Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2(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 1-Uand 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N2NN′ (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-Uand 1-Thare structurally very similar, they display disparate reactivities with TMS3SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N2NN′)UCl2(1-U) and 1 equiv of TMS3SiK in THF unexpectedly formed [Cl(N2NN′)U]2O (2-U) featuring an unusual bent U–O–U moiety. In contrast, a salt elimination reaction between (N2NN′)ThCl2(1-Th) and 1 equiv of TMS3SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Thserves as a synthon for preparing dimetallic bis-azide complex 3-Thby reaction with NaN3. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-Ufrom 1-Usuggest 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 1-Thversus 1-U. Given that reactants 1-Uand 1-Thand products 2-Uand 2-Thall comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-Uand 3-Thprovide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.