A Theoretical 4f Field Effect of Rare-Earth Complexes on Dinitrogen Reduction

Given the importance of rare-earth complexes in small-molecule activation, such as dinitrogen reduction, a comprehensive understanding of their bonding properties and interactions is crucial. However, a lack of knowledge of 4f interactions may limit further development of lanthanide compounds. Herein, the electronic structures of 30 rare-earth dinitrogen complexes (Ln2N2) were investigated by the all-electron DFT method. The CASSCF/NEVPT2 method was also employed to determine stable states. The predicted anomalous trend of Ln2N2 complexes was found to be under the control of significant 4f interactions classified by 4F and 4F/5D groups. Further energy decomposition analysis reveals that the trend is dominated by Pauli repulsion with slight assistance of orbital interaction, which demonstrates the crucial 4f field effect. Furthermore, 4f stabilizations of low-valence rare-earth metals would weaken the dinitrogen reduction due to the internal electron transfer for half/full occupied electrons, leading to the transformation from [Ln3+]-N22--[Ln3+] to [Ln2+]-N20-[Ln2+] complex. Reducing Pauli repulsion of 4f electrons can also stabilize the reductive dinitrogen. This uncovered 4f field effect may provide a theoretical foundation for their applications in catalysis and novel materials.