Generation and Reactivity of a High-Spin Iron(IV)-Oxo Complex That Is Stable at Ambient Temperatures

Nature operates a variety of challenging oxidation reactions through intermediates bearing tetravalent iron centers bound to a terminal oxo ligand. The high-spin (<i>S</i> = 2) electronic configuration is believed to be particularly important in C-H activation reactions mediated by iron(IV)-oxo species. Coordination environments that promote high-spin ground states obviate the need for spin-state crossing to access this state and can promote rapid oxidation reactivity. As a result, however, synthetic iron(IV)-oxo species with <i>S</i> = 2 ground states tend to exhibit poor thermal stabilities, which has hampered a broader elucidation of their reactivity profiles. In this work, we report the synthesis of a remarkably stable high-spin iron(IV)-oxo complex that localizes the Fe═O unit within a rigid organic macrocycle. This design results in essentially unlimited stability at ambient temperatures and a half-life of 21 h at 70 °C in CH₃CN, endowing this compound with the highest thermal stability for a high-spin Fe^IV═O complex reported to date. The ligand's steric profile shuts down intermolecular reactivity with potential O atom acceptors and hydrocarbons bearing weak C-H bonds, but proton-coupled electron transfer reactivity with 2,4,6-tri-<i>tert</i>-butylphenol (TTBP) occurs readily at room temperature despite its steric bulk.