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-Me₃TPA)-(OH_x)(OO^tBu)]^x+ (1; TPA = tris(2-pyridylmethyl)amine, ^tBu = 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/Å for the O-O and 2.87 mdyn/Å 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 π_v* 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 σ interaction between π_h* 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^tBu)]^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 σ 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.