Electronically Tunable Low-Valent Uranium Metallacarboranes

Uranium metallocenes have played a pivotal role in advancing the understanding of low-valent uranium chemistry since the inception of this field, and they still find continued use today. Functionalization strategies for cyclopentadienyl (Cp) ligands used in uranium metallocenes have predominately focused on modifying the steric properties of the ligand through the incorporation of alkyl or silyl groups, which offer limited control over the electronic properties. Moreover, due to the flat, two-dimensional nature of Cp, functional groups will affect the coordination geometry of the uranium metallocene and can potentially present challenges in decoupling steric and electronic effects. In comparison, uranium metallacarboranes, which are boron cluster-based metallocene analogues that feature three-dimensional dianionic dicarbollide (dc) ligands, present a versatile platform that is potentially capable of not only stabilizing the low-valent uranium center but also providing control over the electronic properties of the resulting complex without significantly modifying the coordination geometry through the incorporation of a diverse range of groups onto the dc ligand at vertices directed away from the uranium center. In this work, we synthesized a series of uranium metallacarboranes featuring B-functionalized dc ligands with increasingly electron withdrawing aryl groups. A combination of cyclic voltammetry and density functional theory studies confirms that this strategy offers predictable control over the electronic properties of the uranium center. More broadly, this work establishes uranium metallacarboranes as a highly tunable class of complexes potentially capable of unlocking new insights into low-valent uranium chemistry.