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 (S = 2) electronic configuration is believed to be particularly important in C–H activation reactions mediated by iron(IV)-oxo species, often via spin crossover from an intermediate-spin (S = 1) ground state. Coordination environments that promote high-spin ground states obviate the need for spin-state crossing and can promote rapid oxidation reactivity. As a result, however, molecular iron(IV)-oxo species with S = 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 ºC in CH3CN, endowing this compound with the highest thermal stability for a high-spin FeIV=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-tert-butylphenol (TTBP) occurs readily at room temperature despite its steric bulk, suggesting a step-wise PCET mechanism for this more acidic substrate.