Iron and oxygen interact in many important processes throughout the human body. The most well-known example is hemoglobin. In hemoglobin, iron combines with dioxygen. This means that the two oxygen atoms have joined together, meaning that the oxygen is transported through the bloodstream.
Iron also forms highly reactive compounds known as iron oxos. These compounds are involved in other biological functions, such as reactions carried out by liver enzymes that help break down drugs.
Exploration of oxygen chemistry beyond iron
Rice University chemist Raul Hernandez Sánchez wanted to know whether oxygen could form similar compounds with different metal groups near the bottom of the periodic table. These are known as f-block metals. Lanthanides occupy the top row, and actinides appear below them.
Hernández-Sánchez proposed that if lanthanides could combine with oxygen in the right way, highly reactive lanthanide oxo compounds could be produced. Such molecules could serve as synthetic substitutes for iron oxo and provide small molecule chemists with new tools to study biologically relevant reactions.
A big obstacle stood in the way. F-block metals, especially the lanthanides, were not thought to interact with small molecules such as oxygen through pi interactions. These interactions are important in many biological materials, including proteins.
In a study published in Journal of the American Chemical SocietyHernández-Sánchez and colleagues reported how dioxygen can form π interactions with the lanthanide metal neodymium. This process enabled the creation of lanthanide oxo compounds.
Neodymium molecular basket
“We had a ligand platform that we developed several years ago,” said Hernández Sánchez, assistant professor of chemistry. “You can think of this as a basket that traps metals and allows them to be placed in a way that promotes certain types of bonding.”
Each molecular basket was large enough to hold a single f-block metal atom. The researchers placed two of these baskets facing each other. Between them, six atoms were placed, including a dioxygen molecule, and two neodymium atoms were bonded.
This arrangement created an eight-coordinated ligand environment and gave the scientists a way to fine-tune the position of the metal.
“Once we put lanthanide into the ligand basket, we started investigating its reactivity towards small molecule substrates until we found the right conditions to find dioxygen in an unprecedented way,” said Hong Lei Hsu, postdoctoral researcher and first author of the paper.
Unlock new types of oxygen bonds
Under the right conditions, neodymium and dioxygen formed a previously unsuspected π interaction. This reaction produced lanthanide oxo molecules.
Researchers can now investigate whether these highly reactive compounds can replace iron oxo in synthetic chemistry. We can also test whether lanthanide oxo provides functionality that iron-based compounds do not.
Although this study focused only on neodymium, Hernández-Sánchez and his team believe that the same ligand backbone could support similar reactions in most lanthanides and possibly actinides.
“The ability to bind dioxygen to f-block metals and break the bond between two oxygen atoms reveals highly reactive lanthanide oxos with the potential to form high-value-added chemicals. It could open a new chapter in lanthanide chemistry,” said Hernandez-Sánchez.
This research was supported by start-up funding from Rice University, the Robert A. Welch Foundation, and a Welch Foundation grant (C-2142-20230405).
