| Abstract
| - The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)−alkylperoxomodel complex [Fe(TPA)(OHx)(OOtBu)]x+ (1; TPA = tris(2-pyridylmethyl)amine, tBu = tert-butyl, x = 1 or2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O−O stretch (796cm-1), the Fe−O stretch (696 cm-1), and a combined O−C−C/C−C−C bending mode (490 cm-1) that ismixed with v(FeO). The corresponding force constants have been determined to be 2.92 mdyn/Å for theO−O bond which is small and 3.53 mdyn/Å for the Fe−O bond which is large. Complex 1 is characterizedby a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from thealkylperoxoto a t2g d orbital of Fe(III). This metal−ligand π bond is probed by MCD and resonanceRaman spectroscopies which show that the CT state is mixed with a ligand field state (t2g → eg) by configurationinteraction. This gives rise to two intense transitions under the broad 600 nm envelope with CT characterwhich are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Ramanprofiles of the 796, 696, and 490 cm-1 vibrations. Additional contributions to the Fe−O bond arise from σinteractions between mainly O−O bonding donor orbitals of the alkylperoxo ligand and an eg d orbital ofFe(III), which explains the observed O−O and Fe−O force constants. The observed homolytic cleavage ofthe O−O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. TheO−O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)Ospecies formed is highly stabilized (for spin states S = 1 and 2) by two strong π and a strong σ bond betweenFe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting theO−O bond. In comparison, Cu(II)−alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon,E. I. J. Am. Chem. Soc.2000, 122, 10177] are much less suited for O−O bond homolysis, because the resultingCu(III)O species is less stable. This difference in metal−oxo intermediate stability enables the O−O homolysisin the case of iron but directs the copper complex toward alternative reaction channels.
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