Syntheses, Structures and Magnetic Properties of M₂ (M = Fe, Co) Complexes with N₆ Coordination Environment: Field-Induced Slow Magnetic Relaxation in Co₂

Abstract

Two dinuclear complexes [M₂(H₂L)₂](ClO₄)₄·2MeCN (M = Co for Co₂ and Fe for Fe₂) were synthesized using a symmetric hydrazone ligand with the metal ions in an N₆ coordination environment. The crystal structures and magnetic properties were determined by single-crystal X-ray diffraction and magnetic susceptibility measurements. The crystal structure study revealed that the spin centers were all in the high-spin state with a distorted octahedron (O_h) geometry. Dynamic magnetic properties measurements revealed that complex Co₂ exhibited field-induced single-molecule magnet properties with two-step relaxation in which the fast relaxation path was from QTM and the slow relaxation path from the thermal relaxation under an applied field.

1. Introduction

Since the discovery of the first single-molecule magnet (SMM) Mn₁₂ cluster [1], research on SMMs has aroused interest among the scientific community due to the fact of their potential applications such as high-density information storage, quantum computing, and spintronics [2,3,4,5,6,7,8,9,10,11,12]. Subsequently, a large number of SMMs based on transition mental [13,14,15,16,17], lanthanide [18,19,20,21,22,23,24,25], and mixed-metal ions [26,27,28,29,30,31] have been designed and reported. For SMMs, the combination of a large negative zero-field splitting parameter (D < 0) or strong uniaxial magnetic anisotropy (g_z) and large spin ground state (S) could result in an SMM with a large effective energy barrier (U_eff) [32]. The energy barrier can be calculated by U_eff = |D|S² or |D|(S² − 1/4) for integer and half-integer spin, respectively [33].

For transition metal complexes, crystal field splitting is much stronger than spin-orbit coupling; therefore, the orbital angular momentum is almost completely quenched [34]. However, the zero-field splitting parameter is usually only several dozens of wavenumbers for transition metals, and most of the complexes based on 3d ions are field-induced single-molecule magnets, especially in the presence of quantum tunneling of magnetization (QTM) [35,36,37]. The divalent cobalt ion is in the d⁷ configuration, and there is an important orbital contribution to the magnetization. The coordination number of Co^II ions ranges from two coordination to eight coordination with different coordination geometries [38,39,40,41,42,43,44]. As a result, the Co^II ion is a good candidate for the construction of SMMs due to the fact of its magnetic anisotropy. Up to now, numerous SMMs based on Co^II ions have been reported [45,46,47,48].

By reducing the coordination number [49] or regulating the coordinated atoms [50], it is possible to obtain a relatively weak ligand field and increase the orbital contribution. However, low-coordination molecular magnets are usually unstable in air. Therefore, the design of high-coordination and stable single-molecule magnets has attracted much attention [41,48]. Herein, we designed and synthesized a symmetrical hydrazone ligand 2,6-bis((E)-(2-(6-(1H-pyrazol-1-yl)pyridin-2-yl)hydrazineylidene)methyl)pyridine (H₂L) in which only nitrogen acts as donor atoms to coordinate with metal ions. Two new dinuclear complexes [M₂(H₂L)₂](ClO₄)₄·2MeCN (M = Co for Co₂ and Fe for Fe₂) were obtained from the ligand. The crystal structures and magnetic properties were determined by single-crystal X-ray diffraction and magnetic susceptibility measurements. The spin centers were all in the high-spin state with a distorted octahedron (O_h) geometry. Importantly, dynamic magnetic properties measurements revealed that complex Co₂ exhibited field-induced single-molecule magnet properties with two-step relaxation in which the fast relaxation (FR) path was from the QTM and the slow relaxation (SR) path from the thermal relaxation.

2. Materials and Methods

2.1. Syntheses of Ligand and Complexes

All chemicals and solvents were obtained commercially without further purification. 2-Hydrazineyl-6-(1H-pyrazol-1-yl)pyridine and pyridine-2,6-dicarbaldehyde were synthesized according to the previously reported literature [51,52]. As a caution, the salts Co(ClO₄)₂·6H₂O and Fe(ClO₄)₂·6H₂O should be used carefully, as they are potentially explosive.

2.1.1. Synthesis of Ligand H₂L

2-Hydrazineyl-6-(1H-pyrazol-1-yl)pyridine (1.75 g, 10 mmol) was added into a solution of pyridine-2,6-dicarbaldehyde (0.67 g, 5 mmol) in ethanol (50 mL). Then, the reaction mixture was allowed to reflux overnight giving a dark orange solid. After filtering, the product was washed with ice ethanol and dried in a vacuum, yielding the ligand 2,6-bis((E)-(2-(6-(1H-pyrazol-1-yl)pyridin-2-yl)hydrazineylidene)methyl)pyridine (H₂L) (1.78 g, 79.4%). ¹H NMR (400 MHz, DMSO-d₆): δ = 11.41 (s, 2H), 8.51–8.49 (m, 2H), 8.14 (s, 2H), 7.96–7.93 (m, 2H), 7.89–7.84 (m, 3H), 7.81 (d, J = 1.0, 2H), 7.35 (d, J = 7.5, 2H), 7.26 (d, J = 8.1, 2H), and 6.58 (dd, J = 2.5, 1.7, 2H). Selected IR (solid, ATR) ṽ(cm⁻¹): 543 (m), 566 (w), 578 (w), 588 (w), 605 (m), 644 (m), 667 (w), 688 (w), 715 (w), 728 (m), 738 (m), 769 (s), 790 (s), 811 (w), 863 (w), 883 (w), 902 (m), 916 (m), 964 (m), 989 (w), 1047 (m), 1072 (w), 1087 (w), 1130 (m), 1141 (m), 1176 (m), 1253 (m), 1270 (w), 1284 (w), 1321 (m), 1336 (w), 1346 (w), 1392 (m), 1427 (m), 1452 (s), 1508 (s), 1575 (s), 1606 (s), 2954 (w), 3089 (w), and 3317 (w).

2.1.2. Synthesis of Co₂

A mixture of H₂L (0.1 mmol) and Co(ClO₄)₂·6H₂O (0.1 mmol) in acetonitrile (15 mL) was stirred for one hour, yielding a dark red clear solution after filtering. Subsequently, the filtrate was allowed to stand and evaporate for four days to obtain red crystals suitable for single-crystal X-ray diffraction. Yield: 15.99 mg, (21.36%, based on metal salts). Elemental analysis calculated for C₅₀H₄₄Cl₄Co₂N₂₄O₁₆ (%): C, 40.12; H, 2.96; N, 22.46. Found (%): C, 40.08; H, 2.91; N, 22.42.

2.1.3. Synthesis of Fe₂

A mixture of H₂L (0.1 mmol) and Fe(ClO₄)₂·6H₂O (0.1 mmol) in acetonitrile (15 mL) was stirred for one hour, yielding a dark red clear solution after filtering. Subsequently, the filtrate was allowed to stand and evaporate for four days to obtain red crystals suitable for single-crystal X-ray diffraction. Yield: 13.82 mg, (18.54%, based on metal salts). Elemental analysis calculated for C₅₀H₄₄Cl₄Fe₂N₂₄O₁₆ (%): C, 40.29; H, 2.98; N, 22.55. Found (%): C, 40.23; H, 2.91; N, 22.53.

2.2. Physical Measurements

The ¹H NMR spectrum of H₂L was recorded on a Bruker Avance 400 MHz spectrometer (Bruker, Switzerland, Figure S1). Elemental analyses (i.e., C, H, and N) were measured on a PerkinElmer 2400 analyzer (PerkinElmer, United States). Fourier transform infrared spectrometer (FTIR) spectra were obtained using a Nicolet 6700 Flex FTIR spectrometer (Thermo Fisher, United States) equipped with a smart iTR attenuated total reflectance (ATR) sampling accessory (Figures S2 and S3). Powder X-ray diffraction (XRD) measurements were carried out using a Bruker D8 advance X-ray diffractometer (Bruker AXS GMBH, Germany) with Cu-Kα radiation.

2.2.1. Crystallography

Single-crystal X-ray diffraction data were collected by the Bruker D8 venture CCD diffractometer (Bruker AXS GMBH, Germany) using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å). In the Olex2 package, the structures were solved using SHELXT [53] (direct methods), and all non-hydrogen atoms were refined using SHELXL [54] (full-matrix least squares techniques) on F2 with anisotropic thermal parameters. All hydrogen atoms were introduced in calculated positions and refined with fixed geometry relative to their carrier atoms. The crystallographic data for Co₂ and Fe₂ are listed in Table S1. CCDC 2116748 and 2116749 contain the supplementary crystallographic data for this paper.

2.2.2. Magnetic Measurements

Magnetic measurements were measured by using a Quantum Design MPMS-XL-7 SQUID magnetometer (Quantum Design, United States) equipped with a 7 T magnet. Susceptibility measurements were carried out on the polycrystalline sample of the two complexes. In the temperature range 2–300 K, the direct current (dc) susceptibility measurements were obtained under an applied field of 1000 Oe. Diamagnetic corrections were made with Pascal’s constants [55] for all constituent atoms and the contributions of the sample holder. The field-dependent magnetizations were obtained in the field range of 0−7 T. In the frequency range of 1–1488 Hz, the alternating current (ac) susceptibility measurements were obtained in a 3 Oe ac oscillating field under 0 and 3500 Oe dc fields.

3. Results and Discussions

3.1. Structures of Co₂ and Fe₂

The crystal structures of Co₂ and Fe₂ were determined by single-crystal X-ray diffraction at 173 and 180 K, respectively. The two complexes were isostructural; therefore, the structure of Co₂ is described here only. Co₂ crystallized in the triclinic space group P$\overline{1}$ with the crystallographic data and refinement details shown in Table S1. The asymmetric unit of the complex consisted of one neutral ligand H₂L, one crystallographically independent Co^II center, two ClO₄⁻ anions, and one MeCN solvent molecule in the lattice. The spin centers were in the N₆ coordination pocket from the two H₂L ligands, forming a Co₂ core (Figure 1 and Figure S5). Four perchloride anions crystallized in the crystal lattice to balance the positive charges.

The Co–N bond distances were in the range of 2.05–2.25 Å, which indicates that the Co^II ions were in the high spin (HS) state (Table S2). The coordination geometry of the Co^II center was evaluated by the SHAPE software [56,57] (Figure S4 and Table S3). The coordination geometry of the Co^II ion was closest to an octahedron (O_h) with a CShM value of 5.37. Such a large value suggests a large distortion of the coordination geometry. For Fe₂, the Fe–N bond distances were in the range of 2.11–2.27 Å, which also indicates that the Fe^II ions were in the HS state. The coordination geometry of the Fe^II ion was closest to an octahedron (O_h) with a CShM value of 6.34, suggesting a larger distortion of the coordination geometry than the Co^II ion in Co₂ (Figure S4 and Table S3). Due to the long coordination bonds, complex Fe₂ did not exhibit spin-crossover properties [58]. As depicted in Figure S5 and S6, the intramolecular distance and the shortest intermolecular distance between two Co^II ions were 7.45 and 9.01 Å, respectively. The intermolecular interaction could be ignored because of the relatively long intermolecular distance (9.01 Å) [59], while dinuclear cobalt complexes with shorter Co···Co distances (3.10–3.11 Å) usually show strong ferromagnetic interaction with the coupling parameter regulated by changing the ligand field of one Co^II center as well as the effects on the dc magnetic susceptibility [60]. The intramolecular Co^II···Co^II ions distance of 7.45 Å probably induced very weak intramolecular interaction and, thus, played a role in the magnetic properties. The intramolecular and the shortest intermolecular Fe···Fe distances were 7.47 and 9.05 Å for Fe₂, respectively. As shown in Figure S7, the phase purity of the bulk samples of the two complexes was confirmed by powder XRD analyses.

3.2. Magnetic Properties of Co₂ and Fe₂

3.2.1. Static Magnetic Properties of Co₂ and Fe₂

Direct current magnetic susceptibility measurements were measured on polycrystalline samples with a 1000 Oe field in the temperature range of 2–300 K (Figure 2). At room temperature, the χ_MT values (χ_M is molar magnetic susceptibility) were 4.99 and 7.23 cm³Kmol⁻¹ for Co₂ and Fe₂, respectively. These values were higher than the expected values for the two spin-only HS ions (Co^II, S = 3/2, χ_MT = 1.875 cm³Kmol⁻¹; Fe^II, S = 2, χ_MT = 3 cm³Kmol⁻¹), which may be due to the existence of orbital contribution. In low-temperature regions, the χ_MT values decreased gradually for Co₂ and sharply for Fe₂, which probably resulted from the magnetic interaction or zero-field splitting of the spin center. The temperature-dependent magnetic susceptibility plots were approximately simulated by using a spin Hamiltonian (Equation (1)) [61,62]:

$$\widehat{H}=-2J{\widehat{S}}_1{\widehat{S}}_2+{\displaystyle \sum \{D_i[S_{Z,i}^2-S_i(S_i+1)/3]+\mathrm{E}[S_{x,i}^2}-S_{y,i}^2]\}+\mathrm{g}{\mu }_B{\displaystyle \sum \overrightarrow{B}\overrightarrow{S_i}}$$

(1)

where J, D, E, g, μ_B, and B correspond to magnetic exchange, the axial and rhombic zero-field splitting parameter, Landé factor, Bohr magneton, and magnetic field vector, respectively. The best fit provided large |D| values of 30 cm⁻¹ and rhombic a parameter E (E/D = 0.15) for Co₂, indicating the presence of magnetic anisotropy, which probably relates to the large distortion of the coordination geometry. The g values were 2.32, revealing the presence of orbital contribution. It is worth noting that the sign of D cannot be determined by the simulation of temperature-dependent magnetic susceptibility plots but can be further determined by theoretical calculation or EPR measurements [63,64,65]. In addition, not only Co^II complexes with negative D values can exhibit single-molecule magnet properties [50,62,66,67], but some cobalt complexes with positive D values can also act as SMMs [68,69,70,71]. The two Co^II ions were ferromagnetically coupled, with J = 0.08 cm⁻¹. In contrast, the D values of Fe₂ were smaller (|D| = 1.0 cm⁻¹, E/D = 0.2, g = 2.2), suggesting the possible absence of SMM properties, which is probably because of the difference in the electron structure. Moreover, the magnetic interaction parameter (J = −0.25 cm⁻¹) was different from Co₂, indicating the presence of weak antiferromagnetic interaction.

The field-dependent magnetization for Co₂ was performed in the range of field 0−7 T at 1.9, 3.0, and 5.0 K. The magnetization value did not reach saturation value (6.96 μ_B) at 7 T. The plots of M vs. H/T at various temperatures were non-superimposable, suggesting the presence of magnetic anisotropy (Figure 3).

3.2.2. Dynamic Magnetic Properties of Co₂

To investigate the dynamic magnetic properties, ac susceptibility measurements were performed on Co₂ under various applied dc fields. No out-of-phase (χ″) susceptibilities signals were observed under a zero dc field (Figure S8), possibly due to the presence of QTM. We then measured field-dependent ac susceptibility at 1.9 K to determine the optimal dc field. The peak of out-of-phase (χ″) susceptibilities appealed at 3500 Oe (Figure S9); thus, temperature-dependent and frequency-dependent ac susceptibility were then carried out under this dc field (Figure 4). In the temperature-dependent out-of-phase (χ″) plots, the maximum appeared up to 6.0 K at a frequency of 1488 Hz, suggesting the slow relaxation of magnetization. In the low-temperature region, an upturning appeared, which is probably ascribed to QTM. The peaks of frequency-dependent out-of-phase (χ″) susceptibilities shifted to high frequency when increasing the temperature, suggesting the typical field-induced single-molecule magnet properties.

From the frequency-dependent ac susceptibility data in the temperature region of 1.9–8.0 K, Cole–Cole plots were represented as χ″ vs. χ′ and fitted with the double-relaxation Debye model [72] and CC-FIT program [73] (Figure 5). The Cole–Cole plots show two semi-circular profiles, suggesting the presence of two-step relaxation. Extracting the relaxation time from the frequency-dependent susceptibility provided two relaxation regimes (Table S4). The fast relaxation (FR) path showed typical thermal relaxation, while the slow relaxation (SR) path was almost temperature-independent, which resulted from QTM (Figure 6). To analyze the relaxation procedure, the plots of τ vs. T⁻¹ were fitted using the following equation [74,75,76]:

$$\frac{1}{\tau }=\frac{1}{{\tau }_{QTM}}+AT+C{T}^n+{\tau }_0^{-1}\exp \left(-U_{eff}/T\right)$$

(2)

where 1/τ_QTM, AT, CTⁿ, and τ₀⁻¹exp(–U_eff/T) correspond to quantum tunneling, direct, Raman, and Orbach relaxation processes [77,78], respectively. The best fit of the FR path included direct, Raman, and Orbach relaxation processes with U_eff = 43 K, τ₀ = 1.07 × 10⁻⁷ s, A = 432, C = 1.72 × 10⁻¹² s⁻¹·K⁻ⁿ, n = 3. The quantum tunneling relaxation time of the SR path was linearly fitted for the τ versus 1/T plot, giving τ_QTM = 0.14 s.

The effective barrier obtained from the fitting above (approximately 43 K) for the FR path was much lower than the energy level 2|D| = 60 cm⁻¹. Therefore, we ignored the Orbach relaxation process and fitted the FR path with Equation (3) (where 1/τ_QTM, AT, and CTⁿ correspond to quantum tunneling, direct, and Raman relaxation processes) for the whole temperature region (Figure 7). The best fit gives τ_QTM = 1.48 × 10⁻³ s, A = 124, C = 0.106 s⁻¹·K⁻ⁿ, and n = 6.3.

$$\frac{1}{\tau }=\frac{1}{{\tau }_{QTM}}+AT+C{T}^n$$

(3)

4. Conclusions

In conclusion, we successfully designed and synthesized two dinuclear complexes, Co₂ and Fe₂, using a symmetric hydrazone ligand with the metal ions in an N₆ coordination environment. The crystal structures of the two complexes revealed that the spin centers were in the N₆ coordinated environment with a distorted octahedron (O_h) geometry. The analysis of the crystal structures and the dc susceptibility measurements indicated that the spin centers were in the high-spin state. Dynamic magnetic properties measurements revealed that complex Co₂ exhibited field-induced single-molecule magnet properties due to the magnetic anisotropy of Co^II ions. The complex exhibited two-step relaxation with the FR and SR paths resulting from QTM and thermal relaxation, respectively. This work may open a new opportunity for the design of dinuclear 3d-SMMs based on octahedron coordination geometry.