Crystal structure, magnetic, fluorescent, electrochemical properties and thermal stability of a new copper(II) coordination polymer [Cu₂(C₅H₄NCOO)₂(C₇H₅N₄)₂]_n

2.1 Preparation of [Cu₂(C₅H₄NCOO)₂(C₇H₅N₄)₂]_n (1)

A mixture of 4-pyridinecarboxylic acid (24.6 mg, 0.2 mmol), 3-(pyridin-2-yl)-1,2,4-triazole (14.6 mg, 0.1 mmol), and copper(II) acetate (39.9 mg, 0.20 mmol) was dissolved in 12 mL water. The solution was poured into a 25 mL hydrothermal reaction autoclave and kept at 433 K for 4 days. After cooling to room temperature within 12 h, blue block crystals of 1 suitable for X-ray diffraction analysis were obtained in 43 % yield. M.p. 561–563 K. Anal. for [Cu₂(C₅H₄NCOO)₂(C₇H₅N₄)₂]_n: calculated C 47.20, H 2.74, N 21.17; found: C 47.15, H 2.73, N 21.15. Selected IR data (KBr pellet, cm^–1): v= 3398.6(m), 1625.9(s), 1610.6(s), 1355.9(vs), 1143.8.9(m), 690.5(m), 418.6(m).

2.2 X-ray structure determination

The X-ray diffraction measurements for 1 were carried out on a Bruker SMART CCD area detector at 296(2) K by using graphite-monochromatized MoK_α radiation (λ= 0.71073 Å). The structure was solved by Direct Methods and refined by a full-matrix least-squares technique using the programs Shelxs-97 and Shelxl-97 [21, 22]. All hydrogen atoms were generated geometrically and refined isotropically using the riding model.

CCDDC 1001517 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

3.1 Structure description

The crystal structure of 1 is depicted in Fig. 1, a summary of crystal data and refinement conditions is listed in Table 1 and the main bond lengths and bond angles of the complex are listed in Table 2. As shown in Fig. 1, in 1, two neighboring copper(II) ions are coordinated with two 3-(pyridin-2-yl)-1,2,4-triazolyl anions to form a binuclear centrosymmetrical structure. Adjacent binuclear units are linked by 4-pyridinecarboxylate anions (not shown in Fig. 1) to form a three-dimensional network structure. The copper(II) ions are coordinated with three nitrogen atoms from two 3-(pyridin-2-yl)-1,2,4-triazolyl anions, one oxygen atom, and one nitrogen atom from two 4-pyridinecarboxylate anions to give a distorted square-pyramidal coordination geometry, where N1, N4, O2, and N2A define the base plane and N5B occupies the vertex. The bond angles N1-Cu1-N4, N4-Cu1-O2, O2-Cu1-N2A, and N2A-Cu1-N1 are 80.04(11)°, 92.15(11)°, 89.52(11)°, and 97.36(11)°, respectively, and their sum is 359.07°. The bond length Cu1-O2 is 1.960(2) Å. The bond lengths Cu1-N1, Cu1-N4, Cu1-N2A, and Cu1-N5B are 1.992(3), 2.042(3), 1.973(3), and 2.362(3) Å, respectively, and their average length is 2.092 Å, which is in the normal range. The bond length Cu1-N5B [2.362(3) Å] is longer than that of the other Cu-N probably because N5B is on the top of a distorted square-pyramid.

Fig. 1:

Part of the crystal structure of 1 with 30 % displacement ellipsoids and with crystallographic atomic numbering scheme adopted.

All hydrogen atoms are omitted for clarity.

3.2 Magnetic properties

The temperature dependence of the magnetic susceptibility of 1 was investigated from 2 to 300 K with an applied magnetic field of 2 kOe. The curve 1/χ_M versus T is shown in Fig. 2. 1/χ_M is proportional to T in the range of 115–300 K. The linear regression equation is 1/χ_M = 0.145T + 133.94 with a correlation coefficient of 0.9990. According to the Curie–Weiss law, from χ_M = C/(T–θ), the Weiss constant θ can be obtained, being –923.7 K. Its value is negative, which indicates that 1 exhibits antiferromagnetic properties in the paramagnetic regime.

Fig. 2:

Temperature dependence of the magnetic susceptibility of 1 in the form of 1/χ_M vs. T.

3.3 Fluorescence

As illustrated in Fig. 3, the fluorescence of complex 1 was studied in the solid state at room temperature. There are emission bands at 430 nm (λ_ex = 318 nm) for 1. Such fluorescence emissions may be assigned to intraligand π–π* transitions because the free HPT ligand exhibits a similar broad emission at 450 nm upon excitation at 340 nm. The emission bands of 1 are blue shifted by 20 nm compared to the HPT ligand, which is attributed to the coordinative interactions between the metal atom and the ligand. Such emission bands may be tentatively assigned to ligand-to-metal charge transfer. This suggests that the complex may be a good candidate of blue-fluorescent materials.

Fig. 3:

Fluorescence of 1 in the solid state at room temperature (curve a: emission spectrum of 1 and curve b: absorption spectrum of 1).

3.4 Electrochemical properties

A conventional three-electrode system was employed for the cyclic voltammetric measurement, where an Ag/AgCl electrode, a glassy carbon electrode, and a platinum electrode were chosen as the reference electrode, the working electrode, and the counter electrode, respectively. Complex 1 was dissolved in a mixture of methanol and water (volume ratio of 1:1), the resulting solution having a concentration of 1 × 10^–3 mol L^–1. A 0.1 m NaClO₄ solution was used as the supporting electrolyte. The scan range was –0.50 to 0.50 V. The cyclic voltammogram of 1 at a potential scan rate 0.10 V s^–1 is shown in Fig. 4a. There exists an oxidation peak with 0.014 V oxidation potential, which corresponds to the Cu(I)/Cu(II) oxidation process [23].

Fig. 4:

(a) Cyclic voltammogram of 1 in aqueous methanol (scan rate: 0.10 V s^–1) and (b) effect of the potential scan rate (v) on the oxidation peak current (i_pa) and the oxidation peak potential (E_pa).

Under the same conditions, the electrochemical properties of 1 were also measured by linear sweep stripping voltammetry. In the potential scan rate range of 0.10–0.90 V s^–1, the influence of the potential scan rate (v) on the oxidation peak current (i_pa) and the oxidation peak potential (E_pa) was studied. i_pa is proportional to v, and the linear regression equation is i_pa = 24.407v–1.027 (i_pa in μA; v in V s^–1) with a correlation coefficient of 0.9932 (Fig. 4b), which indicates that the electrode reaction process of 1 was controlled by adsorption. In addition, E_pa shifts to a more positive value with increasing v, and it is proportional to ln v. The linear regression equation is E_pa = 0.111 ln v + 0.262 (E_pa in V; v in V s^–1) with a correlation coefficient of 0.9958 (Fig. 4b).

3.5 Thermal analysis

Thermal stability studies of 1 were performed in air. The thermogravimetric-derivative thermogravimetric (TG-DTG) curve is shown in Fig. 5. There are two weight-loss stages from room temperature to 700 °C. The first stage takes place from 240 °C to 340 °C with a weight loss of 43.9 %, corresponding to the release of 3-(pyridin-2-yl)-1,2,4-triazole molecules (calculated 43.9 %). There is a strong endothermic peak near 290 °C, which can be attributed to an endothermic melting of the complex. The second stage occurs at 340 °C–450 °C with a weight loss of 32.0 % resulting from the loss of 4-pyridinecarboxylic acid molecules (calculated 32.1 %). In air, the final product is copper oxide with the residual weight being approximately 24.1 % (calculated 24.0 %).

Fig. 5:

TG-DTG curve of 1.