Synthesis and properties of a monomeric and a μ-oxo-bridged dimeric iron(III) complex with a tetradentate pyridine amide in-plane ligand. X-ray structure of [Fe(bpc)Cl(DMF)] [H₂bpc=4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene]

Abstract

The controlled nucleophilic halide displacement reaction of [NEt₄][Fe(bpc)Cl₂] [H₂bpc=4,5-dichloro-1,2-bis(pyridine-2-carboxamido) benzene] with AgClO₄ in MeCN afforded a crystalline iron(III) complex Fe(bpc)Cl·H₂O 1. The mixed chloro-dimethylformamide (DMF) axially ligated complex [Fe(bpc)Cl(DMF)] (obtained during recrystallization of 1 from DMF; however, it loses DMF quite readily to revert back to 1) has been structurally characterized. It belongs to only a handful of mononuclear high-spin iron(III) complexes having deprotonated picolinamide ligand. The iron(III) centre is co-ordinated in the equatorial plane by two pyridine nitrogens and two deprotonated amide nitrogens of the ligand, and two axial sites are co-ordinated by a chloride ion and a DMF molecule. The metal atom has a distorted octahedral geometry. Reaction of 1 with [ⁿBu₄N][OH] in MeOH afforded a μ-oxo-bridged diiron(III) complex, [Fe(bpc)]₂O·DMF·2H₂O, 2. The spin state and the co-ordination environment of the iron(III) centres in 1 and 2 have been determined by temperature-dependent (25-300 K) magnetic susceptibility measurements in the solid state (Faraday method) and Mossbauer spectral studies at 300 K. Complex 1 behaves as a perfect S=5/2 system, in the solid-state as well as in DMF solution. The two iron(III) centres in 2 are antiferromagnetically coupled (J=-117.8 cm^-1) and the bridged dimeric structure is retained in DMF solution. Bridge-cleavage reactions of 2 have been demonstrated by its ready reaction with mineral acids such as HCl and MeCO₂H to generate authentic S=5/2 complexes, [Fe(bpc)Cl₂]^- and [Fe(bpc)(O₂CMe)₂]^-, respectively.