A coordinatively saturated ruthenium(II) complex having tetradentate tris(2-pyridylmethyl)amine (TPA) and bidentate 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)](2+) (1), was oxidized by a Ce(IV) ion in H(2)O to afford a Ru(IV)-oxo complex, [Ru(O)(H(+)TPA)(bpy)](3+) (2). The crystal structure of the Ru(IV)-oxo complex 2 was determined by X-ray crystallography. In 2, the TPA ligand partially dissociates to be in a facial tridentate fashion and the uncoordinated pyridine moiety is protonated. The spin state of 2, which showed paramagnetically shifted NMR signals in the range of 60 to -20 ppm, was determined to be an intermediate spin (S = 1) by the Evans' method with (1)H NMR spectroscopy in acetone-d(6). The reaction of 2 with various oraganic substrates in acetonitrile at room temperature afforded oxidized and oxygenated products and a solvent-bound complex, [Ru(H(+)TPA)(bpy)(CH(3)CN)], which is intact in the presence of alcohols. The oxygenation reaction of saturated C-H bonds with 2 proceeds by two-step processes: the hydrogen abstraction with 2, followed by the dissociation of the alcohol products from the oxygen-rebound complexes, Ru(III)-alkoxo complexes, which were successfully detected by ESI-MS spectrometry. The kinetic isotope effects in the first step for the reaction of dihydroanthrathene (DHA) and cumene with 2 were determined to be 49 and 12, respectively. The second-order rate constants of C-H oxygenation in the first step exhibited a linear correlation with bond dissociation energies of the C-H bond cleavage.

The proximal heme axial ligand plays an important role in tuning the reactivity of oxoiron(IV) porphyrin π-cation radical species (compound I) in enzymatic and catalytic oxygenation reactions. To reveal the essence of the axial ligand effect on the reactivity, we investigated it from a thermodynamic viewpoint. Compound I model complexes, (TMP(+•))Fe(IV)O(L) (where TMP is 5,10,15,20-tetramesitylporphyrin and TMP(+•) is its π-cation radical), can be provided with altered reactivity by changing the identity of the axial ligand, but the reactivity is not correlated with spectroscopic data (ν(Fe═O), redox potential, and so on) of (TMP(+•))Fe(IV)O(L). Surprisingly, a clear correlation was found between the reactivity of (TMP(+•))Fe(IV)O(L) and the Fe(II)/Fe(III) redox potential of (TMP)Fe(III)L, the final reaction product. This suggests that the thermodynamic stability of (TMP)Fe(III)L is involved in the mechanism of the axial ligand effect. Axial ligand-exchange experiments and theoretical calculations demonstrate a linear free-energy relationship, in which the axial ligand modulates the reaction free energy by changing the thermodynamic stability of (TMP)Fe(III)(L) to a greater extent than (TMP(+•))Fe(IV)O(L). The linear free energy relationship could be found for a wide range of anionic axial ligands and for various types of reactions, such as epoxidation, demethylation, and hydrogen abstraction reactions. The essence of the axial ligand effect is neither the electron donor ability of the axial ligand nor the electron affinity of compound I, but the binding ability of the axial ligand (the stabilization by the axial ligand). An axial ligand that binds more strongly makes (TMP)Fe(III)(L) more stable and (TMP(+•))Fe(IV)O(L) more reactive. All results indicate that the axial ligand controls the reactivity of compound I (the stability of the transition state) by the stability of the ground state of the final reaction product and not by compound I itself.

High-valent metal-oxo complexes are key reactive species for oxidation and oxygenation of organic compounds in nature as well as in the laboratory. [1, 2] Although iron is the most common metal species among high-valent metal-oxo complexes, [3] there are also manganese-oxo, [4] ruthenium-oxo, [5] and other metal-oxo complexes.[6] High-valent metal-oxo species are produced by reductive activation of molecular oxygen coupled with proton transfer. [7][8][9] Peroxides such as hydrogen peroxide can provide a so-called "peroxide shunt" to produce high-valent metal-oxo species.[1-3] High-valent metal-oxo species can also be produced by proton-coupled electron transfer (PCET), in which deprotonation of a coordinated water molecule and oxidation of the metal center occur concertedly. [10][11][12][13][14] The reactivity of high-valent metal-oxo species varies depending on the type of metal, the oxidation state of the metal center, ligands, and the spin state. Theoretical studies proposed that the reactivity of high-valent metal-oxo species may be determined by two closely lying spin states, which have different activation barriers for the reactions with substrates. [15][16][17] The most straightforward way to clarify the effects of spin states on the reactivity of highvalent metal-oxo species is to examine the reactivity of an analogous series of metal-oxo complexes that have different spin states. There have been extensive studies on Ru IV

High-valent metal-oxo complexes are key reactive species for oxidation and oxygenation of organic compounds in nature as well as in the laboratory. [1, 2] Although iron is the most common metal species among high-valent metal-oxo complexes, [3] there are also manganese-oxo, [4] ruthenium-oxo, [5] and other metal-oxo complexes.[6] High-valent metal-oxo species are produced by reductive activation of molecular oxygen coupled with proton transfer. [7][8][9] Peroxides such as hydrogen peroxide can provide a so-called "peroxide shunt" to produce high-valent metal-oxo species.[1-3] High-valent metal-oxo species can also be produced by proton-coupled electron transfer (PCET), in which deprotonation of a coordinated water molecule and oxidation of the metal center occur concertedly. [10][11][12][13][14] The reactivity of high-valent metal-oxo species varies depending on the type of metal, the oxidation state of the metal center, ligands, and the spin state. Theoretical studies proposed that the reactivity of high-valent metal-oxo species may be determined by two closely lying spin states, which have different activation barriers for the reactions with substrates. [15][16][17] The most straightforward way to clarify the effects of spin states on the reactivity of highvalent metal-oxo species is to examine the reactivity of an analogous series of metal-oxo complexes that have different spin states. There have been extensive studies on Ru IV

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