Chinese Chemical Letters  2014, Vol.25 Issue (06):829-834   PDF    
Two lanthanide(III)-copper(II) chains based on [Cu2Ln2] clusters exhibiting high stability, magnetocaloric effect and slow magnetic relaxation
Xiao-Hong Miao, Song-De Han, Sui-Jun Liu, Xian-He Bu     
Department of Chemistry and TKL of Metal-and Molecule-Based Material Chemistry, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
Abstract: Two 3d-4f heterometallic one-dimensional chains with neutral 4,4'-bipyridine ligands as linkers and [Cu2Ln2] clusters (Ln=Gd for 1, Dy for 2) as nodes have been hydrothermally synthesized and structurally characterized. Magnetic studies indicate that complex 1 exhibits a relatively large magnetocaloric effect, with an entropy change -△Smmax=24:8 J kg-1 K-1, whilst, complex 2 features slow magnetic relaxation at low temperature.
Key words: 3d-4f     High stability     Magnetocaloric effect     Slow magnetic relaxation    
1. Introduction

Over the last decades,investigating magnetic properties of 3d- 4f heterometallic complexes has witnessed flourishing development [1]. Because of their differences in electron clouds (d and f orbitals),coordination geometries/numbers,and coordination preferences to donor atoms (N or O-atom),huge effort has been paid to integrating such different spin centers to fabricate optimized materials that could combine the superiority of both of them for practical applications [2].

Recently,molecular magnetic cryogenic materials based on magnetocaloric effect (MCE),as an alternative to the expensive and rare He-3 in some ultra-low temperature cooling applications, have received great attention because of not only their highefficient, energy-saving and environmentally friendly nature,but also their synthetic tunability and tailorability [3]. Owing to its large spin ground state S,negligible magnetic anisotropy (Dion = 0) and low-lying excited spin states,GdIII ion is a competitive candidate to construct desirable molecular magnetic coolers [4]. Additionally,due to the efficient shielding of the 4f orbitals of the GdIII ion,the magnetic exchange interactions of GdIII-GdIII/3d-GdIII are usually expected to be weak,which is also favorable to enhance MCE [4]. Therefore,Gd-containing complexes attract intensive interest and have been extensively exploited [4, 5].

On the other hand,since slow relaxation of magnetization and hysteresis loops were first discovered in {Mn12} cluster,considerable attention has been focused on the study of single-molecule magnets (SMMs) in molecular materials [6]. It is known that energy barriers for the slow magnetic relaxation are mainly dependent on the spin ground state and uniaxial magnetic anisotropy. Previous publications have shown that large spin ground state and significant uniaxial anisotropy are hard to be achieved simultaneously in pure 3d complexes [7]. In contrast,the incorporation of lanthanide ions into 3d-4f systems provides the possibilities to give rise to high spin and/or large anisotropy,to some extent,which may overcome the drawback of 3d homometallic ones. Recently,assembling SMMs into extended structures with the goal of investigating the effects of inter-SMMs magnetic interactions has been reported,which shed new light on the study of SMMs [8]. To the best of our knowledge,most of the corresponding cases are based on the homometallic SMMs,like trinuclear {Mn3},tetranuclear {Mn4},and hexanuclear {Mn6} [8]. Cases based on 3d-4f heterometallic SMMs are still limited [9].

Although notable progress has been made in the synthetic and theoretic approaches [10],the design and synthesis of 3d-4f magnetic materials are still a challenging task for synthetic chemists due to the fact that the assembly process at a molecular level does not fully follow the will of chemists. When building 3d-4f magnetic materials,one of the basic principles needed to be considered is the distinct affinity and/or preference of the 3d and 4f metal ions to N/O-donors [11]. In terms of the hard-soft acid base theory,3d transition metal ions have a strong tendency to bind to N-donor and 4f lanthanide ions with high coordination numbers and variable coordination geometries preferentially bond to O-donor [12]. In this work,we utilized a dicarboxylic acid phthalic acid (H2pta) as main ligand and 4,4'-bipyridine (4,4'- bipy) as co-ligand,to synthesize 3d-4f complexes based on the following considerations: (1) The two carboxyl groups of H2pta may be completely or partially deprotonated,yielding a variety of coordination modes,which is helpful to form interesting structures; (2) The phenyl group may exhibit additional steric hindrance,which is profitable to produce low dimensional complexes; (3) Neutral co-ligand 4,4'-bipy as a linker can reduce the competition between phthalic anions and 4,4'-bipy when binding metal ions,which is also beneficial to the process of assembly. Herein,we report the synthesis,structures, and magnetic properties of two cluster-based 1D chains, [CuLn(pta)2(Hpta)(4,4'-bipy)0.5(H2O)]n (Ln = Gd for 1; Ln = Dy for 2). Blue block crystals of 1 and 2 were obtained by the hydrothermal reaction of Cu(NO3)23H2O,Ln(NO3)3·6H2O,H2pta and 4,4'-bipy at 140 ℃. Magnetic measurements revealed that these two isostructural heterometallic chains exhibited different magnetic properties,depending on the different anisotropies of the lanthanide spin carriers: complex 1 displayed a relatively large MCE,with an entropy change -△Smmax ¼ 24:8 J kg-1 K-1 (T = 3 K,DH = 7 T),while complex 2 exhibited slow magnetic relaxation at low temperature.

2. Experimental 2.1. Materials and measurements

All chemicals were commercially purchased and used without further purification. Elemental analysis (C,Hand N) was performed on a Perkin-Elmer 240C analyzer (Perkin-Elmer,USA). The PXRD spectra were 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 PXRD spectra were carried out by the singlecrystal data and diffraction-crystal module of the Mercury (Hg) program available free of charge via the Internet at http:// www.iucr.org. 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 measured by a Quantum Design MPMS superconducting quantum interference device (SQUID). Diamagnetic corrections were estimated by using Pascal constants and background corrections by experimental measurement on sample holders.

2.2. Synthesis of complexes 1 and 2

[CuGd(pta)2(Hpta)(4,4'-bipy)0.5(H2O)]n (1). A mixture of Cu(NO3)2·3H2O (0.0483 g,0.20 mmol),Gd(NO3)3·6H2O (0.181 g, 0.40 mmol),H2pta (0.199 g,1.2 mmol),4,4'-bipy (0.125 g, 0.8 mmol) and H2O (10 mL) were placed in a Teflon-lined stainless container,then heated to 140 ℃ and kept at that temperature for 48 h,finally cooled to 30 ℃. Yield: ca. 60% with respect to Cu. Elemental analysis (%): Calcd. for C29H19CuGdNO13 (810.25): C, 42.95; H,2.34; N,1.73. Found: C,42.65; H,2.83; N,1.56. IR (KBr pellets,cm-1): 3521(w),3406(w),1624(s),1573(s),1539(s), 1488(m),1425(s),1390(s),1305(m),1289(m),1220(w), 1143(w),1083(w),973(w),879(w),841(w),803(w),773(w), 752(m),727(w),701(w),684(w),645(w),531(w).

[CuDy(pta)2(Hpta)(4,4'-bipy)0.5(H2O)]n (2). Complex 2 was prepared by a similar procedure as described for 1,but Dy(NO3)3·6H2O (0.183 g,0.40 mmol) was used instead of Gd(NO3)3·6H2O. Yield: ca. 65% with respect to Cu. Elemental analysis (%): Calcd. for C29H19CuDyNO13 (815.50): C,42.67; H, 2.33; N,1.72. Found: C,42.32; H,2.70; N,1.52. IR (KBr pellets, cm-1): 3518(m),3416(m),1629(s),1573(s),1539(s),1493(m), 1420(s),1395(s),1305(m),1289(m),1220(w),1143(w),1083(w), 973(w),879(w),841(w),803(w),773(w),752(m),727(w),701(w), 684(w),645(w),531(w).

2.3. X-ray diffraction studies

The single-crystal X-ray diffraction data of 1 and 2 were collected on a Rigaku SCX-mini diffractometer at 293(2) K with Mo-Kα radiation (λ = 0.71073Å ) by ω scan mode. The program CrystClear [13] was used for the integration of the diffraction profiles. Both the structures were solved by direct method using the SHELXS program of the SHELXTL package and refined by fullmatrix least-squares methods with SHELXL (semi-empirical absorption corrections were applied by using the SADABS program) [14]. 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. All hydrogen atoms of organic ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. The hydrogen atoms of water molecules in 1 and 2 were added by the difference Fourier maps and refined with suitable constrains. Detailed crystallographic data are summarized in Table 1 and the selected bond lengths and angles are given in Tables S2 and S3 in Supporting information.

Table 1
Crystallographic data and refinement details for 1 and 2.
3. Results and discussion 3.1. Powder X-ray diffraction (PXRD) and the thermogravimetric analyses (TGA)

The phase purity of crystalline powders of complexes 1 and 2 was confirmed by comparing experimental PXRD peaks with the simulated PXRD peaks from the single-crystal X-ray data (Fig. 1).In addition,the solvent stability of 1 was examined given that materials stability is one of important evaluation criteria in practical application. The simulated PXRD pattern of 1 reaches beautiful agreement with the corresponding experimental ones after being soaked in common solvents,such as H2O,CH3OH, CH3CH2OH,CH3CN,CH2Cl2,acetone,N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMA),tetrahydrofuran (THF), and cyclohexane solution for 2 days,indicating the excellent and extensive solvent stability of 1 (Fig. 1).

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Fig. 1. The simulated and experimental PXRD patterns of 1 and 2,and 1 in different solvents.

Thermogravimetric analysis studies were also performed under N2 atmosphere at a heating rate of 10 ℃ min-1 for complexes 1 and 2,as shown in Fig. 2. Owing to all two complexes exhibiting similar thermal stabilities,complex 1 was selected to be discussed in detail. The TGA diagram reveals that no weight loss can be found from room temperature to about 100 ℃. Complex 1 showed a gradual weight loss of 2.20% between 100 ℃ and 254 ℃ in good agreement with the theoretical value of removing one coordinated water molecule (theoretically Calcd. 2.22% for 1). Further heating above 300 ℃ causes the framework to collapse accompanied by the decomposition of the organic ligands.

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Fig. 2. TGA patterns of 1 and 2.
3.2. Description of crystal structures

Because of the structural similarities,only the structure of complex 1 was described here. Crystallographic studies revealed that 1 crystallizes in a triclinic system with space group P-1 and its asymmetric unit contains one CuII ion,one GdIII ion,two pta2- ligands,one Hpta- ligand,and one coordinated H2O molecule (Fig. 3). The whole structure of 1 can be comprehensived as a 1D chain composed of [Cu2Gd2] secondary building units (SBUs) linked by 4,4'-bipy ligands. All CuII sites are tetra-coordinate and all GdIII sites are nona-coordinate. Interestingly,the coordination modes of H2pta ligands fall into three groups: the first adopts a μ2- η11 mode in a syn-syn fashion chelating one CuII ion and one GdIII ion (Fig. 4a); the second exhibits a μ211 mode in a syn-syn fashion chelating one CuII ion and one GdIII ion with one carboxyl group,and simultaneously chelates one GdIII ion with the other carboxyl group (Fig. 4b); the third chelates one GdIII ion with one carboxyl group,one GdIII ion with two internal O-atoms of the two carboxyl groups,and one CuII ion with the remaining O-atom. (Fig. 4c). It is notable that the H2pta ligands feature three kinds of coordination modes in 1,which is rare in the H2pta-containing complexes. The diverse coordination modes of the H2pta ligands play an important role in the formation of the [Cu2Gd2] SBUs. Cu1/ Cu1A is four-coordinate with a square-planar [CuNO3] geometry formed by the coordination of one N-atom from one 4,4'-bipy ligand (Cu1· · ·N1 = 1.995(3)Å ),and three O-atoms from three carboxyl groups of three phthalic anions (Cu1· · ·O1 = 1.934(3), Cu1· · ·O5 = 1.995(3),Cu1· · ·O10 = 1.967(3)Å ). Gd1/Gd1A presents a nona-coordinated environment with one O-atom from one coordinated aqua molecule and eight O-atoms from five phthalic anions. The Gd· · ·O distances fall in the range of 2.333(3)- 2.540(3)Å . Neighboring CuII ions and GdIII ions are connected by two syn,syn-carboxylate groups and one syn,anti-carboxylate group. Adjacent GdIII ions are bridged by carboxylate groups in a μ212 mode. Three pairs of symmetry-related phthalic anions bridge two CuII ions and two GdIII ions,holding together the tetrameric [Cu2Gd2] units,which are further connected by 4,4'- bipy ligands to form a 1D chain (Fig. 4d). 1D chains are connected through the intermolecular hydrogen bonds between the uncoordinated O12 atom of the monodentate carboxylate and the coordinated O4 atom of the carboxylate chelating Gd1 (O-H· · ·O = 2.554Å ),thereby resulting in a pseudo-2D plane arrangement parallel to the ac plane (Fig. S2 in Supporting information). The 2D planes are interlinked to produce a 3D supramolecular framework through very weak supramolecular interactions.

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Fig. 3. Ball and stick view of the [Cu2Gd2] unit in 1. H atoms are omitted for clarity. Symmetry operator (-x,1 - y,1 - z) generates equivalent atoms marked with ‘‘A’’.

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Fig. 4. The three coordination modes adopted by the H2pta ligands in 1 (top); A view of the 1-D polymeric chain in 1 (bottom).
3.3. Magnetic studies

Magnetic susceptibility data were measured on microcrystalline samples of complexes 1 and 2 in the temperature range 2.0- 300 K in a 0.1 T of magnetic field (Fig. 5a for 1 and Fig. 5b for 2). The values of χMT at room temperature are 8.63 (1) and 14.5 (2) cm3 K mol-1,respectively,which are close to the corresponding theoretical values (8.25 cm3 K mol-1 for one isolated GdIII (8S7/2, g = 2) and one isolated CuII (S = 1/2,g = 2) in 1,and 14.54 cm3 K mol-1 for one isolated DyIII (6H15/2,g = 4/3) and one isolated CuII in 2). As the temperature is lowered,the χMT value remains roughly constant up to ca. 40 K before it increases gradually to a value of 9.39 cm3 K mol-1 at 2 K for 1. The curve of 1/ χM vs. T obeys the Curie-Weiss law in equation χM = C/(T - u) with C = 8.65 cm3 K mol-1 and θ = 0.21 K for 1,indicating the weak ferromagnetic coupling between the spin carriers. For 2,on lowering the temperature,the χMT product is first almost constant in the range of 300-150 K and then gradually decreases below 150 K to reach 9.47 cm3 K mol-1 at 2 K. The magnetic data are also fitted by Curie-Weiss law with C = 14.94 cm3 K mol-1 and θ = -3.47 K for 2,thus suggesting the presence of antiferromagnetic coupling and/or spin-orbit coupling and crystal-field effect for DyIII. Importantly,it seems necessary for us to synthesize the analogous frameworks containing diamagnetic YIII or LaIII ions to further investigate the magnetic interactions within the clusters, however,our attempts are fruitless.

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Fig. 5. (a) Temperature dependence χMT for 1 measured (*) at 0.1 T and χM-1 vs. T plots (&) for 1 (the red solid line for the Curie-Weiss fitting); (b) temperature dependence xT M for 2 measured (*) at 0.1 T and χM-1 vs. T plots (&) for 2 (the red solid line for the Curie-Weiss fitting).

The field dependencies of magnetization measurements were carried out in the range of 0-7 T at 2-10 K for 1 and 2 (Figs. S3a and S3b in Supporting information). The plots of M vs. H display a gradual increase with the increasing field. For 1,the M increases rapidly at low magnetic field and reaches a saturation value of 8.33 Nβ at 2 K and 7 T,which is close to the expected value for a CuII-GdIII dinuclear system with ferromagnetic interactions (S = 4). For 2,the M value reaches 9.85 Nβ at 2 K and 7 T without saturation,most likely owing to the crystal-field effect on the DyIII ions [15].

Considering the weak ferromagnetic interaction between the spin carriers within the [Cu2Gd2] units and negligible magnetic anisotropy of the spin carriers,complex 1 could be a possible candidate for low-temperature magnetic cooling. To explore the magnetic behaviors of 1 at low temperature,we investigated the temperature dependencies of field-cooled (FC) and zero-field-cooled (ZFC) magnetization under a field of 50 Oe from 2 K to 20 K (Fig. S4 in Supporting information). The superposition of the FC curve and the ZFC curve indicates the absence of any magnetic ordering above 2 K in 1. Hence, the magnetic entropy change ΔSm of 1 is measured according to the M vs. H data at 2-10 K and 0-7 T to evaluate the MCE. According to the Maxwell equation ΔSm(T)ΔH = ∫ [∂M(T,H)/∂T]HdH [16],we obtained the curves of -ΔSm which are depicted in Fig. 6. The maximal entropy change value of 24.8 J kg-1 K-1 is obtained at 3 K for DH = 7 T. Two extreme situations are taken into consideration: (1) only Smax = 4 is achieved when the coupling between CuII centers and GdIII centers is completely ferromagnetic. In this case,the theoretical maximum magnetic entropy of 22.5 J kg-1 K-1 can be given by Rln(2Smax + 1). (2) Given that the spin carriers are fully magnetically isolated,the expression should be calculated as Rln(2SGd + 1) + Rln(2SCu + 1). Therefore the possiblemaximum magnetic entropy of 28.4 J kg-1 K-1 can be obtained for one isolated CuII and one isolated GdIII centers, assuming SGd = 7/2 and SCu = 1/2. It is clearly that the experimental -ΔSm is between these two extreme situations [17],which further confirms the weak ferromagnetic coupling between CuII and GdIII ions. It is worth noting that among the reported Cu-Gd complexes which were developed as potential cryogenic molecule- based magnetic refrigerant materials,-△Smmax above 20.0 J kg-1 K-1 is limited (summarized in Table S1 in Supporting information).

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Fig. 6. Experimental -ΔSm obtained from magnetization data of 1 at different fields and temperatures.

In view of the considerable single-ion anisotropy nature of DyIII, further exploration was devoted to the magnetic dynamics of 2,the frequency and temperature dependencies of the alternating current (ac) susceptibilities have been investigated under zero direct current (dc) field and a 3 Oe ac magnetic field oscillating at frequencies between 500 Hz and 1200 Hz for 2 (Fig. 7). The out-ofphase component (χ'') of ac susceptibilities exhibits frequency dependent signals below 8 K,implying that 2 possesses a slow magnetic relaxation behavior,which might be an indicator of SMM behavior. However,there are no peaks in the out-of-phase susceptibility data as observed in some reported Dy-based complexes in the technically available temperature range,which may be ascribed to strong quantum tunneling effect [18]. Therefore,the energy barrier and relaxation time cannot be calculated by Arrhenius formula fitting.

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Fig. 7. Temperature dependence of the out-of-phase (χ'') ac susceptibility components for 2 at the indicated frequencies with Hdc = 0 Oe (Inset: temperature dependence of the in-phase (x0) ac susceptibility components for 2 at the indicated frequencies with Hdc = 0 Oe).
4. Conclusion

In summary,two new isostructural 1D lanthanide(III)-copper( II) chains based on [Cu2Ln2] clusters have been synthesized via hydrothermal method,and a series of characterizations and magnetic properties have been measured. Magnetic investigations indicate that complex 1 is an interesting magnetic refrigerant with an entropy change -△Smmax ¼ 24:8 J kg-1 K-1,whereas complex 2 exhibits frequency-dependent out-of-phase signal χ'' M blow ~8 K, suggestive of slow relaxation of the magnetization for the strong anisotropy of DyIII ion. Further efforts are in progress to search for other cluster-based 3d-4f complexes with large MCE and SMM properties in our group.

Acknowledgments

This work was financially supported by the 973 Program of China (Nos. 2012CB821700 and 2014CB845600),the NNSF of China (Nos. 21031002 and 21290171),and MOE Innovation Team (No. IRT13022) of China.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.05.025.

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