Ln^III ion dependent magnetism in heterometallic Cu–Ln complexes based on an azido group and 1,2,3-triazole-4,5-dicarboxylate as co-ligands

Abstract

Four Cu–Ln coordination polymers [Cu₂Ln(tda)₂N₃(H₂O)₄·2H₂O] (Ln = Gd (1), Tb (2), Dy (3), Sm (4), tda = 1,2,3-triazole-4,5-dicarboxylate) have been synthesized by employing H₃tda and azido as co-ligands. Complexes 1–4 exhibit the hydrogen bonded three-dimensional (3D) network and intriguing magnetic properties. Among them, 1, 3 and 4 involve antiferromagnetic interaction between metal ions, while 2 exhibits ferromagnetic behavior, demonstrating that magnetic properties could be mediated by a variety of rare earth metal ions.

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Introduction

Molecule-based magnets have attracted considerable attention over the past decades, due to their unique properties and potential applications in high-density data storage technologies, quantum computing and molecular spintronics.^1–5 Thus, many efforts have been devoted to this active field. Among various types of molecule-based magnets, a family of 3d–4f heterometallic coordination complexes are of great interest, since the incorporation of lanthanide and transition metal ions always leads to versatile properties.^6–9 However, on the other hand, due to the competitive coordination between 3d and 4f ions to the same ligand, which favors to form homometallic rather than heterometallic coordination complexes, the synthesis of 3d–4f heterometallic magnets is still challenging. To address this issue, one promising way is to employ the polydentate ligands that allow the coordination with both lanthanide and transition metal ions simultaneously, bearing in mind that the lanthanide ions are oxyphile, while the transition metal ions preferentially coordinate with nitrogen atoms.

On the other hand, azido has proved to be a promising and versatile bridging ligand, as it can readily coordinate to two or more metal ions, thus offering effective superexchange pathways for magnetic couplings among spin carriers.¹⁰ Furthermore, azido has a strong trend to coordinate with transition metals, according to hard–soft acid–base (HSAB) theory,¹¹ thus giving rise to many intriguing structures with excellent magnetic properties.¹² Besides that, azido can also provide E–E and/or E–O bridging mode and thereby has a capability of mediating the ferromagnetic or antiferromagnetic magnetic coupling between metal ions.

Based on above considerations, imidazolecarboxylate containing both N and O coordination sites has been elected here as main ligand, together with azido as co-ligand, to fabricate new 3d–4f magnets.^13,14 Among a class of polydentate ligands, H₃tda (tda = 1,2,3-triazole-4,5-dicarboxylate) should be a good candidate, because it involves both tri-N-donor and two carboxylate and thus can provide distinct coordination sites for 3d and 4f metal ions (Scheme 1). However, only a few 3d–4f coordination polymers based on H₃tda has been documented hitherto.^15,16 During the past decade, our group has devoted to fabricating the azido and carboxylate-mediated magnetic complexes,¹⁷ which would be helpful for research in this context (Scheme 2). By employing H₃tda and azido as co-ligands, we herein report the syntheses of four new isostructural 3d–4f coordination complexes with three-dimensional (3D) structures, [LnCu₂(tda)₂(N₃) (H₂O)₄·2H₂O] (Ln = Gd (1), Ln = Tb (2), Ln = Dy (3), Ln = Sm (4)), together with a comprehensive study of their magnetic properties.

Scheme 1 The H₃tda ligand.

Scheme 2 Binding mode of tda⁻ in complexes 1–4.

Experimental

Materials and physical measurements

The H₃tda ligand was synthesized according to the reported method.¹⁸ All the chemicals used for synthesis are of analytical grade and commercially available. Cu(NO₃)₂·3H₂O, Ln(NO₃)₃·6H₂O and sodium azido were purchased from commercial sources and used as received.

Caution: azido metal complexes are potentially explosive, only a small amount of material should be prepared with care.

Elemental analyses (C, H, N) were performed on a Perkin-Elmer 240C analyzer. The X-ray powder diffraction (XRPD) was recorded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA for a Cu-target tube and a graphite monochromator. Simulation of the XRPD spectra was carried out by the single-crystal data and diffraction-crystal module of the mercury (Hg) program available free of charge via the internet at http://www.iucr.org.

The thermal gravimetric analyses (TGA) was performed with a TGA Q500 instrument under N₂ from room temperature to 700 °C with a heating rate of 10 °C min⁻¹ using aluminium crucibles.

IR spectra were measured in the range of 400–4000 cm⁻¹ on a Tensor 27 OPUS FT-IR spectrometer using KBr pellets (Bruker, German).

Magnetic data were collected using crushed crystals of the sample on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7T magnet. The data were corrected using Pascal's constants to calculate the diamagnetic susceptibility; an experimental correction for the sample holder was applied.

General synthetic procedure for [LnCu₂(tda)₂N₃(H₂O)₄]·2H₂O, Ln^III = Gd, Tb, Dy, Sm: a water solution of CuNO₃·3H₂O, LnNO₃·6H₂O (Ln = Gd for 1, Tb for 2 and Dy for 3, Sm for 4), H₃tda and NaN₃ with the ratio 2 : 1 : 1 : 1 were sealed in a Teflon-lined autoclave and heated to 140 °C. After maintained for 48 h, the reaction vessel was cooled to room temperature in 12 h, dark green crystals were collected with ca. 12% yields based on CuCl₂·2H₂O. FT-IR (KBr pellets, cm⁻¹): for 1: 3421(s), 2358(m),1624(vs), 1541(s), 1436(m), 1388(s), 1315(m), 1156(s), 873(m), 828(s), 790(m); for 2: 3412(s), 2360(m), 1625(vs), 1541(s), 1434(m), 1389(s), 1303(m), 1151(s), 878(m), 831(s), 787(m); for 3: 3410(s), 2358(m), 1627(vs), 1540(s), 1432(m), 1391(s), 1311(m), 1154(s), 873(m), 829(s), 788(m); for 4: 3419(s), 1623(vs), 1624(s), 1432(m), 1392(s), 1308(m), 1154(s), 878(m), 829(s), 788(m); Elemental analytical data correspond to: C₈H₁₂Cu₂N₉O₁₄Gd (1) calcd: C, 12.91; H, 1.60; N, 16.95%. Found: C, 13.21; H, 1.78; N, 17.65%. C₈H₁₂Cu₂N₉O₁₄Tb (2) calcd: C, 12.89; H, 1.61; N, 16.91%. Found: C, 13.35; H, 1.82; N, 17.68%. C₈H₁₂Cu₂N₉O₁₄Dy (3) calcd: C, 12.82; H, 1.60; N, 16.83%. Found: C, 13.18; H, 1.75; N, 17.35%. C₈H₁₂Cu₂N₉O₁₄Sm (4) calcd: C, 13.03; H, 1.63; N, 17.11%. Found: C, 13.25; H, 1.75; N, 18.24%.

X-ray data collection and structure determinations

X-ray single-crystal diffraction data for complexes 1–4 were collected on a Rigaku SCX-mini diffractometer at 293(2) K with Mo-Kα radiation (λ = 0.71073 Å) by ω scan mode. The program CrystClear¹⁹ was used for integration of the diffraction profiles. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL (semi-empirical absorption corrections were applied using SADABS program).²⁰ Metal atoms in each complex were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F². The hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. Table 1 shows crystallographic crystal data and structure processing parameters. Selected bond lengths and bond angles are listed in Table 2.

Table 1 Crystal data and structure refinement for complexes 1–4

	1-Gd	2-Tb	3-Dy	4-Sm
a R = Σ||Fo| − |Fc||/Σ|Fo|.b Rw = [Σ[w(Fo^2 − Fc^2)^2]/Σw(Fo^2)^2]^1/2.
Chemical formula	C8H12Cu2N9O14Gd	C8H12Cu2N9O14Tb	C8H12Cu2N9O14Dy	C8H12Cu2N9O14Sm
Formula weight	742.60	744.27	747.85	735.70
Space group	P2/c	P2/c	P2/c	P2/c
a (Å)	11.827(2)	11.819(2)	11.787(2)	11.878(2)
b (Å)	7.4867(15)	7.3367(15)	7.4590(15)	7.4838(15)
c (Å)	11.557(2)	11.533(2)	11.559(2)	11.582(2)
β/deg	103.03(3)	103.19	103.14(3)	103.09(3)
V/Å^3	997.0(3)	973.7(3)	989.7(3)	1002.8(3)
Z	2	2	2	2
GOF	1.140	1.058	1.177	1.336
D/g cm^−3	2.510	2.539	2.436	2.474
μ/mm^−1	5.495	5.852	5.960	5.084
T/K	293	293	293	293
R^a/wR^b (I > 2σ(I)	0.0447/0.0686	0.0634/0.1344	0.0549/0.1221	0.0476/0.1052
R1/wR2 (for all data)	0.0573/0.0715	0.0801/0.1430	0.0596/0.1244	0.0523/0.1066

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Table 2 Selected bond lengths [Å] and angles [°] for complexes 1–4^a

	1-Gd	2-Tb	3-Dy	4-Sm
a Symmetry codes: (#1) x, y, z; (#2) y, x, z; (#3) y, x, z; (#4) x, y, z; (#5) y, x, z; (#6) x, y1, z; (#7) x, y1, z.
Ln–O(3)	2.273(4)	2.244(8)	2.261(7)	2.311(7)
Ln–O(3)#3	2.273(4)	2.244(8)	2.261(7)	2.311(7)
Ln–O(1W)#3	2.384(5)	2.344(7)	2.368(7)	2.419(6)
Ln–O(1W)	2.384(5)	2.344(7)	2.368(7)	2.419(6)
Ln–O(1)	2.452(4)	2.428(7)	2.431(7)	2.467(6)
Ln–O(1)#3	2.452(4)	2.428(7)	2.431(7)	2.467(6)
Ln–O(2W)#3	2.456(5)	2.437(8)	2.449(5)	2.488(4)
Ln–O(2W)	2.456(5)	2.437(8)	2.449(5)	2.488(4)
Cu(2)–N(1)	1.954(4)	1.952(9)	1.954(9)	1.963(7)
Cu(2)–O(2)	1.973(4)	1.954(8)	1.972(7)	1.972(6)
Cu(2)–N(2)#2	1.980(4)	1.969(9)	1.979(8)	1.973(7)
Cu(2)–N(4)	2.103(4)	2.196(13)	2.116(5)	2.102(4)
Cu(2)–N(3)#4	2.183(4)	2.136(9)	2.170(8)	2.191(7)
N(3)–Cu(2)#1	2.183(4)	2.136(9)	2.170(8)	2.191(7)
N(2)–Cu(2)#2	1.980(4)	1.969(9)	1.979(8)	1.973(7)
N(4)–Cu(2)#5	2.103(4)	2.196(13)	2.116(5)	2.102(4)
Cu(2)–N(4)–Cu(2)#5	105.0(3)	99.5(8)	104.5(2)	105.45(19)

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Results and discussion

Description of crystal structure

Complexes 1–4 are essentially isostructural and crystallize in the monoclinic P2/c space group confirmed by their very similar PXRD patterns and also the single crystal structural analysis (Table 1). Therefore, we will only describe the structure of 3 as a representation for clarity. As shown in Fig. 1, the asymmetric unit of 3 contains half Dy^III ion, one Cu^II ions, half azido anion, one tda³⁻ anion, and two coordinating as well as one non-coordinating water molecule. Each Cu^II ion is pentacoordinated with three nitrogen atoms from three individual tda³⁻ anions, one carboxyl oxygen atom and one azido anion. In forming a one-dimensional (1D) copper chain, the involved Cu^II ions connect to each other by sharing a common side (Fig. 2a), where the azido anions interconnect diagonal copper ions in E–O mode that could further stable this chain structure. On the other hand, each Dy^III anion is coordinated by four carboxyl oxygen atoms from the tda³⁻ ligands of the copper chain, which tends to link two adjacent copper chains to form a two-dimensional (2D) 3d–4f coordination network (Fig. 2b). In the resulting 2D complex, hydrogen bond interaction plays a vital role in consolidating the crystalline structure. As shown in Fig. 2c, the twisted layers are interconnected together via hydrogen bonds to produce a 3D supramolecular network. Moreover, each Ln^III ion exhibits a square antiprism configuration, which is coordinated by four carboxyl oxygen atoms of two tda³⁻ ligands and four water molecules (Fig. 2a), with the distances of Gd–O bonds ranging from 2.273(4) to 2.456(5) Å (Table 1) and the O–Gd–O angles in the range of 70.61(19)–145.64(18)° (Table 2).

Fig. 1 Coordination modes of metal ions in complexes 1–4.

Fig. 2 (a) The 1D chain connected by azido in 3. (b) A 2D Ln^III coordinated 3d–4f network. (c) 3D supramolecular network connected by hydrogen bonds.

PXRD patterns and thermal stabilities

PXRD measurements were carried out at room temperature to confirm their crystalline phase purities. The diffraction peaks of bulk samples are consistent with the simulated patterns based on the single crystal data, thus indicating the presence of mainly one crystalline phase in the corresponding samples of 1–4 (Fig. 3). To examine their thermal stabilities, TGA was performed under N₂ atmosphere. The TGA results of 1 and 2 are quite similar, both showing a two-step weight loss before 300 °C (Fig. 4). In detail, the weight loss quantity during the first step (90–300 °C) is 3.77% for 1 and 4.51% for 2, which agrees well with the calculated value when releasing two uncoordinated water molecules (calcd: 4.84% for 1; 4.83% for 2). Similar analyses also indicate that the second step is ascribed to the release of four coordinated water molecules (found: 9.13% for 1, 8.95% for 2; calcd: 9.68% for 1, 9.87% for 2). After that, further elevating the temperature above 300 °C then leads to the decomposition of these two complexes. On the other hand, the TGA plots of complexes 3 and 4 are also in a similar pattern, which indicate that no obvious weight loss occur between room temperature and 300 °C, as compared with those of complexes 1 and 2. Further heating also leads to the collapse of the frameworks, which occurs in accompany with the loss of coordinated water and free water molecules (Fig. 4).

Fig. 3 Comparisons of the experimental PXRD patterns of as-synthesized 1–4 with that simulated from single crystal data.

Fig. 4 The TGA curves of complexes 1–4.

Magnetic properties

The direct current (dc) magnetic susceptibilities of complexes 1–4 were measured in an applied magnetic field of 1000 Oe in the temperature range of 300–2 K. The profiles of χ_MT vs. T plots and M vs. H are shown in Fig. 5 and 6. For 1, the χ_MT value at room temperature is 10.09 cm³ K mol⁻¹, little larger than the sum of one isolated Gd^III and two Cu^II ions (8.63 cm³ K mol⁻¹). With the decreasing temperature, the value of χ_MT slowly decreases from 10.09 cm³ K mol⁻¹ at 300 K to 8.13 cm³ K mol⁻¹ at 2 K. The Curie–Weiss fitting to the magnetic data between 2 K and 300 K gives the Weiss constant of θ = −4.49 K, which suggests an overall antiferromagnetic interaction in 1. Moreover, the plot of M vs. H reveals that the magnetization of 1 approaches to a maximum value 7.41 Nβ when H increases to 5 T at 2 K, which is slightly smaller than the theoretical saturation value, 9 Nβ, for a single Gd^III (S = 7/2, g = 2) and two Cu^II ions (S = 1/2, g = 2).

Fig. 5 Plots of χ_MT vs. T for complexes 1–4.

Fig. 6 Plots of M vs. H for complexes 1–4.

For 2, the observed χ_MT value of 13.06 cm³ K mol⁻¹ at room temperature is close to the sum of one isolated Tb^III and two Cu^II ions (12.57 cm³ K mol⁻¹). Upon lowering the temperature, the value of χ_MT initially increases to 20.47 cm³ K mol⁻¹ at 10 K and then gradually decreases to 19.47 cm³ K mol⁻¹ at 2 K. The Curie–Weiss fitting of the magnetic data gives C = 12.29 cm³ K mol⁻¹ and θ = 22.15 K, thus indicating that there exists a ferromagnetic coupling between the metal ions. The decline at low temperatures might be due to the magnetic saturation and/or zero-field splitting. The comparison with the reported complex {[Cu₂Tb(tda)₂Cl(H₂O)₄]·2H₂O}_n,^16a which exhibits an antiferromagnetic behaviour, indicates that it is possible to realize a transformation from antiferromagnetic to ferromagnetic interaction by changing Cl⁻ to N₃⁻. According to literatures, ferromagnetic interactions usually occur between Tb^III and Cu^II ions mediated by bridging ligands.²¹ Fig. 6 shows that for 2, the field-dependent magnetizations at 2 K abruptly increase at low fields (<1 T) and then steadily increase to a maximum value 7.65 Nβ, which is smaller than the theoretical saturation value of 11 Nβ for one Tb^III (J = 6, g = 3/2) and two Cu^II ions (S = 1/2, g = 2).

The χ_MT value of complex 3 at room temperature is found to be 15.15 cm³ K mol⁻¹, which is close to the sum of one isolated Dy^III and two Cu^II ions (14.92 cm³ K mol⁻¹) and thus also evidences a transformation from antiferromagnetic to ferromagnetic interaction by changing Cl⁻ to N₃⁻. With the temperature lowering, χ_MT decreases gradually from 15.15 cm³ K mol⁻¹ at 300 K to 11.36 cm³ K mol⁻¹ at 2 K, corresponding to a Weiss constant of θ = −3.64 K estimated from the Curie–Weiss fitting to the magnetic data. However, it is difficult to identify the magnetic interaction between the metal ions of 3, since the strong spin–orbit coupling of Dy^III ions always leads to the thermotropic depopulation of the excited states. Further lowering the temperature will cause a decrease in χ_MT, which might be due to a selective depopulation of the excited crystal field state and/or antiferromagnetic interaction between Ln^III and Cu^II ions. The magnetization of 3 features a steady increase with the increasing H and the experimental saturation value is found to be 6.20 Nβ at 5 T and 2 K, which is much smaller than the theoretical value of 12 Nβ for one individual Dy^III (J = 15/2 and g = 4/3) and two Cu^II ions (S = 1/2, g = 2).

For 4, the χ_MT value observed at room temperature is 1.61 cm³ K mol⁻¹, which is close to the sum of one isolated Sm^III and two Cu^II ions (0.84 cm³ K mol⁻¹). With the decreasing temperature, the value of χ_MT slowly decreases to 0.069 cm³ K mol⁻¹ at 2 K. Strikingly, the Curie–Weiss fitting of the magnetic data gives the Weiss constant of θ = −176.96 K. This abnormal negative value of θ is much larger than that of complexes 1–3. This is mainly due to the coupling between spin and orbital magnetic moment that are coupled antiparallel by the spin–orbital interaction and almost cancel out each other.²² Furthermore, the plot of M vs. H does not follow the Brillouin curves. Instead, M rises with the increasing H. Evidently, the saturation value of 0.25 Nβ is much smaller than the theoretical value of 2.71 Nβ for one Sm^III (J = 5/2, g = 2/7) ion and two Cu^II (S = 1/2, g = 2), thus indicating that there exist antiferromagnetic interactions along with spin competition in the system (frustration index, f = |θ|/T_N = 177/2 = 88.5).

Conclusions

Four new 3d–4f heterometallic coordination polymers based on azido and N/O mixed-donor-bridging ligand 1,2,3-triazole-4,5-dicarboxylate have been successfully synthesized under hydrothermal condition. These isostructural heterometallic complexes exhibit unique 3D coordination networks and intriguing magnetic properties. While 1, 3 and 4 involves antiferromagnetic interactions, complex 2 exhibits a ferromagnetic behavior. These results thus indicate that the magnetic properties could be mediated by a variety of rare earth metal ions.

Acknowledgements

This work was supported by the 973 Program of China (grant 2014CB845600), the NSF of China (grants 21421001, 21290171, and 21403116) and MOE Innovation Team (IRT13022) of China.

Notes and references

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Footnote

† CCDC 1061711 (1), 1061713 (2), 1061710 (3) and 1061712 (4) contain the supplementary crystallographic data for this paper. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07972a

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