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

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.