full formula unit in the asymmetric unit cell. In the crystal structure, each Gd(III) center is coordinated by eight oxygen atoms from two mono-deprotonated Hglc⁻ ligands, three di-deprotonated glc²⁻ ligands and one water molecule, resulting in a triangular dodecahedron geometry (Fig. 1) with a CShM value of 0.79 calculated by SHAPE 2.1.¹² The Gd–O bond lengths range from 2.283(3) to 2.521(3) Å.

Fig. 1 The coordination environments of Gd(III) in complex 1. The hydrogen bonds are displayed as purple dashed lines. Color codes: Gd (green), O (red), C (gray) and H (blue). For clarity, the hydrogen atoms on methylene are omitted.

Both Hglc⁻ and glc²⁻ ligands are observed in the structure of 1 and employ μ₃-η¹:η¹:η² and μ₂-η¹:η¹:η¹ bridging modes respectively between the Gd(III) ions (Scheme 1). Upon the bridging of the hydroxyl μ₂-oxygen atoms from glc²⁻ ligands between two Gd(III) ions, [Gd₂] dinuclear units are formed and connected with each other through four glc²⁻ ligands, extending to a 4-connected 2D layer on the bc plane of unit cell (Fig. 2a and b). The Gd(III) ions in a dinuclear unit are separated by 3.7880(3) Å and the Gd–O–Gd angle is 110.37(11)°. Additionally, the [Gd₂] dinuclear units in the 2D layers are bridged by Hglc⁻ ligands along a axis of the unit cell, forming a 3D structure with a typical pcu topology (Fig. 2c and d).

Fig. 2 The 2D layer constructed by [Gd₂] dinuclear units and glc²⁻ ligands (a) and its simplified structure (b); the extended 3D structure of 1 (c) bridged by Hglc⁻ between layers and its simplified topological structure (d). Blue spheres represent [Gd₂] dinuclear units as 6-connected nodes.

Complex 2, which is synthesized in the absence of tetramethylammonium hydroxide, crystallizes in the monoclinic space group P2₁. The asymmetric unit of 2 contains one Gd(III) ion and three mono-deprotonated Hglc⁻ ligands. In contrast with 1, only mono-deprotonated Hglc⁻ ligands are observed in 2, indicating the degree of deprotonation of H₂glc is pH-dependent and thus making the “fine-tuning” of the coordinating structure possible. In 2, each Hglc⁻ ligand not only chelates a Gd(III) ion with the carboxyl and hydroxyl groups, but also extends into a μ-bridging mode by an syn–anti carboxyl bridge (Gd–OCO–Gd, 6.3784(11) Å). As far as Gd(III) is concerned, each Gd(III) ion is coordinated with nine oxygen atoms from six Hglc⁻ ligands, giving a spherical capped square antiprism geometry with a CShM value of 0.59. More precisely, three of the ligands chelate the Gd(III) center with two oxygen atoms from a hydroxyl and carboxylic group respectively, whereas the other three ligands are coordinated in the opposite direction through one carboxyl oxygen atom (Fig. 3). Topologically, the Gd(III) ions can be simplified as 6-connected nodes (Fig. 4a and b) and the ligands as linkers, which gives rise to a 3D acs topology (Fig. 4c and d).¹³

Fig. 3 The coordination environments of Gd(III) in complex 2.

Fig. 4 Each Gd(III) ion is connected with six other Gd(III) ions through the bridging Hglc⁻ ligands (a) and can be simplified as a 6-connected node (b), so the 3D structure of 2 (c) could be simplified as a 3D acs topology (d).

The variable-temperature magnetic susceptibilities of 1 and 2 were collected from 1.8 to 300 K and 2 to 300 K for 1 and 2 respectively in an applied direct-current (dc) field of 0.1 T. At room temperature, the χ_mT values for complexes 1 and 2 are 7.87 cm³ K mol⁻¹ and 7.84 cm³ K mol⁻¹, respectively, which are both in good agreement with the spin-only value expected for a free Gd(III) ion with g = 2 (7.875 cm³ K mol⁻¹). However, as the temperature decreases, both complexes exhibit different magnetic behaviours. For 1, the χ_mT value remains essentially constant upon lowering of the temperature to about 30 K, after which it decreases sharply to a minimum value of 2.85 cm³ K mol⁻¹ at 1.8 K (Fig. 5a), indicating AF coupling in 1, which is in agreement with the Curie–Weiss fitting for 1 (C = 7.94 cm³ mol⁻¹ K and θ = −2.07 K). The χ_mT value for 2 is essentially constant, between 7.51 and 7.84 cm³ K mol⁻¹, with very weak interactions as indicated by the Curie–Weiss fit (C = 7.85 cm³ K mol⁻¹, θ = −0.6 K in the range 1.8–300 K, Fig. 5b). To further understand the magneto-structural correlations of both complexes,^14,15 the fitting of the variable-temperature magnetic susceptibilities and magnetization of 1 and 2 was performed using PHI software.¹⁶ For 1, the dominant antiferromagnetic interaction is inside the [Gd₂] dinuclear unit with J = −0.103 cm⁻¹, accompanied by a negligible zJ < 0.001 cm⁻¹. For 2, the antiferromagnetic interaction can be easily rationalized by an intermetallic zJ = −0.005 cm⁻¹ (Fig. 5). It is obvious that the AF interactions in 1 are stronger than in 2, such observation is mainly attributed to the single-oxygen bridged Gd(III)⋯Gd(III) coupling in the [Gd₂] unit of 1, in which the Gd(III)⋯Gd(III) distance (3.7880(3) Å for 1) is obviously shorter than in 2 (6.3036(6) Å), resulting in stronger AF coupling in 1.¹⁷

Fig. 5 Temperature-dependencies of the magnetic susceptibility products, χ_MT, and the inverse magnetic susceptibilities, 1/χ_M, at 1.8–300 K with a dc field of 0.1 T for complexes 1 (a) and 2 (b). Field-dependencies of the magnetization for 1 (c) and 2 (d) at 2–10 K. The solid lines represent the magnetic coupling parameter (J) (black) and Curie–Weiss (blue) fitting, respectively.

Magnetization measurements were performed at T = 2–10 K for 1 and 2, in the field range of 0.01–7 T (Fig. 5c and d). The magnetization increased steadily with the external field and reached saturation values of 6.90 and 7.02Nβ for 1 and 2 respectively at 1.8 K and 7 T, which are in good agreement with the theoretical value of 7Nβ for a Gd(III) ion (S = 7/2, g = 2).

According to experimental magnetization data, the magnetic entropy changes (−ΔS_m) for 1 and 2 can be calculated by applying the Maxwell equation.¹⁸

The magnetic field dependent magnetic entropy change (−ΔS_m) of each complex at various temperatures is shown in Fig. 6. It is obviously observed that the −ΔS_m values of 1 and 2 gradually increase with the lowering of the temperature and increase of the applied field. For 1, the −ΔS_m value reaches a maximum value of 40.7 J kg⁻¹ K⁻¹ with ΔH = 7 T at 2 K (Fig. 6a), which is far smaller than its theoretical limiting value of 50.5 J kg⁻¹ K⁻¹ calculated from R ln(2S_Gd + 1) with S_Gd = 7/2. For 2, due to the relatively smaller metal/ligand ratios of 2 in contrast to 1, the calculated theoretical maximum −ΔS_m with the value 45.2 J kg⁻¹ K⁻¹ is smaller than that of 1, however, the experimental result indicates the MCE effect for 2 is more promising than for 1, as confirmed by the larger −ΔS_m with an experimental value of 43.7 J kg⁻¹ K⁻¹ at T = 2 K and ΔH = 7 T (Fig. 6b). Such observation mainly originates from the different strength of magnetic coupling in these two systems as confirmed by their different magnetic coupling parameters (J). As a result, the obvious antiferromagnetic coupling in 1 can decrease the number of low-lying excited spin states upon the lowering of temperature, leading to a decrease of magnetic entropy changes while the magnetic coupling in 2 is much weaker, which makes the Gd(III) ions keep mainly paramagnetic states at lower temperature and gives a large MCE close to its paramagnetic limits. The volumetric −ΔS_m values for 1 and 2 are 107.7 and 113.6 mJ cm⁻³ K⁻¹ respectively. Both 1 and 2 display considerable MCE in comparison with some reported 4f magnetic coolants as shown in Table 1.

Fig. 6 Magnetic entropy changes obtained from the magnetization data for 1 (a) and 2 (b).

Table 1 Magnetic entropy change for selected 4f-based magnetic coolants

Complex	ΔH (T)	−ΔSm,max
		J kg^−1 K^−1	mJ cm^−3 K^−1
1 (this work)	7	40.7	110
2 (this work)	7	43.7	114
[{Gd(OAc)3(H2O)2}2]·4H2O^8a	7	40.6	82.8
[Gd(HCOO)(OAc)2(H2O)2]n ^9a	7	45.9	110
[Gd2(N-BDC)3(dmf)4]n ^9a	7	29.0	41.2
[Gd(HCOO)(bdc)]n ^8b	9	47.0	125
[Gd2(OH)2(suc)2(H2O)]n·2nH2O^7a	7	42.8	120
[Gd6(OH)8(suc)5(H2O)2]n·4nH2O^7a	7	48.0	144
[Gd(HCOO)3]n ^9b	7	55.9	215.7
{[Gd6O(OH)8(ClO4)4(H2O)6](OH)4}n ^8c	7	46.6	215.6
[Gd(C2O4)(H2O)3Cl]^5b	7	48.0	144
[Gd4(SO4)4(μ3-OH)4(H2O)] n^8d	7	51.3	198.9
Gd(OH)3 ^8e	7	62.0	346
[Gd(OH)CO3]n ^9c	7	66.4	355
GdF3 ^8f	7	71.6	506

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Heat capacity

To further investigate the MCE of 1 and 2, low-temperature heat capacity (C) measurements were performed at applied fields of 0, 3, 5 and 7 T down to approximately 0.5 K. As shown in Fig. 7, the higher temperature regions for the two complexes are dominated by lattice contributions arising from the thermal vibration, which fits well with Debye's model and yields Debye temperatures (θ_D) of 272 and 460 K for 1 and 2 respectively. At lower temperatures, the heat capacities for the two complexes are dominated by the field-sensitive Schottky-type magnetic contributions. The observed increase of C upon cooling is attributed to the calculated Schottky anomaly, however, no obvious λ peaks are found in the measured temperature range at zero field, indicating the absence of magnetic ordering for both complexes above 0.5 K.

Fig. 7 Temperature dependent heat capacities (C) for 1 (a) and 2 (b) under various fields. The dotted blue lines represent the estimated lattice contribution.

From the experimental C, the temperature dependent entropies (S) for 1 and 2 are obtained by numerical integration using . Additionally, a compensation to the zero-field entropy has to be added for each complex based on the saturation value of magnetic entropy (S_m,sat = R ln(2S + 1) = 2.08R) (Fig. S2, ESI†). Therefore, the isothermal magnetic entropy change (−ΔS_m) and the adiabatic temperature change (ΔT_ad) for each complex (Fig. 8 and 9) can be extracted from the temperature and field dependencies of the relative S_total(T₀,H) data by vertical and horizontal subtraction, respectively.

Fig. 8 Magnetic entropy change (solid circles) and adiabatic temperature change obtained from the heat capacity for 1 at various temperatures and field changes. The hollow circles represent magnetic entropy obtained from magnetization.

Fig. 9 Magnetic entropy change (solid circles) and adiabatic temperature change obtained from the heat capacity for 2 at various temperatures and field changes. The hollow circles represent magnetic entropy obtained from magnetization.

With the information available in the lower-temperature region, one can easily recognize the peaks of the −ΔS_m–T curves and the expected shift to higher temperature with an increasing field. The ΔS_m of each complex shows good agreement from both independent evaluations, that is, heat capacity and magnetization respectively, confirming the consistency of the two measurements of magnetic entropy change. Due to the weaker magnetic coupling in 2, the observed peaks of −ΔS_m for 2 (Fig. 7) are obviously located at lower temperature regions than 1 (Fig. 8). At ΔH = 7 T, the corresponding maximum of −ΔS_m for 2 is observed at 1.4 K, obviously lower than that of 1 (2.6 K). Additionally, it is noted the maximum of −ΔS_m for 2 at a mild field of ΔH = 3 T gives a considerable value of 41.1 J kg⁻¹ K⁻¹ at 0.78 K, which is also close to its maximum of 43.7 J kg⁻¹ K⁻¹ at 7 T, indicating the energy efficiency and practical applicability of 2. What's more important, on lowering the temperature to 0.5 K, which is the lowest measured temperature, complex 2 still maintains a high −ΔS_m of 39.5 J kg⁻¹ K⁻¹ while the −ΔS_m of 1 sharply decreases to 17.1 J kg⁻¹ K⁻¹, such observation indicates that the weak magnetic coupling avoids the magnetic ordering of 2 at low temperature, thus making it a potential candidate for ultra-low temperature application.

When considering the other parameter, ΔT_ad, complex 2 with a maximum of 24.5 K for ΔH = 7 T is also obviously superior to 1 (ΔT_ad = 16.9 K at T = 3.2 K and ΔH = 7). Such larger ΔT_ad of 2 in contrast to 1 is attributed to not only its higher −ΔS_m value, but also the higher Debye temperature of 2, which leads to less lattice contribution of the heat capacity at the corresponding temperature, and therefore a relatively greater temperature variation (ΔT_ad) can be obtained.

Conclusions

In summary, through controlling the pH environment in the hydrothermal reaction of H₂glc ligands and Gd(III) ions, we successfully obtain two Gd(III)-based polymers, 1 and 2. In the structure of 1, due to the deprotonation of the α-OH group of H₂glc, each hydroxyl oxygen atom bridges two Gd(III) ions in the μ₂ model (Gd–O–Gd, 110.37(11)°), resulting in a dinuclear [Gd₂], thus giving a stronger magnetic coupling than 2 in which the Gd(III) ions are more separated at 6.3784(11) Å through the Gd–COC–Gd bridging model. A theoretical calculation suggested a higher −ΔS_m value of 1 than 2, however, the following MCE measurement indicates both the maxima of −ΔS_m and ΔT_ad of 2 at ΔH = 7 T (−ΔS_m,max = 43.7 J kg⁻¹ K⁻¹, ΔT_ad,max = 24.5 K) are larger than 1 (−ΔS_{m, max} = 39.8 J kg⁻¹ K⁻¹, ΔT_{ad, max} = 16 K). Additionally, the weak magnetic coupling in 2 makes it maintain a considerable value of −ΔS_m,max at moderate field (−ΔS_m,max = 41.1 J kg⁻¹ K⁻¹ at ΔH = 3 T) and has a lower working temperature region. This observation indicates the importance of controlling not only the large metal/ligand ratio but also weak magnetic interactions for successful design of molecular refrigerants.

Acknowledgements

This work was supported by the “973 Project” (2012CB821704 and 2014CB845602), project NSFC (Grant no. 91122032, 21371183, 21121061 and 21201137), the NSF of Guangdong (S2013020013002), Program for Changjiang Scholars and Innovative Research Team in University of China (IRT1298).

Notes and references

1. (a) E. Warburg, Ann. Phys., 1881, 249, 141; (b) P. Debye, Ann. Phys., 1926, 386, 1154; (c) W. F. Giauque, J. Am. Chem. Soc., 1927, 49, 1864; (d) W. F. Giauque and D. P. MacDougall, J. Am. Chem. Soc., 1935, 57, 1175.
2. (a) M. Evangelisti and E. K. Brechin, Dalton Trans., 2010, 39, 4672; (b) M. Evangelisti, F. Luis, L. J. de Jongh and M. Affronte, J. Mater. Chem., 2006, 16, 2534; (c) Y.-Z. Zheng, G.-J. Zhou, Z.-P. Zheng and R. E. P. Winpenny, Chem. Soc. Rev., 2014, 43, 1462; (d) J.-L. Liu, Y.-C. Chen, F.-S. Guo and M.-L. Tong, Coord. Chem. Rev., 2014, 281, 26.
3. R. L. Carlin, Magnetochemistry, Springer-Verlag, Germany, 1986.
4. (a) F. Torres, J. M. Hernández, X. Bohigas and J. Tejada, Appl. Phys. Lett., 2000, 77, 3248; (b) X.-X. Zhang, H.-L. Wei, Z.-Q. Zhang and L. Zhang, Phys. Rev. Lett., 2001, 87, 157203.
5. (a) F.-S. Guo, J.-D. Leng, J.-L. Liu, Z.-S. Meng and M.-L. Tong, Inorg. Chem., 2012, 51, 405; (b) Y. Meng, Y.-C. Chen, Z.-M. Zhang, Z.-J. Lin and M.-L. Tong, Inorg. Chem., 2014, 53, 9052.
6. Y.-C. Chen, F.-S. Guo, J.-L. Liu, J.-D. Leng, P. Vrábel, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, Chem. – Eur. J., 2014, 20, 3029.
7. (a) Y.-C. Chen, F.-S. Guo, Y.-Z. Zheng, J.-L. Liu, J.-D. Leng, R. Tarasenko, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, Chem. – Eur. J., 2013, 19, 13504; (b) L.-X. Chang, G. Xiong, L. Wang, P. Cheng and B. Zhao, Chem. Commun., 2013, 49, 1055; (c) F.-S. Guo, Y.-C. Chen, L.-L. Mao, W.-Q. Lin, J.-D. Leng, R. Tarasenko, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, Chem. – Eur. J., 2013, 19, 14876; (d) M. Wu, F. Jiang, X. Kong, D. Yuan, L. Long, S. A. Al-Thabaiti and M. Hong, Chem. Sci., 2013, 4, 3104; (e) J. W. Sharples, Y.-Z. Zheng, F. Tuna, E. J. L. McInnes and D. Collison, Chem. Commun., 2011, 47, 7650; (f) R. J. Blagg, F. Tuna, E. J. L. McInnes and R. E. P. Winpenny, Chem. Commun., 2011, 47, 10587; (g) F.-S. Guo, Y.-C. Chen, J.-L. Liu, J.-D. Leng, Z.-S. Meng, P. Vrábel, M. Orendáč and M.-L. Tong, Chem. Commun., 2012, 48, 12219; (h) J.-Z. Qiu, L.-F. Wang, Y.-C. Chen, Z.-M. Zhang, Q.-W. Li and M.-L. Tong, Chem. – Eur. J., 2015, 21 DOI:10.1002/chem.201503796.
8. (a) M. Evangelisti, O. Roubeau, E. Palacios, A. Camín, T. N. Hooper, E. K. Brechin and J. J. Alonso, Angew. Chem., Int. Ed., 2011, 50, 6606; (b) R. Sibille, T. Mazet, B. Malaman and M. François, Chem. – Eur. J., 2012, 18, 12970; (c) Y.-L. Hou, G. Xiong, P.-F. Shi, R.-R. Cheng, J.-Z. Cui and B. Zhao, Chem. Commun., 2013, 49, 6066; (d) S.-D. Han, S.-H. Miao, S.-J. Liu and X.-H. Bu, Inorg. Chem. Front., 2014, 1, 549; (e) Y. Yang, Q.-C. Zhang, Y.-Y. Pan, L.-S. Long and L.-S. Zheng, Chem. Commun., 2015, 51, 7317; (f) Y.-C. Chen, J. Prokleška, W.-J. Xu, J.-L. Liu, J. Liu, W.-X. Zhang, J.-H. Jia, V. Sechovský and M.-L. Tong, J. Mater. Chem. C, 2015, 2, 12206–12211.
9. (a) G. Lorusso, M. A. Palacios, G. S. Nichol, E. K. Brechin, O. Roubeau and M. Evangelisti, Chem. Commun., 2012, 48, 7592; (b) G. Lorusso, J. W. Sharples, E. Palacios, O. Roubeau, E. K. Brechin, R. Sessoli, A. Rossin, F. Tuna, E. J. L. McInnes, D. Collison and M. Evangelisti, Adv. Mater., 2013, 25, 4653; (c) Y.-C. Chen, Z.-S. Meng, L. Qin, Y.-Z. Zheng, J.-L. Liu, F.-S. Guo, R. Tarasenko, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, J. Mater. Chem. A, 2014, 2, 9851; (d) L. Sedláková, J. Hanko, A. Orendáčová, M. Orendáč, C.-L. Zhou, W.-H. Zhu, B.-W. Wang, Z. M. Wang and S. Gao, J. Alloys Compd., 2009, 425; (e) G. Lorusso, M. A. Palacios, G. S. Nichol, E. K. Brechin, O. Roubeau and M. Evangelisti, Chem. Commun., 2012, 48, 7592; (f) P. F. Shi, Y. Z. Zheng, X. Q. Zhao, G. Xiong, B. Zhao, F. F. Wam and P. Cheng, Chem. – Eur. J., 2012, 18, 15086; (g) S. Biswas, A. Adhikary, S. Goswami and S. Konar, Dalton Trans., 2013, 42, 13331; (h) S.-J. Liu, J.-P. Zhao, J. Tao, J.-M. Jia, S.-D. Han, Y. Li, Y.-C. Chen and X.-H. Bu, Inorg. Chem., 2013, 52, 9163.
10. A. T. Skjeltorp, C. A. Catanese, H. E. Meissner and W. P. Wolf, Phys. Rev. B: Solid State, 1973, 7, 2062.
11. SHELXTL 6.10, Bruker Analytical Instrumentation, Madison, WI, USA, 2000.
12. D. Casanova, M. Llunel, P. Alemany and S. Alvarez, Chem. – Eur. J., 2005, 11, 1479.
13. A. C. Sudik, A. P. Côté and O. M. Yaghi, Inorg. Chem., 2005, 44, 2998.
14. T. Rajeshkumar, S. K. Singh and G. Rajaraman, Polyhedron, 2013, 52, 1299.
15. L. Cañadillas-Delgado, Ó. Fabelo, C. Ruiz-Pérez and J. Cano, Magnetic Interactions in Oxo-Carboxylate Bridged Gadolinium(III) Complexes, Nova publishing, Spain, 2010.
16. N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini and K. S. Murray, J. Comput. Chem., 2013, 34, 1164.
17. (a) A. Adhikary, J. A. Sheikh, S. Biswas and S. Konar, Dalton Trans., 2014, 43, 9334; (b) A.-J. Hutchings, F. Habib, R. J. Holmberg, I. Korobkov and M. Murugesu, Inorg. Chem., 2014, 53, 2102; (c) F. Yang, Q. Zhou, G. Zeng, G.-H. Li, L. Gao, Z. Shi and S.-H. Feng, Dalton Trans., 2014, 43, 1238.
18. (a) V. K. Pecharsky and K. A. Gschneidner Jr., J. Magn. Magn. Mater., 1999, 200, 44; (b) M. Evangelisti and E. K. Brechin, Dalton Trans., 2010, 39, 4672.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1426346 (1) and 1426347 (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00208g

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