A Structural and Theoretical Investigation of Equatorial cis and trans Uranyl Phosphinimine and Uranyl Phosphine Oxide Complexes UO2Cl2(Cy3PNH)2 and UO2Cl2(Cy3PO)2

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO₂²⁺) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (O═P). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO₂²⁺ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO₂²⁺ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σ- and π-type interactions with U 5f and 6d orbitals in the UO₂Cl₂(TRPO)₂ complexes. Although both the axial (O_yl) and equatorial (O_eq) oxygen atoms in the UO₂Cl₂(TRPO)₂ complexes contribute to the X-ray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O_eq and is assigned to the transition from O_eq 1s orbitals to the unoccupied molecular orbitals of 1b_1u + 1b_2u + 1b_3u symmetries resulting from the σ- and π-type mixing between U 5f and O_eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O_eq 2p character in these orbitals for the four UO₂Cl₂(TRPO)₂ complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U-TRPO bonding interactions in the UO₂Cl₂(TRPO)₂ complexes. The covalent bonding interactions between TRPO and UO₂²⁺, especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

Synthesis, characterization, and theoretical analysis of a plutonyl phosphine oxide complex

The interplay of bond strength and covalency are examined in AnO₂Cl₂(OPcy₃)₂ (An = Pu, U) complexes. The synthesis of trans-PuO₂Cl₂(OPcy₃)₂, 1-Pu, has been carried out and confirmed by single crystal X-ray diffraction along with UV-vis-NIR, and ³¹P NMR spectroscopies. Theoretical analysis finds that despite a higher calculated covalency for the Pu-Cl interaction, the Pu-OPcy₃ interaction is stronger due to the accumulation of electron density in the interatomic region. The coordination of equatorial ligands slightly decreases the strength of the PuO_yl interactions relative to the free gas phase (PuO₂)²⁺ ion.

Interaction with Simple Monopyridinecarboxylic Ligands Revealing Unexpected Structural Types of Uranyl Halides

The inclusion of monopyridinecarboxylic acids into the structures of uranyl halides results in the formation of unexpected structural units. Molecules of picolinic and nicotinic acids acting as bridging ligands favor the formation of unprecedented dinuclear units in two uranyl bromide complexes, which comprise two metal centers in different, tetragonal- and pentagonal-bipyramidal, coordination geometries. Moreover, the different positions of the nitrogen atom in the molecule of nicotinic acid induce significant bending of the heterodimer. The uranyl chloride complex with isonicotinic acid also exhibits a structure containing metal atoms in two unique geometries. The structure consists of cationic and anionic isolated fragments. The anionic part is unprecedented and represents the first example of a 1:3 uranyl halide unit with a tetragonal bipyramid surrounding the central atom.

Exploring the Coordination of Plutonium and Mixed Plutonyl-Uranyl Complexes of Imidodiphosphinates

The coordination chemistry of plutonium(IV) and plutonium(VI) with the complexing agents tetraphenyl and tetra-isopropyl imidodiphosphinate (TPIP^- and TIPIP^-) is reported. Treatment of sodium tetraphenylimidodiphosphinate (NaTPIP) and its related counterpart with peripheral isopropyl groups (NaTIPIP) with [NBu₄]₂[Pu^IV(NO₃)₆] yields the respective Pu^IV complexes [Pu(TPIP)₃(NO₃)] and [Pu(TIPIP)₂(NO₃)₂] + [Pu^IV(TIPIP)₃(NO₃)]. Similarly, the reactions of NaTPIP and NaTIPIP with a Pu(VI) nitrate solution lead to the formation of [PuO₂(HTIPIP)₂(H₂O)][NO₃]₂, which incorporates a protonated bidentate TIPIP^- ligand, and [PuO₂(TPIP)(HTPIP)(NO₃)], where the protonated HTPIP ligand is bound in a monodentate fashion. Finally, a mixed U(VI)/Pu(VI) compound, [(UO₂/PuO₂)(TPIP)(HTPIP)(NO₃)], is reported. All these actinyl complexes remain in the +VI oxidation state in solution over several weeks. The resultant complexes have been characterized using a combination of X-ray structural studies, NMR, optical, vibrational spectroscopies, and electrospray ionization mass spectrometry. The influence of the R-group (R = phenyl or ⁱPr) on the nature of the complex is discussed with the help of DFT studies.

The strength of actinide-element bonds from the quantum theory of atoms-in-molecules

[AnX(3)](2)(μ-η(2):η(2)-N(2)) (An = Th-Pu; X = F, Cl, Br, Me, H, OPh) have been studied using relativistic density functional theory. Geometric and vibrational data suggest that metal→N(2) charge transfer maximises at the protactinium systems, which feature the longest N-N bonds and the smallest σ(N-N), as a result of partial population of the N-N π* orbitals. There is very strong correlation of the standard quantum theory of atoms-in-molecules (QTAIM) metrics - bond critical point ρ, ∇(2)ρ and H and delocalisation indices - with An-N and N-N bond lengths and σ(N-N), but the correlation with An-N interaction energies is very poor. A similar situation exists for the other systems studied; neutral and cationic actinide monoxide and dioxides, and AnL(3+) and AnL(3)(3+) (L = pyridine (Py), pyrazine (Pz) and triazine (Tz)) with the exception of some of the ∇(2)ρ data, for which moderate to good correlations with energy data are sometimes seen. By contrast, in almost all cases there is very strong correlation of interaction and bond energies with |ΔQ(QTAIM)(An)|, a simple QTAIM metric which measures the amount of charge transferred to or from the actinide on compound formation.

Terminal U≡E (E = N, P, As, Sb, and Bi) bonds in uranium complexes: a theoretical perspective

The compound L-U-N [L = [N(CH2CH2NSiPr(i)3)3](3-), Pr(i) = CH(CH3)2] containing a terminal U-N triple bond has been synthesized and isolated successfully in experiments. To investigate the trend in the bonding nature of its pnictogen analogues, we have studied the L-U-E (E = N, P, As, Sb, and Bi) complexes using the scalar relativistic density functional theory. The terminal U-E multiple bond length increases in the order of U-N ≪ U-P < U-As < U-Sb < U-Bi, which can be supported by the hard and soft acids and bases (HSAB) theory. The U-E bond length, molecular orbital (MO), and natural bond orbital (NBO) reveal that the terminal U-E bonds should be genuine triple bonds containing one σ- and two π-bonding orbitals. Quantum theory of atoms in molecules (QTAIM) topological analysis and the electron localization function (ELF) suggest that the terminal U-E bond possesses covalent character and the covalency of U-E bonds decrease sharply when the terminal atom becomes heavier. This work presents a comparison about the bonding characteristic between the terminal U≡N bond and its heavier pnictogen (P, As, Sb, and Bi) analogues. It is expected that this work would shed light on the evaluation of the amount of 5f orbital participation in multiple bonds and further facilitate our deeper understanding of f-block elements.