Iron(iii) complexes of multidentate pyridinyl ligands: synthesis, characterization and catalysis of the direct hydroxylation of benzene

Abstract

Three multidentate ligands, L₁–L₃, derived from bis(pyridin-2-ylmethyl)amine (L₁) were synthesized. Reaction of these ligands with FeCl₃·6H₂O in methanol led to the formation of the iron complexes Fe₁–Fe₃ (Fe₁: [FeL₁Cl₃]; Fe₂: [FeL₂Cl₃]; Fe₃: [FeL₃Cl₃]) in good yields. These complexes have been fully characterized. The structures of complexes Fe₁–Fe₃ have been determined using X-ray single crystal diffraction analysis. Electrochemical investigation revealed that complex Fe₃ partially converts to Fe₄ ([FeL₃Cl₂]PF₆) by the replacement of one of its three chlorides with its pendant triazolyl group in solution. Fe₄ was also synthesized by dechlorination using AgPF₆ as the Cl⁻ abstractor and its composition was further confirmed by both elemental analysis and X-ray single crystal diffraction analysis. All four complexes catalyze the direct hydroxylation of benzene to phenol with hydrogen peroxide as an oxidant in a mixed medium of water and acetonitrile. The reactivity of the complexes correlates well with their reduction potentials. The more negative the potential, the more reactive (high conversion rate) the catalysts. These complexes catalyze not only the oxidation of benzene, but also the further oxidation of the product, phenol. In the oxidation, a radical mechanism is certainly involved but an alternative pathway may also exist.