Kinetic Studies on the Oxoiron(IV) Complex with Tetradentate Aminopyridine Ligand PDP*: Restoration of Catalytic Activity by Reduction with HO.

Oxoiron(IV) is a common catalytic byproduct observed in the oxidation of alkenes by the combination of HO and nonheme iron catalysts including complex , FePDP* (where PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(,)-2,2'-bipyrrolidine). The oxoiron(IV) species have been proposed to arise by O-O homolysis of the peroxyiron(III) or acylperoxyiron(III) intermediates formed during the presumed Fe-Fe catalytic cycle and have generally been regarded as off-pathway. We generated complex (λ = 730 nm, ε = 350 M cm) directly from and an oxygen atom donor IBX (isopropyl 2-iodoxybenzoate) in acetonitrile in the temperature range from -35 to +25 °C under stopped-flow conditions. Species is metastable (half-life of 2.0 min at +25 °C), and its decay is accelerated in the presence of organic substrates such as thioanisole, alkenes, benzene, and cyclohexane. The reaction with cyclohexane- is significantly slower (KIE = 4.9 ± 0.4), suggesting that a hydrogen atom transfer to is the rate limiting step. With benzene-, no significant isotope effect is observed (KIE = 1.0 ± 0.2), but UV-vis spectra show the concomitant formation of an intense 580 nm band likely due to the Fe(III)-phenolate chromophore, suggesting an electrophilic attack of on the aromatic system of benzene. Treatment of with HO resulted in rapid decay of its 730 nm visible band ( = 102.6 ± 4.6 M s at -20 °C), most likely occurring by a hydrogen atom transfer from HO. In the presence of excess HO, the oxoiron(IV) is transformed into peroxyiron(III), as seen from the formation of a characteristic 550 nm visible band and = 2.22, 2.16, and 1.96 electron paramagnetic resonance (EPR) spectroscopy signals. Reductively formed was able to re-enter the catalytic cycle of alkene epoxidation by HO, albeit with lower yields versus those of oxidatively formed (i.e., + HO) peroxyiron(III) owing to a loss of ca. 40% active iron. As such, the oxoiron(IV) species can be reintroduced to the catalytic cycle with extra HO, acting as an iron reservoir. Alternatively, peroxycarboxylic acids, which have a stronger O-H bond dissociation energy, do not reduce , ensuring that more oxidant is productively employed in substrate oxidation. While this reaction with HO may occur for other nonheme oxoiron(IV) complexes, the only previously reported examples are and , which are reduced by hydrogen peroxide 130- and 2900-fold more slowy, respectively (as in 2012 , 51 ( 22 ), 5376 - 5380 , DOI: 10.1002/anie.201200901 ).