Infrared Ion Spectroscopic Characterization of the Gaseous [Co(15-crown-5)(H2O)]2+ Complex

We report fingerprint infrared multiple-photon dissociation spectra of the gaseous monohydrated coordination complex of cobalt(II) and the macrocycle 1,4,7,10,13-pentaoxacyclopentadecane (or 15-crown-5), [Co(15-crown-5)(H2O)]2+. The metal-ligand complexes are generated using electrospray ionization, and their IR action spectra are recorded in a quadrupole ion trap mass spectrometer using the free-electron laser FELIX. The electronic structure and chelation motif are derived from spectral comparison with computed vibrational spectra obtained at the density functional theory level. We focus here on the gas-phase structure, addressing the question of doublet versus quartet spin multiplicity and the chelation geometry. We conclude that the gas-phase complex adopts a quartet spin state, excluding contributions of doublet species, and that the chelation geometry is pseudo-octahedral with the six oxygen centers of 15-crown-5 and H2O coordinated to the metal ion. We also address the possible presence of higher-energy conformers based on the IR spectral evidence and calculated thermodynamics.

Trends in the Electronic Structure and Chemical Bonding of a Series of Porphyrinoid-Uranyl Complexes

In this paper, we have explored the relativistic density functional theory study on a series of deprotonated porphyrinoid (Lⁿ) complexes of uranyl to investigate the geometrical structures and chemical bonding. The ligands bound with uranyl in the 1:1 complexes [UO₂(Lⁿ)]^x (n = 4, 5, 6; x = 0, -1, -2), showing more thermodynamic stability for "in-cavity" structures of L⁵ and L⁶ than that of the "side-on" structure of L⁴ and an increase in stability with the increase of negative charges, L^2- < L^3- < L^4-. Among the six ligands, the cyclo[6]pyrrole presents the best selectivity toward uranyl. Based on chemical bonding analyses, the U-N_L bond in the in-cavity complexes adopts a typical dative N_L → U bond with mainly ionic bonding and significant covalency, which comes from the significant orbital interaction of U 5f_ϕ6d_δ7s hybrid AOs and N_L 2p-based MOs. This work provides a systematic understanding of the coordination chemistry in uranyl pyrrole-containing macrocycle complexes and the nature of chemical bonding in such systems, which may provide inspirations for the future design of synthetic targets that could be relevant to actinide separations or in the remediation of spent nuclear fuel.

Theoretical Probing of Size-Selective Crown Ether Macrocycle Ligands for Transplutonium Element Separation

Effective separation and recovery of chemically similar transplutonium elements from adjacent actinides is extremely challenging in spent fuel reprocessing. Deep comprehension of the complexation of transplutonium elements and ligands is significant for the design and development of ligands for the in-group separation of transplutonium elements. Because of experimental difficulties of transplutonium elements, theoretical calculation has become an effective means of exploring transplutonium complexes. In this work, we systematically investigated the coordination mechanism between transplutonium elements (An = Am, Cm, Bk, Cf) and two crown ether macrocyclic ligands [N,N'- bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H₂bp18c6) and N,N'-bis[(6-methylphosphinic-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H₂bpp18c6)] through quasi-relativistic density functional theory. The extraction complexes of [Anbp18c6]⁺ and [Anbpp18c6]⁺ possess similar geometrical structures with actinide atoms located in the cavity of the ligands. Bonding nature analysis indicates that the coordination ability of the coordinating atoms in pendent arms is stronger than that in the crown ether macrocycle because of the limitation of the macrocycle. Most of the coordination atoms of the H₂bp18c6 ligand have a stronger ability to coordinate with metal ions than those of the H₂bpp18c6 ligand. In addition, the bonding strength between the metal ions and ligands gradually weakens from Am to Cf, which is mainly attributed to the size selectivity of the ligands. Thermodynamic analysis shows that the H₂bp18c6 ligand has a stronger extraction capacity than the H₂bpp18c6 ligand, while the H₂bpp18c6 ligand is superior in terms of the in-group separation ability. The extraction capacity of the two ligands for metal ions gradually decreases across the actinide series, indicating that these crown ether macrocycle ligands have size selectivity for these actinide cations as a result of steric constraint of the crown ether ring. We hope that these results offer theoretical clues for the development of macrocycle ligands for in-group transplutonium separation.

Relativistic Effects Stabilize the Planar Wheel-like Structure of Actinide-Doped Gold Clusters: An@Au7 (An = Th to Cm)

Despite the chemistry of actinide-ligand bonding is continuing and of burgeoning interest, investigations of the chemical bonding of bimetallic complexes involving transuranics remain relatively less, and there are rarely studies on the bonding features between actinide and coinage metals (CM). We present a systematic research on the series of An@Au₇ (An = Th to Cm), UCM₇ (CM = Cu, Ag, Au), and WAu₇ clusters to investigate the unique geometries, electronic structures, and chemical bonding between An 5f6d orbitals and CM ns orbitals, and to find their periodicity across the actinides and within the group of transition metals. A unique planar wheel-like structure for An@Au₇ clusters with the help of actinide metals encapsulation via spin-orbit coupling, resulting in An(III). Instead, the transition-metal (TM) element W retains its usual six-gold-coordination structure in WAu₇, thus forcing the seventh Au out of plane. The An-CM interactions, depending on the ion radii, become stronger with the increase of the atomic number of the actinide metals, as well as the CM. These results show that the presence of actinides in clusters can lead to unique electronic and geometrical structures.

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.

Characterization of holmium(iii)-acetylacetonate complexes derived from therapeutic microspheres by infrared ion spectroscopy

Microspheres containing radioactive ¹⁶⁶holmium-acetylacetonate are employed in emerging radionuclide therapies for the treatment of malignancies. At the molecular level, details on the coordination geometries of the Ho complexes are however elusive. Infrared ion spectroscopy (IRIS) was used to characterize several ¹⁶⁵Ho-acetylacetonate complexes derived from non-radioactive microspheres. The coordination geometry of four distinct ionic complexes were fully assigned by comparison of their measured IR spectra with spectra calculated at the density functional theory (DFT) level. The coordination of each acetylacetonate ligand is dependent on the presence of other ligands, revealing an asymmetric chelation motif in some of the complexes. A fifth, previously unknown constituent of the microspheres was identified as a coordination complex containing an acetic acid ligand. These results pave the way for IRIS-based identification of microsphere constituents upon neutron activation of the metal center.

Organic Compounds of Actinyls: Systematic Computational Assessment of Structural and Topological Properties in [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, Am; n = 1-3) Complexes

Exploring the bonding features between organics and actinide elements is a fundamental topic in nuclear waste separation. In this work, [AnO₂(C₂O₄) _n]^{(2 n-2)-} (An = U, Np, Pu, and Am; n = 1-3) complexes have been characterized by density functional theory. The actinyl oxalate complexes are found to exhibit the typical An-O_yl, An-O_eq bonds and O_yl-An-O_yl angles. Interatomic interaction analyzed by electron density difference, charge decomposition analysis, charges population, bond order, electron localization function, and quantum theory of atom in molecules indicates that An-O_eq bonds are ionic (closed-shell) bonding interaction with a small degree of covalent character. The similarities and differences between isomers have been discussed in the actinide coordination chemistry, and the orbital interactions also have been investigated through total, partial, and overlap population density of state diagrams. Besides, the electrostatic potential was used to predict the adsorption sites on the molecular vdW surface.

Theoretical studies on the oxidation states and electronic structures of actinide-borides: AnB12 (An = Th-Cm) clusters

As B12 clusters exhibit significant structural stability due to double aromaticity, metal doped-B12 clusters often prefer a half sandwich structure. Herein, we report a systematic theoretical study on the geometric and electronic structures, and chemical bonding of the half sandwich AnB12 (An = Th to Cm) clusters to explore the stability and extent of covalency of the An-B bonds of these actinide borides. We have shown that in the gas-phase clusters, the significant stability of AnB12 is determined by electrostatic and orbital interactions between the An 5f6d7s orbitals and π-type molecular orbitals from B 2p orbitals of the B12 unit. A change-over of An-B bond length from An = Th to Cm is found at An = Pa as a result of actinide contraction combined with weakening An-B bonding due to an energy decrease and orbital localization of the 5f orbitals. Consistently, the oxidation states of the An atoms at first increase from Th(f0)IV to Pa(f0)V, and then due to the 5f-AO contraction, they smoothly decline to U(f2)IV, Np(f4)III and Pu(f5)III, and then eventually to Am(f7)II but Cm(f7)III, both with a half-filled 5f shell.

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

Computational characterization of AnO₂²⁺–(15-crown-5) complexes (An = U, Np, Pu, Am, and Cm) reveals actinyl insertion coordination to crown ether.