Mechanism of Ferric Oxalate Photolysis

Iron­(III) oxalate, Fe³⁺(C₂O₄)₃^3–, is a photoactive metal organic complex found in natural systems and used to quantify photon flux as a result of its high absorbance and reaction quantum yield. It also serves as a model complex to understand metal carboxylate complex photolysis because the mechanism of photolysis and eventual production of CO₂ is not well understood for any system. We employed pump/probe mid-infrared transient absorption spectroscopy to study the photolysis reaction of the iron­(III) oxalate ion in D₂O and H₂O up to 3 ns following photoexcitation. We find that intramolecular electron transfer from oxalate to iron occurs on a sub-picosecond time scale, creating iron­(II) complexed by one oxidized and two spectator oxalate ligands. Within 40 ps following electron transfer, the oxidized oxalate molecule dissociates to form free solvated CO_2(aq) and a species inferred to be CO₂^• – based on the appearance of a new vibrational absorption band and ab initio simulation. This work provides direct spectroscopic evidence for the first mechanistic steps in the photolysis reaction and presents a technique to analyze other environmentally relevant metal carboxylate photolysis reactions.