Photochemical and spectroscopic studies of complexes, of iron(III) with citric acid and other carboxylic acids

The quantum yields for the photoreduction of iron(III) carboxylate complexes vary with the nature of the carboxylate ligand and solution pH. With [carboxylate]=0.05 M, [Fe(III)]=0.30 mM and pH=2.9, the quantum yields are in the order oxalate (0.32)>tartrate>malate>citrate>isocitrate>succinate>formate (0.12). Fe(III) acetate shows no photoactivity. The photoreduction of Fe(III) to Fe(II) is accompanied by the oxidative decarboxylation of the carboxylate ligand, and can even be observed in the solid state. The efficiency of the photoreduction reaction in solution depends on two factors: the pH and the initial ligand-to-metal ratio. For a lower ligand:Fe(III) ratio (=5; [carboxylate]=0.0015 M, [Fe(III)]=0.30 mM) the order from highest to lowest is oxalate>tartrate>citrate>malate> isocitrate when the pH of the reaction media is 2.7. Increasing the pH to 4.0 leads to 50% increases in the quantum yields for all listed carboxylates except oxalate, which decreases by 50%. More detailed studies of pH and ligand/iron ratio were done using citric and isocitric acids. The pH dependence is interpreted in terms of a photoactive Fe(III) citrate dimer formed above pH 2 and a photo-inactive monomer present between pH values of 0.5 and 3.0. Magnetic susceptibility data collected as a function of solution pH show that the paramagnetism of the iron carboxylate solutions decreases with increasing pH, presumably because of increased FeFe coupling. The organic intermediate in the photochemical decomposition of Fe(III) citrate can be monitored by HPLC and is shown to be acetone dicarboxylic acid (ADA). The ultimate decarboxylation product of Fe(III) citrate is acetone.