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.
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