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 L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]­heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 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, ΔEQ = 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 1, with E1/2(FeIII/FeII) = 0.49 V vs Fc+/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 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 1 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: λmax = 705 nm, ε ≈ 240 M–1 cm–1; Mössbauer: δ = 0.03(2) mm/s, ΔEQ = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)­FeIVO intermediate was established in rapid (k2 = 26.5 M–1 s–1 for oxidation of PPh3 at 0 °C), associative (ΔH⧧ = 53 kJ/mol, ΔS⧧ = −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 (k2 = 0.17 M–1 s–1 at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)­FeIVO that could be generated from (L1)­FeII 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, (L1)­FeIVO, is a kinetically competent intermediate for cyclooctene epoxidation with H2O2 at room temperature. Partial ligand oxidation of (L1)­FeIV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.