Crystal Structure and Characterization of a New Eight Coordinated Cadmium Complex

Abstract

In this work, a new cadmium complex $[Cd(L)(CH_3COO)_2].2H_2O$ (1) with the ligand L, N,N'-bis(2-pyridinecarboxalidene)-1,2-cyclohexanediamine was prepared and identified by elemental analysis, FT-IR, Raman, $^1H$ NMR spectroscopy and single-crystal X-ray diffraction. The cadmium atom in the crystal structure of 1 has distorted triangular dodecahedral geometry by coordination of the four nitrogen atoms of L and four oxygen atoms of the two acetate ions. Two water molecules are also incorporated in the crystal network. The $O-H{\cdots}O$ hydrogen bonds present in the crystal structure of 1. In this work, three structural surveys including coordination numbers of the cadmium atom, coordination modes of L and resonance in pyridine-2-ylmethanimine-based compounds are presented.

INTRODUCTION

Schiff bases are an important class of ligands that coordinate to metal ions via the azomethine nitrogen and have been studied extensively for their important properties, e.g. their ability to reversibly bind oxygen,1 transfer of an amino group,2 photochromic properties,3 catalytic activity in the hydrogenation of olefins.4 Also many Schiff base complexes show catalytic activity in various reactions at high temperature and in the presence of moisture.5,6 Schiff bases show strong complexing ability towards some toxic metals7 and diverse biological activities, such as antibacterial and antitumor.8,9

In this work the preparation and characterization of the cadmium(II) complex, [Cd(L)(CH3COO)2].2H2O (1) with N,N'-bis(2-pyridinecarboxalidene)-1,2-cyclohexanediamine (L, Scheme 1) is described.

Scheme 1.The structure of L, this numbering scheme used for the 1H NMR spectrum assignments.

The coordination geometry of the cadmium atom and the nature of the ligands afford the coordination numbers of the cadmium atom in range of 2−10.10 In this paper we endeavor to introduce coordination aspects of the tetradentate ligand with cadmium atom.

 

EXPERIMENTAL

All chemicals and solvents were reagent or analytical grade and used as received. The ligand L was prepared according to the literature.11 The carbon, hydrogen, and nitrogen contents were determined in a Thermo Finnigan Flash Elemental Analyzer 1112 EA. The infrared spectrum as KBr pellet was recorded in the range of 400−4000 cm−1 using a FT-IR 8400-Shimadzu spectrometer. The Raman spectrum was obtained using a Nicolet Model 910 Fourier-transform spectrometer. Melting point was determined using a Barnsted Electrothermal 9200 electrically heated apparatus. 1H NMR spectrum was recorded on a Bruker Aspect 3000 instrument operating at 250 MHz; chemical shifts are given in parts per million, with values in reference to an internal standard of TMS.

Synthesis of [Cd (L)(CH3COO)2].2H2O, (1)

Cd(CH3COO)2.2H2O (1 mmol, 0.26 g) was dissolved in EtOH (10 mL) and added with stirring to a solution of L (1 mmol, 0.29 g) in EtOH (10 mL). The reaction mixture was stirred at 60 oC for 2 h. Colorless crystals suitable for X-ray diffraction were obtained from the solution after standing for few days. Colorless crystals (47%), m.p. 209 oC; Anal. Calc. for C22H30CdN4O6 (558.90): C, 47.28; H, 5.41; N, 10.02. found: C, 46.93; H, 5.45; N, 9.93%; IR (KBr disk): 3472 (νas H2O), 3293 (νs H2O), 3058 (ν CHimine and/or CHar), 3016 (ν CHimine and/or CHar), 2932 (ν CH2), 1659 (ν C=Nimine and/or δ H2O), 1581 (ν C=Npy), 1558 (νas COO), 1475 (νs COO and/or ν C=C), 1419 (δas CH2), 1342 (δs CH2), 1149 (ν C−N), 779 and 663 (γ py and/or δ OCO), 625 (ρr H2O), 584 (ρw H2O) cm−1. Raman: 3019 (ν CHimine and/or CHar), 2901 (ν CH2), 1634 (ν C=Nimine and/or δ H2O), 1511 (ν C=Npy and/or C=C), 1391 (δas CH2), 1150 (ν C−N), 804 and 649 (γ py and/or ρr H2O), 494 (ν Cd−N), 369 (ν Cd−O), 246 (ρt H2O). 1H NMR (250 MHz, [D6]DMSO): δ = 1.6−2.1 (m, 8 H, C8H2−C11H2); 3.5 (d, 2 H, C7H, C12H); 4.1 (s, 3 H, CH3COO−); 7.5−8.1 (m, 4 H, C2H, C3H, C16H, C17H); 8.4 (s, 2 H, C6H, C13H); 8.9−8.6 (d, 2 H, C4H, C15H); 8.7−8.8 (d, 2 H, C1H, C18H).

Crystal Structure Determination and Refinement

A suitable crystal of 1 was placed on an Oxford Diffraction Gemini Ultra diffractometer and kept at 150.0 K during data collection. Using Olex-II,12 the structure was determined with the Shelxs13 structure solution program using Direct Methods and refined with the Shelxl13 refinement package using least-squares minimization.

CCDC 870105 for [Cd(L)(CH3COO)2].2H2O (1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.

 

RESULTS AND DISCUSSION

Synthesis and Spectroscopic Characterization

A study using CSD data for complexes of L showed that complex 1 is the first example of a complex with a CdN4O4 environment in which N4 and O4 belong to a tetradentate ligand and two acetate groups, respectively. This study also showed that complexes of Mn,14 Pr, Sm,15 Cu,16 Gd, Ce, Yb, Nd, Er,17 Pt,18 Mo,19 Ag,20 and Pd21 with L were reported previously. All coordination modes of L are presented in Table 1. Among these modes, the three chelate form which was observed in 1 is most frequent.

Table 1.Different types of coordination modes of L collected by CSD

Reaction between L and aqueous solution of cadmium (II) acetate in a molar ratio of 1:1 gave 1 which was characterized by IR, Raman, 1H NMR spectroscopy and X-ray crystallography. This complex is air-stable and soluble in DMSO and DMF.

In the IR spectra of 1 and L, there are two bands near 1650 and 1600 cm−1 which were assigned to the vibrations of the ν (C=Nimine) and ν (C=Npy) respectively. The ν (C=Nimine) in the IR spectrum of 1 shifted 16 cm−1 to higher energy indicating coordination through the nitrogen atom.

Three bands at 1558, 1475 and 663 cm−1 which were assigned to the νas (COO), νs (COO) and δ (OCO) respectively, confirmed the presence of acetate ion in 1. The differences between asymmetric (νas) and symmetric (νs) stretching of the acetate group (Δ) can reveal its coordination type. The Δ value for acetate salt is 164 cm−1. In unidentate complexes, Δ values are much greater than the acetate salt while in bidentate complexes these values are significantly less than the acetate salt.22a The Δ value for 1 is 83 cm−1, thus acetate ion here acts as a bidentate ligand.

The presence of a water molecule in 1 affects the IR and Raman spectra in three regions including 3200−3550 cm-1 for asymmetric and symmetric OH stretches, 1600-1630 cm-1 for H2O bending and 200−600 cm-1 for “librational modes”. These modes are due to rotational oscillations of the water molecules restricted by interactions with neighboring atoms and they are classified into three types (wagging (ρw), twisting (ρt) and rocking (ρr)) depending upon the direction of the principal axis of rotation.22b,23 We assigned all types of the vibrations for H2O molecules in the IR and Raman spectra of 1 but vibrations such as δ (H2O), ν (C=C) and γ (py) overlapped with vibrations due to ν (C=Nimine), νs (COO) and δ (OCO), respectively.

Information about the low-frequency vibrations of metal-ligand bonds can be obtained by Raman spectroscopy.24 In the Raman spectrum of 1, two bands at 494 and 369 cm-1 were assigned to the ν (Cd−N)25 and ν (Cd−O)26 respectively. Attention to the Cd−N and Cd−O bond lengths confirms the higher stretching vibration of the Cd−N bond in respect to the Cd−O bond.

The numbering scheme used for the 1H NMR spectrum of 1 is given in Scheme 1. In the 1H NMR spectrum of L,11 the signal at 8.3 ppm was assigned to the C6H proton. After coordination to 1, the C6H signal is shifted by 0.1 ppm to lower field.

Description of the Crystal Structure

The crystal structure of [Cd(L)(CH3COO)2].2H2O (1) has been determined by single-crystal X-ray diffraction analysis. Diagrams of the molecular structure and unit cell were created using Ortep-III,27 Mercury28 and Diamond.29 Crystallographic data and details of the data collection and structure refinement are listed in Table 2. Selected bond lengths and angles for complex are listed in Table 3 and hydrogen bond geometries are presented in Table 4.

Table 2.Crystal data and structure refinement for [Cd(L)(CH3COO)2]. 2H2O

Table 3.Selected bond length (Å) and angles (º) for [Cd(L) (CH3COO)2].2H2O with estimated standard deviations in parentheses

Table 4.Hydrogen bond geometries (Å, º) for [Cd(L)(CH3COO)2].2H2O

In the crystal structure of 1 (Fig. 1), the cadmium atom is coordinated by two imine and two pyridine nitrogen atoms of L and four oxygen atoms of the two acetate ions in distorted triangular dodecahedral geometry (Fig. 2).

Figure 1.The Ortep-III diagram of the molecular structure of 1. The displacement ellipsoids are drawn at the 20% probability level. The hydrogen atoms of ligand were omitted for clarity.

Figure 2.Distorted triangulated dodecahedral geometry around the cadmium atom in 1.

The bond length averages of the two Cd−Nimine and two Cd−Npy bonds are 2.404 and 2.432Å respectively, which are larger than those of CSD average (2.307 and 2.354 Å for Cd−Nimine and Cd−Npy, respectively). In the crystal structure of 1, the cyclohexane ring has a chair conformation and two chiral centers on the C7 and C12 with the same enantiomeric forms. The unit cell of 1 contains two complexes with a racemic mixture of the R,R and S,S isomers.

In complex 1 the bond length of C6=N2imine (1.253(6) Å) is shorter than the C=Npy (1.336(5) and 1.340(6) Å for C1=N1 and C5=N1, respectively) and comparable to isolated imine bond lengths in the literature.30 This means that probably there is no resonance or very weak resonance between the pyridine ring and imine group in this structure. The average of all 6-substituted imine bonds in pyridine-2-ylmetha-nimine-based structures (Scheme 2) is 1.264 Å (calculated by Vista31) that confirms this result.

Scheme 2.The base was used for study of imine bond lengths in CSD.

Figure 3.Packing of 1, showing R22(6) hydrogen bond motifs and C(sp3)−H···π, O−H···O hydrogen bonds. Only the hydrogen atoms involved in hydrogen bonds are shown.

In the crystal packing of 1 (Fig. 3), two oxygen atoms of each acetate group coordinate to cadmium atom and at the same time form a R22(6) motif32,33 by O(5)−H(5A)···O2 and O(5)−H(5B)···O3 strong hydrogen bonds34 with the water molecule. The other water molecule acts as hydrogen bond bridge and connects two coordinated acetate groups in adjacent complexes by O(6)−H(6A)···O1 and O(6)− H(6B) ···O4 hydrogen bonds. The Cd−O bond lengths (2.439(4)−2.469(4) Å) and O−Cd−O angles (52.5(1), 53.0(1)o) are comparable with analogues in the CSD (2.413 Å, 53.698o).

 

CONCLUSION

In this work [Cd(L)(CH3COO)2].2H2O (1) was synthesized from the reaction between cadmium acetate and L, (N,N'-bis(2-pyridinecarboxalidene)-1,2-cyclohexanediamine) and its spectral (IR, Raman, 1H NMR) and structural properties were investigated. Structural analysis of 1 exhibits distorted triangular dodecahedral geometry around the cadmium atom. A study of the CSD structures revealed that the cadmium atom has coordination numbers in range of 2−10. Difference between asymmetric (νas) and symmetric (νs) stretching in the IR spectrum of 1 is 83 cm−1 which confirms the acetate ion acts as a bidentate ligand. A survey about complexes of L revealed that this ligand has three coordination modes. The synthesis of new complexes with L is ongoing by our research group.