Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron(III) Complex

In the literature, iron–oxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. Iron–imido 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, LMeFeNAd (LMe = 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 four-coordinate intermediate LMeFe(NAd)( t Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very large substrate H/D kinetic isotope effects around 100 are consistent with C–H bond cleavage. The elementary HAT rate constant is increased by electron-donating groups on the pyridine additive, and by a more polar medium. When combined with the faster rate of HAT from indene versus cyclohexadiene, this trend is consistent with H+ transfer character in the HAT transition state. The increase in HAT rate in the presence of t Bupy may be explained by a combination of electronic (weaker FeN π-bonding) and thermodynamic (more exothermic HAT) effects. Most importantly, HAT by these imido complexes has a strong dependence on the size of the hydrocarbon substrate. This selectivity comes from steric hindrance by the spectator ligands, a strategy that has promise for controlling the regioselectivity of these C–H bond activation reactions.