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Article

Synthesis, Crystal Structures and Magnetic Properties of Lanthanide Complexes with Rhodamine Benzoyl Hydrazone Ligands †

by
Lin Miao
1,
Dong-Mei Zhu
2,
Cai-Ming Liu
3,
Yi-Quan Zhang
2,* and
Hui-Zhong Kou
1,*
1
Engineering Research Center of Advanced Rare Earth Materials (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China
2
Ministry of Education Key Laboratory of NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, China
3
Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
*
Authors to whom correspondence should be addressed.
†
Dedicated to Prof. Dai-Zheng Liao on the occasion of his 85th birthday.
Magnetochemistry 2025, 11(8), 68; https://doi.org/10.3390/magnetochemistry11080068
Submission received: 6 July 2025 / Revised: 31 July 2025 / Accepted: 5 August 2025 / Published: 7 August 2025
(This article belongs to the Special Issue Molecular Magnetism: A Themed Issue in Honor of Professor Dai-Zheng Liao on the Occasion of His 85th Birthday)
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Abstract

Given the outstanding magnetic characteristics of lanthanide ions, the development of mononuclear or multinuclear lanthanide complexes becomes imperative. Previous research showed that a series of mononuclear Dy(III) complexes of rhodamine benzoyl hydrazone Schiff base ligands exhibit remarkable single-molecule magnetic properties and fluorescence. In this study, we used analogous ligands to synthesize lanthanide complexes [Dy(HL1-o)(NO3)2(CH3OH)2]NO3·CH3OH (complex 1·MeOH) and tetranuclear complexes [Ln4(L1-c)2(L2)2(μ3-OH)2(NO3)2(CH3OH)4](NO3)2·2CH3CN·5CH3OH·2H2O (Ln = Dy, complex 2; Ln = Gd, complex 3). Magnetic susceptibility measurements show that 1·2H2O is a single-molecule magnet, 2 shows slow magnetic relaxation and 3 is a magnetic cooling material with the magnetic entropy change of 9.81 J kg−1 K−1 at 2 K and 5 T. The theoretical calculations on 1·MeOH indicate that it shows good magnetic anisotropy with the calculated energy barrier of 194.6 cm−1.

1. Introduction

Single-molecule magnets (SMMs) are a type of molecular complex that exhibits slow magnetic relaxation at the single molecular level [1,2,3,4,5]. They have unique electronic and magnetic structures, making them promising candidates for next-generation data storage devices [6,7]. Due to the potential for high-density data storage, the demand for SMM has increased significantly. However, the main challenge of developing SMMs for practical applications lies in their low effective energy barriers (Ueff) and blocking temperatures (TB), which limits their stability and applicability. In recent years, there have been many efforts to improve the performance of SMMs, including using lanthanide ions with suitable symmetry (D4h, D5h, D6h for Dy(III)) and ligand fields to enhance the magnetic anisotropy required for SMMs [8,9]. For example, the lanthanide complexes of the cyclopentadiene series have excellent SMM properties, constantly breaking the record of Ueff [10,11,12]. Currently, the two highest Ueff are held by the binuclear Dysprosium cyclopentadiene complexes (CpiPr5)2Dy2I3 and a mononuclear complex [Dy{N(SiiPr3)[Si(iPr)2C(CH3)=CHCH3]}{N(SiiPr3)(SiiPr2Et)}][Al{OC(CF3)3}4] with the effective energy barrier of 1631 cm−1 and 1843 cm−1, respectively [13,14].
The research on bifunctional SMMs is currently a hot topic, including chiral SMMs and fluorescent SMMs [15,16,17,18,19,20,21,22]. Combining fluorescence enhances our understanding of SMM properties and broadens their application prospects. For instance, fluorescence-labeled SMMs allow for tracking of their locations and behaviors in a solution or solid, while the f–f emission peak positions reflect the energy level splitting of the lanthanide ion, from which the energy barrier value can be obtained [19]. The fluorescence of lanthanide SMMs can take advantage of the intrinsic fluorescence of 4f ions, and can also be induced by incorporating fluorescent ligands, such as spiropyran, rhodamine, tetraethylene and pyrene chromophores [17]. The studies on fluorescent lanthanide SMMs reveal that the choice of ligands can significantly influence the magnetic and optical properties of the complexes.
Our group has been interested in complexes of rhodamine ligands. We have found that rhodamine Fe(II) complexes [23,24] show the coupling effect of spin crossover and fluorescence, and the rhodamine dysprosium SMMs show improved magnetic properties by incorporating the phenolic hydroxyl group [25,26,27,28,29]. All the above complexes are mononuclear species, as the used tridentate chelate ligands tend to coordinate with the same central metal ion. Here we extend our research to polynuclear lanthanide complexes. We choose the ligand HL1 (Scheme 1a) with a methoxy group at the ortho position of the phenolic hydroxyl group, conducive to the formation of multinuclear structures [30]. We further utilize the compartmental hexadentate ligand H2L2 (Scheme 1b) to coordinate with different lanthanide metal ions. Herein, we report the synthesis, crystal structures and magnetic properties of three lanthanide complexes, i.e., mononuclear [Dy(HL1-o)(NO3)2(CH3OH)2]NO3·CH3OH (complex 1·MeOH) and tetranuclear [Ln4(L1-c)2(L2)2(μ3-OH)2(NO3)2(CH3OH)4](NO3)2·2CH3CN·5CH3OH·2H2O (Ln = Dy, complex 2; Ln = Gd, complex 3).

2. Materials and Methods

2.1. Synthesis

All reagents (Sigma Aldrich, St. Louis, MO, USA) used in this study were commercially available and used without further purification. The ligands HL1 and H2L2 were synthesized by method from the literature [28,29]. All syntheses were performed under ambient conditions at room temperature.

2.1.1. Synthesis of Complexes 1·MeOH and 1·2H2O

Dy(NO3)3·5H2O (0.2 mmol, 112.5 mg) and HL1 (0.2 mmol, 86.5 mg) were stirred in methanol (10 mL) to obtain a red solution. Well-shaped red crystals of 1·MeOH suitable for single-crystal structural determination can be obtained by evaporation of the above solution at room temperature for a week. Anal. Calcd for 1·MeOH (C37H46N7O16Dy): H, 4.60; C, 44.12; N, 9.73. Found: H, 4.55; C, 42.75; N, 9.79. The measured data are consistent with that for [Dy(HL1-o)(NO3)2(CH3OH)2]NO3·2H2O (C36H46N7O17Dy, 1·2H2O): H, 4.58; C, 42.76; N, 9.70.

2.1.2. Synthesis of Complex 2

Mixing Dy(NO3)3·5H2O (0.5 mmol, 216 mg) and HL1 (0.2 mmol, 112.5 mg) in methanol (5 mL) and acetonitrile (5 mL) gave a red solution, to which H2L2 (0.2 mmol, 60 mg) in CH2Cl2 (2 mL) and Et3N (100 μL, 2.16 mmol) were added successively. The solution turned yellow and became turbid. After filtration, the filtrate was put into a glass tube and covered with ether for slow diffusion. After a week, yellow crystals can be obtained for X-ray single-crystal diffraction analysis. Anal. Calcd for 2 (C113H142N18O41Dy4): H, 4.680; C, 44.376; N, 8.244. Found: H, 4.343; C, 43.577; N, 7.926.

2.1.3. Synthesis of Complex 3

The synthetic method is similar to that of complex 2, except that Dy(NO3)3·5H2O is replaced with Gd(NO3)3·6H2O (0.5 mmol, 225 mg). High-quality yellow single crystals were obtained. Anal. Calcd for 3 (C113H142N18O41Gd4): H, 4.712; C, 44.683; N, 8.301. Found: H, 4.511; C, 43.001; N, 7.699.

2.2. Physical Measurements

The IR spectra with a KBr tablet were obtained on WQF-510A FTIR equipment (Beifen-Ruili, Beijing, China) with a sweeping interval of 2 cm−1. The elemental analyses were performed on an Elementar Vario Cario Erballo analyzer (Elementar, Langenselbold, Germany). The powder X-ray diffraction (PXRD) measurements were recorded on a Rigaku Miniflex 600 X-ray diffractometer (Rigaku Oxford Diffraction, Tokyo, Japan) at room temperature with a sweeping speed of 10°/min. The UV-vis spectra were measured with a U-3900 spectrophotometer (Hitachi, Tokyo, Japan) between 280 and 600 nm. The fluorescence spectra were recorded on a Lengguang F98 fluorescence spectrophotometer (Lengguang, Shanghai, China). Single-crystal X-ray data were collected on a Rigaku SuperNova, Dual, Cu at zero, AtlasS2 (Rigaku Oxford Diffraction, Tokyo, Japan). The structure was solved by the program SHELXT-2013 and refined by a full matrix least-squares method based on F2 using SHELXL-2014/7 method. Hydrogen atoms were added geometrically and refined using a riding model. Temperature- and field-dependent magnetic susceptibility measurements were carried out on a Quantum Design MPMS-XL5 SQUID magnetometer (Quantum Design, San Diego, CA, USA). The experimental magnetic susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal’s tables).

3. Results and Discussion

According to the previous reports, the rhodamine-based ligand shows interesting ring-opened and ring-closed properties under different acid-base conditions (Scheme 2). Upon addition of Ln(NO3)3, the reactant solution turned red, indicating formation of ring-opened HL1-o. Red single crystals of 1·MeOH suitable for single-crystal X-ray diffraction were obtained from the resulting red solution. Yellow single crystals of 2 and 3 are obtained when HL1 and H2L2 react with Ln(NO3)3 in the presence of Et3N, showing that the ring-closed ligand (L1-c)− is present in the tetranuclear complexes.

3.1. Crystal Structures

High-quality single crystals of 1·MeOH, 2 and 3 were used for X-ray single crystal diffraction measurements. Since complexes 2 and 3 are isomorphic, the structure of complex 2 is described in detail as a representative. The crystallographic data are given in Table S1, and the selected bond distances and bond angles are listed in Tables S2 and S3. Hydrogen bonding for 1·MeOH, 2 and 3 is listed in Tables S4–S6.
The crystal data show that complex 1·MeOH crystallizes in the triclinic space group of P−1. It consists of the [Dy(HL1-o)(NO3)2(CH3OH)2]+ cation (Figure 1a) and a NO3− charge-balancing anion. Dy(III) has a typical O8N nine-coordination, with two chelating nitrates, two methanol and one ring-opened ligand HL1-o participating in the coordination. Calculated by SHAPE software (version 2.1), the local symmetry of the Dy(III) ion is close to Cs (Table S7), and the deviation parameter is 1.359. The coordination pattern is shown in the Figure S1. The bond length for Dy1–N1 is 2.508(3) Å. The shortest Dy–O bond length is 2.192(3) Å (Dy1–O1), and the longest Dy–O bond length is 2.498(3) Å for Dy1–O7. This indicates that the short Dy–Ophenoxy bond is dominant. The ligand HL1-o is in a ring-opened form (Figure S2), with only three atoms available for coordination, while the oxygen in the methoxyl group is free. It is worth noting that the hydrogen on the phenol of the ligand is removed, but the nitrogen atom (N2) of the carbonyl group is protonated. The hydrazide C9–N2–N1 bond angle is 116.71°, which is a clear proof of the presence of hydrogen on N2. Otherwise, the C–N–N bond angle is smaller than 110° due to the stronger electrostatic repulsion of two bond pairs by lone pair on N2. This results in the entire neutral ligand, HL1-o. This hydrogen (H22) attached to N2 forms a significant hydrogen bond with the free nitrate (O11), with an O11∙∙∙H22 distance of 1.970 Å. Interestingly, the nitrate (O13) also forms another hydrogen bond with the free methanol molecule (O14), with an O13∙∙∙H14A distance of 1.954 Å (Figure S3). The shortest intermolecular Dy∙∙∙Dy distance is 6.637 Å. The short intermolecular Dy∙∙∙Dy distance may lead to appreciable intermolecular magnetic interactions. Complex 1·MeOH also forms a supramolecular one-dimensional chain structure via π-π stacking (Figure S4). The interplane distance between adjacent oxyanthracene planes is 3.311 Å, and the center∙∙∙center distance is 3.692 Å.
The crystal data for complex 2 show that it crystallizes in the monoclinic space group of C2/c. It comprises a C2 rotation axis-related [Dy4(L1-c)2(L2)2(μ3-OH)2(NO3)2(CH3OH)4]2+ cation (Figure 1b), with two charge-balancing NO3− anions. The Dy4O6 core is formed by the μ3-OH− and phenoxyl oxygen bridges (Figure S5). There are two different coordination environments of Dy(III) ions within the Dy4 molecule, as shown in Figure S6. Dy1 exhibits nine-coordination, and Dy2 is an eight-coordinate. Calculated by SHAPE software, Dy1 is close to the local symmetry of C4v (Table S8) with the deviation parameter is 0.375; Dy2 is close to the local symmetry of D4d (Table S9), and the deviation parameter is 1.216. The coordination pattern is shown in the Figure S6. The distance between Dy1∙∙∙Dy2 is 3.538 Å, and between Dy1∙∙∙Dy1A is 3.904 Å. In addition, Dy1∙∙∙Dy2A is 4.327 Å and Dy2∙∙∙Dy2A is 6.757 Å. The four Dy(III) ions are not coplanar (Figure S7), forming a saddle-shaped Dy4 quadrangle with a dihedral angle of 23.0°. The coordination patterns of the ligands are shown in Figure S8. The ligand (L1-c)− is in a ring-closed form, and the four coordination atoms of (L1-c)− are all involved in coordination, bridging Dy1 and Dy2. Ligand (L2)2− is a hexadentate ligand, but only five atoms participate in the coordination and bridge three Dy ions (Figure S8). The N–N single bond in the middle of the ligand (L2)2− undergoes rotation, and the dihedral angle between the two benzene planes is 65.76°. The shortest Dy1–O bond length is 2.328(6) Å, and the longest is 2.442(6) Å with the average Dy1–O bond length of 2.395 Å. The shortest Dy2–O bond length is 2.202(8) Å, and the longest Dy2–O bond length is 2.501(7) Å. In addition, solvent molecules including two CH3CN, five CH3OH and two H2O molecules crystallize in the lattice of the crystal. The nearest intermolecular Dy∙∙∙Dy distance for complex 2 is 9.454 Å, large enough to exclude any intermolecular magnetic interaction.

3.2. Characterizations

The PXRD data for the three complexes show that the XRD patterns of complexes 2 and 3 are in good agreement with that simulated from single-crystal data; however, the data for 1·2H2O shows an inconformity with that of complex 1·MeOH, further confirming the partial desolvation and hydration of the crystals (Figure S9). The infrared spectra of complexes 1·2H2O, 2 and 3 and the ligands HL1 and H2L2 show that the peak at 1693 cm–1 for HL1 disappears (Figure S10), which may be due to the fact that the C=O bond of ligand HL1 participates in the coordination in the enol form, which weakened the vibration peak of C=O. The strong peaks at 1625 cm–1 of HL1 and 1616 cm–1 of complex 1·2H2O may be assigned to the stretching vibration of C=N bond of the ligand. The peak of complex 1·2H2O at 1368 cm–1 is the characteristic peak of nitrate ion compared with ligand HL1. For complexes 2 and 3, the spectra are very similar (Figure S10). The strong peaks at 1090 cm–1 of ligand HL1 and 1070 cm–1 of ligand H2L2 may be attributed to the methoxyl group. Upon coordination of this oxygen atom, the peak intensities undergo a notable weakening, particularly evident in the IR spectra of complexes 2 and 3. The emergence of new peaks at 1382 cm–1 and 1380 cm–1 in complexes 2 and 3, respectively, can be attributed to the asymmetric vibration of nitrate.
UV-Vis spectra of complexes 1·2H2O, 2 and 3 in ethanol (10–5 mol/L) are shown in Figure S11. Complex 1·2H2O is pink in EtOH, and complexes 2 and 3 are nearly colorless in EtOH. The absorption peaks of ligand HL1 and complex 1·2H2O at 302 nm and 306 nm should be due to the ligand-to-ligand charge transfer (LLCT) of the ligand, while the new absorption peak at 534 nm for complex 1·2H2O indicates that ligand HL1 is in the ring-opened form. The oxaanthracene group in HL1-o has a planar conjugated rigid structure, which can cause π-π* transition. Complexes 2 and 3, however, have no obvious absorption peaks in the visible region, which is consistent with the presence of ring-closed HL1-c.
The fluorescence spectra of the complexes in ethanol (10–5 mol/L) are shown in Figures S12 and S13. The fluorescence emission spectrum (λex = 354 nm) of complex 1 shows a maximum emission at 569 nm (Figure 2), indicating that the ring-opened rhodamine ligand emits green light upon excitation. This fluorescence behavior is consistent with that of complexes with ring-opened rhodamine ligand. When excited by the light of 378 nm, the emission spectra of complexes 2 and 3 are obtained. It can be found that complex 2 has almost no fluorescence, and complex 3 still has weak fluorescence at 549 nm, presumably due to the presence of trace ring-opened ligand.

3.3. Magnetic Measurements

The direct current (DC) and alternating current (AC) magnetic susceptibilities of complexes 1·2H2O, 2 and 3 were measured on a MPMS-XL5 SQUID magnetometer. Figure 3 shows the temperature dependence of χMT for the three complexes at 2–300 K under 1000-Oe field. At room temperature, the maximum χMT values are 14.00 cm3 K mol−1, 56.21 cm3 K mol−1 and 31.36 cm3 K mol−1, which are close to the theoretical values of a Dy(III) ion (14.18 cm3 K mol−1, S = 5/2, L = 5, gJ = 4/3), four Dy(III) ions (56.72 cm3 K mol−1) and four Gd(III) ions (31.52 cm3 K mol−1, S = 7/2, L = 0, g = 2.0), respectively. For complex 1·2H2O, the χMT value first decreases slowly with the decrease in temperature from 300 K to 18 K, reaching a minimum of 12.27 cm3 K mol−1 at 18 K. Afterwards, the χMT value rapidly increases to 16.29 cm3 K mol−1 at 2 K. The increase in low-temperature magnetic susceptibility may be attributed to intermolecular ferromagnetic interactions in complex 1·2H2O. The shortest distance between Dy∙∙∙Dy of 6.637 Å should be responsible for the intermolecular magnetic interaction. A similar situation occurs to a 1D Dy(III) coordination polymer, which has stronger intermolecular ferromagnetic coupling, and the shortest intermolecular Dy∙∙∙Dy distances are even greater than that of complex 1·MeOH [31]. For complex 2, χMT value decreases slowly with the decrease in temperature to 100 K, and then quickly to 12 K before χMT slightly increases to 52.40 cm3 K mol−1 at 2 K. The increase of χMT below 12 K may be related to the intramolecular Dy∙∙∙Dy ferromagnetic interaction via the μ3-OH− and phenoxyl bridges. As for complex 3, χMT decreases monotonically throughout the entire temperatures. From 300 K to 10 K, the χMT value decreases slowly, and then rapidly from 10 K to 2 K, reaching 20.74 cm3 K mol−1, which suggests that there is an intramolecular antiferromagnetic interaction between the tetranuclear Gd(III) ions in the complex. Magnetic fitting of tetranuclear gadolinium complex 3 was carried out by MagPack software (version 1.0) [32], and the dark blue solid curve was obtained. The Hamiltonian operator expression based on the molecular structure (Figure S14) is as follows:
Ĥ = −2J1(ŜGd1ŜGd2 + ŜGd1AŜGd2A) − 2J2(ŜGd1ŜGd2A + ŜGd1AŜGd2) − 2J3ŜGd1ŜGd1A
It was found that the fitting curve was in good agreement with the experimental data with the fitting parameters of J1 = J2 = −0.022 cm–1, J3 = −0.036 cm–1 and g = 2.000, which also indicated that there was weak intramolecular Gd∙∙∙Gd antiferromagnetic coupling.
The field-dependent magnetization of complexes 1·2H2O, 2 and 3 in the temperature range of 2–10 K was measured, as shown in Figure 4, Figure S15 and Figure S16. The magnetization of complex 3 increases with the increase in magnetic field intensity at 2 K, and increases rapidly at 0 T to 3 T, and gradually approaches the maximum magnetization of 28.4 Nβ at 5 T (Figure S16). When the temperature increases, the maximum magnetization value decreases. At 10 K, the magnetization of complex 3 is far from saturated at 5 T. Contrary to complexes 1·2H2O and 2 (Figure S15), complex 3 basically reached saturation at a temperature of 2 K and 5 T. The theoretical saturation value of four independent Gd(III) ions is 28 Nβ, which is consistent with the maximum experimental saturation value, indicating that the four Gd(III) ions in complex 3 have negligibly weak magnetic interaction. Magnetic fitting of complex 3 (Figure S16) shows that the experimental M-H curve at 2 K is lower than the calculated Brillouin curve of non-interacting SGd spin, which also indicates that there is weak antiferromagnetic coupling.
Some gadolinium compounds have demonstrated exceptionally outstanding magnetocaloric properties, showcasing remarkable potential in various advanced technological applications [33,34,35,36,37,38,39,40,41,42]. We further characterized the magnetocaloric effect (MCE) of complex 3. Using − ∆ S m = n R l n ( 2 S + 1 ) (R ≈ 8.314 J mol–1 K–1), the theoretical magnetic entropy of four independent Gd(III) can be calculated to be 8.32R (22.77 J kg−1 K−1). We used the Maxwell Equation (2):
∆ S m ( T ) ∆ H =   ∫ [ ∂ M ( T , H ) ∂ T ] H d H
The magnetic entropy change values under different temperatures and different external magnetic fields can be calculated (Figure 5). At 2 K and ∆H = 5 T, the value of −∆Sm is 9.81 J kg−1 K−1, which is attributed to the low Gd(III) content (20.71%) that reduces the density of active magnetic moments and thereby limits the cumulative magnetic response, resulting in an unsatisfactory MCE. Moreover, the presence of weak antiferromagnetic coupling within the system also causes the actual value of −∆Sm to be smaller than the theoretical value.
The AC magnetic susceptibility measurements were performed on 1·2H2O and 2 to study the slow magnetic relaxation. Figure S17 shows temperature dependence of out-of-phase AC susceptibilities (χ″M) for 1·2H2O under 0- and 2000-Oe DC field with the frequency of 997 Hz. Below 10 K, no obvious peak is observed in the χ″M curve under 0 Oe, suggesting a strong quantum tunneling of the magnetization (QTM) effect during the magnetization relaxation process. After applying a 2000-Oe DC field, the peak appears at 4 K, indicating that an external field can effectively suppresses the low-temperature QTM.
In order to find an exact external magnetic field, field dependence of χ″M for complex 1·2H2O was measured at 4 K and 997 Hz (Figure S18). With the increase in external magnetic field, the χ″M first increases rapidly, and then decreases slowly, and the optimal external magnetic field corresponds to the maximum value under 600 Oe.
The AC magnetic susceptibilities of complex 1·2H2O are measured under 600 Oe (Figure 6). As shown in Figure 6b, with the increase in frequency, the peak of the χ″M curve of AC magnetic susceptibility moves to the high temperature region, which is a typical feature of SMMs. As shown in Figure 6c, the Cole–Cole curves in the range of 2.0–4.2 K are approximately semicircular, which is well fitted by the generalized Debye model. The fitting parameters are listed in Table S10, and the α value is in the range of 0.13 to 0.095, indicating that the relaxation time distribution is relatively narrow and has a single relaxation. The lnτ vs. T−1 plot shows a certain curvature, indicating that there are multiple relaxation paths. The plot was well fitted with the formula, including Raman and direct processes:
τ − 1 =   A T + C T n
As can be seen from Figure 6d, the fitted curve can well match the data points with the fitting parameters of A = 39.6(3) s−1 K−1, n =8.0(1) and C = 0.007(1) s−1 K−8.0. The n value is in the normal range of 2–9 for Dy(III) SMMs. The inclusion of Orbach relaxation cannot give a satisfactory fitting result, and nor can it only include the Raman process.
Figure S19 shows the χ′M and χ′′M of AC magnetic susceptibility with temperature at different frequencies of complex 2 under 0 Oe. Within the test temperature range, no obvious peak is observed in the χ′′M curve under 0 Oe, suggesting a strong QTM effect during the relaxation process. The AC plots of complexes 1·2H2O and 2 are markedly different, which may be due to the fact that complex 1·2H2O, as a mononuclear Dy complex with one sole short Dy-Ophenoxy bond, exhibits strong magnetic anisotropy. In contrast, complex 2, being a saddle-shaped tetranuclear Dy species, shows diminished magnetic anisotropy due to the cancellation of the anisotropy of four Dy(III) centers.

3.4. Theoretical Calculations

Complete-active-space self-consistent field (CASSCF) calculations on complex 1·MeOH (Figure S20) on the basis of X-ray determined geometry have been carried out with the OpenMolcas [43] and the SINGLE_ANISO [44,45,46] programs (see the Supporting Information for details). The calculated energy levels, g (gx, gy, gz) tensors and the predominant mJ values of the lowest eight Kramers doublets (KDs) of complex 1·MeOH are shown in Table S11. The predominant mJ components for the lowest eight KDs of 1·MeOH are shown in Table S12, where the ground KD is mostly composed of mJ = ±15/2. The first excited KD of 1·MeOH is composed of mJ = ±13/2, which leads to the large transversal magnetic moment in its first excited KD (Figure 7). The calculations have been validated by fitting the χMT data for 1·2H2O with the intermolecular coupling constant of zJ′ = 0.07 cm−1 (Figure 3).
The corresponding magnetization blocking barrier of complex 1·MeOH is shown in Figure 7, where the transversal magnetic moment in the ground KD is close to 10−2 µB, and thus the QTM in the ground KD is suppressed at low temperature. The transversal magnetic moment in its first excited state of 1·MeOH is 0.5 × 10−1 µB, and therefore allowing the fast thermal-assisted QTM (TA-QTM) in its first excited KD. Thus, the calculated energy barrier is 194.6 cm−1. Due to the unfavorable effects of anharmonic phonons, Raman magnetic relaxation, QTM, et al. on the energy barrier, the experimental effective energy barrier Ueff is usually smaller than the calculated one [47,48,49,50,51].
The calculated crystal-field (CF) parameters B (k, q) and corresponding weights are given in Table S13, where the weights of the axial CF parameters B(2,0) and B(4,0) for 1·MeOH are 20.70% and 7.73%, respectively, which exceed the non-axial components, indicating a perfectly axial symmetry for 1·MeOH. The main magnetic axis on Dy(III) ion of 1 is indicated in Figure 8a. In addition, the main magnetic axis is also calculated by MAGELLAN software (version 1.0) [52], and the obtained results are shown in Figure 8b. It can also be observed that the direction of the main magnetic axis is almost identical with the direction of the phenol–oxygen bond, showing the shortest Dy-O bond is dominant in magnetic anisotropy.
It has been documented that the magnetic anisotropy is the key factor affecting the SMM behavior, while the magnetic anisotropy is related to the axiality and coordination geometry of Dy(III). Complex 1·MeOH has a single phenol–oxygen bond with a short distance of 2.193 Å, dictating the orientation of the magnetic axis and anisotropy. However, the calculated energy barrier for 1·MeOH is 194.6 cm−1, obviously smaller than that (221 cm−1) of a similar Dy(III) complex [Dy(LR)(H2O)4(MeCN)](ClO4)3·2H2O·MeCN (LR = salicylaldehyde rhodamine 6G hydrazone) with a single short Dy-Ophenoxy bond of 2.175 Å [26]. Careful examination of the crystal structure shows that both complexes have eight-coordinate Dy(III) and different coordination geometry: Cs for complex 1 and D2d for the latter. Higher coordination geometry and a shorter Dy-Ophenoxy bond usually favors higher calculated energy barrier for the latter. In addition, the coordination of nitrate anion in the equatorial positions attenuates the magnetic anisotropy of 1·MeOH. The under-barrier relaxation might be related to the weathering of the crystals, causing subtle variations in the crystal-field environment that can significantly influence the magnetic properties.

4. Conclusions

In this work, we have developed new Schiff base ligands HL1 and H2L2 to obtain a series of Ln(III) complexes from mononuclear to tetranuclear. The mononuclear complex 1·MeOH features a ring-opened ligand upon crystallization, and it emits a strong green fluorescence emission in ethanol. Magnetic characterizations show that complex 1·2H2O is a typical field-induced SMM with relaxations under-barrier [53]. The ab initio calculations have revealed that complex 1·MeOH exhibits high magnetic anisotropy. There is an intramolecular ferromagnetic coupling in the Dy4 complex (2), whereas complex 3 is weakly antiferromagnetic. Complex 3 can be used as a magnetic refrigeration material. This work reveals that the careful selection of ligands and the rational control of coordination modes can lead to the development of fluorescent lanthanide single-molecule magnets (Ln-SMMs) and a magnetic refrigeration material. This approach, as demonstrated in recent studies, allows for the fine-tuning of magnetic properties, which is crucial for advancing applications in fields such as information storage, quantum computing and biomedical detection.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/magnetochemistry11080068/s1, Figure S1. The polyhedral structures around the Dy(III) coordination in complex 1·MeOH. Figure S2. Coordination mode of ligand HL1-o in complex 1·MeOH. Figure S3. Hydrogen bonding in complex 1·MeOH. Figure S4. One-dimensional chain structure via π-π stacking in complex 1·MeOH. Figure S5. Dy4O6 core skeleton of complex 2. Figure S6. The polyhedral structures around the Dy(III) coordination in complex 2. Figure S7. Top and side views of the coordination polyhedron of the tetranuclear Dy(III) surrounding structures in complex 2. Figure S8. Coordination mode of ligand (L1-c)− and (L2)2− in complex 2. Figure S9. PXRD pattern for complexes 1·2H2O, 2 and 3. Figure S10. Infrared spectra of the ligand and complexes 1·2H2O, 2 and 3. Figure S11. UV-Vis spectra of the ligands and complexes 1·MeOH, 2 and 3. Figure S12. Fluorescence excitation spectra of complex 1·2H2O. Figure S13. Fluorescence excitation spectra of complexes 2 and 3. Figure S14. Magnetic coupling pathways in complex 3. Figure S15. Magnetization curves of complex 1·2H2O (a) and 2 (b). Figure S16. Magnetization curves and Brillouin curve at 2 K of complex 3. Figure S17. Temperature dependence of χ″M for complex 1·2H2O under 0 and 2000 Oe DC field. Figure S18. Plots of χ″M for complex 1·2H2O at 4 K and 997 Hz. Figure S19. The AC magnetic susceptibility data of complex 2 under 0 Oe: the χ′M (a) and χ″M (b) of AC magnetic susceptibility with temperature at different frequencies. Figure S20. Calculated complete structure of complex 1·MeOH. H atoms are omitted for clarify. Figure S21. Magnetization data at 3.0 K for 1·2H2O. The solid line stands for the fit data. Table S1. Crystal data and refinement parameters for complexes 1·MeOH, 2 and 3. Table S2. Selected bond lengths (Å) and angles (°) for complex 1·MeOH. Table S3. Selected bond lengths (Å) and angles (°) for complexes 2 and 3. Table S4. Hydrogen bonding for complex 1·MeOH. Table S5. Hydrogen bonding for complex 2. Table S6. Hydrogen bonds for complex 3. Table S7. The results of coordination geometric configurations evaluated by SHAPE software for complex 1·MeOH. Table S8. The results of coordination geometric configurations evaluated by SHAPE software for Dy1 of complex 2. Table S9. The results of coordination geometric configurations evaluated by SHAPE software for Dy2 of complex 2. Table S10. Cole-Cole plots fitting parameters for complex 1·2H2O. Table S11. Calculated energy levels (cm−1), g (gx, gy, gz) tensors and predominant mJ values of the lowest eight Kramers doublets (KDs) of complex 1·MeOH using CASSCF/RASSI-SO with the OpenMolcas. Table S12. Wave functions with definite projection of the total moment |mJ> for the lowest eight KDs of complex 1·MeOH. Table S13. Calculated crystal-field parameters B (k, q) and corresponding weights for complex 1·MeOH using CASSCF/RASSI-SO with the OpenMolcas. References [43,54,55,56,57,58] are cited in the Supplementary Materials.

Author Contributions

Conceptualization and formal analysis, L.M. and C.-M.L.; ab initio calculations, D.-M.Z.; writing—original draft preparation, L.M. and D.-M.Z.; writing—review and editing, supervision, project administration and funding acquisition, H.-Z.K. and Y.-Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant numbers 22271171 and 21971142.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SMMsSingle-molecule magnets
PXRDPowder X-ray diffraction
LLCTLigand-to-ligand charge transfer
DCDirect current
ACAlternating current
MCEMagnetocaloric effect
χ″MOut-of-phase AC susceptibilities
QTMQuantum tunneling of the magnetization
CASSCFComplete-active-space self-consistent field
KDsKramers doublets
TA-QTMThermal-assisted QTM
CFCrystal-field

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Magnetochemistry 11 00068 sch001
Scheme 1. Structures of the ligands HL1 (a) and H2L2 (b).
Scheme 1. Structures of the ligands HL1 (a) and H2L2 (b).
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Scheme 2. Transformation process of ring-opened HL1-o to ring-closed (L1-c)−.
Scheme 2. Transformation process of ring-opened HL1-o to ring-closed (L1-c)−.
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Figure 1. The crystal structures of complexes 1·MeOH (a) and 2 (b). The lattice solvents and hydrogen atoms are omitted for clarity.
Figure 1. The crystal structures of complexes 1·MeOH (a) and 2 (b). The lattice solvents and hydrogen atoms are omitted for clarity.
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Figure 2. Fluorescence spectra (λex = 354 nm) for complexes 1·2H2O, 2 and 3 in ethanol (10 μM).
Figure 2. Fluorescence spectra (λex = 354 nm) for complexes 1·2H2O, 2 and 3 in ethanol (10 μM).
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Figure 3. Temperature dependence of χMT for complexes 1·2H2O, 2 and 3 at 2–300 K under 1000 Oe field. The solid lines represent the fitting results.
Figure 3. Temperature dependence of χMT for complexes 1·2H2O, 2 and 3 at 2–300 K under 1000 Oe field. The solid lines represent the fitting results.
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Figure 4. Magnetization curves of complex 3.
Figure 4. Magnetization curves of complex 3.
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Figure 5. Magnetic entropy change in temperature of complex 3 under different external magnetic fields.
Figure 5. Magnetic entropy change in temperature of complex 3 under different external magnetic fields.
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Figure 6. The AC magnetic susceptibility data of complex 1·2H2O under 600 Oe DC field: (a) the χ′M of AC magnetic susceptibility changes with temperature at different frequencies; (b) the χ′′M of AC magnetic susceptibility with temperature at different frequencies; (c) the χ′′M of AC magnetic susceptibility is plotted against the χ′M at different temperatures (Cole–Cole) and fitted by the generalized Debye model; (d) lnτ is plotted against T−1, and the red line is the fitting result.
Figure 6. The AC magnetic susceptibility data of complex 1·2H2O under 600 Oe DC field: (a) the χ′M of AC magnetic susceptibility changes with temperature at different frequencies; (b) the χ′′M of AC magnetic susceptibility with temperature at different frequencies; (c) the χ′′M of AC magnetic susceptibility is plotted against the χ′M at different temperatures (Cole–Cole) and fitted by the generalized Debye model; (d) lnτ is plotted against T−1, and the red line is the fitting result.
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Figure 7. Magnetization blocking barrier of complex 1·MeOH. The path shown by the red arrows represents the most probable path for magnetic relaxation in the corresponding compound. The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of transition magnetic moment.
Figure 7. Magnetization blocking barrier of complex 1·MeOH. The path shown by the red arrows represents the most probable path for magnetic relaxation in the corresponding compound. The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of transition magnetic moment.
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Figure 8. Calculated orientation of the local main magnetic axis on Dy(III) of complex 1·MeOH in the ground KD. (a) Ab initio, (b) MAGELLAN.
Figure 8. Calculated orientation of the local main magnetic axis on Dy(III) of complex 1·MeOH in the ground KD. (a) Ab initio, (b) MAGELLAN.
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Miao, L.; Zhu, D.-M.; Liu, C.-M.; Zhang, Y.-Q.; Kou, H.-Z. Synthesis, Crystal Structures and Magnetic Properties of Lanthanide Complexes with Rhodamine Benzoyl Hydrazone Ligands. Magnetochemistry 2025, 11, 68. https://doi.org/10.3390/magnetochemistry11080068

AMA Style

Miao L, Zhu D-M, Liu C-M, Zhang Y-Q, Kou H-Z. Synthesis, Crystal Structures and Magnetic Properties of Lanthanide Complexes with Rhodamine Benzoyl Hydrazone Ligands. Magnetochemistry. 2025; 11(8):68. https://doi.org/10.3390/magnetochemistry11080068

Chicago/Turabian Style

Miao, Lin, Dong-Mei Zhu, Cai-Ming Liu, Yi-Quan Zhang, and Hui-Zhong Kou. 2025. "Synthesis, Crystal Structures and Magnetic Properties of Lanthanide Complexes with Rhodamine Benzoyl Hydrazone Ligands" Magnetochemistry 11, no. 8: 68. https://doi.org/10.3390/magnetochemistry11080068

APA Style

Miao, L., Zhu, D.-M., Liu, C.-M., Zhang, Y.-Q., & Kou, H.-Z. (2025). Synthesis, Crystal Structures and Magnetic Properties of Lanthanide Complexes with Rhodamine Benzoyl Hydrazone Ligands. Magnetochemistry, 11(8), 68. https://doi.org/10.3390/magnetochemistry11080068

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