Christine Thobie-Gautier

Electrochemical Behavior of Cu(II) on Carbon Paste Electrode Modified by Humic Acid, Cyclic Voltammetry Study

Polarography at dropping mercury electrode and infrared spectroscopy were used to confirm the reaction of copper(II) ions with humic acid in solution. Polarography allowed observing the reaction to follow the concentration of the Cu(II) in solution while infrared permitted one to follow the humic acid complexation. Cyclic voltammetry experiments, at stationary carbon paste electrode, showed original properties of humic acid which adsorbs at the surface of carbon paste electrode that becomes appropriate for the preconcentration and identification of Cu(II) species. Although very promising the use of adsorption showed the disadvantage of not being reproducible therefore we developed a new preparation method for the carbon paste electrodes modified with humic acid that allow highly repeatable measurements. Humic acid was dissolved in base solution before mixing it with carbon graphite powder and freeze-drying to lead to a fine powder that allows reproducible batches of electrodes. The modification of carbon paste electrode with two different humic acids revealed that the electrochemical behavior of Cu(II) solutions depends on the nature of the humic material. The effects on the electrochemical behavior of Cu(II) of the solutions pH and the ratio of humic acid in the bulk of the electrode were also examined. Such carbon paste electrodes modified with humic acid extracted from a peat area in the Mogi GuaAu River at Sao Paulo State in Brazil showed high reproducibility and particular stability in solution (pH range of 3.0 to 5.5) entitling them to be very good candidates for analytical purposes.

An Electrochemical Nickel-Catalyzed Arylation of 3-Amino-6-Chloropyridazines

3-Amino-6-aryl- and 3-amino-6-heteroarylpyridazines have been obtained in generally good yield using a nickel-catalyzed electrochemical cross-coupling between 3-amino-6-chloropyridazines and aryl or heteroaryl halides at room temperature. Comparative experiments involving classical palladium-catalyzed reactions, such as Suzuki, Stille, or Negishi cross-couplings, reveal that the electrochemical method can constitute a reliable alternative tool for biaryl formation. A possible reaction mechanism is proposed on the basis of electrochemical analyses.

Electrochemical Cleavage of Sulfonamides: An Efficient and Tunable Strategy to Prevent β-Fragmentation and Epimerization

The electrochemical reduction of sensitive sulfonamides is described. The addition of a benzoyl group on the nitrogen atom facilitates the reductive cleavage of sulfonamides preventing β-fragmentation and epimerization. This strategy was successfully applied to the cyclopropylamine and to α-amino stannanes.

Electrochemical Synthesis and Characterisation of Alternating Tripyridyl-Dipyrrole Molecular Strands with Multiple Nitrogen-Based Donor-Acceptor Binding Sites

Synthesis of alternating pyridine-pyrrole molecular strands composed of two electron-rich pyrrole units (donors) sandwiched between three pyridinic cores (acceptors) is described. The envisioned strategy was a smooth electrosynthesis process involving ring contraction of corresponding tripyridyl-dipyridazine precursors. 2,6-Bis[6-(pyridazin-3-yl)]pyridine ligands 2a-c bearing pyridine residues at the terminal positions were prepared in suitable quantities by a Negishi metal cross-coupling procedure. The yields of heterocyclic coupling between 2-pyridyl zinc bromide reagents 12a-c and 2,6-bis(6-trifluoromethanesulfonylpyridazin-3-yl)pyridine increased from 68 to 95% following introduction of electron-donating methyl groups on the metallated halogenopyridine units. Favorable conditions for preparative electrochemical reduction of tripyridyl-dipyridazines 2b,c were established in THF/acetate buffer (pH 4.6)/acetonitrile to give the targeted 2,6-bis[5-(pyridin-2-yl)pyrrol-2-yl]pyridines 1b and 1c in good yields. The absorption behavior of the donor-acceptor tripyridyl-dipyrrole ligands was evaluated and compared to theoretical calculations. Highly fluorescent properties of these chromophores were found (ν(em)≈2 × 10(4) cm(-1) in MeOH and CH(2)Cl(2)), and both pyrrolic ligands exhibit a remarkable quantum yield in CH(2)Cl(2) (φ(f)=0.10). Structural studies in the solid state established the preferred cis conformation of the dipyrrolic ligands, which adopting a planar arrangement with an embedded molecule of water having a complexation energy exceeding 10 kcal mol(-1). The ability of the tripyridyl-dipyrrole to complex two copper(II) ions in a pentacoordinate square was investigated.

Sensitive and reproducible quantification of Cu2+ by stripping with a carbon paste electrode modified with humic acid

Abstract The preparation of a humic acid modified carbon paste electrode (HA-MCPE) as well as the behavior of its surface as complexing agent toward Cu2+ cations are described. Electrochemical studies of the reduction of the complexed cations and of the anodic stripping oxidation of the resulting copper are outlined. The anodic stripping current was correlated to the Cu2+ concentrations. A well-defined method for the preparation of reproducible electrodes is described. The effects on the current response obtained by cyclic voltammetry of the humic acid ratio, the pH, the accumulation time, and the speed scan rate were studied. Calibration graphs were linear over the range 3 × 10−8 − 10−5 mol L−1 Cu2+ and the relative standard deviation (R.S.D.) was 1.2% (n = 5) for [Cu2+] = 1.6 × 10−5 mol L−1. 5 min accumulation time for [Cu2+]>10−7 mol L−1 and 10 min for [Cu2+] < 10−7 mol L−1 were sufficient to permit sensitive and reproducible measurements. The electrode was successfully used to measure Cu2+ in real samples and the results were compared to those obtained by the standard method with differential pulse anodic stripping voltammetry.