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/Å). 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 to a t2g d orbital of Fe(III) using resonance Raman and MCD spectroscopies and density functional (DFT) calculations. Compared to low-spin [Fe(TPA)(OHx)(OOtBu)]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 •OtBu 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.