Role of Fe(IV)-Oxo Intermediates in Stoichiometric and Catalytic Oxidations Mediated by Iron Pyridine-Azamacrocycles

An iron­(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand <b>L1</b> (<b>L1</b> = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]­heptadeca-1(17),13,15-triene), <b>1</b>, was prepared and characterized. Complex <b>1</b> contains low-spin iron­(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and Mössbauer spectroscopy (90 K, δ = 0.50(2) mm/s, Δ<i>E</i>_Q = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe­(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in <b>1</b>, with <i>E</i>_1/2(Fe^III/Fe^II) = 0.49 V vs Fc⁺/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex <b>1</b> catalyzed the epoxidation of cyclooctene and other olefins with H₂O₂. Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron­(IV)-oxo intermediate in the reaction of <b>1</b> with H₂O₂ and concomitant partial ligand oxidation. A soluble iodine­(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe­(IV)-oxo intermediates for additional spectroscopic (UV–vis in CH₃CN: λ_max = 705 nm, ε ≈ 240 M^–1 cm^–1; Mössbauer: δ = 0.03(2) mm/s, Δ<i>E</i>_Q = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (<b>L1</b>)­Fe^IVO intermediate was established in rapid (<i>k</i>₂ = 26.5 M^–1 s^–1 for oxidation of PPh₃ at 0 °C), associative (Δ<i>H</i>^⧧ = 53 kJ/mol, Δ<i>S</i>^⧧ = −25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet’s parameter ρ = −1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (<i>k</i>₂ = 0.17 M^–1 s^–1 at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (<b>L1</b>)­Fe^IVO that could be generated from (L1)­Fe^II and H₂O₂ or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl­(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe­(IV)-oxo species, (<b>L1</b>)­Fe^IVO, is a kinetically competent intermediate for cyclooctene epoxidation with H₂O₂ at room temperature. Partial ligand oxidation of (<b>L1</b>)­Fe^IVO occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl­(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl­(IV) intermediates to olefin substrates