Insights into Antiradical Behavior: Crystal Structures and DFT Analysis of 2-Formylpyridine N4-Allylthiosemicarbazone Salts

In this paper, six new salts of 2-formylpyridine N4-allylthiosemicarbazone ([H2L]X⸱nH2O, where X is NO3- (1), NH2SO3- (2), Cl- (3), Cl3CCOO- (4), Cl2CHCOO- (5), ClCH2COO- (6); n = 0 (1, 3, 5, 6), 1 (2, 4)) were synthesized and physico-chemically characterized by elemental analysis, molar conductivity measurements, FT-IR studies, 1H and 13C NMR. The crystal structures of compounds 1-5 were determined by single-crystal X-ray diffraction. Analysis of crystal data shows that the structures of these compounds consist of protonated thiosemicarbazone H2L+, anions of acid residue and water molecules in 2 and 4. These compounds manifest antiradical activity toward ABTS•+ cation radicals that exceeds the activity of non-protonated thiosemicarbazone HL and trolox, that is used in medicine. The most active one is compound [H2L]Cl (3) with IC50 value 9.9 μmol/L. Density Functional Theory calculations showed that the electronic structure of cation H2L+ is more favorable for electron acceptation if compared with HL.