Abstract

Based on tetradentate metalloligand L^Cu ([Cu(2,4-pydca)₂], 2,4-pydca = pyridine-2,4-dicarboxylate) and lanthanides (Sm³⁺, Dy³⁺), two 3d-4fheterometalliccoordination polymers, namely, {[Sm₂ (DMSO)₄ (CH₃OH) ₂][L^Cu]₃·7DMSO·2CH₃OH}_n 1 and {[Dy₂ (DMSO)₃ (CH₃OH)][L^Cu3 (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^Cu connecting with {Sm₂} and {Dy₂} clusters, respectively. The 3D structure of 1 has a 6-connected single-nodal topology with the point symbol {4⁹ × 6⁶}, while 2 features a different framework with the point symbol of {4¹² × 6³}. Thermogravimetric analysis exhibits that the skeleton of both 1 and 2 collapse after 350℃. Magnetic properties of 1 and 2 have also been investigated.

Keywords

Coordination Polymers, {Sm₂}/{Dy₂} Clusters, Metalloligand, Crystal Structure, Magnetic Property

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1. Introduction

Nowadays, a great deal of attention has been dedicated to the wide family of coordination compounds due to their unique molecular structures and potential applications in the field of luminescence, magnetism, catalysis and biochemistry [1] [2] [3] [4] . Current interest in these materials mainly focuses on the coordination polymers (CPs) associated with mixed-metal ions [5] . Among various approaches to the construction of heterometallic CPs, the most efficient one is to employ metalloligands that can connect with other metal centers, such as transition metal [6] [7] [8] and lanthanide ions [9] - [16] . For example, molecular building units [M(CN)₈]^3−/4− (M = W, Mo, Nb) and [MS₄]²⁻ (M = W, Mo) have been successfully applied in the fabrication of heterometallic CPs as metalloligands [17] [18] [19] [20] . Recently, an O-containing metalloligand [Cu(2,4-pydca)₂] (L^Cu, see Scheme 1) based on pyridine-2,4-dicarboxylate (2,4-pydca) has been developed [21] . Several heterobimetallic coordination compounds have been reported from the reaction between metalloligand L^Cu and transition metal (Mn^II, Co^II, Ni^II) [22] [23] [24] [25] [26] , alkaline-earth ions [27] .

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 (Sm³⁺ and Dy³⁺) 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., {[Sm₂(DMSO)₄(CH₃OH)₂][L^Cu]₃・7DMSO・2CH₃OH}_n 1 and {[Dy₂(DMSO)₃(CH₃OH)][L^Cu3(DMSO)]・4DMSO・CH₃OH}_n 2. X-ray crystallographic studies reveals that both CPs 1 and 2 exhibit the 3D framework structures, which are constructed from metalloligands L^Cu connecting with {Sm₂} and {Dy₂} clusters. 1 and 2 possess the different 6-connected single-nodal topology with point symbol {4⁹ × 6⁶} and {4¹² × 6³}, 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.

2. Experimental Section

2.1. 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_mT 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.

2.2. Preparation of {[Sm₂(DMSO)₄(CH₃OH)₂][L^Cu]₃・7DMSO・2CH₃OH}_n (1)

The methanol solution of Sm(NO₃)₃・6H₂O (44 mg, 0.1 mmol) was slowly diffused into a dimethyl sulfoxide solution (DMSO) of L^Cu (62 mg, 0.1 mmol) through 1 mL DMSO blank solvent as the buffer solution. After several days, the obtained blue crystals were collected by filtration, washed with methanol. Yield: 50 mg, 60.6% (based on Cu). Anal. Calcd for C₆₈H₈₈Cu₃N₆O₄₂S₁₁Sm₂(2453.57): C, 33.29; H, 4.11; N, 3.41. Practical found: C, 33.32; H, 4.12; N, 3.40. IR (KBr, cm⁻¹): 3419(s), 2980(m), 2924(m), 2354(m), 1660(s), 1617(s), 1555(s), 1476(m), 1390(s), 1341(s), 1261(m), 1181(w), 1083(w), 1009(s), 942(m), 899(w), 825(w), 783(m), 739(m), 684(s), 530(w), 463(w).

2.3. Preparation of {[Dy₂(DMSO)₃(CH₃OH)][L^Cu3(DMSO)]・4DMSO・CH₃OH}_n (2)

CP-2 was obtained as light blue block crystals with the same synthetic method as that of 1 except that Sm(NO₃)₃・6H₂O was replaced by Dy(NO₃)₃・6H₂O (46 mg, 0.1 mmol). Yield: 6 mg, 18% (base on Cu). Anal. Calcd for C₆₀H₆₀Cu₃N₆O₃₆S₈Dy₂(2195.38): C, 32.83; H, 3.37; N, 3.83. Found: C, 32.56; H, 3.36; N, 3.88. IR (KBr, cm⁻¹): 3434(s), 2992(w), 2912(w), 2372(s), 1654(s), 1611(s), 1537(m), 1476(w), 1402(s), 1329(s), 1248(w), 1089(w), 1022(m), 954(w), 832(w), 783(w), 733(m), 684(m), 524(w).

2.4. 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 F² 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, CH₃OH) 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.

3. Results and Discussion

3.1. Synthetic Method

CPs 1 and 2 were crystallized from the reactions between metalloligand [Cu(2,4-pydca)₂] and Sm(NO₃)₃・6H₂O/Dy(NO₃)₃・6H₂O, 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.

3.2. 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 structure. The asymmetric unit consists of one [Sm(DMSO)₂(CH₃OH)]³⁺ cation, one and half [L^Cu]²⁻ ions.

The coordination mode of Sm atom and the connecting modes of L^Cu metalloligands in CP-1 are shown in Figure 1. As in Figure 1(a), each Sm atom is coordinated by eight O atoms from five L^Cu metalloligands, two DMSO molecules and one CH₃OH molecule forming a square antiprism spatial configuration and exhibiting D_4d symmetry. In accordance with Figures 1(b)-(d), CP-1 have three different L^Cu metalloligands: both L^Cu1 and L^Cu5 units connect with four Sm atoms through the carboxylic oxygen atoms showing a four-connecting mode, while the L^Cu4 unit connect with two Sm atoms exhibiting a single-bridged mode. Attributable to the bidentate coordination mode of carboxylic acid in L^Cu1 and

Symmetry transformations used to generate equivalent atoms for 1: #1 −x, −y, −z − 1; #2 −x + 2, −y + 1, −z + 1; #3 −x + 2, −y + 1, −z; #4 −x + 1, −y, −z; for 2: #1 x − 1, y, z; #2 x, y − 1,z; #3 x, y, z − 1; #4 x, y, z + 1; #5 x, y + 1, z; #6 x + 1, y, z.

L^Cu5 units, a {Sm₂} cluster is constituted by four μ₂-bridged carboxyl groups (Figure 2(a)). These {Sm₂} clusters can be linked by four L^Cu metalloligands to build a quadrangular ring (Figure 2(b)). Extension of such rings through six-connected {Sm₂} clusters leads to the whole 2D layer structure (Figure 2(c)) showing a classic sql network (Figure 2(d)). These 2D layers are further extended to the 3D framework (Figure 2(e)) via the L^Cu4 metalloligands (Figure 2(f)), which can be regarded as the pillars of the 3D structure. The packing diagram showing the 3D extending structure of CP-1 from a axis is displayed in 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 ų (38.3%), which is large enough for hosting the solvent molecules (seven DMSO and two CH₃OH). The network analysis based on TOPOS program reveals that CP-1 can be simplified to a (6,6)-connected network with {4⁹ × 6⁶} topology, which is depicted in Figure 4. As in Figure 4, the {Sm₂} clusters act as

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 1.981(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 {Sm₂} cluster, the shortest distance between two Sm atoms is 4.491(1) Å.

3.3. Crystal Structure of 2

The X-ray crystallography study identifies that CP-2 also crystallizes in the triclinic crystal system with P-1 space group and consists of the porous 3D framework structure. The asymmetric unit of CP-2 contains one [Dy(DMSO)₂]³⁺ cation, one [Dy(DMSO)(CH₃OH)]³⁺ cation, two [L^Cu]²⁻ ions and one [L^Cu(DMSO)]²⁻ ion. Interestingly, one of the terminal coordinated O atom from L^Cu has been replaced by a DMSO molecule during the synthetic process of CP-2.

In accordance with Figure 5(a), Dy atoms in CP-2 have two different coordination environments: Dy1 is coordinated by eight O atoms from four L^Cu metalloligands, one L^Cu (DMSO) metalloligand and two DMSO molecules, while Dy2 is coordinated by eight O atoms from four L^Cu metalloligands, one L^Cu (DMSO) metalloligand, one DMSO molecule and one CH₃OH molecule. According to the concrete crystal structure, there are three kinds of L^Cu metalloligands in CP-2. As shown in Figure 5(b) and Figure 5(c), both L^Cu1 (DMSO) and L^Cu2 units in a four-connected mode are linked with two Dy1 and two Dy2 atoms through the carboxylic oxygen atoms. The L^Cu3 unit in CP-2 connects with one Dy1 and one

Dy2 atom via four carboxylic oxygen atoms with the chelate mode (Figure 5(d)). Similarly to CP-1, Dy1 and Dy2 atom in CP-2 can be built to a {Dy₂} cluster with the help of L^Cu1 (DMSO) and L^Cu2 units (Figure 6(a)). The connection of such {Dy₂} clusters and L^Cu metalloligands (Cu2 and Cu3) through the carboxyl groups also forms a quadrangular configuration (Figure 6(b)). As depicted in Figure 6(c), these quadrangular configurations link with each other to construct the 2D network structure with (4,4) topology. With the assist of L^Cu1 (DMSO) (Figure 6(d)), these 2D networks can also be extended to a 3D framework structure (Figure 6(e)). The overall structure of CP-2 can be viewed as a grid-like frame with square channels traversing the framework (Figure 7). The solvent accessible volume of CP-2 is 2945.2 ų (49.6%), which is large enough for hosting the solvent molecules (four DMSO and one CH₃OH). The regular 6-connected mode of {Dy₂} cluster lead to a {4¹² × 6³} topological network (Figure 8). The topology structural difference between CP-1 and CP-2 is mainly due to the distinct connection modes of L^Cu metalloligands in CPs 1 and 2.

The averagelength (1.971(4) Å) of Cu−N bond in CP-2 is little larger than the value that in CP-1 (1.952(5) Å). As for the Cu-O bond in CP-2, the bond lengths range from 1.933(4) Å to 2.388(5) Å. The distance (2.388(5) Å) between Cu

atom and O atom from DMSO molecule is little larger than the value between Cu atoms and O atoms from water molecules (2.238(9) Å for Cu2, 2.306(8) Å for Cu3). Bond lengths of Dy-O range from 2.260(3) Å to 2.808(5) Å. The shortest distance of Dy・・・Dy in {Dy₂} cluster is 5.020(1) Å in the CP-2.

3.4. 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 1 (5.21%) occurs from 50˚C 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 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 MeOH molecules, which is similar to that of CP-1. The following weight loss (14.07%) occurs from 158˚C to 229˚C, which is corresponding to four solvent DMSO molecules (calcd: 14.23%). The third stage of weight loss (14.96%) starts at 280˚C, after that, four coordinated DMSO molecules escape until 342˚C (calcd: 14.23%). With the increase in temperature, the 3D framework of CP-2 begins to collapse.

3.5. 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_mT values of CP-1 at room temperature is 1.62 cm³ K mol⁻¹, which is a little smaller than the theoretical value (1.69 cm³ K mol⁻¹) for a two isolated Sm³⁺ ions (S = 5/2, g = 2/7) and three Cu²⁺ ions (S = 1/2, g = 2) without magnetic interaction. Upon decreasing the temperature, the χ_mT product drops slowly to a minimum of 0.47 cm³ K mol⁻¹ at 1.8 K. This decrease in χ_mT 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 cm³ K mol⁻¹ and negative Weiss constant of θ = −57.7 K.

As shown in Figure 11, the room temperature χ_mT value of CP-2 is 29.36 cm³ K mol⁻¹, which is a little smaller than the theoretical value of 29.48 cm³ K mol⁻¹ for three Cu²⁺ ion (S = 1/2, g = 2) and two Dy³⁺ ions (S = 5/2, g = 4/3) due to the thermally populated excited states of Dy³⁺ [44] . Upon sample cooling, the χ_mT

value decreases continuously to a minimum value of 17.55 cm³ K mol⁻¹ at 7 K. After that, the χ_mT value increases sharply to18.49 cm³ K mol⁻¹at 1.8 K. The transformation trend of χ_mT 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 - 300 K were fitted to the Curie-Weiss law, yielding C = 29.50 cm³ K mol⁻¹ and θ = −16.59 K, which indicates the weak ferromagnetic interactions between the spincenters at 100 - 300 K.

4. Conclusion

In summary, two interesting 3D heterobimetallic coordination polymers, {[Sm₂(DMSO)₄(CH₃OH)₂][L^Cu]₃・7DMSO・2CH₃OH}_n 1 and {[Dy₂(DMSO)₃-(CH₃OH)][L^Cu3(DMSO)]・4DMSO・CH₃OH}_n 2, have been prepared from the reaction between metalloligand L^Cu and lanthanoid ions (Sm³⁺, Dy³⁺). Both CPs 1 and 2 possess the 3D consecutive framework structures, which are constructed from L^Cu metalloligands connecting with six-connected {Sm₂} and {Dy₂} clusters, respectively. Thermal analysis indicates that the skeleton structures of CPs 1 and 2 begin to collapse after 350˚C. Magnetic measurements reveal that CPs 1 and 2 exhibit anti-ferromagnetic properties due to the contribution of 3d-4f couplings. Further work to explore new metalloligand-based 3d-4f heterobimetallic CPs with interesting magnetic and optical properties are currently in progress.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grants 50925207, 51172100, 51432006, and 51602130), the Ministry of Science and Technology of China for the International Science Linkages Program (Grants 2009DFA50620 and 2011DFG52970), the Ministry of Education of China for the Changjiang Innovation Research Team (Grant IRT13R24), the Ministry of Education and the State Administration of Foreign Experts Affairs for the 111 Project (Grant B13025), 100 Talents Program of CAS, Jiangsu Innovation Research Team and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (Grant16KJD430002) are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

References