Crown ether complexes of actinyls: a computational assessment of AnO₂(15-crown-5)²⁺(An = U, Np, Pu, Am, Cm)

Abstract

For further fundamental understanding of the nature and extent of covalency in actinyl-ligand bonding, and the benefits that this may have in the design of new ligands for nuclear waste separation, there is burgeoning interest in the nature of actinyl complexes with polydentate or multiple-point-donor ligands, such as crown ethers. There are few cases of structurally authenticated molecular actinyl-crown bonds under ambient conditions. We report here the computational characterization of AnO-(15-crown-5) complexes, where An = U, Np, Pu, Am, and Cm, and 15-crown-5 is the cyclic polyether ligand with five ether oxygen atoms. In the gas-phase complex, the actinyl group is located inside of the crown ether, tilted slightly out of the plane of the five equatorial oxygen atoms that coordinate the actinide metal center. The actinyl-cyclic ether complexes are found to exhibit a conventional conformation, with typical An-O and An-O distances and angles. A striking result is the enhanced stability of the insertion complex for UOversus NpO, PuO, AmO and CmO, which is evaluated in the context of An-O binding strengths (esp. bonding covalency), and may have ramifications for the utility of actinyl-crown complexes in separation applications.

Destruction of the Uranyl Moiety in a U(V) “Cation–Cation” Interaction

“…The scalar-relativistic Stuttgart energyconsistent pseudopotential with 32-valence-electrons and the associated ECP60MWB_SEG valence basis set 52 were used for U, and Dunning's correlation consistent all-electron basis sets with polarized triple-zeta (cc-pVTZ) 53 were used for O, C and H. This methodology has been successfully applied to other actinide systems. 17 Geometry optimizations were performed without symmetry restrictions and were followed by vibrational frequency analysis to assign optimized structures as local minima or saddle points. Reaction energies were obtained by combining electronic energies with zero-point vibrational energy corrections.…”

“…14 As a result, crown ethers have potential for actinide partitioning from nuclear waste, [15][16] particularly via size-specificity. [17][18][19][20] Gas-phase actinide crown ether complexes, including of uranyl, present a means to elucidate factors that affect complexation and coordination, including geometric effects that may be supported by distinctive crown geometries. 18,20 In the present work we extend this general line of inquiry from mononuclear uranyl-crown complexes to dimeric uranyl complexes, with an aim to understand how crown ethers might act as bridging ligands and support novel bonding motifs such as cation-cation interactions.…”

Periodic Trends in Actinyl Thio-Crown Ether Complexes

“…ligand as shown in our previous work. [38] Upon coordination to AnO 2 2+ , the An−S L interactions are mainly ionic, with rather weak An−S L covalent interactions which feature a decreasing trend of σ SL1-An bonding in the equatorial plane.…”

“…[35,36] Recently, several studies of crown ethers, both experimental and theoretical, have reported on the structural characterization, [37] the equatorial bonding properties of the actinyl cations, and bonding trends across the actinyl series. [6,38] However, in-cavity complexes and the corresponding bonding features between thio-crown ethers and f-elements are essentially unexplored.…”

New theoretical insights into high-coordination-number complexes in actinides-centered borane

“…Furthermore, the reduced 5f energy levels as well as the increasing orbital interaction between B 2pz and An 5f orbitals when traversing the actinide series from Th to Cm consequently, reveals the An-B bonds generally transit from strengthening to weakening at An = U, which has been a general trend in actinide chemistry emerging in accordance with the atomic number of the actinides. [64][65][66][67] Besides, the net consequence of these orbital interactions upon formation of the An(BH)24 complex can be basically viewed as the An valence orbitals being destabilized energetically in Fig. 8, leading to An cations in the complexes.…”