Acid-Triggered O−O Bond Heterolysis of a Nonheme Fe^III(OOH) Species for the Stereospecific Hydroxylation of Strong C−H Bonds

A novel hydroperoxoiron(III) species [Fe^III(OOH)(MeCN)(PyNMe₃)]²⁺ (3) has been generated by reaction of its ferrous precursor [Fe^II(CF₃SO₃)₂(PyNMe₃)] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C−H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C−H bonds with high stereospecificity. Stopped-flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic-acid-assisted heterolytic cleavage of the O−O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high-valent [(Porph^.)Fe^IV(O)] (Compound I).