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.