A Cd complex with a di(imidazolyl)benzene ligand: synthesis, structural characterization, and fluorescence properties

2.1 Structure description of [Cd(L)(SO₄)] (1)

The complex was synthesized by a hydrothermal reaction of CdSO₄ with 1,3-di(4-imidazolyl)benzene (L) at 120°C. Structural analysis shows that complex 1 exhibits a network structure in the orthorhombic system with space group Pnma and Z=4 (Table 1). The L ligand was not deprotonated to give the imidazolate anion despite the addition of KOH to the reaction mixture (see below).

Scheme 1:

Molecular structure of the ligand L.

In the asymmetric unit of 1, there are one centrosymmetric Cd²⁺ ion with an occupancy of 0.5, half of L, and half of an SO₄²⁻ anion. As is shown in Fig. 1a, each Cd²⁺ ion is hexa-coordinated by two nitrogen atoms from two imidazolyl groups and four oxygen atoms from three SO₄²⁻ anions to furnish a distorted octahedral coordination geometry [CdN₂O₄], within which four O atoms occupy the equatorial plane and two N atoms lie at the apexes. The bond distances around Cd²⁺ vary from 2.1825(18) to 2.536(2) Å; the bond angles around Cd(II) are in the range of 57.11(7) to 163.61(8)° (Table 2). In complex 1, the 4-imidazolyl group is not deprotonated and just links two Cd²⁺ cations. Each SO₄²⁻ anion bridges three Cd²⁺ cations. The interconnection repeats infinitely to yield a neutral 2D network. From the perspective of topology, the Cd²⁺ cations can be regarded as a 5-connector and each SO₄²⁻ anion as a three-connected node. As a result complex 1 is a 2-nodal (3,5)-connected 2D net with the point (Schläfli) symbol of (4².6⁷.8)(4².6) (Fig. 1c) [8]. The layers are further connected by hydrogen bonds to form a 3D extended structure (Fig. 1d and Table 3).

Fig. 1:

(a) The coordination environment of the Cd²⁺ 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) Topological view of the 2D network of 1. (d) The 3D framework structure of 1 extended by hydrogen bonds.

2.2 PXRD and TGA

The phase purity of 1 could be proved by a powder X-ray diffraction (PXRD) measurement. As is shown in Fig. 2, the PXRD pattern of the as-synthesized sample is consistent with the simulated one.

Fig. 2:

PXRD pattern of complex 1.

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 3. No obvious weight loss was found before the decomposition of the framework occurring at about 300°C, confirming the absence of solvent in its structure.

Fig. 3:

TGA curve of complex 1.

2.3 Luminescent properties

Previous studies have shown that inorganic–organic hybrid coordination polymers containing metal centers with d¹⁰ electron configuration such as the Cd²⁺ cation can exhibit luminescent properties and may have potential applications as photoactive materials [9, 10]. In the present work, the photoluminescence of complex 1 and the L ligand has been investigated in the solid state at room temperature. As shown in Fig. 4, intensive photoluminescence emission can be observed under the experimental conditions for the complex and the L ligand with emission bands at 405 nm (λ_ex=346 nm) for 1 and 398 nm (λ_ex=340 nm) for the ligand L. The fluorescent emission of the complex may be tentatively assigned to an intra-ligand transition of coordinated L ligands, since similar emission was observed for the free L ligand [11, 12]. The observation of a red shift of the emission maximum in complex 1 compared with the free ligand L originates from the coordination of the ligands to the metal centers [13, 14].

Fig. 4:

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

3.1 Preparation of [Cd(L)(SO₄)] (1)

A reaction mixture of CdSO₄ (20.8 mg, 0.1 mmol), L (21.0 mg, 0.1 mmol), and KOH (11.2 mg, 0.2 mmol) in 8 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 120°C for 3 days. After cooling to room temperature, colorless block crystals of 1 were collected by filtration and washed with water and ethanol several times; the yield was 45% based on the L. – C₁₂H₁₀N₄O₄SCd (418.70): calcd. C 34.42, H 2.41, N 13.38%; found C 34.60, H 2.26, N 13.60%. – IR (KBr pellet, cm⁻¹): ν=3417(m), 3166(m), 1628(s), 1548(s), 1500(s), 1408(s), 1379(m), 1262(m), 1228(m), 1161(m), 1045(m), 960(m), 936(m), 857(m), 803(m), 792(m), 753(m), 650(m), 590(m).

KOH was added to the reaction mixture with the intention to deprotonate the 4-imidazolyl group in the synthesis. This is not the case, however, and in fact, the product is the same without the addition of KOH.

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 by using the program Saint [15], which was also used for the intensity corrections for Lorentz and polarization effects. A semi-empirical absorption correction was applied using the program Sadabs [16]. The structure was solved by Direct Methods (Shelxs-97 [17, 18]) and all non-hydrogen atoms were refined anisotropically by full-matrix least-squares techniques u on F² sing Shelxl-97 [17, 19]. All hydrogen atoms at the C atoms were generated geometrically. The hydrogen atom at N2 could be found at a reasonable position in difference Fourier maps and was located there. The details of crystal parameters, data collection, and refinements are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1538209 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.