Synthesis and structural characterization of a Ni(II) coordination polymer with a tripodal 4-imidazolyl-functional ligand

2.1 Preparation

Ni(ClO₄)₂·6H₂O reacts with 1,3,5-tri(1H-imidazol-4-yl)benzene(H₃L) in the presence of ammonium hydroxide under hydrothermal conditions at T=100°C to yield the complex [Ni(H₂L)₂]·5.5H₂O (1) which is stable in air.

2.2 Structural description of [Ni(H₂L)₂]·5.5H₂O (1)

Single crystal X-ray structural analysis revealed that 1 crystallizes in the monoclinic space group C2/c (Table 1), exhibiting a 2D network structure. As shown in Fig. 1a with the atom numbering scheme, the asymmetric unit of 1 contains one half of a Ni²⁺ cation (located on an inversion center), and one H₂L⁻ ligand as counter anion. The H₃L was deprotonated to give the H₂L⁻ anion, within which each H atom in the moiety of –NH– has the occupancy of 0.667. Each Ni²⁺ cation is six-coordinated with a slightly distorted octahedral coordination geometry with a (NiN₆) donor set. The Ni–N bond lengths are in the range from 2.126(3)Å to 2.137(3)Å; the bond angles N–Ni–N from 88.87(11) to 180°. The average bond length around Ni²⁺ is 2.132Å (Table 2). Each H₂L⁻ links three Ni²⁺ cations (Scheme 1), and each Ni²⁺ is bound by six H₂L⁻. This kind of connectivity repeats infinitely to yield a 2D network structure (Fig. 1b). Topology can be used to further analyze the structure of 1; each Ni²⁺ is bound by six H₂L⁻ and thus treated as a six-connected node; each H₂L⁻ ligand links three Ni²⁺ cations as a three-connected node. So, the structure of 1 can be simplified as a binodal (3,6)-connected 2D kgd network (Fig. 1c). The Point (Schläfli) symbol is (4³)₂(4⁶.6⁶.8³) [18]. Adjacent layers are superposed and connected by hydrogen bonds to show a 2D+2D→3D stacking structure (Fig. 1d).

Fig. 1:

(a) The coordination environment of Ni(II) cations in complex 1 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Symmetry operations: A–1/2–x, 3/2–y, –z; B–1/2+x, –1/2+y, z; C–x, 1–y, –z; D1/2+x, –1/2+y, z; E1/2–x, 3/2–y, –z. (b) View of the 2D structure of 1. (c) Schematic illustration of the topology of 1. (d) View of 2D+2D network-stacking structure in 1.

2.3 Powder X-ray diffractionand thermogravimetric analysis measurements of complex 1

The phase purity of 1 could be proven by powder X-ray diffraction (PXRD) measurement. As shown in Fig. 2, the PXRD pattern of the as-synthesized sample is consistent with the simulated one.

Fig. 2:

The experimental and simulated powder X-ray diffraction pattern of complex 1.

Thermogravimetric analysis (TGA) was carried out for complex 1, and the result is shown in Fig. 3. A continuous weight loss (13.7%) in the temperature range of 92°C–195°C, corresponding to the release of lattice water (calcd 13.98%), and the decomposition of the residue can be observed starting near 400°C.

Fig. 3:

Thermogravimetric analysis curve of complex 1.

3.1 Preparation of [Ni(H₂L)₂]·5.5H₂O (1)

The reaction mixture of H₃L (0.1 mmol, 27.6 mg), Ni(ClO₄)₂·6H₂O (73.1 mg, 0.2 mmol), and aqueous ammonia (25%, 2 mL) in 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 100°C for 3 days. After cooling to room temperature, green block crystals of 1 were collected by filtration and washed with water and ethanol several times (yield 40% based on the H₃L). –C₃₀H₃₃N₁₂O_5.5Ni (708.39): calcd C 50.87, H 4.70, N 23.73; found C 50.59, H 4.42, N 23.44%.–IR (KBr pellet, cm⁻¹): ν=3466 (m), 1617 (s), 1571 (s), 1495 (s), 1448 (s), 1382 (s), 1318 (s), 1271 (s), 1125 (s), 1089 (s), 992 (m), 949 (m), 870 (m), 831 (s), 759 (m), 626 (s).

Caution: The perchlorate salt must be handled with care for the danger of potential explosion.

3.2 X-ray structure determination

The crystallographic data collection for complex 1 was carried out using a Bruker Smart ApexII CCD area-detector diffractometer using graphite-monochromatized MoKα radiation (λ=0.71073 Å) at T=200 K. The diffraction data were integrated by using the program Saint [19], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [20]. The structure of 1 was solved by direct methods, and all nonhydrogen atoms were refined anisotropically on F² by full-matrix least-squares techniques using the Shelxl-97 crystallographic software package [21], [22], [23]. In 1, all hydrogen atoms at C atoms were generated geometrically, while the ones at N12, N32, and N52 were found at reasonable positions in the difference Fourier maps and located there. The H atoms at the interstitial(lattice) water molecule could not be located and thus were excluded from the refinement. The details of crystal parameters, data collection, and refinement are summarized in Table 1; selected bond lengths and angles are listed in Table 2.

CCDC 1538208 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.