Reduction of coordinated molecular oxygen by organic substrates

Cooperative Electrocatalytic O2 Reduction Involving Co(salophen) with p-Hydroquinone as an Electron-Proton Transfer Mediator

The molecular cobalt complex, Co(salophen), and para-hydroquinone (H₂Q) serve as effective cocatalysts for the electrochemical reduction of O₂ to water. Mechanistic studies reveal redox cooperativity between Co(salophen) and H₂Q. H₂Q serves as an electron-proton transfer mediator (EPTM) that enables electrochemical O₂ reduction at higher potentials and with faster rates than is observed with Co(salophen) alone. Replacement of H₂Q with the higher-potential EPTM, 2-chloro-H₂Q, allows for faster O₂ reduction rates at higher applied potential. These results demonstrate a unique strategy to achieve improved performance with molecular electrocatalyst systems.

Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Co(II)(salophen) complex, a Co(III)-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the Co(III)-O2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co(III)-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O2 reduction and generate the reactive p-benzoquinone co-catalyst.

Novel Pathways in the Reactions of Superoxometal Complexes

The reaction of a 1:1 mixture of (H(2)O)(5)Cr((16)O(2))(2+) and (H(2)O)(5)Cr((18)O(2))(2+) at pH 1 did not yield measurable amounts of (16)O(18)O. This result rules out a Russell-type mechanism (2(H(2)O)(5)CrO(2)(2+) --> 2(H(2)O)(5)CrO(2+) + O(2)) for the bimolecular decomposition reaction. Evidence is presented in support of unimolecular (S(H)1) and bimolecular (S(H)2) homolyses as initial steps in the decomposition of (H(2)O)(5)CrO(2)(2+) in strongly acidic solutions (pH </= 1). In the pH range 4-5, (H(2)O)(5)CrO(2)(2+) undergoes hydrolysis-induced disproportionation to (H(2)O)(5)CrO(2)H(2+), Cr(H(2)O)(6)(3+) and O(2). The first step produces HO(2)(*)/O(2)(*)(-), which in further reaction with (H(2)O)(5)CrO(2)(2+) yields the observed products.

Reactivity of the Coordinated Hydroperoxo Ligand

The reactivity of the hydroperoxo complex [Co(CN)(5)OOH](3)(-) has been studied in aqueous solution. The complex undergoes acid-catalyzed aquation (k = 1.89(5) x 10(-)(2) s(-)(1), pK(a) = 5.21(4), T = 20 degrees C, I = 0.1 M). Assuming an I(d) mechanism, this allows the relative affinity for Co(III) to be deduced as H(2)O(2) < H(2)O < HO(2)(-) and implies H(2)O(2) to be a very weak ligand. At neutral pH the hydroperoxo complex effects efficient oxygen atom transfer to L-methionine to give an intermediate identified as [Co(CN)(5)(L-methionine S-oxide)](2)(-), which then dissociates to [Co(CN)(5)OH(2)](2)(-) and L-methionine S-oxide. The reaction is acid catalyzed and is proposed to take place via nucleophilic attack of sulfur on the proximal oxygen of the hydroperoxo ligand with concerted loss of water. The significance of these results for the interaction of hydrogen peroxide with labile metal ions is discussed.