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, Φ_Fe(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.

Keywords:

• Iron(III) complexes
• Photochemistry
• Thiocyanate
• Quantum yield
• Wavelength dependence
• Peripheral group effect

Acknowledgements

The financial support of the Slovak Grant Agency VEGA is (grant No. 1/2452/05) acknowledged.