Molecular Complexes of Manganese and Chromium as Catalysts for Energy Relevant Reactions: Understanding Reactivity and Mechanisms Through the Application of Electrochemical and Spectrochemical Techniques

The reduction of dioxygen (O2) to water (H2O) or hydrogen peroxide (H2O2) has importance in biological systems, oxidative functionalization reactions, and fuel cells. Herein, the synthesis and electrocatalytic reduction of dioxygen by a molecular manganese (III) complex with a tetradentate dianionic bipyridine-based ligand is described. This complex is competent for the reduction of dioxygen to H2O2 with 81±4% Faradaic efficiency. To understand the mechanism in greater deatil, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H2O2 by an ECEC mechanism, with a strong dependence on the pKa of the proton donor. Under spectrochemical conditions, where the homogeneous reductant decamethylferrocene is used, H2O2 is instead produced via a disproportionation pathway, which does not show a strong acid dependence. Using this Mn-based electrocatalyst with p-benzoquinone (BQ) as an electron-proton transfer mediator (EPTM) precursor for oxygen reduction with 2,2,2-trifluoroethanol (TFE-OH) present as a weak Brønsted acid, quantitative selectivities for the four-electron/four proton reduction product H2O are observed.
Alternatively, the electrocatalytic reduction of CO2 by an earth abundant transition metal catalyst continues to represent an appealing method for addressing global climate change. Herein, a molecular chromium complex with a 2,2′-bipyridine-based ligand capable of selectively transforming CO2 into CO with phenol as a sacrificial proton donor at turnover frequencies of 5.7±0.1 s–1 with high Faradaic efficiency (96±8%) and low overpotential (110 mV) is described. Utilizing a mediator, dibenzothiophene-5,5-dioxide (DBTD), can enhance the reactivity of this Cr complex and also enable new catalytic reactivity where the electrocatalytic reduction of CO2 to CO can be carried out, by the Cr complex and DBTD, in the absence of an added proton donor. The catalytic mechanism was analyzed through chemical and electrochemical experiments, as well as through computational DFT analyses.