Influence of the nitrogen donors on nonheme iron models of superoxide reductase: High-spin Fe^III-OOR complexes

A new five-coordinate, (N₄S(thiolate))Fe^II complex, containing tertiary amine donors, [Fe^II(Me₄[15]aneN ₄)(SPh)]BPh₄ (2), was synthesized and structurally characterized as a model of the reduced active site of superoxide reductase (SOR). Reaction of 2 with tert-butyl hydroperoxide (tBuOOH) at -78°C led to the generation of the alkylperoxo-iron(III) complex [Fe^III(Me ₄[15]aneN₄)(SPh)(OOtBu)]⁺ (2a). The nonthiolate-ligated complex, [Fe^II(Me₄[15]aneN ₄)(OTf)₂] (3), was also reacted with tBuOOH and yielded the corresponding alkylperoxo complex [Fe^III(Me₄[15] aneN₄)(OTf)(OOtBu)]⁺ (3a) at an elevated temperature of -23 °C. These species were characterized by low-temperature UV-vis, EPR, and resonance Raman spectroscopies. Complexes 2a and 3a exhibit distinctly different spectroscopic signatures than the analogous alkylperoxo complexes [Fe^III([15]aneN₄)(SAr)(OOR)]⁺, which contain secondary amine donors. Importantly, alkylation at nitrogen leads to a change from low-spin (S = 1/2) to high-spin (S = 5/2) of the iron(III) center. The resonance Raman data reveal that this change in spin state has a large effect on the ν(Fe-O) and ν(O-O) vibrations, and a comparison between 2a and the nonthiolate-ligated complex 3a shows that axial ligation has an additional significant impact on these vibrations. To our knowledge this study is the first in which the influence of a ligand trans to a peroxo moiety has been evaluated for a structurally equivalent pair of high-spin/low-spin peroxo-iron(III) complexes. The implications of spin state and thiolate ligation are discussed with regard to the functioning of SOR.