A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand: Effect of Ligand Field on Oxygen Atom Transfer and C-H Bond Cleavage Reactivity

A mononuclear iron(II) complex [Fe^II(N4Py^Me2)(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4Py^Me2), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4Py^Me2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [Fe^II(N4Py^Me2)(CH₃CN)](OTf)₂ (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at -10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe^IV(O)(N4Py^Me2)]²⁺ (2). Complex 2 exhibits the Mössbauer parameters δ = 0.05 mm/s and ΔE_Q = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [Fe^IV(O)(N4Py)]²⁺, 2 is more reactive in oxidizing thioanisole and oxygenates the C-H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe^IV(O)(N4Py)]²⁺, as supported by DFT calculations. The second-order rate constants for HAT by 2 is well correlated with the C-H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [Fe^IV(O)(N4Py)]²⁺, and 2 is more reactive only in the case of strong C-H bonds (>86 kcal/mol), but less reactive in the case of weaker C-H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.