## Materials and Methods
### Preparation of Schiff Base Ligands  
Condensation reaction of Terephthalalaldehyde with o-nitroaniline, o-Anisidine and 2-aminophenol in molar ratio 1:1 and 1:2 afforded the corresponding Schiff base ligands as described below:  
MNPS-\(N\)-propyl-benzoguanamine-SO\({}_{3}\)H catalyst was prepared by chemical co-precipitation according to the previous literature [23]. To a mixture of terephthalaldehyde and aniline derivative was added to \(N\)-propyl-benzoguanamine-SO\({}_{3}\)H catalyst (6 mg) in 10 mL ethanol as solvent. The reaction mixture was refluxed (100 \({}^{\circ}\)C) for 2-3 h. The progress of the reaction was checked with TLC. After completion of the reaction the mixture was cooled to room temperature. The catalyst was then separated by using an external magnet. The solvent was evaporated under reduced pressure and the resulting solid was obtained. The resulting was then recrystallized in ethanol. _(1,4-phenylenebis(methanylylidene))bis(2-nitroaniline)_ (**L\({}_{\mathbf{1}}\)**): Dark Yellow solid. Yield: 84%. M.P. 208-210 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 2924, 1630, 1449, 1348, 1012. \({}^{1}\)H-NMR (500 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.04 (s, 2H, CH=N), 8.02-7.97 (q, 12.5 Hz, 8H, Ar-H), 7.90-7.87 (d, 7.5 Hz, 2H, Ar-H), 7.53-7.50 (d, 7.5 Hz, 2H, Ar-H). UV-Vis (DMSO): \(\lambda_{\mathrm{max}}\) (nm) = 260, 340. _(1,4-phenylenebis(methanylylidene))bis(2-methayaniline)_ (**L\({}_{\mathbf{2}}\)**): Orange solid. Yield: 86%. M.P. 190-192 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 3062, 3018, 2965, 2835, 1620, 1116. \({}^{1}\)H-NMR (250 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.08 (s, 2H, CH=N), 8.66-8.60 (m, 2H, Ar-H), 8.14-8.02 (m, 5H, Ar-H), 7.35-6.95 (m, 8H, Ar-H), 3.80 (s, 6H, CH\({}_{3}\)). UV-Vis (DMSO): \(\lambda_{\mathrm{max}}\) (nm) = 290, 390. _(1,4-phenylenebis(methanylylidene))bis(azanylylidene))diphenol_ (**L\({}_{\mathbf{3}}\)**): Brown solid. Yield: 64%. M.P. 295-297 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 3412, 3100, 1614, 1059. \({}^{1}\)H-NMR (500 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.08 (s, 2H, CH=N), 8.66-8.60 (m, 2H, Ar-H), 8.14-8.02 (m, 5H, Ar-H), 7.35-6.95 (m, 8H, Ar-H), 3.80 (s, 6H, CH\({}_{3}\)). UV-Vis (DMSO): \(\lambda_{\mathrm{max}}\) (nm) = 290, 390. _(1,4-phenylenebis(methanylylidene))bis(azanylylidene)diphenol_ (**L\({}_{\mathbf{3}}\)**): Brown solid. Yield: 64%. M.P. 295-297 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 3412, 3100, 1614, 1059. \({}^{1}\)H-NMR (500 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.08 (s, 2H, CH=N), 8.66-8.60 (m, 2H, Ar-H), 8.14-8.02 (m, 5H, Ar-H), 7.  
ppm): 9.72 (s, 2H, CH=N), 8.34-8.32 (d, 5 Hz, 2H, Ar-H), 8.19-8.17 (d, 5 Hz, 2H, Ar-H), 7.86-7.82 (m, 2H, Ar-H), 7.67-7.66 (d, 2.5 Hz, 1H, Ar-H), 7.48-7.42 (m, 2H, Ar-H), 7.07-7.03(m, 2, OH). UV-Vis (DMSO): \(\lambda_{\rm max}\) (nm) = 330. _4-(((2-nitrophenyl)imino)methyl)benzaldehyde_ (**L\({}_{\bf 4}\)**): Yellow solid. Yield: 68%. M.P. 203-205 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 2948, 2900, 1702, 1619, 1568, 1356, 1073. \({}^{1}\)H-NMR (250 MHz, CDCl\({}_{3}\), \(\delta\), ppm): 10.99 (s, 1H, CHO), 10.06 (s, 1H, CH=N), 8.16-8.02 (m, 5H, Ar-H), 7.79-7.69 (m, 3H, Ar-H). UV-Vis (DMSO): \(\lambda_{\rm max}\) (nm) = 250, 320. _4-(((2-methoxyphenyl)imino)methyl)benzaldehyde_ (**L\({}_{\bf 5}\)**): Pale Yellow solid. Yield: 73%. M.P. 181-183 \({}^{\circ}\)C. Selected IR data (v, cm\({}^{-1}\)): 3011, 2966, 2838, 1693, 1620, 1102. \({}^{1}\)H-NMR (500 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.07 (s, 1H, CHO), 8.64 (s, 1H, CH=N), 8.03-8.01 (d, 5 Hz, 4H, Ar-H), 7.21-6.95 (m, 4H, Ar-H), 3.79 (s, 6H, CH\({}_{3}\)). UV-Vis (DMSO): \(\lambda_{\rm max}\) (nm) = 290, 370. _4-(((2-hydroxyphenyl)imino)methyl)benzaldehyde_ (**L\({}_{\bf 6}\)**): Light Green solid. Yield: 65%. M.P. 282-284 \({}^{\circ}\)C Selected IR data (v, cm\({}^{-1}\)): 3431, 3147, 3046, 1703, 1652, 1028. \({}^{1}\)H-NMR (500 MHz, DMSO-\(d_{6}\), \(\delta\), ppm): 10.18 (s, 1H, CHO), 9.73 (s, 1H, CH=N), 8.41-7.45 (m, 8H, Ar-H), 1.21 (s, 1H, OH).