Amplification of Lanthanide f-Orbital Differences by Redox Noninnocent Semiquinone Binding

A deeper understanding of the electronic structure of the lanthanides (Ln) is essential to harness their technological applications and enable responsible recovery/separation. Here, an isostructural series of tris(3,5-dimethyl-1-pyrazolyl)borate (Tp*)-supported LnIII adducts of a redox noninnocent 3,5-di-tert-butyl-o-semiquinone (DTBSQ) ligand, (Tp*)2Ln(DTBSQ), is characterized experimentally and computationally for all the early lanthanides La-Gd, except Pm. Computational simulations, together with spectroscopic measurements on selected systems, show the metals retain the trivalent oxidation state despite semiquinone binding with weak metal-radical coupling. While LnIII-DTBSQ bond distances decrease with decreasing LnIII ionic radii, theory suggests increased 4f-orbital mixing driven by the energetic accessibility of the f-shell, with significant radical contributions to the bonding, occurs for Sm/Eu. Orbital analyses and multireference calculations confirm these effects originate from metal-ligand orbital energy matching rather than through-space overlap. The LnIII 4f interactions are more covalent with the radical ligand than the Tp*, showing remarkably low 6s character. This contrasts with other common ligand-Ln studies and suggests that the DTBSQ-based orbitals have the appropriate symmetry and energy to preferentially interact with the 4f orbitals over the 6s orbital. This work provides new insights into Ln-radical interactions, revealing bonding characteristics for potential selective f-element chelators and extension into actinide chemistry.