## 3 Results and Discussion  
### Characterization of Ligands  
Six Schiff base Ligands (**L\({}_{\mathbf{1}}\)**-**L\({}_{\mathbf{6}}\)**) were synthesized by condensation reaction of terephthalaldehyde and o-aniline derivatives in the presence of N-propyl-benzoganamine-SO\({}_{3}\)H MNPs under optimized condition. The magnetic nanoparticles were prepared according to the previous literature [23]. The method for the synthesis of Schiff base ligands is given in Figure 1. In order to optimize the reaction condition of starting materials, the reaction of terephthalaldehyde (1 mmol) and o-nitroaniline (2 mmol) was carried out in different conditions using different solvents and catalysts with different catalyst content. Figure 1: The method for synthesis of Schiff base ligands. According to Table 1 the type of solvent and amount of catalyst was observed to have a significant effect on the yield of reaction using similar catalyst (\(N\)-propyl-benzoguanamine-SO3H MNPs). The highest yield was obtained up to 84%with a shorter reaction time using ethanol as solvent. In the next step, the amount of catalyst in the reaction was also examined. It is obvious from Table 1 that applying more than the specified quantity of catalyst did not have a positive effect on the yield of product and 6 mg of the catalyst represented the best yield of the reaction. It is noted from Table 1, with increasing the amount of catalyst from 3 mg to 11 mg, reaction yield was reduced 25% with a longer reaction time. The effect of p-Toluenesulfonic acid (PTSA) as catalyst was further examined which exhibited lower reaction yield with much longer reaction time. The ligands were characterized by nuclear magnetic resonance (\({}^{1}\)H-NMR), Fourier-transform infrared spectroscopy (FT-IR) and ultraviolet-visible (UV-Visible). In the \({}^{1}\)H-NMR spectra of the (**L\({}_{1}\)-L\({}_{6}\)**) ligand, the singlet peaks due to the CNH (azomethine) group were observed in the range of 10.08-8.64 ppm as singlet. Regarding the \({}^{1}\)H-NMR spectra of the compounds, it can be observed that the CNH signals of the Schiff base ligands shifted to a lower ppm (shielding) when electron donating (OH) substituent was used and it moved to a higher ppm (deshielding) when a withdrawing group (NO\({}_{2}\)) was used. Condensation of amine groups to Terephthalaldehyde in all of the ligands is confirmed by the absence of the N-H protons. The signals of the methyl groups (-CH\({}_{3}\)) for **L\({}_{2}\)** and **L\({}_{5}\)**, and OH groups for **L\({}_{3}\)** and **L\({}_{6}\)** ligands are observed in the range of 3.80-3.79 ppm and 1.21-1.16 ppm, respectively. In the \({}^{1}\)H-NMR spectra of **L\({}_{4}\)**, **L\({}_{5}\)** and **L\({}_{6}\)** ligands the CHO protons are seen at 10.99, 10.07 and 10.18 ppm, respectively. The aromatic protons of the Schiff base ligands are observed in the range of 8.66-6.82 ppm. The \({}^{1}\)H NMR spectral data of the ligands are summarized in Table 2.  
The FTIR spectra of the ligands showed peaks in the range of 1652-1620 cm\({}^{-1}\) assigned to v(C=N). In the L\({}_{4}\), L\({}_{5}\) and L\({}_{6}\) the v(C=O) (carbonyl) stretching appeared at 1702, 1693 and 1703 cm\({}^{-1}\), respectively. In **L\({}_{1}\)** and **L\({}_{4}\)** Schiff bases the bands observed in the range of 1568-1348 cm\({}^{-1}\) are attributed to the NO\({}_{2}\) groups, while the OH groups of **L\({}_{3}\)** and **L\({}_{6}\)** appeared at 3553 and 3438 cm\({}^{-1}\), respectively. The UV-Visible spectra of all the ligands and compounds are recorded in DMSO and the data are listed in Table 3. In the electronic spectra of **L\({}_{1}\)**, **L\({}_{2}\)**, **L\({}_{4}\)** and **L\({}_{5}\)** two peaks appeared which are attributed to n\(\rightarrow\)n* and n\(\rightarrow\)n* transitions, respectively. In **L\({}_{3}\)** and **L\({}_{6}\)** one intense absorption band