Keywords:Standard Gibbs energy of transfer Standard transfer potential Micro-interface Microhole Hydrophobic ions Hydrophilic ions a b s t r a c t A voltammetric methodology to determine the standard Gibbs energy of transfer of highly hydrophobic and hydrophilic ions has been developed. The electrochemical cell used includes a water|1,2-dichloroethane micro-interface supported on a microhole in a thin polymer film separating an electrolyte-free aqueous phase and an organic phase with an electrolyte at low concentrations. The limiting current and the half-wave potential of these organic ions were determined by fitting the initial part of the ion transfer wave. The methodology was validated using ions with known thermodynamic data, and applied to very hydrophobic and very hydrophilic ions that usually cannot be observed within the potential window.

The fundamental aspects of electrochemistry at liquid-liquid interfaces are introduced to present the concept of molecular electrocatalysis. Here, a molecular catalyst is adsorbed at the interface to promote a proton coupled electron transfer reaction such as hydrogen evolution or oxygen reduction using lipophilic electron donors.

ABSTRACT:The self-assembly of the oppositely charged watersoluble porphyrins, cobalt tetramethylpyridinium porphyrin (CoTMPyP 4+ ) and cobalt tetrasulphonatophenyl porphyrin (CoTPPS 4− ), at the interface with an organic solvent to form molecular "rafts", provides an excellent catalyst to perform the interfacial four-electron reduction of oxygen by lipophilic electron donors such as tetrathiafulvalene (TTF). The catalytic activity and selectivity of the self-assembled catalyst toward the four-electron pathway was found to be as good as that of the Pacman type cofacial cobalt porphyrins. The assembly has been characterized by UV−visible spectroscopy, Surface Second Harmonic Generation, and Scanning Electron Microscopy. Density functional theory calculations confirm the possibility of formation of the catalytic CoTMPyP 4+ / CoTPPS 4− complex and its capability to bind oxygen.

The four-electron reduction of oxygen by tetrathiafulvalene (TTF) in acidified 1,2-dichloroethane and at the acidified water/1,2-dichloroethane interface has been observed. Spectroscopy and ion transfer voltammetry results suggest that the reaction proceeds by the fast protonation of TTF followed by the 4-electron reduction of oxygen to form water. Electronic structure computations give evidence of the formation of a helical tetramer assembly ([TTF(4)H(2)](2+)) of two protonated TTF and two neutral TTF molecules. The protonated tetramer is potentially able to deliver the four electrons needed for the oxygen reduction. The production of water was corroborated by (1)H NMR analysis.

Redox electrocatalysis (catalysis of electron-transfer reactions by floating conductive particles) is discussed from the point-of-view of Fermi level equilibration, and an overall theoretical framework is given. Examples of redox electrocatalysis in solution, in bipolar configuration, and at liquid-liquid interfaces are provided, highlighting that bipolar and liquid-liquid interfacial systems allow the study of the electrocatalytic properties of particles without effects from the support, but only liquid-liquid interfaces allow measurement of the electrocatalytic current directly. Additionally, photoinduced redox electrocatalysis will be of interest, for example, to achieve water splitting.

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The photochemical reactivity of osmocene in a biphasic waterorganic solvent system has been investigated to probe its water splitting properties. The photoreduction of aqueous protons to hydrogen under anaerobic conditions induced by osmocene dissolved in 1,2-dichloroethane and the subsequent water splitting by the osmocenium metal-metal dimer formed during H 2 production were studied by electrochemical methods, UV-visible spectrometry, gas chromatography, and nuclear magnetic resonance spectroscopy. Density functional theory computations were used to validate the reaction pathways.S olar water splitting is undoubtedly a key challenge (1-3), and in the quest for solar fuels, hydrogen is definitely a major target with many industrial applications ranging from transportation to carbon dioxide mitigation (4).Interestingly, some transition metal complexes like metallocenes have been reported to reduce protons to hydrogen. In 1988, cobaltocene was found to produce hydrogen in strong acidic solutions. The mechanism was investigated by pulse radiolysis, and it was found that the reaction kinetics was first order with respect to cobaltocene and the protons pointing to a protonation of the metal as the primary step (5). In 2009, Kunkely and Vogler reported that osmocene dissolved in strong acidic solutions could photogenerate hydrogen and that ½Cp 2 Os IV ðOH − Þ þ could photogenerate oxygen (6). Proton reduction was shown to proceed also by the formation of a hydride followed under UV light by the formation of H 2 and the dimer ½Cp 2 Os III -Os III Cp 2 2þ .One of the major difficulties in studying water splitting by metallocenes is their very poor solubility in aqueous or even acidic solutions. When investigating the reduction of protons in organic phases where metallocenes are soluble, the other major drawback is the low solubility and dissociation of acids in organic solvents. One way to circumvent these issues is to carry out these reactions in biphasic systems.In 2008, we studied the reduction of aqueous protons by decamethylferrocene (DMFc) in 1,2-dichloroethane (1,2-DCE), and, here again the data suggested that the first step is the protonation of the metal (7). In particular, we have shown that hydrogen bubbles can form at the interface and that hydrogen production is associated to the oxidation of the electron donor DMFc. More generally, we have shown that voltammetry at the interface between two immiscible electrolyte solutions (ITIES) is a very useful tool to study proton coupled electron transfer reactions involving aqueous protons and organic electron donors (8-10). This methodology has been also applied to molecular electrocatalysis where an amphiphilic catalyst is used to complex oxygen to facilitate its reduction as recently reviewed (10-13).In this study, the photochemical reactivity of osmocene with water in biphasic systems has been investigated by electrochemical methods on solid electrodes and soft liquid-liquid interfaces to unravel the multistep reaction mechanisms. Density functional theory (DFT)...

Excitation of the weak electron donor decamethylosmocene on illumination with white light produces an excited‐state species capable of reducing organically solubilized protons under biphasic conditions. Insight into the mechanism and kinetics of light‐driven biphasic hydrogen evolution are obtained by analysis with gas chromatography, cyclic voltammetry, and UV/Vis and 1H NMR spectroscopy. Formation of decamethylosmocenium hydride, which occurs prior to hydrogen evolution, is a rapid step relative to hydrogen release and takes place independently of light activation. Remarkably, hydride formation occurs with greater efficiency (ca. 90 % conversion) under biphasic conditions than when the reaction is carried out in an acidified single organic phase (ca. 20 % conversion). Cyclic voltammetry studies reveal that decamethylosmocene has a higher proton affinity than either decamethylferrocene or osmocene.