Mechanism of photoredox reactions of iron(III) complexes containing salen-type ligands

A mechanism of photoredox processes occurring in irradiated methanolic solu tions of trans-[Fe(R-salen)(CH3OH)F], where (R-salen)(2-) are tetradentate open-chain Schiff base N,N ' -ethylenebis(5-R-salicylideneiminato) N2O2-lig ands (R = F, I, CH3, CF3, OCH3, NO2), has been investigated and proposed. T he complexes are redox stable in the dark. The photoreduction of Fe-III to Fe-II induced by ultraviolet irradiation of the complexes is associated wit h the radical (CH2OH)-C-. formation, which is subsequently transformed to f ormaldehyde CH2O, the mole ratio of Fe-II and CH2O approaching 2:1. The eff iciency of the photoredox process is strongly wavelength dependent and infl uenced by the peripheral R groups of the tetradentate ligands. Electrode po tentials E-1/2(Fe-III/II) correlate with Hammett constants of the R substit uents. When compared with analogous halogeno trans- [Fe(R-salen)(CH3OH)X] c omplexes, significant stabilization of Fe-III by F- ligands in the photoche mical reduction is observed.