Hydrogen atom transfer (HAT, eq 1) is an elementary chemical transformation that results in the net transfer of both a proton and an electron. 1,2 MetalÀoxo complexes are widely used to abstract hydrogen atoms from organic compounds through HAT, which leads to the metal hydroxide (Scheme 1). 3 In oxidations by cytochrome P450, the mechanism is generally accepted to be HAT to an ironÀoxo species (FedO) followed by radical rebound. 4 Other enzymatic systems such as soluble methane monooxygenase, 5 ribonucleotide reductases and other B 12 -dependent enzymes, 6 lipoxygenases, 7 isopenicillin-N synthase, 8 and TauD 9 also utilize mechanisms with key HAT steps.Considerable effort has been devoted to synthesizing and studying biomimetic oxoiron complexes 10 in order to help elucidate the enzymatic mechanisms and to develop homogeneous iron-based oxidation catalysts. Studies on heme ironÀoxo complexes in the 1980s by Balch, La Mar, and Groves pioneered this field. 11 More recently, Que, Nam, and co-workers have reported isolable non-heme oxoiron(IV) complexes that react with hydrocarbons via HAT. 12 Reactions proceeding by HAT mechanisms have also been studied for terminal oxo complexes of Mn, Ru, Cr, and V. 13À19 In general, the selectivity of nonenzymatic HAT reactions is thermodynamically controlled, and reaction rates follow a linear correlation with the bond dissociation enthalpy (BDE) of the XÀH bond being broken (the BellÀEvansÀPolanyi relation), 1,20 i.e., homolytically weaker substrate bonds react more rapidly.Imido (NR 2À ) ligands are isoelectronic to oxo (O 2À ) ligands (Scheme 1), and imido complexes are often proposed as intermediates in hydrocarbon amination mechanisms in which the imido species performs the cleavage of the CÀH bond that precedes CÀN bond formation. Imido complexes are also more versatile than oxo complexes, because there is an opportunity to tune the steric and electronic properties of the complex by changing the nitrogen substituent. However, the HAT reactivity of imido complexes (MdNR) has not been investigated in as much detail as that of their oxo counterparts. There has been a recent renaissance of activity in the synthesis of imido complexes of the late transition metals (groups 8À11), 21 and this activity has resulted in the isolation of late transition metal complexes Received: January 18, 2011 ABSTRACT: In the literature, ironÀoxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. IronÀimido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L Me FeNAd (L Me = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ( t Bupy) to form a reactive fourcoordinate intermediate L Me Fe(NAd)( t Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very la...