2.1. Materials and instrumentation.
The chemicals and solvents were of commercial-grade quality. The SHIMADZU FT-IR-8400 spectrometer with KBr pellets was used to capture FT-IR spectra. BRUKER AVANCE III (500MHz) spectrometer was used for recording the 1H-NMR and 13C-NMR spectra of compounds in CDCl3. Proton chemical shifts were measured in ppm with respect to tetramethyl silane, which served as an internal standard. ESI-MS spectra of compounds have been carried out with waters, micro mass Q-Tof micro instrument. E-Merck pre-coated silica gel plates were used for thin-layer chromatography (TLC) to monitor the advancement of all reactions. The plates were visualized with a UV-Visible cabinet. A single crystal X-ray diffraction study was conducted at IIT Madras (India).
2.2- General procedure for preparation of Schiff bases.
All the compounds were synthesized by reacting equimolar quantities of aldehyde and amine. A hot methanolic solution of 2, 5-dihydroxybenzaldehyde (1mmol) was added dropwise to the hot methanolic solution of primary amine (1mmol). The resulting reaction mixture was refluxed for 2–3 hours, during which TLC monitored the reaction. The resultant colored solid was collected, washed with cold methanol, and recrystallized in an appropriate solvent [7, 36, 37]
2.2.1 S1: (Z)-2-((4-Bromo-3-chlorophenylimino) methyl) benzene-1, 4-diol.
Red Crystals, Yield 92%, MP: 225°C. 1H-NMR (DMSO, 500MHz) (δ, ppm): 6.82–6.80 (d, 1H, Ar-H); 6.91–6.89 (dd, 1H, Ar-H); 7.07–7.06 (d, 1H, Ar-H); 7.31–7.29 (dd, 1H, Ar-H); 7.70 (d, 1H, Ar-H); 7.81–7.79 (d, 1H, Ar-H ); 8.88 (s, 1H, HC = N); 9.13 (s, 1H, -OH); 11.73 (s, 1H,-OH).ESI-MS: 327.96 (Obs); 326.5 (Cal). 13C-NMR (CDCl3, 500 MHz) (δ, ppm): 164.43; 153.0; 149.63; 149.33;134.21; 133.75; 122.83; 122.26; 121.72; 119.14; 118.68; 117.23; 116.62; FT-IR (KBr pallete, cm− 1): 3296 (OH); 1621 (C = N); 1591(C = C); 1209(C-O).
2.2.2 S2: (Z)-2-((4-cyclohexylphenylimino) methyl) benzene-1, 4-diol.
Red, solid, Yield-85%, MP-159°C. 1H-NMR (DMSO, 500MHz) (δ, ppm): 1.26–1.22 (m, 1H, -CH); 1.44–1.32 (m, 4H, -CH); 1.71–1.68 (m, 1H, -CH); 1.79–1.77 (m,4H, -CH); 2.53–2.49 (m,1H, -CH); 6.80 (dd, 1H, Ar-H); 6.87–6.85 (dd, 1H, Ar-H); 7.03–7.02 (1H,d,Ar-H); 7.31–7.27 (d, 1H, Ar-H ); 8.83 (s, 1H, -HC = N), 9.07 (s,1H,-OH), 12.39 (s, 1H, OH). ESI-MS: 295.38 (Obs), 295.16 (Cal). 13C-NMR (DMSO,500 MHz) (δ, ppm): 162.32, 153, 149.51, 146.35, 146.04; 172; 121.14; 120.78; 119.21; 117; 116.86; 43.27; 33.86; 26.24; and 25.47. FT-IR (KBr pallete, cm− 1): 3285 (OH); 2924 (OH); 1611 (C = N); 1577 (C = C); 1218 (C-O).
2.2.3 S3: (E)-2-((1-methyl-1H-benzo[d]imidazol-2-yliminoimidazol-2-ylimino)methyl) benzene-1, 4-diol.
Turmeric Yellow crystals, Yield-86%, MP-262°C. 1H-NMR (CDCl3,500 MHz) (δ, ppm):3.85 (s, 3H,-CH3); 6.88–6.87 (d, 1H, Ar-H); 6.97–6.95 (dd, 1H, Ar-H); 7.25–7.21(m, 2H, Ar-H); 7.37–7.36 (d, 1H, Ar-H); 7.55–7.54 (dd, 1H, Ar-H); 7.6–7.59 (dd, 1H, Ar-H); 9.2 (s, 1H, -OH); 9.63 (s, 1H, HC = N); 10.87(s, 1H, -OH). ESI-MS: 268.11 (Obs); 267.10 (Cal). 13C-NMR (CDCl3, 500MHz) (δ, ppm): 163.88; 154.11; 153.43; 149.98; 141.13; 135.39; 123.06; 122.99,121.89; 120.15; 118.51; 117.55; 114.59; 110.06; 28.72. FT-IR (KBr pallete, cm− 1): 3058 (OH); 1616 (C = N); 1573 (C = C); 1223 (C-O).
2.2.4 S4: 2-((E)-(quinolin-3-ylimino) methyl) benzene-1, 4-diol.
Reddish brown crystals, Yield-91%, MP: 239°C, 1H-NMR (DMSO,500 MHz) (δ, ppm): 6.87–6.85(d, 1H, Ar-H); 6.94 (dd, 1H, Ar-H); 7.14 (d, 1H, Ar-H); 7.66–7.63 (dd, 1H, Ar-H); 7.76–7.73 (m, 1H, Ar-H); 8.03–8.01(d, 1H, Ar-H); 8.07–8.05 (d, 1H, Ar-H); 8.33–8.32 (d,1H, Ar-H); 9.01 (d, 1H, Ar-H); 9.08 (s, 1H, -OH); 9.17 (s, 1H, -HC = N); 11.86 (s,1H, -OH). ESI-MS: 265.10 (Obs); 264.28(Cal).13C-NMR (DMSO,500MHz) (δ, ppm): 164.69; 153; 149.68; 146.63; 146.17; 141.98; 128.93; 128.64; 128.14; 127.88; 127.12; 124.64; 121.65; 119.47; 117.29; 116.44. FT-IR (KBr pallete, cm-1): 3431 (OH); 3053 (OH); 1625 (C = N); 1579 (C = C); 1233 (C-O).
2.2.5 S5: 2-((E)-(2-methyl-5-nitrophenylimino) methyl) benzene-1, 4-diol.
Yellow crystals, Yield-93%, MP: 213°C. 1H-NMR (DMSO,500MHz) (δ, ppm): 2.42 (s, 3H, Ar-CH3); 6.84–6.82 (d,1H, Ar-H); 6.92–6.90 (dd, 1H, Ar-H); 7.59–7.57(d, 1H, Ar-H); 8.06–8.04 (dd, 1H, Ar-H); 8.14–8.13(d,1H, Ar-H); 8.90 (s,1H,-HC = N); 9.14 (s,1H, -OH); 11.84 (s,1H, OH).ESI-MS: 273.09 (Obs); 272.26 (Cal).13C-NMR (DMSO,500MHz) (δ, ppm): 164.57; 153.07; 149.64; 148.52; 146.71; 140.06; 131.28; 121.76; 120.61; 119.32; 117.19; 116.67; 112.82; FT-IR (KBr pallet,cm-1): 3467 (OH); 1621 (C = N); 1582 (C = C); 1202 (C-O).
2.2.6 S6: 2-((E)-(cycloheptylimino) methyl) benzene-1, 4-diol.
Yellow crystals, Yield-93%, MP: 140°C. 1H-NMR (DMSO,500 MHz) (δ, ppm): 1.52–1.47 (m, 2H, -CH); 1.58–1.53 (m,4H, -CH ); 1.70–1.60 (m, 4H, -CH); 1.81–1.76 (m, 2H, -CH); 3.47–3.42 (m, 1H, -CH); 6.69–6.67 (d, 1H, Ar-H); 6.76 (dd, 1H, Ar-H); 6.78 (d, 1H, Ar-H); 8.41 (s,1H,HC = N); 8.92 (s,1H,-OH); 12.82 (s,1H,-OH). ESI-MS: 234.15 (Obs); 233.31(Cal). 13C-NMR (DMSO, 500MHz) (δ, ppm):162.60; 152.92; 149.10; 119.41; 118.51; 116.59; 116.34; 68.68; 40.01; 35.85; 27.84; 23.5. FT-IR (KBr pallete, cm1): 3405 (OH); 1640 (C = N); 1159(C-O).
2.2.7 S7: 2-((E)-(2,6-difluorophenylimino)methyl)benzene-1,4-diol.
Red crystals, Yield-93%, MP: 221°C. 1H-NMR (DMSO, 500MHz) (δ, ppm): 6.88 (d, 1H, Ar-H); 6.97 (dd, 1H, Ar-H); 7.15 (d, 1H, Ar-H); 7.31(m, 3H, Ar-H); 8.94 (s, 1H, -HC = N); 9.18 (s, 1H, OH); 11.43 (s, 1H, OH). ESI-MS: 250.07 (Obs); 249.06 (Cal).13C-NMR (DMSO,500MHz) (δ, ppm): 168.51; 155.76; 153.79; 153.01; 149.87; 126.62; 122.28; 119.53; 117.57; 115.87; 112.37. FT-IR (KBr pallete, cm-1): 3307 (OH); 1621 (C = N); 1580 (C = C); 1167(C-O).
2.2.8 S8: 2-((E)-(5-methylthiazol-2-ylimino) methyl) benzene-1, 4-diol.
Yellow crystals, Yieid-84%, MP: 245°C. 1H-NMR (DMSO, 500MHz) (δ, ppm): 2.45 (s, 3H, Ar-CH3); 6.84–6.82(d, 1H, Ar-H); 6.93–6.90 (dd,1H, Ar-H); 7.21–7.20 (d,1H, Ar-H); 7.41 (d,1H, Ar-H); 9.13(s,1H, -HC = N); 9.14 (s,1H, -OH);10.76(s,1H, -OH). ESI-MS: 235.05 (Obs); 234.27(Cal). 13C-NMR (DMSO, 500MHz) (δ, ppm):169.12; 161.61; 153.01; 149.93; 138.94; 132.80; 122.69; 119.63; 117.48; 114.53; 12.02. FT-IR (KBr pallete, cm-1): 3053(OH); 1614 (C = N); 1576 (C = C); 1216 (C-O).