Mechanistic Studies of the Oxidative N-Dealkylation of a Substrate Tethered to Carboxylate-Bridged Diiron(II) Complexes, [Fe2(μ-O2CArTol)2(O2CArTol)2(N,N-Bn2en)2]
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posted on 2006-07-10, 00:00 authored by Sungho Yoon, Stephen J. LippardCarboxylate-bridged diiron(II) centers activate dioxygen for the selective oxidation of hydrocarbon substrates in
bacterial multicomponent monooxygenases. Synthetic analogues of these systems exist in which substrate fragments
tethered to the diiron(II) core through attachment to an N-donor ligand are oxidized by transient species that arise
following the introduction of O2 into the system. The present study describes the results of experiments designed
to probe mechanistic details of these oxidative N-dealkylation reactions. A series of diiron(II) complexes with ligands
N,N-(4-R−Bn)Bnen, where en is ethylenediamine, Bn is benzyl, and R−Bn is benzyl with a para-directing group R
= Cl, F, CH3, t-Bu, or OCH3, were prepared. A Hammett plot of the oxygenation product distributions of these
complexes, determined by gas chromatographic analysis, reveals a small positive slope of ρ = +0.48. Kinetic
isotope effect (KIEintra) values for oxygenation of [Fe2(μ-O2CArTol)2(O2CArTol)2(N,N-(C6H5CDH)2en)2] and
[Fe2(μ-O2CArTol)2(O2CArTol)2(N,N-(C6H5CD2)(C6H5CH2)en)2] are 1.3(1) and 2.2(2) at 23 °C, respectively. The positive
slope ρ and low KIEintra values are consistent with a mechanism involving one-electron transfer from the dangling
nitrogen atom in N,N-Bn2en to a transient electrophilic diiron intermediate, followed by proton transfer and
rearrangement to eliminate benzaldehyde.
