Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O₂ Reduction with a Mononuclear Cobalt Porphyrin Catalyst

Abstract

The selective reduction of O 2 , typically with the goal of forming H 2 O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por OMe ) (por OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e – /2H + or 4e – /4H + reduction of O 2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O 2 reduction to H 2 O 2 or H 2 O in DMF are determined and exhibit a Nernstian dependence on the acid p K a , while the Co III/II redox potential is independent of the acid p K a . The reaction product, H 2 O or H 2 O 2 , is defined by the relationship between the thermodynamic potential for O 2 reduction to H 2 O 2 and the Co III/II redox potential: selective H 2 O 2 formation is observed when the Co III/II potential is below the O 2 /H 2 O 2 potential, while H 2 O formation is observed when the Co III/II potential is above the O 2 /H 2 O 2 potential. Mechanistic studies reveal that the reactions generating H 2 O 2 and H 2 O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus p K a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O 2 to H 2 O.

Combining scaling relationships overcomes rate versus overpotential trade-offs in O 2 molecular electrocatalysis

“…This high selectivity for H 2 O is similar to other iron porphyrin catalysts under comparable conditions (5,7,11). With AcOH/AcO − buffer, catalysis occurs at potentials below the equilibrium potential for O 2 to H 2 O 2 and thus requires thermodynamic selectivity for H 2 O [cf., (9)].…”

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

“…The reaction of Co II -TPFP and O 2 leads to the formation of a formally Co III -O 2 c À species. 60,61 This negatively charged O 2 -adduct unit is likely to interact with the positively charged Co III -TPP unit. Such through-space charge interaction effects have been demonstrated to assist the formation and stabilization of charged intermediates.…”

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

“…The product selectivity for ORR for each catalyst can then be calculated and is plotted as a function of applied potential in Figure B. Co‐TPP displays minimal product selectivity with around 50 % Faradaic efficiency for H 2 O 2 , which is common with mononuclear cobalt porphyrins and may be attributed to catalyst aggregation that creates intermolecular active sites that can catalyze the off‐pathway 4 e − reduction of O 2 into H 2 O. The selectivity of porphyrin ORR catalysts is also known to be highly dependent on the proton source and catalyst medium . In contrast, both Co‐PB‐1(6) and Co‐rPB‐1(6) show high selectivity for the 2 e − reduction of O 2 into H 2 O 2 , attaining 100 % H 2 O 2 generation near the E cat/2 .…”

“…[1] Using electricity offers the possibility to drive this process with sustainable energy input, and in the context of molecular catalysts, structural and functional mimics of cytochrome c oxidase enzymes that perform ORR in biology have focused on the selective formation of H 2 O to better understand multielectron small molecule activation and develop efficient fuel cells. [2] Alternatively, H 2 O 2 is a valuable oxidant, energy carrier, and commodity chemical, and using electricity for H 2 O 2 production presents an environmentally green alternative to the traditional anthraquinone process for its synthesis, [3] with the key challenge to avoid further reduction to more thermodynamically favored H 2 O product. Along these lines, molecular catalysts offer a precise approach to active site engineering, and elegant strategies have been devised to guide ORR to the 4 e À pathway, including constructing bimetallic platforms, [4] appending intramolecular proton sources, [5] and integrating electron reservoirs.…”