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 λmax=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔEQ) 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, kH/kD, 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.
Bibliographical noteFunding Information:
This research was supported by the United States-Israel Binational Science Foundation (grant #2004270 to R. N. and L. Q.) and the US National Institutes of Health (grant GM38767 to L. Q. and postdoctoral fellowship ES017390 to M. A. C.). J.E.M.N.K. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship. We also acknowledge Prof. Christopher J. Cramer and the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing computational resources. Iraklii Ebralidze is thanked for his initial research in this area. R. N. is the Rebecca and Israel Professor of Organic Chemistry.
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- Bridging ligands
- C−H bond activation
- O−O bond activation
- hydrogen-atom abstraction