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Radical Clocks, Solvated Electrons, and Magnesium. Heterogeneous versus Homogeneous Electron Transfer. Selectivity at Interfaces

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journal contribution
posted on 21.02.2008 by Hassan Hazimeh, Frédéric Kanoufi, Catherine Combellas, Jean-Marc Mattalia, Caroline Marchi-Delapierre, Michel Chanon
Ammonia solutions of solvated electrons may be prepared by electrolyzing the solvent in the presence of Mg(BF4)2 and Mg++ provided by a magnesium anode. The reactions of these solutions (−40 °C) with the two radical clocks 1-bromo-2-(3-butenyl)benzene 1 and 1-(allyloxy)-2-bromobenzene 7 (0.007−0.008 M) are examined. Both probes yield very high percentages of cyclized products (>97%) when the concentration of solvated electrons is low (0.028 M). These percentages diminish when the metallic character of the solution is increased (0.183 M concentration in solvated electrons). These results strongly contrast with those obtained for the reactions of these radical probes with magnesium turnings or potassium lumps in THF. Under these conditions, the percentages of cyclized products are lower than 4%. We propose that the main cause of this contrasting behavior is the higher reducing efficiency of the heterogeneous electron transfer with respect to the homogeneous electron transfer. This holds even if the reducing agent in the bulk is one of the strongest known reducing agents (solvated electron). The causes for the striking difference between heterogeneous and homogeneous electron transfer are discussed in terms of selectivity within a double-layer perspective. These differences allow the rationalization of an apparent discordance. The behavior of MgX2 (X = halogen) under pulse radiolysis conditions (Mostafavi's group) shows that MgX2 are so weakly oxidizing that in most solvents they do not react with the solvated electron. Nevertheless, the reduction of MgX2 by alkali metals (Rieke's method) is the basis for the preparation of very active slurries of magnesium. This principle has been widely extended for the preparation of various metallic nanoparticles. The difference between homogeneous and heterogeneous electron transfer to MgX2 lies at the heart of the problem as explained in the framework of molecular electrochemistry at the cathode surface. Kinetic treatment of the selectivity at the metal−liquid interface is contrasted with selectivity obtained in homogeneous solution.