Pentacoordinated isothiocyanate iron(III) complexes supported by asymmetric tetradentate donor and acceptor Schiff base ligands: Spectral, Structural and Hirshfeld Surface Analyses

The present study aims to synthesis and full characterization of two new pentacoordinated isothiocyanate iron(III) mixed ligand complexes featuring dianionic (N₂O₂)²⁻ tetradentate Schiff-base ligands unsymmetrically substituted by either a pair of acceptor (4-fluorophenyl and nitro) or donor (ferrocenyl and methoxy) substituents. The two neutral complexes3 and 4 were prepared in very good yields (~90 %) upon reaction of their respective chloro-iron(III) precursors 1 and 2 with sodium thiocyanate in refluxing ethanol. The two paramagnetic compounds 3 and 4 were characterized by elemental analysis, FT-IR and UV-vis spectroscopy, and mass spectrometry. The stretching frequencies of the thiocyanate group observed at 2014 and 2062 cm⁻¹, respectively, indicate a N-bonded NCS⁻ moiety, in agreement with the HSAB principle. The crystal structure of 3 revealed that in the five-coordinate monomer, the iron atom adopts a slightly distorted square-pyramidal geometry, with the N and O atoms of the Schiff-base ligand occupying the basal sites and the nitrogen atom of the isothiocyanate co-ligand located at the apex of the pyramid. Intermolecular interactions in complex 3 have been addressed with the aid of Hirshfeld surface analysis as well as fingerprint plots. Magnetic susceptibility measurements (2-300 K) showed a high-spin configuration (S = 5/2) for thed⁵ Fe(III) ion in 3.