Tuning Stability and Reactivity of Mononuclear Nonheme Iron(IV)-Oxo Species by Amide Group and N-Substituents on TPEN-Derived Ligands

Four iron(II) complexes, [(6-Amide-BnTPEN)FeII](OTf)₂ (1), [(6-Amide-MsTPEN)FeII](OTf)₂ (2), [(6-Amide-5FBnTPEN)FeII](OTf)₂ (3), and [(6-Amide-MeTPEN)FeII](OTf)₂ (4), of N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine (TPEN)-derived ligands have been isolated to probe the effect of the amide group in one of the pyridine arms and N-substituents (benzyl, 2,4,6-trimethylbenzyl, pentafluorobenzyl, and methyl) on their electronic properties and reactivity. The iron(II) complexes are primarily high spin in the solid state but display a temperature-dependent spin crossover behavior in acetonitrile. The amide group tunes the stability of the corresponding iron(IV)-oxo complexes (1A, 2A, 3A, and 4A). The half-lives of the intermediates at 298 K follow the order 3A ∼ 1A > 2A > 4A. All the iron(IV)-oxo species are more reactive (6- to 24-fold) in OAT and HAT reactions compared to the [(BnTPEN)FeIV(O)]²⁺ complex. Among the iron-oxo species, 3A is the most electrophilic, exhibiting catalytic OAT activity toward thioanisole; however, 4A is the overall most unstable oxidant. Experimental and theoretical studies indicate that N-substituents affect the thermodynamic control of structural reorganization during the formation of iron(IV)-oxo complexes. The destabilizing factor, resulting from the spatial proximity of the tBu group and N-substituents, in conjunction with H-bonding-induced stabilization, determines the stability of the iron(IV)-oxo complexes. Moreover, the second sphere interaction from the amide group tunes stability and reactivity, thereby displaying selectivity in oxygenation reactions.