Syntheses, Crystal Structures and Characterization of Two Coordination Polymers Based on Mixed Ligands①

WANG Yu-Fng HE Cho-Jun



Syntheses, Crystal Structures and Characterization of Two Coordination Polymers Based on Mixed Ligands①

WANG Yu-Fanga②HE Chao-Junb

a(471934)b(471934)

Two new coordination polymers, namely, {[Cd1.5(bc)2(HL)].H2O}2n(1) and [Mn(ip)(H2L)(H2O)]n(2) (H2L = 3-(1H-pyrazol-4-yl)-5-(pyridin-2-yl)-1,2,4-triazole, Hbc = benzoic acid, H2ip = isophthalic acid) were constructed by solvothermal reaction. The compounds were characterized by elemental analysis, FT-IR spectroscopy, and single-crystal X-ray diffraction. Compound 1 displays a two-dimensional plane structure consisting of [Cd3(bc)2(HL)] subunits. Compound 2possesses a one-dimensional chain structure and is further extended into a 3-D supramolecular architecture via hydrogen bonds. Moreover, photoluminescence studies showed compound 1 exhibits luminescent emissions with emission maxima at 375 nm. Magnetic susceptibility measurements of 2 indicate that domain antiferromagnetic interactions exist between Mn(II) ions. In addition, thermogravimetric properties of 1 and 2 were also measured.

cadmium(II), manganese(II), crystal structure, luminescence, magnetic property

1 INTRODUCTION

The design and synthesis of new coordination polymers are nowadays a challenging research topic that attracts increasing interest due to the creation of various intriguing architectures and their potential applications[1-7]of such metal-organic materials. As we all know, the selection of organic ligands is very important in the construction of coordination poly- mers.Recently, N-donor ligands have been widely employed in the construction of coordination poly- mers or metal-organic frameworks with intriguing structure. To modulate the structure, different poly- carboxylate ligands can be used as auxiliary ligands to direct the self-assembly. Although these appealing topology and interesting properties of metal coordination frameworks that have been synthesized by various N-donor ligands and aromatic carboxylic acid have so far been explored to a great extent in the area of crystal engineering to date[8-11], the synthesis of compositionally and structurally designed MOFs and their composites remains a significant challenge nowadays owing to the difficulty in fine-tunning the phase distributions and architectures of the final products. We selected 3-(1H-pyrazol-4-yl)-5-(pyri- dine-2-yl)-1,2,4-triazole as ligand and benzoic and isophthalicacidsas auxiliary ligands in this paper. Herein we synthesized two new coordination poly- mers, {[Cd1.5(bc)2(HL)]·H2O}2n(1) and [Mn(ip)(H2L)(H2O)]n(2). Those structure inducing physical properties, like thermal stability, photolumine- scence, magnetic properties and so forth is also described and discussed.

2 EXPERIMENTAL

2. 1 Materials and characterization

All reagents were of analytical grade and used without further purification. Elemental analysesfor carbon, hydrogen and nitrogen atoms were carried out on a Vario ELⅢelemental analyzer. The IR spectrum was recorded (400～4000 cm-1region) on a SHIMADZU IR Affinity-1S Spectrometer. All fluorescence measurements were carried out on an F-7000 Fluorescence Spectrophometer (220～240V). Thermogravimetric analyses (TGA) were carried out in nitrogen at a heating rate of 10°C·min-1using a TG/DTA 6300 integration thermal analyzer. Variable-temperature magnetic susceptibilities were measured on a MPMS-7 SQUID magnetometer. Diamagnetic corrections were made with Pascal’s constants for all constituent atoms.

2. 2 Preparation of {[Cd1.5(bc)2(HL)]·H2O}2n (1)

A mixture of Cd(NO3)2·4H2O (92.5 mg, 0.30 mmol), H2L (21.2 mg, 0.10 mmol), Hbc (36.6 mg, 0.30 mmol) and KOH (11.2 mg, 0.2 mmol) was dissolved in H2O, added in a 10 mL small glass bottle after stirring and then sealed in a 25 mL Teflon-lined stainless-steel autoclave at 130 °C for 72 h, followed by slowly cooling to room tempera- ture at a rate of 5 °C∙h-1. Light yellow block-shaped crystals were obtained in 54% yield, washed with water and air-dried. IR (cm-1): 3433(w), 3180(w), 2976(w), 2363(m), 2336(m), 1581(m), 1536(m), 1526(m), 1383(m), 1300(m), 1150(m), 1069(m), 1025(m), 952(m), 848(m), 765(m), 710(m), 670(m), 518(m), 473(m). Anal. Calcd. (%) for C24H19Cd1.5N6O5: C, 45.03; H, 2.99; N, 13.13. Found (%): C, 45.55; H, 2.92; N, 13.10.

2. 3 Preparation of [Mn(ip)(H2L)(H2O)]n(2)

The preparation of 2 was similar to that of 1 except that MnSO4·4H2O and Hbc were used instead of Cd(NO3)2·4H2O and H2ip. Light-brown crystals of 2 were obtained in 50% yield. IR (cm-1): 3429 (w), 3056 (w), 2465(m), 2320(m), 1617 (m), 1595 (m), 1558 (m), 1476 (m), 1401 (m), 1308 (m), 1196 (m), 1057 (m),784 (m), 716 (m). Anal. Calcd. (%) for C18H14MnN6O5: C, 48.12; H, 3.14; N, 18.71. Found (%): C, 48.45; H, 3.12; N, 18.58.

2. 4 Crystal structure determination

Single-crystal X-ray diffraction data of compounds 1 and 2 were collected with a Rigaku Oxford SuperNova diffractometer with a Moradiation (= 0.71073 Å). Intensities were collected and reduced on the program CrysAlisPro (Rigaku Oxford, Version 1.171.39.3a), and a multi-scan absorption correction was applied. The structures were solved by direct methods with SHELXS-97[12]and refined on2by full-matrix least-squares with SHELXL-97[13]. All non-hydrogen atoms were refined anisotropically, and all hydrogen atomswere assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restrains. The details of crystal parameters are summarized in Table 1, and the selected bond lengths are listed in Table 2.

3 RESULTS AND DISCUSSION

3. 1 Structure description

3. 1. 1 Crystal structure of {[Cd1.5(bc)2(HL)]·H2O}2n(1)

Compound 1 crystallizes in the monoclinic space group21/and exhibits a 2D layered framework, and its asymmetric unit contains one and a half Cd atoms, one HL−ligand, two bc-ligands and one uncoordinated water molecule. As shown in Fig. 1, the two Cd(II) ions in 1 are both six-coordinated, in which the Cd(1) ion with 1/2 occupation is coor- dinated by two nitrogen atoms (N(4), N(4)#1) of two different H2L ligands and four oxygen atoms (O(2), O(2)#1, O(3), O(3)#1) from four different Hbc ligands. The Cd(1) is located in an inversion center. While Cd(2) is coordinated by three nitrogen atoms (N(1), N(3), N(6)#2) of two different H2L ligands and three oxygen atoms (O(1), O(3) and O(4)) from two different bc-ligands. Besides, the two adjacent Cd(II) atoms are bridged by O(1) and O(2) atoms of carboxylate group from one bc-ligand and a bifurcated O(3) atom of carboxylate group from another bc-ligand, and another O(4) atom of carboxylate group from this bc-ligand is linked to Cd(2) atom. At the same time, the two adjacent Cd(II) ions are bridged by N(3) and N(4) atoms of triazole from HL-ligand, and N(1) atom of pyridine from this HL-ligand is linked to Cd(2) ion. The Cd–O distances fall in the range of 2.1892(19)～2.5890(19) Å, and the Cd–N distances vary from 2.261(2) to 2.403(2) Å. These bond angles and bond distances all fall in the normal ranges[14]. In compound 1, two pairs of oppositely arranged bc-anions and one pair of oppositely arranged HL-anions bind three Cd(II) ions (two Cd(2) and one Cd(1)) to form the [Cd3(bc)2(HL)] subunit, with the Cd(1)×××Cd(2) sepa- ration to be 3.806 Å. Then the trinuclear subunits are linked to form an infinite two-dimensional network by an HL-ligand bridging mode (Fig. 2).

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) of Compounds 1 and 2

Symmetry transformations used to generate the equivalent atoms: #1: 1–, 1–, 1–; #2: –1/2+, 1/2–, –1/2+for 1; #1:, –1+for 2

Fig, 1. Coordination environment of Cd(II) ions in 1

Fig. 2. 2D network structure of 1

In addition, in the crystal packing, there are hydro- gen bonding interactions among uncoordinated water molecules and nitrogen atoms of HL-ligands and oxygen atoms of bc-ligands (N(5)–H(5)×××O(5)#1 (H×××O/N×××O distance = 1.96/2.749(3) Å, angle = 152.3°); O(5)–H(5A)×××N(2)#2 (H×××N/O×××N distance = 1.94/2.789(3) Å, angle = 171.9°); O(5)– H(5B)×××O(4) (H×××O/O×××O distance = 1.94/2.738(3) Å, angle = 157.2)). The 2D architecture of 1 is also reinforced by extensive hydrogen bonding interac- tions.

3. 1. 2 Crystal structure of [Mn(ip)(H2L)(H2O)]n(2)

X-ray single-crystal diffraction analysis reveals that 2 crystallizes in monoclinic system, space group21/and exhibits a 1D framework. As shown in Fig. 3, the asymmetric unit contains one Mn(II) ion, one H2L ligand and one ip2-ligand. The Mn(II) center is five-coordinated to two nitrogen atoms (N(2) and N(3)) from the same H2L ligand, three oxygen atoms (O(1), O(4)#1) from two different H2ip ligands and one oxygen atom (O(5)) from one coordinated water molecule. The Mn–N distances are 2.223(2) and 2.260(2) Å, respectively. And the Mn–O lengths fall in the range of 2.1653(18)～2.2175(18) Å, which fall in the normal range of those observed in manganese complexes[15]. And there are weak coordinated bonds (Mn(1)×××O(2) = 2.665 and Mn(1)×××O(3) = 2.559 Å) in 2,resulting intwo carboxylate groups of ip2-ligand nearly coplanar. The H2L ligand adopts the chelating mode to connect one Mn(II) atom through one imidazole N atom and one pyridine N atom. At the same time, two carboxyl groups of ip2-ligand in compound 2 are completely deprotonated and exhibit coordination mode μ2-η1:η1to link two Mn(II) ions, finally forming a one-dimensional chain with the Mn×××Mn distances of 10.160 Å.

Fig. 3. Coordination environment of the Mn(II) ion in 2

In the crystal packing, there are H-bonding interactions among pyridine N atom, imidazole N atom, coordinated water molecule and carboxyl groups of ip2-ligand: (N(5)–H(5)×××O(2) (H×××O/N×××O distance = 1.995/2.831 Å, angle = 163.6°); O(5)– H(5A)×××O(1) (H×××N/O×××O distance = 1.865/2.747 Å, angle = 153.8°); O(5)–H(5B)×××O(4) (H×××N/O×××O distance = 1.933/2.756 Å, angle = 162.5); N(7)– H(7)×××O(3) (H×××N/N×××O distance = 2.268/2.918 Å, angle = 132.5°)). In addition,capiling interactions exist between the nearest neighboring benzene rings which parallel each other with the interplane distances of 4.070 Å. These hydrogen bonds andπstacking interactions lead to the formation of a 3-D supramolecular network (Fig. 4), in which the Mn×××Mn separation is 4.800 Å.

3. 2 Luminescence spectra of compound 1

As shown in Fig. 5, the complexes with10metal centers have been investigated for fluorescent properties and for potential applications as fluore- scence emitting materials. It should be noted that H2L ligand displays fluorescence in the solid state at room temperature, while compound 1 exhibits dif- ferent fluorescence. The ligand maximum appears at an excitation wavelengthex= 325 nm, with a maximum emission peak atem= 380 nm. In 1, there is an excitation maximum at 270 nm with a maximum emission peak at 375 nm. It can be seen clearly that compared with the ligand H2L, the emission peak of compound 1 has slight blue shift by 380 to 375 nm. The phenomena should be best ascribed to the metal-to-ligand charge transfer according to literatures[16-18].

Fig. 4. 3D supramolecule structure of 2

Fig. 5. Emission spectra for 1 in the solid state

3. 3 Magnetic properties of compound 2

For 2, the solid-state magnetic susceptibility was measured on a polycrystalline sample at 2000 Oe over the temperature range of 2～300 K. A plot of thecMTT susceptibility data for 2 is shown in Fig. 6. The value ofcMT at 300 K is 8.80 emu·mol-1·K, which is slightly higher than the expected spin-only value for two Mn(II) ions (8.75 emu·mol-1·K). As the temperature is lowered, thecMT value decreases slowly to 8.75 emu·mol-1·K at 2 K. This indicates the presence of weak antiferromagnetic interactions within the sample.

Fig. 6. Temperature dependence ofMT () andM(O) for 2 and their corresponding theoretical curves (solid lines)

The crystal packing of 2, which is driven by hydrogen-bonding, reveals intermolecular Mn×××Mn separation distance of 4.800 Å, regarding the intra- chain Mn···Mn separation distance of 10.160 Å. And the Mn–Mn dinuclear fragment formed by the two Mn–COO×××HO(H)–Mn paths is characterized by the shortest metal-metal separation distance (4.800 Å). Moreover, the magnetic exchange between transi- tion metal centers through hydrogen-bonding interac- tions involving coordinated water molecules is well-exemplified in the literature[19, 20]. The Heisen- berg spin Hamiltonian model(= –12, S1= S2= 5/2)[21, 22]for the isotropic magnetic exchange inte- raction in the dinuclear Mn(2) unit is given in Eq. (1).

cM

A = 55exp[30J/KT] + 30exp[20J/KT] +

14exp[12J/KT] + 5exp[6J/KT] + exp[2J/KT]

B = 1 + 11exp[30J/KT] + 9exp[20J/KT] +

7exp[12J/KT] + 5exp[6J/KT] + 3exp[2J/KT] (1)

A good fit was achieved with the fitting parameters as follows:–0.23 cm-1,= 2.004 and the agree- ment factor R =å[(M)obs-(M)calc]2/å(M)2obsis 1.08 × 10-5. The analysis confirms weak antiferro- magnetic interactions between the Mn(II) atoms bridged by hydrogen-bonding. The hydrogen interac- tions provide an effective pathway for the magnetic exchange interaction between Mn(II) atoms. The smallvalue observed may be explained by the fact that the magnetic orbitals are unfavorably oriented to interact.

3. 4 Thermogravimetric analyses of compounds 1 and 2

In order to further characterize compounds 1 and 2, their thermal analyses were performed under N2atmosphere at a heating rate of 10°C/min in the temperature range of 30～800°C. As shown in Fig. 7, the TGA curves of 1 and 2 indicated that the samples undergo two main weight loss steps and the coor- dinated water molecules were lost in the ranges of 140～190 and 160～240°C for 1 and 2, respec- tively. The weight loss of 2.92% and 4.18% is con- sistent with the calculated values (2.81% and 4.01%). Then the framework began to decompose with con- tinuous weight loss above 260 and 340°C for 1 and 2, respectively. The final residues of 29.63% (calcd. 30.06%) for 1 and 15.58% (calcd. 15.78%) for 2 may be the CdO and MnO powder, respectively. The results suggest that the backbone of 2 is more thermally robust than 1, and can resist decomposition at temperature up to 340°C.

Fig. 7. TGA curves of compounds 1 and 2

3. 5 XRD analyses

In order to check whether the crystal structures are truly representative of the bulk materials, powder X-ray diffraction (PXRD) experiments were carried out for 1 and 2 at room temperature. As shown in Fig. 8, the peak positions of the simulated and experi- mental PXRD patterns are in agreement with each other, demonstrating that the bulk synthesized ma- terials and the measured single crystals are the same.

Fig. 8. PXRD patterns of compounds 1 and 2

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5 July 2017;

16 October 2017 (CCDC 1535311 for 1 and 1560119 for 2)

① This work was financially supported by the National Natural Science Foundation of China (21571093), and the Science and Technology Project of Henan Province (No. 162106000025)

. Wang Yu-Fang. E-Mail: wangyf78@163.com

10.14102/j.cnki.0254-5861.2011-1773