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.