3.1. Characterization of Cd complex
3.1.1. Structure description
The molecular structure of the Cd complex was examined using single-crystal X-ray diffraction and revealed the title compound crystallizes in a monoclinic system, space group C2/c and Z = 4 in the unit cell. All information about the crystallographic data and structure refinement of the Cd complex are summarized in Table 1. The selected bond distances and angles are labeled in Table 2. The molecular structure of the Cd complex and the atomic numbering scheme is shown in Fig. 1. The Cd2+ is coordinated to three nitrogen atoms and four oxygen atoms forming a distorted pentagonal bipyramid while atoms O3 and O1 of two coordinated water molecules in the apical position and the rest of the atoms are at the base. A polyhedral representation around Cd2+ is shown in Fig. 2. The Cd–N bond lengths have the values of 2.270 (4) and 2.350(3) Å for Cd1–N2 and Cd1–N1 respectively, in good agreement with the corresponding ones previously reported for cadmium complexes [23, 24]. The Cd-O bond distances varied between 1.963Å and 2.185Å, indicating a single bond character, which supports the values reported for cadmium complexes [25]. The bond angles for N-Cd-N varied in the range of 70.06(12)–144.97 (6), for N-Cd-O in the range of 69.67(6)–145.17(9), and for O-Cd-O in the range of 87.65(10)–179.99(14), which are ranges similar to those presented in cadmium complexes [26]. Furthermore, in the crystal packing of the complex, the neighboring sheets linked together into a three-dimensional structure by a high amount of O–H···O hydrogen bonds (Fig. 3).
Table 1
Crystallographic data for the Cd(II) complex.
Empirical Formula | C17H21CdN3O9 |
Mr | 523.77 |
Crystal system, space group | Monoclinic, C2/c |
a (Å) | 13.2291(6) |
b (Å) | 15.8453(5) |
c (Å) | 10.5638(5) |
β (°) | 119.366(4) |
V (Å3) | 1929.84(16) |
Z | 4 |
F (000) | 1056 |
Dimensions (mm) | 0.30 x 0.25 x 0.25 |
µ (mm− 1) | 1.19 |
ρcalc (g.m− 3) | 1.803 |
T (K) | 120(2) |
Radiation, λ (Å) | Mo-Kα, 0.71073 |
θ range (°) | 2.18–25.37 |
Limiting indices | h = − 15→15 k = − 19→19 l = − 12→12 |
Measured, unique, observed [I > 2σ(I)] data | 7927, 1765, 1608 |
Rint | 0.087 |
Tmin/Tmax | 0.175 |
R[F2 > 2σ(F2)], wR(F2), S | 0.031, 0.070, 1.13 |
Reflections, parameters, restraints | 1765, 186, 224 |
Δρmax, Δρmin (e Å−3) | 1.44, − 1.89 |
CCDC no. | 2027364 |
Table 2
Selected experimental and computational bond distances (Å) and bond angles (°) in the title complex.
Bond distances | Exp. | Comp. | Bond angles | Exp. | Comp. | Bond angles | Exp. | Comp. |
Cd1-O3 | 1.963 | 2.373 | O3-Cd1-O3 | 179.99 | 154.97 | O3-Cd1-N1 | 88.70 | 82.03 |
Cd1-O3 | 1.963 | 2.396 | O3-Cd1-O1 | 92.35 | 63.24 | O3-Cd1-N1 | 91.30 | 102.18 |
Cd1-O1 | 2.184 | 2.541 | O3-Cd1-O1 | 87.66 | 111.65 | O1-Cd1-N1 | 75.40 | 78.17 |
Cd1-O1 | 2.185 | 2.301 | O3-Cd1-O1 | 87.65 | 92.67 | O1-Cd1-N1 | 145.17 | 144.87 |
Cd1-N2 | 2.270 | 2.393 | O3-Cd1-O1 | 92.35 | 89.94 | N2-Cd1-N1 | 144.97 | 142.41 |
Cd1-N1 | 2.350 | 2.445 | O1-Cd1-O1 | 139.35 | 135.26 | O3-Cd1-N1 | 91.30 | 86.58 |
Cd1-N1 | 2.350 | 2.432 | O3-Cd1-N2 | 90.01 | 91.12 | O3-Cd1-N1 | 88.70 | 93.94 |
O1-C1 | 1.285 | 1.325 | O3-Cd1-N2 | 90.00 | 84.41 | O1-Cd1-N1 | 145.17 | 146.54 |
O2-C1 | 1.224 | 1.261 | O1-Cd1-N2 | 69.68 | 69.93 | O1-Cd1-N1 | 75.41 | 79.23 |
N1-C5 | 1.343 | 1.363 | O1-Cd1-N2 | 69.67 | 65.93 | N2-Cd1-N1 | 144.97 | 149.10 |
3.1.2. FT-IR study
The FT-IR spectrum of the Cd complex is shown in Fig. 4. A broad and strong stretching vibration band in the range of 3200–3600 cm− 1, which is the characteristic region for stretching vibrations of OH groups, shows the presence of coordinated and lattice water molecules [27]. The absorption band at 3006 cm− 1 can be attributed to stretching vibration of aromatic C-H bonds [28]. The absorption band at 1620 cm− 1 is due to the asymmetric [ʋas(COO)] of carboxylate groups and advocates the presence of acetate groups bonded to the cadmium center. This bond is overlapped with the bending vibrations of coordinated and lattice water molecules [29]. The characteristic absorption band at 1412 cm− 1 is assigned to symmetric [ʋs(COO)] stretching vibrations of carboxylate groups [30]. The Δν is the frequency difference between the asymmetric and symmetric stretching modes of carboxylate groups. Δʋ=208 cm− 1 suggests that the carboxylate groups are coordinated in a chelate mode to the cadmium center [31]. The characteristic absorption bands at 1105 and 916 cm− 1 are assigned to the C-H in-plane and out-of-plane bending modes, respectively [32, 33]. The appearance of absorption bands at 858 and 759 cm− 1 can be related to the ring-wagging vibrations of the pyridine [34]. Finally, absorption bands at 659 and 602cm− 1 are attributed to the stretching vibration of Cd-O and Cd-N, respectively.
3.1.3. Thermal stability
The thermal studies (TGA and DTA) provide useful data on the thermal stability of metal complexes. The thermal analysis curve of Cd complex was recorded from 25 to 600°C under N2 atmosphere (Fig. 5). The 48 mg of Cd compound was heated at a rate of 10°C.min− 1 while α-Al2O3 powder was used as reference. TGA curves of Cd complex showed three decomposition steps. The initial weight loss that happened between 25–225 ◦C, corresponded to the loss of lattice and coordinated water molecules. All organic parts of Cd complex are decomposed in two stages above 225 ◦C. The weight loss continued up to 525 ◦C while the complete collapse of the complex happened. After all decomposition steps, the final weight of the remaining residue corresponded to cadmium oxide. Differential thermal analysis (DTA) curve for Cd complex shows three endothermic peaks in the temperature range of 25–230°C, which may be ascribed to the loss of lattice and coordinated water molecules. Five exothermic peaks in the temperature interval from 250 to 600°C can be attributed to the successive burning of organic parts. There is no weight loss after this step. This fact shows that the decomposition of the Cd complex completes at this temperature.
3.1.4. SEM Analysis
The surface morphology and particle size of the Cd complex have been investigated by scanning electron microscope technique (SEM). Figure 6 exhibits typical SEM photographs of the Cd complex obtained by hydrothermal method. According to the SEM photographs, the synthesized complex has a spherical like morphology.
3.2 Computational studies
Geometry of the Cd(II) complex was designed using GaussView 5.0. All calculations were performed at the B3LYP/Lanl2dz level of theory using Gaussian 09 program package [35]. The calculated structural parameters of the Cd(II) complex were reported in the Table 2. Results showed that there is good linear relation between the selected experimental and calculated structural parameters with R2 = 0.97. Quantum theory of atoms in molecules (QTAIM) was employed to calculate electron charge densities at bond critical points (ρBCP) and ring critical points (ρRCP) using AIM2000 program [36]. The population analysis was implemented by natural bond orbital (NBO) method [37] using NBO program put into operation in Gaussian 09 [38]. Interaction energy (ΔE) of the Cd(II) complex was calculated using supramolecular approach. In fact, interaction energy of this complex was evaluated as difference between its energy and sum of energies of the constructing monomers. Results revealed that interaction energy of the Cd(II) complex is -681.90 kcal mol− 1. As can be seen in Fig. 1, Cd+ 2 ion is coordinated with three nitrogen atoms and four oxygen atoms in the Cd(II) complex. It seems that stability of this complex arises from interactions between the ligands and Cd+ 2 ion. Thus, it is necessary to understand nature and magnitude of such interactions. Results of the AIM study pointed out that construction of the Cd(II) complex is accompanied by formation of three new ring critical points that symbolized with a, d, and e. Typical molecular graph of the Cd(II) complex is depicted in Fig. 7. The ρRCP value at a, d, and e is 8.533×10− 3, 7.267×10− 3, and 1.659×10− 2 au, respectively. On the other hand, the ρRCP value at b and c is 8.533×10− 3 and 1.1750×10− 2 au, respectively. Results indicated that these ρRCP values are lower than that corresponding values in dipic by 0.01 and 0.001 au, respectively. Also, ρRCP value at A, Aʹ, and B in the complex is 2.286×10− 2, 2.286×10− 2, and 2.319×10− 2 au, respectively. It should be noted that formation of the complex is followed by decrease of ρRCP values at A and Aʹ in comparison to phen. However, the ρRCP value at B in the complex is larger than that in dipic. This outcome implies that oxygen atoms of dipic (O1) more contribute in electron charge transfer to the Cd+ 2 ion than its nitrogen atom (N2). Also, nitrogen atoms of phen contribute suitably in the mentioned electron charge transfer and assist to development of new ring critical point e. Therefore, formation of the new ring critical points and electron charge transfers to the Cd+ 2 ion make good stability for the Cd(II) complex. Moreover, aromaticity of the rings A, Aʹ, and B was calculated by means of para delocalization index (PDI) benchmark [39]. The PDI value at A, Aʹ, and B in phen and dipic is 0.0998, 0.0998, and 0.0986, respectively. However, formation of the Cd(II) complex leads to decrease of aromaticity at the mentioned rings due to contribution of the N1 and N2 atoms in the electron charge transfer process to the Cd+ 2 ion.
The values of electron charge density (ρBCP) at bond critical points of D-Cd (D = N1, N2, O1, and O3), Laplacian of electron charge density (▽2ρ), potential energy (VBCP), total electronic energy (HBCP), and eigenvalues of Hessian matrix (λ2) were gathered in the Table 3. As can be see, the ▽2ρ and HBCP values are negative in all cases. Also, sign(λ2)ρ(r) (product of electron charge density and λ2) is negative in all of interactions. This outcome is an indicative of attractive nature of the interactions between ligands and Cd+ 2 ion in the complex.
Table 3
The topological properties of electron charge densities (au × 10− 2) at bond critical points between donor atoms of ligands (D) and Cd.
| ρ | ▽2ρ | VBCP | HBCP | λ2 |
Cd1-O3 | 3.849 | -5.145 | -4.780 | -5.099 | -3.987 |
Cd1-O3 | 3.618 | -5.080 | -4.780 | -4.930 | -3.987 |
Cd1-O1 | 2.835 | -3.370 | -2.951 | -3.160 | -2.764 |
Cd1-O1 | 4.754 | -6.081 | -6.742 | -6.411 | -5.380 |
Cd1-N2 | 4.493 | -5.017 | -5.867 | -5.442 | -4.873 |
Cd1-N1 | 3.995 | -4.488 | -4.918 | -4.703 | -4.309 |
Cd1-N1 | 4.117 | -4.626 | -5.147 | -4.886 | -4.418 |
The NBO method was exploited to discover character of electron charge transfers between molecular orbitals of the Cd(II) complex. Accordingly, donor-acceptor interaction energy (E(2)) values were assessed. Sum of the E(2) values for LPD to LP*Cd donor-acceptor interactions for
D = N1, N2, O1, and O3 is 48.62, 26.95, 81.47, and 54.77 kcal mol− 1, respectively. As can be realized, oxygen atoms of dipic (O1) are the best electron donors among other donor atoms. On the other hand, nitrogen atom of dipic (N2) is a member of pyridine ring and has the smallest electron donation to the Cd+ 2 ion. Moreover, the O3 and N1 atoms are relatively good electron donors. Subsequently, all of the mentioned interactions have helpful role on stability of the Cd(II) complex. Actually, ligands such as phen and dipic have satisfying coordination function and can be applied to removal of metal ions from wastewater or detection of them in other situations.