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 degrees C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe-IV(O)(N4Py(Me2))](2+) (2). Complex 2 exhibits the Mossbauer parameters delta = 0.05 mm/s and Delta E-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 degrees 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 CH 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.