Photochemical behaviour of iron(III) oxalato complexes with N,N′-ethylenebis(benzoylacetoneiminato) Schiff base derivatives

The complexes Na[Fe(4-R-benacen)(C2O4)], where 4-R-benacen2− are tetradentate chelating N,N’-ethylenebis(4-R-benzoylacetoneiminato) N2O2-ligands (R = H, Cl, Br, OCH3) are redox stable in the dark in methanolic solutions. Continuous irradiation of such solutions by ultraviolet and/or visible radiation leads to photophysical and photochemical deactivation processes yielding Fe(II) and CH2O as final products. However, the molar ratio of Fe(II): CH2O is much lower than that being typical for other methanolic iron(III) complexes. Using EPR spin trapping technique CH2OH and a methine carbon localized radicals (C−) of the benacen-ligand were identified in the irradiated solutions of the benacen-containing complex as intermediates. Contrary, carbon dioxide anion radicals CO2 C− radicals were found in a system with structurally similar Na[Fe(salen)(C2O4)], where salen2− is N,N′-ethylenebis (salicylaldiminato) N2O2-ligand. The efficiency of the photoredox processes, expressed by the quantum yield of Fe(II) formation, ΦFe(II) slightly decreases with increased wavelength of the incident radiation, and is influenced by the peripheral groups R of the tetradentate ligands. Contrary to the salen-containing complex, 4-R-benacen-ligands did not behave as innocent ones and participated in radical photoprocesses.