Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnCl (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand.

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes and , in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand NNN' (1,1,1-trimethyl--(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although and are structurally very similar, they display disparate reactivities with TMSSiK (tris(trimethylsilyl)silylpotassium). The reaction of (NNN')UCl () and 1 equiv of TMSSiK in THF unexpectedly formed [Cl(NNN')U]O () featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (NNN')ThCl () and 1 equiv of TMSSiK led to thorium complex , in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex serves as a synthon for preparing dimetallic bis-azide complex by reaction with NaN. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of from suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of versus . Given that reactants and and products and all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes and provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.