Three copper-based complexes, copper complexes of nicotinic-phthalic acids (CuNA/Ph,
1), nicotinic-salicylic acids (CuNA/Sal,
2) and nicotinic-anthranilic acids (CuNA/Ant,
3), were synthesized by the reaction of 1:1:1 ratio of cupric chloride with nicotinic acid and aromatic carboxylic acids (phthalic, salicylic and anthranilic acids). The complexes were obtained in excellent yields (82-91%) as aquamarine powder, highly polar, insoluble in methanol and water, but soluble in dimethyl sulfoxide (DMSO). Their melting points (m.p.) and magnetic moments (µ
eff) are listed in
Table 1.
Infrared spectra
Coordination of the copper atom with the functional groups of the ligands was established from the IR spectra (
Table 2). From the data of complex
1, it can be seen that the C-O stretching vibration frequency was located at 1,298 cm
-1, while the free phthalic acid and nicotinic acid absorptions υ(C-O) appeared at 1,282 and 1,299 cm
-1, respectively. In addition, disappearance of the hydroxyl group bending vibration (δO-H) of Ph at 1,404 cm
-1 was observed. For the carbonyl group (C=O), the stretching vibrations of both NA and Ph still exhibited intensive bands at 1,685, 1,701, and 1,718 cm
-1. The weak band at 1333 cm
-1 could be ascribed to C-N stretching vibration of complex
1 as compared with the spectra of NA, which shows strong band υ(C-N) at 1324 cm
-1. Thus, complex
1 was formed using pyridine ring nitrogen atom of NA and two hydroxyl groups of Ph to coordinate with copper atom. Structures of ligands are shown in
Figure 1.
Table 2.
IR spectra of the free ligands and the copper coordination complexes.
Table 2.
IR spectra of the free ligands and the copper coordination complexes.
Cpd | υC=O | υC-O | υC-N | υO-H | δO-H | υNH | δNH |
---|
NA | 1,718 (s) | 1,299 (s) | 1,324 (s) | 3,072-2,449 (w) | 1,418 (s) | − | − |
| 1,700 (s) | | | | | | |
Ph | 1,700 (s) | 1,282 (s) | − | 3,072-2,524 (br) | 1,404 (s) | − | − |
| 1,685 (s) | | | | | | |
1 | 1,718 (s) | 1,298 (m) | 1,333 (w) | 3,080-2,565 (w) | 1,418 (m) | − | − |
| 1,701 (s) | | | | | | |
| 1,685 (s) | | | | | | |
Sal | 1,662 (s) | 1,296 (s) | − | 3,471 (br) | 1,446 (s) | − | − |
| 1,655 (s) | 1,250 (s) | | 3,240 (sh) | 1,484 (s) | | |
| | 1,211 (s) | | | | | |
| | 1,211 (s) | | | | | |
2 | 1,717 (s) | 1,298 (s) | 1,332 (m) | 2,682 (sh, w)) | 1,454 (m) | − | − |
| 1,700 (s) | 1,201 (w) | | 2,567 (sh, w) | 1,420 (m) | | |
| 1,685 (s) | 1,144 (w) | | | | | |
| | 1,126 (w) | | | | | |
Ant | 1,679 (m) | 1,277 (m) | 1,371 (s) | 2,869 (br) | 1,458 (m) | 3,325 (sh, s) | 754 (vs) |
| 1,654 (sh, m) | 1,238 (m) | 1,319 (s) | 2,580 (br) | | 3,239 (sh, s) | |
| | | | 2,362 (sh, w) | | | |
| | | | 3,449 (br) | | | |
3 | 1,718 (s) | 1,298 (s) | 1,386 (s) | 3,400 (br) | 1,458 (m) | 3,275 (m) | 756 (s) |
| 1,700 (s) | | 1,332 (m) | 2,681 (w) | 1,420 (m) | | |
| | | | 2,565 (w) | | | |
In a similar fashion, the IR spectra of complex 2 were interpreted by comparison of its spectra with those of the free ligands. It was apparent that the hydroxyl and carbonyl absorptions of salicylic acid were at 3,471 cm-1 υ(O-H) and 1,655, 1,662 cm-1 υ(C=O), respectively, suggesting that Cu-complex formation took place via the phenolic and carbonyl functionalities. The IR spectra of complex 2 still displayed strong absorptions at 1,700, 1,717 cm-1 υ(C=O), 1,298 cm-1 υ(C-O) and 1,420 cm-1 δ(O-H). However, the C-N stretching vibration of complex 2 was observed at 1,332 cm-1 whereas the C-N absorption of free NA appeared at 1,324 cm-1. Based on the IR spectra, complex 2 was observed to be formed through the carbonyl and the phenolic hydroxyl group of Sal and the pyridine ring nitrogen atom.
Figure 1.
Molecular structure of ligands: nicotinic acid (a), phthalic acid (b), salicylic acid (c), and anthranilic acid (d).
Figure 1.
Molecular structure of ligands: nicotinic acid (a), phthalic acid (b), salicylic acid (c), and anthranilic acid (d).
The IR spectra of complex 3 exhibited disappearance of the anthranilic acid carbonyl (C=O) absorption at 1,654 and 1,679 cm-1. Additionally, the amino (NH) absorption of complex 3 appeared at 3,275 cm-1, while the NH2 absorption of Ant was observed at 3,239 and 3,325 cm-1. Strong C-N stretching of Ant was displayed at 1,371 and 1,319 cm-1 with a very strong NH bending observed at 754 cm-1. The IR spectra of complex 3 showed strong absorptions at 1,718, 1,700 (C=O), 1,298 (C-O), medium bending band of OH at 1,420 cm-1 and C-N stretching of ring N-atom at 1,332 cm-1. The characteristic free NA C-N stretching absorption of the pyridine ring (υ 1,324 cm-1) was shifted to higher frequency at 1332 cm-1. The C-N stretching of NH group of complex 3 was observed at 1,386 cm-1. It was suggested that complex 3 was formed through coordination of pyridine ring nitrogen atom; carbonyl and NH groups of Ant with Cu centered atom.
It is noted that these Cu-complexes were formed via monodentate interaction of the ring nitrogen atom of NA resulting in a υ(C-N) shift from 1,324 cm-1 to a higher frequency range of 1,332-1,333 cm‑1. The υ(C=O), υ(C-O) and δ(O-H) absorptions of the NA ligand of the complexes remained unperturbed. Aside from NA, other ligands coordinated with the central Cu atom via bidentate carboxylates. With regards to complex 1, Ph is the bidentate ligand using two hydroxyls of dicarboxylic acid to coordinate with the Cu atom. This is observed by the disappearance of C-O stretching at 1,282 cm-1 and O-H bending at 1,404 cm-1. As for complex 2, stretching at 1,662 and 1,655 cm-1 (C=O) and at 3,471 and 3,240 cm-1 (O-H) disappeared with a shift of C-O stretching from 1,250, 1,211 and 1,156 cm-1 to lower frequencies. This suggested that the Cu atom formed a complex by means of a bidentate phenolic carboxylate. Similarly, the Cu atom of complex 3 was found to be coordinated with the amino carboxylate of Ant ligand, which is due to the disappearance of C=O and NH2 stretching absorptions of Ant.
High resolution mass spectra (HRMS) measurements for complexes 1-3 were performed, but their molecular ions were not detected. Fragmented ions were observed at m/z of 124.0399 corresponding to free nicotinic acid as calculated for C6H5NO2 [M+H]+. Additionally, m/z of 124.0399 was quantified for complexes 1 and 2 while 124.0396 was detected for complex 3.
Complexes 1-3 were paramagnetic, with magnetic moments (µeff) of 1.69, 1.68 and 1.80 B.M., respectively. The µeff value indicates that the complexes were of tetrahedral conformation with the Cu(II) center. Therefore, on the basis of both IR spectra and µeff, we can confirm that complexes 1-3 were of tetrahedral geometry with the Cu(II) center.
Molecular modeling of SOD mimics
To elucidate the mechanisms of radical scavenging activities of the copper complexes, density functional theory calculations at the B3LYP/LANL2DZ level was employed. Previous efforts have indicated that the calculation of theoretical parameters, binding energies and electron affinities, at such theoretical levels are suitable for characterizing the superoxide radical scavenging activity [
29,
30,
31,
32]. The molecular structures were constructed on the basis of IR and magnetic moment data which indicated that the coordination complex was of distorted tetrahedral conformation. Geometrically optimized structure of complexes
1-3 are presented in
Figure 2.
Figure 2.
Molecular structures of copper coordination complexes 1 (a), 2 (b), and 3 (c).
Figure 2.
Molecular structures of copper coordination complexes 1 (a), 2 (b), and 3 (c).
Electron affinity, in particular, is an appropriate theoretical parameter accounting for the electron transfer rate from superoxide anion to copper atom [
30,
31,
32]. The lower the EA becomes the higher the electron transfer rate is, which correspondingly leads to higher superoxide radical scavenging activity. EA was calculated according to equation (1) by taking the difference of the total energy of Cu(II) and Cu(I) coordination complexes. The EA for complexes
1-
3 as presented in
Table 5 were calculated to be -125.502, -139.283 and -212.679 kcal/mol, respectively.
Table 5.
Theoretical parameters of the copper complexes.
Table 5.
Theoretical parameters of the copper complexes.
Complex | TECu(II)a (hartree) | TECu(I)b (hartree) | EA* (kcal/mol) |
---|
1 | -1316.673 | -1316.873 | -125.502 |
2 | -1203.223 | -1203.445 | -139.283 |
3 | -1183.292 | -1183.631 | -212.679 |
The experimental SOD activities of complexes
1-3 had an IC
50 of 34.42, 42.79 and 47.49 µM, respectively. The results indicated that there exists a positive correlation between EA and SOD activity where high EA gives rise to high SOD activity, which is inversely correlated in previous reports [
30,
31]. This is presumably due to differences in the coordination geometry of the copper complexes used in this study (tetragonally distorted) and the copper complexes (distorted square-planar and square-pyramidal) reported previously [
30,
31]. Similar observation was deduced by Branco
et al. in their studies on active site distortion of CuZnSOD [
33,
34].
Additionally, the usefulness of quantum chemical descriptors such as energy of the highest occupied molecular orbital and energy of the lowest unoccupied molecular orbital in elucidating the radical scavenging activity were also investigated. Examples on the usage of HOMO and LUMO energies in accounting for chemical reactivities of molecules and their relevance to electron transfer complexes can be found in an excellent review by Karelson and Lobanov [
35]. The HOMO energies of the ligands were calculated to be -5.818, -6.729 and -7.644 eV, respectively with the following order: Ant > Sal > Ph (
Table 6).
Table 6.
HOMO and LUMO energies of the free ligands and copper complexes.
Table 6.
HOMO and LUMO energies of the free ligands and copper complexes.
Compound | HOMO (eV) | LUMO (eV) |
---|
Ph | -7.644 | -2.252 |
Sal | -6.729 | -1.749 |
Ant | -5.818 | -1.553 |
1 | -7.302 | -4.898 |
2 | -6.293 | -4.595 |
3 | -5.829 | -4.086 |
Likewise, the calculated value of HOMO for the complexes exhibited similar trend:
3 >
2 >
1 with the corresponding values of -5.829, -6.293 and -7.302 eV, respectively. It is well established that HOMO accounts for the electron donating ability while LUMO characterizes the ability to accept electron [
36]. From the frontier molecular orbital approximation, high HOMO energy value infers that the molecule or ligand can easily release electrons to the unoccupied orbital of the metal ion, indicating strong binding affinity [
37]. Thus, Ant possesses the strongest interaction with copper leading to the best binding capacity. Such degree of binding capacity is found to be Ant > Sal > Ph.
In addition, the selected bond lengths and angles of the geometrically optimized structures are given in
Table 7. The average bond distance at axial position for Cu-N1 of complexes
1-3 was 2.029 Å. The longest axial bond length for Cu-O1 and Cu-O2 were observed for complex
1 with values of 1.872 and 1.870 Å, respectively. The axial bond lengths of Cu-O1 and Cu-O2 for complex
2 was found to be shorter than that of complex
1 with values of 1.838 and 1.857 Å, respectively. This is explained by the inductive effect of the two carbonyl groups of complex
1 withdraws electrons from the Cu atom giving rise to low electron donating ability of the ligand as also indicated by the lower HOMO energy. Therefore, complex
1 exhibited the highest SOD activity. Finally, the shortest axial bond lengths for Cu-N2 and Cu-O1 were found in complex
3 to be 1.809 and 1.830 Å, respectively. This can be attributed to the greater electron donating ability of the amino group of anthranilic acid than the phenolic group of salicylic acid. This results in the lower HOMO energy of
2 than
3. It was previously reported by Li
et al. [
38] that axial bond lengths were crucial for SOD activity where long bond lengths being advantageous for the dismutation of superoxide anion. In addition, correlation between the binding capacity and the SOD activity of metal complexes was previously studied using molecular modeling and quantum chemical calculation [
8]. It was found that molecules exhibiting higher metal binding affinity displayed lower SOD activity [
8,
39]. This was observed for complex
3 which possessed the lowest SOD activity and the highest calculated HOMO value. On the other hand, complex
1 exhibited the highest SOD activity with the lowest calculated HOMO value (
Table 3 and
Table 6). Furthermore, the calculated energies of HOMO and LUMO were well correlated with the SOD activity as observed from
r = 0.999 and 0.953, respectively. Such results demonstrate the practical usage of HOMO and LUMO as theoretical parameters for the characterization of SOD activity in terms of charge- or electron-transfer of the complex.
Table 7.
Selected bond distances (Å) and angles (°) of complexes 1-3.
Table 7.
Selected bond distances (Å) and angles (°) of complexes 1-3.
Complex | Bond Lengths (Å) | Angles (°) |
---|
1 | Cu-N1 | 2.035 | N1-Cu-O1 | 111.475 |
| Cu-O1 | 1.872 | N1-Cu-O2 | 111.235 |
| Cu-O2 | 1.870 | N1-Cu-O3 | 107.711 |
| Cu-O3 | 1.829 | O1-Cu-O2 | 105.661 |
| | | O1-Cu-O3 | 110.275 |
| | | O2-Cu-O3 | 110.513 |
2 | Cu-N1 | 2.035 | N(1)-Cu-O1 | 109.012 |
| Cu-O1 | 1.838 | N(1)-Cu-O2 | 111.596 |
| Cu-O2 | 1.857 | N(1)-Cu-O3 | 109.467 |
| Cu-O3 | 1.836 | O(1)-Cu-O2 | 107.727 |
| | | O(1)-Cu-O3 | 109.929 |
| | | O(2)-Cu-O3 | 109.355 |
3 | Cu-N1 | 2.016 | N1-Cu-O1 | 109.926 |
| Cu-N2 | 1.809 | N1-Cu-O2 | 109.365 |
| Cu-O1 | 1.830 | N1-Cu-N2 | 110.023 |
| Cu-O2 | 1.835 | N2-Cu-O2 | 109.091 |
| | | N2-Cu-O1 | 108.241 |
| | | O1-Cu-O2 | 109.581 |