Syntheses , Structures and Properties of 3 d-4 f Heterometallic Coordination Polymers Based on Tetradentate Metalloligand and Lanthanoid Ions

Based on tetradentate metalloligand L ([Cu(2,4-pydca)2], 2,4-pydca = pyridine-2,4-dicarboxylate) and lanthanides (Sm, Dy), two 3d-4fheterometalliccoordination polymers, namely, {[Sm2(DMSO)4(CH3OH)2][L]3·7DMSO·2CH3OH}n 1 and {[Dy2(DMSO)3(CH3OH)][L(DMSO)]·4DMSO·CH3OH}n 2 (DMSO = dimethyl sulfoxide), have been synthesized and well characterized by elemental analysis, Fourier-transform infrared spectroscopy, thermogravimetric and single-crystal X-ray diffraction analysis. Single-crystal X-ray analysis reveals that both 1 and 2 crystallize in the triclinic crystal system with P-1 space group and possess the 3D framework structures, which are constructed from metalloligands L connecting with {Sm2} and {Dy2} clusters, respectively. The 3D structure of 1 has a 6-connected single-nodal topology with the point symbol {4 × 6}, while 2 features a different framework with the point symbol of {4 × 6}. Thermogravimetric analysis exhibits that the skeleton of both 1 and 2 collapse after 350 ̊C. Magnetic properties of 1 and 2 have also been investigated.

On the other hand, the molecular structures and properties of CPs are also highly influenced by several critical factors during the synthetic process, such as pH values, metal-ligand ratio, solvent polarity, auxiliary ligands and synthetic strategy.For example, although several 3d-4f heterobimetallic CPs containing L Cu structure have been reported [28]- [33], in which the L Cu come from the reactions between cupric oxide/cupric nitrate and pyridine-2,4-dicarboxylic acid via the hydro-thermal reaction.However, the direct application of metalloligand L Cu in the construction of 3d-4f heterometallic CPs has not been reported yet.In addition, the coexistence of 3d transition metal and 4f lanthanide ions in one molecule may lead to various structures and physical properties due to the rich coordination environments of lanthanide and transition metal ions, which will finally affect the spatial configurations and magnetic couplings [34] [35] [36].
Meanwhile, benefited from the large spin values, spin-ion anisotropy and large spin-orbit couplings of lanthanide ions, 3d-4f heterometallic CPs may exhibit fascinating and complicated magnetic behaviors [37] [38] [39].Therefore, metalloligand L Cu and lanthanide centers (Sm 3+ and Dy 3+ ) will be introduced for the construction of 3d-4f coordination polymers with magnetic properties through Scheme 1. Structure of L Cu .Hydrogen atoms are omitted for clarity.the inter-diffusion method.
In this work, we have successfully synthesized two new 3d-4f heterometallic CPs from the metalloligand L Cu , i.e., graphic studies reveals that both CPs 1 and 2 exhibit the 3D framework structures, which are constructed from metalloligands L Cu connecting with {Sm 2 } and {Dy 2 } clusters.1 and 2 possess the different 6-connected single-nodal topology with point symbol {4 9 × 6 6 } and {4 12 × 6 3 }, respectively.According to the molecular formula of CP-2, which has been determined definitively from the crystal structure, one of the metalloligand L Cu was changed into L Cu (DMSO) during the reaction process.Further, the TGA behaviors of two CPs have been measured in the temperature range of 25˚C -800˚C, while the magnetic properties of 1 and 2 have also been investigated.

Materials and Physical Measurements
All the chemicals and solvents were reagent grade and purchased from commercial sources and used without further purification.Pyridine-2,4-dicarboxylate acid and metalloligand L Cu were synthesized according to procedures already reported outlined in the literature [21].Element analyses for C, H and N were performed with a PerkineElmer 240C elemental analyzer.Infrared spectra were obtained from a sample powder pelletized with KBr disks on a Nicolet Nexus 470 spectrometer (Germany) over a range of 400 -4000 cm −1 .Thermogravimetric analysis (TGA) measurements were carried out in the temperature range of 25˚C -800˚C on a PerkineElmer Pyis 1 system in a nitrogenpurge with a heating rate of 10˚C/min.The temperature dependence of molar magnetic susceptibility was measured under an applied field of 1000 G in the form of χ m T versus T in the range of 1.8 -300 K by Quantum Design MPMS XL-5.The influence of sample holder background was subtracted by the automatic subtraction feature of the software.

Single-Crystal Structure Determination
Sizeable and high-quality single crystals of two compounds were selected carefully from little glass tubes, and mounted on a glass fiber with epoxy resin covered.All measurements were obtained by a Rigaku Saturn 724 + CCD imaging plate diffractometer with graphite-monochromated Mo-Ka radiation (λ = 0.71073 Å) at room temperature.The two crystals structures were solved by direct methods, while the non-hydrogen atoms were subjected to anisotropic refinement on F 2 through full-matrix least-squares with SHELX-97 package [40] [41] [42].All the non-hydrogen atoms were determined with anisotropic thermal displacement coefficients.Hydrogen atoms were treated isotropically according to a riding model, beyond that the hydrogen atoms were located in idealized positions.The contribution of missing solvent molecules (DMSO, CH 3 OH) to the diffraction pattern was subtracted from the reflection data by the "SQUEEZE" method as implemented in PLATON [43].Details of the crystal parameters, data collection and refinement of CPs 1 and 2 are listed in Table 1, while the selected bond lengths are listed in Table 2.

Synthetic Method
CPs 1 and 2 were crystallized from the reactions between metalloligand [Cu(2,4-pydca) 2 ] and Sm(NO 3 ) 3 •6H 2 O/Dy(NO 3 ) 3 •6H 2 O, respectively.According to the literature, the reported pydca-based 3d-4f structures were obtained from rare earth hydrates, copper oxide/copper acetate hydrate and pyridine-2,4-dicarboxylic acid through the hydro-thermal synthetic approach [28]- [33].Compared to the hydro-thermal syntheses of above pydca-based 3d-4f structures, the inter-diffusion method was applied as a mild way for the crystallization of 1 and 2. In this work, our synthetic strategy uses DMSO as the buffer solution, which can slow the interactions between L Cu and lanthanide ions.As a result, well shaped crystals of 1 and 2 can be obtained from the cushion breaker.
It is obviously that the use of blank solvent as buffer solution provides a stable condition for the reaction between two different reactive components [21].
Compared with the conventional hydro-thermal/solvent-thermal synthetic approach, the inter-diffusion method here plays an important role in the crystallization process of CPs 1 and 2.

Crystal Structure of 1
The result of single crystal X-ray structural analysis reveals that CP-1 crystallizes in the triclinic crystal system with P-1 space group and exhibits a 3D framework    the vertexes of the geometry, while the sides are formed by L Cu metalloligands.
The selected bond lengths of CPs 1 and 2 are listed in Table 2.As shown in Table 2, the lengths of Cu-N bond range from 1.

Crystal Structure of 2
The X-ray crystallography study identifies that CP-2 also crystallizes in the tric-

Thermogravimetric Analysis
Thermal properties of CPs 1 and 2 were examined by thermogravimetric analysis (TGA) from 25˚C to 800˚C in a nitrogen atmosphere with a heating rate of 10˚C/min.Thethermogravimetric curve of CPs 1 and 2 are shown in Figure 9.
As shown in the Figure 9, the first weight loss of CP

Magnetic Properties
The temperature dependence of magnetic susceptibility is recorded for crystalline samples of CPs 1 and 2 at an applied magnetic field of 1000 Oe in the temperature range of 1.8 -300 K.The measurement results are shown in Figure 10 and Figure 11, respectively, in which χ m is the molar magnetic susceptibility.As in Figure 10, The χ m T values of CP-1 at room temperature is 1.62 cm 3 K mol −1 , which is a little smaller than the theoretical value (1.69 cm 3 K mol −1 ) for a two isolated Sm 3+ ions (S = 5/2, g = 2/7) and three Cu 2+ ions (S = 1/2, g = 2) without magnetic interaction.Upon decreasing the temperature, the χ m T product drops slowly to a minimum of 0.47 cm 3 K mol −1 at 1.8 K.This decrease in χ m T may originate in the antiferromagnetic interaction between metal centers.The magnetic data in the range of 50 -300 K followed the Curiee-Weiss fitting with a Curie constant of C = 1.8 cm 3 K mol −1 and negative Weiss constant of θ = −57.7 K.As shown in Figure 11, the room temperature χ m T value of CP-2 is 29.36 cm 3 K mol −1 , which is a little smaller than the theoretical value of 29.48 cm 3 K mol −1 for three Cu 2+ ion (S = 1/2, g = 2) and two Dy 3+ ions (S = 5/2, g = 4/3) due to the thermally populated excited states of Dy 3+ [44].Upon sample cooling, the χ m T   value decreases continuously to a minimum value of 17.55 cm 3 K mol −1 at 7 K.
After that, the χ m T value increases sharply to18.49cm 3 K mol −1 at 1.8 K.The transformation trend of χ m T curve below 7 K suggests the presence of weak intramolecular ferromagnetic correlation.And this magnetic difference is due to the different electron spin of the two center metals.The data in the range 100 -

Figure 3 . 1 Å 3 (
Figure 3.As in Figure 3, CP-1 exhibits a porous structure with various channels traversing the framework.Due to the small steric hindrance of DMSO molecules and long lengths of L Cu units, 3D framework of 1 possesses big cavities in these channels.The solvent accessible volume of CP-1 calculated by PLATON is 855.1 Å 3 (38.3%),which is large enough for hosting the solvent molecules (seven DMSO and two CH 3 OH).The network analysis based on TOPOS program reveals that CP-1 can be simplified to a (6,6)-connected network with {4 9 × 6 6 } topology, which is depicted in Figure 4.As in Figure 4, the {Sm 2 } clusters act as

Figure 3 .
Figure 3. Packing structure of CP-1 (a axis).Hydrogen atoms and DMSO molecules are omitted for clarity.
981(3) to 1.984(3) Å, while the value of Cu-O fall into the range of 1.971(3)-2.238(4)Å.Among the Cu-O bond, the lengths between Cu atoms and O atoms from water molecules are larger than the distances between Cu atoms and O atoms from carboxylates.Bond lengths of Sm-O vary from 2.316(4) to 2.531(8) Å.In the {Sm 2 } cluster, the shortest distance between two Sm atoms is 4.491(1) Å.
Figure 5. (a) Coordination modes of Dy1 and Dy2 atoms; (b) Connecting mode of L Cu unit (Cu1) in CP-2; (c) Connecting mode of L Cu unit (Cu2) in CP-2; (d) Connecting mode of L Cu unit (Cu3) in CP-2.Hydrogen atoms are omitted for clarity.

Figure 7 .
Figure 7. Packing structure of CP-2 (c axis).Hydrogen atoms and DMSO molecules are omitted for clarity.

Figure 10 .
Figure 10.Plots of the temperature dependence of χ m T and χ m (insert) for CP-1.

Figure 11 .
Figure 11.Plots of the temperature dependence of χ m T and χ m (insert) for CP-2.

Table 1 .
Crystal data for 1 and 2. Cu 3 N 6 O 42 S 11 Sm 2 C 60 H 60 Cu 3 N 6 O 36 S 8 Dy 2 273˚C -341˚C, four coordinated and one free DMSO molecules (15.84%) lose with the rise of temperature (calcd: 15.92%).After 350˚C, the organic groups of CP-1 start to lose and the skeleton structure starts to crumble.As for CP-2, the first weight loss of 2.86% (calcd: 2.92%) is observed from 50˚C to 110˚C for two to 118˚C, corresponding to the loss of four methanol molecules (calcd: 5.22%).Further weight loss (19.05%) appears from 142˚C to 236˚C, corresponding to the loss of six free DMSO molecules (calcd: 19.10%).In the temperature range of Journal of Materials Science and Chemical Engineering