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) comp lexes 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 hyd rogens). Reactions of the completely deprotonated POPY4- with certain iron( III) starting materials in DMF afford pentagonal bipyramidal complexes of t he type [Fe(POPY)X-2](n-) (X = 1-MeIm, SCN-), in which the pentadentate POP Y4- Ligand occupies the equatorial plane. When smaller amounts of base are used, the partially deprotonated POPYH22- ligand gives rise to the tetrahed ral bis complex [Fe(POPYH2)(2)](-) in which only the phenolic oxygens of th e ligand are bound to iron(III). With [Fe2OCl6](2-), one obtains [Fe(POPYH2 )(Cl)](2)O, a new species that contains a (mu-oxo)diiron(III) core. Here al so, POPYH22- employs the phenolic oxygens to bind iron and acts as a bident ate ligand. The structures of these complexes have been determined by X-ray crystallography. The conditions for interconversions among the four comple xes have also been elucidated. Both [Fe(POPY)X-2](n-) complexes with coordi nated carboxamido nitrogens are stable in air and in aqueous solution. The redox properties of these species indicate that ligated carboxamido nitroge ns provide enhanced stability to iron(III) centers. (C) 2000 Elsevier Scien ce S.A. All rights reserved.