Novel Method for Grafting Alkyl Chains onto Glassy Carbon. Application to the Easy Immobilization of Ferrocene Used as Redox Probe

Primary alkyl iodides (RI) have been found to react with a cathodically charged glassy carbon surface at potentials more negative than −1.7 V vs Ag/AgCl. In aprotic solvents, this reaction results in grafting of the alkyl chains onto carbon. It is proposed that the process corresponds to the cathodic charge of graphitized and fullerenized zones present in carbon followed by a displacement reaction (analogous to a nucleophilic attack) toward alkyl iodides. This new mode of grafting is applied to the immobilization of ferrocene used as an electrochemical probe. The present work points out the reaction of ω-iodoalkylferrocenes and quantifies the level of grafting of alkyl chains via this promising method for modification of carbon surfaces. Coverage levels were found to be high, reaching the apparent surface concentrations of 8 × 10^–9 mol cm^–2. These large values are explained on the basis of swelling of the interface provoked by progressive charging of the carbon surface via insertion of tetraalkylammonium cations concomitantly with the substitution process. Alkylferrocene layers deposited onto carbon were found to be chemically and electrochemically stable

Efficient Anodic Allylation and Benzylation of Carbons Using Allyl and Benzyl Trimethylsilanes

An easy process for allylation and benzylation of different carbon materials, primarily of glassy carbon, in acetonitrile solutions containing tetraalkyammonium salts is described. The method relies on the capability of C­(sp²) zones of glassy carbon (graphite and fullerene-like inclusions) to be anodically charged at potentials >1.5 V versus Ag/AgCl to form electrophilic centers reacting with substituted trimethylsilanes RSiMe₃. Great propensity of the trimethylsilyl group (TMS⁺) to act as a cationic leaving group facilitates electrophilic reactions of the charged anodic surface with R-carrying silylated precursors, permitting efficient grafting of a large variety of R groups. The present preliminary work focuses only on the efficient grafting of benzyl and allyl moieties

Electron Transfer-Induced Conformational Changes of Highly Hindered Aromatic Compounds. The Case of Hexakis(alkylsulfonyl)benzenes

The monoelectronic reduction of hexakis(alkylsulfonyl)benzenes (alkyl = methyl 1a, ethyl 1b, butyl 1c, iso-propyl 1d, and iso-butyl 1e) to the corresponding radical anion in dimethylformamide involves two widely separated redox systems, except in the case of 1e that shows a single perfectly reversible system. Electrochemical data supported by calculations of molecular modeling (DFT and PM3 methods) show the existence of a four-member square scheme for which the neutral and radical anion species can both exist under chair and boat conformations. The relative stability of the conformers was found to be strongly dependent on the nature of the alkyl substituents. Generally, the most stable neutral forms adopt a chairlike geometry, and the radical anions adopt a boatlike geometry. For the most hindered compound 1e, the steric contribution of the (iso-butylsulfonyl) substituents becomes so strong that the conformational changes are considerably slowed, resulting in a frozen chair conformation

Reactivity of Platinum Metal with Organic Radical Anions from Metal to Negative Oxidation States

The reaction of platinum metal with an organic molecular radical anion leads to the formation of iono-metallic phases where Pt exists under negative oxidation states. This puzzling transformation of a “noncorrodible metal” was examined using localized electrochemical techniques in dimethylformamide containing different tetra-alkylammonium salts chosen as test systems. Our experiments demonstrate that the platinum metal is locally reduced as soon as the Pt faces relatively moderate reducing conditions, for example, when the Pt is used as a negative electrode or when the metal is in the presence of a reducing agent such as an organic radical anion. Scanning electrochemical microscopy (SECM) analysis, current−distance curves, and transient mode responses provide detailed descriptions of the reactivity of Pt to form negative oxidation states (the key step is the reaction of the metal with a molecular reducing agent), of the insulating nature of the “reduced” solid phases of the thermodynamics and kinetics conditions of the Pt conversion. The passage from the conductor to insulator states controlled the spatial development of the reaction that always remains in competition with the other “natural” roles of a metallic electrode. Formally, the phenomena can be treated by analogy with the C. Amatore's model previously developed for the mediated reduction of the poly(tetrafluoroethylene). Consequences of this general reactivity of Pt are discussed in view of a wide utilization of this metal in reductive conditions and the possible applications of such processes in the micropatterning of metallic surfaces

Electrochemical Immobilization of a Benzylic Film through the Reduction of Benzyl Halide Derivatives: Deposition onto Highly Ordered Pyrolytic Graphite

The reactivity of electrogenerated benzyl radicals at carbon surfaces was examined through the cathodic reduction of the corresponding bromide derivatives. 4-Nitrobenzyl bromide and benzyl bromide were reduced in N,N-dimethylformamide (DMF) on highly ordered pyrolytic graphite (HOPG) surfaces. Electroproduced films were examined using electrochemistry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Experiments show the formation of strongly adherent deposits and the occurrence of electrografting processes. They are based on radical generation and the reaction of the radical with the substrate. As expected, the thickness of the organic film increases with deposition time but the deposit displays a lower compactness than previously reported for the electroreduction of aryl diazonium salts. Interestingly for benzyl derivatives, the reduction potential required for the electrografting could be rendered much more positive by simply using an iodide-type supporting electrolyte