Valence Tautomerism in a High-Valent Manganese-Oxo Porphyrinoid Complex Induced by a Lewis Acid
ABSTRACT Addition of the Lewis acid Zn(2+) to (TBP(8)Cz)Mn(V)(O) induces valence tautomerization, resulting in the formation of [(TBP(8)Cz(+•))Mn(IV)(O)-Zn(2+)]. This new species was characterized by UV-vis, EPR, the Evans method, and (1)H NMR and supported by DFT calculations. Removal of Zn(2+) quantitatively restores the starting material. Electron-transfer and hydrogen-atom-transfer reactions are strongly influenced by the presence of Zn(2+).
SourceAvailable from: Chivukula V Sastri[Show abstract] [Hide abstract]
ABSTRACT: Nature often utilizes molecular oxygen for oxidation reactions through monoxygenases and dioxygenases. In many of these systems a high-valent iron(IV)-oxo active species is found. In recent years evidence has accumulated of possible iron(IV)-imido and iron(V)-nitrido intermediates in enzymatic catalysis, although little is known on their activity. In this work we report a detailed combined kinetics and computational study on the difference in reactivity and chemical properties of nonheme iron(IV)-oxo vis-à-vis iron(IV)-tosylimido. We show here that iron(IV)-tosylimido complex is much more reactive with sulfides than the corresponding iron(IV)-oxo complex, however, the reverse trend is obtained for hydrogen atom abstraction reactions. The latter proceed with a relatively small kinetic isotope effect of kH/kD = 7 for the iron(IV)-tosylimido complex. Moreover, a Hammett analysis of hydrogen atom abstraction from para-X-benzyl alcohol reveals a slope of close to zero for the iron(IV)-oxo, whereas a strong negative slope is found for the iron(IV)-tosylimido complex. These studies implicate dramatic changes in the reaction mechanisms and suggest a considerable charge-transfer in the transition states. Density functional theory calculations were performed to support the experiments and confirm an initial long-range electron transfer for the iron(IV)-tosylimido complex with substrates, due to a substantially larger electron affinity as compared to the iron(IV)-oxo species. As a consequence, it also reacts more efficiently in electrophilic addition reactions such as those with sulfides. By contrast, the long-range electron transfer for the iron(IV)-tosylimido complex results in a rate constant that is dependent on the *xz *z2 excitation energy, which raises the hydrogen atom abstraction barrier above that found for the iron(IV)-oxo. On the other hand, sulfimidation has much earlier electron transfer steps as compared to sulfoxidation. All data has been analyzed and rationalized with valence bond models and thermochemical cycles. Our studies highlight the catalytic potential of iron(IV)-tosylimido complexes in chemistry and biology.Journal of the American Chemical Society 11/2014; 136(49). DOI:10.1021/ja508403w · 11.44 Impact Factor
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ABSTRACT: Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating reactivities of oxygen-containing metal complexes in a variety of biological and biomimetic reactions, including dioxygen activation/formation and functionalization of organic substrates. Mononuclear nonheme iron(III)-peroxo species are invoked as active oxygen intermediates in the catalytic cycles of dioxygen activation by nonheme iron enzymes and their biomimetic compounds. Here, we report mononuclear nonheme iron(III)-peroxo complexes binding redox-inactive metal ions, [(TMC)Fe(III)(O2)](+)-M(3+) (M(3+) = Sc(3+) and Y(3+); TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), which are characterized spectroscopically as a 'side-on' iron(III)-peroxo complex binding a redox-inactive metal ion, (TMC)Fe(III)-(μ,η(2):η(2)-O2)-M(3+) (2-M). While an iron(III)-peroxo complex, [(TMC)Fe(III)(O2)](+), does not react with electron donors (e.g., ferrocene), one-electron reduction of the iron(III)-peroxo complexes binding redox-inactive metal ions occurs readily upon addition of electron donors, resulting in the generation of an iron(IV)-oxo complex, [(TMC)Fe(IV)(O)](2+) (4), via heterolytic O-O bond cleavage of the peroxide ligand. The rates of the conversion of 2-M to 4 are found to depend on the Lewis acidity of the redox-inactive metal ions and the oxidation potential of the electron donors. We have also determined the fundamental electron-transfer properties of 2-M, such as the reduction potential and the reorganization energy in electron-transfer reaction. Based on the results presented herein, we have proposed a mechanism for the reactions of 2-M and electron donors; the reduction of 2-M to the reduced species, (TMC)Fe(II)-(O2)-M(3+) (2'-M), is the rate-determining step, followed by heterolytic O-O bond cleavage of the reduced species to form 4. The present results provide a biomimetic example demonstrating that redox-inactive metal ions bound to an iron(III)-peroxo intermediate play a significant role in activating the peroxide O-O bond to form a high-valent iron(IV)-oxo species.Chemical Science 07/2013; 4(10):3917-3923. DOI:10.1039/C3SC51864G · 8.60 Impact Factor
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ABSTRACT: Tetraazaporphyrin (TAP) complexes with group 15 elements (phosphorus(V) or arsenic(V)) containing two axial OH ligands showed reversible spectroscopic changes with acid or base doping. Spectroscopic and theoretical analysis revealed that the modification of axial ligands can tune the interaction between peripheral substituents and the TAP macrocycle.Chemical Communications 10/2014; 50(95). DOI:10.1039/C4CC07408D · 6.38 Impact Factor