Three Gd-based magnetic refrigerant materials with high magnetic entropy: From di-nuclearity to hexa-nuclearity to octa-nuclearity.

Magnetocaloric effect (MCE) is one of the most promising features of molecular-based magnetic materials. We reported three Gd-based magnetic refrigerant materials, namely, Gd2(L)(NO3)(H2O)‧CH3CN‧H2O (1, H2L = (Z)-N-[(1E)-(2-hydroxy-3-methphenyl)methylidene]pyrazine-2-carbohydrazonic acid), {Gd6(L)6(CO3)2(CH3OH)2(H2O)3Cl}Cl‧4CH3CN (2), and Gd8(L)8(CO3)4(H2O)8‧2H2O (3). Complex 1 contains two GdIII ions linked by two η 2:η 1:η 1:η 1:μ 2-L2- ligands, which are seven-coordinated in a capped trigonal prism, and complex 2 possesses six GdIII ions, contributing to a triangular prism configuration. For complex 3, eight GdIII ions form a distorted cube arrangement. Moreover, the large values of magnetic entropy in the three complexes prove to be excellent candidates as cryogenic magnetic coolants.

Introduction

Ln-based complexes play a critical role in molecular-based materials not only due to the charming geometrical structures but also because of the extensive applications such as luminescence, catalysis, especially for magnetic materials including magnetocaloric effect (MCE) (Wu D et al., 2020; Shang et al., 2021; Wei et al., 2021), and single-molecule magnets (SMMs) (Liu et al., 2014; Liu et al., 2016; Zhang and Cheng, 2016; Reis, 2020). As a member of the Ln elements, the Gd ion is a perfect candidate in the synthesis of molecular-based magnetic refrigeration materials because of the large magnetothermal effects (Evangelisti et al., 2011; Chen et al., 2013; Chen et al., 2014; Wang et al., 2020a; Li et al., 2021; Lin et al., 2021; Wu T et al., 2021; Zhou et al., 2021). Some of the reported magnetic materials even possess a large cryogenic MCE, which is comparable to that of the commercial coolant {Gd3Ga5O12} (Pecharsky and Gschneidner, 1997; Zhang S. et al., 2015; Zhang S.,-W. et al., 2015).

It is worth mentioning that in the pure 4f system, improving magnetic density is the ideal method to gain MCE performance (Zhang et al., 2016; Reis, 2020). Therefore, organic ligands play an important role in the building units of the complexes. In previous studies, various organic ligands (e. g. Schiff-based ligands (Aronica et al., 2006; Boulon et al., 2013; Mannini et al., 2014; Burgess et al., 2015; Nava et al., 2015; Wang et al., 2015; Lakma et al., 2019; Li et al., 2019; Wang et al., 2020b; Wang J. et al., 2021; Wang M. et al., 2021), carboxylates (Milios et al., 2007; Dermitzaki et al., 2015; Yin et al., 2015; Botezat et al., 2017; Feltham et al., 2017; Li et al., 2019; Zheng et al., 2020; Han et al., 2021; Zhou et al., 2021), diketones (Zhu et al., 2014; Yao et al., 2018; Wang et al., 2019a; Wang et al., 2019b; Shi et al., 2021), and diamines (Neves et al., 1992; Zhang et al., 2013; Cornia et al., 2014; Oyarzabal et al., 2014; Feltham et al., 2015; Luan et al., 2015; Lu et al., 2019) etc.) have been successfully utilized in the synthesis of MCE materials. Among them, Schiff-based ligands comprise rich O and N sites, which are widely used in the synthesis of many Ln complexes because of the simple synthesis and structural diversity.

In this work, three Gd-based magnetic refrigerant materials based on Schiff-based ligands (Z)-N-[(1E)-(2-hydroxy-3-methphenyl) methylidene]pyrazine-2-carbohydrazonic acid (H2L) were synthesized, namely, Gd2(L) (NO3) (H2O)‧CH3CN‧H2O (1), {Gd6(L)6(CO3)2(CH3OH)2(H2O)3Cl}Cl‧4CH3CN (2), and Gd8(L)8(CO3)4(H2O)8‧2H2O (3). Magnetic studies indicate that all complexes exhibit antiferromagnetic interactions between the spin centers and display large magnetic entropies.

Materials and methods

Materials

All reactions and manipulations were performed in the ambient atmosphere. The Schiff-based H2L ligand was prepared by condensation with o-vanillin and hydrazine-2-carbohydrazide in methanol according to the literature (Chandrasekhar et al., 2013; Chen et al., 2016). Metal salts and other reagents were commercially available and used without further purification.

Synthesis

Synthesis of Gd2(L)2(NO3)2(H2O)2‧CH3CN‧H2O (1): a mixture of H2L (0.1 mmol, 27.2 mg) and Gd(NO3)3·6H2O (0.1 mmol, 45.7 mg) was dissolved in CH3CN (5 ml) and CH3OH (2.5 ml). After stirring for 5 min, pyridine (0.04 ml) was added and stirred for another 10 min. The solution was filtered and left to slowly evaporate. Well-shaped orange crystals were obtained after 1 week. Yield: 20 mg, 36% based on Gd. Elemental analysis (EA) calc. (%) for Gd2C30H30N12O16, C: 31.91, H: 2.68, N: 14.89; found (%), C: 32.03, H: 2.61, N: 14.93.

{Gd6(L)6(CO3)2(CH3OH)2(H2O)3Cl}Cl‧4CH3CN (2): a mixture of H2L (0.2 mmol, 54.4 mg) and GdCl3.6H2O (0.2 mmol, 74.3 mg) was dissolved in CH3CN (10 ml) and CH3OH (5 ml). After stirring for 5 min, NaHCO3 (0.2 mmol, 33.6 mg) was added and stirred for another 3 h. Well-shaped orange crystals were obtained after 1 week. Yield: 32 mg, 32% based on Gd. Elemental analysis (EA) calc. (%) for Gd6C90H92N28O29Cl2, C: 35.51, H: 3.05, N: 12.88; found (%), C: 35.72, H: 2.99, N: 12.92.

Gd8(L)8(CO3)4(H2O)8‧2H2O (3): a mixture of H2L (0.2 mmol, 13.6 mg) and GdCl3.6H2O (0.2 mmol, 18.6 mg) was dissolved in CH3CN (5 ml) and CH3OH (2.5 ml). After stirring for 5 min, NaCO3 (0.2 mmol, 10.6 mg) was added and stirred for another 2 h. Well-shaped orange crystals were obtained after 1 week. Yield: 28 mg, 29% based on Gd. Elemental analysis (EA) calc. (%) for Gd8C108H100N32O46, C: 33.78, H: 2.62, N: 11.67; found (%), C: 33.83, H: 2.51, N: 11.84.

Physical measurements

The C, H, and N elemental analyses were performed using an Elementar Vario-EL CHNS elemental analyzer. The Fourier transform-infrared (FT-IR) spectra were carried out from KBr pellets in the range 4,000–400 cm−1 using an EQUINOX 55 spectrometer. Powder X-ray diffraction (PXRD) patterns were performed using the Bruker D8 Advance diffractometer (Cu–Kα, λ = 1.54056 Å). Magnetic susceptibility measurements were measured with a Quantum Design MPMS-XL7 SQUID. Polycrystalline samples were embedded in vaseline to prevent torquing. Data were corrected for the diamagnetic contribution calculated from Pascal constants.

Crystallographic study

Suitable single crystals for 1–3 were selected for single-crystal X-ray diffraction analysis. Data were collected using a Rigaku Oxford diffractometer with a Mo–Kα radiation (λ = 0.71073 Å) at 120 K. The structures were solved by direct methods and refined by least-squares on F 2 utilizing the SHELXTL program suite and Olex2 (Dolomanov et al., 2009; Sheldrick, 2015a,b). The hydrogen atoms were set in calculated positions and refined as riding atoms with common fixed isotropic thermal parameters. EA was used to detect the content of C, H, and N atoms. Detailed information about the crystal data and structure refinements is summarized in Table 1. Selected bond lengths and angles of complexes 1–3 are listed in Supplementary Table S1–S3.

TABLE 1

Crystallographic data and structural refinement parameters for complexes 1–3.

Complex	1	2	3
Formula	Gd2(L)2(NO3)2(H2O)2‧CH3CN‧H2O	{Gd6(L)6(CO3)2(CH3OH)2(H2O)3Cl}Cl‧4CH3CN	Gd8(L)8(CO3)4(H2O)8‧2H2O
M r [g·mol−1]	1127.17	3044.31	3840.19
T [K]	120 (2)	120 (2)	120 (2)
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P-1	P-1	P-1
a [Å]	8.8137 (8)	13.5408 (9)	17.84766 (16)
b [Å]	9.4123 (9)	18.8679 (15)	18.2321 (2)
c [Å]	13.3026 (12)	23.2844 (16)	28.0616 (3)
α [°]	95.912 (3)	90.424 (3)	73.0978 (10)
β [°]	109.101 (3)	92.888 (2)	77.7530 (8)
γ [°]	107.900 (3)	106.296 (2)	61.2890 (11)
V [Å3]	966.55 (16)	5701.2 (7)	7634.03 (16)
Z	1	2	2
ρ calcd [g·cm−3]	1.936	1.773	1.671
μ [mm−1]	3.487	3.569	3.506
F (000)	550.7	2956.0	3704.0
Refl.collected/unique	8376/3895	70871/25093	124216/39110
GOF on F 2	1.0385	1.030	1.033
R 1/wR 2 [I > 2σ(I), squeeze] a	0.0379/0.0963	0.0393/0.0943	0.0378/0.0887
R 1/wR 2 (all data, squeeze)	0.0424/0.1001	0.0454/0.0995	0.0512/0.0946
CCDC No.	2174539	2174540	2174541

R 1 = ∑||F o| − |F c||/∑|F o|. wR 2 = [∑w (F o 2–F c 2)2/∑w (F o 2)2]1/2.

Results and discussion

Description of the structures of 1–3

Complexes 1–3 are synthesized by the evolution method with H2L and gadolinium salt in the solution of CH3CN/CH3OH (V 1:V 2 = 2:1) under the existence of alkali. The alkali is added to be conducive to protonate the ligand H2L, which is beneficial to incorporate GdIII ions. The H2L ligand in all complexes is completely dehydrogenated adopting the μ 2:η 2:η :η :η -mode (Scheme 1A), which is similar to the reported literature (Chandrasekhar et al., 2013; Chen et al., 2016; Zhang et al., 2017; Zhang  et al., 2016; Jiang et al., 2016).

SCHEME 1

Coordination modes of L2- ligand (A) and CO3 2- (B,C).

Complex 1 is crystalized in the triclinic P-1 space group. As shown in Figure 1, the crystallography independent unit of 1 contains half of the molecule, including one GdIII ion, one L2 - ligand, one NO3 − anion, and half of CH3CN and H2O molecules. The metallic GdIII ions (Gd1 and Gd1A) are surrounded by two L2 - ligands using the aforementioned mode, two NO3 − anions and two H2O molecules located above and below the plane, respectively. The average bond lengths of Gd-O and Gd-N are 2.379 (5) Å and 2.460 (5) Å (Supplementary Table S1), respectively, which are in accordance with those of the reported Gd-based complexes (Chen et al., 1995; Zhao et al., 2017; Mayans and Escuer, 2021; Ren et al., 2021). In complex 1, the Gd ion is seven-coordinated to form a capped trigonal prism, which is confirmed by CShM calculations (Alvarez et al., 2005; Casanova et al., 2005) (Supplementary Figure S1, Supplementary Table S4).

FIGURE 1

Crystal structure of complex 1. The hydrogen atoms are omitted for clarity. Color codes: Gd, purple; O, pink; N, blue; and C, grey. Symmetric code: A, 1-x, 1-year, and 1-z.

Complex 2 crystalizes in the same space group as complex 1, and the asymmetric unit comprises the whole molecule with six crystallographically independent GdIII ions (Figure 2A). The six Gd ions are held together to form a {Gd6} triangular prism metallic skeleton (Figure 2B). Therein, three Gd ions in the plane (Gd1, Gd2, and Gd3 or Gd4, Gd5, and Gd6) contribute a triangular configuration, which are bridged by one CO3 2- anion in μ 3-η 2:η 2:η 2-mode (Scheme 1B). The two triangular metallic skeletons are then linked together by six μ 2-O bridges from ligands.

FIGURE 2

Crystal structure (A) and metallic core (B) of complex 2. The hydrogen atoms are omitted for clarity. Color codes: Gd, purple; O, pink; N, blue; and C, gray.

All Gd ions are eight coordinated, showing two kinds of coordination geometry confirmed by CShM calculations (Alvarez et al., 2005; Casanova et al., 2005) (Supplementary Table S5). The Gd1, Gd2, Gd3, Gd5, and Gd6 ions are in {O6N2} environment with six O atoms and two N atoms from two chelated L2- ligands, one CO3 2- anion and one CH3OH/H2O molecule, which display a biaugmented trigonal prism configuration (Supplementary Figure S2). The average Gd-O and Gd-N distances are 2.352 (4) Å and 2.475 (4) Å, respectively (Supplementary Table S2), which are consistent with those reported Gd-based complexes (Chen et al., 1995; Zhao et al., 2017; Mayans and Escuer, 2021; Ren et al., 2021). However, Gd4 has triangular dodecahedron coordination geometry and is located in an {O5N2Cl} environment with five O and two N atoms from two chelated L2- ligands and one Cl− anion. The bond length of Gd4-Cl1 is 2.746 (1) Å, which is longer than that of Gd-O and Gd-N.

For complex 3, the synthetic method is the same as complex 2; except NaHCO3 was used in place of Na2CO3. Surprisingly, complex 3 possesses an octa-nuclearity structure, which crystalizes in the triclinic P-1 space group. The asymmetric unit consists of a completed molecule, and there are eight crystallographically independent Gd atoms in the molecular structure (Figure 3A). As shown in Figure 3B, the eight GdIII ions contribute to a cubic trapezoid metallic core. Gd1, Gd4, Gd5, and Gd8 ions lie in the four vertices of the plane below the cubic trapezoid, while Gd2, Gd3, Gd6, and Gd7 ions situate in the upper plane. The metallic core is held together by four CO3 2- anions in μ 3-η 2:η 2:η -mode (Scheme 1C). The periphery of the metal core is ligated by eight L2- ligands, eight H2O molecules, and two lattice H2O molecules.

FIGURE 3

Crystal structure (A) and metallic core (B) of 3. The hydrogen atoms are omitted for clarity. Color codes: Gd, purple; O, pink; N, blue; and C, gray.

There are two coordination numbers of GdIII ions in complex 3 (Supplementary Figure S3). Gd1, Gd3, Gd5, and Gd7 are eight-coordinated ions in {O6N2} donor set from two L2- ligands, two CO3 2- anions, and one H2O molecule, while Gd2, Gd4, Gd6, and Gd8 ions are nine-coordinated in the {O7N2} donor set. The difference between the two kinds of Gd ions is the diverse coordination modes of the CO3 2- anion. There is only one coordination bond of O atom in CO3 2- anion, which is adopted in Gd1, Gd3, Gd5, and Gd7 ions. For Gd2, Gd4, Gd6, and Gd8 ions, the bonding mode of the CO3 2- anion is adopted in the bidentate mode. The eight metal ions exhibit three coordination geometries: biaugmented trigonal prism (Gd1), triangular dodecahedron (Gd3, Gd5, and Gd7), and muffin (Gd2, Gd4, Gd6, and Gd8) (Supplementary Tables S5,6). The average Gd-O distance is 2.361 (4) Å, which is shorter than that of Gd-N (2.564 (4) Å) lengths. The O/N-Gd-O/N angles are in the range of 60.99°–154.86°, which are in the normal range (Chen et al., 1995; Zhao et al., 2017; Mayans and Escuer, 2021; Ren et al., 2021).

It is worth mentioning that the use of different alkalis can affect the number of formed metal nuclearity. For the organic weak alkali triethylamine, which is used in complex 1, it only facilitates protonation of the ligand H2L but is not involved in the final formation of complex 1. However, for complexes 2 and 3, the inorganic alkalis not only deprotonate the ligand but also participate in the construction of the molecules. Compared to NaHCO3 in complex 2, the alkalinity of Na2CO3 is relatively strong. Moreover, mainly due to the degree of hydrolysis of carbonates being higher, there are more carbonate triangle skeletons in complex 3, making it easier to coordinate with Gd ions, thus forming an octa-nuclearity complex.

IR spectra and PXRD studies

The FT-IR spectra of complexes 1–3 were acquired (v = 4,000–500 cm−1), which are shown in Supplementary Figure S4. Powder X-ray diffraction (PXRD) measurements for complexes 1–3 were performed for the crystalline crystals (Supplementary Figure S5), and the experimental patterns are in good agreement with the simulated ones from the crystallographic data. The minor inconsistencies in the intensity and shape of the peaks indicate the phase purity of complexes 1–3.

Magnetic studies

The direct current magnetic susceptibilities of complexes 1–3 were studied for polycrystalline samples in the temperature range of 2–300 K at an external magnetic field of 1000 Oe (Figure 4A). At room temperature, the χ M T values of complexes 1–3 are 15.77, 47.16, and 62.81 cm3 K mol−1, respectively, which is in good agreement with the expected spin-only values (GdIII ion: 7.875 cm3 K mol−1, g = 2). Upon cooling, the χ M T values in all cases stay essentially unchanged until approximately 25 K and then followed by an obvious decrease to the minimum values of 13.29, 38.46, and 58.30 cm3 K mol−1, indicating antiferromagnetic interactions (Kahn et al., 2000). Fitting the curve of χ M −1 vs. T with the Curie–Weiss Law (Figure 4B) gives the resulting C and θ values, which are listed in Supplementary Table S7. The negative θ values imply the presence of weak antiferromagnetic interaction within complexes 1–3.

FIGURE 4

χ M T products measured under a 1000 Oe DC applied field (A) and the plots of 1/χ M vs. T (B) for complexes 1–3. The solid lines represent the best fitting.

The field dependence of the magnetization plots for complexes 1–3 was performed in the field range of 1–7 T at 2–8 K (Supplementary Figure S6). Magnetizations in all complexes are increased gradually at the entire field region, reaching saturation values of 13.81, 41.75, and 55.83 Nμ B at 7 T and 2 K, respectively, close to the theoretical value (1: 14 Nμ B; 2: 42 Nμ B; 3: 56 Nμ B). The reduced magnetization plots (M vs. HT −1) in all complexes are superposable due to the isotropic system (Supplementary Figure S7).

Due to the complicated systems in complexes 2 and 3, only complex 1 is attempted to analyze the magnetic interactions by using a simplified spin Hamiltonian with the PHI program (Eq. 1):

The best-fit parameters are J = -0.022 (2) cm−1 and g = 1.98 (Figure 4A; Supplementary Figure S8). The negative J value confirms the antiferromagnetic interactions between the GdIII ions, which is in accordance with the trend of the χ M T product with cooling and the result of the Curie–Weiss Law.

The isothermal magnetization for complexes 1–3 was measured from 2 to 8 K in an applied DC field up to 7 T to calculate the magnetic entropy (-∆S m) according to the Maxwell equation (Pecharsky and Gschneidner, 1999) (Eq. 2). It can be seen that the curves of -∆S m of complexes 1–3 gradually increase with decreasing temperature and increasing of magnetic field without saturation, the maximum -∆S m values are 25.05 J kg−1 K−1, 27.21 J kg−1 K−1, and 30.79 J kg−1 K−1 at 2 K, ∆H = 7 T, respectively (Figure 5). These values are smaller than the theoretical values of 34.57 J kg−1 K−1 for 1, 34.07 J kg−1 K−1 for 2, and 36.01 J kg−1 K−1 for 3, which are calculated using Eq 3, (n = 2, 6, and 8 for 1, 2, and 3, respectively; S = 7/2 and the R value is 8.314 J mol−1 K−1), owing to the existence of antiferromagnetic coupling. The maximum -∆S m of 1 in di-nuclearity complex is among the highest observed to date for 4f clusters appeared at low temperature (Table 2). Although complexes 2 and 3 do not possess the highest -∆S m values, they are still comparable in the same nuclear complexes.

FIGURE 5

-∆S m at various fields and temperatures, calculated from the magnetization data for 1(A), 2(B), and 3(C).

TABLE 2

Summary of -∆S m in different ∆H at a given temperature for reported di-nuclearity, hexa-nuclearity, octa-nuclearity, and other multinuclear Gd-based complexes.

Complex	-∆S m [J kg−1 K−1]	∆H [T], T [K]	Ref
{Gd2(OAc)6(H2O)4}·4H2O	40.6	7, 1.8	Evangelisti et al. (2011)
Gd2(OAc)2(Ph2acac)4 (MeOH)2	23.7	7, 2.4	Guo et al. (2012)
Gd2 (hfac)4 (fpmoq)2	17.1	8, 3.0	Wang et al. (2015)
Gd2 (hfac)4 (btoq)2	16.9	8, 2.0	Shen et al. (2015)
Gd2 (L1) (dbm)5	17.69	8, 2.0	Wang et al. (2021a)
Gd2 (iba)6 (bipy)2	29.3	7, 2.0	Zhou et al. (2021)
Gd2 (nic)6(H2O)4	27.4	7, 2.0	Zhou et al. (2021)
Gd2(L)2(CH3OH)2	24.75	7, 2.0	Shi et al. (2020)
{Gd2(L)2 (dbm)2(H2O)2}·nCH3OH	23.2	7, 2.0	Shi et al. (2021)
Gd2 (dnba)6 (phen)2	16.8	7, 2.0	Zheng et al. (2020)
Gd2(Hnsa)2 (nsa)2 (phen)2(H2O)2	22.2	7, 2.0	Zheng et al. (2020)
1	25.05	7, 2.0	This work
{Gd6 (bobdz)2(HCO2)4 (μ3-OH)4 (DMF)6(H2O)2}Cl2·4H2O	33.5	7,3.0	Adhikary et al. (2014)
Gd6(L)2 (acac)6(OH)4(NO3)2(CH3OH)2	35.3	7, 2.0	Wang et al. (2020b)
{H2 [Gd6(OH)8(H2O)6 (p-BDC-F4)6]}·3 (2,2′-bpy)·6H2O	28.27	7, 2.0	Wei et al. (2019)
{H2 [Gd6(OH)8(H2O)6 (m-BDC-F4)6]}·3 (4,4′-bpy)·6H2O	29.20	7, 2.0	Wei et al. (2019)
2	27.21	7, 2.0	This work
{Gd8(IN)14 (μ 3-OH)8 (μ 2-OH)2(H2O)8}·11H2O	31.77	7, 2.0	Shi et al. (2020)
{Gd8 (μ 3-O)4(L)8(CH3COO)4(CO3)2}·15H2O	32.49	7, 2.0	Li et al. (2019)
3	30.79	7, 2.0	This work
{Gd3 (dbm)5(HL)2}·4CH3OH·3CH2Cl2	20.60	7, 2.0	Wang et al. (2020c)
{Gd3(HL) (H2L) (NO3)4}·C2H5OH	30.22	7, 2.0	Wang et al. (2019c)
{Gd4(H3L)2(OAc)3(F6acac)3}·4MeOH·2.5H2O	21.88	5, 2.0	Liu and Hao, (2022)
{Gd4 (acac)4 (μ3-OH)2L6}·2CH3CN	14.57	7, 3.0	Hou et al. (2020)
{Gd4(HL)4(CH3O)4}·3CH3OH	30.42	7, 2.0	Li et al. (2020)
{Gd4(L)4 (m2-CH3O)4}·CH3OH	28.50	7, 2.0	Xu et al. (2021)
{Gd4 (acac)4(L)6 (μ3-OH)2}·CH3CN	24.46	7, 2.0	Wang et al. (2020d)

Conclusion

In conclusion, three clusters 1-{Gd2}, 2-{Gd6}, and 3-{Gd8} based on Schiff ligand H2L were synthesized. Complex 1 contains two GdIII ions, and magnetic measurement indicates antiferromagnetic interactions between the metal core, which is also confirmed by PHI fitting. Complexes 2 and 3 are hexa-nuclearity with a biaugmented trigonal prism configuration and octa-nuclearity with a cubic trapezoid structure. Magnetic investigations indicate the antiferromagnetic interactions between GdIII ions are observed in complexes 2 and 3. Magnetocaloric studies for complexes 1–3 show that the magnetic entropies of complexes 1–3 are smaller than the theoretical values, which is mainly caused by antiferromagnetic coupling. Furthermore, complex 1 exhibits a large magnetic entropy of 25.05 J kg−1 K−1 at 2.0 K in di-nuclearity magnetic refrigerant materials, while complexes 2 and 3 belong to the normal range in hexa-nuclearity and octa-nuclearity complexes, respectively, demonstrating that they are promising molecular magnetic coolants for low-temperature cooling applications.