Electronic structure and reactivity of low-spin Fe(III)-hydroperoxo complexes: Comparison to activated bleomycin

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-hydroperoxo complex [Fe(N4Py)(OOH)]²⁺ (1, N4Py = N, N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) are investigated in comparison to those of activated bleomycin (ABLM). Complex 1 is characterized by Raman features at 632 (Fe-O stretch) and 790 cm^-1 (O-O stretch), corresponding to a strong Fe-O bond (force constant 3.62 mdyn/Å) and a weak O-O bond (3.05 mdyn/A). The UV-vis spectrum of 1 shows a broad absorption band around 550 nm that is assigned to a charge-transfer transition from the hydroperoxo π*_v to a t_2g d orbital of Fe(III) using resonance Raman and MCD spectroscopies and density functional (DFT) calculations. Compared to low-spin [Fe(TPA)(OH_x)(OO^tBu)]^x+ (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2), an overall similar Fe-OOR bonding results for low-spin Fe(III)-alkylperoxo and -hydroperoxo species. Correspondingly, both systems show similar reactivities and undergo homolytic cleavage of the O-O bond. From the DFT calculations, this reaction is more endothermic for 1 due to the reduced stabilization of the ·OH radical compared to ·O^tBu and the absence of the hydroxo ligand that helps to stabilize the resulting Fe(IV)=O species. In contrast, ABLM has a somewhat different electronic structure where no π donor bond between the hydroperoxo ligand and iron(III) is present [Neese, F.; Zaleski, J. M.; Loeb-Zaleski, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11703]. Possible reaction pathways for ABLM are discussed in relation to known experimental results.