Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Assembly of Lanthanide(III) Cubanes and Dimers with SingleMolecule Magnetism and Photoluminescence Ho-Yin Wong, Wesley Ting Kwok Chan, and Ga-Lai Law* Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon 999077, Hong Kong S Supporting Information *
ABSTRACT: Discrete lanthanide(III) tetranuclear cubane-like clusters seldom occur throughout the LnIII series and behave as single-molecule magnets (SMMs). Herein, a series of cubanes, [Ln4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (1− 9, Ln = La−Dy (except Pm), tfa = trifluoroacetate, hfa = hexafluoroacetylacetonate, phen = 1,10-phenanthroline), and dinuclear clusters, [Ln2(μOH) 2 (hfa)4 (phen) 2 ] (10−16, Ln = Tb−Lu), were synthesized and characterized. Two types of clusters were formed due to the change of preferred coordination geometry for lighter and heavier LnIII ions which favor nine-coordinated cubanes and eight-coordinated dimers, respectively. A magnetic study shows that 8-Tb4 and 9-Dy4 are ferromagnetically coupled and SMM in nature because of the larger Ln···Ln distance compared to other discrete cubanes. The anisotropic barriers, Ueff, of 9-Dy4 are determined to be 67.0 K. In addition, the photophysical properties of 6-Eu4, 8-Tb4, and 10-Tb2 owing to tfa, hfa, and phen sensitization and O−H quenching are discussed.
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INTRODUCTION With potential for high-density information storage, molecular qubits and spintronic devices,1 single-molecule magnets (SMMs) have been deeply exploited. Lanthanide(III) compounds are one of the more promising candidates owing to the large magnetic anisotropy and magnetic moment of LnIII ions.2 Since the model for Ln-SMMs with high anisotropic barrier (Ueff) and blocking temperature (TB) was revealed,3 many DyIII single-ion magnets (SIMs) with axial ligand field and large ground spin state were obtained with sizable SMM properties.4 Nevertheless, some polynuclear DyIII compounds display decent SMMs,5 such as the Dy5 pyramid5a and Dy2 hydroxo cluster5b with Ueff up to 530 and 754 K, respectively. Although their relaxation of magnetization mainly arises from single-ion anisotropy,2a quantum tunneling of magnetization (QTM), the cause of fast relaxation, is possibly suppressed by exchange interactions.6 This demonstrates that polynuclear assemblies can provide insight into the complicated magnetic mechanism. Thus far, only a few examples of discrete LnIII cubane-like tetranuclear cluster SMMs are reported,7 and these are often weak. Here, new Ln-SMM cubanes were achieved by hydrolytic synthesis which is well-known for oxo/hydroxo clusters.8 β-Diketones were used as the ligand, because not only are they generally good sensitizers for LnIII ions but they also act as shielding ligands to stabilize the cluster core and prevent polymerization.9,10 While dibenzylmethane11 and acetylacetone12 are commonly employed to yield polynuclear clusters, for hexafluoroacetylacetonate (hfa) cubane, only [Gd4(μ3OH)(μ-H2O)2(H2O)4(hfa)8]·2C6H6·H2O has been reported.13 Therefore, in this work, hfa-based clusters were synthesized with trifluoroacetate (tfa) and 1,10-phenanthroline (phen) © XXXX American Chemical Society
serving as coligands. Apart from the magnetic properties, the lanthanide contraction was also investigated, so the whole series of LnIII clusters was synthesized, resulting in cubane-like clusters 1−9 and dinuclear compounds 10−16 (Figure 1).
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RESULTS AND DISCUSSION Synthesis and Characterization. Two types of clusters, [ L n 4 (μ 3 -OH) 4 (μ-tfa) 4 (hfa) 4 ( ph e n) 4 ] and [Ln 2 (μOH)2(hfa)4(phen)2], were obtained by the reaction in Figure 1, in which a precipitate was rapidly formed and purified by recrystallization with a mixture of acetone and ethanol. Compounds 1−7 and 10−16 were obtained through the reaction of lanthanide(III) salt, hfa, phen, and NaOH in the ratio 1:3:1:3. Single-crystal structure analysis affirmed that 1−7 are tetranuclear cubane-like clusters and 10−16 are dinuclear complexes (see the section on Crystal Structures). The formation of the cluster can be explained by the theory of “ligand-controlled hydrolytic approach”.10 Since LnIII ions are Lewis acids in nature, under high pH, hydrolysis is triggered and Ln-OH units are formed. Next, olation would occur; i.e., one OH ligand would participate in bridging interactions with additional LnIII ions. Finally, the organic ligands (hfa, tfa, and phen) chelate to the LnIII ions which controls the olation process and results in desirable structures. In the case of this work, [Ln4(μ3-OH)4]8+ and [Ln2(μ-OH)2]4+ cores were generated. The formation of tfa in 10−16 is likely due to the hydrolysis of hfa.14 In an attempt to obtain the tetranuclear clusters of TbIII to LuIII, tfa was introduced to the synthesis. Received: February 25, 2018
A
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Synthetic route of the clusters: cubane (left); dimer (right). Color code: Ln (blue), O (red). When Ln = La and Ce, x = 7; when Ln = Pr− Lu, x = 8.
that of 10−16. Further increase in temperature to 880 °C leads to the formation of Ln2O3 residue. Crystal Structures. Single-crystal X-ray structure analysis was conducted to gain structural insight into the clusters. All single crystals were obtained by slow evaporation of an acetone/ethanol mixture. For the cubane series, structural analysis revealed that 1−5 crystallize in the I4/a space group, and 6−9 are in Pbcn (Table S1). Despite being not isomorphous, these clusters are isostructural and share the same cubane-like tetranuclear core (Figure 3a). Their coordination environments are the same. In general, there are four metal centers, and each LnIII ion is ninecoordinated by three μ3-OH groups, one hfa, one μ-tfa, and one phen. The cubane core is formed by the μ3-OH groups bridging three LnIII ions. The difference between 1−5 and 6−9 can be viewed in the asymmetric units where there is only one crystallographically independent metal center for the former while there are two for the latter. To describe the cubane core, 1-La4 is used as an example. Within the core, all bond angles deviate from the ideal 90°, indicating a distorted cube. As it is cube-like, there are 12 La− O−La and O−Ln−O angles, respectively, with average values of 108.34 ± 3.86° and 67.45 ± 2.01°. The edges are the 12 La− OH bonds with an average bond length of 2.515 ± 0.010 Å. The average La···La distance is 4.076 ± 0.089 Å. For comparison, all bond lengths and angles within the core of 1−9 are summarized in Table 1. When the LnIII series moves from 1-La4 to 9-Dy4, Ln−OH bond lengths and Ln···Ln interactions decrease along the series due to lanthanide contraction while Ln−O−Ln and O−Ln−O bond angles remain almost unchanged. In contrast, the Ln2 clusters 10−16 are isostructural with the same space group P1̅ (Table S1), and the structure of 13-Er2 was reported elsewhere.17 For illustration, 16-Lu2 is used as an example. There are two metal centers, and each LuIII ion is eight-coordinated and surrounded by two μ-OH groups, two hfa, and one phen (Figure 3b). The core is rhombic with Lu− O−Lu and O−Lu−O angles being 109.98° and 70.02° respectively. The average Lu−OH bond length is 2.194 ± 0.010 Å, and the Lu···Lu distance is 3.594 Å. For 10−16, the bond lengths and angles within the cluster core are also included in Table 1. Similar to the cubane, as the LnIII series goes down from 10-Tb2 to 16-Lu2, the Ln−OH bonds and Ln···Ln distance become shorter because of lanthanide contraction while the Ln−O−Ln and O−Ln−O angles remain almost unchanged. The structural change from cubane to dimer arouses curiosity because, aside from 8-Tb4 and 9-Dy4, the synthetic procedures are the same for all compounds. Literature examples of such
Surprisingly, 8-Tb4 and 9-Dy4 were isolated but not for the series of HoIII to LuIII. Since the ionic radii of HoIII to LuIII are smaller, their sizes may not favor the formation of this type of Ln4 cluster.15 Further explanation is included in the section on crystal structures. Hence, the attempt to synthesize the tetranuclear clusters for the later LnIII series, even by adding more equivalence of base or switching to different bases such as organic bases like trimethylamine, was not successful. All complexes were characterized by CHN elemental analysis, FT-IR spectroscopy (Figures S1−S16), and thermogravimetric analysis (TGA, Figure S17). For 1-La4, 5-Sm4, and 6Eu4, 1H NMR was also employed for additional characterization (Figures S20). While the purity is ensured by CHN and NMR, the FT-IR spectra can differentiate cubane and dimer by the absorption peaks attributed to the stretching of the bridging OH bonds.16 The tetranuclear clusters show a sharp peak at around 3600 cm−1 while that of dinuclear compounds is at ∼3700 cm−1, as shown from the TbIII and DyIII compounds (Figure 2). The lower wavenumber of 8-Tb4 and 9-Dy4 is attributed to the μ3 binding mode which weakens the O−H bonds more than the μ2 mode of 10-Tb2 and 11-Dy2.
Figure 2. FT-IR spectra of compounds 8−11 in the range 3500−3800 cm−1.
Additionally, thermal properties of 1−16 were disclosed by TGA. Two sets of thermal profiles, 1−9 and 10−16, are similar. Before around 210 °C, both of the two curves are flat, indicating the absence of solvent molecules trapped inside the lattice. The major degradation happens between around 220 and 300 °C. For 1−9, the major mass loss is slightly less than B
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Crystal structures of (a) 1-La4 and (b) 16-Lu2. The cluster cores are highlighted in yellow. Hydrogen atoms are omitted for clarity.
Table 1. Bond Lengths and Angles within the Cluster Cores of Compounds 1−16
a
Color code: Ln (blue), O (red). bAverage values within the cluster cores.
structural rearrangement are rarely available.18 For example, Zheng’s group established a series of LnIII hydroxo clusters using acetylacetone as a chelator.18a For LaIII and PrIII, ninecoordinated polymeric complexes were favorable whereas for the heavier LnIII, eight-coordinated cubane-like or nonanuclear clusters were assembled instead. To gain more insight into the trend, the coordination geometries of 1−16 were examined by the Shape Analysis program developed by Raymond’s group (Table S2).19 For the cubane-like cluster, 1−5 are closer to an idealized tricapped trigonal prism (D3h) while 6−9 are more likely capped square antiprism (C4v) (Figure S21). For the Ln2 clusters, 10-Tb2, 11Dy2, and 12−16 resemble trigonal dodecahedron (D2d), bicapped trigonal prism (C2v), and square antiprism (D4d), respectively (Figure S21). Note that these all have a distorted geometry which is an intermediate between one another. Interestingly, the geometries 10-Tb2 and 11-Dy2 deviate from the rest of the dimer series, and by having a more distorted intermediate geometry, they are likely to form the cubane-like 8-Tb4 and 9-Dy4. Further analysis is made by comparing the bond lengths and angles of 10-Tb2 and 11-Dy2 with those of 8-Tb4 and 9-Dy4. Within the cluster core, the Ln−O−Ln and O−Ln−O angles are similar, but the Ln−OH bonds and Ln···Ln distances of Ln2 are shorter. Also, the Ln−Ohfa and Ln−Nphen bonds of the Ln2
are shorter than those of Ln4, showing that the dimer is probably more stable and favorable for the later LnIII series. The intermolecular interactions may further account for the structural discrepancy (Figure S22). Compounds 10−16 form an extensive phen ring stacking to build a 1D chain which gives rise to extra stability over 6−9 which only contains intramolecular π-interactions of phen rings. Therefore, this stable feature may pull the heavier LnIII ions, which also prefer a lower coordination number, into the dimer assembly. Despite the stability, a TbIII and DyIII cubane can be formed because of the immediate geometry of 10-Tb2 and 11-Dy2. Another structural feature is that for the cubane with lighter LnIII, 1−5: A 3D network is formed because the intermolecular π-interactions of phen rings are oriented in a parallel-displaced fashion (Figure S22). Solvent-accessible voids of around 260 Å3 exist, apart from 5-Sm4 because its crystal is air-sensitive (but the bulk compound is not as demonstrated by FT-IR and NMR spectra). For 5-Sm4, the X-ray diffraction data was collected at 150 K, and as a result, acetone molecules are present in the lattice. To further investigate this feature, data of the 2-Ce4 crystal were collected at 150 K right after leaving the mother liquid. Likewise, acetone molecules were found. When the temperature was raised to room temperature, the acetone molecules evaporated, and a cavity was formed again (Figure S23), indicating that the 3D structures were strongly C
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry maintained by the intermolecular and intramolecular πinteractions at room temperature. This feature is also supported by the powder XRD measurement. In Figure S24, the PXRD patterns of 1−4 and 10−16 match the simulated pattern, confirming the phase purity of the bulk sample. However, for 5−9, the PXRD patterns consistently deviated from the simulated pattern. Upon detailed investigation (see further explanation on PXRD in Supporting Information), we found as suggested above, the single crystal of 5-Sm4 degraded quickly upon leaving the mother solution. This may lead to phase transition and is also noted for compounds 6−9 when drying under vacuum. Magnetic Properties. Since TbIII and DyIII ions possess large magnetic anisotropy, and the spin-only GdIII ion is applicable for determining exchange coupling constant, the magnetic properties of 7−11 were investigated. Here in our discussion, we will focus on TbIII and DyIII due to their unique magnetic properties. The magnetic properties of the other compounds are reported in Figure S25 and Table S3. The magnetic interaction between metal ions is examined by temperature-dependent magnetic direct current (dc) susceptibility measurements under 1000 Oe applied field between 300 and 2 K. The plots of χMT vs T are displayed in Figure 4, where
GdIII (8S7/2, g = 2), TbIII (7F6, g = 3/2), and DyIII (6H15/2, g = 4/ 3) are 31.5, 47.3, and 56.7 cm3 mol−1 K, and those of two uncoupled TbIII and DyIII are 23.6 and 28.3 cm3 mol−1 K, respectively. The experimental values are slightly smaller than but close to the expected ones. The χMT values of 7-Gd4 and 10-Tb2 remain almost constant within the temperature range 300−50 K and then decrease to a minimum of 18.6 and 9.0 cm3 mol−1 K at 2 K. This phenomenon is due to a progressive depopulation of excited Stark sublevels or the weak antiferromagnetic interactions between the LnIII ions.20 A more detailed analysis of 7-Gd4 using the spin-Hamiltonian Ĥ = ̂ ŜGd2 + ŜGd1ŜGd3 + ŜGd1ŜGd4 + ŜGd2ŜGd3 + ŜGd2ŜGd4 + 2Jex(SGd1 ̂ = SGd2 ̂ = SGd3 ̂ = SGd4 ̂ = 7/2 and Jex is the ŜGd3ŜGd4), where SGd1 exchange constant, afforded the best fit curve with Jex = −0.052 cm−1 and g = 1.95, confirming the antiferromagnetic nature.21 In contrast, for 8-Tb4, 9-Dy4, and 11-Dy2, the χMT value decreases gradually upon cooling, and increases at lower temperatures until 2 K. This behavior is due to the presence of intramolecular weak ferromagnetic interactions within the complexes, which are strong enough to overcome the crystal field effects.21,22 The SMM behavior of 8−11 is determined by the measurement of the dynamic magnetic properties in a zerodc field. Due to the large magnetic anisotropy of TbIII and DyIII ions, the temperature- and frequency-dependences of the inphase (χM′) and out-of-phase (χM″) alternating current (ac) susceptibility displayed a slow relaxation of the magnetization. The result suggested an SMM nature of 11-Dy2, and also, surprisingly, 8-Tb4 and 9-Dy4, since cubane-like LnIII SMMs are rarely known or very weak.7 Furthermore, peak maxima exist in the out-of-phase plots of 9-Dy4 and 11-Dy2 (Figure 5 and Figure S2); thus, extra insight into the relaxation dynamics can be gained by plotting ln τ versus 1/T, where τ is the relaxation time (Figure 6). The curvature of the plots indicates multiple relaxation processes, and these two SMMs display different properties. At the high-temperature regime, both compounds show linear dependence of ln τ on 1/T, suggesting relaxation via an Orbach mechanism. However, for 11-Dy2, ln τ becomes independent of 1/T at lower temperature where the quantum regime begins, while for 9-Dy4, a weak temperature dependency can still be observed, indicating the Raman relaxation process. The data is fitted with the equation τ−1 = τ0−1 exp(−Ueff/kBT) + CTn + τQTM−1 to consider the Orbach and Raman relaxation processes and QTM,23 affording Ueff = 67.0 K and τ0 = 1.22 × 10−8 s for 9-Dy4, and Ueff = 82.5 K and τ0 = 1.73 × 10−7 s for 11-Dy2, where Ueff is the anisotropic barrier and τ0 is the
Figure 4. Temperature-dependence of magnetic susceptibilities, χMT vs T, for 7−11 at 1000 Oe. The red solid line of 7-Gd4 corresponds to the best fit curve.
χM is the molar magnetic susceptibility. At 300 K, the χMT values of 7−11 are 29.8, 44.0, 54.6, 22.9, and 24.0 cm3 mol−1 K, respectively. The theoretical values of four magnetically isolated
Figure 5. Frequency dependence of the out-of-phase ac susceptibility under zero-dc field of 9-Dy4 and 11-Dy2. The solid lines are the visual guides. D
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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107.8 ± 2.2°, which is in general larger compared to other cubanes and further supports this hypothesis. Apart from that, the average Dy···Dy distance of 9-Dy4, 3.87 ± 0.04 Å, is the largest due to the longer Dy−OH bonds, and larger Dy−O−Dy and smaller O−Dy−O angles which pull the DyIII ions apart. This slight structural difference may alter the magnetic interaction,7f leading to the weak ferromagnetic interaction within 9-Dy4. Another example is [Dy4(OH)4(SO4)3(ox)(H2O)6], which has comparable Dy−O−Dy and O−Dy−O angles, 107.0 ± 1.7° and 69.9 ± 0.8°.25a Although ferromagnetically coupled, its SMM was not reported, which may be due to the shorter Dy···Dy intramolecular distance, 3.823 ± 0.039 Å. It is noticeable that when this average distance is smaller than 3.85 Å, either weak SMM or non-SMM results. This is likely to be due to a stronger exchange interaction within the cubane core. About the structural feature of SMM Dy2 clusters, according to the work of Tong and co-workers, the Dy−O−Dy bond can influence the SMM property.25 For 11-Dy2, that angle is 110.10°, which is comparable to that of other [Dy2(μ-OH)2]4+ clusters (Table S4).5b,26,27 Further, to boost the SMM property, having a low coordination environment is effective. For example, 5-CN [Dy(μ-OH)(DBP)2(THF)]2 exhibits the largest Ueff, 754 K, for the Dy2 hydroxo cluster to date.5b Photophysical Properties. The UV−vis absorption spectra of 1−16 were recorded in acetonitrile solution, 10 μM (Figure S31). Expectedly, cubanes 1−9 and dimers 10−16 have different profiles, which can be illustrated by 8-Tb4 and 10-Tb2 (Figure 8). Three peaks at ca. 200, 225, and 270 nm exist in both profiles which are attributed to the π → π* absorption of 1,10-phenanthroline. For 10-Tb2, the peak at ca. 290 nm is due to the hfa, and for 8-Tb4, the broad band at ca. 310 nm is the combination of the transition of tfa and hfa. Solid-state photoluminescence study at room temperature was performed. Only 6-Eu4 and 8-Tb4 displayed measurable emission spectra. The excitation and emission spectra are displayed in Figure S32 and Figure 9. Both excitation spectra show broad bands with a maximum at around 350 nm, indicating the ligand-centered transitions of tfa, hfa, and phen. For 6-Eu4, the two strong narrow lines at 395 nm (5L6 ← 7F0) and 465 nm (5D2 ← 7F0) are characteristic of direct excitation of the EuIII ion.28 For 8-Tb4, the direct excitation peak is located at 485 nm (5D4 ← 7F6).29
Figure 6. Arrhenius plots of relaxation time data for 9-Dy4 and 11-Dy2. The solid lines correspond to fits to the equation τ−1 = τ0−1 exp(−Ueff/ kBT) + CTn + τQTM−1.
attempt time. The fitting parameters are displayed in Table S5. The molar susceptibility measurement was also conducted in an optimized dc field of 2000 Oe (Figures S27−S28) to reduce QTM. The same calculation conferred Ueff = 78.6 K and τ0 = 1.3 × 10−8 s for 9-Dy4 and Ueff = 86.1 K and τ0 = 1.3 × 10−8 s for 11-Dy2 (Figure S29 and Table S5), which do not deviate much from the zero-field values. Cole−Cole diagrams of 9-Dy4 and 11-Dy2 were also constructed to resolve the distribution of the relaxation time. In Figure 7, the plots show a general arc formation in the temperature range 2−7 K, which can be fitted using the generalized Debye model.24 The afforded α parameter is in the range of 0.09 to 0.18 for 9-Dy4 and 0.13 to 0.27 for 11-Dy4, indicative of a narrow width of distribution in the single relaxation process for both compounds (Tables S6 and S7). Although the ac measurement demonstrated the SMM behavior of 8-Tb4, 9-Dy4, and 11-Dy2, no hysteresis loops were observed in the magnetic study at 2 K (Figure S30). To correlate the magnetic and structural properties of SMM DyIII cubane-like clusters, a summary of bond lengths and angles and intramolecular Dy···Dy distances of 9-Dy4 and some literature examples were included in Table 2. Ke et al. proposed that DyIII cubanes are likely to display SMM behavior if their Dy−O−Dy angles are greater than 99°.7f 9-Dy4 species are found to be in the range 105.15−110.80° with an average of
Figure 7. Cole−Cole plots for 9-Dy4 (left) and 11-Dy2 (right) obtained using the ac susceptibility data. The solid lines correspond to the best fits obtained with a generalized Debye model.24 E
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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ref
a Average values of the cluster core. bSMM positive if it displays any out-of-phase ac susceptibility signal. Blank entries mean no dynamic magnetic properties were reported. cValues under zero external dc field. Blank entries indicate no peak maxima can be found in the out-of-phase ac susceptibility plot. dLn−Oalkoxy bond length.
67.0 43.4 yes yes yes yes yes yes yes yes no 0.039 0.216 0.038 0.035 0.033 0.048 0.006 0.057 0.114 0.039 0.045 0.044 0.080 0.079 0.109 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 3.867 3.858 3.808 3.795 3.784 3.776 3.755 3.769 3.751 3.823 3.799 3.779 3.769 3.748 3.737 1.2 4.3 0.7 1.2 0.9 1.3 1.1 1.2 2.4 0.8 1.9 1.46 2.2 2.0 2.8 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 68.4 71.7 69.9 70.4 71.1 70.6 70.5 70.8 71.2 69.9 68.9 70.27 70.2 71.1 72.3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 9-Dy4 [Ln4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] [Dy4(L)4(μ2-η1η1Piv)4]·4H2O·6CH3OH [Ln4(μ3-OH)4(L)4(μ2-piv)4(MeOH)4] [Dy4(μ3-OH)4(Acc)6(H2O)7(ClO4)]·(ClO4)7·11H2O [Dy4(μ3-OH)2(μ3-O)2(cpt)6(MeOH)6(H2O)]2 [Dy4(μ3-OH)4(nic)6(py)(MeOH)7][ClO4]2·py·4CH3OH [Dy4(μ3-OH)3(μ3-O)(NOPyCOO)6(OH)(H2O)3·(DMF)3(H2O)12]∞ Dy8(HL)10(C6H4NH2COO)2(μ3-OH)8(OH)2(NO3)2(H2O)4 [Dy4(HL)4(C6H4NH2COO)2(μ3-OH)4(μ-OH)2(H2O)4]3·4CH3CN3·12H2O [Dy4(OH)4(SO4)3(ox)(H2O)6] [Dy4(OH)4(SO4)4(H2O)3] {[Dy4(OH)4(asp)3(H2O)8](ClO4)2·10H2O}n (H3O)4[Dy4(OH)5(H2O)4(μ2-IN)(IN@CB[6])2]Br [Dy4(OH)8(H2O)3(IN@CB[6])2{Ag(H2O)2(HIN)}{Ag(H2O)2-(IN)}][Ag(H2O)2(CB[6])](NO3)4·28H2O [Dy4(μ3-OH)4(nmc)8]
2.393 2.440 2.372 2.369 2.37 2.359 2.351 2.36 2.355 2.380 2.356 2.357 2.352 2.348 2.360
0.020 0.054d 0.019 0.049 0.03 0.029 0.040 0.026 0.012 0.040 0.054 0.033 0.028 0.042 0.049
107.8 104.7 106.8 106.5 106.1 106.4 106.2 106.2 105.7 107.0 107.5 106.61 106.6 106.0 104.8 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
2.2 6.8 1.5 2.4 1.6 2.0 1.6 2.0 4.8 1.7 2.6 1.92 3.2 3.4 4.6
Ueff (K)c SMMb Dy···Dy distance (Å)a O−Dy−O (deg)a Dy−O−Dy (deg)a Dy−OH (Å)a compd
Table 2. Comparison of Magnetic Properties of Some LnIII Cubane-like Clusters
this work 7a 5b 7b 7c 7d 7e 7f 7f 25a 25a 25b 25c 25c 25d
Inorganic Chemistry
Figure 8. UV−vis absorption spectra of 8-Tb4 and 10-Tb2 in acetonitrile, 10 μM.
Figure 9. Emission spectra of 6-Eu4 and 8-Tb4 at room temperature. Measurement condition: λex = 350 nm, long pass filter = 420 nm.
The emission spectrum of 6-Eu4 is assigned to 5D0 → 7FJ (J = 0−4) transitions under excitation at 350 nm. The asymmetric ratio (the intensity ratio of the transitions J = 2 and J = 1) of 6Eu4 is 7.58; the large value is attributed to the low symmetry of the EuIII center, and this is in agreement with the crystal structure.30 The single peak at 580 nm (5D0 → 7F0 transition) indicates only one EuIII emitting species in the complex.30 The luminescence decay profile of the 5D0 → 7F2 transition was fitted with a monoexponential decay, indicating the presence of a single emitting species in the 6-Eu4 sample, and the resulting lifetime is 674 μs (Figure S33). In the case of 8-Tb4, the emission spectrum shows the characteristic 5D4 → 7FJ (J = 6−2) transition. The intensity ratio of the 5D4 → 7F5 and 5D4 → 7F6 transitions is 4.17, suggesting a low symmetry for the TbIII ion site, the same situation as in the isostructural 6-Eu4. The luminescence decay lifetime of the 5D4 → 7F5 transition is also monoexponential and determined to be 90 μs (Figure S33). The much shorter luminescence lifetime of 8-Tb4 than 6-Eu4 is due to the TbIII ion having experienced a more effective nonradiative quenching via OH oscillator. To account for the luminescence, an energy level diagram was constructed by using the triplet energies of the ligands (Figure 10). The literature values are 22 000 cm−1 (hfa), 23 000 cm−1 (tfa), and 22 100 cm−1 (phen).31 The phosphorescence spectrum of 1-La4 was also measured at 20 K (Figure S34) which is dominated by the hfa triplet at 459 nm (21 800 cm−1). The energy gaps between the hfa triplet and the 5D1 excited state of the EuIII ion were determined to be 2600 cm−1, which F
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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oscillation. Work on developing other functional polynuclear compounds is currently being undertaken to further improve upon the optical and magnetic properties.
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EXPERIMENTAL SECTION
General Considerations. All chemicals used for synthesis were obtained from Sigma-Aldrich and Acros Organics and used without further purification. Elemental analysis was performed by using an Elementar Vario Micro Cube elemental analyzer. FT-IR spectra were recorded by using the Nicolet iS 50 FT-IR spectrometer with a KBr pellet. Thermogravimetric analysis was performed on a PerkinElmer Pyris 6 thermogravimetric analyzer. The samples were heated in N2 atmosphere from room temperature to 880 °C at 10 °C/min. Powder X-ray diffraction (PXRD) data were obtained on a Bruker AXS D8 advance X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å), in the range 2θ = 5−30°. The HR-ESI mass spectra were obtained from Orbitrap Fusion Lumos Tribrid mass spectrometer. Crystallographic Data. A suitable crystal was picked and mounted using the “oil-drop mounting” technique, and its data were collected using either the Bruker Smart Apex II or Bruker D8-Venture single-crystal diffractometer. Multiscan absorption correction was then applied to the data using the Bruker SAINT/SADABS software.33,34 The SHELX program suite35 was used to calculate the initial structural solution through either the direct or the Patterson method, and which was then refined by full matrix least-squares on F2. Magnetic Measurement. Magnetic susceptibility measurements were obtained by using a Quantum Design MPMS-XL7 SQUID magnetometer and a Quantum Design PPMS-XL9 VSM. Alternating current susceptibility data were collected with a 5 Oe switching field at frequencies between 1 and 1488 Hz. Data correction for diamagnetic contribution was calculated from Pascal constants. Photophysical Measurement. Absorption spectra of complexes were measured with an Agilent HP UV-8453 spectrophotometer. Steady-state room temperature solid-state photoluminescence measurements were performed with a PTI QuantaMaster 50 spectrophotometer equipped with a 75 W xenon arc lamp, a double emission monochromator using 400 nm blazed 1200 lines per mm, and a Hamamatsu R928 PMT thermoelectrically cooled. A solid sample was prepared by crushing the crystal, and the powder was clipped between two quartz plates, held by a rotatable sample holder which was oriented at 30°. Low-temperature measurements were achieved by using the Advanced Research System (ARS) DE-202 Cryocooler connected with the water-cooled ARS-2HW compressor that circulated helium. The sample was clipped between two quartz plates and held in the cryocooler. The luminescence decay lifetimes were recorded by using the PTI GL-3300 nitrogen laser delivering a pulse at 337 nm. The monoexponential curve was fitted using OriginPro. Synthesis of Clusters. [La4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (1-La4). To a mixture of trifluoroacetic acid (0.11 g, 1 mmol), hexafluoroacetylacetone (0.42 g, 2 mmol), and 1,10-phenanthroline (0.18 g, 1 mmol) in ethanol (5 mL) in a vial at 60 °C was added freshly prepared 1 M NaOH solution (3.5 mL), resulting in a yellow solution. The mixture was heated and stirred for 20 min. Addition of aqueous solution (1 mL) of lanthanum(III) chloride heptahydrate (0.37 g, 1 mmol) followed. A white solid was formed immediately, and the mixture was heated for 3 h. The solid was then filtered, washed with water and petroleum ether, and recrystallized with the mixture of acetone and ethanol to obtain single crystals. The same approach was applied for all the LnIII clusters in this study. Yield: 60% (based on the LaIII salt). 1H NMR (400 MHz, DMSO-d6) δ 5.60 (s, 4H), δ 7.79 (m, 8H), δ 8.01 (m, 8H), δ 8.51 (m, 8H), δ 9.12 (m, 8H). Anal. (%) Calcd for C76H40N8O20F36La4: C, 34.78; H, 1.54; N, 4.27. Found: C, 34.25; H, 1.58; N, 4.31. IR (KBr, cm−1): 3597 (m, υ(OH)), 1686 (s, υ(CO)), 1497 (s, υ(CN)), 1255, 1209, 1137, 1103 (s, br, υ(C F)), 842 (s), 795 (s), 662 (s), 581 (s). [Ce4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (2-Ce4). Yield: 58% (based on the CeIII salt). Anal. (%) Calcd for C76H40N8O20F36Ce4: C, 34.71; H, 1.53; N, 4.26. Found: C, 34.62; H, 1.67; N, 4.25. IR (KBr, cm−1): 3595 (m, υ(OH)), 1686, 1654 (s, υ(CO)), 1520 (s, υ(CN)), 1255,
Figure 10. Energy level diagram of the excited state of EuIII and TbIII ion and the triplet state of the ligands.
allows efficient energy transfer.32 However, the triplet of hfa and phen is close to the 5D4 state of the TbIII ion, and therefore, efficient sensitization is only caused by tfa with the energy gap of 2500 cm−1. This explains that, without tfa, 10-Tb2 has an inefficient sensitization, resulting in weak luminescence at room temperature. Another influential factor of the luminescence is that the O− H oscillating phonons can quench 10-Tb2 emission nonradiatively.32 The IR spectrum in Figure 2 determines that the vibrational energies of the bridging O−H of 8-Tb4 (μ3-OH) and 10-Tb2 (μ-OH) are 3619 and 3693 cm−1, respectively. While the free TbIII energy gap is 14 800 cm−1, four vibrational quanta of the μ-OH (∼14800 cm−1) bond can deactivate TbIII luminescence more efficiently than those of the μ3-OH (∼14500 cm−1) bond. This stronger deactivation accounts for why 10-Tb2 is weakly luminescent compared with 8-Tb4. At low temperature, since oscillation is suppressed, and back energy transfer from the first excited state to triplet state is reduced, 10-Tb2 a displays characteristic TbIII transition with a lifetime (5D4 → 7F5) of 854 μs (Figure S35). High-resolution spectra of 6-Eu4 and 8-Tb4 at 20 K were also recorded, and they revealed the C1 site symmetry of the cubane clusters (Figures S36 and S37). Furthermore, their lifetimes become longer with the values of 763 μs for 6-Eu4 and 1505 μs for 8-Tb4 (Figures S36 and S37). A more than 10-fold increase in lifetime from 90 to 1505 is observed for 8-Tb4, implying that the O−H bond oscillation quenches TbIII more efficiently than EuIII.
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CONCLUSION This work highlighted the synthesis and characterization of a series of lanthanide(III) hydroxo clusters with tetranuclear cubane-like and dinuclear cores. The assembly was accomplished by hydrolytic synthesis using hydroxide base, βdiketones, carboxylic acids, and phenanthroline. The preference of coordination geometry determines the cluster type; i.e., for a larger lanthanide (LaIII−DyIII), nine-coordinated cubane is formed, whereas for the smaller (TbIII−LuIII), eight-coordinated dimer is obtained. For the magnetic property, as a result of the long Ln···Ln distance, 8-Tb4, 9-Dy4, and 11-Dy2 are ferromagnetically coupled and exhibit SMM behavior, and the anisotropic barriers are measured to be 67.0 and 82.5 K for 9Dy4 and 11-Dy2, respectively. For the photophysical property, due to sensitization of tfa, hfa, and phen, 6-Eu4 and 8-Tb4 display characteristic red and green emission under photoexcitation at room temperature. Luminescence of 10-Tb2 is only detectable at 20 K because of quenching by O−H G
DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
H)), 1660 (s, υ(CO)), 1510 (s, υ(CN)), 1258, 1197, 1143, 1101 (s, br, υ(CF)), 841 (s), 796 (s), 726 (s), 661 (s), 586 (s). [Tm2(μ-OH)2(hfa)4(phen)2] (14-Tm2). Yield: 64% (based on the TmIII salt). Anal. (%) Calcd for C44H22N4O10F24Tm2: C, 33.87; H, 1.42; N, 3.59. Found: C, 33.88; H, 1.48; N, 3.55. IR (KBr, cm−1): 3702 (m, υ(OH)), 1661 (s, υ(CO)), 1510 (s, υ(CN)), 1258, 1198, 1143, 1101 (s, br, υ(CF)), 841 (s), 796 (s), 662 (s), 586 (s). [Yb2(μ-OH)2(hfa)4(phen)2] (15-Yb2). Yield: 69% (based on the YbIII salt). Anal. (%) Calcd for C44H22N4O10F24Yb2: C, 33.69; H, 1.41; N, 3.57. Found: C, 33.73; H, 1.24; N, 3.57. IR (KBr, cm−1): 3704 (m, υ(OH)), 1663 (s, υ(CO)), 1511 (s, υ(CN)), 1258, 1197, 1139, 1101 (s, br, υ(CF)), 841 (s), 796 (s), 662 (s), 586 (s). [Lu2(μ-OH)2(hfa)4(phen)2] (16-Lu2). Yield: 51% (based on LuIII salt). Anal. (%) Calcd for C44H22N4O10F24Lu2: C, 33.61; H, 1.41; N, 3.56. Found: C, 33.99; H, 1.15; N, 3.58. IR (KBr, cm−1): 3707 (m, υ(OH)), 1663 (s, υ(CO)), 1511 (s, υ(CN)), 1258, 1198, 1144, 1102 (s, br, υ(CF)), 841 (s), 796 (s), 662 (s), 587 (s).
1205, 1136, 1103 (s, br, υ(CF)), 843 (s), 796 (s), 662 (s), 581 (s). HRMS (m/z) calcd for C74H40F33N8O18Ce4 [M − tfa]+: 2514.8151. Found: 2514.8115. [Pr4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (3-Pr4). Yield: 67% (based on the PrIII salt). Anal. (%) Calcd for C76H40N8O20F36Pr4: C, 34.67; H, 1.53; N, 4.26. Found: C, 34.52; H, 1.62; N, 4.23. IR (KBr, cm−1): 3600 (m, υ(OH)), 1689, 1655 (s, υ(CO)), 1498 (s, υ(CN)), 1255, 1206, 1136, 1104 (s, br, υ(CF)), 842 (s), 795 (s), 663 (s), 584 (s). HRMS (m/z) calcd for C74H40F33N8O18Pr4 [M − tfa]+: 2518.8240. Found: 2518.8186. [Nd4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (4-Nd4). Yield: 57% (based on the NdIII salt). Anal. (%) Calcd for C76H40N8O20F36Nd4: C, 34.50; H, 1.52; N, 4.23. Found: C, 34.58; H, 1.67; N, 4.30. IR (KBr, cm−1): 3605 (m, υ(OH)), 1691, 1655 (s, υ(CO)), 1499 (s, υ(CN)), 1255, 1208, 1135, 1104 (s, br, υ(CF)), 842 (s), 795 (s), 664 (s), 583 (s). HRMS (m/z) calcd for C74H40F33N8O18Nd4 [M − tfa]+: 2530.8320. Found: 2530.8362. [Sm4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (5-Sm4). Yield: 55% (based on the SmIII salt). Anal. (%) Calcd for C76H40N8O20F36Sm4: C, 34.18; H, 1.51; N, 4.20. Found: C, 33.98; H, 1.62; N, 4.20. IR (KBr, cm−1): 3610 (m, υ(OH)), 1694, 1655 (s, υ(CO)), 1501 (s, υ(CN)), 1255, 1208, 1135, 1104 (s, br, υ(CF)), 841 (s), 794 (s), 668 (s), 583 (s). HRMS (m/z) calcd for C74H40F33N8O18Sm4 [M − tfa]+: 2561.8722. Found: 2561.8766. [Eu4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (6-Eu4). Yield: 56% (based on the EuIII salt). Anal. (%) Calcd for C76H40N8O20F36Eu4: C, 34.10; H, 1.51; N, 4.19. Found: C, 34.15; H, 1.51; N, 4.19. IR (KBr, cm−1): 3615 (m, υ(OH)), 1697, 1657 (s, υ(CO)), 1503 (s, υ(CN)), 1255, 1205, 1136, 1105 (s, br, υ(CF)), 842 (s), 794 (s), 669 (s), 583 (s). [Gd4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (7-Gd4). Yield: 59% (based on the GdIII salt). Anal. (%) Calcd for C76H40N8O20F36Gd4: C, 33.83; H, 1.49; N, 4.15. Found: C, 33.82; H, 1.25; N, 4.16. IR (KBr, cm−1): 3620 (m, υ(OH)), 1698, 1655 (s, υ(CO)), 1503 (s, υ(CN)), 1255, 1208, 1135, 1105 (s, br, υ(CF)), 842 (s), 794 (s), 718 (s), 679 (s), 583 (s). HRMS (m/z) calcd for C74H40F33N8O18Gd4 [M − tfa]+: 2585.8906. Found: 2584.9012. [Tb4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (8-Tb4). Yield: 65% (based on the TbIII salt). Anal. (%) Calcd for C76H40N8O20F36Tb4: C, 33.75; H, 1.49; N, 4.14. Found: C, 33.72; H, 1.54; N, 4.14. IR (KBr, cm−1): 3620 (m, υ(OH)), 1698, 1659 (s, υ(CO)), 1503 (s, υ(CN)), 1255, 1208, 1136, 1105 (s, br, υ(CF)), 841 (s), 794 (s), 719 (s), 681 (s), 584 (s). [Dy4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (9-Dy4). Yield: 55% (based on the DyIII salt). Anal. (%) Calcd for C76H40N8O20F36Dy4: C, 33.57; H, 1.48; N, 4.12. Found: C, 33.54; H, 1.47; N, 4.10. IR (KBr, cm−1): 3621 (m, υ(OH)), 1701, 1659 (s, υ(CO)), 1504 (s, υ(CN)), 1255, 1208, 1135, 1105 (s, br, υ(CF)), 841 (s), 794 (s), 719 (s), 685 (s), 584 (s). [Tb2(μ-OH)2(hfa)4(phen)2] (10-Tb2). Yield: 63% (based on the TbIII salt). Anal. (%) Calcd for C44H22N4O10F24Tb2: C, 34.31; H, 1.44; N, 3.64. Found: C, 34.30; H, 1.41; N, 3.63. IR (KBr, cm−1): 3694 (m, υ(OH)), 1659 (s, υ(CO)), 1507 (s, υ(CN)), 1258, 1197, 1146, 1099 (s, br, υ(CF)), 841 (s), 796 (s), 726 (s), 682 (s), 585 (s). [Dy2(μ-OH)2(hfa)4(phen)2] (11-Dy2). Yield: 71% (based on the DyIII salt). Anal. (%) Calcd for C44H22N4O10F24Dy2: C, 34.15; H, 1.43; N, 3.62. Found: C, 34.26; H, 1.38; N, 3.62. IR (KBr, cm−1): 3696 (m, υ(OH)), 1660 (s, υ(CO)), 1509 (s, υ(CN)), 1258, 1197, 1146, 1100 (s, br, υ(CF)), 841 (s), 796 (s), 726 (s), 661 (s), 585 (s). [Ho2(μ-OH)2(hfa)4(phen)2] (12-Ho2). Yield: 64% (based on the HoIII salt). Anal. (%) Calcd for C44H22N4O10F24Ho2: C, 34.04; H, 1.43; N, 3.61. Found: C, 35.05; H, 1.35; N, 3.70. IR (KBr, cm−1): 3699 (m, υ(OH)), 1660 (s, υ(CO)), 1509 (s, υ(CN)), 1256, 1198, 1144, 1100 (s, br, υ(CF)), 841 (s), 796 (s), 726 (s), 661 (s), 585 (s). [Er2(μ-OH)2(hfa)4(phen)2] (13-Er2). Yield: 63% (based on ErIII salt). Anal. (%) Calcd for C44H22N4O10F24Er2: C, 33.94; H, 1.42; N, 3.60. Found: C, 34.00; H, 1.28; N, 3.58. IR (KBr, cm−1): 3700 (m, υ(O
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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.8b00505. 1 H NMR spectra, FT-IR spectra, TGA profiles, crystal structures, and luminescent and magnetic data (PDF) Accession Codes
CCDC 1460824−1460825, 1553239−1553251, and 1553524− 1553525 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 data_
[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 Author
*E-mail:
[email protected]. ORCID
Ga-Lai Law: 0000-0002-2192-6887 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Prof. Jing Wang of Sun-Yat Sen University for the magnetic properties measurements. The authors acknowledge the financial support from the Research Grants Council (Bruker D8-Venture X-ray diffractometerPolyU11/CRF/13E) and the central research grants from The Hong Kong Polytechnic University (PolyU 5096/13P) and the University Research Facility in Life Sciences (ULS). H.Y.W. acknowledges the receipt of a postgraduate studentship administered by The Hong Kong Polytechnic University.
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DOI: 10.1021/acs.inorgchem.8b00505 Inorg. Chem. XXXX, XXX, XXX−XXX
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