Daniel J. Graham

Reversible Reduction of Oxygen to Peroxide Facilitated by Molecular Recognition

Boxing in Peroxide Hydrogen peroxide (H2O2) is a powerful oxidant, and its reactivity is exploited in numerous biological, as well as synthetic, contexts. Lopez et al. (p. 450) have now managed to capture its dianion (O22-) in a cryptand—essentially a molecular box assembled from benzamide derivatives—keeping the dianion stable in organic solution for days through a net of well-placed internal hydrogen-bond donors. The encapsulated dianion exhibited clean oxidative reactivity back to O2 either by chemical or by electrochemical means. The highly reactive peroxide dianion (O22–) can be captured and stabilized by hydrogen bonding in a molecular box. Generation of soluble sources of peroxide dianion (O22–) is a challenge in dioxygen chemistry. The oxidizing nature of this anion renders its stabilization in organic media difficult. This Report describes the chemically reversible reduction of oxygen (O2) to cryptand-encapsulated O22–. The dianion is stabilized by strong hydrogen bonds to N-H groups from the hexacarboxamide cryptand. Analogous stabilization of peroxide by hydrogen bonding has been invoked recently in crystalline saccharide and protein systems. The present peroxide adducts are stable at room temperature in dimethyl sulfoxide (DMSO) and N,N′-dimethylformamide (DMF). These adducts can be obtained in gram quantities from the cryptand-driven disproportionation reaction of potassium superoxide (KO2) at room temperature.

Electrochemical polymerization of pyrene derivatives on functionalized carbon nanotubes for pseudocapacitive electrodes

Electrochemical energy-storage devices have the potential to be clean and efficient, but their current cost and performance limit their use in numerous transportation and stationary applications. Many organic molecules are abundant, economical and electrochemically active; if selected correctly and rationally designed, these organic molecules offer a promising route to expand the applications of these energy-storage devices. In this study, polycyclic aromatic hydrocarbons are introduced within a functionalized few-walled carbon nanotube matrix to develop high-energy, high-power positive electrodes for pseudocapacitor applications. The reduction potential and capacity of various polycyclic aromatic hydrocarbons are correlated with their interaction with the functionalized few-walled carbon nanotube matrix, chemical configuration and electronic structure. These findings provide rational design criteria for nanostructured organic electrodes. When combined with lithium negative electrodes, these nanostructured organic electrodes exhibit energy densities of ∼350 Wh kg−1electrode at power densities of ∼10 kW kg−1electrode for over 10,000 cycles.

Reactivity of a Pd(I)−Pd(I) Dimer with O2: Monohapto Pd Superoxide and Dipalladium Peroxide in Equilibrium

The Pd(I)-Pd(I) dimer [((F)PNP)Pd-](2) reacts with O(2) upon exposure to light to produce either the superoxide ((F)PNP)PdO(2) or the peroxide [((F)PNP)PdO-](2), which exist in equilibrium with free O(2). Both complexes contain square-planar Pd(II) centers. The unpaired electron density in ((F)PNP)PdO(2) is localized on the superoxide ligand.

Electron-Transfer Studies of a Peroxide Dianion

A peroxide dianion (O2(2-)) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)⊂mBDCA-5t-H6](2-), stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)⊂mBDCA-5t-H6](2-) with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O2(2-) occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O2(2-) within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O2)⊂mBDCA-5t-H6](2-) as a chemical reagent.

Post-synthetic modification of hangman porphyrins synthesized on the gram scale.

We report a multi-gram scale synthesis of methyl 6-formyl-4-dibenzofurancarboxylate and its subsequent use in the gram scale synthesis of a dibenzofuran-functionalized hangman porphyrin containing a pendant carboxylic acid (HPD-CO2H). HPD-CO2H can be isolated as a free carboxylic acid in high purity with minimal purification. Post-synthetic modification of HPD-CO2H allows for the introduction of any desired pendant group in good yields, resulting in a practical amount of hangman porphyrin ligand with an easily customizable second coordination sphere. The cobalt complexes of these hangman porphyrins are shown to be active proton reduction electrocatalysts.