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Singh, Reena; Ganguly, Gaurab; Malinkin, Sergey O; Demeshko, Serhiy; Meyer, Franc; Nordlander, Ebbe; Paine, Tapan Kanti
Inorganic chemistry, 02/2019, Volume: 58, Issue: 3Journal Article
A mononuclear iron(II) complex FeII(N4PyMe2)(OTf)(OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4PyMe2), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4PyMe2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to FeII(N4PyMe2)(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, FeIV(O)(N4PyMe2)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 FeIV(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 FeIV(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 FeIV(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.
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