Radical-anions, metal surface

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

Eggins and McNeill compared the solvents of water, dimethylsulfoxide (DMSO), acetonitrile, propylene carbonate, and DMF electrolytes for C02 reduction at glassy carbon, Hg, Pt, Au, and Pb electrodes [78], The main products were CO and oxalate in the organic solvents, while metal electrodes (such as Pt) which absorb C02 showed a higher production for CO. In DMF, containing 0.1 M tetrabutyl ammonium perchlorate and 0.02 M C02 at a Hg electrode, Isse et al. produced oxalate and CO with faradaic efficiencies of 84% and 1.7%, respectively [79], Similarly, Ito et al. examined a survey of metals for C02 reduction in nonaqueous solution, and found that Hg, Tl, and Pb yielded primarily oxalate, while Cu, Zn, In, Sn, and Au gave CO [80, 81]. Kaiser and Heitz examined Hg and steel (Cr/Ni/Mo, 18 10 2%) electrodes to produce oxalate with 61% faradaic efficiency at 6 mA cm-2 [82]. For this, they examined the reduction of C02 at electrodes where C02 and reduction products do not readily adsorb. The production of oxalate was therefore explained by a high concentration of C02 radical anions, COi, close to the surface. Dimerization resulted in oxalate production rather than CO formation. 

The reactions of these anion radicals and anions with positively charged metal surfaces, forming films protecting the surface from wear... 

O2 is able to abstract an electron from an Fsurface superoxide O2 species. This process has been investigated by EPR spectroscopy (238, 241-243) it may play an important role in many catalytic oxidation reactions (244-248). The formation of the superoxide species via the reaction F,f + O2 F + + 0-7 (146,243) is of course favored on metal-doped MgO surfaces. In a combined EPR and IR study, Giamello et al. (240) demonstrated that CO is able to abstract an electron from a surface F center to form novel dimeric species produced by C202. CO- radical anions have also been reported (237). 

Coal and many coal-derived liquids contain polycyclic aromatic structures, whose molecular equivalents form radical cations at anodes and radical anions at cathodes. ESR-electrolysis experiments support this (14). Chemically, radical cations form by action of H2SO4 (15,19), acidic media containing oxidizing agents (15,20,21,22), Lewis acid media (18,23-35) halogens (36), iodine and AgC104 (37,38), and metal salts (39,40). They also form by photoionization (41,42,43) and on such solid catalytic surfaces as gamma-alumina (44), silica-alumina (45), and zeolites (46). Radical anions form in the presence of active metals (76). 

It is well known that ACN reacts with active metals (Li, Ca) to form polymers [48], These polymers are products of condensation reactions in which ACIST radical anions are formed by the electron transfer from the active metal and attack, nucleophilically, more solvent molecules. Species such as CH3C=N(CH3)C=N are probably intermediates in this polymerization. ACN does not react on noble metal electrodes in the same way as with active metals. For instance, well-re-solved Li UPD peaks characterize the voltammograms of noble metal electrodes in ACN/Li salt solutions. This reflects a stability of the Li ad-layers that are formed at potentials above Li deposition potentials. Hence, the cathodic limit of noble metal electrodes in ACN solutions is the cation reduction process (either TAA or active metal cations). As discussed in the previous sections, with TAA-based solutions it is possible that the electrode surfaces remain bare. When the cations are metallic (e.g., Li+), it is expected that the electrode surfaces become covered with surface films originating from atmospheric contaminants reduction if the electrodes are polarized below 1.5 V (Li/Li+). As Mann found [13], in the presence of Na salts the polarization of metal electrodes in ACN solutions to sodium deposition potentials leads to solvent decomposition, with evolution of H2, CH4 and sodium cyanide (due to reaction with metallic sodium). 

A similar situation may be obtained when alkali metals are immersed in ultrapure ethers containing benzophenone [53], The metal thus dissolves via formation of stable ketal radical anions in solution (and metal ions as well). It should be emphasized that the above processes occur even when the active metal is initially introduced into the solution covered by surface films (due to reactions with atmospheric contaminants). We assume that electron tunneling through the films enables the initiation of the dissolution process. This process breaks the film on the metal (as metal is depleted beneath the rigid surface film), thus enabling solvent molecules to reach the active surface and solvate more electrons. This increases the metal solubilization and the further breakdown of the initial surface films. Hence, an equilibrium between a bare metal and the blue solution can finally be reached, as explained above (Eq. 13). 

Pyridines undergo a variety of reactions with radical reagents, and at surfaces many of these parallel the corresponding reactions of benzenes however, the reaction of nucleophilic radicals with protonated pyridines is important (the Minisci reaction). Electron uptake from a metal to form a radical anion occurs readily. 

Following the prophetic speculations of Kharasch and Reinmuth [A] and the crucial CIDNP experiments of the Amsterdam group [22], the main features of the mechanism are now clear. Electron transfer from the magnesium surface to adsorbed organic halide gives rise to alkyl radicals and halide ions. Further recombination steps lead to the alkylmagnesium halide. Two matters especially remain controversial whether the electron transfer step involves a discrete radical anion intermediate, and to what extent radicals leave the metal surface to diffuse into the solution. Recent arguments have been summarized [23, 24], commented on [25], and pursued [26]. The implications for practical work may be summarized briefly as follows ... 

Kapton films reduced for 15 seconds in the above benzil bath, followed by immersion in 0.05 M PdCl2 / DMF solution have a 45 10 percent charge transfer efficiency for Pd deposition based on the amount of radical-anion form available at the surface. Increasing the amount of film reduction (> 30 sec immersion in the reducing bath) results in increased metal deposition and a visibly darkened surface after exposure to the metal complex solution. Highly... 

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