Proton-Coupled Electron Transfer at the Pu5+/4+ Couple

The synthesis and solution and solid-state characterization of [Pu4+(NPC)4], 1-Pu, (NPC = [NPtBu(pyrr)2]-; tBu = C(CH3)3; pyrr = pyrrolidinyl) and [Pu3+(NPC)4][K(2.2.2.-cryptand)], 2-Pu, is described. Cyclic voltammetry studies of 1-Pu reveal a quasi-reversible Pu4+/3+ couple, an irreversible Pu5+/4+ couple, and a third couple evincing a rapid proton-coupled electron transfer (PCET) reaction occurring after the electrochemical formation of Pu5+. The chemical identity of the product of the PCET reaction was confirmed by independent chemical synthesis to be [Pu4+(NPC)3(HNPC)][B(ArF5)4], 3-Pu, (B(ArF5)4 = tetrakis(2,3,4,5,6-pentafluourophenyl)borate) via two mechanistically distinct transformations of 1-Pu: protonation and oxidation. The kinetics and thermodynamics of this PCET reaction are determined via electrochemical analysis, simulation, and density functional theory. The computational studies demonstrate a direct correlation between the changing nature of 5f and 6d orbital participation in metal-ligand bonding and the electron density on the Nim atom with the thermodynamics of the PCET reaction from Np to Pu, and an indirect correlation with the roughly 5-orders of magnitude faster Pu PCET compared to Np for the An5+ species.

Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of protactinium (Z=91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, protactinium in the pentavalent state diverges by not forming the typical dioxo protactinyl moiety PaO2 + in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH)2 (X)(H2 O) and Pa(OH)4 (X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs free energy for the reaction PaO(OH)2 (X)(H2 O)→Pa(OH)4 (X), we find that the PaO(OH)2 (X)(H2 O) complex is stabilized with Cl- , Br- , I- , NCS- , NO3 - , and SO4 2- ligands, while it is not favored with OH- , F- , and C2 O4 2- ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl- , Br- , I- , NCS- , and NO3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

Actinide-ligand bonds with high multiplicities remain poorly understood. Decades ago, an effect known as 6p pushing from below (PFB) was proposed to enhance actinide covalency. A related effect-also poorly understood-is inverse trans influence (ITI). The present computational study of actinide-ligand covalent interactions with high bond multiplicities quantifies the energetic contributions from PFB and identifies a hitherto overlooked fourth bonding interaction for 2nd-row ligands in the studied organometallic systems. The latter are best described by a terminal O/N ligand exhibiting quadruple bonding interactions with the actinide. The 4th interaction may be characterized as a multi-center or charge-shift bond involving the trans ligand. It is shown in this work that the 4th bonding interaction is a manifestation of ITI, assisted by PFB, and provides a long-sought missing piece in the understanding of actinide chemistry.

Uranyl Analogue Complexes—Current Progress and Synthetic Challenges

Uranyl ions, {UO2}n+ (n = 1, 2), display trans, strongly covalent, and chemically robust U-O multiple bonds, where 6d, 5f, and 6p orbitals play important roles. The synthesis of isoelectronic analogues of uranyl has been of interest for quite some time, mainly with the purpose of unveiling covalence and 5f-orbital participation in bonding. Significant advances have occurred in the last two decades, initially marked by the synthesis of uranium(VI) bis(imido) complexes, the first analogues with a {RNUNR}2+ core, later followed by the synthesis of unique trans-{EUO}2+ (E = S, Se) complexes, and recently highlighted by the synthesis of the first complexes featuring a linear {NUN} moiety. This review covers the synthesis, structure, bonding, and reactivity of uranium complexes containing a linear {EUE}n+ core (n = 0, 1, 2), isoelectronic to uranyl ions, {OUO}n+ (n = 1, 2), incorporating σ- and π-donating ligands that can engage in uranium–ligand multiple bonding, where oxygen may be replaced by heavier chalcogenido, imido, nitride, and carbene ligands, or by a transition metal. It focuses on synthetic methods of well-defined molecular uranium species in the condensed phase but also references gas-phase and low-temperature-matrix experiments, as well as computational studies that may lead to valuable insights.

Intrinsic reactivity of [OUCH]+ : Apparent synthesis of [OUS]+ by reaction with CS2

RATIONALE

Building on our report that collision-induced dissociation (CID) can be used to create the highly reactive U-alkylidyne species [O=U≡CH]⁺ , our goal was to determine whether the species could be as an intermediate for synthesis of [OUS]⁺ by reaction with carbon disulfide (CS₂ ).

METHODS

Cationic uranyl-propiolate precursor ions were generated by electrospray ionization, and multiple-stage CID in a linear trap instrument was used to prepare [O=U≡CH]⁺ . Neutral CS₂ was admitted into the trap through a modified He inlet and precision leak valves.

RESULTS

The [O=U≡CH]⁺ ion reacts with CS₂ to generate [OUS]⁺ . CID of [OUS]⁺ causes elimination of the axial sulfide ligand to generate [OU]⁺ . Using isotopically labeled reagent, we found that [OUS]⁺ reacts with O₂ to create [UO₂ ]⁺ .

CONCLUSIONS

[O=U≡CH]⁺ proves to be a useful reagent ion for synthesis of [OUS]⁺ , a species that to date has only been created by gas-phase reactions of U⁺ and U²⁺ . Dissociation of [OUS]⁺ to create [OU]⁺ , but not [US]⁺ , and the efficient conversion of the species into [UO₂ ]⁺ , is consistent with the relative differences in U-O and U-S bond energies.

Uranium: The Nuclear Fuel Cycle and Beyond

This review summarizes the recent developments regarding the use of uranium as nuclear fuel, including recycling and health aspects, elucidated from a chemical point of view, i.e., emphasizing the rich uranium coordination chemistry, which has also raised interest in using uranium compounds in synthesis and catalysis. A number of novel uranium coordination features are addressed, such the emerging number of U(II) complexes and uranium nitride complexes as a promising class of materials for more efficient and safer nuclear fuels. The current discussion about uranium triple bonds is addressed by quantum chemical investigations using local vibrational mode force constants as quantitative bond strength descriptors based on vibrational spectroscopy. The local mode analysis of selected uranium nitrides, N≡U≡N, U≡N, N≡U=NH and N≡U=O, could confirm and quantify, for the first time, that these molecules exhibit a UN triple bond as hypothesized in the literature. We hope that this review will inspire the community interested in uranium chemistry and will serve as an incubator for fruitful collaborations between theory and experimentation in exploring the wealth of uranium chemistry.

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

X-ray absorption near edge structure (XANES) spectroscopy, coupled with ab initio calculations, has emerged as the state-of-the-art tool for elucidating the metal-ligand bonding in f-element complexes. This highlight presents recent efforts in calculating XANES spectra of lanthanide and actinide compounds with relativistic multiconfiguration wavefunction approaches that account for differences in donation bonding in the ground state (GS) versus a core-excited state (ES), multiplet effects, and spin-orbit-coupling. With the GS and ES wavefunctions available, including spin-orbit effects, an arsenal of chemical bonding tools that are popular among chemists can be applied to rationalize the observed intensities in terms of covalent bonding.

Covalency in Actinide Compounds

Covalency in actinides has emerged as a resounding research topic on account of the technological importance in separating minor actinides from lanthanides for spent nuclear fuel processing, and utilization of their distinct bonding properties has been realized as a route towards overcoming this challenge. Because of the limited radial extent of the 4f orbitals, there is almost no 4f electron participation in bonding in lanthanides; this is not the case for the actinides, which have extended 5f orbitals that are capable of overlapping with ligand orbitals, although not to the degree of overlap as in the d orbitals of transition metals. In this concept paper, a general description of covalency in actinide compounds is provided. After introducing two main approaches to enhance covalency, either by exploiting increased orbital overlap or decreasing energy differences between the orbitals causing orbital energy degeneracy, the current state of the field is illustrated by using several examples from the recent literature. This paper is concluded by proposing the use of actinide chalcogenides as a convenient auxiliary tool to study covalency in actinide compounds.

High coordination number actinide-noble gas complexes; a computational study

The geometries, electronic structures and bonding of early actinide-noble gas complexes are studied computationally by density functional and wavefunction theory methods, and by ab initio molecular dynamics. AcHe183+ is confirmed as being an 18-coordinate system, with all of the He atoms accommodated in the primary coordination shell, and this record coordination number is reported for the first time for Th4+ and Th3+. For Pa and U in their group valences of 5 and 6 respectively, the largest number of coordinated He atoms is 17. For AnHe17q+ (An = Ac, q = 3; An = Th, q = 4; An = Pa, q = 5; An = U, q = 6), the average An-He binding energy increases significantly across the series, and correlates linearly with the extent of He → Anq+ charge transfer. The interatomic exchange-correlation term Vxc obtained from the interacting quantum atoms approach correlates linearly with the An-He quantum theory of atoms-in-molecules delocalization index, both indicating that covalency increases from AcHe173+ to UHe176+. The correlation energy in AnHe163+ obtained from MP2 calculations decreases in the order Pa > Th > U > Ac, the same trend found in Vxc. The most stable complexes of Ac3+ with the heavier noble gases Ar-Xe are 12 coordinate, best described as Ng12 cages encapsulating an Ac3+ ion. There is enhanced Ng → Ac3+ charge transfer as the Ng gets heavier, and Ac-Ng covalency increases.

Covalency in AnCl3 (An = Th-No)

The geometric and electronic structures of AnCl3 are studied computationally using scalar relativistic, hybrid density functional theory (PBE0). The An-Cl bond lengths generally decrease across the 5f series, although there is a slight lengthening from Fm-Cl to No-Cl as the metal ions display increasing M(ii) character. Covalency in the An-Cl bond is studied using a wide range of metrics drawn from the Natural Bond Orbital, Natural Resonance Theory and Quantum Theory of Atoms-in-Molecules (QTAIM) methods, including bond order, orbital composition, orbital overlap and electron density topology data. Most metrics agree that the later An-Cl bonds are less ionic than might be anticipated on the basis of trends in the first half of the series, due to energy degeneracy-driven covalency in the β spin manifold; for example, the An-Cl QTAIM delocalisation index (bond order) for MdCl3 (0.88) is almost exactly the same as for NpCl3 (0.89). By contrast, the ratio of the kinetic to potential energy densities at the An-Cl bond critical points indicates that ionicity increases across the series, suggesting that the delocalisation index measures both orbital overlap and energy degeneracy-based covalency, while the bond critical point metric gauges only the former. Recalculation of all the data using the generalised gradient approximation PBE functional finds larger energy degeneracy-driven covalency in the later actinides than using hybrid DFT. Hence, we find that conclusions concerning the covalency of the An-Cl bond are dependent not only on the metric used to evaluate it, but also on the underlying electronic structure method.

Series of Tetravalent Actinide Amidinates: Structure Determination and Bonding Analysis

Two series of isostructural tetravalent actinide amidinates [AnX((S)-PEBA)₃] (An = Th, U, Np; X = Cl, N₃) bearing the chiral (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) ligand have been synthesized and thoroughly characterized in solid and in solution. This study expands the already reported tetravalent neptunium complexes to the lighter actinides thorium and uranium. Furthermore, a rare Ce(IV) amidinate [CeCl((S)-PEBA)₃] was synthesized to compare its properties to those of the analogous tetravalent actinide complexes. All compounds were characterized in the solid state using single-crystal XRD and infrared spectroscopy and in solution using NMR spectroscopy. Quantum chemical bonding analysis including also the isostructural Pa and Pu complexes was used to characterize the covalent contributions to any bond involving the metal cation. Th shows the least covalent character throughout the series, even substantially smaller than for the Ce complex. For U, Np, and Pu, similar covalent bonding contributions are found, but a natural population analysis reveals different origins. The 6d participation is the highest for U and decreases afterward, whereas the 5f participation increases continuously from Pa to Pu.

The duality of electron localization and covalency in lanthanide and actinide metallocenes

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C8H8)2, have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C8H8)2 is analyzed experimentally using carbon K-edge and cerium M5,4-edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C8H8 2- ligands. Specifically, the Ce M5,4-edge XAS and CI calculations show that the contribution of the 4f1, or Ce3+, configuration to the ground state of Ce(C8H8)2 is similar to strongly correlated materials such as CeRh3 and significantly larger than observed for other formally Ce4+ compounds including CeO2 and CeCl6 2-. Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e2u symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e2u orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C8H8)2 is comparable in magnitude to values reported previously for U(C8H8)2. An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes.

Sulfur-substituted uranyl stabilized by fluoride ligands: matrix preparation of U(O)(S)F2via oxidation of U(0) by SOF2

A neutral sulfur-substituted uranyl complex [U(O)(S)F₂] in which the SUO²⁺ moiety is stabilized by electron withdrawing fluoride ligands was prepared via oxidation of U(0) by SOF₂ in cryogenic matrixes.

Quantum chemical topology and natural bond orbital analysis of M–O covalency in M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, Pa, U, Np)

V_XC(M,O): the exchange–correlation metric quantifies covalency between M and O atomic basins in M(OC₆H₅)₄ (M = Ti, Zr, Hf, Ce, Th, Pa, U, Np).