Synthesis, structure and magnetocaloric properties of a new two-dimensional gadolinium(III) coordination polymer based on azo­benzene-2,2′,3,3′-tetra­carb­oxy­lic acid

Wei, W.-W.; Lu, L.-P.; Feng, S.-S.; Zhu, M.-L.; Englert, U.

doi:10.1107/S2053229621008871

research papers

STRUCTURAL
CHEMISTRY

ISSN: 2053-2296

Volume 77| Part 10| October 2021| Pages 591-598

https://doi.org/10.1107/S2053229621008871

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Synthesis, structure and magnetocaloric properties of a new two-dimensional gadolinium(III) coordination polymer based on azobenzene-2,2′,3,3′-tetracarboxylic acid

Wen-Wen Wei,^a Li-Ping Lu,^a ^* Si-Si Feng,^a Miao-Li Zhu ^a and Ulli Englert ^a,^b ^*

^aInstitute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Shanxi University, 92 Wucheng Road, Taiyuan, Shanxi 030006, People's Republic of China, and ^bInstitute of Inorganic Chemistry, RWTH Aachen University, Landoltweg. 1, Aachen 52074, Germany
^*Correspondence e-mail: luliping@sxu.edu.cn, ullrich.englert@ac.rwth-aachen.de

Edited by E. Y. Cheung, Moderna Inc., USA (Received 28 June 2021; accepted 24 August 2021; online 10 September 2021)

A new Gd³⁺ coordination polymer (CP), namely, poly[diaqua[μ₄-1′-carboxy-3,3′-(diazene-1,2-diyl)dibenzene-1,2,2′-tricarboxylato]gadolinium(III)], [Gd(C₁₆H₇N₂O₈)(H₂O)₂]_n, (I), has been synthesized hydrothermally from Gd(NO₃)₃·6H₂O and azobenzene-2,2′,3,3′-tetracarboxylic acid (H₄abtc). The target solid has been characterized by single-crystal and powder X-ray diffraction, elemental analysis, IR spectroscopy and susceptibility measurements. CP (I) crystallizes in the monoclinic space group C2/c. The structure features a 4-connected topology in which Gd³⁺ ions are connected by carboxylate groups into a linear chain along the monoclinic symmetry direction. Adjacent one-dimensional aggregates are bridged by Habtc³⁻ ligands to form a two-dimensional CP in the (10 $[\overline{1}]$ ) plane. A very short hydrogen bond [O⋯O = 2.4393 (4) Å] links neighbouring layers into a three-dimensional network. A magnetic study revealed antiferromagnetic Gd⋯Gd coupling within the chain direction. CP (I) displays a significant magnetocaloric effect (MCE), with a maximum −ΔS_m of 27.26 J kg⁻¹ K⁻¹ for ΔH = 7 T at 3.0 K. As the MCE in (I) exceeds that of the commercial magnetic refrigerant GGG (Gd₃Ga₅O₁₂, −ΔS_m = 24 J kg⁻¹ K⁻¹, ΔH = 30 kG), CP (I) can be regarded as a potential cryogenic material for low-temperature magnetic refrigeration.

Keywords: azobenzenetetracarboxylic acid; Gd³⁺ coordination polymer; bridging carboxylate; antiferromagnetism; magnetocaloric effect; magnetic refrigeration; crystal structure.

CCDC reference: 2105211

1. Introduction

Coordination polymers (CPs), a class of compounds based on repetition of metal cations connected by coordinated linkers, have developed rapidly in the past 20 years (Chakraborty et al., 2021 ) due to their interesting structures and variable applications in gas storage and separation (Roztocki et al., 2020 ), catalysis (Kang et al., 2019 ), sensing (Lustig et al., 2017 ) and magnetic materials (Yang et al., 2019a ). In particular, due to the unique 4f electron configuration of Ln³⁺ ions, lanthanide coordination polymers (Ln-CPs) usually exhibit a high coordination number, flexible coordination geometry and strong spin-orbit coupling (Sorace et al., 2011 ; Liu et al., 2016 ). These properties suggest their application in luminescence sensing (Ye et al., 2017 ), molecular magnetism (Liu et al., 2019 ), magnetic resonance imaging (Debroye & Parac-Vogt, 2014 ) and related fields (Kumar et al., 2019 ).

Magnetic refrigeration represents a focus area in the field of magnetism. This approach is based on the magnetocaloric effect (MCE) (Yang et al., 2015 ; Wu et al., 2021 ) and is considered a highly efficient and energy-saving, hence environmentally friendly, technology. Key factors for success comprise a high-spin ground state S, negligible magnetic anisotropy and low-lying excited spin states (Evangelisti et al., 2006 ; Liu et al., 2014a ). The basic principle of magnetic refrigeration is realized through repeated cycles of isothermal magnetization and adiabatic demagnetization through the MCE displayed by the magnetic materials (Han et al., 2018 ). Magnetic refrigeration has potential for the generation of ultra-low temperatures. The magnitude of the MCE is usually measured by magnetic entropy change (−ΔS_m) and adiabatic temperature change (ΔT_ad) under certain conditions (Franco et al., 2018 ). A large ΔS_m under a relatively low magnetic field is mandatory for an attractive cryogenic magnetorefrigerant (Liu et al., 2017 ). The −ΔS_m value of the well-known commercial low-temperature magnetic refrigeration material GGG (Gd₃Ga₅O₁₂) is 24 J kg⁻¹ K⁻¹ (ΔH = 30 kG) (Daudin et al., 1982 ).

The Gd³⁺ ion meets the requirements of a high-spin ground state S (S = 7/2), of low-lying excited spin states and magnetic isotropy (Niu et al., 2019 ). The magnetic coupling between Gd³⁺ centres is relatively weak, which allows the system to achieve a large MCE (Zhang et al., 2021 ). Therefore, the Gd³⁺ ion represents an ideal choice for the construction of molecular-based low-temperature magnetic refrigeration materials (Wang et al., 2019 ). At present, molecular materials of cryogenic magnetic refrigeration mainly include Gd-based clusters and Gd-based CPs. However, the exploration of MCE for one-dimensional (1D) linear Gd³⁺ CPs has only rarely been documented (Liu et al., 2014b ).

In view of the above-mentioned promising properties, we report the new two-dimensional (2D) Gd³⁺ complex, [Gd(Habtc)(H₂O)₂]_n, (I), for which we selected azobenzene-2,2′,3,3′-tetracarboxylic acid (H₄abtc) as the ligand. The four carboxylic acid groups of this rigid H₄abtc linker may be partially or completely deprotonated and thus show flexible and diverse coordination patterns. In one of these coordination modes, the O atoms of a carboxylate group can bridge Gd³⁺ ions and thus ensure magnetic exchange and transfer between adjacent Gd³⁺ ions, at the same time maintaining an overall rigid product (Zhang et al., 2015c ). In this article, we communicate the synthesis, structure and magnetic properties of (I).

2. Experimental

All reagents and solvents used were commercially available and were used without further purification. H₄abtc was purchased from Jinan Trading Company (China). FT–IR spectra were obtained with a Bruker TENSOR27 spectrometer on KBr disks in the 4000–400 cm⁻¹ region. Elemental analyses (EAs) were performed using a PerkinElmer 240 elemental analyzer. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance X-ray diffractometer (Cu Kα, λ = 1.5418 Å) at a rate of 10° min⁻¹ in the 2θ range 5–50°. Based on the results of the single-crystal X-ray diffraction experiment, the simulated pattern was obtained with Mercury (Macrae et al., 2020 ) assuming Cu Kα₁ radiation (λ = 1.54056 Å). The thermogravimetric analysis was performed on a Dupont thermal analyzer between room temperature and 1045 K under an N₂ flow with a heating rate of 10 K min⁻¹. Magnetic susceptibility was measured from a microcrystalline sample using a SQUID magnetometer (Quantum Design MPMS) in the range 2–300 K with a direct-current field of 1000 Oe. Isothermal field-dependent magnetization M(H) was measured in the range 0–7 T from 2 to 10 K.

2.1. Synthesis and crystallization

The reaction route to (I) is shown in Scheme 1. Gd(NO₃)₃·6H₂O (67.7 mg, 0.15 mmol) and H₄abtc (35.8 mg, 0.1 mmol) were dissolved in a mixture of N,N-dimethylformamide (DMF, 2 ml), acetonitrile (CH₃CN, 2 ml) and distilled water (H₂O, 6 ml). The solution was sealed in a stainless steel container and heated under autogenous pressure at 393 K for 72 h. After this period, heating was suspended and the container was allowed to cool to room temperature. Yellow block-shaped crystals of the product were obtained by filtration, washed with water and dried in the air (yield 67%). Analysis calculated (%) for C₁₆H₁₁GdN₂O₁₀: C 35.01, H 2.01, N 5.10; found: C 35.05, H 2.02, N 5.13.

2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. Carbon-bound H atoms were placed in calculated positions and refined using a riding model, with aromatic C—H = 0.93 Å and U_iso(H) = 1.2U_eq(C). The water H-atom positions were fixed as found (O—H distances are approximately 0.82 Å), with U_iso(H) = 1.5U_eq(O). A difference Fourier map (Fig. 1) suggested Wyckoff position 4b for atom H4A in the short O⋯O contact, albeit as a very broad residual electron-density maximum. Our structure model with H4 in this special position therefore assumes a short symmetric hydrogen bond. In the absence of high-resolution or neutron data, we can neither disprove nor support a split-atom alternative and an asymmetric hydrogen bond. Şerb et al. (2011 ) have compiled structures featuring very short O⋯O bonds. The reflection conditions for the correct space group C2/c are also compatible with the subgroup Cc; tentative refinements in this noncentrosymmetric subgroup resulted in numerous high correlations and anticorrelations for positional and displacement parameters: 26 elements of the final inverted refinement matrix showed correlation coefficients with a modulus >0.9 and more than 100 with a modulus >0.8. These high correlations resulted in an unrealistically broad range of C—C bonds, and no convergence for physically meaningful displacement parameters could be achieved.

Table 1
Experimental details

Crystal data
Chemical formula	[Gd(C₁₆H₇N₂O₈)(H₂O)₂]
M_r	548.52
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	298
a, b, c (Å)	25.725 (4), 5.0236 (9), 17.274 (3)
β (°)	127.393 (4)
V (Å³)	1773.6 (5)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	3.80
Crystal size (mm)	0.20 × 0.15 × 0.15

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.600, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	6278, 1568, 1508
R_int	0.024
(sin θ/λ)_max (Å⁻¹)	0.598

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.013, 0.034, 1.11
No. of reflections	1568
No. of parameters	133
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.29, −0.33
Computer programs: APEX2 (Bruker, 2009 ), SAINT (Bruker, 2009), SHELXS97 (Sheldrick, 2008 ), SHELXL2014 (Sheldrick, 2015 ) and SHELXTL (Bruker, 2009).

Figure 1
Difference Fourier map (PLATON; Spek, 2020

) for (I)

before inclusion of H4A into the structure model. Contour lines are drawn at an electron density of 0.1 e Å⁻³. [Symmetry code: (i) −x, −y + 1, −z + 1.]

3. Results and discussion

3.1. IR spectroscopy

The IR spectra of the ligand and (I) in the range 4000–400 cm⁻¹ are presented in Fig. 2. The broad band at 3405 cm⁻¹ indicates O—H stretching of the hydroxy groups and the coordinated water molecules in (I) (Yang et al., 2019b ). The characteristic absorption peaks of the asymmetric and symmetric stretching vibrations of the carboxylate groups appear at 1383 and 1563 cm⁻¹ for (I) (Du et al., 2016 ; Li et al., 2012 ; Zhang et al., 2015a ). They are clearly shifted to lower wavenumbers in comparison with free H₄abtc (1426 and 1572 cm⁻¹), suggesting that the carboxylate groups in the complex are coordinated to the Gd³⁺ ions (An et al., 2018 ). The absorption observed at 1468 cm⁻¹ is caused by the N=N stretching vibration of the ligand (Goel & Kumar, 2018 ). The structural features of the complex deduced from IR spectra match the results of the single-crystal X-ray analysis. IR (KBr, ν, cm⁻¹, s = strong, m = medium and w = weak): 3405 (m), 1709 (w), 1563 (s), 1468 (s), 1383 (s), 1298 (w), 1147 (w), 1072 (m), 934 (w), 840 (m), 769 (s), 684 (w), 571 (s), 500 (s).

Figure 2
IR spectra of the ligand and (I)

3.2. Structure description

Coordination polymer (I) crystallizes in the monoclinic space group C2/c, adopting a 2D framework based on coordination and covalent bonds; we originally expected a three-dimensional (3D) solid from the reaction between Gd(NO₃)₃·6H₂O and H₄abtc. The asymmetric unit of (I) contains a Gd³⁺ ion situated on a twofold axis (Wyckoff position 4e), one half of the Habtc³⁻ ligand and a coordinated H₂O molecule. As shown in Fig. 3, each Gd³⁺ ion is eight-coordinated by O atoms in a {GdO₈} environment, in which six O atoms [O1, O2, O3, O1ⁱ, O2ⁱ and O3ⁱ; symmetry code: (i) −x, −y + 1, −z + 1] are derived from the carboxylate groups of four Habtc³⁻ moieties and two O atoms (O5 and O5ⁱ) represent aqua ligands. The Gd—O distances are in the range 2.3449 (15)–2.4503 (16) Å and the O—Gd—O angles vary from 68.66 (5) to 149.37 (5)° (Table 2), consistent with values observed in related compounds (Nakamura et al., 2021 ). The coordination polyhedron about the Gd³⁺ ion displays a dicapped trigonal prismatic geometry, in which each Habtc³⁻ links four Gd³⁺ ions in a μ₄-η¹:η¹:η¹:η⁰:η¹:η¹:η¹:η⁰ coordination mode and all Gd³⁺ ions are connected via four bridging Habtc³⁻ ligands. Adjacent Gd³⁺ atoms are linked by the carboxylate groups of Habtc³⁻, forming a linear Gd chain along [010]; the Gd⋯Gd separation corresponds to the lattice parameter b of 5.0236 (9) Å [Fig. 4(a)]. The 1D Gd chains are bridged by the central azo group of the Habtc³⁻ ligands to form a layer structure [Fig. 4(b)]. Two Habtc³⁻ ligands share the proton H4 which is located on a centre of inversion [see Refinement (§2.2) and Fig. 1] and plays the decisive role in linking adjacent coordination layers to a 3D framework [Fig. 4(c)]. In addition to this very short and symmetric hydrogen bond, the aqua ligand O5 acts as a hydrogen-bond donor towards carboxylate O atoms of a neighbouring layer. Detailed information of the intermolecular hydrogen bonds is summarized in Table 3. In order to obtain better insight into the nature of the intricate structure of CP (I), the network was simplified and its topology was analyzed with the help of the program TOPOS (Blatov & Shevchenko, 2006 ). As shown in Fig. 4(d), each Habtc³⁻ ligand can be perceived a four-connected node towards Gd³⁺ ions and, vice versa, each Gd³⁺ ion is coordinated by four Habtc³⁻ ligands. The overall network can thus be described as a 4-connected net with the point symbol (4⁴·6²).

Table 2
Selected geometric parameters (Å, °)

Gd1—O1	2.3449 (15)	Gd1—O3	2.4446 (15)
Gd1—O2	2.3722 (15)	Gd1—O5	2.4502 (16)

O1—Gd1—O1ⁱ	88.98 (8)	O3—Gd1—O3ⁱ	139.00 (8)
O1—Gd1—O2	92.22 (6)	O1—Gd1—O5ⁱ	140.25 (5)
O1—Gd1—O2ⁱ	149.36 (5)	O1—Gd1—O5	80.69 (6)
O2—Gd1—O2ⁱ	101.87 (8)	O2—Gd1—O5ⁱ	78.93 (6)
O1—Gd1—O3	71.73 (5)	O2—Gd1—O5	69.85 (6)
O1—Gd1—O3ⁱ	79.27 (5)	O3—Gd1—O5ⁱ	68.66 (5)
O2—Gd1—O3	72.12 (5)	O3—Gd1—O5	131.45 (5)
O2—Gd1—O3ⁱ	138.46 (5)	O5—Gd1—O5ⁱ	129.63 (9)
Symmetry code: (i) $[-x, y, -z+{\script{1\over 2}}]$ .

Table 3
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H4A⋯O3ⁱⁱ	1.22	2.42	3.130 (2)	114
O4—H4A⋯O4ⁱⁱ	1.22	1.22	2.439 (4)	180
O5—H5A⋯O1ⁱⁱⁱ	0.82	2.03	2.756 (2)	147
O5—H5B⋯O4^iv	0.82	2.01	2.821 (3)	173
Symmetry codes: (ii) ; (iii) ; (iv) $[x, -y+1, z-{\script{1\over 2}}]$ .

Figure 3
Expanded asymmetric unit and coordination environment of the Gd³⁺ ion in (I)

. Displacement ellipsoids are drawn at 30% probability and H atoms are represented as spheres of arbitrary radius. [Symmetry codes: (i) −x, y, −z + $[{1\over 2}]$ ; (ii) x, y + 1, z; (iii) −x + $[{1\over 2}]$ . −y + $[{5\over 2}]$ , −z + 1.]

Figure 4
(a) Distances between adjacent Gd³⁺ ions in the 1D metal chain constructed by Gd³⁺ ions and the carboxylate groups of the Habtc³⁻ ligands (H atoms have been omitted for clarity). (b) The 2D layer of (I)

. The inset is the local coordination geometry of the Gd³⁺ ion of (I)

. [Symmetry code: (i) −x, y, −z + $[{1\over 2}]$ .] (c) The 3D framework formed by hydrogen bonds in (I)

(different colours represent different layers and H4A atoms are shown in red). (d) The 2D topology of (I)

with point symbol (4⁴·6²).

3.3. Powder X-ray diffraction (PXRD) and thermal stability

To verify the phase purity of the compound, the as-synthesized samples were characterized by PXRD at room temperature. As shown in Fig. 5(a), the experimental PXRD pattern of (I) is in excellent agreement with the simulated one, demonstrating the phase purity of the bulk sample. Minor differences in line intensities can probably be attributed to preferred orientation of the powder sample. Thermal stability was investigated by a thermogravimetric analysis (TGA) under an N₂ atmosphere. Fig. 5(b) summarizes the weight loss for (I) between room temperature and 1045 K. In the temperature range 325–471 K, the TGA curve shows a weight loss of 6.88% which may be attributed to the elimination of two coordinated water molecules (calculated 6.56%). At higher temperatures, the framework of (I) gradually collapses.

Figure 5
(a) Experimental and simulated PXRD patterns of (I)

in the range 5–50°. (b) Thermogravimetric analysis for (I)

3.4. Magnetic properties

Magnetic properties of (I) were studied in order to understand potential magnetic interactions. Variable-temperature magnetic susceptibility measurements of (I) were conducted in the range 2–300 K with an applied magnetic field of 1000 Oe. As shown in Fig. 6, the experimental χ_mT value for (I) amounts to 8.00 cm³ mol⁻¹ K at 300 K, close to the expected value of 7.88 cm³ mol⁻¹ K calculated for an isolated Gd³⁺ ion (S = 7/2, g = 2) (Xi et al., 2020 ). As the temperature is decreased, the χ_mT value of (I) decreases slowly to 7.93 cm³ mol⁻¹ K around 10 K, and then increases gradually to 8.14 cm³ mol⁻¹ K at 2 K. The data in the whole temperature range 2–300 K fit well the Curie–Weiss law with C = 8.06 cm³ K mol⁻¹ and θ = −0.08 K. The negative θ value indicates the existence of weak antiferromagnetic interactions between the metal centres in the 1D chain of (I). To further quantitatively analyze the magnetic interactions, the molar susceptibility of (I) can be described by a Fisher expression for a classical spin chain which allows an evaluation of the magnetic coupling (J) between adjacent Gd³⁺ ions (Farger et al., 2018 ). The best least-squares fit parameters are g = 2.01 and J = −0.02 cm⁻¹, with an agreement factor R = 6.27 × 10⁻⁵ in the range 35–300 K. The value for J further proves the existence of weak antiferromagnetic interactions between adjacent Gd³⁺ ions in (I).

Figure 6
Plots of χ_m, χ_mT and χ_m⁻¹ (inset) as functions of T for (I)

. Red solid lines represent best fits.

The magnetization of (I) was measured in the interval between 0 and 7 T at temperatures between 2 and 10 K (Fig. 7a). The M values for (I) show a steady increase with increasing H and a saturation value of 7.14 Nβ at 7 T and 2 K, which is close to the expected value of S×g = 7/2×2 = 7 Nβ for an isolated Gd³⁺ ion (S = 7/2, g = 2). To evaluate the magnetocaloric effect (MCE), the magnetic entropy change (−ΔS_m) of (I) was calculated for a field between 0 and 7 T in the temperature range 2–10 K, and it can be obtained (Fig. 7b) by the Maxwell relation in the equation ΔS_m(T) = [M(T,H)/T]_HdH. The resulting maximum value of −ΔS_m amounts to 27.26 J kg⁻¹ K⁻¹ for ΔH = 7 T at 3.0 K, which is smaller than the theoretical value of 31.52 J kg⁻¹ K⁻¹, as calculated from the equation −ΔS_m = N_Gd Rln(2s + 1)/M_W, with S = 7/2. In this equation, M_W is the formula mass of 548.52 g mol⁻¹ and N_Gd is the number of Gd³⁺ ions present per mole of (I). The difference in −ΔS_m between the theoretical and experimental values may be attributed to the existence of antiferromagnetic interactions between Gd³⁺ ions. The experimental −ΔS_m value is also smaller than several previously prepared 1D linear-chain Gd³⁺ complexes (Table 4), which can be ascribed to the large M_W/N_Gd ratio arising from the large H₄abtc ligand and the antiferromagnetic interactions between the neighbouring Gd³⁺ ions in (I).

Table 4
Comparison of −ΔS_m for (I) and several previously reported 1D Gd³⁺ complexes

OAc is acetate, pda is propanedionate, ox is oxalate, cit is citrate, piv is pivalate, MMA is methylmalonate, INA is isonicotinate, glu is glutamate, HPA is homophtalate, azdc is 4,4′-azobenzoate, phen is 1,10-phenanthroline, 2,5-TDA is thiophene-2,5-dicarboxylate, DMA is dimethylacetamide, DMF is dimethylformamide, N-BDC is 2-aminobenzene-1,4-dicarboxylate, mnba is m-nitrobenzoate, PAA is phenylacetate, HIN is isonicotinic acid and IN is isonicotinate.

Complex	Dimensionality	−ΔS_m^max ( J kg⁻¹ K⁻¹)	Gd⋯Gd (Å)	M_W/N_Gd	Reference
[Gd(OAc)₃(H₂O)_0.5]_n	One-dimensional	50.4	4.0	343	Guo et al. (2012 )
[Gd(pda)(ox)_0.5]_n	Three-dimensional	46.8	4.1–6.1	303	Liu et al. (2017)
[Gd(pda)(ox)_0.5(H₂O)]_n	Three-dimensional	46.1	4.3–6.3	321	Liu et al. (2017)
[Gd(HCOO)(OAc)₂(H₂O)₂]_n	One-dimensional	45.9	5.9	572	Lorusso et al. (2012 )
[Gd(OAc)₃(MeOH)]_n	One-dimensional	45.0	4.1	366	Guo et al. (2012)
[Gd(pda)(ox)_0.5(H₂O)₂]_n	Two-dimensional	45.0	4.2–6.2	339	Liu et al. (2017)
[Gd(cit)(H₂O)]_n	Two-dimensional	43.6	4.5	363	Liu et al. (2014b)
[Gd₂(piv)₅(μ₃-OH)(H₂O)]_n	One-dimensional	37.5	3.7	427	Liu et al. (2014b)
[Gd(MMA)(INA)(H₂O)₂]_n	Two-dimensional	36.0	4.7	431	Li et al. (2017a )
{[Gd₂(glu)₃(H₂O)₂]·4H₂O}_n	Three-dimensional	36.0	4.2	406	Zheng et al. (2017 )
{[Gd(HPA)(NO₃)(H₂O)₂]·H₂O}_n	One-dimensional	35.6	3.9	415	Li et al. (2017b )
{[Gd₂(HPA)₃(H₂O)₂]·H₂O}_n	Two-dimensional	35.4	3.9	415	Li et al. (2017b)
[Gd(azdc)(HCOO)]_n	Three-dimensional	34.9	3.9	470	Zhang et al. (2015b )
[Gd₂(MMA)₂(INA)₂(H₂O)₃]_n	Two-dimensional	34.3	4.8	844	Li et al. (2017a)
[Gd₂(SO₄)₃(phen)₂(H₂O)₂]_n	One-dimensional	31.7	4.3	499	Zheng et al. (2017)
[Gd₂(2,5-TDA)₃(DMA)₂]_n	Three-dimensional	31.0	4.1	499	Kumar et al. (2020 )
{[Gd₂(OH)₂L₂]·DMF·4H₂O}_n	Three-dimensional	30.3	3.8–3.9	417	Peng et al. (2018 )
[Gd₂(N-BDC)₃(DMF)₄]_n	Three-dimensional	29.0	10.5–12.1	366	Lorusso et al. (2012)
[GdL_1/2(H₂O)₂]_n	Two-dimensional	27.3	5.0	548	This work
[Gd₂(mnba)₄(μ-OH)₂(H₂O)]_n	One-dimensional	27.1	3.8	515	Liu et al. (2014b)
[Gd(PAA)₃(H₂O)]_n	One-dimensional	26.7	4.0	580	Li et al. (2017c )
{Gd[IN][HIN][CH₂OCH₂O]}_n	One-dimensional	26.2	3.7	462	Li et al. (2020 )
{[Gd₂(azdc)₃(DMA)₂]·2DMA}_n	Three-dimensional	22.3	4.6	734	Zhang et al. (2014 )

Figure 7
(a) M versus H plots from 2 to 10 K. (b) Calculated −ΔS_m from the magnetization data of (I)

at various fields and temperatures.

3.5. Conclusion

In summary, the novel coordination polymer (I) has been successfully constructed under hydrothermal conditions via the combination of Gd³⁺ ions and the H₄abtc linker. The underlying structural principles in (I) comprise a 1D [Gd₂(COO)₄]_n chain and the linking of neighbouring chains via the organic ligand into a 2D structure with point symbol (4⁴·6²). Further crosslinking into a 3D framework occurs via very short hydrogen bonds. The new CP offers potential for application; magnetic studies reveal that (I) displays intrachain antiferromagnetic Gd⋯Gd coupling and a cryogenic MCE with the maximum −ΔS_m of 27.26 J kg⁻¹ K⁻¹ for ΔH = 7 T at 3.0 K. This small −ΔS_m value can be ascribed to the high M_W/N_Gd ratio arising from the large H₄abtc ligand and the antiferromagnetic interactions between neighbouring Gd³⁺ ions in (I). The selection of low molecular-weight ligands that transfer weak coupling may be a promising approach for obtaining Gd³⁺ complexes as molecule-based magnetic refrigerants. Further studies on Gd³⁺ complexes for magnetic refrigeration are underway in our laboratory.

Supporting information

CCDC reference: 2105211

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2053229621008871/cu3174sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2053229621008871/cu3174Isup2.hkl

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Bruker, 2009); software used to prepare material for publication: SHELXTL (Bruker, 2009).

Poly[diaqua[µ₄-1'-carboxy-3,3'-(diazene-1,2-diyl)dibenzene-1,2,2'-tricarboxylato]gadolinium(III) top

Crystal data top

[Gd(C₁₆H₇N₂O₈)(H₂O)₂]	F(000) = 1060
M_r = 548.52	D_x = 2.054 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 25.725 (4) Å	Cell parameters from 5374 reflections
b = 5.0236 (9) Å	θ = 3.0–25.2°
c = 17.274 (3) Å	µ = 3.80 mm⁻¹
β = 127.393 (4)°	T = 298 K
V = 1773.6 (5) Å³	Block, yellow
Z = 4	0.20 × 0.15 × 0.15 mm

Data collection top

Bruker APEXII CCD diffractometer	1508 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.024
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 25.2°, θ_min = 3.0°
T_min = 0.600, T_max = 0.747	h = −30→29
6278 measured reflections	k = −5→5
1568 independent reflections	l = −20→20

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.013	Hydrogen site location: mixed
wR(F²) = 0.034	H atoms treated by a mixture of independent and constrained refinement
S = 1.11	w = 1/[σ²(F_o²) + (0.0155P)² + 1.5784P] where P = (F_o² + 2F_c²)/3
1568 reflections	(Δ/σ)_max = 0.001
133 parameters	Δρ_max = 0.29 e Å⁻³
0 restraints	Δρ_min = −0.33 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Single-crystal X-ray diffraction data for (I) were collected on a Bruker APEXII diffractometer equipped with 1 K CCD instrument, using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å) at room temperature. Absorption corrections were performed via the SADABS program (Bruker, 2001). All the structures were solved by means of direct methods with SHELXS-97 program (Sheldrick, 2008) and refined on F² with full-matrix least-squares techniques using the program SHELXL-2014 program (Sheldrick, 2015). All non-H atoms were refined anisotropically.

H positions and isotropic displacement parameters constrained; H4a with refined Uiso on center of symmetry, hence coordinates fixed

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Gd1	0.000000	0.56130 (3)	0.250000	0.01598 (6)
O1	0.07997 (8)	0.8943 (3)	0.31251 (11)	0.0227 (3)
O2	0.07378 (8)	0.2637 (3)	0.37795 (11)	0.0262 (4)
O3	0.02231 (7)	0.7317 (3)	0.40008 (10)	0.0235 (3)
O4	0.05656 (10)	0.5732 (4)	0.54375 (14)	0.0499 (6)
H4A	0.000000 (1)	0.500000 (1)	0.500000 (1)	0.13 (3)*
O5	0.07156 (8)	0.3538 (4)	0.21824 (12)	0.0327 (4)
H5A	0.078437	0.195005	0.232767	0.049*
H5B	0.064136	0.369310	0.165148	0.049*
N1	0.22255 (9)	1.1967 (4)	0.47574 (13)	0.0253 (4)
C1	0.21412 (11)	1.0106 (5)	0.53013 (17)	0.0224 (5)
C2	0.14953 (11)	0.9340 (4)	0.48480 (16)	0.0187 (4)
C3	0.13547 (11)	0.7600 (5)	0.53305 (15)	0.0233 (5)
C4	0.18693 (13)	0.6585 (5)	0.62331 (17)	0.0339 (6)
H4	0.177983	0.544331	0.656255	0.041*
C5	0.25089 (12)	0.7240 (5)	0.66474 (18)	0.0360 (6)
H5	0.284663	0.647226	0.723624	0.043*
C6	0.26517 (12)	0.9030 (5)	0.61938 (18)	0.0316 (6)
H6	0.308258	0.950938	0.648062	0.038*
C7	0.09661 (10)	1.0408 (4)	0.38454 (16)	0.0186 (5)
C8	0.06676 (11)	0.6860 (5)	0.48759 (16)	0.0259 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Gd1	0.01432 (9)	0.01587 (9)	0.01428 (8)	0.000	0.00689 (7)	0.000
O1	0.0215 (8)	0.0262 (9)	0.0185 (8)	−0.0045 (6)	0.0112 (7)	−0.0014 (6)
O2	0.0275 (9)	0.0244 (9)	0.0251 (8)	0.0078 (7)	0.0151 (7)	0.0057 (7)
O3	0.0197 (8)	0.0306 (9)	0.0207 (8)	−0.0040 (6)	0.0125 (7)	−0.0025 (7)
O4	0.0400 (12)	0.0872 (17)	0.0256 (10)	−0.0310 (11)	0.0216 (9)	−0.0025 (10)
O5	0.0396 (10)	0.0347 (9)	0.0334 (9)	0.0130 (8)	0.0272 (9)	0.0112 (8)
N1	0.0181 (9)	0.0278 (11)	0.0261 (10)	−0.0058 (8)	0.0114 (8)	0.0019 (9)
C1	0.0185 (11)	0.0233 (11)	0.0210 (11)	−0.0035 (9)	0.0097 (10)	0.0010 (9)
C2	0.0176 (11)	0.0180 (11)	0.0174 (10)	−0.0015 (8)	0.0091 (9)	−0.0010 (9)
C3	0.0233 (12)	0.0259 (12)	0.0192 (11)	−0.0049 (9)	0.0122 (10)	0.0000 (9)
C4	0.0373 (14)	0.0352 (14)	0.0237 (12)	−0.0080 (12)	0.0157 (11)	0.0071 (11)
C5	0.0265 (13)	0.0404 (16)	0.0221 (12)	−0.0015 (11)	0.0048 (11)	0.0099 (11)
C6	0.0178 (12)	0.0374 (15)	0.0273 (13)	−0.0039 (10)	0.0074 (11)	0.0036 (11)
C7	0.0143 (10)	0.0218 (12)	0.0204 (11)	−0.0040 (9)	0.0108 (10)	0.0019 (9)
C8	0.0300 (13)	0.0303 (13)	0.0218 (12)	−0.0102 (11)	0.0180 (11)	−0.0041 (10)

Geometric parameters (Å, º) top

Gd1—O1ⁱ	2.3449 (15)	O5—H5B	0.8191
Gd1—O1	2.3449 (15)	N1—N1ⁱⁱⁱ	1.243 (4)
Gd1—O2	2.3722 (15)	N1—C1	1.432 (3)
Gd1—O2ⁱ	2.3722 (15)	C1—C6	1.391 (3)
Gd1—O3	2.4446 (15)	C1—C2	1.393 (3)
Gd1—O3ⁱ	2.4446 (15)	C2—C3	1.398 (3)
Gd1—O5ⁱ	2.4502 (16)	C2—C7	1.509 (3)
Gd1—O5	2.4502 (16)	C3—C4	1.392 (3)
O1—C7	1.275 (3)	C3—C8	1.481 (3)
O2—C7ⁱⁱ	1.237 (3)	C4—C5	1.377 (4)
O3—C8	1.243 (3)	C4—H4	0.9300
O4—C8	1.281 (3)	C5—C6	1.381 (4)
O4—H4A	1.2196	C5—H5	0.9300
O5—H5A	0.8224	C6—H6	0.9300

O1—Gd1—O1ⁱ	88.98 (8)	C8—O3—Gd1	132.45 (15)
O1ⁱ—Gd1—O2	149.37 (5)	C8—O4—H4A	112.33
O1—Gd1—O2	92.22 (6)	Gd1—O5—H5A	113.3
O1ⁱ—Gd1—O2ⁱ	92.22 (6)	Gd1—O5—H5B	120.9
O1—Gd1—O2ⁱ	149.36 (5)	H5A—O5—H5B	107.1
O2—Gd1—O2ⁱ	101.87 (8)	N1ⁱⁱⁱ—N1—C1	113.7 (2)
O1ⁱ—Gd1—O3	79.27 (5)	C6—C1—C2	121.1 (2)
O1—Gd1—O3	71.73 (5)	C6—C1—N1	124.3 (2)
O1ⁱ—Gd1—O3ⁱ	71.73 (5)	C2—C1—N1	114.6 (2)
O1—Gd1—O3ⁱ	79.27 (5)	C1—C2—C3	119.3 (2)
O2—Gd1—O3	72.12 (5)	C1—C2—C7	118.92 (19)
O2ⁱ—Gd1—O3	138.46 (5)	C3—C2—C7	121.8 (2)
O2—Gd1—O3ⁱ	138.46 (5)	C4—C3—C2	118.8 (2)
O2ⁱ—Gd1—O3ⁱ	72.11 (5)	C4—C3—C8	121.3 (2)
O3—Gd1—O3ⁱ	139.00 (8)	C2—C3—C8	119.9 (2)
O1ⁱ—Gd1—O5ⁱ	80.69 (6)	C5—C4—C3	121.2 (2)
O1—Gd1—O5ⁱ	140.25 (5)	C5—C4—H4	119.4
O1ⁱ—Gd1—O5	140.25 (5)	C3—C4—H4	119.4
O1—Gd1—O5	80.69 (6)	C4—C5—C6	120.4 (2)
O2—Gd1—O5ⁱ	78.93 (6)	C4—C5—H5	119.8
O2ⁱ—Gd1—O5ⁱ	69.86 (6)	C6—C5—H5	119.8
O2—Gd1—O5	69.85 (6)	C5—C6—C1	119.0 (2)
O2ⁱ—Gd1—O5	78.93 (6)	C5—C6—H6	120.5
O3—Gd1—O5ⁱ	68.66 (5)	C1—C6—H6	120.5
O3ⁱ—Gd1—O5ⁱ	131.45 (5)	O2^iv—C7—O1	125.0 (2)
O3—Gd1—O5	131.45 (5)	O2^iv—C7—C2	118.32 (19)
O3ⁱ—Gd1—O5	68.66 (5)	O1—C7—C2	116.69 (18)
O5—Gd1—O5ⁱ	129.63 (9)	O3—C8—O4	122.9 (2)
C7—O1—Gd1	122.79 (13)	O3—C8—C3	121.22 (19)
C7ⁱⁱ—O2—Gd1	136.28 (14)	O4—C8—C3	115.9 (2)

N1ⁱⁱⁱ—N1—C1—C6	−16.0 (4)	C2—C1—C6—C5	−2.1 (4)
N1ⁱⁱⁱ—N1—C1—C2	166.3 (3)	N1—C1—C6—C5	−179.7 (2)
C6—C1—C2—C3	4.4 (3)	Gd1—O1—C7—O2^iv	−94.9 (2)
N1—C1—C2—C3	−177.8 (2)	Gd1—O1—C7—C2	86.9 (2)
C6—C1—C2—C7	−175.3 (2)	C1—C2—C7—O2^iv	−83.3 (3)
N1—C1—C2—C7	2.4 (3)	C3—C2—C7—O2^iv	97.0 (3)
C1—C2—C3—C4	−2.9 (3)	C1—C2—C7—O1	95.1 (2)
C7—C2—C3—C4	176.8 (2)	C3—C2—C7—O1	−84.7 (3)
C1—C2—C3—C8	177.4 (2)	Gd1—O3—C8—O4	−114.6 (2)
C7—C2—C3—C8	−2.9 (3)	Gd1—O3—C8—C3	65.3 (3)
C2—C3—C4—C5	−0.8 (4)	C4—C3—C8—O3	−165.5 (2)
C8—C3—C4—C5	178.9 (3)	C2—C3—C8—O3	14.2 (4)
C3—C4—C5—C6	3.1 (4)	C4—C3—C8—O4	14.4 (4)
C4—C5—C6—C1	−1.6 (4)	C2—C3—C8—O4	−165.9 (2)

Symmetry codes: (i) −x, y, −z+1/2; (ii) x, y−1, z; (iii) −x+1/2, −y+5/2, −z+1; (iv) x, y+1, z.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4A···O3^v	1.22	2.42	3.130 (2)	114
O4—H4A···O4^v	1.22	1.22	2.439 (4)	180
O5—H5A···O1ⁱⁱ	0.82	2.03	2.756 (2)	147
O5—H5B···O4^vi	0.82	2.01	2.821 (3)	173

Symmetry codes: (ii) x, y−1, z; (v) −x, −y+1, −z+1; (vi) x, −y+1, z−1/2.

Comparison of -ΔS_m for (I) and several previously reported 1D Gd³⁺ complexes top

Complex	Dimensionality	-ΔS_m^max [J Kg K^-1]	Gd···Gd (Å)	M_W/N_Gd	Reference
[Gd(OAc)₃(H₂O)_0.5]_n	1D	50.4	4.0	343	Guo et al. (2012)
[Gd(pda)(ox)_0.5]_n	3D	46.8	4.1-6.1	303	Liu et al. (2017)
[Gd(pda)(ox)_0.5(H₂O)]_n	3D	46.1	4.3-6.3	321	Liu et al. (2017)
[Gd(HCOO)(OAc)₂(H₂O)₂]_n	1D	45.9	5.9	572	Lorusso et al. (2012)
[Gd(OAc)₃(MeOH)]_n	1D	45.0	4.1	366	Guo et al. (2012)
[Gd(pda)(ox)_0.5(H₂O)₂]_n	2D	45.0	4.2-6.2	339	Liu et al. (2017)
[Gd(cit)(H₂O)]_n	2D	43.6	4.5	363	Liu et al. (2014a,b)
[Gd₂(piv)₅(µ₃-OH)(H₂O)]_n	1D	37.5	3.7	427	Liu et al. (2014a,b)
[Gd(MMA)(INA)(H₂O)₂]_n	2D	36.0	4.7	431	Li et al. (2017a)
{[Gd₂(glu)₃(H₂O)₂].4H₂O}_n	3D	36.0	4.2	406	Zheng et al. (2017)
{[Gd(HPA)(NO₃)(H₂O)₂].H₂O}_n	1D	35.6	3.9	415	Li et al. (2017b)
{[Gd₂(HPA)₃(H₂O)₂].H₂O}_n	2D	35.4	3.9	415	Li et al. (2017b)
[Gd(azdc)(HCOO)]_n	3D	34.9	3.9	470	Zhang et al. (2015c)
[Gd₂(MMA)₂(INA)₂(H₂O)₃]_n	2D	34.3	4.8	844	Li et al. (2017a)
[Gd₂(SO₄)₃(phen)₂(H₂O)₂]_n	1D	31.7	4.3	499	Zheng et al. (2017)
[Gd₂(2,5-TDA)₃(DMA)₂]_n	3D	31.0	4.1	499	Kumar et al. (2020)
{[Gd₂(OH)₂L₂].DMF.4H₂O}_n	3D	30.3	3.8-3.9	417	Peng et al. (2018)
[Gd₂(N-BDC)₃(DMF)₄]_n	3D	29.0	10.5-12.1	366	Lorusso et al. (2012)
[GdL_1/2(H₂O)₂]_n	2D	27.3	5.0	548	This work
[Gd₂(mnba)₄(µ-OH)₂(H₂O)]_n	1D	27.1	3.8	515	Liu et al. (2014a,b)
[Gd(PAA)₃(H₂O)]_n	1D	26.7	4.0	580	Li et al. (2017c)
{Gd[IN][HIN][CH₂OCH₂O]}_n	1D	26.2	3.7	462	Li et al. (2020)
{[Gd₂(azdc)₃(DMA)₂].2DMA}_n	3D	22.3	4.6	734	Zhang et al. (2014)

Acknowledgements

Open access funding enabled and organized by Projekt DEAL.

Funding information

Funding for this research was provided by: National Natural Science Foundation of China (grant No. 21671124 to MLZ; grant No. 21571118 to LPL); One Hundred-Talent Program of Shanxi Province (award to UE); Shanxi University for academic research in Germany (award to WWW).

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