By Emma N. Cook.

UVA ChemSciComm

These summaries describe a research project for five different levels of understanding, going from primary school to expert. Here I describe a recent publication:

Cook, E.N.; Dickie, D.A.; Machan, C.W.^# “Catalytic Reduction of Dioxygen to Water by a Bioinspired Non-Heme Iron Complex via a 2+2 Mechanism” J. Am. Chem. Soc. 2021, 143, 16411–16418.

Primary School:

Oxygen is everywhere in our atmosphere and is very important in our lives. For example, we breathe in oxygen so our bodies can turn what we eat into energy, which keeps us alive and able to move. Oxygen is also very important to plants, where they use oxygen to make other substances that keep them alive. There are special chemicals in our bodies that allow us and plants to use oxygen to produce energy. Scientists have been working to understand how these chemicals work with oxygen and have made their own substances to copy them. We have made one of the first substances that looks like one of these special chemicals and have figured out how it turns oxygen into water. It is important for us to understand how oxygen works in nature so that we can use it in a similar way to make energy to power our homes, cars, and technology.  

High School:

Oxygen is important to aerobic life, where it plays important roles in energy conversion. For example, when we breathe in oxygen our bodies use it to generate energy during cellular respiration. Researchers have been working to understand how oxygen reacts in nature and have discovered that we could use oxygen to produce energy. Specific proteins and enzymes in nature react with oxygen, protons and electrons to make water. Scientists have been using them as inspiration for synthetic molecules with similar structures to the ones found in nature to make catalysts, a molecule that speeds up a chemical reaction, to convert oxygen into water. We have made a catalyst and have determined how it reacts with oxygen, protons and electrons to make water. It is important for us to understand oxygen reactivity so that we could use it in alternative energy processes.

College:

Oxygen is critical to aerobic life, where it plays important roles in energy conversion. For example, oxygen reduction to water upon the addition of protons and electrons drives energy release during cellular respiration. Oxygen reduction is a field of interest as it can be used in alternative energy applications and researchers have used nature as inspiration to study oxygen reactivity. Efforts to mimic metalloproteins that bind and transport oxygen, such as hemoglobin, have resulted in many large planar molecular complexes that have four N atoms bound to the metal center. However, there are many enzymes that react with oxygen that have a different structure, generally they are non-planar and contain three N atoms and one O atom bound to the metal center. Inspired by these, we have synthesized one of the first catalysts for oxygen reduction that contains an iron metal center bound to three N atoms and one O atom. Using spectroscopic and electrochemical techniques, we studied the mechanism in which our catalyst reacts with protons, electrons, and oxygen to make the final product, water. These results are important for scientists to better understand how oxygen reacts at different active sites in nature as well as being able to use oxygen efficiently in alternative energy processes.

Graduate Student in the Discipline:

Dioxygen reduction and activation are important in biological energy conversion and can be used in alternative energy applications as well as an abundant oxidant. The field of oxygen reduction (ORR) has been dominated by heme complexes containing planar, macrocyclic complexes containing four N atoms bound to the metal center that are inspired by metalloproteins that bind and transport O₂ in nature, such as hemoglobin. However, there are a number of enzymes containing “non-heme” ligands that bind and reduce O₂. Inspired by this, we have synthesized a bioinspired iron complex (Fe(PMG)(Cl)₂) containing a non-heme, N₃O ligand framework that is electrocatalytically active for the ORR. Under electrochemical and chemical conditions, Fe(PMG)(Cl)₂ selectively produces water via 2+2 mechanism, where O₂ is reduced by 2H⁺/2e^- to H₂O₂, which is further reduced by an additional 2H⁺/2e^– to form two equivalents of water. UV-vis stopped-flow mechanistic studies revealed that a peroxo dimer species is the resting state of the catalyst in solution. Additional electrochemical, spectroscopic, and DFT studies to further elucidate the catalytic cycle revealed that the carboxylate moiety of the ligand hydrogen-bond with the proton donor upon reduction of Fe(III/II).

Expert:

Dioxygen reduction and activation is important for biological energy conversion. Despite the number of non-heme iron enzymes that activate O₂ in nature, there has been a lack of reported mononuclear, non-macrocyclic iron electrocatalysts for ORR. We have synthesized a bioinspired non-heme Fe complex (Fe(PMG)(Cl)₂) with a tripodal [N₃O]^- ligand framework that is electrocatalytically active toward the ORR. Under electrochemical and chemical conditions in acetonitrile solution with acetic acid as a proton source, Fe(PMG)(Cl)₂ selectively produces H₂O via a 2+2 mechanism, where H₂O₂ produced during catalysis is further reduced to two equivalents of water. UV-vis stopped-flow mechanistic studies revealed that an off-cycle peroxo dimer species is the resting state of the catalyst in solution. Spectroscopic analysis of the singly reduced Fe^II(PMG)Cl species showed formation of an Fe(III)-hydroxide species upon exposure to H₂O₂ and O₂. Electrochemical, spectrochemical and DFT studies suggest that hydrogen-bonding of acetic acid with the carboxylate moiety of the ligand upon reduction of Fe(III)/(II) facilitates Cl^- loss and plays an important role in catalysis.