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 (LMe) and N,N′-bis­(2,6-diisopropylphenyl)-2,2–6,6-tetramethylheptane-3,5-diiminate (LtBu) toward ThX4(THF)4 (X = Br, I) and UCl4 has been investigated. The reaction between K­[LMe] and ThX4(THF)4 (X = Br, I) afforded the mono­(β-diketiminate)­thorium­(IV) halide complexes (LMe)­ThX3(THF) (X = Br (7), I (8)). The same reaction carried out with the more sterically demanding K­[LtBu] gave (LtBu)­ThBr3(THF) (9) and (LtBu)­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[LMe] or K­[LtBu]. However, complex 9 was shown to react with smaller anions such as K­[C5H4Me] to give the mixed-ligand methylcyclopentadienyl β-diketiminate complex (LtBu)­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 LtBu framework. In a similar manner, both K­[LMe] and K­[LtBu] were shown to react with UCl4 to give the corresponding mono­(β-diketiminate)­uranium­(IV) chloride complexes (LMe)­UCl3(THF) (12) and (LtBu)­UCl3 (13). Complex 13 represents the first example of a uranium complex featuring the LtBu framework. Efforts to prepare the bis­(β-diketiminate)­uranium­(IV) complex (LMe)2UCl2 by reacting 2 equiv of K­[LMe] with UCl4 led instead to the interesting cationic diuranium complex [{(LMe)­(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 LMe complexes and is asymmetrically bound to the actinide metal in the LtBu 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[LMe], (LMe)­ThI3(THF) (8), and (LMe)­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.