Syntheses, structures, and properties of zinc(II) and copper(II) coordination polymers with imidazole-containing ligands

2.1 Synthesis and crystallization of [Zn(bib)(HCOO)₂·H₂O] (1)

All reagents and solvents were obtained from commercially available sources and of analytical grade. A mixture of Zn(NO₃)₂·6H₂O (0.0297 g, 0.1 mmol), bib (0.0210 g, 0.1 mmol), dimethylformamide (DMF, 2.0 mL), formic acid (2.0 mL), and H₂O (4.0 mL) was stirred and heated in a 20-mL Teflon-lined autoclave at T=373 K for 5 days, and then slowly cooled down to room temperature for crystallization. Colorless block crystals were obtained. Yield: 47% (based on Zn). Elemental analysis C₁₄H₁₄N₄O₅Zn (%): Calcd. C 43.83, H 3.68, N 14.60; found C 43.69, H 3.56, N 14.51. IR (KBr, cm⁻¹): 3442 (vs), 3138 (s), 1618 (vs), 1423 (s), 1234 (s), 1145 (s), 1068 (s), 958(m), 839 (s), 769 (w), 646 (vs).

2.2 Synthesis and crystallization of [Cu(biyb)Cl₂] (2)

A mixture of CuCl₂·2H₂O (0.0170 g, 0.1 mmol), biyb (0.0238 g, 0.1 mmol), DMF (2.0 mL), formic acid (2.0 mL), and H₂O (4.0 mL) was stirred and heated in a 20-mL Teflon-lined autoclave at 373 K for 5 days, and then slowly cooled down to room temperature for crystallization. Blue block crystals were obtained. Yield: 45% (based on Cu). Elemental analysis C₁₄H₁₄N₄Cl₂Cu (%): Calcd. C 45.11, H 3.78, N 15.03; found C 45.02, H 3.76, N 14.94. IR (KBr, cm⁻¹): 1516 (s), 1441 (m), 1276 (m), 1231 (vs), 1102 (vs), 1093 (s), 936 (m), 834 (m), 754 (s), 665 (vs).

2.3 Crystal structure determinations

Diffraction intensities for complexes 1 and 2 were collected at T=296(2) K on a Bruker SMART APEX-II CCD diffractometer employing graphite-monochromatized MoK_α radiation (λ=0.71073 Å). The structure was solved by Direct Methods and refined by full-matrix least-squares on F² using the programs Shelxt-2014 [16] and Shelxl-2014 [17], respectively. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in the calculated positions. H atoms bonded to C atoms were treated using a riding model approximation, with C–H=0.93/0.97 Å and U_iso(H)=1.2U_eq(C) for aromatic H atoms. Water H atoms were refined in a riding mode with the constraint O–H=0.81Å and with U_iso(H)=1.5U_eq(O). The crystallographic data for compounds 1 and 2 are listed in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1936085 (1) and 1936086 (2) contain the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Crystal and molecular structure of compound 1

Single-crystal X-ray structure determination revealed that compound 1 crystallizes in the space group P2/c with Z=2. It consists of one Zn^II ion, two formate anions, one bib ligand, and one free water molecule. The Zn^II atoms reside on crystallographic twofold axes. Each Zn^II center is four-coordinated by two oxygen atoms from two formate anions and two nitrogen atoms from two bib ligands, yielding a distorted ZnO₂N₂ tetrahedral geometry (Fig. 1). The Zn–O/N bond lengths are 1.9540(17) Å/2.0045(17) Å. The O/N–Zn–O/N bond angles are in the range 100.85(11)–113.02(8)° (Table 2). In compound 1, the adjacent Zn^II centers are bridged by bib ligands to form a chain along the crystallographic a-axis, with the Zn···Zn distance of 13.522(3) Å, where all formate anions bristle out from two sides of the chain (Fig. 2). Hydrogen-bonding and aromatic π-π stacking interactions play the major role in packing the molecules into a higher dimensional network. So, through intermolecular O–H···O hydrogen bonds between the free water and a carboxylate O atom of the formate anion [O3–H3A···O2 distance: 2.840(3) Å, angle: 175.79(4)°], the adjacent chains are joined to generate a layer structure (Fig. 3). Further, through aromatic π-π stacking interactions between phenyl and imidazole ring of the bib ligands (centroid-to-centroid distance: 3.755(3) Å), the adjacent layers generate a 3D supramolecular structure (Fig. 4).

Fig. 1:

Coordination environment of Zn(II) in compound 1. (Symmetry code: #1 1 – x, y, 1/2−z.)

Fig. 2:

Chain in crystals of 1.

Fig. 3:

Layer structure in crystals of 1.

Fig. 4:

3D structure in crystals of 1.

3.2 Crystal and molecular structure of compound 2

Compound 2 crystallizes in the space group Pbcn with Z=4. The asymmetric unit of 2 contains one crystallographically unique Cu^II ion, one biyb ligand, and two Cl atoms. As in complex 1, the metal atom resides on a crystallographic twofold axis. Each Cu^II center is four-coordinated by two nitrogen atoms from two biyb ligands and two Cl atoms, yielding a distorted CuN₂Cl₂ square planar coordination [18] (Fig. 5). The Cu–N bond length is 1.9420(16) Å and the Cu–Cl bond length is 2.2634(5) Å. The Cl/N–Cu–N/Cl bond angles are in the range 96.92(6)–144.10(11)° (Table 2). In compound 2, Each Cu^II ion is linked by two different μ₂-bridging biyb ligands with its two neighboring Cu^II ions to form a chain. Interestingly, these chains have two orientations in space (Fig. 6). So, through intermolecular C–H···Cl hydrogen bonds between the Cl atom and a CH₂ unit of the biyb ligand [C4–H4B···Cl1 distance: 3.626(2) Å, angle: 132.5(2)°], these adjacent chains with different orientations are joined to generate a 3D supramolecular structure (Fig. 7).

Fig. 5:

Coordination environment of Cu(II) in compound 2. (Symmetry code: #1 2−x, y, 3/2−z)

Fig. 6:

Chains in crystals of 2.

Fig. 7:

3D structure in crystals of 2.

3.3 IR spectra

In the IR spectrum of compound 1, the broad, strong bands in the frequency region near 3442 cm⁻¹ are assigned to the characteristic stretching peaks of OH of free water molecules. The strong bands that appear around 1618 and 1423 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of the carboxylic group. The strong peaks that appear around 1234, 1145, and 1068 cm⁻¹ correspond to the C=C, C–N, and C–H vibrations of the imidazole ring. In compound 2, the absorption peaks near 1441 cm⁻¹ are assigned to the C–H vibration of the –CH₂– groups. The strong bands that appear around 1231, 1102, and 1093 cm⁻¹ correspond to the C=C, C–N, and C–H vibrations of the imidazole ring.

3.4 Thermogravimetric analysis

Thermogravimetric (TG) analyses were carried out to examine the thermal stability of 1 and 2 from T=20–700°C at a heating rate of 5 K·min⁻¹ under nitrogen atmosphere (Fig. 8). The TG curve of 1 shows two weight loss steps: the first one in the range 80–120°C (obsd. 4.8%, calcd. 4.70%) is assigned to the loss of free water, and the second one of 78.4% in the temperature range 210–460°C corresponds to the decomposition of the bib ligands and formate anions (calcd. 78.26%). The TG curve of 2 shows no weight loss from 20°C until about 180°C, above which a significant weight loss of 83.1% is observed and ended at about 440°C, indicating the release the ligands (calcd. 82.95%). The final decomposition products of 1 and 2 were confirmed to be ZnO and CuO, respectively, by their PXRD patterns.

Fig. 8:

TG curves of compounds 1 and 2.

3.5 Fluorescence properties of compound 1

The luminescence properties of compound 1 and of the free bib ligand were studied in the solid state at room temperature. As shown in Fig. 9, the photoluminescence (PL) spectrum of compound 1 displays a relatively wide emission band with a maximum at 433 nm (λ_ex=307 nm). Upon excitation at 374 nm, the free bib ligand shows an intense emission peak at 444 nm. Compared to the bib ligand, compound 1 shows a blue shift of 11 nm, probably due to the ligand-to-metal charge transfer (LMCT) [19], [20].

Fig. 9:

Emission spectra of the free bib ligand and compound 1.

3.6 Magnetic properties of compound 2

The magnetic behavior of compound 2 was investigated over the temperature range 2–300 K in a field of 10 kOe (1 kOe=7.96×10⁴ A m⁻¹). The magnetic susceptibilities χ_MT and 1/χ_M vs. T plots are shown in Fig. 10. The χ_MT value of 2 at T=300 K is 0.386 cm³ K mol⁻¹, which is only slightly larger than 0.375 cm³ K mol⁻¹ expected for the spin-only Cu(II) ion. Upon cooling, the value of χ_MT decreases continuously down to 50 K, and then rapidly drops near 2 K. The temperature dependence of the reciprocal susceptibility (1/χ_M) obeys the Curie-Weiss law with θ=−1.43 K, C=0.389 cm³ K mol⁻¹, and R=2.38×10⁻⁴ (R=∑[(χ_M)_obs−z(χ_M)_calcd]²/∑(χ_M)_obs²). The value of θ indicates weak antiferromagnetic interactions between adjacent Cu(II) (S=1/2) ions.

Fig. 10:

Thermal variation of 1/χ_M and χ_MT for 2 (∆, 1/χ_M experimental values; ○, χ_MT experimental values and solid lines represent the best fit obtained from the Hamiltonian given in the text).