Elucidating the Role of Energy- and Electron-Transfer Processes and the Route to Highly Reduced Products in the Dual IrIII/FeIII Photocatalyzed CO2 Reduction Reaction

Abstract Mechanistic elucidation of a photochemically driven CO2 reduction reaction (CO2RR) using an IrIII photosensitizer and an iron metalloporphyrin reduction catalyst, which has been proposed to transform CO2 into CH4, has been conducted using various steady-state and time-resolved absorption and emission spectroscopic methods, cyclic voltammetry, reaction kinetics, quantum yield measurements, spectroelectrochemistry, and isotope-labeling experiments. A dynamic reactivity landscape was subsequently uncovered where the key photochemical mechanistic steps are altered as a function of reaction progress from an initial photoinduced single electron transfer to an energy transfer mechanism, which is supported by the in situ generation of a ferrous-carbonyl intermediate that was subjected to Stern–Volmer analysis and transient absorption spectroscopy to determine the overall photochemical mechanism using a select IrIII photosensitizer. This alteration of the photochemical mechanistic steps stymies further photoinduced reduction of the ferrous-carbonyl intermediate, which is a proposed on-cycle step toward the photoevolution of CH4. Instead, cyclic voltammetry experiments implicated a plausible CH4 evolution mechanism involving degradation of the porphyrin structure through attack of a second equivalent of the reduced metalloporphyrin catalyst. This mechanistic picture is consistent for the observed formation of ethane under steady-state irradiation conditions, which also does not arise from the CO2RR but instead represents a degradation product of the sacrificial electron donor, triethylamine, following single electron oxidation.