Amplification of Lanthanide - f-orbital differences by a Redox-Non-Innocent Semiquinone

A deeper understanding of lanthanides (Ln) fundamental electronic structure is essential to both fully harness their technological applications and to enable their environmentally responsible recovery and separation. Molecules in which electrons can occupy and interact with the (f-) orbitals of the metal and in a redox-active ligand offer an exceptional chance to tune these physicochemical properties. Here, an isostructural series of tris(3,5-dimethyl-1-pyrazolyl)borate (Tp*)-supported LnIII adducts of a redox non-innocent 3,5-di-tert-butyl-o-semiquinone (DTBSQ) ligand, (Tp*)2Ln(DTBSQ), is characterized experimentally and computationally for the first half of the series (La-Gd, except Pm), despite the redox activity of the semiquinone ligand. While the overall LnIII-DTBSQ bond distances decrease in line with decreasing LnIII ionic radii, theory suggests increased 4f -orbital mixing in the LnIII-DTBSQ are driven by the energetic accessibility of the f -shell. Deviations from trends are noticed experimentally and computationally, with critical radical contributions affecting the SmIII and EuIII bonding. Orbital analyses and multireference calculations confirm these originate from metal-ligand orbital energy matching, rather than through-space overlap. The interactions of the LnIII 4f orbitals are much larger with the polarizable radical ligand than with the ionic ancillary Tp* ligands, and show remarkably low 6s content, in contrast to all other common Ln-bound ligands studied, suggesting 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 fundamental insights into Ln-radical interactions, which, despite being predominantly ionic, reveal unusual bonding characteristics that hold potential for the design of new selective f-element chelators, and extension into actinide chemistry.