The magneto-structural correlations of ferric and ferrous neutral complexes with the pyruvic acid thiosemicarbazone ligand.

Neutral iron(III) and iron(II) complexes based on the pyruvic acid thiosemicarbazone (H2thpy) ligand [FeIII(Hthpy)(thpy)] (1) and [FeII(Hthpy)2] (2) were synthesized, and deeper insights into magneto-structural correlation were gained by FT-IR spectroscopy, single crystal X-ray crystallography, dc magnetic characterization, 57Fe Mössbauer spectroscopy, and DFT calculations. The X-ray structures of complex 1 were established for the HS (S = 5/2) state at 295 K and the LS (S = 1/2) state at 150 K. The crystal packing of 1 at these temperatures corresponds to the triclinic P1̄ symmetry and contains pairs of [FeIII(Hthpy)(thpy)] complexes interconnected by a shortened S⋯S contact. The crystal structure of 2 at 120 K is described by monoclinic I2/a symmetry and contains extended 1D chains of interconnected Fe(II) complexes in the HS (S = 2) state. According to dc magnetic measurements, a complete, abrupt spin-crossover with T1/2 = 226 K and a hysteresis loop of 8 K occurs in the temperature range of 210-240 K for complex 1, whereas complex 2 is stabilized in the HS state in the temperature range of 2-300 K. DFT calculations showed that the absence of spin-crossover for complex 2 is due to insufficient energy of splitting of d orbitals by the field of the Hthpy- ligand. BS-DFT calculations confirmed the presence of a weak antiferromagnetic exchange in the structures of 1 and 2.