Synthesis, characterization and Mössbauer studies of Fe(II) and Fe(III) complexes of 2-acetylpyridine thiosemicarbazone

Abstract

Three iron complexes of 2-acetylpyridine thiosemicarbazone were obtained, namely [Fe(HAPT)(APT)]Cl2 (I), [Fe(HAPT)2]Cl2 (II), and [Fe(APT)2] (III), in which HAPT stands for the ligand protonated at the N(2') level in the side chain and APT- for the ligand deprotonated at N(2'). The coordination to the metal ion occurs through the tridentate ligating Npy ∩ N(1') ∩ (S), system with formation of 2:1 ligand-to-metal complexes having meridional stereochemistry. The Mossbauer parameters δ (isomer shift) and ∆ (quadrupole splitting) are characteristic of Fe(III) for complex I and of Fe(II) for both complexes II and III, all low spin with octahedral geometry. The isomer shift value decreases in going from [Fe(HAPT)2]Cl2 (II, δ = 0.264 ± 0.001 mm/s) to [Fe(APT)2] (III, δ = 0.227 ± 0.001 mm/s), due to the higher s electron density of the latter. This results from the electron donor inductive effect of the acetyl methyl group and the negative charge of the thiolate sulfur, which is consequently a better sigma donor than the thione sulfur. The quadrupole splitting increases in going from II (∆ = 0.537 ± 0.002 mm/s) to III (∆ = 0.755 ± 0.002 mm/s), probably due to ligand inequivalencies, a significant contribution being attributed to differences in the metal-ligand bond order.