Synthesis and structural characterization of a novel copper(II)/lead(II) heterometallic organic–inorganic hybrid

2.1 Preparation and characterization of the copper(II)/lead(II) complex 1

The complex [(terpy)₃CuPb₅Br₁₂] (1) was prepared as a green crystalline product under a hydrothermal condition via the combination of 2,2′:6′,2″-terpyridine, CuCl₂, and PbBr₂. The complex is air stable and insoluble in common organic solvents such as acetonitrile, acetone, alcohol, and dichloromethane.

Complex 1 was characterized by infrared (IR), elemental analysis, and single-crystal X-ray diffraction. Weak IR bands centered at 3057 cm^–1 can be assigned to ν(C–H) of the terpy ligands. A large group of bands in the region 1630–1430 cm^–1 are characteristic of ν(C=N) and ν(C=C) stretches of aromatic groups and reveal the presence of terpy ligands.

The structure of the title complex 1 is shown in Fig. 1 with the atom labeling scheme. The crystal structure data are summarized in Table 1. Selected bond lengths and angles are tabulated in Table 2. Crystals are orthorhombic with space group Pnnm with four formula units. The asymmetric unit contains five lead atoms, 1.5 terpy ligands, one copper atom, and nine independent bromine atoms.

Fig. 1:

Coordination environments of complex 1 and atom numbering adopted. The hydrogen atoms are omitted for clarity. For symmetry operations see Table 2.

In the crystal clusters, [(terpy)₄Pb₉Br₂₀]^2– are observed (Fig. 2), which to our knowledge are unprecedented. These building blocks are connected by a hexabromo-coordinated octahedral atom Pb2 resulting in triply bridging PbBr₃Pb units through edge sharing of octahedra. This leads to the formation of a zigzag chain running along the a axis (Fig. 3a). The clusters are further connected by the dinuclear copper units (terpy)Cu₂Br₄ through four-membered CuBr₂Pb rings leading to a two-dimensional network (Figs. 3b and 4a). The layers are further linked by terpy ligands through weak π–π interactions of Cg(1) and Cg(2) (Fig. 4b) with the distance of 3.774(11) Å (symmetry operation: 1/2 + x, 1/2 – y, 12 – z; Cg(1) stands for ring C1-C2-C3-C4-C5-N1, Cg(2) for ring C11-C12-C13-C14-C15-N3) resulting in a weakly bound 3D network.

Fig. 2:

The [(terpy)₄Pb₉Br₂₀]^2– bromoplumbate(II) cluster in complex 1.

Fig. 3:

Illustrations of the crystal structure of 1: (a) chains running along the crystallographic a axis; atom Pb2 referred to in the text is the hexa-coordinated Pb atom connecting the [(terpy)₄Pb₉Br₂₀]^2– units; (b) chains running along the b axis; the unit connecting the [(terpy)₄Pb₉Br₂₀]^2– is the dinuclear building block (terpy)Cu₂Br₄.

Fig. 4:

Illustrations of the crystal structure of 1: (a) sheets as seen down the crystallographic c axis; (b) projection of the crystal structure as seen down the a axis.

In the following, some key features of the crystal and molecular structure shall be briefly discussed. The Pb–Br distances are in the range 2.8805(13) to 3.799(5) Å, which is consistent with the data reported in the literature [3, 7, 11–14, 30]. The Cu1–Br6 and Cu1–Br7 of 1 are 2.342(2) and 2.7842(19) Å, and the Cu1–N4 and Cu1–N5 bond lengths are 2.030(8) and 2.014(10) Å, respectively, which are in agreement with those reported in other copper complexes [15]. The ligand terpy is almost planar with N–C–C–N torsion angles of 1.74° (Cu) and 1.18° (Pb). The metal ions Pb2, Pb3, Pb4, and Pb5 are in a distorted octahedral coordination environment of six bromine atoms, while Pb1 are coordinated by five bromine atoms and three nitrogen atoms of a terpy ligand. The bromine atoms that are bound to Pb atoms have different connectivities. The Cu1 atom is coordinated to three terpyridyl N atoms, one μ₂-bromine atom (Br7), and two μ₃-bromine atoms (Br6 and Br6^#6, symmetry operation: –x, –y, z), thus forming a Cu₂Br₄(terpy)₂ unit. The Pb1 atom is coordinated to two μ₂-Br atoms (Br8 and Br9) and three μ₃-Br atoms (Br5, Br3, and Br3^#1, symmetry operation: –x, –y+ 1, z). The Pb2, Pb3, and Pb4 atoms are bonded to six bridging bromine atoms. The coordination sphere of Pb5 includes the Br4 (μ₂), Br5 (μ₄), Br7 (μ₂), Br8 (μ₂), Br8 (μ₂), and Br6 (μ₃) atoms.

4.1 Materials and measurements

All solvents, CuCl₂·2H₂O, KBr, PbBr₂, and 2,2′:6′,2″-terpyridine (terpy) were commercially available and purchased from Aladdin Industrial Corporation (Shanghai, China) and used as received. Elemental analyses were performed on an Elementar Vario ELIII elemental analyzer. The IR spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the 4000–400 cm^–1 region.

4.2 Synthesis of the complex [(terpy)₃CuPb₅Br₁₂] (1)

CuCl₂·2H₂O (51 mg, 0.3 mmol) and terpy (139.8 mg, 0.6 mmol) were dissolved in 5 mL H₂O, and the mixture was stirred for 20 min. PbBr₂ (110.0 mg 0.3 mmol) and KBr (35.7 mg, 0.3 mmol) in 5 mL H₂O was added to the above solution. The mixture was stirred for another 20 min. Then it was placed in a Teflon reactor (40 mL) and heated at 150 °C for 24 h. Finally, the temperature was lowered at a rate of 10 °C h^–1 to room temperature to give block-shaped crystals with dark green color. C₄₅H₃₃Br₁₂CuN₉Pb₅ (1) (13 mg, yield based on Cu: 15.7 %). – Analysis for C₄₅H₃₃Br₁₂CuN₉Pb₅: C 19.60, H, 1.21, N, 4.57; found: C 19.58, H 1.18, N 4.61 %. – IR (cm^–1): 3057(w), 2959(w), 1621(m), 1591(s), 1575(s), 1563(m), 1475(s), 1446(s), 1430(w), 1304(w), 1238(w), 1156(m), 1051(w), 1007(m), 971(m), 899(w), 827(w), 775(s), 722(w), 648(m), 633(w), 509(w).

4.3 Crystal structure determination

All measurements were made on a Bruker SMART Apex II CCD diffractometer using graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) by using an φ–ω scan mode at T = 293(2) K. An absorption correction was applied using the program SADABS [31]. The structure was solved by Direct Methods [32] with the program SHELXTL (version 6.10) [31] and refined by full-matrix least-squares techniques on F² with SHELXTL [32, 33]. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically. Disorder of atoms C5, C6, C7, C8, C9, and C10 was found which was modeled with displacement parameter restraints.

CCDC 1036238 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.