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]aneN₄)(SPh)]BPh₄ (<b>2</b>), was synthesized and structurally characterized as a model of the reduced active site of superoxide reductase (SOR). Reaction of <b>2</b> with <i>tert</i>-butyl hydroperoxide (<i>t</i>BuOOH) at −78 °C led to the generation of the alkylperoxo-iron(III) complex [Fe^III(Me₄[15]aneN₄)(SPh)(OO<i>t</i>Bu)]⁺ (<b>2a</b>). The nonthiolate-ligated complex, [Fe^II(Me₄[15]aneN₄)(OTf)₂] (<b>3</b>), was also reacted with <i>t</i>BuOOH and yielded the corresponding alkylperoxo complex [Fe^III(Me₄[15]aneN₄)(OTf)(OO<i>t</i>Bu)]⁺ (<b>3a</b>) at an elevated temperature of −23 °C. These species were characterized by low-temperature UV−vis, EPR, and resonance Raman spectroscopies. Complexes <b>2a</b> and <b>3a</b> 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 (<i>S</i> = 1/2) to high-spin (<i>S</i> = 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 <b>2a</b> and the nonthiolate-ligated complex <b>3a</b> 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.