Chemistry has a new answer to an old question: can the rare earths, the f-block metals that fill the lower rows of the periodic table, do what iron does with oxygen? For decades the answer appeared to be no. Iron and its transition-metal neighbors bind oxygen through the sharing of electrons in directional bonds, a flexibility that lets biology build hemoglobin, cytochrome P450 enzymes, and the rest of the oxygen-handling machinery of life. The f-block metals were thought to be too ionic, their electrons too diffuse and buried, to participate in such interactions. A team at Rice University has now shown otherwise, using a molecular basket of their own design to coax the rare earth neodymium into binding and even splitting oxygen in ways that were considered out of reach. The work, published in the Journal of the American Chemical Society, also delivers a surprise about how the oxygen-oxygen bond breaks: the milder chemical reagent does the splitting, while the more powerful one leaves the bond intact.
The platform is a ligand the Rice group introduced in earlier work, a cyclen-based molecule with an eight-nitrogen pocket just large enough to hold a single f-block metal atom. The researchers loaded it with neodymium and exposed the resulting compounds to dry oxygen under reducing conditions, in the presence of two different alkali metals. The results, reported by first author Hong-Lei Xu, postdoctoral researcher Alejandro Fuentes Beltran, and principal investigator Raul Hernandez Sanchez, diverged dramatically depending on which alkali metal was standing by.
With potassium, in the form of the powerful reductant KC8, the reaction produced a cluster in which one oxygen molecule, O2, sits end-on between two neodymium atoms, each held in its basket, with four potassium ions arrayed around the pair. The oxygen-oxygen distance of 1.48 angstroms marks it as a peroxide, a state in which the bond is stretched but intact. End-on binding of this kind had never been seen at an f-block metal; the authors describe it as the first example of a trans-end-on peroxo coordination to a lanthanide. The binding implies a small but real donation of electron density from the neodymium into the oxygen, a backbonding interaction that 4f metals were long assumed incapable of, since they form predominantly ionic bonds. The potassium ions are not spectators: they cradle the peroxide and stabilize a geometry that the metal alone could not hold.
With sodium, the plot inverts. The team used sodium naphthalenide, which is a weaker reductant than KC8, with a smaller reduction potential. The product is a cluster in which the oxygen-oxygen bond has been broken outright, leaving a single oxygen atom, an oxo, buried at the center of an octahedral arrangement of two neodymium and four sodium atoms. Isotopic labeling with oxygen-18 confirmed the assignment, shifting a vibrational band from 424 to 404 wavenumbers, the signature of neodymium-oxygen stretching. The weaker reagent severs the bond that the stronger reagent preserved. The authors attribute the difference not to driving force but to Lewis acidity: the sodium ion is a harder, more acidic partner than potassium, and that acidity, not the reducing power of the reagent, decides whether dioxygen is parked or split.
The oxo itself would ordinarily be unmanageable. Mononuclear lanthanide oxos are so reactive that they are difficult to isolate at all, one reason this chemistry has lagged so far behind transition-metal chemistry. In the new cluster, the oxo is tamed by dimerization and by the four sodium ions that sit in an equatorial plane around it, sharing the burden of the metal’s oxidation. X-ray photoelectron spectroscopy shows the neodymium centers in both clusters sit at slightly higher oxidation levels than in the starting material, consistent with the metals participating in binding rather than merely standing nearby. The result, the authors conclude, demonstrates a synergy between the rare earth and its alkali metal partner that opens a new route into lanthanide peroxo and oxo chemistry.
The wider significance is about the periodic table itself. Small-molecule activation, the business of grabbing a stable molecule like oxygen and doing something useful with it, has been the domain of transition metals, both in biology and in industry. The f-block elements, despite their abundance in modern technology, from magnets to catalysts to the separation chemistry of nuclear fuel, have been largely sidelined from this arena. The Rice work shows the field a path in: put the metal in the right pocket, pair it with the right alkali metal, and reactions that seemed impossible become routine enough to isolate and study. The hope, the authors say, is that highly reactive lanthanide oxos could one day stand in for the iron-based oxidants of biology and chemical manufacturing, or do things iron cannot. A basket built to hold one metal atom has turned out to hold a much larger question about what rare earths are capable of.
References
Hong-Lei Xu, Alejandro Fuentes Beltran and Raul Hernandez Sanchez, Activation of Dioxygen via Neodymium-Alkali Metal Clusters. Journal of the American Chemical Society 148, 12463-12469 (2026). DOI: 10.1021/jacs.5c22234.
Rachel Leeson, A tiny molecular basket unlocks a powerful new kind of chemistry. Rice University / ScienceDaily, 29 July 2026.