Towards redox-switchable organocatalysts based on bidentate halogen bond donors.

Redox-active bidentate halogen bond donors based on halopyridinium groups as halogen-bond donating units were synthesized and their structures were elucidated by X-ray diffraction analyses and DFT calculations. Via reversible twofold reduction, these dicationic species can be transformed to neutral compounds which should be much weaker Lewis acids. The corresponding electrochemical data were obtained, and CV as well as UV-vis and NMR techniques were also used to determine binding constants of these halogen bond donors to halides. While all titrations agree on the relative order of binding strengths (with chloride being bound strongest), there are marked deviations in the overall affinity constants which are discussed. In contrast to earlier azo-bridge analogues, the ethylene-linked variants presented herein do not oxidize halides, and thus the novel halogen bond donors could also be used as Lewis acidic organocatalysts in a halide abstraction benchmark reaction, yielding a performance similar to bis(haloimidazolium)-derived catalysts.

Electrochemical activation of a tetrathiafulvalene halogen bond donor in solution.

The halogen bond donor properties of iodo-tetrathiafulvalene (I-TTF) can be electrochemically switched and controlled via reversible oxidation in the solution phase. Interestingly the activation of only one single halogen bond yielded already a strong and selective interaction, quantified by cyclic voltammetry. The standard potentials of the redox couples I-TTF(0/1+) and I-TTF(1+/2+) were observed to shift upon the addition of halides. These anions selectively stabilize the cationic I-TTF species through halogen bonding in polar liquid electrolytes. The thermodynamic affinity constants for chloride and bromide binding to the oxidized species have been determined. Competition in halide binding between I-TTF(1+) and other halogen bond donors allowed for comparing the relative donor strength of the respective electrophilic species. Furthermore it has been shown that halogen bonding can prevail over hydrogen bonding in the investigated system.

Determination of horseradish peroxidase and a peroxidase-like iron porphyrin at a Nafion-modified electrode.

A catalytic coupling reaction between 4-amino antipyrine and a N,N-disubstituted aniline derivative has been exploited in the indirect electrochemical detection of horseradish peroxidase (HRP) and of a biomimetic catalyst, the iron(III) sulfonated tetraphenyl porphyrin. In the presence of hydrogen peroxide and one of the two catalysts a cationic electroactive quinone-iminium dye P+ was formed and detected by linear scan voltammetry using a screen-printed electrode coated with a Nafion film. Detection limits of 10(-12) M for HRP and 4 x 10(-10) M for the iron porphyrin have been achieved. In conclusion the iron porphyrin is considered to be a promising alternative to the HRP label in enzyme immunoassays with electrochemical detection.

Simultaneous detection of three drugs labeled by cationic metal complexes at a nafion-loaded carbon paste electrode.

The synthesis of the pentaammine ruthenium(II) complex of N-isonicotinoyl-nortriptyline (NORPy-Ru(2+)) was performed and its electrochemical properties at a nafion-loaded carbon paste electrode were examined. The anodic oxidation of the positively charged labeled antidepressant proceeded at -0.06 V (versus Ag/AgCl, 0.05 M Cl(-)). A detection limit of 0.075 muM (S/N=3) was achieved at physiological pH by square-wave voltammetry after a 5-min preconcentration step, with a linear response over the range 0.075-5.0 muM. With a view to a future triple-analyte immunoassay, the detection of NORPy-Ru(2+) was also examined in the presence of two labeled antiepileptics previously synthesised, i.e. phenytoin labeled by a ferroceneammonium salt (oxidation potential at 0.26 V) and phenobarbital labeled by a cobaltocenium salt (reduction potential at -1.05 V). The simultaneous detection of the three labeled drugs proceeded with analytical performances similar to those corresponding to the separate accumulation of each tracer. However it was observed that the pentaammine ruthenium(II) complexes of pyridine and its derivatives were not stable in the presence of serum, which does not allow for their use as redox cationic labels in a multi-analyte immunoassay to be envisaged.

Adsorption of Glycosidic Surfactants at the Mercury Electrode

The adsorption of glycosidic surfactants from aqueous electrolyte solutions on a mercury electrode was studied by means of differential capacitive measurements (tensammetric method). The adsorption behavior of the mono- and disaccharidic surfactants studied is discussed in relation to their micellar properties, in particular their respective critical micellar concentration (CMC). With monosaccharidic surfactants, a broad and bell-shaped peak is observed on the tensammetric curves and it suggests the formation of a monolayer called hemimicelle at the mercury electrode. For disaccharidic compounds, the split capacity peak observed at concentrations above the CMC suggests the formation of a multilayer of micelles throughout the electrical double layer.