Distinct Reactivity Differences of Metal Oxo and Its Corresponding Hydroxo Moieties in Oxidations: Implications from a Manganese(IV) Complex Having Dihydroxide Ligand
ABSTRACT The M n+-OH moiety in a manganese(IV) complex has more powerful electron-transfer capability than its corresponding M n+=O moiety. An M n+=O moiety can abstract hydrogen from a substrate, then rebind the OH group from its reduced M (n-1)+-OH form to the substrate radical. In contrast, the active center with an M n+-OH cannot perform similar rebound from its reduced M (n-1)+-OH 2 group (see scheme; HAT=hydrogen abstraction).
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ABSTRACT: One-electron oxidation of the tetragonal Cu(II) complex [Bu(4)N][LCuOH] at -80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH(2)). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M(-1) s(-1) at -80 °C, ΔH(‡) = 5.4(2) kcal mol(-1), ΔS(‡) = -30(2) eu) and with very large kinetic isotope effects (cf. k(H)/k(D) = 44 at -70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.Journal of the American Chemical Society 11/2011; 133(44):17602-5. DOI:10.1021/ja207882h · 12.11 Impact Factor
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ABSTRACT: Iron complexes with the tetradentate N-donor ligand N,N'-di(phenylmethyl)-N,N'-bis(2-pyridinylmethyl)-1,2-cyclohexanediamine (bbpc) are reported. Despite the benzyl groups present on the amines, the iron compounds catalyze the oxygenation of cyclohexane to an extent similar to those employing less sterically encumbered ligands. The catalytic activity is strongly dependent on the counterion, with the highest activity and the strongest preference for alkane hydroxylation correlating to the most weakly coordinating anion, SbF(6)(-). The selectivity for the alcohol product over the ketone is amplified when acetic acid is present as an additive. When hydrocarbon substrates with both secondary and tertiary carbons are oxidized by H(2)O(2), the catalyst directs oxidation toward the secondary carbons to a greater degree than other previously reported iron-containing homogeneous catalysts.Inorganic Chemistry 11/2011; 50(24):12651-60. DOI:10.1021/ic201695a · 4.76 Impact Factor
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ABSTRACT: Clear elucidation of the oxidative relationships of the active metal hydroperoxide moiety with its corresponding metal oxo and hydroxo intermediates would help the understanding of the different roles they may play in redox metalloenzymes and oxidation chemistry. Using an Mn(Me(2)EBC)Cl(2) complex, it was found that, in t-butanol-water (4 : 1) with excess H(2)O(2) at pH 1.5, the Mn(IV)-OOH moiety may exist in the catalytic solution with a mass signal of m/z = 358.1, which provides a particular chance to investigate its oxidative properties. In catalytic oxidations, the Mn(IV)-OOH moiety demonstrates a relatively poor activity in hydrogen abstraction from diphenyl methane and ethylbenzene with TOF of only 1.2 h(-1) and 1.1 h(-1) at 50 °C, whereas it can efficiently oxygenate diphenyl sulfide, methyl phenyl sulfide and benzyl phenyl sulfide with TOF of 13.8 h(-1), 15.4 h(-1) and 17.8 h(-1), respectively. In mechanistic studies using H(2)(18)O and H(2)(18)O(2), it was found that, in the Mn(IV)-OOH moiety mediated hydrogen abstraction and sulfide oxygenations, the reaction proceeds by two parallel pathways: one by direct oxygen insertion/transfer, and the other by plausible electron transfer. Together with a good understanding of the corresponding manganese(IV) oxo and hydroxo intermediates, this work provides the first chance to compare the reactivity differences and similarities of the active metal oxo, hydroxo and hydroperoxide intermediates. The available evidence reveals that the Mn(IV)-OOH moiety has a much more powerful oxidizing capability than the corresponding Mn(IV)=O and Mn(IV)-OH functional groups in both hydrogen abstraction and oxygenation.Dalton Transactions 03/2012; 41(9):2612-9. DOI:10.1039/c2dt11814a · 4.20 Impact Factor