SHORT COMMUNICATION DOI: 10.1002/ejic.201000398 The Effect of an Electrical Bias on the Mechanism of Decomposition of Transients with Metal–Carbon σ Bonds Irena Rusonik, [a] Haim Cohen, [b] Alex Lugowskoy, [b] Alexander Krasnopolski, [b] Michael Zinigrad, [b] and Dan Meyerstein* [a,b] Keywords: Dehalogenation / Iron / Electrochemistry / Radicals / Metal–carbon bonds When CCl 3 CO 2 – reacts thermally with porous iron elec- trodes, the radical · CCl 2 CO 2 – is formed. This radical reacts with the surface of the electrode to form a transient with a Fe–C σ bond, Fe 0 (s) –CCl 2 COO – . The mechanism of decompo- Introduction Radicals or radical ions are formed as primary interme- diates in a large variety of organic electrochemical processes including oxidations (e.g. the Kolbe Reaction) and re- ductions (e.g. reduction of alkyl halides and diazonium salts). When the radical is formed at the electrode surface, for example, by reduction of a diazonium salt at the open circuit potential, then the radical is expected to react with the electrode to form an intermediate or a stable product with a covalently bound organic molecule to the elec- trode. [1] Thus the reduction of aryldiazonium salts, mainly in aprotic media, is used to prepare covalently bound or- ganic layers to conductive or semiconductive surfaces. [2] For metals forming stable organometallic compounds, for ex- ample, Hg, Cd, and Pb, results point out that “in a few electrochemical reactions, the initial electron-transfer step does generate a σ radical at the electrode surface, and orga- nometallic compounds are formed.” [3a] The binding of radi- cals to other metals, semiconductors, and metal oxides was also recently reported. [3b–3d] However, it was suggested that “In the majority of electroorganic reactions, the working electrode is an inert material. Electron transfer generates a radical-ion species with sufficient lifetime to migrate away from the electrode surface. Further reactions then generate more reactive free radical species, and these undergo ter- minal reactions before they are able to react with the elec- trode surface.” [3a] Recent results [4–7] suggest that the latter assumption has to be reconsidered, as radicals formed at the surface of metals react with them to form short-lived [a] Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel [b] Department of Biological Chemistry, Ariel University Center of Samaria, Ariel, Israel Fax: +972-3-9067440 E-mail: danmeyer@bgumail.bgu.ac.il © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2010, 3252–3255 3252 sition of this transient depends on the electrical bias applied to the electrode: at a low bias of E  –0.9 V vs. SCE, HCCl 2 COO – is the major final product, whereas at a higher bias of E  –1.2 V vs. SCE, HC(O)CO 2 – is the major product. intermediates with metal–carbon σ bonds; this is of special importance in the dehalogenation of halo-organic pol- lutants by Fe 0 . [4–7] Furthermore, for the Kolbe-type oxidations of RCO 2 – , it was pointed out [8] that “the reaction is controlled by a vari- ety of factors including anode material, anode potential, current density, solvent, supporting electrolyte, structure of R, and temperature.” Similarly, the reduction of acetone in 6  H 2 SO 4 was shown to yield 2-propanol on Hg cathodes (95%), propane on Zn and Cd electrodes, and mixture of 2-propanol (68%), propane (25 %), and the corresponding pinacol (7%) on Pb electrodes. [9] The observation that the nature of the electrode affects the composition of the prod- ucts clearly points out that the intermediate, the radical, formed in these processes interacts with the electrode. It was also shown that the mechanism of reduction of dihaloalk- anes greatly depends on the cathode potential, that is, the yield of cyclic products increases as the potential of the cathode becomes more negative. [10] Thus, for example, the reduction of 1,4-dibromobutane in DMF yields 90% cyclo- butane and 10% butane at –2.3 V vs. SCE, but 26% cyclo- butane and 74% butane at –1.75 V vs. SCE. [10] Again, this effect is difficult to explain if no bond is formed between the electrode and the initially formed radical. The electrolytic reduction of CCl 4 on Fe 0 electrodes was reported to yield directly CH 4 without the formation of CHCl 3 , CH 2 Cl 2 , and CH 3 Cl as intermediates, [11] though the latter compounds are reduced slower than CCl 4 by Fe 0 . [5,12–15] It should be noted that other authors reported the formation of CHCl 3 when CCl 4 is treated with iron powder. [5,12–15] Results and Discussion It seemed of interest to study the effect of an electrical bias on the dehalogenation of polychlorinated compounds.