Journal Information
Journal ID (publisher-id): chemical
Title: Journal of the Korean Chemical Society
Translated Title (ko): 대한화학회지
ISSN (print): 1017-2548
ISSN (electronic): 2234-8530
Publisher: Korean Chemical Society대한화학회
INTRODUCTION▼
Recently we have reported a mixed metal Cd/Hg complex, [Cd(HEAC)2][HgI4], in reaction between cis-2-((2-((2-hydroxyethyl)amino)ethyl)amino)cyclohexan-1-ol (HEAC) and CdI2/HgI2 mixture. In this reaction, two HEAC ligand have been coordinated selectively to the cadmium atom and iodo ligands coordinated to the mercury atom. Based on this observation we decided to examine the reaction between HEAC with other mixtures of the group 12 halide salts. It is interesting issue that the HEAC and halido ligands prefer which metal from the mixture.
Study of the Cambridge Structural Database (CSD)1 revealed that there are a few examples for mixed metal complexes of the group 12 elements. Also this study revealed that there is no example containing Zn/Cd/Hg metals at the same time. Although the mixed metal compounds of the group 12 elements have been considered rarely, but they can form a variety of compounds including heterometallic host-guest cages,2 sensors for detecting the palladium,3 coordination polymers,4−9 non-centrosymmetric (NCS) materials,10 metal clusters,11 luminescence active compounds,12 MOFs13 and nonlinear optical (NLO) materials.6
In this work, the reaction between HEAC with HgCl2/CdCl2, HgCl2/CdI2, ZnCl2/CdCl2 and ZnCl2/CdCl2/HgCl2 mixtures is investigated by spectral (FT-IR, Raman, 1H NMR), structural (X-ray) and theoretical (DFT) methods along with structural investigations of the CSD database.
EXPERIMENTAL▼
All starting chemicals and solvents were obtained from Merck and were used as received. The HEAC ligand was synthesized according to the literature.14,15 Infrared spectra (from KBr pellets) in the range 4000–400 cm−1 were recorded with an FT-IR 8400-Shimadzu spectrophoto-meter. Raman spectra were obtained using a Nicolet Model 910 Fourier-transform spectrometer. The carbon, hydrogen and nitrogen contents were determined using a Thermo Finnigan Flash Elemental Analyzer 1112 EA. The melting point was measured with a Barnsted Electrothermal 9200 electrically heated apparatus.
Synthesis of [Cd2(HEAC)2(µ-Cl)2Cl2], (1)
In all reactions the complex [Cd2(HEAC)2(μ-Cl)2Cl2] (1) was formed as a main product and crystallized from the reaction mixture. Thus its spectral data is gathered at the end of this section to avoid repetition.
HEAC + HgCl2/CdCl2
HEAC (0.20 g, 1 mmol), HgCl2 (0.14 g, 0.50 mmol) and CdCl2·2.5(H2O) (0.11 g, 0.50 mmol) were placed in the large arm of a branched tube. Methanol was carefully added to fill both arms. The tube was then sealed and the ligand-containing arm was immersed in a bath at 60 C while the other was maintained at ambient temperature.16-18
After a few days, colorless crystals that were deposited in the cooler arm were filtered off and dried in air (Yield (0.17 g) 88%). Also same crystals were obtained from the filtrated solution (Yield (0.03 g) 16%).
HEAC + HgCl2/CdI2
The procedure for this reaction was similar to above except that CdCl2 was replaced CdI2 (0.18 g, 0.50 mmol). After five days did not form any product in the branched tube and reaction mixture was filtrated. Suitable crystals for X-ray diffraction studies were obtained by slow evaporation of the solution for three days and collected by filtration. Yield (0.07 g) 36%.
HEAC + ZnCl2/CdCl2
The procedure for this reaction was similar to the first method except that HgCl2 was replaced by ZnCl2 (0.07 g, 0.50 mmol). Yield (0.16 g) 83% for branched tube crystals and (0.02 g) 10% for crystals from the filtrated solution.
HEAC + ZnCl2/CdCl2/HgCl2
The procedure for this reaction was similar to the first method along with addition of ZnCl2 (0.11 g, 0.79 mmol) to the reaction mixture and using of 0.79 mmol for all reagents. After five days did not form any product in the branched tube and reaction mixture was filtrated. An oily compound was formed after evaporation of the solution. Suitable crystals for X-ray diffraction studies were obtained by recrystallization of this compound in the branched tube using the ethanol. Yield (0.06 g) 20%. mp. 222 °C. Anal. Calcd for C20H44Cd2Cl4N4O4 (%): C, 31.15; H, 5.75; N, 7.26. Found: C, 30.97; H, 5.69; N, 7.21. IR (cm−1, KBr): 3355 s (ν OH), 3224 s (ν NH), 2931 s (ν CH), 2862 m (ν CH2), 1450 w (δas CH2), 1380 m (δs CH2), 1200 w (ν CO), 1110 m (ν C6N), 1056 m (ν C7N). Raman (cm−1): 2979 w (ν CH), 2869 w (ν CH2), 1550 m, 1308 m (ν CO), 1130 m (ν CN), 986 m, 848 m, 694 m, 550 m (ν CdN), 415 w (ν CdO), 311 w (ν CdClterminal), 249 w (ν CdClbridging). 1H NMR (300 MHz, DMSO): δ 6.5 (1H, O1H), 4.8 (1H, O2H), 3.5 (2H, N1H, N2H), 2.8-2.9 (7H, C6H, C7H2, C8H2 & C9H2), 1.9-2.0 (3H, C1H, C10H2), 1.6 (2H, C2H2), 1.1 (6H, C3H2, C4H2 & C5H2).
Crystal Structure Determination and Refinement
Diffraction data were collected at 173 K on a Rigaku Oxford Diffraction Gemini Ultra diffractometer. Data processing and absorption correction was carried out using Crysalis Pro.17
The structures were solved with direct methods and refined with least squares using the OLEX2 package.19 All hydrogen atoms were placed at their calculated positions. Selected crystallographic data are presented in Table 1. Diagrams of the molecular structure and unit cell were created using Ortep-III.20,21
Table1.
Crystal data and structure refinement for 1
RESULTS AND DISCUSSION▼
Reaction between HEAC and mixtures of the group 12 metals including, HgCl2/CdCl2, HgCl2/CdI2, ZnCl2/CdCl2 and ZnCl2/CdCl2/HgCl2 were investigated. All crystals obtained from the four reactions were characterized by spectral, elemental analysis and single crystal X-ray diffraction. Our observations revealed that all crystals were same with formula of [Cd2(HEAC)2(μ-Cl)2Cl2] (1). The result of the HEAC + HgCl2/CdI2 reaction was the complex 1. This product is also formed in reaction between cadmium(II) iodide with mercury(II) chloride, confirming that the chloride ions transfer from mercury to the cadmium atom during the coordination process. It seems that the binuclear complex of the cadmium (1) is thermody-namically stable than the other products. The driving force of this reaction could be Hard-Soft-Acid-Base interaction. HEAC has hard O and N (softer than O) donor atoms in the second period and soft Hg(II) likes soft I‾ (HgI2 is more stable than HgCl2).
Based on this assumption a series of the DFT calculations were run to find the most thermodynamically stable product among the six possible homo- and hetero-atomic binuclear structures of the group 12 metals for an isolated molecule (Table 2). The complex 1 is air-stable, and soluble in DMSO. We have reported the structure of the complex 1 previously in reaction between HEAC with cadmium(II) chloride.15
Table2.
Optimized structure for possible homo- and hetero-atomic binuclear compounds of group 12 metals with HEAC along with their CSD average
In all CSD searches which have been presented, for more precise results, the structures containing any error or disorder have been omitted.
Spectroscopic Characterization
The ν (O–H) and ν (N–H) in the IR spectrum of the 1 shifted 88 and 127 cm−1 to the higher frequency than those of the free ligand,15 confirming coordination through these donor sites. In the Raman spectrum, the bridging Cd–Cl stretching frequency is observed at 249 cm−1 which was lower than the terminal Cd–Cl stretching frequency (311 cm−1) and both are in agree with the literature values.27
Numbering used for the 1H NMR spectra of the free ligand is given in Scheme 1. After coordination, O1H signal shifted by 1.73 ppm to lower magnetic field compared to the free ligand.15
Similar shifts were observed for the N1H and N2H signals (1.45 ppm) while no significant shift was observed for the O2H. These observations revealed that the potentially N2O2-donor HEAC act as the N2O-donor in reaction toward the cadmium(II) chloride.
Theoretical Studies
Based on the experimental data, it seems that the complex 1 is the most stable product that can be formed in reaction between HEAC with group 12 metals mixture. In the binuclear structure of 1, (Fig. 1) the cadmium atom has an octahedral geometry by coordination of one HEAC, one terminal and two bridging chloro ligands. Based on this structure, other possible isostructural binuclear complexes were optimized to determine the most stable ones (Table 2). For finding a reasonable structure for all studied compounds, all theoretically structures were compared with the CSD analogues average. In all optimized structures, the M(μ-Cl)2M unit has a rectangular-like structure and thus all M−Cl bond lengths are not equal. In 1opt, the Cd−Cl bond lengths are longer than the CSD average but are in range that have been observed for their analogues (Table 2), thus the formula of [Cd2(HEAC)2(µ-Cl)2Cl2] is reasonable for 1opt. In 2opt, only one of the Zn−Cla (2.463 Å) can be consider as a coordinate bond thus this structure has not a binuclear structure. In 3opt, although two Hg−Cl bond lengths are longer than their analogues average whilst are within the acceptable range for Hg−Cl bond lengths and thus this structure can be considered as a binuclear. In 4opt, the short M−Cla bond lengths of cadmium and zinc atoms are within the CSD range whilst the longer one is out of this range. This observation conducts us to conclude that this structure does not have a binuclear structure and cadmium and zinc complexes are placed in side of each other separately. There is not any CSD example containing Cd(μ-Cl)2Zn unit. All coordinated bond lengths in 5opt are acceptable thus this structure has a hetero atomic binuclear structure. Study of the CSD revealed that there is only one example for complexes containing the Cd(µ-Cl)2Hg unit.28
Figure1.
The ortep diagram of the molecular structure of the complex 1. The ellipsoids are drawn at the 50% probability level.
In the last optimized structure, 6opt, among the four M−Clbridging bond lengths, only Zn−Clb distance (3.556 Å) is not acceptable as coordinate bond and this structure can be considered as a hetero atomic binuclear with one chloro bridge. There is not any example for complexes with Hg(μ-Cl)2Zn moiety. Based on these DFT calculations, among the six possible binuclear complexes only three of them really can be consider as a binuclear structure with formula of [MCl (HEAC)(μ-Cl)2M'Cl(HEAC)] including 1opt, 3opt and 5opt. The total energy of 1opt, 3opt and 5opt were calculated to be −1464.65, −1453.91 and −1459.28 a.u., respectively, confirming that the 1opt is thermodynamically stable than the others and in agree with the result of the experiments.
CONCLUSION▼
In this work, reaction of cis-2-((2-((2-hydroxyethyl) amino)ethyl)amino)cyclohexan-1-ol (HEAC) with group 12 metals mixtures including HgCl2/CdCl2, HgCl2/CdI2, ZnCl2/CdCl2 and ZnCl2/CdCl2/HgCl2 was studied. Based on the structural (X-ray), spectral (FT-IR, 1H NMR) and theoretical (DFT) investigations a binuclear cadmium complex, [Cd2(HEAC)2(μ-Cl)2Cl2] (1) was determined as a most stable compound. In the structure of 1, the cadmium atom has a CdN2O(μ-Cl)2Cl environment and distorted octahedral geometry in which two chloro ligands were located in the axial positions and a N2O-donor HEAC was lied on the equatorial plane along with another chloro ligand. Theoretical and CSD studies revealed that among the six possible homoand hetero-atomic binuclear structures only three of them can form [MCl(HEAC)(μ-Cl)2M'Cl(HEAC)] structure (1opt, 3opt and 5opt). Also these studies revealed that the mixed metal structure containing Cd(μ-Cl)2Hg unit is more stable than the other hetero-atomic mixtures. Qualitatively HEAC seems not like hardest Zn2+ and softest Hg2+, and the computation results may reflect this. So HEAC seems like Cd2+.