Crystal structure and theoretical investigation of bis(cis-1,2-diaminocyclohexane)zinc(II) tetrachloridozincate(II)

2.1 Chemicals

ZnCl₂ and Dach were obtained from Merck Chemical Co. (Germany).

2.2 Synthesis

A solution of 0.12 g (1 mmol) cis-1,2-diaminocyclohexane in 15 mL methanol was added to 0.14 g ZnCl₂ (1 mmol) in 5 mL distilled water. A clear solution was obtained on mixing. After stirring for 15 min, the light yellow solution was filtered and the filtrate was kept for crystallization at room temperature. After 24 h light yellow crystals were obtained (yield=72%, 0.18 g). Physical and spectroscopic data: m.p. 261–263°C. – IR: ν=3288, 3245, 3131, 2933, 2831, 1579, 1129, 1100, 698 cm⁻¹. – Zn₂C₁₂H₂₈N₄Cl₄ (500.9): calcd. C 28.77, H 5.63, N 11.18; found C 28.91, H 5.08, N, 10.60.

2.3 X-ray structure determination

Single crystal data collection was performed at 120 K on an Oxford Gemini S diffractometer equipped with a four-circle goniometer and using graphite monochromated CuK_α radiation. The structure was solved by Direct Methods with Shelx-2013 and refined by the full-matrix least-squares procedure on F² using Shelxl-2014 [17], [18]. All non-hydrogen atoms were refined anisotropically. All C-bonded hydrogen atoms were geometrically placed and refined in riding modes using default parameters. The positions of N-bonded hydrogen atoms were taken from difference Fourier maps and refined isotropically with one averaging U_iso parameter and at fixed N–H distances. The OMIT restraint was used to omit disagreeable reflections with an Error to estimated standard deviation (ESD) ratio exceeding 8. OMIT was used furthermore to cut off a limited number of weak reflections above 2θ=132.0°. Crystal data and details of the data collection are summarized in Table 1.

CCDC 1522955 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.

2.4 DFT calculations

The calculations were performed at the same level of theory as reported previously for other zinc(II) complexes of diamines [14], [20].

3.1 Crystal and molecular structure

An Ortep view of the title complex (1) is shown in Fig. 1 and the geometrical features are listed in Table 2. The crystal structure of compound 1 is built up from discrete bis(cis-1,2-diaminocyclohexane)zinc(II) cations and tetrachloridozincate(II) anions. The Zn(II) ion in [Zn(Dach)₂]⁺² is coordinated by four nitrogen atoms of two chelating Dach ligands displaying a distorted tetrahedral geometry. The angles around the zinc atom are in the range of 85.36(17)–130.91(17)°. The distortion from the regular tetrahedral geometry may be associated with the steric constraints of the cyclic Dach molecules. The two chelating N–Zn–N bite angles in 1 are almost identical (Table 2). The five-membered chelate rings in the dication of 1 approximate to an envelope. The Dach entities exhibit a regular spatial conformation (chair conformation for both cyclohexane rings) with normal distances and angles. The [ZnCl₄]²⁻ anion adopts a slightly distorted tetrahedral stereochemistry with the variation of bond angles from 105.63(5) to 113.45(6)°. The average Zn–N and Zn–Cl bond lengths are in agreement with the values reported for other Zn(II)–Dach [6], [9], [10], [11], [12], [13], [14] and [ZnCl₄]²⁻ complexes [21], [22], [23], [24]. Among the reported Zn–Dach complexes, only one, [Zn(Dach)₂](ClO₄)₂, involves a bis-chelated environment as observed in 1 [6].

Fig. 1:

Ortep diagram (ellipsoids at the 50% probability) of the molecular structure of 1 shows the orientation of the [Zn(dach)₂]²⁺ cation towards the [ZnCl₄]²⁻ anion in the asymmetric unit. All carbon-bonded hydrogen atoms are omitted for clarity. Dotted lines refer to interionic hydrogen bonds.

The packing of the ionic components is stabilized by extensive hydrogen bond interactions between the N–H groups (or C–H) of the Dach ligands and [ZnCl₄]²⁻ anions. All N–H hydrogen atoms are involved in hydrogen bonding. The details of hydrogen bonding are given in Table 3. The linking of cations and anions through N–H···Cl hydrogen bonds leads to the formation of a two-dimensional network. The hydrogen bonding pattern is believed to stabilize the encountered distorted tetrahedral geometry.

3.2 DFT calculations

The fully DFT-optimized structure of [Zn(Dach)₂][ZnCl₄] (1) in vacuo leads to a final structure different from the experimental one. However, in a larger model, {[Zn(Dach)₂][ZnCl₄]}₃ (1)₃, the optimized structure of the central [Zn(Dach)₂][ZnCl₄] complex, is similar to that obtained in the experiment as shown in Figure S1 (Supporting Information). The selected experimental and B3LYP-D3 calculated interatomic distances, and bond angles of the {[Zn(Dach)₂][ZnCl₄]}₃ (1)₃ complex are collected in Table S1 (Supporting Information). The natural bond order (NBO) charges on the selected atoms of the central [Zn(Dach)₂][ZnCl₄] complex are collected in Table S2.