Homogeneous and heterogeneous catalytic redox processes: Solution and solid state voltammetry of lead complexes at carbon electrodes
Marken F., Leslie WM., Compton RG., Moloney MG., Sanders E., Davies SG., Bull SD.
In this study, conditions suitable for the electrochemical recycling of Pb(II) to Pb(IV) benzoate complexes, which are important redox reagents in organic chemistry, are reported. The electrode process was found to be governed by surface chemical steps involving the formation of films of Pb(IV)-containing deposits. Only on glassy carbon and on basal plane pyrolytic graphite electrodes was a well-defined diffusion-controlled oxidation response observed for Pb(II) benzoate dissolved in acetonitrile and in the presence of an excess of both benzoic acid and pyridine. Benzoic acid acts as a ligand. The role of pyridine in the oxidation process was investigated by comparison of the effect of pyridine with that of 4,4′-bipyridine, 2,2′-bipyridine, and 1,10-phenanthroline. The coordination of the nitrogen donor ligand is suggested to stabilize and trap intermediates in the electrochemical process. The homogeneous oxidation of ethylene glycol by Pb(IV) benzoate coupled to the electrode process is reported; a channel flow cell experiment with data analysis by computer modelling, employing the backwards implicit method, was used to determine the second order rate constants k for the corresponding catalytic EC′-type process in the presence of varying amounts of pyridine. A linear increase of k with pyridine concentration suggests a direct involvement of pyridine in or before the rate determining step in the oxidative cleavage of ethylene glycol. Further, in a study of three insoluble lead-containing materials, PbO2, Pb2O3, and Pb2Fe(CN)6, mechanically attached to a basal plane pyrolytic graphite electrode in the form of microcrystalline particles and immersed in acetonitrile containing benzoic acid and pyridine, the novel catalytic oxidation of ethylene glycol in a heterogeneous electrode process could be detected. Possible prospects of Pb(IV) electrocatalysis in electro-organic synthesis are discussed.