Electrochemical Preparation and Characterization of Meisenheimer Complexes of Trinitrofluorenone with Pyrrole, Indole, and Carbazole

The mechanism of Meisenheimer complex formation by a novel electrochemical route was investigated. The synthetic route utilizes a CH3CN solution consisting of trinitrofluorenone (TNF), electron-donative molecules (EDM = pyrrole, indole, and carbazole), and a supporting electrolyte. Electroreduction of the solution causes a complexation of TNF with the EDMs to yield a green anionic σ complex. To understand this electrode reaction, the effects of electrolysis and solution conditions (solvent, supporting electrolyte, the role of O2, etc.) on the complex formation were investigated. As a result, it was revealed that the σ-complex formation proceeds in the following successive steps: (1) charge-transfer (CT) complex formation between TNF and the EDMs, (2) two-electron reduction of the CT complex to yield double-anionic radical species, (3) attack of O2 on the radical species to induce complexation of TNF with the EDMs, and (4) stabilization of the resulting σ complex by hydrophobic electrolyte cations. In addition, kinetic and X-ray photoelectron spectroscopic measurements revealed that the σ complexes possess unique electronic structures with high stability (first-order decomposition rate constant of 2.3 × 10-6−1.0 × 10-5 s-1 at 20 °C in acetone; no decomposition at 0 °C in the solid state and in acetone).