Iron and oxygen bind together throughout the body. Most famously, iron binds dioxygen, or two oxygens paired with each other, in hemoglobin that transports oxygen through blood. But iron-oxo compounds, as they're called, are found in many other places throughout the body. For example, the highly reactive iron-oxo is used in liver enzymes that metabolize drugs.
Rice University chemist Raúl Hernández Sánchez was interested in how oxygen could react with other types of metals — ones that reside on the lowest section of the periodic table, known as f-block metals, with lanthanides on the upper row and actinides on the lower. If lanthanides could bind with oxygen, he theorized, it would form a highly reactive lanthanide-oxo compound that potentially could be used as a synthetic replacement for iron-oxo, opening up a new toolbox for small molecule chemists interested in studying these biological reactions.
The only issue is that f-block metals, especially lanthanides, couldn't engage with small molecules like oxygen through pi interactions, a type of interaction essential for biological materials like proteins. In a recent publication in the Journal of the American Chemical Society , Hernández Sánchez and his team described a way to enable pi interactions between dioxygen and a lanthanide metal called neodymium, enabling the creation of lanthanide-oxos.
"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."
The basket was just big enough to hold one f-block metal atom. The research team placed two baskets across from each other with six carefully placed atoms, including a dioxygen molecule, in between, bridging the two neodymium atoms. This creates an octacoordinate ligand environment, which can be used to adjust 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.
The right conditions, contrary to previous thought, enabled pi interactions between the neodymium and dioxygen, resulting in a lanthanide-oxo molecule. Chemists can now start testing to see if these highly reactive molecules can be used as a synthetic replacement for iron-oxo, and if so, what options they offer that iron-oxo does not.
While the paper only looked at neodymium, Hernández Sánchez's team hypothesizes that similar chemistry can be extended to most lanthanides and likely actinides using the same ligand scaffold.
"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.
This research was supported with startup funds provided by Rice University, the Robert A. Welch Foundation and a Welch Foundation Grant (C-2142-20230405).
