Salen-type Schiff bases spaced by the highly flexible and hydrophobic tetramethyldisiloxane motif. Some synthetic, structural and behavioral particularities

An unusual highly flexible, organic-inorganic diamine, 1,3-bis(3-aminopropyl)tetramethyldisiloxane (APDS), is reacted with 3,5-dichloro-, 3,5-dibromo- and 3-hydroxysalicylaldehyde resulting the salen-type Schiff bases H2L1, H2L2 and H2L3. For comparison, organic homologues derived from 3-hydroxysalicylaldehyde with hexamethylenediamine (H2L4) and dodecamethylenediamine (H2L5) as well as from 2,3-dichlorosalycylaldehyde with hexamethylenediamine (H2L6) and dodecamethylenediamine (H2L7) are prepared. The effects of the presence of siloxane motif in the amine structure on the reaction rate of Schiff base formation and equilibrium conversion, as well as on their behaviour in solution, are studied. Thus, the values of the rate constant determined in the reaction with salicylaldehyde derivatives indicate for these nine atoms spaced-diamine, only slightly lower reactivity than that of the aliphatic diamine (hexamethylenediamine), while the equilibrium conversion is greater than the latter. The high flexibility and hydrophobicity of the tetramethyldisiloxane spacer makes the three resulting compounds to be oily, and show surface activity being able of reducing the surface tension of the DMF and to aggregate in solution, when concentrations exceed the critical micellar concentration. The metal binding capacity of Schiff bases as ligands is assessed by spectrophotometric titration and Benesi-Hildebrand method. The results indicate that the stoichiometry of the complexation remains unchanged during the chemical process, no secondary reactions occur during the considered time range. It has been found that the metal binding constant values for H2L1 vary in order: CuL1 > NiL1 > ZnL1 > CoL1. The binding constant of copper by H2L3 is higher than in the case of organic homologues derived from and hexamethylenediamine and dodecamethylenediamine. [Display omitted] •Siloxane-spaced salen type Schiff bases are prepared and compared with organic ones.•The compounds are able to self-assemble in solution even in the reaction medium.•Surface activity of the diamine and products influences reaction rate and equilibrium.•The products are oily, surface active, and able to coordinate metals.•Metal binding capacity depends on both the metal and the salicylaldimine substituents.