Synthesis, structural characterization, and fluorescence of a Cd(II) coordination polymer with N-donor ligands

2.1 Preparation

The hydrothermal reaction of CdSO₄ with 2,5-di(1H-imidazol-4-yl)thiophene (L) in the presence of aqueous ammonium hydroxide at 100°C gives rise to the complex [Cd(L)(SO₄)]_n, which is stable in air.

2.2 Structural description of [Cd(L)(SO₄)]_n (1)

The determination of the structure of complex 1 by X-ray crystallography has shown that it crystallizes in the monoclinic crystal system with the space group C2/c and Z=4 (Table 1).

As shown in Fig. 1a, in the asymmetrical unit of complex 1 there are one Cd(II) with occupancy of 0.5, one half of centrosymmetric L, and one half of SO₄²⁻. Each Cd(II) cation is six-coordinated by two nitrogen atoms from two different L ligands and four oxygen atoms from three different SO₄²⁻ to furnish a distorted octahedral coordination geometry [CdN₂O₄]: four O atoms define the equatorial plane and two N atoms lie in the apexes. The bond lengths around Cd(II) range from 2.194(2) to 2.438(2) Å; the bond angles around Cd(II) are in the range from 57.49(7)° to 161.95(8)° (Table 2).

Fig. 1:

(a) The coordination environment of the Cd(II) ions in 1 with displacement ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity. (b) The network structure of 1, (c) view of a sulfate-bridged Cd(II) chain in 1, (d) scheme illustrating the topology of 1.

In complex 1, the 1H-imidazol-4-yl groups are not deprotonated, and each of them coordinates to one Cd(II), so the ligand L acts as a bidentate bridging ligand. Two O atoms of SO₄²⁻ chelate to one Cd(II); each of them is also bridging to another Cd(II). The other two O atoms in SO₄²⁻ are free of coordination. Interestingly, the SO₄²⁻ anions bridge the Cd²⁺ cations to form SO₄²⁻-bridged metal chains (Fig. 1b). The ligand L links different metal chains to form a 2D network (Fig. 1c). Given as μ₂ bridges, the ligand L can be considered as a linear connector in the view of topology; SO₄²⁻ anions link three metal atoms as three-connected nodes; Cd²⁺ cation are surrounded by two L and three SO₄²⁻, and can be treated as five-connected nodes. The structure of 1 can be simplified as a 2-nodal (3,5)-connected 2D network with (4².6⁷.8)(4².6) topology (Fig. 1d) [8].

2.3 Thermal stability of complex 1

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 2. No obvious weight loss can be observed before the decomposition of the framework, which further confirms the absence of solvent in the structure. Its framework is stable up to 280°C.

Fig. 2:

TGA curve of complex 1.

2.4 Luminescent properties

Previous studies have shown that coordination compounds containing d¹⁰ metal centers such as Cd(II) may exhibit luminescence properties [9], [10]. Therefore, the luminescence of complex 1 and the L ligand has been investigated in the solid state at room temperature. As shown in Fig. 3, an intensive fluorescence can be observed with emission bands at 406 nm (λ_ex=346 nm) for 1 and at 415 nm (λ_ex=350 nm) for the L ligand. This fluorescence may be tentatively assigned to intra-ligand transitions of the coordinated L ligands, because a similar emission was observed for the free L [9], [10]. The observed blue shift of the emission maximum for 1 versus L may be considered to originate from the coordination interactions between a metal atom and ligand [11], [12].

Fig. 3:

Emission spectra of 1 and the uncoordinated ligand L in the solid state at room temperature.

3.1 Preparation of [Cd(L)(SO₄)]_n (1)

A reaction mixture of CdSO₄ (20.8 mg, 0.1 mmol), L (76.4 mg, 0.2 mmol), and aqueous ammonia (25%, 2 mL) in 10 mL H₂O was stirred for 2 h in air, then transferred to a 25 mL Teflon-lined reactor, and heated in an oven to 100°C for 72 h. After cooling to room temperature, colorless block crystals of 1 were collected by filtration and washed with water and ethanol several times; yield 50% (based on L). – C₁₀H₈N₄O₄S₂Cd (424.72): calcd. C 28.28, H 1.90, N 13.19; found C 28.06, H 2.16, N 12.90%. – IR (KBr pellet, cm ^{− 1}): ν=3446 (m), 3144 (m), 1636 (s), 1513 (m), 1477 (m), 1367 (s), 1265 (m), 1161 (s), 1065 (s), 953 (m), 832 (m), 797 (m), 655 (m), 532 (m).

3.2 X-ray structure determination

The crystallographic data collection for complex 1 was carried out on a Bruker Smart ApexII CCD area-detector diffractometer using graphite-monochromatized MoK_α radiation (λ=0.71073 Å) at 293(2) K. The diffraction data were integrated using the program Saint [13], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [14]. The structure of 1 was solved by Direct Methods, and all non-hydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the program Shelxl-97 [15], [16], [17]. In 1, all hydrogen atoms at C atoms were generated geometrically. The hydrogen atoms at N atoms could be found at reasonable positions in difference Fourier maps. The details of crystal parameters, data collection, and refinement are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1510891 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.