Evaluation of Chemical Bonding in Actinyl(VI/V) Oxo-Crown-Ether Complexes for Actinide Series from Uranium to Curium

The separation and management of nuclear waste is one of the problems that needs to be solved urgently, so finding a new radiation-proof and durable extractant to deal with nuclear waste is a difficult but desirable task. Since the successful isolation of the first pentavalent plutonium crown ether complex recently (Wang et al. CCS Chem. 2020, 2, 425–431), complexes with actinyl­(V/VI) inserted into the cavity of 18-crown-6 ether (oxo-18C6), as well as their bonding character, need to be explored. Here we present a series of novel crown ether complexes containing actinyl­(V/VI) and oxo-18C6 via computational prediction and analysis. On the basis of the calculations, actinyl­(V/VI) are thermodynamically feasible and can be stabilized by oxo-18C6 ligand via six dative bonds between An ions and the oxo-18C6 O atoms in the “insertion” structure of [AnO₂(18C6)]^2+/+ complexes. The stability of actinyl­(VI) species generally falls at minor actinides, ascribed to the reduced highest possible oxidation states of curium, which is mainly attributed to the mixing of bonding orbitals and non-bonding orbitals as well as the increase of occupation on partially 5f antibonding orbitals. It is found that the interactions between the actinyl­(V/VI) and oxo-18C6 are mainly electronic interactions, with the well-known covalency contributions generally decreasing from uranium to curium due to energy degeneracy and spatial orbital contraction. This work would give a basic understanding of the coordination chemistry of actinyl­(V/VI), which also provides inspirations on the design of new extractants for actinide separations.