A series of monomeric carboxylate ferrous complexes with a tripodal N-3 ligand HB(3,5-iPr(2)pz)(3) hn synthesized and characterized to model the iron site in non-heme iron proteins which bind or activate dioxygen. The structures of Fe(OAc)(HB(3,5-iPr(2)pz)(3)) (OBz)(MeCN)(HB(3,5-iPr(2)pz)(3)) and Fe(OOCtBu)(HB(3,5iPr(2)pz)3) (13) were determined by X-ray crystallography. The five-coordinate complex Fe(OBz) (HB(3,5-iPr(2)pz)(3)) (4) was found to bind a variety of sigma-donating ligands such as dimethyl sulfoxide and pyridine at the open coordination site. The reaction between the ferrous complexes and dioxygen has been explored. Fe(OBz)(HB(3,5-iPr(2)pz)(3)) (4) was found to bind dioxygen to form an adduct which is reasonably stable below -20 degrees C. The dioxygen adduct was characterized by dioxygen uptake measurement, and UV-vis, resonance Raman, H-1-NMR, and X-ray absorption spectroscopy. On the basis of these results, the Fe:O-2 stoichiometry of 2:1, an intense absorption band at 682 nm, nu(O-O) at 876 cm(-1), and Fe***Fe separation of 4.3 Angstrom estimated from EXAFS, the dioxygen adduct was identifed as a mu-peroxo dinuclear ferric complex. The variable temperature magnetic susceptibility measurement of the isolated mu-peroxo complex indicates that the complex is antiferromagnetically coupled with J = -33 cm(-1), which is weaker than those known for other mu-peroxo dinuclear ferric complexes. This characteristic feature may be associated with the structurally unique Fe-O-O-Fe frame, which is discussed on the basis of the extended Huckel calculations. Above -20 OC, the reaction of 4 with dioxygen causes irreversible oxidation, resulting in formation of a trimeric ferric complex (HB(3,5-iPr(2)pz)(3))Fe(OBz)(2)(O)Fe(OH)(OBz)(2)Fe(HB(3,5-iPr(2)pz)(3)) (16). This assignment is consistent with the magnetic property, Mossbauer spectrum, and X-ray analysis.