Introduction

There has been particular emphasis on developing new metal complexes with extended planar aromatic ligands to enhance noncovalent binding to transition metal ions. Ligands such as 2,2-bipyridine (bpy), 2,2-bipyridylamine (bpa), and 1,10-phenanthroline (phen) and their derivatives have been found very suitable in this respect [1–5]. As a result of rotational flexibility around the carbon–carbon single bond bridging the two pyridine rings of bpy, and the central amine bridging the two pyridine rings of bpa, the two pyridine planes adopt either a coplanar or tilted conformation in coordination with the metal centers [6, 7]. However, such rotational flexibility is not possible for phen ligands, which remain coplanar even after metal-coordination.

Cadmium(II) ions have been found to induce a variety of pathological conditions, for example cardiovascular diseases, hypertension, and cancer [8, 9]. Cadmium can replace the zinc of superoxide dismutase (SOD) and this results in a drop in the biological activity of SOD [10].

As a part of our ongoing studies on the synthesis of new ligands and their coordination transition metal complexes, and use of these for studies of structural, medicinal, and catalytic application [11–21], we report here the synthesis and crystal structure of the novel di-μ-chloro-bis[chloro(4,7-dimethyl-1,10-phenanthroline)cadmium(II)] dimer complex for the first time. Elemental analysis, FAB-MS (mass spectrometry), FT-IR, electronic absorption, thermal, and NMR studies are also reported.

Experimental

Materials and instrumentation

4,7-Dimethyl-1,10-phenanthroline (dmphen) ligand and cadmium chloride hemipentahydrate CdCl2·2.5H2O were purchase from Across and used as received. Elemental analysis was performed on an Elementar Vario EL analyzer. High-resolution liquid 1H, 13C{1H} NMR spectra were recorded on a Bruker DRX 250 spectrometer at 298 K. Frequencies were: 1H NMR: 250.12 MHz, 13C{1H} NMR: chemical shifts in the 1H and 13C{1H} NMR spectra were measured relative to partially deuterated solvent peaks which are reported relative to TMS. IR data were obtained on a Bruker IFS 48 FT-IR spectrometer. Mass spectra were obtained by FAB-MS with a Finnigan 711A (8 kV), modified by AMD and are reported as mass-to-charge ratio (m/z).

General procedure for preparation of the dimer complex

A solution of dmphen (0.08 g, 3.51 mmol) in CH2Cl2 (5 ml) was added to a solution of CdCl2·2.5H2O (0.07 g, 3.51 mmol) in dry ethanol (10 ml) and the mixture was then stirred for 3 h at room temperature in an open atmosphere. The solution obtained was concentrated to approximately 1 ml under reduced pressure and mixed with 40 ml diethyl ether (yield; 0.12 g, 78 %). Suitable crystals for X-ray diffraction experiments were obtained by diethyl ether diffusion to a solution in CH2Cl2. Colorless prisms were isolated after two days.

CdCl2·2.5H2O was treated with dmphen ligand to produce the dimer complex of formula [CdCl2(C14H12N2)]2 as a white powder, mp = 208 °C. 1H NMR (CDCl3): δ (ppm) 2.91 (br, 12H, CH3), 7.40–8.20 (3m, 12H, Ph, Py), 13C{1H} NMR (CDCl3): δ (ppm) 25.78 (s, 4C, CH3), 122.23–160.22 (6s, 24C, Ph, Py). MS m/z = 783 [M+] for C28H24Cl3N4Cd2. Calculated: C, 42.94; H, 3.09; N, 7.15; Cl, 18.11 %. Found: C, 42.58; H, 3.22; N, 7.01; Cl, 18.07 %.

X-ray structural analysis of the dimer complex

Crystallographic details obtained from determination of the structure of the complex are summarized in Table 1.

Table 1 Crystal data and structure refinement for the dimer complex
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The X-ray data for the crystal of the dimer complex were collected with graphite-monochromated Ag Kα radiation (λ = 0.56087 Å) which was used for measurement of intensity data in the ω-scan mode at a temperature of 298 K. The structure was solved by direct methods and refined by full-matrix least-squares with anisotropic temperature factor, for the non-hydrogen atoms. The hydrogen atoms were located on the difference Fourier map and adjusted to 0.97 Å for bonded atoms. Cell parameters were determined from 35 to 50 automatically centered reflections. The intensity data were corrected for polarization and Lorentz effects. Structure solution and refinement of the structure were realized by use of crystallography software [22].

Results and discussion

Synthesis of the dimer complex

The title complex, which has a dimer structure, was prepared by use of a modification of our literature method [13, 14]. The complex was isolated in good yield from a simple, 3 h, RT reaction of one equivalent of dmphen ligand with CdCl2·2.5H2O under gentle, stirred, open atmosphere conditions, using mixture of dichloromethane and ethanol as solvent (Scheme 1). The white powder complex product is soluble in chlorinated solvents, for example chloroform and dichloromethane, and insoluble in alcohols, water, ethers, and n-hexane.

Scheme 1
scheme 1

Synthesis of the dimer complex

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The structure of the complex was deduced from elemental analysis, infrared spectroscopy, electron-impact FAB-MS, TG/DTA, 1H and 13C{1H} NMR spectroscopy and X-ray single-crystal structure measurements.

Elemental analyses and FAB-MS

The elemental analysis of the complex was in a good agreement with the proposed composition which is further confirmed by the molecular ion peak at m/z = 783 (“Experimental” section). The FAB-MS spectrum confirms the presence of the toxic Cd(II) ion in the complex. The spectrum contains eight peaks assigned to the formula [Cd2Cl4(C14H12N2)2]+ (at m/z = 783, 100 % [M]+; m/z = 781, 83.3 %; m/z = 782, 66.7 %; m/z = 780, 60.5 %; m/z = 779, 45.2 %; m/z = 784, 48.4 %; m/z = 785, 33.9 %; m/z = 786, 16.7 %) as seen in Fig. 1.

Fig. 1
figure 1

FAB-MS spectrum of the dimer complex

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IR spectral investigation

IR analysis of the dimer complex revealed no water or solvent of coordination or adsorbed on the complex lattice. The IR spectra of the complexes were compared with of the spectra of the dmphen free ligand. IR spectra of the complex contained four characteristic absorption peaks at 3090, 2890, 820, and 290 cm−1, which can be assigned to, Ph–CH, Me–CH, Cd–N, and Cd–Cl stretching vibrations, respectively. All other functional group vibrations appeared at their expected positions (Fig. 2).

Fig. 2
figure 2

IR spectrum of the dimer complex (KBr disk)

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Electronic absorption spectral study

The electronic absorption spectrum of the complex was acquired in chloroform at room temperature. Intense transitions were observed for the complex in the UV–visible region. Bands at the high-energy end at 200–290 nm were assigned to intra-ligand electron transfer; no other bands were detected elsewhere (Fig. 3).

Fig. 3
figure 3

UV–visible spectrum of dimer complex dissolved in CHCl3 at RT

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Thermal analysis

The thermal properties of the dimer complex were investigated in the 0–800 °C temperature range under an open atmosphere at a heating rate of 10 °/min. Typical thermal TG/DTA curves of the dimer complex are given in Fig. 4.

Fig. 4
figure 4

TG and DTA thermal curves of the complex

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No coordinated or uncoordinated water molecules were detected, because no weight loss was recorded in the range 25–200 °C; this confirmed IR and XRD results. This complex undergoes simple one-step decomposition with weight loss experimentally 96 %. The coordinated chloride and dmphen ligands were lost from the complex structure between 370 and 600 °C, with a major exothermic DTA peak at 423.7 °C.

NMR investigation

The 1H and 13C{1H} NMR spectra of the complex were recorded in CDCl3 solution to confirm the binding of the dmphen ligand to the cadmium(II); the assignments are given in the “Experimental” section. The 1H and 13C{1H} NMR spectra corroborate the structure of the dimer complex; only two functional group (CH3 and phenyl) signals belonging to the dmphen ligand coordinated with CdCl2 were recorded, No water was recorded in the composition of the complex by 1H NMR (Fig. 5).

Fig. 5
figure 5

1H NMR spectrum in CDCl3 at RT corroborates the structure of the dimer complex

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X-ray structural determination of the complex

Solvent-free crystals suitable for X-ray structural analysis were prepared from the complex. The molecular structure and packing view are shown in Figs. 6 and 7, respectively, selected bond distances and angles are given in Tables 2 and 3, respectively.

Fig. 6
figure 6

Molecular structure of the dimer complex, with atom labeling. Thermal ellipsoids are drawn at the 50 % probability level

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Fig. 7
figure 7

View of packing the complex chains extending along the a axis. The H-atoms not involved in H-bonding have been omitted

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Table 2 Selected bond distances (Å) of the dimer complex
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Table 3 Selected bond angles (°) of the dimer complex
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The coordination environment of the Cd(II) ion in the binuclear desired complex crystallizes in the monoclinic system with the P21/c space group. Each Cd(II) ion is five-coordinated by three Cl and two N atoms, Cd···Cd metal ions are separated by a distance of 3.978 Å. X-ray single-crystal diffraction analysis shows that complex [CdCl2(C14H12N2)]2, is a centrosymmetric dimer bridged through the Cl atoms. The bridging Cd2Cl2 unit is planar and each Cd(II) ion has two different distances to the bridging and terminal Cl atoms. The overall geometry around the cadmium atoms is in a slightly distorted square-pyramidal disposition. Several hydrogen bonds are formed by the terminal Cl atoms and H-Me/H-Ph groups, which may stabilize the structure.

Conclusion

A binuclear cadmium(II) complex has been prepared, and its crystal and molecular structure have been determined by X-ray diffraction. The crystal is monoclinic, space group P21/c, overall distorted square-pyramidal. The structure of the dimer complex was characterized on the basis of elemental analysis, FAB-MS, IR, UV–visible, 1H and 13C NMR spectroscopy, and TG/DTA.

Supplementary material

Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 892180. Copies of this information may be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).