Oxygen and proton reduction by decamethylferrocene in non-aqueous acidic mediaw

O2 reduction by ferrocene and its derivatives in organic media in the presence of an acid, such as trichloroacetic and trifluoroacetic acids and perchloric acid, has been known for many years. Recently, O2 reduction has also been studied in biphasic systems composed of an organic solvent containing a ferrocene derivative in contact with an aqueous inorganic acid solution containing a very lipophilic anion. At such liquid–liquid interfaces, O2 reduction takes place by involving the lipophilic electron donors i.e. ferrocene derivatives located in the organic phase and aqueous protons. Moreover, it has been found that in the same biphasic system the electron-rich decamethylferrocene (DMFc) can also reduce aqueous protons under anaerobic conditions, leading to the evolution of hydrogen. In both cases, the reaction has been proposed to proceed in two steps: first a heterogeneous proton transfer facilitated by DMFc from the aqueous to the organic phase as observed by voltammetry experiments at liquid–liquid interfaces, followed by a homogenous proton/oxygen reduction in the organic phase, the mechanism of which being yet unresolved. In this communication, we present experimental results for oxygen/proton reductions by DMFc in bulk 1,2-dichloroethane (DCE) in the presence of organic acids. In the case of oxygen reduction, DFT computations support a reaction pathway involving protonated DMFc, DMFcH, as an intermediate species, which reacts with oxygen to produce hydrogen peroxide. Fig. 1a illustrates an experiment performed in DCE under aerobic conditions. A dilute solution of DMFc appeared yellow, to which addition of an acid, hydrogen tetrakis(pentafluorophenyl)borate (HTB),y led to an immediate colour change to bright green. In the UV-visible spectrum, the absorption band of DMFc centered at 425 nm is replaced by a strong band of DMFc at 779 nm. Moreover, by shaking the green organic solution with pure water and then by titrating this aqueous phase with sodium iodide (NaI), triiodide was detected with a spectroscopic signature at 286 nm and 330 nm. Triiodide was generated by oxidation of iodide by H2O2. 7 These results suggest the occurrence of oxygen reduction to H2O2 by DMFc in DCE in the presence of the organic acid: