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The voltammetric response of seleno-l-cystine has been studied at gold substrates under physiological conditions. The reactivity of diselenides is utilized to generate a modified electrode surface, which in turn allows for electroanalysis of solution-based selenium species. Stable and reproducible voltammetry is observed for selenocystine reduction at pH 7 on selenium-modified gold substrates (Emid = −486 mV vs. Ag/AgCl) and is consistent with a diffusionally controlled process. Modification of the gold surface is readily achieved via electrochemical cycling in the presence of a diselenide source at conventional scan rates. These studies afford voltammetric characterization of the selenocystine/selenocysteine redox couple under physiologically relevant conditions and highlight the potential utility of selenium-modified substrates for electroanalysis of chalcogen-containing species. Keywords: Selenium, Selenocystine, Diselenide, Voltammetry

Two series of homoleptic phenolate complexes with fluorinated aryloxide ligands A2[M(OAr)4] with M = Co2+ or Cu 2+, OAr- = (OC6F5)- (OArF) or {3,5-OC6H3(CF3) 2}- (OAr′), A+ = K (18-crown-6) +, Tl+, Ph4P+, Et3HN +, or Me4N+ have been synthesized. Two related complexes with nonfluorinated phenoxide ligands have been synthesized and studied in comparison to the fluorinated aryloxides demonstrating the dramatic structural changes effected by modification of OPh to OArF. The compounds {K(18-crown-6)}2[Cu(OArF)4], 1a; {K(18-crown-6)}2[Cu(OAr′)4], 1b; [Tl 2Cu(OArF)4], 2a; [Tl2Cu(OAr′) 4], 2b; (Ph4P)2[Cu(OArF) 4], 3; (nBu4N)2[Cu(OAr F)4], 4; (HEt3N)2[Cu(OAr F)4], 5; {K(18-crown-6)}2[Cu 2(μ2-OC6H5)2(OC 6H5)4], 6; {K(18-crown-6)} 2[Co(OArF)4], 7a; {K(18-crown-6)} 2[Co(OAr′)4], 7b; [Tl2Co(OAr F)4], 8a; [Tl2Co(OAr′)4], 8b; (Me4N)2[Co(OArF)4], 9; [Cp 2Co]2[Co(OAr′)4], 10; and {K(18-crown-6)}2[Co2(μ2-OC6H 5)2(OC6H5)4], 11, have been characterized with UV-vis and multinuclear NMR spectroscopy and solution magnetic moment studies. Cyclic voltammetry was used to study 1a, 1b, 7a, and 7b, X-ray crystallography was used to characterize 1b, 3, 4, 5, 6, 7a, 7b; 10, and 11. The related [MX4]2- compound (Ph 4P)2[Co(OArF)2Cl2], 12, has also been synthesized and characterized spectroscopically, as well as with conductivity and single-crystal X-ray diffraction. Use of fluorinated aryloxides permits synthesis and isolation of the mononuclear, homoleptic phenolate anions in good yield without oligomerized side products. The reaction conditions that result in homoleptic 1a and 7a with OArF upon changing the ligand to OPh result in μ2-OPh bridging phenoxides and the dimeric complexes 6 and 11. The [M(OArF)4]2- and [M(OAr′)4]2- anions in 1a, 1b, 3, 4, 5, 7a, 7b, 9, and 10 demonstrate that stable, isolable homoleptic phenolate anions do not need to be coordinatively or sterically saturated and can be achieved by increasing the electronegativity of the ligand.

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C 4 mim][NTf 2 ]), and the conventional aprotic solvent, acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim ® was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2 Br - → Br 2 + 2 e - Br 2 + Br - ⇌ k b k f Br 3 - K eq Athigher potentials, the tribromide anion dissociates to bromine, Br 2 , and bromide, Br − , which is immediately oxidised, leading to the emergence of a second anodic wave. Irreversible electrode kinetics were inferred from Tafel analysis and removal of the first electron is believed to be the rate-determining step in the formation of bromine. The equilibrium constant, K eq , where K eq = [ Br 3 - ] [ Br 2 ] [ Br - ] was found to be 3 × 10 3 and 9 × 10 6 M −1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, K complex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M −1 .

The reduction of oxygen in the presence of carbon dioxide has been investigated by cyclic voltammetry at a gold microdisk electrode in the two room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222] [N(Tf)2]). With increasing levels of CO2, cyclic voltammetry shows an increase in the reductive wave and diminishing of the oxidative wave, indicating that the generated superoxide readily reacts with carbon dioxide, The kinetics of this reaction are investigated in both ionic liquids. The reaction was found to proceed via a DISP1 type mechanism in [EMIM][N(Tf)2], with an overall second-order rate constant of 1.4 ± 0.4 × 103 M-1 s-1. An ECE or DISP1 mechanism was determined to be the most likely pathway for the reaction in [N6222][N(Tf)2], with an overall second-order rate constant of 1.72 ± 0.45 × 103 M-1 s -1.

The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis-((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2·-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2·- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2·- = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM]-[N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2·- = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.

The electrochemical oxidation of cobalt(II) at gold, boron-doped diamond, basal and edge plane pyrolytic graphite, and highly oriented pyrolytic graphite electrodes in aqueous solutions containing NH3 has been studied using cyclic voltammetry, with subsequent chemical and electrochemical processes explained in detail. Furthermore, the electroreduction of [Co(NH 3)6]3+ in the presence of different electrolytes has also been studied to obtain a better understanding of the oxidation pathway of the Co(II)-ammine complexes. In aqueous solution the mechanism can be described by the following scheme: (Chemical presented) © 2006 Wiley-VCH Verlag GmbH and Co. KGaA.

The electrochemical windows of acetonitrile solutions doped with 0.1 M concentrations of several ionic liquids were examined by cyclic voltammetry at gold and platinum microelectrodes. These results were compared with those observed in the commonly used 0.1 M tetrabutylammonium perchlorate/acetonitrile system as well as with neat ionic liquids. The use of a trifluorotris(pentafluoroethyl)phosphate-based ionic liquid, specifically, as supporting electrolyte in acetonitrile solutions affords a wider anodic window, which is attributed to the high stability of the anionic component of these intrinsically conductive and thermally robust compounds.