Effects of wavelength and substituents on iron(III) photoreduction in trans-[Fe(R-sal-R‵-en)(CH3OH)(NCS)] complexes

Abstract

Photoredox reactions in irradiated methanolic solutions of trans-[Fe(R′-sal-R2-en) (CH3OH)(NCS)] complexes, where R′-sal-R2-en2− are tetradentate open-chain N,N′-1,1-R2-ethylenebis(R′-salicylideneiminato) ligands (R = H or CH3; R′ = H, 5-Cl, 5-Br, 3,5-Br2, 3-OCH3, 4-OCH3), have been explored and a mechanism suggested. The complexes are redox-stable in the dark in methanol. Continuous irradiation of solutions in the region of intraligand (IL) or ligand-to-metal charge transfer (LMCT) transitions leads to photoreduction of Fe(III) to Fe(II) and formation of formaldehyde (CH2O). Formation of polystyrene-containing bonded NCS, when irradiating the complexes in the presence of styrene used as a radical scavenger, indicates that the primary photoreduction of Fe(III) to Fe(II) is accompanied by oxidation of NCS− to the •NCS radical. R′-sal-R2-en ligands have little effect on the photoinduced redox processes. The quantum yield of Fe(II) formation, ΦFe(II), as a quantitative parameter of photoredox efficiency, decreases significantly with increasing wavelength of incident radiation, and is slightly influenced by the peripheral groups, R, of R′-sal-R2-en.