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

A novel coordination polymer (CP) has been successfully constructed under hydrothermal conditions via the combination of Gd3+ ions and the azobenzene-2,2′,3,3′-tetracarboxylic acid linker. The underlying structural principles 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 it displays intrachain antiferromagnetic Gd⋯Gd coupling and a cryogenic magnetocaloric effect.


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 Ln 3+ 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 ISSN 2053ISSN -2296 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 (ÀÁS m ) and adiabatic temperature change (ÁT ad ) under certain conditions (Franco et al., 2018). A large ÁS m under a relatively low magnetic field is mandatory for an attractive cryogenic magnetorefrigerant . The ÀÁS m value of the well-known commercial low-temperature magnetic refrigeration material GGG (Gd 3 Ga 5 O 12 ) is 24 J kg À1 K À1 (ÁH = 30 kG) (Daudin et al., 1982).
The Gd 3+ 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 Gd 3+ centres is relatively weak, which allows the system to achieve a large MCE (Zhang et al., 2021). Therefore, the Gd 3+ ion represents an ideal choice for the construction of molecular-based low-temperature magnetic refrigeration materials . 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 Gd 3+ 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) Gd 3+ complex, [Gd-(Habtc)(H 2 O) 2 ] n , (I), for which we selected azobenzene-2,2 0 ,3,3 0 -tetracarboxylic acid (H 4 abtc) as the ligand. The four carboxylic acid groups of this rigid H 4 abtc 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 Gd 3+ ions and thus ensure magnetic exchange and transfer between adjacent Gd 3+ 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).

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 À3 . [Symmetry code: (i) Àx, Ày + 1, Àz + 1.] 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 K 1 radiation ( = 1.54056 Å ). The thermogravimetric analysis was performed on a Dupont thermal analyzer between room temperature and 1045 K under an N 2 flow with a heating rate of 10 K min À1 . 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.

Synthesis and crystallization
The reaction route to (I) is shown in Scheme 1. Gd(NO 3 ) 3 Á6H 2 O (67.7 mg, 0.15 mmol) and H 4 abtc (35.8 mg, 0.1 mmol) were dissolved in a mixture of N,N-dimethylformamide (DMF, 2 ml), acetonitrile (CH 3 CN, 2 ml) and distilled water (H 2 O, 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 C 16 H 11 GdN 2 O 10 : C 35.01, H 2.01, N 5.10; found: C 35.05, H 2.02, N 5.13.

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 OÁ Á ÁO 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 highresolution or neutron data, we can neither disprove nor support a split-atom alternative and an asymmetric hydrogen bond. Ş erb et al. (2011) have compiled structures featuring very short OÁ Á ÁO 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.

IR spectroscopy
The IR spectra of the ligand and (I) in the range 4000-400 cm À1 are presented in Fig. 2. The broad band at 3405 cm À1 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 À1 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 H 4 abtc (1426 and 1572 cm À1 ), suggesting that the carboxylate groups in the complex are coordinated to the Gd 3+ ions . The absorption observed at 1468 cm À1 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, , cm À1 , 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).

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 threedimensional (3D) solid from the reaction between Gd(NO 3 ) 3 Á6H 2 O and H 4 abtc. The asymmetric unit of (I) contains a Gd 3+ ion situated on a twofold axis (Wyckoff position 4e), one half of the Habtc 3À ligand and a coordinated H 2 O molecule. As shown in Fig. 3  . Two Habtc 3À ligands share the proton H4 which is located on a centre of inversion [see Refinement (x2.2) 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 Habtc 3À ligand can be perceived a four-connected node towards Gd 3+ ions and, vice versa, each Gd 3+ ion is coordinated by four Habtc 3À ligands. The overall network can thus be described as a 4-connected net with the point symbol (4 4 Á6 2 ).

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 N 2 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.

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 m T value for (I) amounts to 8.00 cm 3 mol À1 K at 300 K, close to the expected value of 7.88 cm 3 mol À1 K calculated for an isolated Gd 3+ ion (S = 7/2, g = 2) (Xi et al., 2020). As the temperature is decreased, the m T value of (I) decreases slowly to 7.93 cm 3 mol À1 K around 10 K, and then increases gradually to 8.14 cm 3 mol À1 K at 2 K. The data in the whole temperature range 2-300 K fit well the Curie-Weiss law with C = 8.06 cm 3 K mol À1 and = À0.08 K. The negative 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 Gd 3+ ions (Farger et al., 2018). The best least-squares fit parameters are g = 2.01 and J = À0.02 cm À1 , with an agreement factor R = 6.27 Â 10 À5 in the range 35-300 K. The value for J further proves the existence of weak antiferromagnetic interactions between adjacent Gd 3+ ions in (I).   Symmetry codes: (ii) Àx; Ày þ 1; Àz þ 1; (iii) x; y À 1; z; (iv) x; Ày þ 1; z À 1 2 .

Figure 3
Expanded asymmetric unit and coordination environment of the Gd 3+ ion in (I). Displacement ellipsoids are drawn at 30% probability and H atoms are represented as spheres of arbitrary radius. [Symmetry codes: (i) Àx, y, Àz + 1 2 ; (ii) x, y + 1, z; (iii) Àx + 1 2 . Ày + 5 2 , Àz + 1.] 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 N at 7 T and 2 K, which is close to the expected value of SÂg = 7/2Â2 = 7 N for an isolated Gd 3+ ion (S = 7/2, g = 2). To evaluate the magne-    tocaloric effect (MCE), the magnetic entropy change (ÀÁS 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 ÁS m (T) = [M(T,H)/
T] H dH. The resulting maximum value of ÀÁS m amounts to 27.26 J kg À1 K À1 for ÁH = 7 T at 3.0 K, which is smaller than the theoretical value of 31.52 J kg À1 K À1 , as calculated from the equation ÀÁS m = N Gd R ln(2s + 1)/M W , with S = 7/2. In this equation, M W is the formula mass of 548.52 g mol À1 and N Gd is the number of Gd 3+ ions present per mole of (I). The difference in ÀÁS m between the theoretical and experimental values may be attributed to the existence of antiferromagnetic interactions between Gd 3+ ions. The experimental ÀÁS m value is also smaller than several previously prepared 1D linear-chain Gd 3+ complexes (Table 4), which can be ascribed to the large M W /N Gd ratio arising from the large H 4 abtc ligand and the antiferromagnetic interactions between the neighbouring Gd 3+ ions in (I).

Conclusion
In summary, the novel coordination polymer (I) has been successfully constructed under hydrothermal conditions via the combination of Gd 3+ ions and the H 4 abtc linker. The underlying structural principles in (I) comprise a 1D [Gd 2 (COO) 4 ] n chain and the linking of neighbouring chains via the organic ligand into a 2D structure with point symbol (4 4 Á6 2 ). 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 GdÁ Á ÁGd coupling and a cryogenic MCE with the maximum ÀÁS m of 27.26 J kg À1 K À1 for ÁH = 7 T at 3.0 K. This small ÀÁS m value can be ascribed to the high M W /N Gd ratio arising from the large H 4 abtc ligand and the antiferromagnetic interactions between neighbouring Gd 3+ ions in (I). The selection of low molecular-weight ligands that transfer weak coupling may be a promising approach for obtaining Gd 3+ complexes as molecule-based magnetic refrigerants. Further studies on Gd 3+ complexes for magnetic refrigeration are underway in our laboratory.

Acknowledgements
Open access funding enabled and organized by Projekt DEAL.  Table 4 Comparison of ÀÁS m for (I) and several previously reported 1D Gd 3+ complexes.

Poly[diaqua[µ 4 -1′-carboxy-3,3′-(diazene-1,2-diyl)dibenzene-1,2,2′-tricarboxylato]gadolinium(III)
Crystal data [Gd(C 16   Special details 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 F 2 with full-matrix least-squares techniques using the program SHELXL-2014 program (Sheldrick, 2015