Two copper(II) coordination polymers constructed by bis(4-(1H-imidazol-1-yl)phenyl)methanone and dicarboxylate ligands

2.1 Preparation and characterization of the complexes

The two new copper(II)-based coordination polymers were synthesized by solvothermal reactions of Cu(OH)₂ with H₂npa (H₂pa) and bipmo in good yields. The complexes were characterized by elemental analyses and FT-IR spectroscopy. The asymmetric stretching vibrations υ_as(COO⁻) were observed in the range of 1680–1665 cm⁻¹ and the symmetric stretching vibrations υ_s(COO⁻) with 1338–1364 cm⁻¹. The above stretching vibrations are shifted to lower values, compared to the carboxyl frequencies of free H₂npa (H₂pa). The peaks for 1 and 2 from 3057 to 3171 cm⁻¹ are attributed to C–H stretching vibrations.

2.2 Molecular structures of 1 and 2

Numerical details of the crystal structure determinations of 1 and 2 are given in Table 1. Selected bond lengths and angles for 1 and 2 are listed in Table 2. Compound 1, {[Cu(bipmo)(npa)]}_n, features a porous 2D metal organic frameworks, with the asymmetric unit comprising one Cu²⁺ center, one npa²⁻, and one bipmo ligand. As shown in Fig. 1, the Cu1 center is surrounded by three carboxylate oxygen atoms from two crystallographically equivalent npa²⁻ ligands in which the dicarboxylate groups of npa²⁻ present μ₁-η¹:η⁰ monodentate and μ₁-η¹:η¹ chelate coordination modes, and two imidazole nitrogen atoms from two crystallographically equivalent bipmo ligands. It displays a coordination geometry that resembles a distorted square pyramid [τ=0.224; τ=(β−α)/60, where α and β are the two largest bond angles around the Cu(II) center; τ=0 for an ideal square pyramid; and τ=1 for an ideal trigonal bipyramid [19]: O2, O3#1, N1, and N4#2 are in the equatorial positions [Cu1–O=2.013(3), 1.931(3) Å; and Cu1–N=1.984(4), 1.986(3) Å, symmetry code: #1=1−x, 1−y, 1/2+z; #2=1+x, y, −1+z] and O1 is in the axial position [Cu1–O1=2.445(3) Å] (Table 2). These values lie within the normal range for Cu–N and Cu–O bond lengths in similar complexes [20], [21], [22].

Fig. 1:

Coordination environments of complex 1. The hydrogen atoms are omitted for clarity. Symmetry codes: ^#1 1−x, 1−y, 1/2+z; ^#2 1+x, y, z−1.

In 1, the five-coordinated Cu ions connected npa²⁻ and bipmo ligands forming a layer (Fig. 2). In order to better understand the complicated framework, the network topology of the complex was analyzed by the freely available computer program Topos [23]. As depicted in Figs. 1–3, each Cu1 center acts as a 4-connected node to connect two npa ligands and two bipmo ligands. Each npa²⁻ and bipmo unit serves as a bridging linker for the Cu²⁺ ions. From a topological point of view, the framework of 1 can be classified as a 4-connected 2D network with the Schläfli symbol of {6⁵·8} (Fig. 3).

Fig. 2:

A view of the layers in complex 1.

Fig. 3:

View of the 2D simplified framework of 1 and 2.

Compound 2 displays a similar layer structure. As shown in Fig. 4, the asymmetric unit 2 consists of one Cu²⁺ cation, one pa²⁻, and one bipmo ligand. Cu1 is four-coordinated by two oxygen atoms from two different pa²⁻ dianions and two nitrogen atoms from two different bipmo ligands, showing a square-planar coordination geometry. The Cu–N and Cu–O bond lengths lie in the ranges of 1.990(2)–1.975(2) and 1.9386(17)–1.9626(18) Å, respectively.

Fig. 4:

Coordination environments of complex 2. The hydrogen atoms are omitted for clarity. Symmetry codes: ^#1 −x, 1/2+y, −1/2–z; ^#2x+1, y+1, z.

In compound 2, two pa²⁻ dianions connect two Cu²⁺ cations via the carboxylate groups (μ₁-η¹:η⁰ monodentate mode) to afford a [Cupa] unit. Neighboring [Cupa] units are linked by two bipmo ligands to furnish a 2D network (Fig. 5). The bipmo and pa²⁻ ligands, like in 1, connect the Cu²⁺ ions to generate a similar 2D network with the Schläfli symbol of {6⁵·8} (Fig. 3).

Fig. 5:

A view of the layers in complex 2.

It is observed that in the crystals of 1 and 2 a large number of hydrogen bonds are formed between the hydrogen atoms at the nitrogen atoms and the oxygen atoms of the imidazole rings, carbonyl groups, and carboxylate units. The lengths of these bonds are very different, varying from 2.816(5) to 3.785(6) Å, reflecting the difference in strength of these bonds. Obviously, these hydrogen bonds sustain the stability of the three-dimensional structure of the crystals. In addition, C–H···π and π···π interactions are present in the structures.

4.1 Materials and measurements

Reagents and solvents were purchased from Aladdin Industrial Corporation of Shanghai, China, and used as received. Bipmo was prepared according to the literature method [14]. Elemental analyses were performed on an Elementar Vario ELIII elemental analyzer. The infrared (IR) spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the 4000–400 cm⁻¹ region.

4.2 X-ray crystallography

All measurements were made on an Agilent Technology SuperNova Eos Dual system with a micro focus source (MoK_α, λ=0.71073 Å) and focusing multilayer mirror optics. The data were collected at a temperature of 293 K and processed using CrysAlis^Pro [24]. Absorption corrections were applied using the SADABS program [25]. The structures were solved by Direct Methods [26] with the program Shelxtl (version 6.10) [26], [27] and refined by full matrix least-squares techniques on F² with Shelxtl [26], [27]. All non-hydrogen atoms were refined anisotropically. The ligand hydrogen atoms were localized in their calculated positions and refined using a riding model.

CCDC 1504730 and 1504734 contain 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.