First heterometallic Ga III – Dy III single-molecule magnets: implication of Ga III in extracting Fe – Dy interaction †

The compounds of the system [M 4 M ’ 2 ( μ 3 -OH) 2 ( n bdea) 4 (C 6 H 5 CO 2 ) 8 ]·MeCN, where M = Ga III , M ’ = Dy III ( 2 ), M = Fe III , M ’ = Y III ( 3 ) are isostructural to the known [Fe 4 Dy 2 ] compound ( 1 ). Those of the system [M 4 M ’ 4 ( μ 3 -OH) 4 ( n bdea) 4 ( m -CH 3 C 6 H 4 CO 2 ) 12 ]· n MeCN, where M = Ga III , M ’ = Dy III , n = 4 ( 5 ), M = Fe III , M ’ = Y III , n = 1 ( 6 ) are isostructural to the [Fe 4 Dy 4 ] compound ( 4 ). This allows for comparisons between single ion e ﬀ ects of the paramagnetic ions. The structures were determined using single crystal analysis. Magnetic susceptibility measurements reveal that the Ga III – Dy III compounds 2 and 5 are SMMs. The energy barrier for 2 is close to that for the known isostructural Fe 4 Dy 2 compound ( 1 ), but with a signi ﬁ cantly increased relaxation time.


General procedures
Unless otherwise stated, all chemicals and solvents were obtained from commercial sources and were used without further purification.All reactions were carried out under aerobic conditions.
were prepared according to the literature procedure. 41,42Compound 5 was synthesised by sealing the reaction mixture in transparent 20 mL Biotage Microwave Reaction Kits (http://www.biotage.com) and placing the vials in an oven at 120 °C under normal solvothermal conditions, rather than under microwave conditions.Elemental analyses for C, H, N were performed using an ElementarVario EL analyzer and were carried out at the Institute of Inorganic Chemistry, Karlsruhe Institute of Technology.IR spectra were measured on a Perkin-Elmer Spectrum One spectrometer using KBr pellets and the X-ray powder diffraction patterns were measured at room temperature using a Stoe STADI-P diffractometer with a Cu-Kα radiation.The synthetic procedures for compounds 1 and 4 were previously reported. 40eparation of [Ga 4 Dy 2 (μ 3 -OH) 2 (nbdea) 4 (C 6 H 5 CO 2 ) 8 ]•MeCN (2)  Compound 6 was obtained using the similar procedure as for 3, but using

Physical measurements
X-Ray crystallography.X-ray crystallographic data for 2 and 5 were collected on an Oxford Diffraction SuperNova Ediffractometer using graphite-monochromated Mo-Kα radiation, and data were corrected for absorption.The structures were solved using dual-space direct methods (SHELXT), followed by fullmatrix least-squares refinement against F 2 (all data) using SHELXTL-2014. 43Anisotropic refinement was used for all ordered non-hydrogen atoms.Organic hydrogen atoms were placed in calculated positions, the coordinates of hydroxo hydrogen atoms were either placed in calculated positions or located from the difference Fourier map and then refined with O-H restrained to 0.88(4) Å.
Crystallographic and structure refinement data are summarised in Table 1.Crystallographic data (excluding structure factors) for the structures of 2 and 5 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no.CCDC 1460736 and 1460737.The previously published structures of compounds 1 and 4 have deposition numbers CCDC 1000674 and 1000675. 40agnetic measurements.The magnetic susceptibility measurements were collected on a Quantum Design SQUID magnetometer MPMS-XL.This magnetometer works between 1.8 and 400 K for dc applied fields ranging from −7 to 7 T. Measurements were carried out on finely ground polycrystalline samples constrained with eicosane.The dc magnetic susceptibility data for compounds 2, 3, 5 and 6 were collected in the 1.8-300 K temperature range at 1000 Oe.AC susceptibility measurements were measured with an oscillating ac field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz.The magnetic data were corrected for diamagnetic contributions of the sample holder.

Synthesis and crystal structures
The reaction of Dy(NO 3 ) 3 with the [Ga 3 O] 7+ benzoate triangle and nbdeaH 2 at 80 °C in MeCN afforded the hexanuclear Ga III 4 -Dy III 2 (2), while the octanuclear Ga III 4 Dy III 4 (5) was synthesized at 120 °C under solvothermal conditions for 24 h using Ga(NO 3 ) 3 /Dy(NO 3 ) 3 /nbdeaH 2 /m-CH 3 C 6 H 4 COOH in a molar ratio of 1 : 1 : 8 : 8. Compounds 3 and 6 can be obtained through the identical procedure as that reported for compound 1 or 4. 40 The reactions of [Fe III 3 O] 7+ /Y(NO 3 ) 3 /nbdeaH 2 in a molar ratio of 1 : 1 : 8 in MeCN at room temperature produced compounds 3 and 6, respectively.By comparison of their X-ray powder diffraction patterns (Fig. S1 †) and unit cells, compounds 1-3 were found to crystallise isotypically.Compound 6 was shown to be crystallise isotypically with compound 4, with both having one lattice MeCN per cluster.Although compound 5 has by contrast four lattice MeCN per cluster, the unit cell is in fact rather similar to that of 4 and 6, apart from a corresponding increase in volume, and the packing is also not disimilar.
Single-crystal X-ray diffraction studies reveal that compounds 1-3 crystallise in the monoclinic space group P2 1 /c and compounds 4-6 in the triclinic space group P1 ˉ.

Magnetic properties
The dc magnetic susceptibility data for compounds 2, 3, 5 and 6 were measured in the temperature range 1.8-300 K at 1000 Oe, respectively.For 2 (Fig. 3), the χT product is 27.0 cm 3 K mol −1 at 300 K, which is in relatively good agreement with the value (28.4 cm 3 K mol −1 ) corresponding to two uncoupled Dy III (S = 5/2, L = 5, 6 H 15/2 , g = 4/3) and four Ga III (S = 0) ions.On lowering temperature the χT product continuously decreases, reaching the minimum value of 18.8 cm 3 K mol −1 at 1.8 K.This type of behaviour suggests that the magnetic interaction between the central Dy III 2 unit is weakly antiferromagnetic.The thermal depopulation of the Dy III excited states could also contribute to the decrease of the χT product.In the case of compound 3 (Fig. 3), the χT product of 14.5 cm 3 K mol −1 at 300 K is lower than the expected value of 17.5 cm 3 K mol −1 for four Fe III (S = 5/2, g = 2) and two Y III (S = 0) non-interacting ions, which indicates the presence of antiferromagnetic interactions between spin carriers.The χT value decreases gradually with decreasing temperature, reaching the minimum value of 0.3 cm 3 K mol −1 at 1.8 K, suggesting that the dominant antiferromagnetic interactions between Fe III ions lead to a spin ground state of zero.
However, the magnetic behaviour of the reported isostructral Fe 4 Dy 2 compound 1 indicates the presence of ferromagnetic interactions between spin centres at very low temperature (Fig. 3). 40Since the Dy-Dy and the Fe-Fe exchange interactions are all antiferromagnetic, it can be concluded that ferromagnetic Fe-Dy interactions are revealed at low temperature.This could be one of the origins for the slow relaxation observed in the bulk magnetic data and the Mössbauer spectra for Fe III containing compounds with antiferromagnetically coupled Fe-Fe pairs as seen in compound 1 40 and in the similar Fe 4 Dy 2 compound recently reported. 30,34It is worth to mention that, using Co III as a diamagnetic ion, recent studies have proved the strong magnetic exchange between the Cr III and Dy III ions. 44he field dependences of the magnetisation at low temperatures for compounds 2 and 3 are shown in Fig. S2.† For the Ga 4 Dy 2 compound 2 (Fig. S2, † left), the magnetisation increases rapidly at low fields below 10 kOe, followed by an almost linear increase till 70 kOe.The lack of saturation even up to 70 kOe suggests the thermally and/or field-induced population of low lying excited states, as well as the presence of significant magnetic anisotropy.However, the very low value of 10.1μ B at 2 K and 70 kOe is in good agreement with that expected for two Dy III single ions in polycrystalline samples (each ∼5-6μ B ).The magnetisation measurements with varying scan rate did not show hysteresis.The field dependence of the magnetisation for the Fe 4 Y 2 compound 3 shows a very slow increase with the applied fields and at 2 K only reaches 0.25μ B at 70 kOe (Fig. S2, † right), which confirms the antiferromagnetic coupling between Fe III ions.
To investigate the dynamics of the relaxation, ac susceptibility measurements were performed under a zero dc applied field in the 1-1500 Hz frequency range between 1.8 and 20 K.Both temperature-and frequency-dependent in-phase and outof-phase signals were observed for the Ga 4 Dy 2 compound 2 (Fig. 4 and 5), revealing slow relaxation of magnetization expected for a SMM.In the χ″ vs. T plot, no maximum is observed at lower frequencies.However, clear peaks and some shoulders are observed at higher frequencies (Fig. 4, right).In there is evidence for one peak which develops at temperatures below 1.8 K.This phenomenon suggests the presence of quantum tunnelling effects and at least two additional relaxation processes in this system.The linear fitting of Arrhenius plots (Fig. S3 †) of the data from frequency dependent measurements (Fig. 5, right) give an extracted   energy barrier U eff 20.9 K, which is very close to the value of 21.4 K for the previously reported isostructural Fe 4 Dy 2 compound 1. 40 However, the relaxation time increases significantly from τ 0 = 2.7 × 10 −8 s (for 1) 40 to τ 0 = 1.5 × 10 −5 s (for 2).This behaviour was also observed in the cases of the reported {Cr III 2 -Dy III 2 } and {Co III 2 Dy III 2 } systems, 45,46 indicating possible suppression of the quantum tunnelling.The Cole-Cole plots in Fig. S4 † show nearly symmetric semicircles and were fitted to a generalised Debye function.The resulting α parameter ranges from 0.17 to 0.26 in the temperature range between 1.9 and 7.9 K, indicating a wider distribution of the relaxation time in comparison to the value (α = 0.04-0.13 between 2.4 and 2.9 K) for 1, 40 and the presence of multi-relaxation processes in the system.
A dc field of 1500 Oe was applied to further investigate the relaxation dynamic in compound 2 (Fig. 6 and 7).Clear shoulders are observed in the χ″ vs. T plot (Fig. 6, right).The data from frequency dependent measurements (Fig. 7) were analysed using an Arrhenius law, which gives a characteristic energy barrier U eff of 41.2 K and a relaxation time τ 0 of 2.2 × 10 −6 s (Fig. S5 †).As shown in Fig. S6, † the Cole-Cole plots for 2 at 1500 Oe can be fitted to a generalised Debye function at high temperature, giving large α parameters in the range 0.31-0.38.At low temperature between 1.9 and 5.5 K, the Cole-Cole plots cannot be fitted well, confirming that more than one relaxation process occurs in this system.
The χT values for the Ga 4 Dy 4 5 and Fe 4 Y 4 6 with the "square-in-square" core topology are 55.7 and 18.4 cm 3 K mol −1 at 300 K, which are close to the expected values (56.7 and 17.5 cm 3 K mol −1 ) for non-interacting spin centres: four Dy III (S = 5/2, L = 5, 6 H 15/2 , g = 4/3) and four Ga III (S = 0) ions in 5 or four Fe III (S = 5/2, g = 2) and four Y III (S = 0) ions in 6, respectively.On lowering the temperature from 300 to 50 K, the χT products for both compounds remain almost constant and then rapidly drop to 33.8 and 14.9 cm 3 K mol −1 with further cooling to 1.8 K, respectively (Fig. 8, top).The overall behaviour suggests very weak antiferromagnetic interactions between Dy III centres in 5 or between Fe III centres in 6.As shown in Fig. 8 47 interactions Fe III and the adjacent Dy III centres must be weakly ferromagnetic.The orientation of the anisotropy axis for each Dy III ion in the Ga 4 Dy 4 compound 5 was calculated using the program, MAGELLAN, 47 and shown in Fig. 8 (bottom).All four axes are nearly perpendicular to the Dy 4 plane and nearly parallel to each other, similarly to the situation reported for [Cr III 4 Dy III 4 ], for which the directions of main anisotropy axes were determined from ab initio calculations. 48he field dependent magnetisation measurements at low temperature for 5 show that the magnetisation increases steadily with the application of the external field without saturation even at 70 kOe (Fig. S7, † left).This behaviour indicates the presence of magnetic anisotropy and/or low lying excited states in this system.However, the value of 22.6μ B at 2 K and 70 kOe is in good agreement with the expected saturation value for four Dy III isolated ions in polycrystalline samples (each ∼5-6μ B ).For 6, the magnetisation at 2 K under a field of 70 kOe is 20.7μB (Fig. S7, † right), which is in very good agreement with the presence of four isolated S = 5/2 Fe III ions aligned parallel to the dc field suggesting a possible S = 10 ground state for 6.Although the ac susceptibilities for the Fe III 4 Dy III 4 compound 4 did not show any sign of slow relaxation of the magnetisation, 40 the ac susceptibility measurement for 5 in zero dc field shows weak out-of-phase ac signal with no maximum is observable above 1.8 K (Fig. S8 †).In order to check any quantum tunnelling effects above 1.8 K, the frequency dependence of the ac susceptibility was measured under different applied dc fields at 1.8 K (Fig. S9 †).As shown in Fig. S9 † (right), the maximum in the frequency dependent out-of-phase plot is only slightly moved to lower frequency, indicating the absence of quantum tunnelling effects above 1.8 K.
To study the system further, an external dc field of 1000 Oe was applied and both the in-phase and out-of-phase signals show temperature and frequency dependence (Fig. 9 and 10).Although there is no peak observed in the χ″ vs. T plot (Fig. 9, right), clear peaks are observed in the χ″ vs. v data (Fig. 10, right).Fitting the data using an Arrhenius law leads to an estimation of the energy gap U eff = 5.4 K and relaxation time τ 0 = 4.1 × 10 −5 s (Fig. S10 †).

Fig. 4
Fig. 4 Temperature dependence of the in-phase (χ') (left) and out-of-phase (χ'') (right) ac susceptibility components at different frequencies in zero dc field for 2.

Fig. 5
Fig. 5 Frequency dependence of the in-phase (χ') (left) and out-of-phase (χ'') (right) ac susceptibility components at different temperatures in zero dc field for 2.
(top), the increase of the χT vs. T curve on decreasing the temperature in the 4-16 K temperature range suggests the presence of weak ferromagnetic interactions as observed in the reported isostructral Fe III 4 Dy III 4 compound 4. 40 If both Fe-Fe and Dy-Dy interactions are antiferromagnetic within 4, then the shape of χT vs. T plot for compound 4 at low temperature (Fig. 8, top) leads to the conclusion that the

Table 1
Crystallographic and structure refinement data for compounds 2, 3, 5 and 6