RESUMEN

Manganese catalysts that activate hydrogen peroxide have seen increased use in organic transformations, such as olefin epoxidation and alkane C-H bond oxidation. Proposed mechanisms for these catalysts involve the formation and activation of MnIII-hydroperoxo intermediates. Examples of well-defined MnIII-hydroperoxo complexes are rare, and the properties of these species are often inferred from MnIIIalkylperoxo analogues. In this study, we show that the reaction of the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ (1) with hydrogen peroxide and acid results in the formation of a dark-green MnIII-hydroperoxo species [MnIII(OOH)(6Medpaq)]+ (2). The formulation of 2 is based on electronic absorption, 1H NMR, IR, and ESI-MS data. The thermal decay of 2 follows a first order process, and variable-temperature kinetic studies of the decay of 2 yielded activation parameters that could be compared with those of a MnIII-alkylperoxo analogue. Complex 2 reacts with the hydrogen-atom donor TEMPOH two-fold faster than the MnIII-hydroxo complex 1. Complex 2 also oxidizes PPh3, and this MnIII-hydroperoxo species is 600-fold more reactive with this substrate than its MnIII-alkylperoxo analogue [MnIII(OOtBu)(6Medpaq)]+. DFT and time-dependent (TD) DFT computations are used to compare the electronic structure of 2 with similar MnIII-hydroperoxo and MnIII-alkylperoxo complexes.

RESUMEN

In this work, we report the generation and characterization of two new Co(III)-peroxo complexes 2 and 3. 2 is best described as a mononuclear CoIII-(O2) complex that exhibits an 18O-isotope sensitive OO bond stretching vibration at 845(-49) cm-1, indicating a relatively weak peroxo moiety compared to those of other CoIII-(O2) complexes reported previously. Complex 3 is a CoIII-peroxo-arylboronate species having a rare {CoIIIOOBO} five-membered metallocycle, which is structurally characterized using X-ray crystallography. Investigations of the reaction mechanism using density functional theory calculations show that 2 likely undergoes a nucleophilic attack to an arylboronic acid, which is generated by hydrolysis of the BPh4- anion in wet acetonitrile solution, to first form a CoIII-peroxo-arylboronic acid adduct, followed by the loss of one benzene molecule to generate the five-membered metallocycle. The entire reaction is thermodynamically favorable. Taken together, the conversion of 2 to 3 represents the discovery of a novel nucleophilic reactivity that can be carried out by mononuclear Co(III)-peroxo complexes.

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RESUMEN

In order to shed light on metal-dependent mechanisms for O-O bond cleavage, and its microscopic reverse, we compare herein the electronic and geometric structures of O2-derived binuclear Co(III)- and Mn(III)-peroxo compounds. Binuclear metal peroxo complexes are proposed to form as intermediates during Mn-promoted photosynthetic H2O oxidation, and a Co-containing artificial leaf inspired by nature's photosynthetic H2O oxidation catalyst. Crystallographic characterization of an extremely activated peroxo is made possible by working with substitution-inert, low-spin Co(III). Density functional theory (DFT) calculations show that the frontier orbitals of the Co(III)-peroxo compound differ noticeably from the analogous Mn(III)-peroxo compound. The highest occupied molecular orbital (HOMO) associated with the Co(III)-peroxo is more localized on the peroxo in an antibonding π*(O-O) orbital, whereas the HOMO of the structurally analogous Mn(III)-peroxo is delocalized over both the metal d-orbitals and peroxo π*(O-O) orbital. With low-spin d6 Co(III), filled t2g orbitals prevent π-back-donation from the doubly occupied antibonding π*(O-O) orbital onto the metal ion. This is not the case with high-spin d4 Mn(III), since these orbitals are half-filled. This weakens the peroxo O-O bond of the former relative to the latter.

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RESUMEN

We report the Cu(I)/O2 chemistry of complexes derived from the macrocylic ligands 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and 12-TMC (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). While [(14-TMC)Cu(I)](+) is unreactive towards dioxygen, the smaller analog [(12-TMC)Cu(I)(CH3CN)](+) reacts with O2 to give a side-on bound peroxo-dicopper(II) species ((S)P), confirmed by spectroscopic and computational methods. Intriguingly, 12-TMC as a N4 donor ligand generates (S)P species, thus in contrast with the previous observation that such species are generated by N2 and N3 ligands. In addition, the reactivity of this macrocyclic side-on peroxo-dicopper(II) differs from typical (S)P species, because it reacts only with acid to release H2O2, in contrast with the classic reactivity of Cu2O2 cores. Kinetics and computations are consistent with a protonation mechanism whereby the TMC acts as a hemilabile ligand and shuttles H(+) to an isomerized peroxo core.

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RESUMEN

Redox-inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal-oxygen intermediates, such as metal-oxo and metal-peroxo complexes. The mechanisms of the oxidative CH bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)-oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)3 have been unified as rate-determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)3 ) by iron(III)-oxo complexes. All logarithms of the observed second-order rate constants of Lewis acid-promoted oxidative CH bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)-oxo complexes exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)3 to the metal-oxo moiety has been confirmed for Mn(IV) -oxo complexes. The enhancement of the electron-transfer reactivity of metal-oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox-inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)-peroxo complexes, resulting in acceleration of the electron-transfer reduction but deceleration of the electron-transfer oxidation. Such a control on the reactivity of metal-oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca(2+) in the oxidation of water to dioxygen by the oxygen-evolving complex in photosystem II.