Chemical structure and bonding in a thorium(iii)-aluminum heterobimetallic complex

Thorium sits at a unique position on the periodic table. On one hand, there is little evidence that its 5f orbitals engage in bonding as they do in other early actinides; on the other hand, its chemistry is distinct from Lewis acidic transition metals. To gain insight into the underlying electronic structure of Th and develop trends across the actinide series, it is useful to study Th(iii) and Th(ii) systems with valence electrons that may engage in non-electrostatic metal-ligand interactions, although only a handful of such systems are known. To expand the range of low-valent compounds and gain deeper insight into Th electronic structure, we targeted actinide bimetallic complexes containing metal-metal bonds. Herein, we report the syntheses of Th-Al bimetallics from reactions between a di-tert-butylcyclopentadienyl supported Th(iv) dihalide (Cp^‡₂ThCl₂) and an anionic aluminum hydride salt [K(H₃AlC(SiMe₃)₃) (1)]. Reduction of the [Th(iv)](Cl)-[Al] product resulted in a [Th(iii)]-[Al] complex [Cp^‡₂Th(μ-H₃)AlC(SiMe₃)₃ (4)]. The U(iii) analogue [Cp^‡₂U(μ-H₃)AlC(SiMe₃)₃ (5)] could be synthesized directly from a U(iii) halide starting material. Electron paramagnetic resonance studies on 4 demonstrate hyperfine interactions between the unpaired electron and the Al atom indicative of spin density delocalization from the Th metal center to the Al. Density functional theory and atom in molecules calculations confirmed the presence of An→Al interactions in 4 and 5, which represents the first examples of An→M interactions where the actinide behaves as an electron donor.