Thorium(IV) and Uranium(IV) Halide Complexes Supported by Bulky β‑Diketiminate Ligands

The coordination behavior of the bulky β-diketiminate ligands N,N′-bis­(2,6-diisopropylphenyl)­pentane-2,4-diiminate (L^Me) and N,N′-bis­(2,6-diisopropylphenyl)-2,2–6,6-tetramethylheptane-3,5-diiminate (L^tBu) toward ThX₄(THF)₄ (X = Br, I) and UCl₄ has been investigated. The reaction between K­[L^Me] and ThX₄(THF)₄ (X = Br, I) afforded the mono­(β-diketiminate)­thorium­(IV) halide complexes (L^Me)­ThX₃(THF) (X = Br (7), I (8)). The same reaction carried out with the more sterically demanding K­[L^tBu] gave (L^tBu)­ThBr₃(THF) (9) and (L^tBu)­ThI₃ (11). All attempts to install two β-diketiminate ligands on thorium­(IV) were unsuccessful, giving the mono­(β-diketiminate)­thorium­(IV) halide complex and unreacted K­[L^Me] or K­[L^tBu]. However, complex 9 was shown to react with smaller anions such as K­[C₅H₄Me] to give the mixed-ligand methylcyclopentadienyl β-diketiminate complex (L^tBu)­Th­(C₅H₄Me)­Br₂ (10). Complexes 7–11 represent rare examples of thorium complexes featuring only one β-diketiminate ligand, and complexes 9–11 are the first examples of thorium and halide complexes supported by the L^tBu framework. In a similar manner, both K­[L^Me] and K­[L^tBu] were shown to react with UCl₄ to give the corresponding mono­(β-diketiminate)­uranium­(IV) chloride complexes (L^Me)­UCl₃(THF) (12) and (L^tBu)­UCl₃ (13). Complex 13 represents the first example of a uranium complex featuring the L^tBu framework. Efforts to prepare the bis­(β-diketiminate)­uranium­(IV) complex (L^Me)₂UCl₂ by reacting 2 equiv of K­[L^Me] with UCl₄ led instead to the interesting cationic diuranium complex [{(L^Me)­(Cl)­U}₂(μ-Cl)₃]­[Cl] (14). Complexes 7–14 have been characterized by a combination of ¹H and ¹³C­{¹H} NMR spectroscopy, elemental analysis, electrochemistry, and UV–visible–near-IR spectroscopy. Several complexes have also been characterized by X-ray crystallography, and a discussion of their structures is presented. NMR spectroscopy and the X-ray structures demonstrate that the β-diketiminate ligand is symmetrically bound to the actinide metal in the L^Me complexes and is asymmetrically bound to the actinide metal in the L^tBu complexes. In all cases the actinide­(IV) metal centers lie out of the plane of the β-diketiminate ligand NCCCN backbone by ∼1–2 Å. The electronic spectroscopy data on K­[L^Me], (L^Me)­ThI₃(THF) (8), and (L^Me)­UCl₃(THF) (12) suggest relatively weak metal–(β-diketiminate) ligand bonding interactions, although small perturbations in the characteristics of the β-diketiminate π–π* bands with changes in the the metal ion are consistent with some metal–ligand orbital interactions. This new class of mono­(β-diketiminate)­thorium and -uranium halide complexes promises to provide a robust platform for developing new chemistry of the actinides.