Electrochemical recognition of anionic guest species by redox-active receptor molecules

3 Electrochemical recognition of anionic guest species by redox-active receptor molecules 

A first venture into this field was to utilize the pH-independent, positively charged, redox-active cobaltocenium moiety (Beer and Keefe, 1989 Beer et 

The addition of tetrabutylammonium chloride to H solutions of [68] and [69] in deuteriated acetonitrile resulted in remarkable nmr shifts of the respective proton signals of both receptors. Of particular note were the substantial downfield shifts of the amide protons (AS = 1.28 ppm for [68] and 1.52 ppm for [69]) on addition of one equivalent of chloride. These results suggest that a significant —CO—NH-Cr hydrogen-bonding interaction contributes to the overall anion complexation process. Subsequent nmr titration curves suggesting 1 1 stoichiometry with anion complexes of [68] and [69] were found in all cases. Negligible shifts were observed under identi- 

To test this hypothesis further, simple acyclic cobaltocenium derivatives (Fig. 38) containing tertiary amide groups ([72] and [73]) were prepared and complexation with Cl- and Br anions was investigated in solution by nmr spectroscopy. No shifts of the host s proton resonances were observed, emphasizing again the importance of the amide N—H group in anion binding. 

The cathodic shifts of the redox potentials of compounds [68], [69] and [71] on addition of halide anions are due to the stabilization of the cobaltocenium cation by the bound anion which causes the redox couple to shift to a more 

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