Hydrate of neutral iron(III) complex based on pyruvic acid thiosemicarbazone ligand with abrupt spin-crossover with T1/2=340 K and wide hysteresis loop of 45 K

The hydrate of neutral iron(III) complex based on pyruvic acid thiosemicarbazone ligand [Fe^III(Hthpy)(thpy)]·H₂O (1) was synthesized and characterized using FT-IR spectroscopy, powder and single-crystal X-ray diffraction, dc magnetic measurements, EPR and ⁵⁷Fe Mössbauer spectroscopy. The crystal structure of 1 was determined for the first time. Two distinct chelating ligands Hthpy^ˉ and thpy^2ˉcoordinate the Fe(III) ion to form the FeN₂O₂S₂ octahedron which shows a low spin geometry at 150-350 K. The crystal packing contains infinite chains of the Fe(III) complexes as well as water molecules located in cavities. Along the chain, π-π interacting pairs of the Fe complexes are linked by H-bonding. According to the dc magnetic measurements, the complete abrupt spin-crossover with half-transition temperature T_1/2=340 K and a hysteresis loop of 45 K occurs in the temperature range of 300‒363 K. Based on the X-ray structure of 1, the Bleaney–Bowers equation for the isolated dimer model was used to approximate the temperature dependence of the magnetic susceptibility in the range of 2–50 K. The defined intradimer exchange constant J_exp= −0.498(1) K corresponds to a weak antiferromagnetic exchange between the iron(III) magnetic centers. DFT calculations of H- and π-π bonded fragments of the crystal structure of 1 in the HS and LS states were carried out. Moreover, BS-DFT calculations confirm the presence of antiferromagnetic exchange J_calc = – 0.92 K in the π-π bonded pairs of the ferric complexes, and show the exchange pathway between Fe(III) ions by the calculated spin density distribution.