Photolysis of Fe(III) carboxylato complexes: Fe(II) quantum yields and reaction mechanisms

• A number of quantum yields for the photolysis of iron-carboxylato complexes have been determined.
• Reaction mechanisms are discussed.
• The quantum yields are of interest in environmental chemistry.

Fe(III) carboxylato complex photochemistry can be of interest for environmental aqueous systems, oxidative processing of wastewater or laboratory photochemical applications in general. A comprehensive dataset of Fe2+ quantum yields from the photolysis of aqueous Fe(III) complexes with malonate, succinate, glutarate, dl-tartrate, tartronate, gluconate, dl-lactate, dl-malate, pyruvate and glyoxalate has been measured. Irradiation techniques included single laser flash photolysis at 308 and 351 nm and continuous photolysis with a Hg(Xe) lamp-monochromator system at 313, 366, 405 and 436 nm. Complexes with ligands having a higher oxygen to carbon ratio tend to exhibit better photoreduction ability. Ligands containing OH, keto or diol functional groups in the α-position exhibit higher quantum yields than unsubstituted carboxylates (R-CH2-COOH). Generally, dissolved O2 lowers the Fe2+ quantum yield but at certain wavelengths, for some ligands this is the opposite. The influence of transient decay pathways and secondary red-ox reactions including interactions with dissolved O2 has been investigated for Fe(III) glyoxalato complexes using kinetic simulations. Some complexes show a dependence of Fe2+ quantum yield on the irradiation energy.