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+).
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ABSTRACT: Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidation-state tetranuclear-dioxido clusters comprising three manganese centres and a redox-inactive metal (M). Crystallographic studies show an unprecedented Mn3M(µ4-O)(µ2-O) core that remains intact on changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds demonstrate a linear dependence between reduction potential and acidity with a slope of ∼100 mV per pKa unit. The Sr(2+) and Ca(2+) compounds show similar potentials, an observation that correlates with the behaviour of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present.Nature Chemistry 04/2013; 5(4):293-9. · 21.76 Impact Factor
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ABSTRACT: One and two scandium ions (Sc(3+)) are bound strongly to nonheme manganese(IV)-oxo complexes, [(N4Py)Mn(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [(Bn-TPEN)Mn(IV)(O)](2+) (Bn-TPEN = N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), to form Mn(IV)(O)-(Sc(3+))1 and Mn(IV)(O)-(Sc(3+))2 complexes, respectively. The binding of Sc(3+) ions to the Mn(IV)(O) complexes was examined by spectroscopic methods as well as by DFT calculations. The one-electron reduction potentials of the Mn(IV)(O) complexes were markedly shifted to a positive direction by binding of Sc(3+) ions. Accordingly, rates of the electron transfer reactions of the Mn(IV)(O) complexes were enhanced as much as 10(7)-fold by binding of two Sc(3+) ions. The driving force dependence of electron transfer from various electron donors to the Mn(IV)(O) and Mn(IV)(O)-(Sc(3+))2 complexes was examined and analyzed in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The smaller reorganization energies and much more positive reduction potentials of the Mn(IV)(O)-(Sc(3+))2 complexes resulted in remarkable enhancement of the electron-transfer reactivity of the Mn(IV)(O) complexes. Such a dramatic enhancement of the electron-transfer reactivity of the Mn(IV)(O) complexes by binding of Sc(3+) ions resulted in the change of mechanism in the sulfoxidation of thioanisoles by Mn(IV)(O) complexes from a direct oxygen atom transfer pathway without metal ion binding to an electron-transfer pathway with binding of Sc(3+) ions.Journal of the American Chemical Society 06/2013; · 10.68 Impact Factor
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ABSTRACT: This Minireview aims to shed light on the emergent field of inducing a change in the magnetic properties of a solution-phase sample by exposing it to a chemical analyte. A considerable body of knowledge exists on materials that alter their magnetic characteristics after a change in the surrounding physical conditions and a number of cases even exist of solution-phase samples that do so under these same circumstances. However, examples of dissolved molecules or particles that react in this fashion under constant conditions and in response to an analyte are limited. Although some cases in organic solvents are discussed, the emphasis of this Minireview is on water. Our aim is to provide the reader with guidelines for designing new magnetogenic probes for the detection of the desired chemical analyte.Angewandte Chemie International Edition 11/2013; · 11.34 Impact Factor