Measurements of standard potentials for nucleophiles by fast cyclic voltammetry: I: 9-substituted fluorenide ions in dimethyl sulphoxide

Abstract

Standard potentials E Fl ./Fl − for 12 different 9-substituted fluorenide ions were measured in dimethyl sulphoxide (DMSO) by fast cyclic voltammetry. 9-Amino-fluorenide ions are found to exhibit two reversible single-electron waves in cyclic voltammetry showing that the corresponding 9-aminofluorenyl radicals and fluorenium cations are stable. Standard potentials for the 9-aminofluorenide ion and for the 9-aminofluorenyl radical were measured. For 9-alkyl-, phenyl-, methoxy-, phenylthiofluorenide ions, only one single-electron irreversible wave was found and linear sweep voltammetric results indicate a DIM1 mechanism for less sterically hindered fluorenyl radicals. For severely sterically hindered fluorenyl radicals a more complicated mechanism was found to occur. The rate constants for the dimerization of the fluorenyl radicals were determined by comparison of experimental and simulated voltammograms, from the potential shift of oxidation potentials relative to standard potentials or by double potential step chronoamperometry. 9-aminofluorenyl radicals are found to be stable owing to spin delocalization to the nitrogen present and it was possible to measure the standard potentials for the 9-aminofluorenide ion and the 9-aminofluorenyl radical. The former potentials are quite close to the oxidation potentials published by Bordwell. For the other group of 9-substituted fluorenide ions we find that the corresponding fluorenyl radicals dimerize. If the steric hindrance is not too severe, two fluorenyl radicals will dimerize and 9,9′-disubstituted-9,9′-bifluorene is obtained. When steric hindrance becomes more important the dimerization in the 9-position becomes slower and an alternative dimerization of one fluorenyl radical and one fluorenide ion results in the anion radical of the bifluorene. This anion radical is finally oxidized by a fluorenyl radical to the bifluorene in a rate-determining solution electron transfer.