Mechanism of Photoinduced Processes in Solutions of Iodo Iron(III) Complexes Containing Schiff Base Ligands

Summary.  The mechanisms of photoinduced processes occurring in methanolic solutions of trans-Fe(4-R-benacen)(CH₃OH)I] (4-R-benacen ²⁻ : N,N′-ethylene-bis-(4-R-benzoylacetoneiminato) tetradentate open-chain Schiff bases with R = H, Cl, Br, CH₃, OCH₃, or NO₂) were investigated by electronic absorption spectroscopy and EPR spin trapping. The complexes are redox-stable in the dark both in the solid state and in methanolic solutions. Ultraviolet and/or visible irradiation in methanol induces photoreduction of Fe(III) to Fe(II). No formation of I^˙ or was observed. ^˙CH₂OH radicals and/or solvated electrons were identified in irradiated systems using nitrosodurene or 5,5-dimethyl-1-pyrroline-N-oxide as spin traps. The final product of the photooxidation coupled with the photoreduction of Fe(III) is formaldehyde, the molar ratio of Fe(II) and CH₂O being close to 2:1. The efficiency of the photoredox process is strongly wavelength dependent and influenced by the peripheral groups R of the tetradentate ligands. It is suggested that the primary photoredox step starts from thermally nonequilibrated ligand-to-metal charge transfer excited states. Received May 2, 2001. Accepted May 30, 2001