Intermediates in Photochemistry of Fe(III) Complexes in Water

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The photochemistry of Fe(OH)2+ complex and complexes formed by Fe(III) and pyruvic (Pyr), tartaric (Tart), sulfosalycilic (SSA) and oxalic (Ox) acids in aqueous solutions were studied by means of stationary and nanosecond laser flash photolysis. The application of different scavengers of transient radicals has shown that the hydroxyl radical is the primary photochemical species in photochemistry of the FeOH2+ complex. In the photochemistry of FePyr2+ and FeTart+ complexes a weak absorption was found in the red spectral region which was attributed to [FeII…R-COO]2+ radical complexes. Laser flash photolysis of FePyr2+ and FeTart+ complexes in the presence of methyl viologen (effective scavenger of different free radicals) gave evidence of MV•2+ radical cation formation with concentration as small as ~2% of Fe(III) complex disappeared. The reaction mechanism including inner-sphere electron transfer with the formation of [FeII…R-COO]2+ radical complex and its transformation to the reaction products is proposed. The main photochemical process for FeIII(C2O4)33— complex in aqueous solutions was found to be intramolecular electron transfer from the ligand to Fe(III) ion with the formation of a primary radical complex [(C2O4)2FeII(C2O4•)]3−. The yield of free radical species (i.e., CO2•− and C2O4•−) was found to be less then 6% of FeIII(C2O4)33— disappeared after a laser pulse.