Hydrogen-Atom Transfer Oxidation with H₂O₂ Catalyzed by [FeII(1,2-bis(2,2′-bipyridyl-6-yl)ethane(H₂O)₂]²⁺: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate

The iron(II) triflate complex (1) of 1,2-bis(2,2′-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30 % H₂O₂ to an acetonitrile solution of 1 yielded 2, a green compound with λ_max=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔE_Q) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm⁻¹ that downshifted to 832, 540 and 441 cm⁻¹ when 1 was treated with H₂ ¹⁸O₂. All the spectroscopic data support the initial formation of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0 °C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C−H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C−H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, k_H/k_D, of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.