Generation of a Nonheme Oxoiron(IV) Intermediate and Its Reactivities in Oxidation Reactions

Abstract

High-valent iron(IV)-oxo species have been invoked as the key reactive intermediates in the catalytic cycles of nonheme iron enzymes involved in dioxygen metabolism. Direct experimental evidence for such intermediates has been obtained in the chemistry of methane monooxygenase and taurine-2-oxoglutarate dioxygenase. To gain insight into this interesting chemistry, we have made much effort in the design and synthesis of functional models for such systems. Recently, it has been reported that synthetic mononuclear oxoiron(IV) complexes were generated using tetradentate N4 ligands such as macrocyclic TMC and tripodal TPA ligands and pentadentate N5 ligands such as N4Py and Bn-tpen. While the TMC oxoiron(IV) intermediate can effect oxygen-atom transfer to PPh3 and the TPA complex is able to epoxidize cyclooctene, the N4Py and Bn-tpen complexes are capable of activating C-H bonds of alkanes. As the TMC ligand provides relatively weak reactivity to oxoiron(IV) species, we have tried to modify cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) with other substituents on nitrogens of the ligand, in order to understand the effect of ligand environment by cyclam derivatives on the spectroscopic properties and reactivities of oxoiron(IV) species. We now report the synthesis and characterization of an Fe(IV)-oxo complex bearing a tetradentate macrocyclic TBC (TBC = 1,4,8,11-tetrabenzyl-1,4, 8,11-tetraazacyclotetradecane) which has benzyl groups instead of methyl on TMC. This new Fe=O species exhibits an ability to oxidize a variety of organic substrates such as triphenylphosphine, thioanisole, N,N-dimethylaniline, and triphenylmethane. The tetradentate TBC ligand (1) was prepared from cyclam by alkylation with benzyl bromide (Scheme 1). After purification by column chromatography, the TBC ligand was obtained in 60% yield. Reaction of 1 with Fe(OTf)2 afforded white powder of [Fe(TBC)(CH3CN)2](OTf)2 (2). Complex 2 collected by recrystallization was subjected to comprehensive spectroscopic analysis. We then tried to test the generation of oxoiron(IV) species