First crystal structure of an Fe(iii) anionic complex based on a pyruvic acid thiosemicarbazone ligand with Li+: synthesis, features of magnetic behavior and theoretical analysis

Abstract

The iron(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with the lithium cation Li[Fe^III(thpy)₂]·3H₂O (1) has been synthesized and characterized by FTIR spectroscopy, powder and single crystal X-ray diffraction, direct current magnetic susceptibility measurements, and ⁵⁷Fe Mössbauer spectroscopy. Moreover, the molecular structure of the [Fe(thpy)₂]⁻ anion has been determined for the first time. The [Fe(thpy)₂]⁻ units in the triclinic P lattice of 1 are assembled into layers parallel to the bc plane. The Li⁺ cations and water molecules are located between the layers and the structure is stabilized by hydrogen bonding. The [Fe(thpy)₂]⁻ anions form interconnected dimer pairs through hydrogen bonds and short contacts with Fe⋯Fe separation of 6.7861(4) Å. According to dc magnetic measurements, compound 1 demonstrates an incipient spin-crossover transition from the LS (S = 1/2) to the HS (S = 5/2) state above 250 K. The Bleaney–Bowers equation for a model of an isolated LS dimer with a mean-field correction was applied to fit the experimental data of magnetic susceptibility dependence on temperature in the temperature range of 2–250 K. The intra-dimer J₁ = −1.79(1) K and inter-dimer J₂ = −0.24(3) K antiferromagnetic coupling constants were defined. The analysis of the ⁵⁷Fe Mössbauer spectra at 80 K and 296 K confirms the presence of the shortened distances between the iron nuclei. Moreover, the influence of the lithium cation on the stabilization of the LS state was shown for the [Fe(thpy)₂]⁻ anion. BS-DFT calculations for the optimized structure of two isolated [Fe(thpy)₂]⁻ anions also correctly predict a weak exchange J₁(calc) = −0.92 K. DFT calculations revealed the OPBE (GGA-type) functional that correctly predicts the spin-crossover transition for the iron(III) thpy compounds. Besides, the effect of the N₂O₄, N₂S₂O₂, and N₂Se₂O₂ coordination environments on the energy stabilization of the LS state of iron(III) anionic thpy complexes was noted as well.