Redox properties of photoexcited (nBu(4)N)(3)PW12O40/Fe-III porphyrins composite systems

The photochemical and photocatalytic properties of (nBu(4)N)(3)PW12O40 (PW12O403-) was investigated in the presence of several Fe-III-meso-tetrarylporphyrins. The photochemical excitation of the polyoxotungstate in the presence of 2-propanol or cyclohexane leads to its reduction and simultaneous oxidation of the organic substrate. The reduced polyoxotungstate is able, in turn, to transfer one electron to the Fe(III)porphyrin to give the ferrous complex. In the presence of anaerobic cyclohexane, sigma-alkyl-Fe-III complexes are obtained as a consequence of the reaction between cyclohexyl radicals and the iron porphyrin in its reduced form. Subsequent oxidative migration of the sigma-bound cyclohexyl group from the iron to one of the pyrrole nitrogens leads to the formation of N-substituted porphyrin. The results obtained by laser flash photolysis experiments indicate that the electron transfer from the photoreduced polyoxotungstate to the Fe(III)porphyrin complexes is faster than to O-2. Therefore, the photoexcited polyoxotungstate can operate as a source of electrons for the reduction of the Fe(III)porphyrin to the ferrous form, thus creating the site at which O-2 is reductively activated. Continuous irradiation of the composite systems under aerobic conditions leads to the oxidation of cyclohexane to cyclohexanol and cyclohexanone. In comparison with the Fe(III)porphyrins alone, the composite photocatalysts present higher yields of the oxidation products. In comparison with PW12O403- alone, they produce higher amounts of the hydroxylated product.