In an effort to gain more insight into the factors controlling the formation of low-spin non-heme Fe-III-peroxo intermediates in oxidation catalysis, such as activated bleomycin, we have synthesized a series of iron complexes based on the pentadentate ligand N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). The following complexes have been prepared: [(N4Py)Fe-II(CH3CN)](ClO4)2 (1), [(N4Py)(FeCl)-Cl-II](ClO4) (2), [(N4Py)(FeOMe)-O-III](ClO4)(2) (3), and [(N4Py)(2)Fe2O] (ClO4)(4) (4) Complexes 1 and 2 have low- and high-spin Fe-II centers, respectively, whereas 3 is an Fe-III complex that undergoes a temperature-dependent spin transition. The iron centers in the ore-bridged dimer 4 are antiferromagnetically coupled (J = - 104 cm(-1)). Comparison of the crystal structures of 1, 3, and 4 shows that the ligand is well suited to accommodate both Fe-II and Fe-III in either spin state. For the high-spin Fe-III complexes 3 and 4 the iron atoms are positioned somewhat outside of the cavity formed by the ligand, while in the case of the low-spin Fe-II complex 1 the iron atom is retained in the middle of the cavity with approximately equal bond lengths to all nitrogen atoms from the ligand. On the basis of UV/vis and EPR observations, it is shown that 1, 3, and 4 all react with H2O2 to generate the purple low-spin [(N4Py)(FeOOH)-O-III](2+) intermediate (6). In the case of 1, titration experiments with H2O2 monitored by UV/vis and H-1 NMR reveal the formation of [(N4Py)(FeOH)-O-III](2+) (5) and the ore-bridged diiron(III) dimer (4) prior to the generation of the Fe-III-OOH species (6). Raman spectra of 6 show distinctive Raman features, particularly a v(O-O) at 790 cm(-1) that is the lowest observed for any iron-peroxo species. This observation may rationalize the reactivity of low-spin Fe-III-OOH species such as "activated bleomycin".