Beyond CO2•-: Elusive Relaxation and Distinct Redox-Upconverting Intermediates in Ferrioxalate Photolysis via...

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.