Substituent effect evaluation on spectroscopic properties of Schiff bases derived from diaminomaleonitrile and salicylaldehydes

Schiff bases are organic compounds that are usually prepared by the reaction between a primary amine and an active carbonyl compound, first described by Hugo Schiff in 1864.[1] Diaminomaleonitrile is a hydrogen cyanide tetramer which is an electron rich diamine with low basicity and low nucleophilicity, due to the electron withdraw effect caused by its conjugated cyano groups.[2] So it reacts with carbonyl compounds to give monocondensated Schiff bases even in excess of carbonylic compound.[3] In the work, seven monoiminated Schiff bases were obtained from the reaction between diaminomaleonitrile and salicylaldehyde or substituted salicylaldehyde. The products were characterized by physical methods and the substituent effect was evaluated by infrared and UV-Vis spectrometry and 1H NMR. The medium yield of the reactions was 90 % and the compounds were soluble in polar organic solvents such as DMSO, N,N-DMF and THF and were insoluble in water and hexane. Vibrational spectra shows characteristic νN-H2 (3484-3192 cm-1), νC≡N (2246-2205 cm-1), νC=N (1645-1624 cm-1) and νC-O (1304-1230 cm-1) bands. Characteristic 1H NMR peaks were observed as follows: 8,95-10,66 ppm (s, OH); 8,45-8,61 (s, -N=CH-) and 7,56-7,90 (-NH2). When substituted in the position 3, the imine hydrogen is observed at higher chemical shifts. 4-substitued Schiff bases have the higher chemical shift for phenolic hydrogen. The methoxy-substituted compounds have higher chemical shift for amine hydrogen. The Electronic spectra morphologies were defined by the substituent position rather than its chemical nature, however, methoxy-substituted compounds shower a hyperchromic effect.