Pentacoordinated chloro-iron(III) complexes with unsymmetrically substituted N₂O₂ quadridentate schiff-base ligands: Syntheses, structures, magnetic and redox properties

Since their development in the 1930s, Schiff-base complexes have played an important role in the field of coordination chemistry. Here, we report the synthetic, spectral, structural, magnetic and electrochemical studies of two new pentacoordinated neutral chloro-iron(III) complexes (3,5) supported by dianionic [N₂O₂]^2- tetradentate Schiff-base ligands unsymmetrically substituted by either a pair of acceptor (F and NO₂) or donor (ferrocenyl and OCH₃) groups. The electron-withdrawing Schiff-base proligand 2 and the complexes 3 and 5 were prepared in good yields (79-86%). Complex 3 was readily obtained upon reaction of 2 with anhydrous iron chloride under basic conditions, while the bimetallic derivative 5 was synthesized by condensation of the free amino group of the ferrocenyl-containing O,N,N-tridentate half-unit 4 with 5-methoxysalicylaldehyde in the presence of FeCl₃. The three new compounds were characterized by elemental analysis, FT-IR, UV-Vis, mass spectrometry and in the case of 2 by multinuclear NMR spectroscopy. The crystal structures of 3 and 5 revealed that in the two five-coordinate monomers, the iron atom showed distorted square-pyramidal geometry, with the N and O atoms of the Schiff-base ligand occupying the basal sites and the chlorine atom at the apex of the pyramid. Magnetic measurements showed a high-spin configuration (S = 5/2) for the Fe(III) ion in 3 and 5. Reduction associated with the Fe(III)/Fe(II) redox couple occurred at -0.464 and -0.764 V vs. Ag/Ag⁺, and oxidation taking place at the Schiff-base ligand was observed at 1.300 and 0.942 V vs. Ag/Ag⁺ for 3 and 5, respectively. A high-electronic delocalization of the Schiff-base ligand substituted by fluoro and nitro groups stabilizes the Fe(II) oxidation state and shifts the redox potential anodically.