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Nanosized Chiral [Mn6Ln2] Clusters Modeled by Enantiomeric Schiff Base Derivatives: Synthesis, Crystal Structures, and Magnetic Properties Peng Hu,† Xiao-ning Wang,† Cheng-gang Jiang,† Fan Yu,‡ Bao Li,*,† Gui-lin Zhuang,*,§ and Tianle Zhang*,†

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†

Key Laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, School of Chemical and Environmental Engineering, Jianghan University, Wuhan, Hubei 430056, People’s Republic of China § Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310023, People’s Republic of China S Supporting Information *

ABSTRACT: Two enantiomeric pairs of new 3d−4f heterometallic clusters have been built from two enantiomer Schiff base derivatives, labeled as R-/S-H2L, in situ obtained from the condensation reactions with o-vanillin and R-/S-2-phenylglycinol. The formulas of the series clusters are [Mn6Ln2(μ3-OH)4(μ4-O)(Ac)4(H2O)2(R-L)6]·NO3·OH (Ln = Dy (1R), Gd (2R)), [Mn6Ln2(μ3-OH)4(μ4-O)(Ac)4(H2O)2(SL)6]·NO3·OH (Ln = Dy (1S), Gd (2S)), whose crystal structures and magnetic properties have been characterized. Structural analysis indicated that the above clusters consisted of a [Mn6Ln2] core, featuring a sandwich configuration. The results of magnetic measurements showed the presence of slow magnetic relaxation with the effective energy barrier of 14.85 K in two Dy derivatives under the condition of zero-dc field, while the significant magnetocaloric effect of Gd analogues was found in a wide temperature range.

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INTRODUCTION Chirality plays the significant role in many fields as biological, chemical, and materials sciences. In recent years, it has been proved that introducing chiral characteristic into molecular materials would be of the desirable strategy for the assembly of multifunctional molecule-based magnets.1 Normally, chiral molecule-based magnets could be obtained by utilizing enantiopure chiral organic ligands with a specific coordination environment, which would endow astonishing properties, for example, the effect of magnetochiral dichroism (MChD),2 ferroelectric properties,3 or second harmonic generation (SHG).4 In terms of molecule-based magnets, since the first single-molecule magnets (SMM) [Mn12] had been discovered,5 SMMs have received wide attention not only because they could present distinct models for understanding the phenomenon of quantum but also due to their wide potentials in the fields of quantum computing, spintronics, and highdensity information storage.6 Up to now, numerous SMMs, containing 3d, 3d−4f, or 4f polynuclear clusters and mononuclear 3d or 4f single-ion magnets (SIMs) with the anticipated coordination conformation, have been constructed with the aim of increasing the blocking temperature (TB).7 Compared to the pure 3d-based SMMs or SIMs, most of 4f © XXXX American Chemical Society

systems have more significant performance because of crystal field (CF) splitting, strong spin−orbit coupling, and large spin number of 4f ions.8 However, as an inescapable weakness of the contracted 4f orbitals, the extremely weak interactions of magnetic coupling are usually exhibited between the adjacent 4f ions.9 Therefore, considerable efforts have been shifted toward the reasonable construction of 3d−4f clusters to increase the strength of magnetic coupling and suppress the effect of quantum tunneling of magnetization (QTM).10 To effectively transfer the coupling effect of different magnetic centers in SMMs, it has been validated that the selected chelating ligands play the crucial role in constructing 3d−4f clusters with the desired properties. It is generally agreed that chelating ligands should possess multiple coordination sites and flexible skeletons to match the requirements of SMMs. Due to the distinct abilities of coordination sites, two enantiomeric Schiff base derivatives, (R/S)-2-[(2-hydroxy-1-phenylethylimino)methyl]-5-methoxyphenol (R-/S-H2L), in situ obtained during the reactions of R-/ S-2-phenylglycinol and o-vanillin, would be a proper selection Received: May 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Structural view of 1R with partial atoms labeling (hydrogen atoms are omitted for clarity). (b) Coordination environment of [Mn6Dy2] core.

encapsulated in the final structures. The amount of triethylamine must be controlled due to the important role in the deprotonation of R-/S-H2L, and enhances the coordination ability to incorporate two different metal ions together into one cluster. The structures of a series of clusters had been obtained via the analysis of single-crystal X-ray diffraction, and the corresponding crystal data and structural parameters of crystalline states are gathered in Table S1. Crystal Structures of Four Complexes. All of the four clusters exhibit the isostructural characteristic via the analysis of single-crystal and power X-ray diffraction (Table S1, Figure S2), except for the chiral configuration. Herein, only the specific representation of 1R has been structurally described as the example. 1R crystallizes in the tetragonal space group P41, whose asymmetric unit exhibits the complete structure of [Dy2Mn6] in the cluster (Figure 1 and Figure S3). Two Dy ions locate in the center of the cluster and are encompassed by nine oxygen atoms to form the model of distorted tricapped triangular prism (Figure S4). The nine oxygen atoms separately belong to three μ3-OH, one μ4-O, three aliphatic oxygen of different L2− ligands, and two bridging acetates. The Dy−O bond lengths fall in the range from 2.261(5) to 2.647(6) Å, consistent with the ones of other DyIII clusters.14 Two Dy ions are interconnected with each other via sharing one triangular base consisting of two μ3-OH and one μ4-O bridges. The angles of Dy−O−Dy are 91.40(17), 94.93(13), and 95.34(19)°, and the distance of Dy···Dy is 3.601(1) Å. Based on the oxygen atoms of sharing triangular base, two types of dinuclear Mn units are decorated onto the Dy2 template. Two Mn ions in one dinuclear Mn node are bound together through two aliphatic oxygen atoms from different L2− ligands, and adhere on the Dy2 core via two μ3OH, acetate, and aliphatic oxygen groups. The other two are interconnected via a μ4-O atom, and fixed on the Dy2 core through μ4-O and other two μ3-OH groups of different triangular base. These four Mn atoms adopt a distorted octahedral coordination configuration (Figure S4), and chelate with one Schiff base ligand. In addition, another two Mn ions that exhibited the coordination environment of square prism (Figure S4) have been separately settled on the private triangular base of each Dy ion via μ3-OH, acetate, and aliphatic

for the construction of heterometallic clusters. The N−O chelating site shows an inclination for 3d metal ions, and aliphatic hydroxyl favors the hard oxyphilic 4f metal ions.11 The multiple coordination sites and abilities of the derivatives satisfy the assembly requisitions of SMMs, and have been utilized in a series of chiral 3d-based magnetic clusters, such as [CuII6],12 [MnIII8NaI2],12 [MnIIMnIII3NaI],13 and [MnIIMnIII6NaI2],13 which exhibited the absence of any phenomenon of slow magnetic relaxation that is typical for SMMs. Furthermore, very limited 3d−4f or 4f systems have been constructed by the utilization of H2L since it exhibits the excellent coordination ability, let alone the corresponding complexes with slow magnetic relaxation. Subsequently, more and more attention should be paid on constructing novel magnetic systems with slow magnetic relaxation and optical activity to explore the coordination abilities of the derived Schiff base ligands. With all the above considerations in mind, we selected the chiral ligands R-/S-H2L (Scheme S1) to act as the chelating ligands to explore the novel 3d−4f magnetic clusters. Herein, four new chiral 3d−4f heterometallic complexes based on the corresponding chiral ligands R-/S-H2L have been successfully constructed, namely, [Mn6Ln2(μ3-OH)4(μ4-O)(Ac)4(H2O)2(R-L)6]·NO3·OH (Ln = Dy (1R), Gd (2R)), [Mn6Ln2(μ3OH)4(μ4-O)(Ac)4(H2O)2(S-L)6]·NO3·OH (Ln = Dy (1S), Gd (2S)), which exhibit the similar sandwich structures. Magnetic studies reveal that Gd analogues exhibit low values of magnetic entropy change, and Dy ones show the phenomenon of slow magnetic relaxation, which are very rare in the systems containing o-vanillin derivative. Herein, the synthesis, structural descriptions, and magnetic studies of these chiral [Mn6Ln2] clusters have been described as below.

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RESULTS AND DISCUSSION Synthesis of Chiral Heterometallic Clusters. The title series of chiral 3d−4f heterometallic clusters had been synthesized via the reaction of Ln(NO3)3·6H2O (Ln = Dy, Gd), Mn(OAc)2·4H2O, o-vanillin, corresponding 2-phenylglycinol, and triethylamine according to the definite molar ratio in methanol with the yields of 48%. The corresponding enantiomeric Schiff base derivatives in situ generated could be B

DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

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The plots of M vs H/T curves (Figure S8, inset) show the nonsuperimposed curves. All manifest the possibility of significant magnetic anisotropy and/or low-lying excited states of DyIII and MnIII ions.18 No hysteresis could be detected from the plot of M vs H at 2 K. Differently, the measurement condition of magnetization for 2R had been carried out from 0 to 7 T and 1.8 to 14 K. As shown in Figure S9, the tendency of M vs H shows a gradual increase with the change of external magnetic field. At 7 T and 2 K, the low-temperature reduced magnetization approaches 20.07 Nβ, far from the expected values of 38 Nβ, which might be originated from the phenomenon of progressive population of the Zeeman sublevels of low-lying excited states in GdIII ions along with magnetic fields increasing.19 To probe the possibility of magnetic dynamics of magnetization, the data of temperature-dependent alternating current (ac) susceptibility were collected for 1R/1S under zero applied dc field. As shown in Figure 2c and Figure S10, the split curves in the in-phase (χ′) and out-of-phase (χ″) ac susceptibility signals demonstrate the existence of the phenomenon of slow magnetic relaxation, which is the typical characteristic of a single-molecule magnet. However, there were no obvious peaks observed, due to the fast QTM via the spin reversal barrier. To overcome the degeneracy of ground state and suppress the effect of QTM, the data of temperaturedependent ac susceptibilities have been collected again with the same condition excepted for the application of the external dc field of 2000 Oe (Figure S11). Similar plots of χ′ and χ″ vs T had been obtained with the comparison of data under a zerodc field, indicating the ineffective suppression of QTM by the application of an external field. Similar magnetic behavior has been commonly observed in other polynuclear 4f and 3d−4f clusters consisting of eight- or nine-coordinated lanthanide ions,20 but rarely observed in the systems consisting of L2−. In this situation, the relaxation time (τ0) and effective energy barrier (Ueff) cannot be simulated through the Arrhenius method. But supposing only one relaxation process is exhibited in 1R, the rough values of τ0 and Ueff could be fitted by the model of Debye with the equation ln(χ″/χ′) = ln(ωτ0) + Ea/ kBT.21 The best fitting values could be obtained via using the data of frequencies 97, 396, and 597 Hz, which gives the Ueff of 14.85 K and τ0 of 2.38 × 10−7 s (Figure S12). The τ0 value is consistent with the typical range for SMMs (10−6 to 10−11 s).22 The relatively small energy barrier might be caused by the low symmetry of the coordination configuration of DyIII ions in the final structure. The magnetic entropy change ΔSm parameter was also explored for 2R based on magnetization data (Figure S9) by applying the Maxwell equation of ΔSm(T)ΔH = ∫ [∂M(T,H)/ ∂T]H dH.23 The calculated maximum value of −ΔSm has been calculated as 10.3 J kg−1 K−1 at 6 K and 7 T (Figure 2b). As far as we know, the maximum of the −ΔSm for GdIII-based clusters is usually located at 2 K and shows the tendency of decrease with increasing temperature. Herein, for the Gd cluster, the peak has shifted to 6 K, which is possibly owing to the antiferromagnetic exchange and/or the magnetic anisotropy of high spin that split the spin multiples.19 Although the maximum value of −ΔSm is smaller, which could be ascribed to the antiferromagnetic coupling and the evidently unsaturated magnetization, it may offer a way to acquire a large magnetocaloric effect (MCE) at high temperature and under a small field.

oxygen groups. In this way, the [Dy2Mn6] core had been constructed, whose periphery is further decorated with six L2− with a [2110.3] model according to the Harris notation,15 four syn-syn acetate bridges (Scheme S2), two coordination water, and counteranions. The bonds distance of Mn−O and −N ranges from 1.871(5) to 2.379(6) Å. Referring to the results of bond valence sum analysis16 (Table S2) and balance the charge of the whole cluster, all of Mn ions should be defined as a MnIII state, which were generated upon oxidation by atmospheric O2 from the exclusive starting sources of MnII ions. The distances of adjacent Dy···Mn and Mn···Mn fall in the range of 3.347(23)−3.982(38) Å and 3.258(32)−6.753(26) Å. What’s more, the dimensions and thickness of the whole clusters are measured as 1.68 × 1.73 and 2.08 nm according to the spacefilling model, indicating the presentation of nanoclusters (Figure S5). Magnetic Properties of Four Clusters. The magnetic properties of four complexes were investigated by utilizing the crystalline samples from the reaction solution. The two Gd or Dy enantiomers exhibited similar magnetic behaviors (Figure 2, Figures S6−S12), and herein the properties of 1R and 2R

Figure 2. (a) Plots of χMT vs T for 1R and 2R. (b) Experimental −ΔSm of 2R at various temperatures and magnetic field change. (c) Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility data for 1R under a zero-dc field.

have been detailed. At room temperature, χMT values of complexes 1R and 2R are 45.25 and 29.97 cm3 mol−1 K, respectively, similar to the expected parameters of 46.34 and 33.76 cm3 mol−1 K for six uncoupled MnIII (S = 2, g = 2) ions and two respective LnIII ions. Upon cooling to 75 K, the χMT product of 1R keeps a constant, and then decreases rapidly to 34.82 cm3 mol−1 K at 2 K, which might be ascribed to a combination of the depopulation of the excited Stark sublevels and non-negligible antiferromagnetic coupling between the adjacent metal ions.17 For complex 2R, the χMT curve shows a similar tendency. The χMT decreases smoothly along with the decreasing of temperature to 30 K, and then distinctly drops to 18.25 cm3 mol−1 K at 2 K (Figure 2a). The plots of M vs H for complexes 1R at 2−5 K were obtained (Figure S8). From 0 to 1.5 T, the values of M increase rapidly, and become slowly above 1.5 T. At 7 T and 2 K, the value of M reaches 30.18 Nβ, which is smaller than the theoretical 44 Nβ, indicating the unsaturated magnetization. C

DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

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= 4.85 cm−1, D2 = 1.24 cm−1, and R = 1.32 × 10−4. Weak ferromagnetic coupling (0.12 cm−1) between GdIII ions is associated with the bond-distance difference of two Gd−O bonds (2.503 and 2.546 Å). It well matched with those result obtained experimentally27 and theoretically.28 Bridges of Gd− Mn and Mn−Mn feature very weak antiferromagnetic coupling, coincident with experimental results.29 Two positive D values of Mn(III) ions are associated with compression effects of coordination geometry.29 The weak ferromagnetic coupling between the adjacent Gd ions might be responsible for the presentation of described MCE behaviors. It is wellknown that larger MCE at high temperature and under small magnetic field can indeed be caused by a negative ZFS parameter (D) and ferromagnetic coupling, which favor a large spin ground state. However, for compound 2R, a positive D value arose from MnIII ions (DMn(III) = 3.91 cm−1). Therefore, the magnetocaloric behavior is most likely due to ferromagnetic coupling between the adjacent Gd ions; the small values of magnetic entropy change might be attributed to the dominant antiferromagnetic coupling among the whole molecule. Circular Dichroism (CD) Spectrum of Clusters. To validate the chiral characteristic of the [Mn6Ln2] heterometallic complexes, the enantiomeric essence of 1R and 1S was investigated via the spectra of circular dichroism (CD) in solution and the solid state (Figure 4 and Figure S16). In the

In addition, similar magnetic behaviors of 1S and 2S could be also observed, and the corresponding behaviors have been listed in the Supporting Information (Figure S7), indicating that the introduction of chiral factor into 3d−4f clusters brings about less effect to control the magnetic anisotropy and coupling in these clusters. To probe the magnetic property mechanism of the cluster and set up the corresponding magnetic models, magnetic exchange parameters of the [Mn6Gd2] core were investigated by fitting the curve of variable-temperature dc susceptibility (Figure 3a). However, it

Figure 3. (a) Variable-temperature susceptibility curves (χMT vs T and χ−1 vs T) of polycrystalline samples of 2R with experiment (scatter) and simulation (red solid line for QMC simulation; blue solid line for Curie−Weiss law fitting). (b) Geometrical connecting mode of magnetic ions.

is very difficult to conduct the traditional method of irreducible tensor operator (ITO)24 because of the huge energy matrix of 106 × 106. Therefore, we applied a combination of Genetic Algorithm25 with the method of Quantum Monte Carlo (QMC), involving directed loop algorithm with stochastic series expansion representation of ALPS.26 Inspecting the whole structure of 2R, magnetic property is simplified as a mixed contribution from GdIII−GdIII (d(Gd···Gd) = 3.60 Å) with two μ3-OH and one μ4-O bridges (J1), MnIII−MnIII (d(Mn···Mn) = 3.282−3.447 Å) with μ3-OH bridges (J2), and GdIII−MnIII (d(Gd···Mn) = 3.461−3.864 Å) with syn-syn carboxylate, μ3-OH, and μ3-Oligand (J3) (Figure 3b and Figure S13). Concerning the coordination-geometry difference, two single ion anisotropies (D1 and D2) of MnIII ions were taken into account. The whole Hamilton operator is presented as eq 1.

Figure 4. CD spectra of 1R and 1S at 298 K (2 × 10−5 M, CH3OH).

solution of methanol, the spectra of 1R presents the positive effect of Cotton with λmax equals to 280 and 410 nm, and the negative peak at λmax = 310 nm is associated with the π−π* transitions. Comparably, 1S exhibits the effect of Cotton at the opposite positions with the same wavelengths. In addition, the solid-state CD spectrum also exhibits the good symmetry peaks similar to the signals in solution. The signals in CD spectra for two clusters present mirror-symmetric shapes, indicating the corresponding enantiomers.

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CONCLUSIONS In summary, a series of enantiomeric pairs of 3d−4f heterometallic [Mn6Ln2] clusters with enantiomer Schiff base ligands have been constructed and structurally characterized. Magnetic results declare the dominant existence of antiferromagnetic coupling between the adjacent metal ions. The Dy derivatives should be considered as the types of SMMs due to the presentation of slow magnetic relaxation under zero applied dc field. The Gd analogues present the interesting effect of magnetocaloric in the high temperature range. All of

H = −2J1∑ SGdiSGdj − 2J2 ∑ SMniSMnj − 2J3∑ SGdiSMnj 2 2 + D1∑ SMniz + D2∑ SMniz

(1)

Following the least reliability factor R (∑[(χMT)obs − (χMT)calcd]2/∑[(χMT)obs]2), the best set of optimal parameters after 200 evolution generations are presented as follows: J1 = 0.12 cm−1, J2 = −0.24 cm−1, and J3 = −0.07 cm−1, g = 1.96, D1 D

DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

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these results would provide a viable pathway for the construction of expected chiral 3d−4f molecule-based magnets, which could act as bifunctional molecular materials in nanoscale.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01423. The detailed experimental description, characterization, and physical measurements (PDF) Accession Codes

CCDC 1837730−1837733 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.). *E-mail: [email protected] (T.Z.). *E-mail: [email protected] (G.-l.Z.). ORCID

Bao Li: 0000-0003-1154-6423 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471062, 21371064) and the Open Project of the State Key Laboratory of Physical Chemistry of the Solid Surface (Xiamen University) (201616). We gratefully acknowledge the Analytical and Testing Center, Huazhong University of Science and Technology, for analysis and spectral measurements. We also thank the staffs from BL17B beamline of the National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility, for assistance during data collection.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01423 Inorg. Chem. XXXX, XXX, XXX−XXX