Electron transfer reactions of extended o-, p-quinones—voltammetric and EPR/ENDOR spectroscopic investigations

The electrochemical properties of four ortho-, para-extended quinones/quinonemethides † 1–4 have been investigated using cyclic voltammetry (CV) and normal pulse voltammetry (NPV). The species are reduced by addition of two, four or six electrons to form a dianion (1), tetraanions (2, 3) and a hexaanion (4), all of which can be re-oxidized to the corresponding quinonoid species. These compounds, thus, exhibit a multistep redox behaviour, acting as good (quasi)reversible electron acceptors with well separated potential steps, some of them corresponding to a formal two-electron transfer. The radical anions 1˙––4˙– and the radical trianion 3˙3–, prepared electrochemically in the first and third reduction steps, are persistent for several hours in the absence of air. They have been characterized by EPR and ENDOR spectroscopy, revealing a delocalization of the odd electron. The areas of preferential spin residing, however, are quite different in radical trianion 3˙3– as compared to radical monoanion 3˙–, indicating a strong influence of the charge state in these radical anions