Oxometallate(VI) (M = Mo, W) Bridged Cadmium(II) Cyclam Dimers Bearing Cis-V Configurations

EXPERIMENTAL

Materials and Methods

All chemicals except solvents were purchased from Aldrich, and were used without further purification. Distilled water was used for all procedures. Infrared spectra of solid samples were recorded on a Perkin-Elmer Spectrum X spectrophotometer between 4000 cm−1 and 400 cm−1 as Nujol mulls on KBr discs. The elemental analyses (C, H, N) were performed on crystalline samples using a CE Instruments EA-1110 Elemental Analyzer at KRICT, Korea. Thermogravimetric analysis was performed on a Perkin-Elmer Model TGA-7 under N2 sweep from 30 to 700 ℃ at a heating rate of 10 ℃/min. The starting material Cd(cyclam)(ClO4)2 was prepared according to the literature procedures.7 Caution! The perchlorate salts are potentially explosive and should be handled with care in small quantities.

Synthesis of [(μ-MoO4)2{Cd(cyclam)}2]·9H2O (1)

To a DMF (5 mL) solution of Cd(cyclam)(ClO4)2 (126 mg, 0.2 mmol) was added an aqueous solution (10 mL) of K2MoO4 (48 mg, 0.2 mmole). The mixture was allowed to stand in an open beaker at room temperature. The colorless blocks of 1 were obtained in a day, which were collected for X-ray diffraction studies and other measurements. Yield ＞ 90%. Found C, 21.56%; H, 5.87%; N, 10.09%. Anal. Calcd. for C20H66Cd2Mo2N8O17 (2): C, 21.67%; H, 5.96%; N, 10.11%. IR (Nujol, cm-1): 3341 (νO−H), 3214 (νN−H), 1668 (δH2O), 846, 827, 814, 802 (νMo−O).

Syntheses of [(μ-WO4)2{Cd(cyclam)}2]·9H2O (2)

Color blocks of 2 were prepared according to a similar procedure described in the synthesis of 1 using K2WO4 instead of K2MoO4. Yield ＞ 90%. Found C, 18.81%; H, 5.11%; N, 8.75%. Anal. Calcd. for C20H66Cd2W2N8O17 (2): C, 18.70%; H, 5.14%; N, 8.72%. IR (Nujol, cm-1): 3392 (νO−H), 3216 (νN−H), 1668 (δH2O), 847, 829, 814, 805 (νW−O).

X-ray Crystallography

X-ray data were collected on a Nonius Kappa CCD diffractometer, using graphite monochromated Mo Kα radiation(λ = 0.71073Å). A combination of 1º ϕ and ω (with κ offsets) scans were used to collect sufficient data. The data frames were integrated and scaled using the Denzo-SMN package.11 The structures were solved and refined using the SHELXTL\PC V6.1 package.12 Refinement was performed by full-matrix least squares on F2, using all data (negative intensities included). For 1 and 2, O5W was refined as a solvent water molecule. The O atom lies on a twofold rotation axis and the H atoms are not visible. The H atoms are most likely disordered and have not been included in the refinement but are included in the molecular formula. For 1 and 2, hydrogen atoms were included in calculated positions. Crystal data and refinement details for 1 and 2 are listed in Table 1.

Table 1.Crystal data and structure refinement parameters for 1 and 2

 

RESULTS AND DISCUSSION

Description of Structures

The dimeric structure of 1, as illustrated in Fig. 1, consists of two cadmium(II) cyclam units and two bridging molybdate ligands. Each molybdate ligand bridges cadmium(II) cyclams in monodentate fashion. The coordination environment around the cadmium(II) ion can be described as a distorted octahedron with four Cd−N and two Cd−O bonds, where the bond distances are Cd−N = 2.312(6), 2.344(5), 2.337(5), 2.325(6) Å and Cd−O = 2.310(4), 2.271(4) Å. The Cd−N and Cd−O distances are comparable to those found in the folded cadmium(II) cyclam complex.7 The Mo−O bond distances of 1.773(4), 1.747(5), 1.758(4), 1.748(4) are observed in 1. A list of selected bond distances and angles is given in Table 2. Since the macrocycle in the cadmium(II) cyclam unit is folded, one of the oxygen atoms from the molybdate ligand occupies an axial position and the other one resides on the equatorial position in the distorted octahedron. The configuration of four nitrogen atoms in 1 shows a Cis-V (S,S,S,S) configuration. The isolation and the understanding of metal cyclams bearing the Cis-V (S,S,S,S) configuration are important to design and develop the drugs exhibiting anti-HIV activity.5 It has been reported that one of the zinc cyclam unit in the Zn2-AMD 3100 complex is folded, showing the Cis-V configuration.9 Under such configuration, the carboxylate group of aspartate residue of CXCR4 coreceptor effectively interacts by covalent bond as well as hydrogen bonds. Two factors are believed to affect the configuration in the metallocyclam. One is the metal ion radius of metal ion which is coordinated to a cyclam and the other is the nature of an additional ligand involving in the coordination to the metallocyclam. The most stable Trans III (S,S,R,R) configuration is commonly observed in many cases.13 However, other configurations such as Trans-I (R,S,R,S), Cis-I (R,S,R,S), and Cis-V (S,S,S,S) are rarely found in the metal complexes of cyclam in the solid state.7−913 The solid state structures of folded cadmium(II) cyclam complexes with Cis-I configuration have also been reported.7−9 The present molybdate bridged cadmium cyclam dimer 1 is another type of a folded cadmium(II) cyclam, but the configuration is Cis-V. The difference between the Cis-V and Cis-V configuration is that all four hydrogen atoms on nitrogen atoms point to the same direction in the Cis-V configuration, whereas two of the four hydrogen atoms on nitrogen atoms face the opposite directions in the Cis-V configuration.

Figure 1.Molecular structure of 1 with atom-labeling scheme. Hydrogen atoms other than those on nitrogen atoms are omitted for clarity.

Table 2.Symmetry transformations used to generate equivalent atoms: #1 −x+1, −y+1, z

In the structure 2, two cadmium cyclam units are linked by bridging of two tungstate ligands, forming a dimer (Fig. 2). The complex also shows a folded cadmium cyclam unit with a Cis-V configuration. But, interestingly enough the absolute configuration of four nitrogen atoms in 2 are all inverse compared to those found in 1, resulting in Cis-V (R,R,R,R) configuration. The selective isolation of Cis-V (R,R,R,R) is unexpected, and it is under current investigation for the possible presence and isolation of a Cis-V (S,S,S,S) enantiomer. A list of selected bond distances, angles and hydrogen bonds for complexes 2 is given in Table 3. The dimeric complexes 1 and 2 extend their structures in the solid state by hydrogen bonds to form 2D supramolecules, respectively (Figs. 3 and 4).

Figure 2.Molecular structure of 2 with atom-labeling scheme. Hydrogen atoms other than those on nitrogen atoms are omitted for clarity.

Table 3.Symmetry transformations used to generate equivalent atoms: #1 −x+1, −y+1, z

Analytical, Spectroscopic and Thermal Properties

The microanalytical results for 1and 2 were consistent with the structures determined by X-ray diffraction methods. The solid state infrared spectrum of 1 shows strong absorptions at 846, 827, 814, 802 cm-1 (νMo−O) due to bridging molybdate ligands.14 In addition, bands at 3341 cm-1 (νO−H), 3214 cm-1 (νN−H) and 1668 (δH2O) are observed, indicating the presence of the macrocycle and the lattice water molecules. Similarly, the infrared spectrum of 2 shows bands at 847, 829, 814, 805 (νW−O),15 3392 cm-1 (νO−H), 3216 cm-1 (νN−H), and 1668 (δH2O) in 2. In the infrared spectra of 1 and 2, no strong absorptions associated with perchlorate ions due to the remaining of starting materials are observable.

Figure 3.Extended structure of 1 by hydrogen bonds.

Figure 4.Extended structure of 2 by hydrogen bonds.

TGA curves for complexes 1 and 2 further support the structures. Complex 1 shows weight losses of 14.54% (calculated 14.64%) over ~54-280 ℃ by three endothermic processes, corresponding to the loss of nine lattice water molecules. On further heating, significant weight loss accompanies due to the loss of macrocyclic ligand. Finally, the CdO and MoO3 residues (observed 51.04%, calculated 49.18%) remain above 700 ℃. For complex 2, the TGA curve shows the loss of nine solvated water molecules (12.63%, calculated 12.63%) over ~74-235 ℃ by two endothermic processes. Subsequent loss of the macrocycle is also observed on further heating. Finally, the CdO and WO3 residues (observed 54.82%, calculated 56.14%) remain above 630 ℃.

In conclusion, by the reaction of Cd(cyclam)(ClO4)2 and oxometallate ligands, two new cadmium(II) cyclam dimers 1 and 2 in which each cadmium(II) cyclam unit shows a folded structure. Both complexes show a Cis-V configuration. However, the absolute configuration of nitrogen atoms is different from each other, showing (S,S,S,S) in 1 and (R,R,R,R) in 2. The identification of folded macrocycles in 1 and 2 by the interactions between cadmium(II) cyclams and bridging oxometallate lignads may provide insights into the design and development of macrocyclic metal complexes in potential anti-HIV agents.