Synthesis, characterization, structural optimization using density functional theory and superoxide ion scavenging activity of some Schiff bases

Four Schiff bases, 2-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-2), 4-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-4), (1H-indol-3-ylmethylene)-(4-nitro-phenyl)-amine (IYNPA), (3-nitro-benzylidene)-(4-nitro-phenyl)-amine (NBNPA) with experimental clue of binding with superoxide ion have been synthesized by the condensation of p-nitroaniline with o-hydroxy benzaldehyde, p-hydroxybenzaldehyde, indole-3-carboxaldehyde and m-nitrobenzaldehyde respectively. These were characterized by IR, H-1 NMR, CHN, mass spectroscopy and UV-visible spectroscopy. The Schiff bases thus synthesized exhibited evidence for their binding with superoxide ion in the form of a high intensity charge transfer band beyond 500 nm in their respective UV-vis spectra on the addition of two drops of N/10 NaOH to their respective 10(-2) M solutions in DMSO. On further addition of two drops of N/10 HCl solution, the charge transfer band vanished and the original spectral pattern of the Schiff bases in DMSO was observed. Out of the four Schiff bases, the binding of NPIMP-4 with O-2(-) was observed to be reversible throughout both the cycles of measurements, but the remaining Schiff bases, namely NPIMP-2, IYNPA and NBNPA were found to show reversibility in only one cycle. The variable temperature 1H NMR (RT-175 degrees C) in DMSO-d(6) revealed reversible intramolecular proton transfer in NPIMP-2 and NPIMP-4 leading to existence of several tautomeric structures. Besides these synthetic and spectroscopic studies, the density functional theory (DFT) calculations were performed for two Schiff bases i.e., NPIMP-2 and NPIMP-4. These calculations provided some important information about the relative stability of various tautomeric forms of NPIMP-2 and NPIMP-4. (c) 2007 Elsevier B.V. All rights reserved.