Photoelectrochemical reduction of p-halonitrobenzenes

Abstract

A detailed and comparative mechanistic study of the photoelectrochemical dehalogenation of the four p-halonitrobenzenes, X-C₆H₄-NO₂(X = F, Cl, Br or I) in acetonitrile solution is reported. In the case of p-iodonitrobenzene, iodide loss is accomplished by electrochemical reduction alone. The formation of I^– is shown to take place via an ECE process with the ultimate generation of the radical anion of nitrobenzene. Dual photo- and electro-chemical activation of p-chloronitrobenzene and p-bromonitrobenzene leads to halide loss through a photo-ECE mechanism. This proceeds via absorption of light by the radical anions, [X-C₆H₄-NO₂]˙^–, which is followed by fragmentation forming the ˙C₆H₄NO₂ aryl radical. The latter is shown to react with the solvent system forming nitrobenzene which is further reduced at the electrode with the generation of [C₆H₄-NO₂]˙^–. The aryl radical is demonstrated to be able to undergo (partial) recombination with added Br^– or Cl^–. The effectiveness of different electronic transitions in the radical ions, [X-C₆H₄-NO₂]˙^–(X = Br, Cl) towards dehalogenation are compared; both radical anions-exhibit transitions centred near 330 and 470 nm in the near UV–VIS part of the spectrum. For the chloro-compound only the former band is effective in stimulating chloride release, whereas for the bromo-compound the excitation of either band causes bromide loss. The long wavelength band of the latter is quantified as being some 5.6 times more effective towards fragmentation on a per photon absorbed basis and this is rationalised using spin selection rules. No loss of fluoride from [F-C₆N₄-NO₂]˙^– was observed at any wavelength in the visible region of the spectrum.