Synthesis and spectroscopic properties of novel asymmetric Schiff bases

Three novel diimine Schiff bases including two asymmetric imines (2-OH)R-CHN-C6H4-CHN-R’(2-OH) type [where R = R’ = phenyl for H2L1; R = naphthyl, R’ = phenyl for H2L2 and R = R’ = naphthyl for H2L3] have been synthesized with a new two step method. For this purpose, the starting Schiff bases 4-nitrobenzylidene-2-hydroxyaniline (SB1-NO2) and 4-nitrobenzylidene-2-hydroxy-3-naphthylamine (SB2-NO2) have been synthesized, previously. Nitro groups of them have been reduced into their amino derivatives (SB1-NH2 and SB2-NH2) with sodium dithionite as selective reductant and the other imino groups have been formed by adding salicylaldehyde or 2-hydroxy-1-naphthaldehyde to the same solutions. The structures of the diimine Schiff bases were confirmed by elemental analyses, ESI-MS, FT-IR, 1H NMR and 13C NMR spectroscopy. The phenol-imine and keto-amine tautomerism of the Schiff bases were investigated by FT-IR, 1H NMR, 13C NMR techniques and UV–vis spectra in different solvents (DMSO, methanol, chloroform, toluene and cyclohexane). The effects of acidic and basic media on the tautomeric equilibria were discussed.