Electrochemical behaviour and redox reactivity of some 4-R-1,2,4-triazolin-3,5-diones

Cyclic voltammetry with stationary and rotating disk electrodes results in two 4-R-1,2,4-triazolin-3,5-diones (RTAD) and the corresponding urazoles in dimethylsulphoxide and acetonitrile are reported and discussed. The results show a first reversible reduction step, corresponding to fairly stable anion-radicals, characterised by well resolved EPR spectra. From the electrochemical and spectral data, a reduction mechanism for 4-R-triazolindiones is proposed, leading finally to the corresponding urazoles. Anodic oxidation of 4-methyl urazole gives the corresponding dione, accounting for the reversibility of the reduction process and furnishing further experimental support for the proposed mechanism. Possible intermediates in these redox processes indicated by the electrochemical data are the anion-radical (A−), the dianion (A−2), the urazolyl radical (AH) and the urazolide anion (AH−). MO calculations at the semiempirical AM1 level of the electronic structure of these species account reasonably for the differences in the redox reactivity between the compounds investigated.