Iron Chemistry of a Pentadentate Ligand That Generates a Metastable FeIII−OOH Intermediate

In an effort to gain more insight into the factors controlling the formation of low-spin non-heme FeIII−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)FeII(CH3CN)](ClO4)2 (1), [(N4Py)FeIICl](ClO4) (2), [(N4Py)FeIIIOMe](ClO4)2 (3), and [(N4Py)2Fe2O](ClO4)4 (4). Complexes 1 and 2 have low- and high-spin FeII centers, respectively, whereas 3 is an FeIII complex that undergoes a temperature-dependent spin transition. The iron centers in the oxo-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 FeII and FeIII in either spin state. For the high-spin FeIII 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 FeII 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)FeIIIOOH]2+ intermediate (6). In the case of 1, titration experiments with H2O2 monitored by UV/vis and 1H NMR reveal the formation of [(N4Py)FeIIIOH]2+ (5) and the oxo-bridged diiron(III) dimer (4) prior to the generation of the FeIII−OOH species (6). Raman spectra of 6 show distinctive Raman features, particularly a ν(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 FeIII−OOH species such as “activated bleomycin”.