Synthesis, electrochemical investigation and EPR spectroscopy of a reversible four-stage redox system based on mesoionic 5,5′-azinobis(1,3-diphenyltetrazole) and related mesoionic compounds‡
Mesoionic 5,5′-azinobis(1,3-diphenyltetrazole) 1 was prepared, and its chemical oxidation gave stable crystals of the corresponding radical cation 1˙+ and dication 12+, which reversibly gave back azine 1 on reduction with zinc. Electrochemical investigations of 1 using cyclovoltammetry and differential pulse voltammetry in pyridine (Py) or dichloromethane (DCM) also revealed the two reversible successive one-electron oxidations leading to dication 12+via radical cation 1˙+, both of which can be reduced to the neutral state 1. In the cathodic process, 1 was reduced by two consecutive one-electron transfers at only slightly different potentials up to the corresponding dianion 12– which could be re-oxidized to the neutral state; thus constituting a reversible four-stage redox system. Radical cation 1˙+ and anion 1˙– were characterized by EPR spectroscopy. In order to get more insights into the spin-density distribution of 1˙+, the bis- and tetra-15N-labelled species 1a˙+, 1b˙+ and 1c˙+ were synthesized and investigated by EPR and 15N as well as 14N ENDOR spectroscopy, revealing that the largest N hyperfine coupling constants are due to the nitrogen atoms of the central bridge. According to 1H ENDOR there seems to be a small coupling with the protons of both phenyl rings which cannot be resolved in the EPR spectrum.
The electrochemical properties of the related mesoionic compounds 5–10 were also investigated in Py or DCM solutions. In the cathodic process, a reduction peak of 9 and 10 was observed due to their reversible one-electron reduction to the corresponding radical anions. The radical obtained on reduction of 10 was characterized by EPR spectroscopy. On the other hand, 5–8 can be reduced by a formal two-electron transfer up to the corresponding dianions which are re-oxidizable to the neutral state. In the anodic process, 9/10 undergo irreversible one-electron oxidations whereas 5/6 (in DCM) and 7/8 (in Py) experience reversible or irreversible step by step two-electron oxidations leading to the dications. In Py the oxidation products of 5/6 react to further species revealing two more oxidation and several rereduction peaks. On the other hand, the oxidation products of 7/8 are instable in DCM (one main oxidation and rereduction peak). The electrochemical data are discussed in terms of delocalization in the cations and conjugation in the dications.