Synthesis, crystal structures, and properties of three coordination polymers of 5-(1H-imidazol-1-yl) isophthalic acid

Abstract

Three coordination polymers have been obtained by hydrothermal synthesis, namely [Zn(imip)(H₂O)₂]·0.5H₂O (1), [Co(imip)(H₂O)₂]·0.5H₂O (2), and [Cd₂(imip)₂(H₂O)₃] (3) [H₂imip = 5-(1H-imidazol-1-yl) isophthalic acid]. The coordination polymers were characterized by IR spectra, elemental analysis, powder X-ray diffraction, and thermogravimetric analysis. Furthermore, single-crystal X-ray analysis reveals that they have 2D structures, which are extended into 3D networks via O–H···O hydrogen-bonding interactions. The luminescent properties of these coordination polymers were investigated.

Introduction

The design and synthesis of coordination polymers (CPs) has been an important area of research in recent decades [1–9]. These materials not only form intriguing architectures and topologies, but also find potential applications in the areas of luminescence, gas adsorption, catalysis, magnetic materials, drug delivery, and so on [10–21]. However, the prediction of final structures of such crystalline products is still a big challenge, since there are many factors that influence the self-assembly process, such as choice of organic ligands, metal ions, temperature, solvent system, and pH of the reaction system [22, 23]. Hydrothermal synthesis was proved to be an efficient strategy to provide a stable reaction environment [24, 25]. Some transition metals (Zn, Co, Cd) can be effectively chelated or bridged with carboxylic acid ligands [26]; hence, it is very important to choose appropriate metal ions and suitable organic nitrogen-donor or oxygen-donor ligands in the preparation of such materials [27].

Multi-carboxylate ligands have often been selected as multi-functional organic linkers, because of their variable coordination modes, which allow them to adopt various structural topologies [28]. 5-(1H-imidazol-1-yl) isophthalic acid, (H₂imip) which has two carboxylate and one nitrogen donors, has captured the attention of chemists of late [29]. Its coordination polymers have a wide range of structural diversities and potential applications as porous and magnetic materials; furthermore, multi-carboxylate ligands are also capable of functioning as hydrogen bond donors and/or acceptors [30].

In this work, H₂imip was chosen as a ligand for the preparation of functional coordination polymers with the transition metals Zn(II), Co(II), and Cd(II). This led to the successful synthesis of three new CPs, namely [Zn(imip)(H₂O)₂]·0.5H₂O (1), [Co(imip)(H₂O)₂]·0.5H₂O (2), and [Cd₂(imip)₂(H₂O)₃] (3). All three CPs have been characterized by their IR spectra, elemental analysis, PXRD, and single-crystal X-ray crystallography. In addition, the solid-state photoluminescent properties at room temperature and the thermal decomposition processes of the complexes were investigated.

Experimental

All starting materials were purchased commercially and used without further purification. These included H₂imip (Xiya, 99 %), Zn(OAc)₂·2H₂O, CoSO₄·7H₂O, Cd(NO₃)₂·4H₂O (Aladdin, 99 %), and NaOH (Kelon, 99 %). Elemental analyses for C, H, and N were obtained on a PerkinElmer 2400 II elemental analyzer. FTIR spectra were obtained on a PE Spectrum One FTIR spectrometer in the 4000–400 cm⁻¹ region, using KBr pellets. A PerkinElmer Diamond TG/DTA thermal analyzer was used to record simultaneous TG and DTG curves in a static air atmosphere at a heating rate of 10 K min⁻¹ in the temperature range of 25–1000 °C using platinum crucibles. Fluorescence spectra were recorded on an F-4600 FL spectrophotometer analyzer. Powder X-ray diffraction patterns were obtained using a pinhole camera (Anton Paar) operating with point-focused Ni-filtered Cu Kα radiation in the 2θ range from 10° to 50° at a scan rate of 0.08° per second.

Synthesis of complex 1

A mixture of Zn(OAc)₂·2H₂O (43.9 mg, 0.2 mmol), H₂imip (46.4 mg, 0.2 mmol), NaOH (16 mg, 0.4 mmol), and water (9 mL) was sealed in a Teflon-lined stainless steel autoclave reactor (25 mL) and heated at 140 °C for 3 days. After cooling to room temperature at a rate of 5 °C h⁻¹, colorless block crystals of complex 1 were obtained in a yield of 56 % based on H₂imip. Calcd. for C₂₂H₂₂N₄O₁₃Zn₂: C, 38.75; H, 3.23; N, 8.22; Found: C, 38.81; H, 3.28; N, 8.27; IR (KBr, cm⁻¹): 3420(s), 3143(m), 1634(s), 1582(s), 1558(m), 1541(m), 1509(m), 1456(m), 1433(m), 1385(s), 1317(m), 1265(m), 1072(s), 776(s), 744(s), 722(s).

Synthesis of complex 2

Complex 2 was prepared by a similar procedure to that for complex 1, by using a mixture of CoSO₄·7H₂O (56.2 mg, 0.2 mmol), H₂imip (46.4 mg, 0.2 mmol), and NaOH (16 mg, 0.4 mmol). Red block crystals were obtained in 70 % yield based on H₂imip. Calcd. for C₂₂H₂₀Co₂N₄O₁₃: C, 39.50; H, 3.29; N, 8.38; Found: C, 39.57; H, 3.23; N, 8.42; IR (KBr, cm⁻¹): 3446(s), 3137(m), 1626(s), 1579(s), 1508(s), 1434(s), 1393(s), 1316(m), 1265(m), 1072(s), 776(s), 744(s), 722(s).

Synthesis of complex 3

Complex 3 was prepared by a similar procedure to that used for complex 1, using a mixture of Cd(NO₃)₂·4H₂O (61.7 mg, 0.2 mmol), H₂imip (46.4 mg, 0.2 mmol), and NaOH (16 mg, 0.4 mmol). Colorless block crystals were obtained in 68 % yield based on H₂imip. Calcd. for C₂₂H₁₈Cd₂N₄O₁₁: C, 35.71; H, 2.44; N, 7.57; Found: C, 35.76; H, 2.48; N, 7.62; IR (KBr, cm⁻¹): 3448(s), 3150(m), 1626(s), 1584(s), 1548(s), 1464(s), 1401(s), 1120(m), 1067(s), 774(s), 744(s), 670(s).

Crystal structure determination

Suitable single crystals with approximate dimensions (0.11 × 0.11 × 0.11 mm³) were mounted on a glass fiber and used for X-ray diffraction analyses. The data were collected on a Bruker SMART CCD diffractometer at 293(2) K, using graphite-monochromatized Mo Kα radiation (λ = 0.71069 Å). Absorption corrections were applied using SADABS. All structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXTL-97 program package [31]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon were placed in geometrically idealized position and refined using a riding model. The crystallographic data as well as details of the data collection and refinement are listed in Table 1. Selected bond lengths and angles are given in Table 2, and hydrogen bond geometries are given in Table 3.

Table 1 Crystallographic data for polymers 1–3

Table 2 Selected bond lengths (Å) and angles (°) for 1–3

Table 3 Hydrogen bond geometry (Å) for polymers 1–3

Results and discussion

Description of the structures

Single-crystal X-ray diffraction analysis reveals that in spite of their different metal centers, complexes 1 and 2 are isostructural; hence, only the structure of complex 1 is discussed here. Complex 1 crystallizes in the monoclinic system, space group P2(1)/c. As shown in Fig. 1, the Zn atom has a six-coordinate mode, provided by three oxygen atoms from different imip ligands plus a further two oxygen atoms from two coordinated water ligands. The remaining coordination site is occupied by a nitrogen atom from the third imip ligand. The relevant bond lengths are Zn–O3 = 2.027(7) Å, Zn–O5 = 2.147(7) Å, Zn–O6 = 2.461(8) Å, Zn–O1(W) = 2.075(6) Å, Zn–O2(W) = 2.051(6) Å, and Zn–N2 = 2.036(7) Å. The O1W–Zn–N2, O1W–Zn–O3, O1W–Zn–O2W, and O1W–Zn–O5 bond angles are 96.4(3)°, 88.7(3)°, 168.7(2)°, and 90.7(3)°, respectively. The two water ligands occupy the axial positions, in a slightly distorted octahedral geometry overall. In complex 2, the relevant bond lengths are Co–O3 = 2.077(5) Å, Co–O5 = 2.159(5) Å, Co–O6 = 2.341(5) Å, Co–O1(W) = 2.098(6) Å, and Co–O2(W) = 2.098(6) Å, while the O1W–Co–N2, O1W–Co–O3, O1W–Co–O2W, and O1W–Co–O5 band angles are 94.6(2)°, 88.3(2)°, 169(2)°, and 89.7(2)°, respectively (Figs. 2, 3).

Fig. 1

Coordination environment of Zn in complex 1. All the hydrogen atoms are omitted for clarity

Fig. 2

An interesting infinitely extended S curve chain of complex 1. Unnecessary atoms are omitted for clarity

Fig. 3

One-dimensional linear chain of complex 1 along a axis. Unnecessary atoms are omitted for clarity

In complexes 1 and 2, the carboxylate groups of the imip²⁻ ligands adopt two different coordination modes (mode A, scheme 1) to connect the metal atoms. As shown in Fig. 4, it is apparent that the organic anions play an important role in determining the structures of these metal–organic complexes. Furthermore, each imip ligand joins three metal centers together and each metal center is coordinated by three imip ligands, resulting in infinite two-dimensional network structures. There are several different hydrogen bonds, namely O(2)–H(2D)···O(6)#6, O(2)–H(2C)···O(4)#7, O(1)-H(1D)···O(6)#8, and O(1)–H(1C)···O(4)#9. The O···O distances all fall in the range of 2.51(5)–2.93(2) Å. The 2D structure is extended into a 3D framework via O–H···O hydrogen-bonding interactions (Fig. 5).

Scheme 1

Coordination Modes of the carboxylate group in H₂imip

Fig. 4

Two-dimensional network structures of complex 1 in the ab plane. Unnecessary atoms are omitted for clarity

Fig. 5

Three-dimensional polymeric frameworks of complex 1 connected by O–H···O hydrogen bonds (black dashed line). Unnecessary atoms are omitted for clarity

Complex 3 crystallizes in the triclinic system, space group P-1. There are two Cd atoms, two imip ligands, and three water molecules in the asymmetric unit. As shown in Fig. 6, in coordination polymer 3, the two symmetrically independent cadmium atoms adopt distinct coordination environments. The Cd1 atom has a six-coordinate mode with four oxygen atoms from three different imip ligands and a further oxygen atom from a coordinated water ligand, while the remaining coordination site is occupied by a nitrogen atom from the fourth imip ligand. The Cd2 atom has a six-coordinate mode, provided by three oxygen atoms from two different imip ligands and two more oxygen atoms from two coordinated water ligands; the final coordination site is occupied by a nitrogen atom from the third imip ligand. Unlike complexes 1–2, the imip²⁻ ligand in 3 adopts a novel coordination mode (mode B, Scheme 1). In mode B, the imip acts as a imidazole ligand to coordinate with four cadmium ions. As shown in Fig. 7, the three carboxylic oxygen atoms in bidentate and monodentate coordination modes, plus one nitrogen atom of each imip ligand, link the metal centers to form an interesting infinitely extended 2D network structure. In addition, we observe two neighboring planes connected by a group of carboxylate oxygen atoms in chelating-bridging tridentate coordination modes (Fig. 8). The hydrogen bond lengths are O(1)–H(1D)···O(3)#6 = 2.652(4) Å, O(1)–H(1A)···O(4) = 3.228(4) Å, O(2)–H(2C)···O(1)#7 = 2.975(4) Å, O(2)–H(2C)···O(1)#7 = 3.001(4) Å, O(2)–H(2D)···O(10)#9 = 2.721(3) Å, O(7)–H(7C)···O(6)#10 = 2.973(3) Å, and O(7)–H(7D)···O(8)#9 = 2.737(3) Å. Figure 7 shows the 3D polymeric frameworks formed by these OH···O hydrogen-bonding interactions.

Fig. 6

Coordination environment of Cd in complex 3. All the hydrogen atoms are omitted for clarity

Fig. 7

Two-dimensional network structures of complex 3 in the bc plane. All the hydrogen atoms are omitted for clarity

Fig. 8

Two neighboring planes of complex 3 connect by Cd1-O6. All the hydrogen atoms are omitted for clarity

Spectroscopic and XRD properties

In the IR spectra of all three complexes, strong and broad absorption bands at about 3420–3448 cm⁻¹ are attributed to the water ligand symmetric O–H stretching and O–H bending modes, respectively. The medium intensity bands at 3150–3137 cm⁻¹ are assigned to the stretching vibrations of the aromatic C–H groups, while the bands at about 1582, 1509, 1456, and 1385 cm⁻¹ are assigned to the stretching vibrations of the aromatic C=C and C=N bonds. The COO⁻ of the coordinated carboxyl group takes give rise to asymmetric and symmetric stretches at 1630 and 1265 cm⁻¹ in 1, 1626 and 1265 cm⁻¹ in 2, and at 1626 and 1120 cm⁻¹ in 3 (Fig. 9).

Fig. 9

Three-dimensional polymeric frameworks of complex 3 connected by O–H…O hydrogen bonds (purple dashed line). Unnecessary atoms are omitted for clarity. (Color figure online)

In view of the excellent luminescence properties of coordination polymers with d10 metal centers and their potential applications in chemical sensors and photochemistry displays, the luminescence properties of the free H₂imip ligand and complexes 1 and 3 were investigated in the solid state at room temperature. As shown in Fig. 10, an emission peak was observed at 359 nm for H₂imip (λ _ex = 274 nm), which can be ascribed to ligand-centered transitions [32]. Under the same excitation conditions, emission peaks were observed at 363 nm for complex 1 (λ _ex = 268 nm), and 365 nm for complex 3 (λ _ex = 294 nm). Thus, compared to H₂imip, the emission maxima of 1 and 3 are redshifted. The most likely emission mechanism is ligand-to-metal charge transfer (LMCT) [33], given the similar emission bands observed for the free ligand and its complexes. The enhanced luminescence in the complexes can be attributed to the ligand coordination to the metal center, which enhances its rigidity and thus reduces the loss of energy through radiationless pathways [34].

Fig. 10

Solid-state fluorescence emissions recorded at room temperature for free ligand and complexes 1 and 3

The observed and simulated powder XRD patterns of the complex are depicted in Fig. 11. The measured powder XRD patterns are in good agreement with the patterns simulated from the X-ray single-crystal data, confirming the phase purities of the samples. The differences in reflection intensities between the simulated and experimental patterns can be explained by the different orientations of the crystals in the bulk powder sample.

Fig. 11

XRD patterns of the complexes 1–3

Thermogravimetric analyses

Thermogravimetric (TG) analysis was performed in air on freshly grown air-dried crystals of the complexes, and the resulting TG curves are shown in Fig. 12. The first stage indicates that complexes 1 and 2 lose 16.2 and 17.1 % of their respective total weights at temperatures ranging from room temperature to 115 °C for 1 and 131 °C for 2. These weight losses are related to the loss of five water molecules (Calcd. 13.2, 13.5 %, respectively). The second stage occurs between 400 and 560 °C for both complexes 1 and 2 and corresponds to the release of the imip ligand. The corresponding weight losses are 60.7 and 60.1 % (Calcd. 62.9 and 64.1 % for 1 and 2, respectively). After 560 °C, there is no further weight loss. The residues (23.9, 22.0 %) are identified as ZnO and CoO, respectively (Calcd. 23.1 and 22.4 %, respectively). As for complex 3, the host framework was stable up to ca. 91 °C. The first weight loss of 6.4 % occurred between 91 and 160 °C, which is related to the removal of the three coordinated water ligands (Calcd. 7.3 %). The second stage came between 363 and 560 °C, corresponding to the release of the imip ligand; the observed weight loss was 60.7 % (Calcd. 58.0 %). No further weight loss was observed above 560 °C. The residue (obs. 32.9 %) is assigned to CdO (calcd. 34.7 %).

Fig. 12

TG curves for complexes 1–3

Conclusion

In summary, Zn, Co, and Cd coordination polymers were prepared under solvothermal conditions by using H₂imip as a ligand. The results show that H₂imip is a good candidate for the construction of coordination polymers with stable and intriguing structures. The coordination polymers of these different metals show different properties with respect to their thermal stability and fluorescence.