Ambiphilicity of a mononuclear cobalt(III) superoxo complex

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a Mn^III-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn^IV-hydroperoxo intermediate, [Mn(BDP^BrP)(OOH)]⁺ (1, H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn^III-superoxo complex, Mn(BDP^BrP)(O₂^•) (2) with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of 1 are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of 2 with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDP^BrP)(OO)Sc(OTf)_n]^(3-n)+ (4). Furthermore, it is found that deprotonation of 1 quantitatively regenerates 2, and that one-electron oxidation of the corresponding Mn^III-hydroperoxo species, Mn(BDP^BrP)(OOH) (3), also yields 1.

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn₂ RNRs superoxide (O₂ ^.- ) was postulated to react with the Mn^II ₂ core to yield a Mn^II Mn^III -peroxide moiety. The reactivity of complex 1 ([Mn^II ₂ (O₂ CCH₃ )₂ (BPMP)](ClO₄ ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O₂ ^.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λ_max =440, 590 nm) characteristic of a Mn^II Mn^III -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a Mn^II Mn^III -peroxide complex. Unlike all other previously reported Mn₂ -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O₂ -activation in Class Ib Mn₂ RNRs, and the highly reactive intermediates in their catalytic cycle.

Mimicking Class I b Mn2 -Ribonucleotide Reductase: A MnII2 Complex and Its Reaction with Superoxide

A fascinating discovery in the chemistry of ribonucleotide reductases (RNRs) has been the identification of a dimanganese (Mn₂ ) active site in class I b RNRs that requires superoxide anion (O₂^.- ), rather than dioxygen (O₂ ), to access a high-valent Mn₂ oxidant. Complex 1 ([Mn₂ (O₂ CCH₃ )(N-Et-HPTB)](ClO₄ )₂ , N-Et-HPTB=N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane) was synthesised in high yield (90 %). 1 was reacted with O₂^.- at -40 °C resulting in the formation of a metastable species (2). 2 displayed electronic absorption features (λ_max =460, 610 nm) typical of a Mn-peroxide species and a 29-line EPR signal typical of a Mn^II Mn^III entity. Mn K-edge X-ray absorption near-edge spectroscopy (XANES) suggested a formal oxidation state change of Mn^II₂ in 1 to Mn^II Mn^III for 2. Electrospray ionisation mass spectrometry (ESI-MS) suggested 2 to be a Mn^II Mn^III -peroxide complex. 2 was capable of oxidizing ferrocene and weak O-H bonds upon activation with proton donors. Our findings provide support for the postulated mechanism of O₂^.- activation at class I b Mn₂ RNRs.

A Chromium(III)-Superoxo Complex as a Three-Electron Oxidant with a Large Tunneling Effect in Multi-Electron Oxidation of NADH Analogues

Metal-superoxo species are involved in a variety of enzymatic oxidation reactions, and multi-electron oxidation of substrates is frequently observed in those enzymatic reactions. A Cr^III -superoxo complex, [Cr^III (O₂ )(TMC)(Cl)]⁺ (1; TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), is described that acts as a novel three-electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a Cr^IV -oxo complex, [Cr^IV (O)(TMC)(Cl)]⁺ (2), is formed by a heterolytic O-O bond cleavage of a putative Cr^II -hydroperoxo complex, [Cr^II (OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p-chloranil (Cl₄ Q) indicates that oxidation of NADH analogues by 1 proceeds by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.

Characterization and reactivity of a terminal nickel(III)-oxygen adduct

High-valent terminal metal-oxygen adducts are hypothesized to be the potent oxidizing reactants in late transition metal oxidation catalysis. In particular, examples of high-valent terminal nickel-oxygen adducts are scarce, meaning there is a dearth in the understanding of such oxidants. A monoanionic Ni(II)-bicarbonate complex has been found to react in a 1:1 ratio with the one-electron oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate, yielding a thermally unstable intermediate in high yield (ca. 95%). Electronic absorption, electronic paramagnetic resonance, and X-ray absorption spectroscopies and density functional theory calculations confirm its description as a low-spin (S = 1/2), square planar Ni(III)-oxygen adduct. This rare example of a high-valent terminal nickel-oxygen complex performs oxidations of organic substrates, including 2,6-di-tert-butylphenol and triphenylphosphine, which are indicative of hydrogen atom abstraction and oxygen atom transfer reactivity, respectively.