Accessibility of Uranyl-Plutonium Complex Supported by a Polypyrrolic Macrocycle: An Implication for Experimental Synthesis.

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The reaction of (THF)(H2L)(UVIO2) (L is a tetra-anion of polypyrrolic macrocycle) with AnIIICp3 (Cp = cyclopentadienyl) afforded two intriguing cation-cation interaction (CCI) complexes (i.e., uranyl-Np and -U), but did not yield the uranyl-Pu analogue. To complement and extend experimental results, a scalar relativistic density functional theory has been performed on the formation reactions and various relevant properties of (THF)(A2L)(OUO)-An(CpX)3 (A = Li and H; An = Pu, Np, and U; X = Me, H, Cl, and SiMe3). Inspired by a strategy that improves uranyl precursor reactivity, we utilized (THF)(Li2L)(UVIO2) instead to gain a uranyl-Pu complex. Reaction free energy is reduced even to be negative (i.e., undergoing an exergonic process), which provides the thermodynamic possibility for experimental synthesis. This manner is further rationalized by the lithiated precursor showing the increased Li-Oendo bond, uranium oxidation ability (VI → V), and exo-oxo basicity, as well as the lithiated uranyl-Pu product having more amount of electron transfer and a stronger Oexo-Pu bond (i.e., representing the CCI). Electronic structures and electron-transfer analyses reveal a UV-PuIV oxidation state for the new complex. Applying the more reactive lithiated precursor also decreases the formation reaction energies of uranyl-An (An = Np and U) complexes. The second strategy via exploiting substituted Cp to raise the reactivity of the plutonium reactant does not work well.