Mononuclear high-spin Fe(iii) complexes: synthesis, crystal structures, magnetic and optical properties

Abstract

The coordination agent H₂L = 2,6-diacetylpyridine bis(picolinoylhydrazone) was used for the synthesis of two mononuclear iron(III) complexes [Fe(H₂L)(H₂O)₂]X₃·2.5H₂O, where X = NO₃⁻ (1) and ClO₄⁻ (2). In the studied crystals of 1 and 2, characterized by IR spectroscopy and X-ray diffraction, the pentadentate Schiff base ligands are coordinated in mononuclear cationic complexes [Fe(H₂L)(H₂O)₂]³⁺ by donor atoms N₃O₂ as double zwitterions. In 1, the cations are joined to centrosymmetric dimers via hydrogen bonds (HB), forming chains in the crystal, while in 2, the cations are assembled into tetramers through HB owing to the anions and water molecules. The crystal structures of the studied compounds were stabilized via hydrogen bonding that yielded supramolecular 3D networks via coordinated and crystallization water molecules, PyH⁺ groups of H₂L and NO₃⁻ or ClO₄⁻ anions. The theoretical calculations, performed using the new DFT-TB2J approach revealed the magnetic exchange interactions between the iron ions, which were mediated through intermolecular hydrogen bonds. The UV-vis absorption spectra of 1 and 2 were not overlapped with their photoluminescence emission spectra because of the spin-gapless characteristic of the band structure of these compounds.