ADF reveals how 4f electrons shape dinitrogen activation by rare-earth metals

Breaking the strong N≡N bond is one of the central challenges in nitrogen chemistry. Rare-earth metals can reduce and activate N₂, but the role of their highly localized 4f electrons in this process has remained difficult to understand.

In a recent JACS study, Wang and co-workers computationally investigated 30 rare-earth dinitrogen complexes across the lanthanide series. Rather than treating the 4f electrons as part of an effective core, the authors used all-electron DFT with ADF to explicitly capture their contribution to Ln–N₂ interactions.

The ADF calculations reveal an unusual periodic trend. Depending on the rare-earth element, N₂ interacts predominantly with 5d orbitals, 4f orbitals, or mixed 4f/5d character. Surprisingly, stronger calculated Ln–N₂ interactions can coincide with longer Ln–N₂ bonds—a counterintuitive behavior traced back to the 4f electrons.

This is where ETS-NOCV energy decomposition analysis in ADF becomes particularly useful. By separating the interaction energy into Pauli repulsion, electrostatic attraction, and orbital interactions, the authors show that the unusual 4f effect is governed primarily by changes in Pauli repulsion, rather than simply by stronger covalent bonding.

The calculations also identify another important effect: for elements such as Eu and Yb, internal electron transfer can favor low-valent Ln²⁺ configurations and effectively deactivate the reduced N₂ unit. Taken together, the analysis suggests concrete ligand-design principles: reducing unfavorable 4f Pauli repulsion and controlling internal electron transfer could help stabilize activated N₂.

Technically, ADF 2019.304 was used for the main DFT calculations with PBE-D3(BJ), ZORA relativistic treatment, Slater-type basis sets, and ETS-NOCV analysis. Selected electronic states were additionally checked with other DFT and multireference methods.