PHOTOCHEMICAL AND SPECTROSCOPIC STUDIES OF COMPLEXES OF IRON(III) WITH CITRIC-ACID AND OTHER CARBOXYLIC-ACID

The quantum yields for the photoreduction of iron(III) carboxylate com plexes 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 rtrate>malate>citrate>isocitrate>succi nate>formate (0.12). Fe(III) acetate shows no photoactivity. The photo -reduction of Fe(III) to Fe(II) is accompanied by the oxidative decarb oxylation of the carboxylate ligand, and can even be observed in the s olid 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>citrat e>malate> isocitrate when the pH of the reaction media is 2.7. Increas ing the pH to 4.0 leads to 50% increases in the quantum yields for all listed carboxylates except oxalate, which decreases by 50%. More deta iled studies of pH and ligand/iron ratio were done using citric and is ocitric acids. The pH dependence is interpreted in terms of a photoact ive Fe(III) citrate dimer formed above pH 2 and a photo-inactive monom er present between pH values of 0.5 and 3.0. Magnetic susceptibility d ata collected as a function of solution pH show that the paramagnetism of the iron carboxylate solutions decreases with increasing pH, presu mably because of increased Fe-Fe coupling. The organic intermediate in the photochemical decomposition of Fe(III) citrate can be monitored b y HPLC and is shown to be acetone dicarboxylic acid (ADA). The ultimat e decarboxylation product of Fe(III) citrate is acetone.