Beyond CO2•–: Elusive Relaxation and Distinct Redox-Upconverting Intermediates in Ferrioxalate Photolysis via Multireference Quantum Chemistry

Abstract Ferrioxalate photolysis is critical in geochemistry and engineered advanced oxidation and recently emerges in iron-catalyzed two-electron transfer reduction. Yet the primary excited-state relaxation pathways and distinct redox-upconverting intermediates during photolysis remain uncharacterized. Here, we target these elusive species and mechanisms through multireference quantum chemistry investigations on photolysis of tris(oxalato)ferrate(III) [FeIII(C2O42–)3]3–. We identify multiple relaxation pathways and transient intermediates upon relaxation of the ligand-to-metal charge transfer (LMCT) and ligand-centered states and uncover a localized-to-delocalized state crossover governing C–C bond cleavage. The photolytic intermediate [(C2O42–)2FeII(CO2•–)]3– may undergo either CO2•– elimination or further transformation into other intermediates, including both sextet [(C2O42–)2FeII(–C•O2–)]3–/[(C2O42–)2FeIII(–OC••O2–)]3–/[(C2O42–)2FeIII(–C••O22–)]3–/[(C2O42–)2FeIII–O2–]3– and quartet [(C2O42–)2FeI(–CO2)]3– species. Solvation effect may influence the kinetic formation of these radical species. The ligand excited state relaxation is found to proceed through nonadiabatic crossing to LMCT states, or formation of a ferric diradical intermediate [(C2O42–)2FeIII(–OC•O–)2]3–, which may further either releases CO2•– or regenerates the initial reactant. This work gives additional insights to ferrioxalate photolysis and provides preliminary evidence for iron(III)-oxalate complexes as a facile, cost-effective photochemical platform to access highly reactive species.