The generation of an onheme oxoiron(IV) intermediate,[ (cyclam)Fe IV (O)(CH 3 CN)] 2+ (2;c yclam = 1,4,8,11tetraazacyclotetradecane), is reported in the reactions of [(cyclam)Fe II ] 2+ with aqueous hydrogen peroxide( H 2 O 2 )o r as oluble iodosylbenzene (sPhIO) as ar are example of an oxoiron(IV) species that shows ap reference for epoxidation over allylic oxidation in the oxidation of cyclohexene.Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo-and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)-Fe II ] 2+ and H 2 O 2 .Inspite of the predominance of the oxoiron-(IV) cores in biology,t he present study is ar are example of high-yield isolation and spectroscopic characterization of ac atalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.

High-valent first-row transition-metal–oxo complexes are important intermediates in biologically and chemically relevant oxidative transformations of organic molecules and in the water splitting reaction in (artificial) photosynthesis. While high-valent Fe– and Mn–oxo complexes have been characterized in detail, much less is known about their analogues with late transition metals. In this study, we present the synthesis and detailed characterization of a unique mononuclear terminal Ni–O complex. This compound, [Ni(TAML)(O)(OH)]3–, is characterized by an intense charge-transfer (CT) band around 730 nm and has an S t = 1 ground state, as determined by magnetic circular dichroism spectroscopy. From extended X-ray absorption fine structure (EXAFS), the Ni–O bond distance is 1.84 Å. Ni K edge XAS data indicate that the complex contains a Ni(III) center, which results from an unusually large degree of Ni–O π-bond inversion, with one hole located on the oxo ligand. The complex is therefore best described as a low-spin Ni(III) complex (S = 1/2) with a bound oxyl (O•–) ligand (S = 1/2), where the spins of Ni and oxyl are ferromagnetically coupled, giving rise to the observed S t = 1 ground state. This bonding description is roughly equivalent to the presence of a Ni–O single (σ) bond. Reactivity studies show that [Ni(TAML)(O)(OH)]3– is a strong oxidant capable of oxidizing thioanisole and styrene derivatives with large negative ρ values in the Hammett plot, indicating its electrophilic nature. The intermediate also shows high reactivity in C–H bond activation of hydrocarbons with a kinetic isotope effect of 7.0(3) in xanthene oxidation.

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co(O)] (1; HTAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co]·3(HO) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λ = 393 nm) resulted in generation of the excited state S* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S* converted to a lower-lying excited state, S* (λ = 580 nm), with rate constant k = 7(2) × 10 s (S* → S*). S* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant k = 1.7(3) × 10 s (S* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf) (OTf = triflate anion), [Fe(bpy)] (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA). The electron-transfer reactivity of S* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( E vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the E values of electron donors allowed the estimation of the one-electron reduction potential of S* ( E*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( E = 0.86 V vs SCE).

We report the catalytic oxidation of organic substrates by a cobalt complex, [Co III (TAML)] À (TAML = tetra-amido macrocyclic ligand), and meta-chloroperbenzoic acid (m-CPBA) under the mild reaction conditions and the identification of a cobalt(IV)-oxo intermediate, [Co IV (TAML)(O)] 2À , as an active oxidant in the catalytic oxidation reactions. In the hydroxylation of cyclohexane, cyclohexanol is the sole product and the hydroxylation reaction occurs via an oxygen non-rebound mechanism. Kinetic isotope effect value of 3.1, obtained in the oxidation of cyclohexane-h 12 /d 12 , indicates that a hydrogen atom abstraction from cyclohexane by the cobalt(IV)-oxo complex is the rate-determining step. The catalytic oxidation of cyclohexene prefers the C═C bond epoxidation to the C-H bond activation, yielding cyclohexene oxide as a sole product. In the epoxidation of cis-and transstilbenes, cis-and trans-stilbene oxides, respectively, are yielded stereoselectively. The present study reports a highly selective catalytic oxidation of organic substrates by a cobalt complex and m-CPBA and the involvement of a cobalt(IV)-oxo intermediate as an active oxidant in the catalytic oxidation reactions.

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)FeIV(O)(CH3CN)]2+ (2; cyclam=1,4,8,11‐tetraazacyclotetradecane), is reported in the reactions of [(cyclam)FeII]2+ with aqueous hydrogen peroxide (H2O2) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo‐ and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)FeII]2+ and H2O2. In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high‐yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.