Evidence for a two-step electron-transfer process in the electrode reactions of tetraisopropylhydrazine, tetracyclohexylhydrazine and their radical cation salts

Abstract

The heterogeneous electron-transfer kinetics of tetraisopropylhydrazine (4), tetraisopropylhydrazine radical cation hexafluoroantimonate (4+radical dot SbF_6^−), tetracyclohexylhydrazine (5) and tetracyclohexylhydrazine radical cation hexafluoroantimonate (5+radical dot SbF_6^−) have been studied by cyclic voltammetry at a gold working electrode in acetonitrile containing 0.10 M tetrabutylammonium hexafluorophosphate. Results were obtained at eight scan rates between 0.2 and 40 V s^−1 and six temperatures ranging from −15 to 50°C. The results were analyzed according to two models: (1) A direct, one-step electron transfer in which structural change and electron transfer are concerted. (2) A two-step process in which structural change is considered as a separate chemical reaction that precedes or follows the electron-transfer event. Specifically, a square scheme is proposed in which the favored untwisted radical cation can convert to a twisted version, which then receives an electron to form the favored twisted neutral hydrazine. Also, the favored twisted neutral can convert to an untwisted version, which can give up an electron to form the favored untwisted radical cation, thus completing the square. It was found that the one-step model was unable to account for the voltammetric data. On the other hand, analysis by the two-step mechanism produced substantially better agreement between simulation and experiment, particularly for 4 and 4+radical dot SbF_6^−. The present experiments provide the first evidence that two-step electron transfer reactions occur with acyclic tetraalkylhydrazines.