Tetranuclear iron(III) complexes with a planar (μ-oxo)(μ-hydroxo)bis(μ-alkoxo)bis(μ-carboxylato)tetrairon core derived from hydrolysis of carboxylic acid amides by diiron(III) center

Reactions of Fe(NO3)3·9H2O with 0.5 equiv. of 2-hydroxy-1,3-diaminopropane N,N,N′,N′-tetraacetic acid (H5dhpta) in a mixed solvent of water and N,N-dimethyl carboxylic acid amide (N,N-Me2NCOR; R=H, CH3, CH2CH3) at 105°C for 22 h gave pale green crystals formulated as (Me2NH2)4[Fe4(dhpta)2(μ-O)(μ-OH)(O2CR)2](NO3) (R=H (1a), CH3 (1b), CH2CH3 (1c)) in good yields. Complexes 1a–c could also be prepared from reaction of Fe(NO3)3·9H2O with H5dhpta in the presence of the corresponding carboxylic acid (RCO2H; R=H, CH3, CH2CH3) at room temperature with the pH adjusted to about 5 by Me2NH. The tetrairon(III) complexes were characterized by elemental analyses, IR, electronic absorption, the 57Fe Mössbauer spectrum, and X-ray crystallographic analyses to comprise a planar (μ-oxo)(μ-hydroxo)bis(μ-alkoxo)bis(μ-carboxylato)tetrairon(III) cluster core bridged by two dhpta and two carboxylate ligands. The central two oxo groups are protonated to form a strongly hydrogen bonded (O–H–O)3− bridge. The 57Fe Mössbauer spectrum for 1b clearly demonstrated that the four iron sites are equivalent as a high-spin octahedral iron(III) state. When Fe(NO3)3·9H2O was treated with H5dhpta (0.5 equiv.) and RCO2H (R=CH3, CH2OH) in N,N-dimethylformamide (dmf) in the absence of Me2NH, diiron(III) complexes bridged by a dhpta and a carboxylate ligand, [Fe2(dhpta)(μ-O2CR)(dmf)2] (R=CH3 (2b), CH2OH (2d)), were obtained in good yields. Complex 2b was further converted to the tetranuclear complex 1b by treatment with Me2NH. These results strongly indicated that amide hydrolysis took place on the dimetal center at an initial step to give the diiron(III) complex with a carboxylate bridge which was subsequently converted into the tetrairon(III) complex.