Chiral Pseudo-D6h Dy(III) Single-Molecule Magnet Based on a Hexaaza Macrocycle

1. Introduction

Single-molecule magnets (SMMs), representing a significant breakthrough in nanomagnetism, integrate slow magnetization relaxation and quantum tunneling effects, and are potentially utilized in the fabrication of nanoscale devices and high-density data storage media.[1,2,3,4,5] Nevertheless, the operational thresholds of these materials predominantly reside in extreme cryogenic regimes, where the anisotropy barrier magnitude and blocking temperature emerge as critical determinants dictating their ambient-temperature functionality.[6,7,8,9,10] Lanthanide-based single-molecule magnets (Ln-SMMs) demonstrate substantial potential to promote iterative evolution in high-performance magnetic materials, that from the enhanced anisotropic magnetic moments and robust magnetic anisotropy inherent to rare-earth elements.[11,12,13,14]

The performance enhancement of Ln-SMMs fundamentally hinges on increasing magnetic anisotropy while suppressing quantum tunneling interference in relaxation dynamics.[15,16,17,18] The magnitude of magnetic anisotropy is predominantly governed by two competing factors: spin-orbit coupling (SOC) strength and electrostatic crystal field effects.[19,20,21] Long and his coworkers revealed that the oblate electron density distribution of Dy³⁺ ions enables strategic ligand engineering through their point-charge electrostatic model.[22] Weak isotropic equatorial coordination minimizes 4f electron cloud repulsion within the equatorial plane, whereas axial coordination of strong-field phenoxide, alkoxide, or siloxide auxiliary ligands maximizes axial magnetic anisotropy through optimized orbital overlap. Quantum tunneling processes can be effectively suppressed by high-symmetry crystal fields (e.g., D_4d D_5h, D_6h coordination geometries).[23,24,25,26] Furthermore, the rigidity of equatorial-plane ligands has been reported to mitigate quantum tunneling interference. A representative case was demonstrated by the Tang group in 2019 through a mononuclear dysprosium single-molecule magnet (Dy-SMM) with D_6h-symmetric coordination.[26] The equatorial plane employs a rigid hexaazamacrocyclic ligand, while the axial positions feature shortened Dy–O_phenoxy bonds (2.21 Å) that are significantly shorter than other coordination bonds (2.35-2.50 Å), creating pronounced axial magnetic anisotropy through ligand-induced orbital compression.

The high coordination number propensity of lanthanide ions predisposes them to isotropic coordination modes, adverse to the magnetic anisotropy for Ln-SMMs. In this work, we synthesized a mononuclear complex [Dy(phenN6)(HL')₂]PF₆·CH₂Cl₂ (H₂L' = R/S-1,1'-binaphthyl-2,2'-diphenol) with local D_6h symmetry. The flexible linear cyclic polydentate ligand provides a weak coordination environment in the equatorial plane, forming a non-planar coordination sphere encapsulating Dy³⁺. Axial positions are occupied by chiral phenoxy ligands (HL')^-, generating short Dy-O_phenoxy coordination bonds. By constructing a high-symmetry D_6h local configuration and dominant strong axial coordination bonds, the magnetic anisotropy along the axial direction is maximized, thereby enhancing the effective energy barrier. AC magnetic susceptibility measurements reveal distinct temperature-dependent and frequency-dependent characteristics in the AC magnetic signals, typical of single-molecule magnet behavior.

2. Results and Discussion

2.1. Synthesis and Characterizations

The electrostatic repulsion model suggests that the most ideal strategy for enhancing the magnetic performance of flattened Dy³⁺ ions involves concentrating ligands in the axial direction to form a monodentate coordination compound (coordination number = 1). However, lanthanide ions exhibit diverse coordination numbers and complicated configurations, making it challenging to obtain even bidentate structures, let alone monodentate ones. In order to obtain a D_6h high-performance Dy(III) SMM, we adopted a two-step strategy, intentionally weakening equatorial coordinating atoms, and then strategically enhancing the axial coordination by positioning the dominant phenoxyl oxygen along the anisotropy axis.[17,21] To this end, we optimized the transverse ligand field by retaining the rigid polyaromatic framework of 1,10-phenanthroline. Through 1:1 condensation with triethylenetetramine, we synthesized a hexa-dentate cyclic neutral symmetric Schiff base ligand, phenN6. This ligand not only preserves the rigidity of phenanthroline but also employs its unique cyclic coordination cavity to encapsulate Dy³⁺ metal centers in the equatorial positions.[21] Simultaneously, the symmetric coordination environment ensures isotropic transverse crystal fields, effectively minimizing interference with axial anisotropy. For axial coordination, functional phenoxy oxygen ligands were employed. Through axial ligand substitution in solution, we successfully obtained a mononuclear complex 1 exhibiting local D_6h symmetry. The two-step synthesis of complex 1 is shown in Scheme 1.

The point chirality of ligands can be transferred through coordination to produce chiral complexes. 1,1'-bi-2-naphthol is a common chiral compound. After coordination with Dy³⁺, a pair of chiral mononuclear complexes 1(R) and 1(S) are produced, which are mirror images of each other in crystal structure. The circular dichroism (CD) in acetonitrile solution further proves that they are enantiomers. As shown in Figure 1, the CD spectrum of the complex 1(R) shows a positive Cotton effect at 221 nm and a negative Cotton effect at 235 nm, which is due to the π→π* charge transfer in the binaphthyl benzene ring. However, the CD spectrum of the complex 1(S) shows a mirror image signal completely opposite to that of 1(R) at the same wavelength, which indicates that 1(R) and 1(S) are chiral and are a pair of Dy(III) enantiomers. These results further prove that the point chirality of ligands can be effectively transferred to the whole complex structure through coordination with metal ions.

We tested the complexes 1(R) and 1(S) by TGA in order to explore their thermal stability. As can be seen from Figure S1, the sample slowly loses the lattice solvent CH₂Cl₂ from room temperature, and above 185 °C solvent completely leaves (weight loss is 4.5 %). Then in the temperature range of 185-207 °C, HPF₆ was completely removed (the weight loss of 8.3 %). When the temperature rises above 253 °C, the complex begins to decompose.

As shown in Figure S2, the infrared spectrum of complex 1 shows a strong absorption peak at 1600 cm^-1, which may be caused by the stretching vibration of Schiff base ligand C=N, while a strong absorption peak at 830 cm^-1 is attributed to the P-F stretching vibration in PF₆^- ion.

Figure 2 shows the fluorescence spectra of complexes 1(R) and 1(S) in acetonitrile solution with excitation wavelength of 320 nm. It can be seen that they all have tiny shoulder peaks around 400 nm, which are derived from the rigid aromatic ring of equatorial plane ligands. 1,1'-binaphthyl-2,2'-diphenol is a good fluorescent chromophore, and the dihedral angle of its monomer is close to 90°, which can effectively prevent intermolecular π-π stacking, thus obtaining strong fluorescence emission. Complex 1(R/S) has the maximum emission peak at 360 nm, which is the characteristic emission peak of naphthalene ring itself.

2.2. Structure

Mononuclear complex 1(R) and 1(S) crystallize in polar space group P1, as shown in Figure 3. The direction of chiral helix corresponding to S/R can be determined by the orientation of uncoordinated phenol oxygen bond in 1,1'-binaphthyl-2,2'-diphenol. The independent asymmetric unit contains a Dy³⁺ ion, a neutral cyclic ligand (phenN6), two negative monovalent axial ligands (HL')^-, a charge-balancing anion PF₆^- and a lattice solvent CH₂Cl₂. As we designed, Dy³⁺ was indeed wrapped in the coordination cavity composed of hexaaza N₆, and the rigid limitation of o-phenanthroline made the whole equatorial position coplanar at the head, while the flexible aliphatic amine chain at the tail deviated from the equatorial plane upon the coordination (N5), which made the equatorial coordination macrocycle not coplanar. Similar situation occurs to less distorted [Dy(bpyN6)(Ph₃SiO)₂](BPh₄) and [Dy(phenN6)(Ph₃SiO)₂](PF₆).[17,21] The axial positions are occupied by the phenoxy group in binaphthol. According to the SHAPE calculation (version 2.1), the local configuration of Dy³⁺ is close to the compressed hexagonal bipyramidal D_6h configuration), and the deviation parameter is 5.478 (Table S3). The axial Dy-O_phenoxy distances are 2.189(5) and 2.144 (5) Å, respectively, while the bond lengths of Dy-N on the equatorial plane are in the range of 2.525(7)–2.719(5) Å, which is obviously longer than the axial Dy-O_phenoxy bond length. The axial O_phenoxy-Dy-O_phenoxy bond angle of 162.89(18)° markedly deviates from the ideal linearity, which might be related to the spatial hindrance induced by the nonplanarity of the equatorial ligand phenN6. At the same time, the free CH₂Cl₂ and PF₆^- are situated in the crystal lattice (Figure 4), and form abundant weak intermolecular interactions. The nearest intermolecular Dy---Dy distance is 9.224 Å.

2.3. Magnetism

Variable-temperature magnetic susceptibility measurements for complex 1 were conducted under a 1000 Oe dc field across the 2-300 K range. As shown in Figure 5a, the room-temperature χ_MT value of 13.9 cm³·mol⁻¹·K is close to the theoretical value (14.17 cm³·mol⁻¹·K) for a non-interacting Dy³⁺ ion. The χ_MT profile of mononuclear complex 1 remains nearly constant upon cooling before gradually decreasing to a minimum of 11.79 cm³·mol⁻¹·K at 2 K, indicative of weak intermolecular antiferromagnetic interactions between Dy³⁺ centers. The magnetization curve of complex 1 at 2.0 K (Figure 5b) shows that the magnetization in the region of 0-1.0 T increases nearly linearly with the increase of external magnetic field, and then gradually to 5.6 Nβ at 5.0 T, which is far lower than the theoretical saturation value of 10 Nβ of rare earth Dy(III) ion complexes. As shown in Figure 5b, complex 1 has a small hysteresis loop at 1.9 K without residual magnetization at the external zero magnetic field.

The temperature-dependent out-of-phase alternating current (ac) magnetic susceptibility of complex 1 under zero dc field is shown in Figure 6(a). The presence of non-zero χ″ signals across the 10–997 Hz range, in the absence of distinct peaks, suggests significant quantum tunneling of magnetization (QTM) within the molecular framework. To mitigate this effect, field-dependent ac susceptibility measurements were conducted at 10 K and 997 Hz to identify the optimal suppressing dc field. As revealed in Figure 6(b), the χ″ response exhibits a maximum near 1800 Oe, indicating effective QTM suppression at this field. Subsequent ac susceptibility tests were thus performed under an applied 1800 Oe dc field.

Figure 7 demonstrates pronounced temperature- and frequency-dependent out-of-phase (χ_M″) ac susceptibility signals for 1, characteristic of single-molecule magnet (SMM) behavior. Cole-Cole plots between 6.0–28.0 K (Figure 7c) display semicircular profiles that were satisfactorily fitted using the generalized Debye model (Table S4). The small distribution parameters (α < 0.17) confirm a narrow relaxation time distribution, consistent with a single dominant relaxation pathway.

Arrhenius analysis of the extracted relaxation times (lnτ vs. T^-1) reveals two distinct regimes: (i) A linear high-temperature region (T > 15 K) governed by the Orbach process, and (ii) a curved low-temperature region (T < 15 K) dominated by a Raman relaxation mechanism. Consequently, the entire temperature range was fitted using Equation ${\tau }^{-1}=C{T}^n+{\tau }_0^{-1}\exp \left(-{\displaystyle \frac{U_{\mathrm{e}\mathrm{f}\mathrm{f}}}{k_{\mathrm{B}}T}}\right)$ incorporating both Orbach and Raman relaxation mechanisms, yielding the parameters: n = 4.24, C = 0.001 s⁻¹·K⁻ⁿ, U_eff = 300.2 K, and τ₀ = 6.7 × 10⁻⁷ s.

For Dy(III) single-molecule magnets, the magnitude of the effective energy barrier is primarily determined by the coordination environment of the rare-earth ion. Subtle alterations in local coordination geometry and variations in charge distribution can both induce modifications in single-molecule magnetic properties.[27,28] Recent reports on Dy-SMMs (37 cases) with the D_6h spatial configuration show that they possess the U_eff values in the range of 35-2437 K (Table S5).[17,18,19,20,21,26,29,30,31,32,33,34,35,36,37,38,39] It is well documented that for mononuclear D_6h-Dy single-molecule magnets, the rigidity and electrical properties of the equatorial ligands, the electronegativity of the coordination atoms and the steric hindrance of the axial ligands will change the structure and local coordination configuration (coplanarity) of the complex, and then alter the strength of the crystal field around Dy(III), thus regulating the magnetic anisotropy of Dy(III). Comparison of the magnetic properties for D_6h high-performance Dy-SMMs in Table S5 suggests that the axial O-Dy-O bond angle plays the most critical role, i.e. larger bond angles correspond to better magnetic anisotropy and higher U_eff. Besides, the coplanarity, electrical neutrality and strong electronegativity of coordination atoms play a favorable secondary role. In complex 1, the Dy³⁺ center adopts a compressed D_6h configuration with a neutral hexaaza macrocyclic ligand, showing a medium U_eff among the similar pseudo-D_6h Dy-SMMs (Table S5). The small axial O-Dy-O bond angle of 162.89(18)º should be responsible for the situation. Previous theoretical calculation results indicate that the large deviation from ideal D_6h usually brings about the QTM process occurred in the first excited state, which leads to the failure of the energy barrier flip to to the second excited state, and finally a small effective energy barrier. In contrast, ideal D_6h could make the anisotropic axis of the first excited state and even the higher excited states coincide with the ground state, and then the QTM in the first or higher excited states can be effectively suppressed, thus significantly improving the effective energy barrier.[31]

3. Materials and Methods

3.1. Synthesis and Preparations

All of the reagents were commercially available and were used without further purification.

3.1.1. Synthesis of the Precursor phenN6-Dy

1.18 g (5 mmol) of 1,10-o-phenanthroline-2,9-dicarbaldehyde was dissolved in 30 mL of hot ethanol, then 800 μL (5 mmol) of triethylenetetramine was added, and the reaction system began to slowly produce turbidity, then 1.92 g (5 mmol) of dysprosium chloride hexahydrate was added. The mixture was heated to reflux for 6 h. After the reaction, a large amount of light brown precipitate was produced at the bottom of the round-bottomed flask. The reaction product was collected by suction filtration and was repeatedly washed with ice ethanol. Finally, the potential coordination solvent was removed by vacuum drying at 80 °C for 5 h, and finally about 1.8 g of brown powder phenN6-Dy was obtained with a yield of about 65%.

3.1.2. Synthesis of Complex [Dy(phenN6)(HL^’)₂]PF₆⋅CH₂Cl₂ (1R/1S)

Precursors phenN6-Dy (0.05 mmol), R/S-1,1'-binaphthyl-2,2'-diphenol (H₂L^’, 0.1 mmol), KPF₆ (0.1 mmol) and triethylamine (0.1 mmol) were respectively added into 10 mL of dichloromethane solvent, and then 10 mL of deionized water was added into the mixed system under stirring at room temperature. Subsequently, the reaction system was refluxed for 2 h. The solution was allowed to cool at room temperature. The dichloromethane layer was separated and filtered, and the crimson filtrate was allowed to stand and evaporate slowly in a small bottle with a hole. Two days later, red flaky crystals were precipitated with a yield of about 53 % (based on the amount of precursor phenN6-Dy).

3.2. Physical Measurements

Single crystal X-ray data were collected by Rigaku SuperNova, Dual, Cu at zero and AtlasS2. Use Olex2 1.3 program to solve the structure, and use the full matrix least square method based on F² to refine it with the method of SHEXL-2018/3. Hydrogen atoms are added geometrically and refined by riding model. The temperature- and field-dependent magnetic susceptibility was measured by MPMS XL5 SQUID magnetometer of Quantum Design Company. Infrared spectra (KBr tablet) in the range of 400 ~ 4000 cm⁻¹ were recorded on WQF 510A FTIR equipment, and the scanning interval was 2 cm⁻¹. TU-1901 spectrophotometer was used to measure the ultraviolet-visible absorption spectra in the range of 280 ~ 600 nm, and the scanning interval was 0.5 nm. The photoluminescence spectrum was measured by Lengguang F98 fluorescence spectrophotometer. The scanning speed is 1000 nm/min and the scanning interval is 1 nm. The circular dichroism (CD) spectrum was measured by JASCO J-1500 spectrometer.

4. Conclusions

A chiral mononuclear complex [Dy(phenN6)(HL')₂]PF₆·CH₂Cl₂ (H₂L' = R/S-1,1'-binaphthyl-2,2'-diphenol) with local D_6h symmetry was successfully constructed. The short Dy–O_phenoxy coordination bonds formed through axial coordination of chiral HL’^- ligands effectively enhance the magnetic anisotropy. AC magnetic susceptibility measurements unambiguously confirm the single-molecule magnet behavior with the effective energy barrier of 300.2 K. This work reemphasizes that perfect D_6h coordination configuration of Dy(III) is in favor of high-performance Dy-SMMs.