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/A) 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 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(t)Bu)](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(t)Bu 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 pi 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.
Lehnert
Nicolai
N
Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA.
Neese
Frank
F
Ho
Raymond Y N
RY
Que
Lawrence
L
Solomon
Edward I
EI
D000903:Antibiotics, Antineoplastic / Q000737:chemistry*; D001761:Bleomycin / Q000737:chemistry*; D002942:Circular Dichroism; D005290:Ferric Compounds / Q000737:chemistry*; D008958:Models, Molecular; D010545:Peroxides / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman / Q000379:methods; D013816:Thermodynamics
D000903:Antibiotics, Antineoplastic; D005290:Ferric Compounds; C431844:N4Py compound; D010545:Peroxides; D011725:Pyridines; D001761:Bleomycin