Mechanistic studies of the oxidative N-dealkylation of a substrate tethered to carboxylate-bridged diiron(II) complexes, [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(N,N-Bn(2)en)(2)]

Abstract

Carboxylate-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 O-2 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 rho = +0.48. Kinetic isotope effect (KIEintra) values for oxygenation of [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(N,N-(C6H5CDH)(2)en)(2)] and [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(N,N-(C6H5CD2)(C6H5CH2)en)(2)] are 1.3(1) and 2.2(2) at 23 degrees C, respectively. The positive slope rho and low KIEintra values are consistent with a mechanism involving one-electron transfer from the dangling nitrogen atom in N,N-Bn(2)en to a transient electrophilic diiron intermediate, followed by proton transfer and rearrangement to eliminate benzaldehyde.