Transfer of highly hydrophilic ions from water to nitrobenzene, studied by three-phase and thin-film modified electrodes.

The study by voltammetry of hydrophilic ion transfers across the interface between an aqueous solution and an immiscible organic solvent is limited by the presence of supporting electrolytes in both phases. Such a study is impossible for ions having a higher affinity for water than ions of the electrolytes. Indirectly, methods based on modified solid electrodes can be used; these are obtained by the deposition of an organic phase containing a molecule having redox properties, the modified electrode being in contact with an aqueous solution of the appropriate electrolyte. The three-phase electrode is very convenient for that purpose. However, this experimental tool also has its own limitation, due mainly to the redox species produced in the organic phase. The oxidized, or reduced, form of the redox molecule must have a very low affinity for water, as otherwise its transfer masks that of the ion under study. Ferrocene is almost useless because of the affinity of the ferrocenium cation for water, decamethylferrocene being a better choice. The present work illustrates how the use of lutetium bisphthalocyanines widely expands the possibilities, as these molecular sandwich complexes can be reduced as well as oxidized, the products of the reactions having a very low affinity for water. This made the determination of the Gibbs energy possible for the transfers of highly hydrophilic ions from water to nitrobenzene: Cl(-) (40 kJ mol(-1)), F(-) (57 kJ mol(-1)), H2PO4(-) (64 kJ mol(-1)). Nothing being really known about the transfer of F(-) or H2PO4(-) from water to organic solvents, these are the first values ever published. H(+), OH(-), and HSO4(-) have also been studied, showing that these species, which have a poor affinity for nitrobenzene, are prone to association reactions with the reduced or oxidized forms of the lutetium bisphthalocyanine.

Lutetium bis(tetra-tert-butylphthalocyaninato): a superior redox probe to study the transfer of anions and cations across the water/nitrobenzene interface by means of square-wave voltammetry at the three-phase electrode.

The redox properties of lutetium bis(tetra-tert-butylphthalocyaninato) (LBPC) have been studied in nitrobenzene that is deposited as a microfilm on the surface of highly oriented pyrolytic graphite electrodes. The behavior of the modified electrode, which is immersed in an aqueous electrolyte solution, is typical for the three-phase electrode (Scholz, F.; Komorsky-Lovric, S.; Lovric, M. Electrochem. Comm. 2000, 2, 112-118). LBPC can be both oxidized and reduced in one electron reversible processes. The oxidation and the reduction of LBPC at the graphite/nitrobenzene interface is accompanied by the transfer of anion or cation, respectively, from the aqueous phase into the organic layer. Thus, using LBPC as a redox probe for the three-phase electrode, the transfer of both anions and cations across the water/nitrobenzene interface can be studied in a single experiment. The hydrophobicity of LBPC is so high that it enables inspection of cations and anions with Delta (nb)(w) (G)(theta)(Cat+) < or = 43 kJ/mol and Delta (nb)(w) (G)(theta)(X-) < or = 50 kJ/mol, respectively. The direct transfer of Na(+) and Li(+) from water to nitrobenzene, mutually saturated, is achieved for the first time at a macroscopic water/nitrobenzene interface.

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes.

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.

From the Single- to the Triple-Decker Sandwich. Effect of Stacking on the Redox and UV-Visible Spectroscopic Properties of Lutetium(III) 1,2-Naphthalocyaninate Complexes.

(1,2-Naphthalocyaninato)lutetium(III), bis(1,2-naphthalocyaninato)lutetium(III), and tris(1,2-naphthalocyaninato)dilutetium(III) have been synthesized in good yields. These complexes were characterized by UV-visible, (1)H NMR, and mass spectroscopies and by electrochemical techniques. Bis(1,2-naphthalocyaninato)lutetium(III) is compared to bis(2,3-naphthalocyaninato)lutetium(III): the good correspondance between the experimental data and theoretical results obtained by others is emphasized in this work. Stacking one to three macrocyclic rings is possible in the series of the 1,2-naphthalocyanine sandwich-like complexes with lutetium(III); as shown by UV-visible spectroelectrochemistry and voltammetry, this influences greatly the pi-conjugated systems, modifying spectacularly their spectroscopic and redox properties.

A comparative study of the anion transfer kinetics across a water/nitrobenzene interface by means of electrochemical impedance spectroscopy and square-wave voltammetry at thin organic film-modified electrodes.

The kinetics of the transfer of a series of hydrophilic monovalent anions across the water/nitrobenzene (W/NB) interface has been studied by means of thin organic film-modified electrodes in combination with electrochemical impedance spectroscopy and square-wave voltammetry. The studied ions are Cl-, Br-, I-, ClO4-, NO3-, SCN-, and CH3COO-. The electrode assembly comprises a graphite electrode (GE) covered with a thin NB film containing a neutral strongly hydrophobic redox probe (decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato)) and an organic supporting electrolyte. The modified electrode is immersed in an aqueous solution containing a supporting electrolyte and transferring ions, and used in a conventional three-electrode configuration. Upon oxidation of the redox probe, the overall electrochemical process proceeds as an electron-ion charge-transfer reaction coupling the electron transfer at the GE/NB interface and compensates ion transfer across the W/NB interface. The rate of the ion transfer across the W/NB interface is the limiting step in the kinetics of the overall coupled electron-ion transfer reaction. Moreover, the transferring ion that is initially present in the aqueous phase only at a concentration lower than the redox probe, controls the mass transfer regime in the overall reaction. A rate equation describing the kinetics of the ion transfer that is valid for the conditions at thin organic film-modified electrodes is derived. Kinetic data measured with two electrochemical techniques are in very good agreement.

Kinetics of anion transfer across the liquid | liquid interface of a thin organic film modified electrode, studied by means of square-wave voltammetry.

The electrochemical oxidation of lutetium bis(tetra-tert-butylphthalocyaninato) (LBPC) and decamethylferrocene (DMFC), as well as the reduction of LBPC, lutetium bis(phthalocyaninato) (LPC), and lutetium (tetra-tert-butylphthalocyaninato hexadecachlorphthalocyaninato) (LBPCl), has been studied in a thin nitrobenzene (NB) film deposited on the surface of a graphite electrode (GE) by means of square-wave voltammetry (SWV). The organic film-modified electrode was immersed in an aqueous (W) electrolyte solution and used in a conventional three-electrode configuration. When the aqueous phase contains ClO4-, NO3-, or Cl- (ClO4-, or NO3- only, in the case of DMFC), both LBPC and DMFC are oxidized to stable monovalent cations in the organic phase. The electron transfer at the GE | NB interface is accompanied by a simultaneous anion transfer across the W | NB interface to preserve the electroneutrality of the organic phase. LBPC, LPC, and LBPCl are reduced to stable monovalent anions accompanied by expulsion of the anion of the electrolyte from the organic into the aqueous phase. In all cases, the overall electrochemical process comprises simultaneous electron and ion transfer across two separate interfaces. Under conditions of SWV, the overall electrochemical process is quasireversible, exhibiting a well-formed "quasireversible maximum" that is an intrinsic property of electrode reactions occurring in a limiting diffusion space. For all the redox compounds that have been studied, the kinetics of the overall electrochemical process is controlled by the rate of the ion transfer across the liquid | liquid interface. Based on the quasireversible maximum, a novel and simple methodology for measuring the rate of ion transfer across the liquid | liquid interface is proposed. A theoretical background explaining the role of the ion-transfer kinetics on the overall electrochemical process at the thin organic film modified electrode under conditions of SWV is presented. Comparing the positions of the theoretical and experimental quasireversible maximums, the kinetics of ClO4-, NO3-, and Cl- across the W | NB interface was estimated. The kinetics of the overall process at the thin organic film modified electrode, represented by the second-order standard rate constant, is 91 +/- 8, 90 +/- 4, and 133 +/- 10 cm(4) s(-1) mol(-1), for the transfer of ClO4-, NO3-, and Cl- respectively.

Substituted tren-capped porphyrins: probing the influence of copper in synthetic models of cytochrome c oxidase.

Tris(2-aminoethylamine) (tren)-capped porphyrins bearing electron donating or withdrawing groups on the amino functions of their tripod have been synthesised and the catalytic activity of each complex was carefully studied both as the iron and the iron-copper complexes: only one of our iron-copper complexes was shown to be active and selective towards the reduction of dioxygen to water.