Peculiar Sequence of the Thermodynamic Protonation Parameters for Bis-Chelate Ligands with Methylhydroxyiminoethanamide Moieties

Abstract

New bis-chelate oxime-and-amide (2E,2'E)-N,N'-(2-hydroxypropane-1,3-diyl)bis[2- (hydroxyimino)propanamide]) ligand has been synthesised and characterised. Protonation constants for it and structurally similar (2E,2'E)-N,N'-propane-1,3-diylbis[2-(hydroxyimino)propanamide] ligand were determined by potentiometric titrations at 25 °C in 0.100 M NMe4Cl aqueous medium and led to the conclusion of a tetraprotic (LH4) behaviour, contrary to previously reported in literature LH2 model. Protonation enthalpies for the two ligands were determined by calorimetric titrations and the tetraprotic behaviour in both cases was confirmed. Complete thermodynamic characterisation of the protonation process revealed peculiar sequence of the stepwise enthalpy and entropy values. The latter were interpreted in terms of structural rearrangement of the ligand forms at various stages of protonation. Results of the UV-Vis spectroscopic titrations for both ligands were analysed and compared with spectra computed at the DFT level. Experimental and calculated spectra were shown to be in satisfactory agreement, and support key assumptions about the nature of protonated species inferred from the thermodynamic studies. Observed spectroscopic transitions were designated as π→π* ones. Keywords: Ab initio DFT calculation of the UV-Vis spectra, bis-chelate oxime-and-amide ligands, calorimetric, potentiometric, and spectroscopic titrations, intramolecular H-bonding, thermodynamics of protonation.