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

Abstract

Experimental studies and density functional theory (DFT) computations suggest that oxygen and proton reduction by decamethylferrocene (DMFc) in 1,2-dichloroethane involves protonated DMFc, DMFcH + , as an active intermediate species, producing hydrogen peroxide and hydrogen in aerobic and anaerobic conditions, respectively.O 2 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. [1][2][3][4][5] Recently, O 2 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. 6-8 At such liquid-liquid interfaces, O 2 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. 9 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 H 2 O 2 . 7 These results suggest the occurrence of oxygen reduction to H 2 O 2 by DMFc in DCE in the presence of the organic acid:However, only 0.03 mM of H 2 O 2 was detected, an amount which is about five times less than that of DMFc + produced (1.5 mM). This indicates a much lower yield than the stoichiometric value for H 2 O 2 . One possible reason is the further reduction of H 2 O 2 by DMFc to finally produce water. Indeed, in biphasic systems, the yield of H 2 O 2 can reach 40% as its extraction to water competes with further reduction. 7 Another possible reason for th...

Scanning electrochemical microscopy determination of hydrogen flux at liquid|liquid interface with potentiometric probe

“…This method was applied to determine the fluxes of hydrogen generated at a liquid|liquid interface with decamethylferrocene as the electron donor [3][4][5]. The proposed method seems to be suitable to other liquid junctions, providing that both oxidized and reduced form of electron donor are not soluble in aqueous phase.…”

“…Recently, it has been shown that hydrogen evolution reaction (HER) occurs at the 1,2-dichloroethane (DCE)| water interface under anaerobic conditions, with decamethylferrocene (DMFc) as a reducing agent present in the organic phase [3][4][5]. This reaction is significantly accelerated by the presence of micro-or nanoparticles [6,7].…”

Molecular electrocatalysis at soft interfaces

“…9, and in fact we have shown that the proton reduction competes with oxygen reduction in aerobic conditions. 24 Of course, the redox potential for the reduction of O 2 being much more positive than for the reduction of the aqueous protons, this is not surprising that with a larger driving force the reduction of O 2 occurs quasi-instantly by contacting the biphasic system whilst the evolution of H 2 is a much slower process.…”

Artificial Photosynthesis at Soft Interfaces

“…4 whereas in aerobic conditions both hydrogen and hydrogen peroxide are formed by oxygen reduction. [13,14] One of the objectives of LEPA in the NCCR MuST project is to investigate the role of the interface and its polarisation in the stabilisation of charge separated states in sensitizer-molecular dyads such as the one illustrated in Fig. 5.…”