Photoelectrochemical Reduction of p-Halonitrobenzenes

A detailed and comparative mechanistic study of the photoelectrochemical dehalogenation of the four p-halonitrobenzenes, X-C,H,-NO, (X = F, CI, 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 Iis shown to take place via an ECE process with the ultimate generation of the radical anion of nitrobenzene. Dual photoand 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-C,H,-NO,]'-, which is followed by fragmentation forming the 'C,H,NO, 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 [C,H,-NO,]'-. The aryl radical is demonstrated to be able to undergo (partial) recombination with added Bror CI-. The effectiveness of different electronic transitions in the radical ions, [X-C,H,-NO,]'(X = Br, CI) towards dehalogenation are compared; both radical anions exhi bit 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-C,N,-NO,]'was observed at any wavelength in the visible region of the spectrum.