Syntheses, structures and magnetic properties of symmetrical Schiff-base supported dinuclear Ln(III) compounds

Abstract

Three dinuclear Ln(III) compounds, [Ln₂(H₃L)₂(PhCOO)₆] (Ln = Sm (1), Dy (2)) and [Dy₂(H₄L′)₂(PhCOO)₄]·2CH₃CN (3) have been synthesized and structurally and magnetically characterized. Structure analyses revealed distinct structural features in compounds with different symmetrical arms of Schiff-base ligands. With the symmetrical two-arm Schiff-base ligand H₃L, dinuclear Ln(III) compounds 1 and 2 were synthesized, where nine-coordinated Ln(III) ions were bridged by two syn–syn η¹:η¹-µ₂-benzoate groups. When using symmetrical Schiff-base ligand H₅L′ with four arms, the dinuclear Dy(III) compound 3 was prepared, in which eight-coordinated Dy ions were bridged by two alkoxido oxygen atoms from additional arms of H₅L′. Direct-current (DC) magnetic susceptibility studies revealed that the Dy(III) ions were very weakly coupled in dysprosium compounds. Alternating-current (AC) magnetic susceptibility studies for compounds 2 and 3 indicated that field-induced slow relaxation phenomenon occured for both compounds. Furthermore, two relaxation phases under the optimal field appeared in compound 3, which are probably associated with the existence of the anisotropic Dy(III) ion for the slow relaxation phase (SR) and significant quantum tunneling for the fast relaxation phase (FR).

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Introduction

Interest in the design and synthesis of new molecular magnetic materials still attracts much attention because of their great potential applications for high-density data storage and nanosized electronics.¹ In this field, lanthanide ions, particularly the heavy lanthanide ions such as Tb(III) and Dy(III), have proven to be very attractive to design molecular magnetism with unique structures and interesting magnetic behaviour due to their large magnetic moments and typically large magnetic anisotropy.² It has indisputably led to the largest number of high anisotropic barrier 4f single-molecule magnets (SMMs) with various nuclear centers.^1a,3 However, discovering magnetostructural correlations and rationalizing the magnetic behaviour of highly anisotropic systems for SMMs is still a significant challenge, which defines a significant impetus for the continuous development of a comprehensive (quasi)isostructural series of complexes. Thus far, successful examples have rarely been achieved in lanthanide SMMs, such as the [Tb(Pc)₂],⁴ Dy₁ series,⁵ Dy₂ compounds⁶ and Dy₆ compounds,⁷ which demonstrated that the ligands played essential roles to achieve SMMs by ways of ligand field and defined geometries. Nevertheless, most multinuclear lanthanide SMMs synthesized to date are obtained through a serendipitous approach based on versatile multidentate ligands,⁸ which is due to the variable and high coordination numbers and the weak stereochemical preferences of lanthanide ions. Thus, in order to improve our knowledge of the structure–property relationships of lanthanide-based SMMs, it is still necessary to design novel structures to enlarge the available database.

The structure of the selected ligand plays a crucial role in forming novel frameworks and tuning magnetic properties.^3a,9 Based on the theory of hard and soft acids and bases, lanthanide ions tend to coordinate with the O-donor ligands. Thus, it is reasonable to design the ligands containing multiple O-donors such as hydroxyl, carbonyl or carboxyl groups for constructing diverse lanthanide compounds. Among numerous ligands, the salen type with rich oxygen and nitrogen donors can stabilize Dy(III) ions in various coordination environments, which exhibit unique molecular structure displaying distinct mononuclear to tetranuclear anisotropic centers.¹⁰ A similar but more donor type ligand, DFMP (2,6-diformyl-4-methylphenol)-based symmetrical Schiff-base ligand, presents a more versatile coordination ability in lanthanide compounds.¹¹ Herein, we synthesized such a symmetrical DFMP-based ligand (H₃L) with two arms, and designed a new symmetrical ligand (H₅L′) containing four arms by introduction of propanediol groups (Scheme 1). Three dinuclear compounds, [Ln₂(H₃L)₂(PhCOO)₆] (Ln = Sm (1), Dy (2)) and [Dy₂(H₄L′)₂(PhCOO)₄]·2CH₃CN have been obtained. Magnetic property studies indicated that the Dy(III) ions are weakly coupled. It is noteworthy that the Dy₂ compounds (compounds 2 and 3) displayed field-induced slow magnetic relaxation.

Scheme 1 Structural representation of the ligands H₃L and H₅L′.

Experimental

General

All chemicals and solvents used for the syntheses were of A.R. Grade and were used without any further purification. Samarium benzoate, dysprosium benzoate and 2,6-diformyl-4-methylphenol (DFMP) were prepared according to a previously published method.¹² The Schiff-base ligands H₃L and H₅L′ were formed in a one pot route by the in situ condensation of DFMP and respective ethanolamine and 2-amino-2-methyl-1,3-propanediol in a 1 : 2 ratio in the presence of the Ln(III) ion.

Elemental analyses for C, H and N were carried out on a Perkin-Elmer 2400 analyzer. IR spectra (4000–300 cm⁻¹) were measured using KBr pellets by a Nicolet 6700 Fourier transform infrared spectrometer. All magnetization data were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The variable-temperature magnetization was measured with an external magnetic field of 1000 Oe in the temperature range of 2–300 K. The experimental magnetic susceptibility data are corrected for the diamagnetism estimated from Pascal's tables and sample holder calibration.

X-ray crystallography

Suitable single crystals were selected for single-crystal X-ray diffraction analysis. Crystallographic data were collected at 185(2) K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined on F² with full-matrix least-squares techniques using SHELXS-97 and SHELXL-97 programs.¹³ The locations of lanthanide ions were easily determined, and O, N, and C atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The H atoms were introduced in calculated positions and refined with a fixed geometry with respect to their carrier atoms. Crystallographic data and refinement details are given in Table 1. CCDC 1487903–1487905 contain the supplementary crystallographic data for this paper.

Table 1 Crystal data and structure refinement for compounds 1–3

Compound	1	2	3
Empirical formula	Sm2C68H66N4O18	Dy2C68H66N4O18	Dy2C66H76N6O18
FW (g mol^−1)	1527.95	1552.25	1566.33
Crystal system	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P21/n
a (Å)	11.1124(5)	11.1487(7)	13.988(2)
b (Å)	11.4404(5)	11.3872(7)	15.402(2)
c (Å)	13.6416(7)	13.6878(8)	15.741(2)
α (^o)	94.291(1)	94.287(1)	90
β (^o)	112.910(1)	113.068(1)	98.823(2)
γ (^o)	97.524(1)	97.695(1)	90
V (Å^3)	1568.60(13)	1568.86(17)	3351.1(8)
Z, ρcalcd (mg m^−3)	1, 1.618	1, 1.643	2, 1.552
F(000), Rint	770, 0.0244	778, 0.0167	1580, 0.0520
R 1, wR2 [I > 2σ(I)]	0.0338, 0.0721	0.0248, 0.0642	0.0374, 0.0830
R 1, wR2 (all data)	0.0428, 0.0765	0.0272, 0.0659	0.0573, 0.0933
GOF	1.024	1.045	1.029

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Synthesis of the compounds

Synthesis of [Sm₂(H₃L)₂(PhCOO)₆] (1). Sm(C₆H₅COO)₃ (0.3 mmol, 156.8 mg) was added to the solution of DFMP (0.15 mmol) and ethanolamine (0.3 mmol) in 15 mL of MeOH in the presence of Et₃N (0.3 mmol). The resulting mixture was stirred for about 2 h at room temperature and then the mixture was filtered and the filtrate was left undisturbed and concentrated by slow evaporation at room temperature. After several days, yellow single crystals of 1, suitable for X-ray diffraction analysis were formed. Yield: 26 mg (23%, based on the DFMP). Elemental analysis (%) calcd for Sm₂C₆₈H₆₆N₄O₁₈: C 53.45, H 4.35, N 3.67; found: C 53.12, H 3.99, N 3.36. IR (KBr, cm⁻¹) shown in Fig. S1:† 3364 (m), 3068 (w), 2934 (m), 2890 (w), 1650 (s), 1632 (m), 1600 (s), 1536 (vs), 1492 (m), 1440 (w), 1403 (vs), 1303 (w), 1207 (w), 1074 (w), 1054 (w), 1023 (w), 848 (m), 717 (m).

Synthesis of [Dy₂(H₃L)₂(PhCOO)₆] (2). This compound was obtained by a similar procedure as described for 1, substituting Dy(C₆H₅COO)₃ for Sm(C₆H₅COO)₃. Yield: 20 mg, (17%, based on the DFMP). Elemental analysis (%) calcd for Dy₂C₆₈H₆₆N₄O₁₈: C 52.61, H 4.28, N 3.61; found: C 52.32, H 3.98, N 3.29. IR (KBr, cm⁻¹) shown in Fig. S1:† 3392 (m), 3069 (w), 2936 (m), 1651 (m), 1633 (m), 1597 (s), 1538 (vs), 1493 (m), 1441 (w), 1404 (s), 1303 (w), 1208 (m), 1073 (w), 1056 (w), 1023 (w), 850 (w), 717 (m).

Synthesis of [Dy₂(H₄L′)₂(PhCOO)₄]·2CH₃CN (3). Dy(C₆H₅COO)₃ (0.1 mmol, 53.4 mg) was added to the solution of DFMP (0.1 mmol) and 2-amino-2-methyl-1,3-propanediol (0.2 mmol) in 15 mL MeOH in presence of Et₃N (0.2 mmol). The resulting mixture was stirred at room temperature and then filtered and the filtrate was left undisturbed. After several days, yellow single crystals of compound 3 were formed. Yield: 23 mg, (29%, based on the DFMP). Elemental analysis (%) calcd. for Dy₂C₆₆H₇₆N₆O₁₈: C 50.61, H 4.89, N 5.36; found: C 50.42, H 4.23, N 5.01. IR (KBr, cm⁻¹) shown in Fig. S1:† 3335 (m), 2936 (w), 2361 (m), 2342 (m), 1653 (s), 11 594 (m), 1543 (vs), 1496 (m), 1404 (s), 1385 (s), 1231 (m), 1071 (s), 1031 (w), 991 (w), 826 (w), 714 (m).

Results and discussion

The symmetrical Schiff-base ligand H₃L with two arms was prepared by the condensation of DFMP and ethanolamine (1 : 2) in methanol. The reaction of H₃L with samarium(III) benzoate and dysprosium(III) benzoate in the presence of Et₃N produced compounds 1 and 2, respectively.

Single crystal X-ray diffraction analyses revealed that compounds 1 and 2 were crystallographically isostructural, with the same dinuclear [Ln₂(H₃L)₂(PhCOO)₆] (Ln = Sm (1), Dy (2)) core; hence, the structure of compound 2 was selected and described as a representative (Fig. 1). Compound 2 crystallized in the triclinic space group P1̄ with Z = 1. The dinuclear dysprosium compound was composed of two [Dy(H₃L) (PhCOO)₂] units with the distance of Dy⋯Dy 5.3316(3) Å that was bridged by two syn–syn η¹:η¹-µ₂-benzoate groups. The ligand H₃L serves as a tridentate ligand and chelates the Dy atom through one phenoxido oxygen atom, one imine nitrogen atom and one alkoxido oxygen. It was remarkable that the terminal ligands of H₃L, instead of bridging benzoate ligands, prevented the formation of a one-dimensional chain and resulted in the isolation of a discrete Dy₂ compound, which was similar to the discrete linear Dy₄ complex.¹⁴ Two η² chelating benzoate ligands completed the coordination sphere of the Dy ion, generating a nine-coordinate center. The exact geometry was determined using the SHAPE 2.1 software¹⁵ and the result indicated that the nine-coordinate Dy center was situated in a distorted coordination sphere between a tricapped trigonal prism (i.e., the basal planes of atoms O1, O9A, N1 and O5, O8, O7 (symmetry code: 1 − x, −y, 1 − z)) and capped square antiprism (i.e., the basal planes made up of atoms N1, O9A, O8, O7 and O1, O4, O5, O6) (Table S1† and Fig. 2). In addition, the distances of Dy–O were in the range of 2.2817(19)–2.583(2) Å and the Dy–N bond length was 2.504(2) Å with all of these being normal coordination bonds.

Fig. 1 Partially labelled crystal structure of compound 2 with hydrogen atoms omitted for clarity.

Fig. 2 Two possible coordination polyhedra for Dy ions observed in compound 2: tricapped trigonal prism (a); capped square antiprism (b).

For compound 1 (Fig. S2†), the distance of Sm⋯Sm was 5.3991(4) Å. The distance of the Sm–N bond length was 2.555(3) Å and that of Sm–O were in the range of 2.314(3)–2.616(3) Å.

Examination of the crystal packing revealed that both molecules of 1 and 2 were in contact through π–π interactions and the separation distances of ring centroids Cg⋯Cg were 3.6569(2) and 3.6667(2) Å for compounds 1 and 2, respectively. Therefore, infinite supramolecular chains were generated with the shortest intermolecular Ln⋯Ln distances of 10.2063(5) and 10.2332(5) Å for 1 and 2, respectively (Fig. 3 and S3†).

Fig. 3 Crystal packing of compound 2 showing the formation of a supramolecular chain through π–π interactions. The distance Cg3⋯Cg4 is 3.6667(2) Å.

To explore the effect of additional alkoxido hydroxyl groups on the lanthanide compound, the symmetrical Schiff-base H₅L′ with four arms was synthesized by the condensation of DFMP and 2-amino-2-methyl-1,3-propanediol (1 : 2) in methanol. Similar to the procedure for compounds 1 and 2, the reaction of H₅L′ with dysprosium(III) benzoate in the presence of Et₃N produced compound 3.

As shown in Fig. 4, compound 3 crystallized in the monoclinic space group P2₁/n with Z = 2. The dinuclear dysprosium compound was composed of two [Dy(H₄L′) (PhCOO)₂] units bridged by two alkoxido oxygens from two additional arms. In contrast to the H₃L with two arms in compounds 1 and 2, the ligand (H₄L′)⁻ with four arms served as a tetradentate ligand and chelated the Dy atom through one phenoxido oxygen atom, one imine nitrogen atom and two alkoxido oxygens. The alkoxido oxygens (O5 and O5a) from additional arms bridged two Dy ions, leading to a nearly rhombic Dy₂O₂ core with a Dy–O–Dy angle of 108.45(13)° and a Dy⋯Dy distance of 3.6850(6) Å. Similar to compounds 1 and 2, only one set of side arms of the ligand were coordinated, whereas the other side arms remain uncoordinated, which may be further used to build a distinct complex. In addition, one bidentate (in η² form) chelating benzoate ligand and one monodentate (in η¹ form) benzoate ligand completed the coordination sphere of the Dy ion, generating an eight-coordinate center. The exact geometry was also determined using the SHAPE 2.1 software.¹⁵ However, the results indicated that the nine-coordinate Dy center was not situated in a traditional coordination sphere listed in SHAPE 2.1 (Table S1† and Fig. 5). The coordination sphere of the Dy center may be close to a hula-hoop-like geometry, but a distorted hula-hoop. Moreover, the lengths of the Dy–O bonds were in the range of 2.265(3)–2.514(4) Å, and that of the Dy–N bond was 2.496(4) Å.

Fig. 4 Partially labelled crystal structure of compound 3 with hydrogen atoms omitted for clarity.

Fig. 5 Coordination polyhedra for Dy1 and Dy1A ions observed in compound 3.

Magnetic properties

To study the possible magnetic interactions of the compounds, temperature-dependent magnetic susceptibility measurements were performed on the polycrystalline sample in the range of 2–300 K under an applied magnetic field of 1000 Oe. The plots of χ_MT versus T for compounds 1–3 are shown in Fig. 6.

Fig. 6 Plots of χ_MT vs. T for compounds 1 (top), 2 and 3 (bottom) in a dc field of 1000 Oe (2–300 K).

For compound 1, the χ_MT value observed at room temperature (300 K) was 0.767 cm³ kmol⁻¹, which is significantly larger than the theoretical one (0.178 cm³ kmol⁻¹). This is because the first or even higher excited states (⁶H_7/2, ⁶H_9/2, …, ⁶H_15/2) of the Sm ion can certainly be populated at room temperature.¹⁶ As the temperature was lowered, the χ_MT values of compound 1 decreased rapidly to 0.062 cm³ kmol⁻¹ at 2.0 K (Fig. 6).

For compounds 2 and 3, the χ_MT values at room temperature were 28.30 and 28.28 cm³ kmol⁻¹, which are very close to the expected value of 28.34 cm³ kmol⁻¹ for two isolated Dy(III) ions (⁶H_15/2, g = 4/3). On cooling, the χ_MT values of 2 and 3 decreased slightly from 300 to 80 K and then further decreased by different degrees. For 2, the χ_MT values decreased slowly to a minimum of 23.85 cm³ kmol⁻¹ at 2 K. Comparing the magnetic properties of compound 2 with a noninteracting complex Dy₄,¹⁴ we found that the χ_MT value of compound 2 almost agreed well with the half χ_MT value of complex Dy₄ at low temperatures (½χ_MT = 23.9 cm³ kmol⁻¹ at 2 K), indicating insignificant exchange interactions between the Dy(III) ions in compound 2. In contrast, the χ_MT product for compound 3 decreased more steadily than that of compound 2, reaching a minimum of 19.93 cm³ kmol⁻¹ at 2 K, which suggests the presence of weak antiferromagnetic interactions in compound 3. In general, the decrease of χ_MT values was associated with the progressive depopulation of the excited m_J sublevels of the Ln(III) ions and/or the weak antiferromagnetic interactions between the Ln(III) ions.¹⁷ Thus, the decrease of the χ_MT values of compound 2 was mainly attributed to the progressive depopulation of the Dy(III) ions, whereas that of compound 3 was attributed to both factors.

To the best of our knowledge, Dy₂ complexes with dialkoxo and dicarboxylato bridging groups (in η¹:η¹-µ₂ coordination mode) have rarely been reported with SMM behavior, and the former complexes usually showed interesting magnetic properties (Table S2†).

To probe the low-temperature magnetic relaxation behaviour, alternating current (AC) magnetic susceptibility measurements were performed for compounds 2 and 3. Under a zero DC field, both compounds 2 and 3 did not exhibit significant slow magnetic relaxation since it is hard to observe an appreciable out-of-phase signal (χ″) at frequencies of up to 997 Hz and at temperatures down to 1.9 K (Fig. S4†). This may be attributed to a fast zero-field quantum tunnelling of magnetizations (QTM), which obliterates the advantage of anisotropic lanthanide ions. This was commonly reported for lanthanide-containing compounds. The high axial coordination geometry around the Dy(III) ions has much influence on the suppression of zero-field QTM and plays a key role for the SMMs with a high barrier, such as the approximate D_4d symmetry in [TbPc₂]¹⁸ and the axial hula hoop-like geometry in asymmetric Dy₂ compounds¹⁹ and the Dy₆ triangular prism.²⁰ On the contrary, the fast QTM observed in compounds 2 and 3 indicated an absence of the highly axial nature of the Dy(III) ions. It is likely that the severely distorted geometry of nine-coordinate Dy in 2 and eight-coordinate Dy in 3 (Fig. 2 and 5) might result in greater importance of the transverse anisotropy terms,²¹ thus largely affecting the dynamic behaviour of both the compounds.

On the other hand, the application of a DC field may probably remove the degeneracy of the ±M_J energy levels and suppress the QTM from the +M_J state to the −M_J state.²² Indeed, field dependent signals for both compounds 2 and 3 were clearly observed at 997 Hz and 2 K with a significant peak around the optimal DC field, indicating that field-induced slow magnetic relaxation was operating (Fig. 7). The optimal fields were 300 and 1300 Oe for compounds 2 and 3, respectively. Specifically, the ratios of the intensities of χ″/χ′ were about 1 : 14.3(0.07) and 1 : 4.9(0.2) at the optimal DC field for compounds 2 and 3, respectively. The latter ratio value shows almost a 3 fold over the former. Thus, the AC susceptibility was measured with the application of the optimal field (1300 Oe) for compound 3 to further investigate the relaxation behaviour.

Fig. 7 Dependence of the out of phase signal of compounds 2 and 3 on applied DC field strength at 2.0 K and 997 Hz.

From the temperature-dependent AC susceptibility under 1300 Oe (Fig. 8 and S5†), temperature-dependent χ′ and χ″ signals were observed with good peak shapes at low temperature, indicating the presence of slow magnetic relaxation under the optimal field, typical of field-induced SMM behaviour. On the other hand, the frequent-dependent AC susceptibility showed two maximal values at low temperature (<3.0 K) and one maximal value at high temperature (>3.0 K), which corresponded to two and one relaxation phases, respectively. The two relaxation phases at low temperature contained the high-frequency signal (fast relaxation phase, FR) and the low-frequency region (slow relaxation phase, SR).

Cole–Cole diagrams under 1300 Oe (Fig. S6†) showed two arc shapes at a low temperature, which were also indicative of the occurrence of the two relaxation processes. It should be noted that multiple relaxation processes for most lanthanide systems are due to the presence of crystallographically independent lanthanide centers and/or to significant quantum tunnelling. Hence, the SR was attributed to the presence of a unique crystallographic Dy ion in the dinuclear structure. The FR was indicative of quantum tunnelling, as the high-frequency peaks were almost temperature independent (shown in the upper part of Fig. 8). In addition, attempts to fit the data by the sum of two modified Debye functions were unsuccessful due to the fact that high frequency and low frequency peaks of the χ″ signals were not observed within the available frequency range of the AC measurements.

Fig. 8 Temperature dependence (upper) and frequency dependence (bottom) of the out-of phase AC susceptibility of compound 3 below 8.0 K, under a 1300 Oe DC field.

The above analysis confirms the importance of a correct analysis of structure–property relationships, particularly the coordination polyhedra on the anisotropy nature of lanthanide ions, which may have steered our efforts to achieve high magnetic axiality by designing the local environment. Furthermore, using the same type Schiff-base ligands with differences in some groups may be an advantageous synthetic methodology to synthesize a comprehensive (quasi)isostructural series of complexes and further analyse the structure–property relationships.

Conclusions

In conclusion, three dinuclear Ln(III) compounds incorporating lanthanide ions (Ln = Sm and Dy) and symmetrical multidentate ligands have been successfully synthesized and characterized. Structural analyses reveal that the introduction of different symmetrical arms in Schiff-base ligands produced distinct dinuclear Ln(III) compounds. Compounds 1 and 2 were synthesized using the symmetrical ligand H₃L with two arms where each center was nine-coordinated with a distorted coordination sphere between a tricapped trigonal prism and capped square antiprism, which were bridged by two syn–syn η¹:η¹-µ₂-benzoato groups. When using a symmetrical H₅L′ ligand with four arms, the dinuclear Dy(III) compound 3 was prepared, in which eight-coordinated Dy ions were bridged by two alkoxido oxygen atoms from the additional arms of H₅L′. Magnetic studies revealed that all compounds exhibited weak magnetic interactions, whereas compounds 2 and 3 exhibited a field-induced slow relaxation phenomenon. Furthermore, two relaxation phases under the optimal field appeared in compound 3, which were probably associated with the existence of an anisotropic Dy ion and significant quantum tunnelling. The discussion indicates that the synthetic methodology of employing the same type Schiff-base ligands with difference in some groups may represent a promising new route toward the design of new lanthanide clusters.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (no. 21401034) and the Natural Science Foundation of Heilongjiang Province (QC2015008).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1487903–1487905. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra16669e

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