WAVELENGTH AND LIGAND PERIPHERAL GROUPS EFFECTS IN PHOTOREDOX CHEMISTRY OF [FE(R-SALEN)(BR-KOJI)] COMPLEXES (SALEN = N,N’-ETHYLENEBIS(R-SALICYLIDENEIMINATO); KOJI = 5-HYDROXY-2-(HYDROXYMETHYL)-4H-PYRAN-4-ONE)

Abstract

Photoredox reactions occurring in irradiated methanolic solutions of the complexes [Fe(R-salen)(Br-koji)], (where R-salen2~ are N,N’-ethylenebis(R-salicylideneiminato) ligands (R = H, 5-F, 5-C1, 5-Br, 3-OCH3, 4-OCH3 or 5-OCH3); Br-kojr is the anion of 5-hydroxy-2-(bromomethyl)-4H-pyran-4-one) have been investigated and their mechanism proposed. The complexes are redox stable in the dark. Continuous irradiation of such solutions in the region of intraligand (IL) and/or ligand-to-metal charge transfer (LMCT) transitions causes a series of photophysical and photochemical deactivations leading to the photoreduction of Fe(III) to Fe(II) and formation of formaldehyde CH20 as final producis. Using EPR spin trapping technique -CH2OH radicais were identified in the irradiated solutions. Both the tetradentate R-salen2~ and bidentate bromokojato Br-kojr ligands behave as innocent moieties virtually not participating in the redox processes of the complexes. The efficiency of the photoredox processes, expressed through the integral quantum yield of Fe(II) formation, (Fen) substantially decreases with increasing wavelength of the incident radiation, and is slightly influenced by the peripheral groups R of the R-salen2~ ligands. A mechanism of the primary photochemical and subsequent thermal reactions is proposed