End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands: Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalytic
cycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases.
Only recently have biomimetic analogues of such reactive species been generated and characterized at low
temperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5
ligands that react with H2O2 to generate transient low-spin FeIII−OOH intermediates. These intermediates have
low-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500−600-nm region. Their
resonance Raman frequencies, νO-O, near 800 cm-1 are significantly lower than those observed for high-spin
counterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the νO-O of the Fe−OOH unit
decreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spin
Fe(III) center weakens the O−O bond, in accord with conclusions drawn from published DFT calculations. The
parent [(N4Py)FeIII(η1-OOH)]2+ (1a) ion in this series (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine)
can be converted to its conjugate base, which is demonstrated to be a high-spin iron(III) complex with a side-on
peroxo ligand, [(N4Py)FeIII(η2-O2)]+ (1b). A detailed analysis of 1a and 1b by EPR and Mössbauer spectroscopy
provides insights into their electronic properties. The orientation of the observed 57Fe A-tensor of 1a can be explained
with the frequently employed Griffith model provided the rhombic component of the ligand field, determined by the
disposition of the hydroperoxo ligand, is 45° rotated relative to the octahedral field. EXAFS studies of 1a and 1b
reveal the first metrical details of the iron−peroxo units in this family of complexes:  [(N4Py)FeIII(η1-OOH)]2+ has an
Fe−O bond of 1.76 Å, while [(N4Py)FeIII(η2-O2)]+ has two Fe−O bonds of 1.93 Å, values which are in very good
agreement with results obtained from DFT calculations.