Author(s): Bouznif, Hajer ; Justino, Licínia L. G. ; Soares, Maria I. L. ; Costa, Telma ; Ramos, Maria Luisa ; Nikitin, Timur ; Pinho e Melo, Teresa M. V. D. ; Zouari, Nabil ; Fausto, Rui
Date: 2025
Persistent ID: https://hdl.handle.net/10316/118690
Origin: Estudo Geral - Universidade de Coimbra
Subject(s): Water soluble Schiff base; enol-imine (O-H...N) // keto-enamine (O...H-N); Hydrolysis; Al(III) complex; DFT calculations
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, ATR-FTIR and Raman spectroscopies, accompanied by DFT calculations. The study discusses the enol-imine/keto-enamine 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.
The authors acknowledge the financial support from the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT), through Projects CQC-IMS UIDB/00313/2020 and UIDP/00313/2020 (national funds; DOI: 10.54499/UIDB/00313/2020 and 10.54499/UIDP/00313/2020) and LA/P/0056/2020. L.L.G.J. acknowledges the financial support from FCT (DL 57/2016/CP1370/CT0032, DOI: 10.54499/DL57/2016/CP1370/CT0032). M.I.L.S. acknowledges the financial support from FCT (DL57/2016/CP1370/CT0041, DOI: 10.54499/DL57/2016/CP1370/CT0041). The authors also thank the Laboratory for Advanced Computing at the University of Coimbra for providing computing resources that have contributed to the research results reported within this paper (URL: https://www.uc.pt/lca). The NMR data was collected at the UC-NMR facility (URL: www.nmrccc.uc.pt) which is supported in part by FEDER – European Regional Development Fund through the COMPETE Programme (Operational Programme for Competitiveness) and by national funds through FCT (Grants REEQ/481/QUI/2006, RECI/QEQ-QFI/0168/2012, CENTRO-07-CT62-FEDER-002012, and Rede Nacional de Ressonância Magnética Nuclear (RNRMN)).
