The flexible small-cavity macrocycle, trans-calix[2]benzene[2]pyrrolide (H₂L), has been found experimentally to complexate low-valent U^III and U^IV with binding pockets (BP) of bis(arene) (Ar) and bis(pyrrolide) (Pl), respectively. This switchable coordination of the uranium center has been explored using relativistic density functional theory (DFT) in this work. Systematic calculations of [(BP-L)An^m(η-H)_nBH_4-n)]^z+ (BP = Ar and Pl; An = U, Np and Pu; m = III and IV; n = 2 and 3; and z = 0 and 1), labeled as BP-An^m-nH, were carried out. Energetics and geometrical/electronic-structure analyses reveal that the size matching between actinide ions and the binding pocket plays a significant role in determining the energetic ordering of isomers. The relatively large-size U^III and Np^III ions are selectively preferred by the large bis(arene) pocket, yielding the most stable isomer of Ar-An-2H; simultaneously formed δ(An-Ar₂) bonding helps stabilizing the system. In contrast, the small-size Pu^III and An^IV are held by the smaller bis(pyrrolide) to show the energetically favored Pl-An-3H isomer. This size argument is further supported by calculations on the related Th and Pa compounds. The formation reactions of BP-An^m-nH demonstrate an endothermic process when using the H₂L ligand reactant. Applying a more basic alkali ligand (A₂L; A = Li, Na and K) as the reactant significantly reduces the reaction energy and presents thermodynamic possibility to prepare the low-valent actinide complexes. This is in agreement with the experimental synthesis where K₂L was utilized. The redox potentials (E⁰) from tri- to tetravalent actinides were calculated while including both solvation and spin-orbit coupling effects. The highly reductive nature of the U^III complex was manifested by the calculated E⁰ of over 1.1 V.

Zhe Bao

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