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)(OO^tBu)]^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 π_v* to a t_2g 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 (t_2g → e_g) 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^-l 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 e_g 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(Il)-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.