No abstract

The electroreductions of Buckminsterfullerene (c60) in aprotic solvents were examined as a function of solvent, supporting electrolyte, and temperature. Altogether, 11 different solvents and 17 different supporting electrolytes were utilized in measurements made between 223 and 348 K. The cations of the supporting electrolytes were Li' and Na+ as well as quaternary ammonium and quaternary phosphonium cations. The anions of the supporting electrolytes were C104-, BF4-, PF6-, and Br-. Cyclic voltammograms, rotating disk electrode voltammograms, and controlled potential coulometry revealed up to five reversible one-electron reductions. A qualitative approach is used to elucidate the effects of solvent, supporting electrolyte, and temperature on the half-wave potentials, E l / z , of the reductions of c 6 0 . E l l 2 for the first reduction correlates well with the Gutmann donor number of the solvent with a positive slope, but it also shows a linear correlation with the Gutmann acceptor number of the solvent with a negatiue slope. In contrast, the third reduction E1/2 correlates fairly with the Gutmann acceptor number with a positive slope. The first three reductions also correlate with the normalized Dimroth-Reichardt solvent parameter. The inorganic anions of the supporting electrolytes do not significantly affect the half-wave potentials, but these values vary substantially with the type and size of the supporting electrolyte cations. The relative magnitudes of the solvent and supporting electrolyte effects on E l I z differ for each redox process of Cdo, and values of shift over a range of 280-600 mV for a given redox couple. The shifts in reduction potentials were rationalized in terms of the following: (i) charge density on the fulleride anions, (ii) solvophobic effects involving Cso (aggregation), (iii) solvophobic type interactions involving Cm anions and the larger cations of the supporting electrolytes in polar solvents, (iv) ion pairing of Cdo anions with smaller cations in nonpolar solvents, and (v) the specific acceptor or donor properties of the solvents. The reversible half-wave potentials were also measured as a function of temperature in eight different solvent/supporting electrolyte systems, and the measured values of AElI2/AT were used to calculate the change of entropy associated with each electron-transfer step. The shifts in E l I z with temperature are relatively large and indicate that an unusually large change of entropy accompanies each electroreduction step. Diffusion coefficients, Stokes radii, and apparent solvation numbers of neutral Cdo were also determined in different solvent systems, and these values are discussed with respect to the nature of the solventsolute interaction.

Three series of cobalt(III) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me(4)Ph(5)Cor)Co, one a face-to-face biscorrole linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge, (BCY)Co(2) (with Y = A, B, X, O or S), and one a face-to-face bismacrocyclic complex, (PCY)Co(2), containing a Co(II) porphyrin and a Co(III) corrole also linked by one of the above rigid spacers (Y = A, B, X, or O). Cyclic voltammetry and rotating ring-disk electrode voltammetry were both used to examine the catalytic activity of the cobalt complexes in acid media. The mixed valent Co(II)/Co(III) complexes, (PCY)Co(2), and the biscorrole complexes, (BCY)Co(2), which contain two Co(III) ions in their air-stable forms, all provide a direct four-electron pathway for the reduction of O(2) to H(2)O in aqueous acidic electrolyte when adsorbed on a graphite electrode, with the most efficient process being observed in the case of the complexes having an anthracene spacer. A relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E(1/2) which was located at 0.47 V vs SCE for (PCA)Co(2) and 0.39 V vs SCE for (BCA)Co(2). The cobalt(III) monocorrole (Me(4)Ph(5)Cor)Co also catalyzes the electroreduction of dioxygen at E(1/2) = 0.38 V with the final products being an approximate 50% mixture of H(2)O(2) and H(2)O.

No abstract

Long live the state! Photoexcitation of a zinc chlorin–fullerene dyad with a short linkage results in the formation of the ultra‐long‐lived charge‐separated (CS) state by a one‐step photoinduced electron transfer without loss of energy, which is inevitable for charge separation by multistep electron‐transfer processes. The lifetime of the charge‐separated state was 120 s in frozen PhCN at −150 °C (see picture).

Eleven free-base corroles with different electron-donating or electron-withdrawing meso substituents were characterized as to their electrochemistry and UV-visible spectroscopy in benzonitrile (PhCN) or pyridine containing tetra-n-butylammonium perchlorate (0.1 M). Six forms of the compounds with different numbers of protons and/or oxidation states were spectroscopically identified and are represented as (Cor)H3, (.Cor)H2, [(Cor)H2]-, [(.Cor)H2]2-, [(Cor)H4]+, and [(.Cor)H4]2+, where Cor is a trianionic corrole macrocycle. The electrochemistry and UV-visible properties are a function of corrole basicity, solvent basicity, and types or sizes of the meso substituents, and the compounds could be subdivided into one of two different groups, one of which comprises sterically hindered corroles and another that does not. The electroactive species in PhCN is (Cor)H3, whereas in pyridine, one inner proton dissociates, generating a mixture of (Cor)H3, [(Cor)H2]-, and pyH+. The addition of one electron to [(Cor)H2]- reversibly gives the [(.Cor)H2]2- pi-anion radical, whereas a reversible oxidation of the same species gives the neutral radical (.Cor)H2. The first one-electron reduction of (Cor)H3 occurs at the macrocycle in PhCN, but the initial product rapidly converts to [(Cor)H2]-, which undergoes additional reversible redox reactions at the conjugated pi-ring system. The first oxidation of (Cor)H3 in PhCN leads to a mixture of (.Cor)H2 and [(Cor)H4]+, both of which could be further oxidized or reduced. The UV-visible spectra of [(Cor)H4]+ were measured in PhCN after titrations with trifluoroacetic acid, after which selected samples were examined as to their electrochemistry. The HOMO-LUMO gaps of [(Cor)H2]-, (Cor)H3, and [(Cor)H4]+ were also determined.

This review describes the known electrochemistry of corroles in nonaqueous media from 1980 until the present. The outline of the review is grouped according to the periodic table, proceeding from left to right, describing first monomeric free-base derivatives and then transition-metal compounds, followed by main-group corroles, before ending with a brief description of lanthanide and actinide corroles. Many similarities exist between the redox properties of metallocorroles and metalloporphyrins, but there are also many differences due, in part, to the different charges of the two conjugated macrocycles and the noninnocence of the corrole ligand in a variety of compounds. One part of this review will focus on describing redox behavior as a function of metal ion and axial ligands, while another will focus on how changes in structure of the macrocycle are associated with changes in redox behavior. It is hoped that this review will answer the majority of the readers' questions as to what has been electrochemically observed for corroles in the past while at the same time enabling the reader to utilize data in the literature to predict and "tune" what might be observed in future electrochemical studies of corroles that have yet to be synthesized and characterized.

No abstract