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

Reactions of Fe(NO3)(3). 9H(2)O with 0.5 equiv. of 2-hydroxy-1,3-diaminopro pane N,N,N',N'-tetraacetic acid (H(5)dhpta) in a mixed solvent of water and N,N-dimethyl carboxylic acid amide (N,N-Me2NCOR; R = H, CH3, CH2CH3) at 10 5 degrees C for 22 h gave pale green crystals formulated as (Me2NH2)(4)[Fe- 4(dhpta)(2)(mu-O)(mu-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). 9H(2)O with H(5)dhpta in the presence of the corresponding carboxy lic acid (RCO2H; R = H, CH3, CH2CH3) at room temperature with the pH adjust ed to about 5 by Me2NH. The tetrairon(III) complexes were characterized by elemental analyses, IR, electronic absorption, the Fe-57 Mossbauer spectrum , and X-ray crystallographic analyses to comprise a planar (mu-oxo)(mu-hydr oxo)bis(mu-alkoxo)bis(mu-carboxylato)tetrairon(III) cluster core bridged by two dhpta and two carboxylate ligands. The central two oxo groups are prot onated to form a strongly hydrogen bonded (O-H-O)(3-) bridge. The Fe-57 Mos sbauer spectrum for Ib clearly demonstrated that the four iron sites are eq uivalent as a high-spin octahedral iron(III) state. When Fe(NO3)(3). 9H(2)O was treated with H(5)dhpta (0.5 equiv.) and RCO2H (R = CH3, CH2OH) in N,N- dimethylformamide (dmf) in the absence of Me,NH, diiron(III) complexes brid ged by a dhpta and a carboxylate ligand, [Fe-2(dhpta)(mu-O2CR)(dmf)(2)] (R= CH3 (2b), CH2OH (2d)), were obtained in good yields. Complex 2b was further converted to the tetranuclear complex Ib by treatment with Me,NH. These re sults strongly indicated that amide hydrolysis took place on the dimetal ce nter at an initial step to give the diiron(III) complex with a carboxylate bridge which was subsequently converted into the tetrairon(III) complex. (C ) 2000 Elsevier Science S.A. All rights reserved.