Chemistry of iron(III) complexes of N,N′-bis(2-hydroxyphenyl)-pyridine-2,6-dicarboxamide: seven-coordinate iron(III) complexes ligated to deprotonated carboxamido nitrogens

As part of our continuing effort to develop the chemistry of iron(III) complexes of ligands that employ carboxamido nitrogens to bind the metal center, we have synthesized the designed ligand N,N′-bis(2-hydroxyphenyl)pyridine-2,6-dicarboxamide (POPYH4, Hs are dissociable phenolic and carboxamido hydrogens). Reactions of the completely deprotonated POPY4− with certain iron(III) starting materials in DMF afford pentagonal bipyramidal complexes of the type [Fe(POPY)X2]n− (X=1-MeIm, SCN−), in which the pentadentate POPY4− ligand occupies the equatorial plane. When smaller amounts of base are used, the partially deprotonated POPYH2 2− ligand gives rise to the tetrahedral bis complex [Fe(POPYH2)2]− in which only the phenolic oxygens of the ligand are bound to iron(III). With [Fe2OCl6]2−, one obtains [Fe(POPYH2)(Cl)]2O, a new species that contains a (μ-oxo)diiron(III) core. Here also, POPYH2 2− employs the phenolic oxygens to bind iron and acts as a bidentate ligand. The structures of these complexes have been determined by X-ray crystallography. The conditions for interconversions among the four complexes have also been elucidated. Both [Fe(POPY)X2]n− complexes with coordinated carboxamido nitrogens are stable in air and in aqueous solution. The redox properties of these species indicate that ligated carboxamido nitrogens provide enhanced stability to iron(III) centers.