Wheel-type Heterometallic Ferromagnetic Clusters: [Ni7xMx(HL)6(μ3-OMe)4(μ3-OH)2]Cl2 (M = Zn, Co, Mn; x = 1, 3)

Fumiya Kobayashi,a Ryo Ohtani,a Sotaro Kusumoto,a Leonard F. Lindoy,b Shinya Hayami*a,c and Masaaki Nakamura*a

Abstract

Wheel-type heptanuclear heterometallic clusters of stoichiometry [Ni7-xMx(HL)6(μ3-OMe)4(μ3-OH)2]Cl2 (L = 1-(2-hydroxy-3methoxybenzamido)-2-(benzylideneamino)ethane ; M = Zn2+, Co2+, Mn2+; x = 1, 3) were synthesized, and their metal ion dependent ferromagnetic properties investigated. It was demonstrated that the central position of each cluster was occupied by a Ni2+ ion while other metal ions (Zn2+, Co2+ or Mn2+) were accommodated in peripheral positions. The magnetic interactions, anisotropies and magnetizations of the resultant clusters were all influenced by the combinations

Introduction

Multinuclear metal complexes (clusters) have attracted much attention because they often show unique magnetic properties arising from magnetic interactions acting between the metal ions; leading to magnetic clusters exhibiting large magnetizations and single molecule magnet (SMMs) behavior that are available for the development of the next generation magnetic devices.1−10 Controlling the magnetic interactions has been a requirement for utilizing such magnetic clusters in the magnetic devices. The magnetic interactions in clusters are sensitive to various factors; for example, to the bond lengths and bridging angles between metal ions influencing the overlapping of orbitals, as well as to the electronic configurations of the metal ions themselves.11−13 The construction of heterometallic clusters has proved to be a very productive approach for not only investigating relationships between the above factors and corresponding magnetic interactions but also for developing novel functional magnetic clusters.
The development of heterometallic clusters incorporating different kinds of metal ions has enabled tuning of magnetic interactions and allowed detailed studies of magneto-structural relationships.14-21 Several researchers have reported details of heterometallic magnetic clusters which were synthesized by exchanging some of the metal ions in homometallic multinuclear structures with different metal ions.22,23 For example, Podgajny, Ohkoshi and co-workers have reported a series of trimetallic cyano-bridged {Fe9−xCox[W(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, …, 8, 9) clusters that exhibit switchable magnetic properties between charge-transfer assisted phase transition and slow magnetic relaxation induced by changing the Co/Fe metal ratios.24 Brechin, Dalgarno, Piligkos and co-workers have reported the systematic replacement of the central Mn2+ ions in a family of calix[4]arene-supported [MnIII2MnII2] clusters with Ln3+ ions, resulting in an increase in magnetization relaxation times and the observation of SMMs behavior.25 This approach produced heterometallic isomorphous clusters exhibiting magnetic properties that differed from that of the pristine homometallic clusters due to ‘tuning’ of the magnetic interactions. Up to the present, [M′x@M7-x] heterometallic clusters have been constructed by incorporating center metal ions that display lower or higher oxidation numbers than those of the peripheral metal ions, for example: [Na@NiII6], [MnII@VIV6], [CrIII@CoII6] and [FeIII@FeII6].26−29 On the other hand, there are only a few reports of [M′x@M7-x] type heterometallic clusters incorporating metal ions with the same oxidation number.30 This might reflect that the local positions of individual metal ions in such mixed-metal clusters can be difficult to determine by X-ray diffraction analysis. Our group has previously reported the heptanuclear Ni2+ complex [Ni7(HL)6(µ3-OMe)6]Cl2 (1; where H2L = 1-(2-hydroxy-3methoxybenzamido)-2-(benzylideneamino) ethane) incorporating a [M@M6] arrangement together with an analysis of its magneto-structural values.31 We anticipated that the introduction of Mn2+ or Co2+ into this Ni cluster would result in new heptanuclear clusters exhibiting large magnetizations and/or magnetic anisotropies. As part of this investigation, we also introduced diamagnetic Zn2+ ions in order to probe the local positions by analyzing the magnetic interactions within the resulting heterometallic clusters. Herein, we report the syntheses of new heterometallic clusters of type [Ni7-xMx(HL)6(µ3-OMe)4(µ3-OH)2]Cl2

Results and discussion

The isomorphous and isostructural crystals of [Ni7xMx(HL)6(µ3-OMe)4(µ3-OH)2]Cl2 (Ni6Zn: (2), Ni4Zn3: (3), Ni6Co: (4), Ni4Co3: (5), Ni6Mn: (6), Ni4Mn3: (7)) were prepared according to the method reported previously.31 The respective crystals were grown by standing solutions of NiCl2·6H2O, MCl2·xH2O, a H2L ligand and triethylamine in methanol in a molar ratio corresponding to the targeted compositions (Table S1, for details, see Experimental Section). The resultant crystals of 1−7 were characterized by powder X-ray diffraction (PXRD), fourier transform infrared spectroscopy (FT-IR) and elemental analyses. The Ni7 cluster 1 has previously been reported as a cationic heptanuclear alkoxo-bridged cluster with a M@M6 arrangement.31 Seven Ni2+ ions are bridged by six µ3OMe, six phenoxo, and six alkoxo groups to form a Ni7 wheel core. Although we confirmed by single crystal X-ray structural analyses that all compounds 2−7 crystallized in the cubic Pa-3 space group, it proved difficult to determine the absolute locations of the metal ions (see discussion parts involving magnetic behavior of 2 to determine its crystal structure). Each of 2−7 showed similar powder X-ray diffraction patterns, demonstrating that they are isostructural with 1 (Figure 1, S1). These compounds were not soluble in suitable solvents for mass spectrum measurements, accordingly average Ni:M:Cl (M = Zn, Co, Mn) ratios were determined by X-ray fluorescent spectroscopy (XRF) measurements (see Experimental Section). The ratios of Ni:M:Cl (M = Zn, Co, Mn) in the powder samples 2−7 were found to be 5.8:1.2:2.0, 3.5:3.5:2.0, 5.6:1.4:2.1, 3.6:3.4:1.9, 5.7:1.3:1.8 and 3.5:3.5:2.0, respectively. Scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDX) elemental mapping showed the co-presence of Ni and Zn for 2, 3, Co for 4, 5 and Mn for 6, 7 in their single particles (Figure 2 and Table 1). These elemental mapping images showed a uniform distribution of metal ions in the particles, corroborating that Zn, Co, and Mn ions were uniformly incorporated into the corresponding compounds.
Variable-temperature direct-current (dc) magnetic susceptibility data for crushed single crystalline samples of 1−7 were collected in the temperature range 2–300 K under an applied field of 5000 Oe. The magnetic data for 1–7 are summarized in Table 2. At room temperature, the χmT value of 8.38 cm3 K mol-1 for 1 is slightly above the expected value for seven uncoupled Ni2+ ions (7 cm3 K mol-1). The room temperature χmT values for 2−7 are 7.74, 5.45, 14.53, 13.53, 14.02 and 17.77 cm3 K mol-1, respectively; these are also larger than spin-only χmT values. In particular, 4 and 5 containing Co2+ ions exhibit much larger values than their spin-only values, likely because of the effects of spin-orbit coupling.32,33 On lowering the temperature, the χmT products for 1−7 gradually increase to reach a maximum of 13.19, 11.57, 9.13, with the presence of ferromagnetic intramolecular interactions in the heptanuclear cores (Figure 3 and Figure S2). ·S6)
We included in the calculation the intermolecular magnetic interaction (zJ) treated using the mean-field approach. The fitting result gave the following parameter values: J1 = +5.78 cm-1, J2 = −2.78 cm-1, gNi = 2.25 and zJ = −0.004 cm-1 with an agreement factor R = 1.2×10-4 [R is defined as R = Σ(χmTexp - χmTcalc)2/Σ(χmTexp)2)]. These values are close to the results of Ni7 cluster (1), J1 = +6.87 cm-1, J2 = −3.41 cm-1, gNi = 2.24.31 Slight differences in the two sets of parameters indicate that the coordination environments around the Ni2+ ions are influenced by the introduction of a Zn2+ ion into the heptanuclear cluster.
According to the information about the location of Zn2+ ion case, intramolecular antiferromagnetic interactions between peripheral Ni2+ ions would occur while of course no magnetic interactions between peripheral Ni2+ ions and the center Zn2+ ion will exist. In the second case (with the Zn2+ ion in a peripheral position), ferromagnetic behavior will result due to ferromagnetic interactions between the Ni2+ ions. In fact, 2 displayed ferromagnetic behavior, indicating that the Zn2+ ion is located in a peripheral position as shown in Figure 4(b). The spin topology of 2 is shown in the following scheme (Figure 5a), assuming that each pairwise interaction is described by following Heisenberg Hamiltonian: Figure 4. Position of the metal ion (M) in the heterometallic cluster Ni6M.
Magnetic interactions in 3 were investigated by a similar manner to that used for 2. The center position is occupied by a Ni2+ ion and not a Zn2+ ion because 3 exhibits ferromagnetic behavior. In this case, three spin topologies need to be considered as shown in Figure 6. A spin topology corresponding to Figure 6(a) including a 1J model was not in accord with the experimental values. On the other hand, the spin topologies shown in Figure 6(b) and 6(c) were in good agreement with the experimental results for 3 (Figure 5b). The Ni7 and Zn7 clusters (rather than Ni6Zn1 or Ni4Zn3 clusters) were present in the crystals of 2 and 3, then magnetic moment values should decrease in inverse proportion to the content of diamagnetic Zn2+ ions present; for example, to 4.9 ×10-5 for Ni7:Zn7 = 6:1 and to 3.3×10-5 emu mg-1 for Ni7:Zn7 = 4:3. These values are not consistent with the observed values for 2 and 3. As a consequence, the di-metallic heptanuclear Ni6Zn1 and Ni4Zn3 clusters occur unambiguously in 2 and 3, respectively.
In the case of 2, two types of spin topologies should be considered; one corresponds to where the Zn2+ is in the central location (as shown in Figure 4a), the other is where it occupies a peripheral position (as shown in Figure 4b). In the former
The data fitting for the spin topology (b) gave the following values: J1 = +6.89 cm-1, J2 = −4.37 cm-1, gNi = 2.36 and zJ = +0.024 cm-1 with an agreement factor R = 2.4×10-5 [R is defined as R = Σ(χmTexp - χmTcalc)2/Σ(χmTexp)2)], while the fitting for the spin topology (c) gave the following values: J1 = +7.81 cm-1, J2 = −2.86 cm-1, gNi = 2.34 and zJ = −0.014 cm-1 with an agreement factor R = 3.1×10-5 [R is defined as R = Σ(χmTexp - χmTcalc)2/Σ(χmTexp)2)].
The field dependence of the magnetizations of 1–7 were measured in the field range –5 to 5 T at 2 K. They showed gradual increases of the magnetization at low fields, without saturation at 5 T for all compounds (Figure 7 and Figure S4). The magnetizations at 5 T for 1−7 are 10.9, 9.6, 7.7, 11.1, 11.2, 15.7 and 17.4 µβ, respectively. 2 and 3 have smaller ground spin state (S ≈ 5 for 2, S ≈ 4 for 3) than incorporation of diamagnetic Zn ions, resulting in decreases in the magnetization values with increasing Zn2+ ion content. The magnetization values for 4 and 5 are close to that of 1 although they have larger total spins (4; St = 15/2, 5; St = 17/2) than 1. This result very likely reflects the magnetic anisotropy (large zero-field splitting (ZFS)) of the Co2+ ion. With respect to this, it is noted that Tong, Herchel and co-workers have reported the large magnetic anisotropy (ZFS parameter; D) for Co2+ in [CoIIICoII6] clusters, obtained by using appropriate spin-
Variable-temperature ac susceptibility measurements for 1, 5 and 7 were performed under zero applied dc field with a 3 Oe ac field oscillating in the frequency range 1−1488 Hz. No outof-phase susceptibility (χ′′) was detected at 1.8 K for 1 and 7 (Figure S5), while 5 displayed a frequency-dependent out-ofphase signal without prominent peaks in χ′ and χ′′ (Figure 8), demonstrating that this cluster shows a slow relaxation of its magnetization. Although a conventional Arrhenius plot could not be carried out for the result involving 5, a modified Arrhenius analysis incorporating the ln(χ”/χ′) vs T−1 plots allowed an estimate of the activation energy (∆) for the magnetization reorientation of 5 (Figure 9). The ∆ value and pre-exponential factor τ0 are defined as ln( χ ) = ln(ωτ ) + ∆/kBT.35 The optimized parameters are ∆/kB τ0 ≈ 6.5 × 10−8 s for 5. Further measurements at lower temperature range (T < 1 K) are required to obtain their precise values. This magnetic slow-relaxation behavior of 5 likely arises from the presence of the magnetic anisotropy of the Co2+ ions.

Conclusions

We have synthesized magnetic heptanuclear clusters of type [Ni7-xMx(HL)6(µ3-OMe)4(µ3-OH)2]Cl2 (M = Zn2+, Co2+, Mn2+). They have alkoxo-bridged structures in which ferromagnetic interaction between the central and peripheral metal ions are observed. We have shown that the Zn2+ ions occupy peripheral positions rather than in the center of the clusters by investigating the magnetic behavior of these clusters in detail. Such local positions of metal ions in dimetallic clusters incorporating metal ions bearing the same oxidation number are difficult to determine by X-ray diffraction. Furthermore, we have demonstrated that the Ni4Co3 cluster shows slow magnetic relaxation phenomenon, leading to a SMM based on the magnetic anisotropies of Co2+ ions.
The present work demonstrates that control of the magnetic properties of heterometallic clusters can be achieved by changing the combination of metals present and provides a useful strategy which will promote both the better understanding and the future development of new magnetic clusters.

Experimental

Synthesis

All reagents were commercially available and used without further purification. 1-(2-Hydroxy-3-methoxybenzamido)-2-(benzylideneamino)ethane (H2L) and [Ni7(HL)6(µ3-OMe)6]Cl2 (1). H2L and 1 were prepared according to methods described previously by us.31 Heterometallic clusters [Ni7-xZnx(HL)6(µ3-OMe)4(µ3-OH)2]Cl2 (x = 1 (2), 3 (3)). Triethylamine (1.0 mmol) in methanol (10 mL) was added with stirring to a methanol solution (30 mL) containing H2L (1.0 mmol) and a mixture of NiCl2·6H2O and ZnCl2. The amounts of Ni2+ and Mn2+ salts employed were adjusted to keep the total number of moles equal to 1.17 mmol and the required Ni2+:Zn2+ molar ratio as expected in the final product: 6:1 (2), 4:3 (3). The reaction mixture was stirred for 2 h at 80 °C in air. Details of the amounts of precursors used in the syntheses are collected in Table S1. Compounds 2 and 3 were obtained as green powders. Both products were recrystallized from CH3OH/CH2Cl2; both solutions were allowed to stand for a few days, during which time green block crystals formed. They were collected by suction filtration, washed with a small amount of diethyl ether, and dried in air. Anal. Calc. for 2·2CH3OH·4H2O: C, 50.54; H, 5.18; N, 6.55. Found: C, 50.32; H, 5.03; N, 6.45%. Anal. Calc. for 3·2CH3OH·4H2O: C, 50.28; H, 5.16; N, 6.51. Found: C, 50.43; H, 5.24; N, 6.41%. XRF for 2: Ni:Zn:Cl = 5.8:1.2:2.0. XRF for 3: Ni:Zn:Cl = 3.7:3.3:2.0.

Physical measurements

Single-crystal X-ray data for 2−7 were recorded on an Oxford Gemini Ultra diffractometer employing graphite monochromated Mo Kα radiation generated from a sealed tube (λ = 0.7107 Å). Data integration and reduction were undertaken with CrysAlisPro. Using CrystalStructure crystallographic software package, the structure was solved with the SHELXT structure solution program using Direct Methods and refined with the SHELXL refinement package using Least Squares minimization. Hydrogen atoms were included in idealized positions and refined using a riding model. Powder X-ray diffraction data (PXRD) were collected on a Rigaku MiniFlex II ultra (30 kV/15 mA) X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5°−30° with a step width of 1.0°. Scanning electron microscope (SEM) images were collected on JEOL JSM-7600F. Elemental analyses (C,H,N) were carried out on a J-SCIENCE LAB JM10 analyzer at the Instrumental Analysis Centre of Kumamoto University. Infrared (IR) spectra measurements were performed on a PerkinElmer Spectrum Two FT-IR equipped with an ATR accessory. Temperature-dependent magnetic susceptibilities for 2−7 between 2 K and 300 K were measured using a superconducting quantum interference 4-MU device (SQUID) magnetometer (Quantum Design MPMS XL) in an external field of 0.5 T. M versus H curves were recorded at 2 K, with a field up to 5 T. The magnetic susceptibility data for 2 and 3 were fitted using the program PHI.36

Notes and references

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