A 3D mixed-valence Cu(I)/Cu(II) coordination polymer constructed by 2-(2-fluorophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline and 1,3-benzenedicarboxylate

2.1 Structure description of 1

Suitable single crystals of 1 were obtained by the in situ hydrothermal redox reaction of Cu(NO₃)₂, HL and 1,3-H₂BDC at 185 °C for 6 days. At high temperature, both water and HL may serve as effective reducing agents for Cu(II) to give Cu(I), and hence to generate a mixed-valence Cu(I)/Cu(II) coordination polymer 1. Single-crystal X-ray diffraction analysis has revealed that compound 1 crystallizes in the monoclinic space group P2₁/c with four formula units [Cu₂(L)(1,3-BDC)] in the unit cell. Hence, as shown in Fig. 1, the asymmetric unit of 1 contains one Cu(I) atom, one Cu(II) atom, one L, and one 1,3-BDC. Selected bond lengths and angles are listed in Table 1. The Cu(I) atom (Cu1) is three-coordinated by two nitrogen atoms from two L ligands and one carboxylate oxygen atom from one 1,3-BDC in a trigonal planar mode. The Cu(II) atom (Cu2) displays a square-pyramidal coordination geometry. The base of the square pyramid geometry is provided by two carboxylate oxygen atoms (O1 and O4C) from two different 1,3-BDC and two nitrogen atoms (N1 and N2) from one L. The apical position of the square pyramid is occupied by one carboxylate oxygen atom (O2D) from a third 1,3-BDC with a substantially longer Cu2–O2D distance of 2.351(3) Å. Each 1,3-BDC anion adopts a tetradentate coordination mode: one carboxylate unit bridges two Cu(II) atoms with a Cu2···Cu2D separation of 4.30 Å (Fig. 1), while the other one coordinates with one Cu(I) atom and one Cu(II) atom. In this fashion, the 1,3-BDC anions link the mixed-valence Cu(I)/Cu(II) atoms to generate a layer structure as is illustrated in Fig. 2. Notably, the HL ligand is deprotonated to L with a negative charge when it coordinates to both Cu(II) and Cu(I) atoms. Each ligand L bridges two Cu(I) atoms and chelates one Cu(II) atom in a tridentate coordination mode. In this way, the ligands L further link the layers into a 3D framework structure (Fig. 3). Topological analysis shows that the coordination polymer 1 is a novel example of the 3D (3,3,4)-connected (4.7.9)(7.8.9)(4.7.8.9³) net with each Cu₂(μ₂-O₂CR)₄ core as a 4-connected node, and each Cu(I) and each L as 3-connected nodes (Fig. 4).

Fig. 1:

Coordination environments of the Cu(II) and Cu(I) atoms in 1. Symmetry transformations used to generate equivalent atoms: (A) x, −y + 3/2, z − 1/2; (B) −x + 1, y + 1/2, −z + 1/2; (C) x, −y + 1/2, z + 1/2; (D) −x + 2, −y + 1, −z + 1; (E) –x + 2, y + 1/2, –z + 1/2.

Fig. 2:

View of the layer structure of 1 constructed by Cu(I), Cu(II) and 1,3-BDC.

Fig. 3:

View of the 3D framework structure of 1.

Fig. 4:

View of the 3D (3,3,4)-connected (4.7.9)(7.8.9)(4.7.8.9³) net in the crystal structure of 1.

2.2 Powder X-ray diffraction

To confirm whether the single crystal examined is truly representative of the bulk material, powder X-ray diffraction (PXRD) was carried out for 1. The experimental and simulated PXRD patterns are shown in Fig. 5. They show that the synthesized bulk material and the measured single crystals are identical within the error limits of the experiment.

Fig. 5:

Simulated (red) and experimental (blue) PXRD patterns of 1.

2.3 Magnetic properties

The temperature-dependent magnetic susceptibility data of compound 1 were measured at an applied magnetic field of 1 kOe (1 kOe = 7.96 × 10⁴ A m^–1) in the temperature range of 2–300 K (Fig. 6). For 1, the χ_mT value at 300 K is 0.859 cm³ mol^–1 K, which falls into the range of two isolated Cu(II) ions (S = 1/2) [26]. Upon cooling, the values of χ_mT decrease quickly, reaching a minimum value of 0.104 cm³ mol^–1 K at 2 K. This feature indicates a strong antiferromagnetic interaction between Cu(II) ions in each binuclear unit [26]. The magnetic susceptibility in the temperature range of 300–90 K obeys the Curie–Weiss law with the Curie constant, C = 2.1 cm³ mol^–1 K, and the Weiss constant, Θ = –457.1 K. The result of the analysis indicates that complex 1 shows a relatively strong exchange interaction between Cu(II) ions in diamagnetic Cu₂(μ₂-O₂CR)₄ units, where the Cu2···Cu2D separation is 4.30 Å (see also Fig. 1). Usually, bridging carboxylate anions induce a strong antiferromagnetic interaction between Cu(II) ions because of the small energy difference of the relevant orbitals of the Cu(II) ion and of the bridging species [27, 28].

Fig. 6:

Plots of the temperature dependence of χ_mT (open squares) and χ_m⁻¹ (open triangles) of 1.

3.1 General

All reagents and solvents used in the experiment were purchased from commercial sources and used without further purification. The C, H and N elemental analyses were conducted on a Perkin-Elmer 240C elemental analyzer. The PXRD patterns of the samples were recorded on a Rigaku Dmax 2000 X-ray diffractometer with graphite monochromatized CuK_α radiation (λ = 0.154 nm). Temperature-dependent magnetic susceptibility data for polycrystalline compound 1 were obtained on a Quantum Design MPMSXL SQUID magnetometer under an applied field of 1 kOe over the temperature range of 2–300 K.

3.2 Synthesis of compound 1

A mixture of Cu(NO₃)₂·3H₂O (0.24 g, 1 mmol), HL (0.32 g, 1 mmol), 1,3-H₂BDC (0.166 g, 1 mmol) and H₂O (10 mL) was stirred for 1 h, and then sealed in an 18 mL Teflon-lined stainless steel container. The mixture was heated at 185 °C for 6 days and then cooled to room temperature at a rate of 10 °C h^–1. Navy blue crystals of 1 were collected in 33 % yield based on Cu. Analysis for C₂₇H₁₄Cu₂FN₄O₄ (604.50): calcd. C 53.64, H 2.33, N 9.27; found C 53.25, H 2.02, N 9.36.

3.3 X-ray structure determination

Single-crystal X-ray diffraction data for 1 were recorded at a temperature of 293(2) K on a Rigaku RAXIS-RAPID image plate diffractometer with MoK_α radiation (λ = 0.71073 Å) using ω scans. Absorption corrections were applied using ψ scans. The structure was solved with Direct Methods (shelxs-97 [29,30]) and refined with full-matrix least squares using shelxl-97 [31,32]. The non-hydrogen atoms of the complex were refined with anisotropic displacement parameters. The hydrogen atoms attached to a carbon atom were generated geometrically. The phenyl ring C22–C27 bearing the fluorine substituent was found to be positionally disordered with respect to the F atom. The distances of C23–F1 and C27–F1′ were restrained to 1.35 ± 0.01 Å.

CCDC 1043326 for 1 contains the supplementary crystallographic data for this paper (Table 2). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.