The valorization of CO 2 via photo-or electrocatalytic reduction constitutes a promising approach toward the sustainable production of fuels or value-added chemicals using intermittent renewable energy sources. For this purpose, molecular catalysts are generally studied independently with respect to the photo-or the electrochemical application, although a unifying approach would be much more effective with respect to the mechanistic understanding and the catalyst optimization. In this context, we present a combined photo-and electrocatalytic study of three Mn diimine catalysts, which demonstrates the synergistic interplay between the two methods. The photochemical part of our study involves the development of a catalytic system containing a heteroleptic Cu photosensitizer and the sacrificial BIH reagent. The system shows exclusive selectivity for CO generation and renders turnover numbers which are among the highest reported thus far within the group of fully earth-abundant photocatalytic systems. The electrochemical part of our investigations complements the mechanistic understanding of the photochemical process and demonstrates that in the present case the sacrificial reagent, the photosensitizer, and the irradiation source can be replaced by the electrode and a weak Brønsted acid.

In a previous paper we have demonstrated that the easily-synthesized class of iron(0) cyclopentadienone complexes constitutes a promising catalyst platform for the electrochemical conversion of CO2 to CO and H2O. One of the unusual features of these catalysts is that catalysis proceeds efficiently in aprotic electrolytes in the absence of acidic additives. Herein we present a detailed study of the underlying catalytic mechanisms. Using a combination of FTIR spectroelectrochemistry, DFT calculations, and nonelectrochemical control experiments, we have identified a number of catalytic intermediates including the active species and the product of catalyst deactivation. On the basis of these insights, we have carried out digital simulations in order to decipher the voltammetric profiles of the iron(0) cyclopentadienones. Further control experiments revealed that the anodic oxidation of the electrolyte constitutes the terminal proton source for the formation of CO and H2O. Taken together, our results suggest a competition between two coexisting catalytic pathways, one of which proceeds via a hitherto unknown Fe–Fe dimer as an active species.

The redox sequences of sensitive aliphatic mono-dithiolene complexes [M II (CO) 2 (cydt)(dppe)] (M = Mo, W) were studied with extensive cyclic voltammetrical and spectro-electrochemical (SEC) investigations using UV-vis and IR spectroscopy. With a newly developed matrix factorisation technique, the pure component spectra and temporal evolution of the developed species were ascertained, including sensitive and transient ones. The concentration of all involved compounds at the working electrode as dependent on the respective applied potential was derived with very high accuracy. The assignment of the species' geometries and electronic structures was further verified by quantum chemical calculations. Overall, the combined electrochemical, spectroscopic, quantum chemical and mathematical approach facilitates a comprehensive understanding of the underlying electrochemical transformations and equilibria. Since only a single CV run is needed to obtain all data required to identify the individual species, the method presented here is expected to be applicable to a wide range of complicated electrochemically responsive systems and thus opens a path towards developing a deeper mechanistic understanding of the underlying chemistry.