Spectroscopic Properties and Electronic Structure of Low-Spin Fe(III)−Alkylperoxo Complexes:  Homolytic Cleavage of the O−O Bond

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)−alkylperoxo model complex [Fe(TPA)(OH x )(OOtBu)] x + (1; TPA = tris(2-pyridylmethyl)amine, tBu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O−O stretch (796 cm-1), the Fe−O stretch (696 cm-1), and a combined O−C−C/C−C−C bending mode (490 cm-1) that is mixed with v(FeO). The corresponding force constants have been determined to be 2.92 mdyn/Å for the O−O bond which is small and 3.53 mdyn/Å for the Fe−O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t2g d orbital of Fe(III). This metal−ligand π bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t2g → eg) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm-1 vibrations. Additional contributions to the Fe−O bond arise from σ interactions between mainly O−O bonding donor orbitals of the alkylperoxo ligand and an eg d orbital of Fe(III), which explains the observed O−O and Fe−O force constants. The observed homolytic cleavage of the O−O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O−O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)O species formed is highly stabilized (for spin states S = 1 and 2) by two strong π and a strong σ bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O−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 resulting Cu(III)O species is less stable. This difference in metal−oxo intermediate stability enables the O−O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.