Novel Tetradentate N2O2 Water-Soluble Schiff Base and Its Al(III) Complex: Synthesis, Structural Characterization, and Correlations Between Structure and Stability Against Hydrolysis

Abstract

A novel water-soluble di-Schiff base, N,N'-bis(3-methoxy-5-sulfonatosalicylidene)-1,2-ethylenediamine disodium salt (MSS), was synthesized under both conventional heating and microwave-assisted conditions. Detailed characterization was conducted using various techniques including NMR, HRMS, UV_vis, SEM/EDX, DSC, ATRFTIR and Raman spectroscopies, accompanied by DFT calculations. The study discusses the enol-imine/ketoenamine tautomerism of MSS based on theoretical, solution and solid-state results, as well as the impact of intra- and intermolecular interactions on the balance between the tautomers. The enol-imine form was predicted as the most stable tautomer in gas phase. Nonetheless, the keto-enamine form was found to be the main tautomer in water, DMSO and in the solid state. To assess the stability of MSS and to study its complexation with Al(III) in aqueous and non-aqueous media, a combined spectroscopic and computational approach was used. The stability of MSS against hydrolysis was analyzed and compared with that of its non-substituted analogue. The differences found were interpreted based on the different tautomeric equilibria of the two Schiff bases and specific structural susceptibilities towards nucleophilic attack by water. The presented results impact on the development of more efficient strategies for the design of water-stable Schiff bases, highlighting the role of tautomerism in determining stability against hydrolysis. NMR data revealed the formation of a stable Al(III) complex with MSS and additional complexes with MSS degradation products in the pH range 4-7. The infrared and Raman results provided additional structural details and supported the aforesaid conclusions.