Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand

Direct comparison of reactivity on fullyanalogous U(IV)and Th(IV) complexes supported by a dianionic ligand toward TMS3SiK through a salt elimination strategy was reported, andthe two gave completely different products. A theoretical calculationwas performed to demonstrate substantial differences in reactivities.The results provide a platform for constructing new actinide complexes. Although synthesis, reactivity, andbonding of U(IV) and Th(IV)complexes have been extensively studied, direct comparison of fullyanalogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supportedby the tetradentate pyridine-decorated dianionic ligand N2NN & PRIME; (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine).Although 1-U and 1-Th are structurally verysimilar, they display disparate reactivities with TMS3SiK(tris(trimethylsilyl)silylpotassium). The reaction of (N2NN & PRIME;)UCl2 (1-U) and 1 equiv of TMS3SiK in THF unexpectedly formed [Cl(N2NN & PRIME;)U](2)O (2-U) featuring an unusual bent U-O-Umoiety. In contrast, a salt elimination reaction between (N2NN & PRIME;)ThCl2 (1-Th) and 1 equiv of TMS3SiK led to thorium complex 2-Th, in which thepyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azidecomplex 3-Th by reaction with NaN3. The complexeswere characterized by X-ray crystal diffraction, solution NMR, FT-IR,and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a keyintermediate for promoting the cleavage of the C-O bonds ofTHF. The inaccessible nature of Th(III) as an intermediate oxidationstate explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprisetetravalent actinides, this is an unusual case of very disparate reactivitydespite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of otherdinuclear actinide complexes with novel reactivity and properties.