Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: reaction coordinate of O-O bond homolysis.
The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.
Lehnert
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Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA.
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Solomon
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D005290:Ferric Compounds / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman / Q000379:methods; D014732:Vibration; D020122:tert-Butylhydroperoxide / Q000737:chemistry
D005290:Ferric Compounds; D005609:Free Radicals; D011725:Pyridines; C081909:iron(III)-tris(N-(2-pyridylmethyl)-2-aminoethyl)amine; D020122:tert-Butylhydroperoxide; D010100:Oxygen