Iron and oxygen interact in many important processes throughout the human body. The best-known example is hemoglobin, where iron binds dioxygen, meaning two oxygen atoms joined together, so oxygen can be carried through the bloodstream.
Iron also forms highly reactive compounds known as iron oxos. These compounds are involved in other biological functions, including reactions carried out by liver enzymes that help break down drugs.
Exploring Oxygen Chemistry Beyond Iron
Rice University chemist Raúl Hernández Sánchez wanted to know whether oxygen could form similar compounds with a different group of metals found near the bottom of the periodic table. These are known as f-block metals. Lanthanides occupy the upper row, while actinides appear below them.
Hernández Sánchez proposed that if lanthanides could bind oxygen in the right way, they might produce highly reactive lanthanide oxo compounds. Such molecules could potentially serve as synthetic alternatives to iron oxos and give small molecule chemists new tools for studying reactions related to biology.
A major obstacle stood in the way. F-block metals, particularly lanthanides, were not believed 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 the Journal of the American Chemical Society, Hernández Sánchez and his colleagues reported a method that allowed dioxygen to form pi interactions with neodymium, a lanthanide metal. The process made it possible to create lanthanide oxo compounds.
A Molecular Basket for Neodymium
“We had a ligand platform that we developed a few years ago,” said Hernández Sánchez, an assistant professor of chemistry. “You can think of it as a basket that allows us to capture metals and position them in ways to encourage specific types of bindings.”
Each molecular basket was large enough to hold a single f-block metal atom. The researchers placed two of these baskets opposite one another. Between them, they arranged six atoms, including a dioxygen molecule, that connected the two neodymium atoms.
This arrangement created an octacoordinate ligand environment, giving the scientists a way to fine-tune the positions of the metals.
“Once we had the lanthanide in our ligand basket, we started to explore its reactivity to small molecule substrates until we found the right conditions to find dioxygen in an unprecedented fashion,” said Hong-Lei Xu, a postdoctoral researcher and first author on the paper.
Unlocking a New Type of Oxygen Bonding
Under the right conditions, neodymium and dioxygen formed pi interactions that had previously been considered unlikely. The reaction produced a lanthanide oxo molecule.
Researchers can now investigate whether these highly reactive compounds could replace iron oxos in synthetic chemistry. They can also test whether lanthanide oxos offer capabilities that iron-based compounds do not.
Although the study focused only on neodymium, Hernández Sánchez and his team believe the same ligand scaffold could support similar reactions with most lanthanides and probably actinides as well.
“The ability to bind dioxygen to f-block metals and cleave the bond between the two oxygen atoms allows us to potentially unveil highly reactive lanthanide oxos and form high value-added chemicals. We could open a new chapter in the chemistry of lanthanides,” Hernández Sánchez said.
The research was supported with startup funding from Rice University, the Robert A. Welch Foundation and a Welch Foundation Grant (C-2142-20230405).