A versatile strategy for the formation of hydride-bridged actinide-iridium multimetallics.

Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H3] with UCl4 and ThCl4(DME)2 led to the complete replacement of the halide ligands to generate multimetallic complexes U{(µ-H)3IrCp*}4 (1) and Th{[(µ-H2)(H)IrCp*]2[(µ-H)3IrCp*]2} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI3(1,4-dioxane)1.5, resulted in the octanuclear complex {U[(µ2-H3)IrCp*]2[(µ3-H2)IrCp*]}2 (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.

Correction: Enhanced 5f-δ bonding in [U(C7H7)2]-: C K-edge XAS, magnetism, and ab initio calculations.

Correction for 'Enhanced 5f-δ bonding in [U(C7H7)2]-: C K-edge XAS, magnetism, and ab initio calculations' by Yusen Qiao et al., Chem. Commun., 2021, 57, 9562-9565, DOI: 10.1039/D1CC03414F.

Enhanced 5f-δ bonding in [U(C7H7)2]-: C K-edge XAS, magnetism, and ab initio calculations.

5f covalency in [U(C7H7)2]- was probed with carbon K-edge X-ray absorption spectroscopy (XAS) and electronic structure theory. The results revealed U 5f orbital participation in δ-bonding in both the ground- and core-excited states; additional 5f ϕ-mixing is observed in the core-excited states. Comparisons with U(C8H8)2 show greater δ-covalency for [U(C7H7)2]-.

Comparison of tetravalent cerium and terbium ions in a conserved, homoleptic imidophosphorane ligand field.

A redox pair of Ce4+ and Ce3+ complexes has been prepared that is stabilized by the [(NP(1,2-bis- t Bu-diamidoethane)(NEt2))]1- ligand. Since these complexes are isostructural to the recently reported isovalent terbium analogs, a detailed structural and spectroscopic comparative analysis was pursued via Voronoi-Dirichlet polyhedra analysis, UV-vis-NIR, L3-edge X-ray absorption near edge spectroscopy (XANES), cyclic voltammetry, and natural transitions orbital (NTO) analysis and natural bond orbital (NBO) analysis. The electrochemical studies confirm previous theoretical studies of the redox properties of the related complex [K][Ce3+(NP(pip)3)4] (pip = piperidinyl), 1-Ce(PN). Complex 1-Ce(PN*) presents the most negative E pc of -2.88 V vs. Fc/Fc+ in THF of any cerium complex studied electrochemically. Likewise 1-Tb(PN*) has the most negative E pc for electrochemically interrogated terbium complexes at -1.79 V vs. Fc/Fc+ in THF. Complexes 1-Ce(PN*) and 2-Ce(PN*) were also studied by L3-edge X-ray absorption near edges spectroscopy (XANES) and a comparison to previously reported spectra for 1-Tb(PN*), 2-Tb(PN*), 1-Ce(PN), and, [Ce4+(NP(pip)3)4], 2-Ce(PN), demonstrates similar n f values for all the tetravalent lanthanide complexes. According to the natural bond orbital analysis, a greater covalent character of the M-L bonds is found in 2-Ce(PN*) than in 1-Ce(PN*), in agreement with the shorter Ce-N bonds in the tetravalent counterpart. The greater contribution of Ce orbitals in the Ce-N bonding and, specifically, the higher participation of 4f electrons accounts for the stronger covalent interactions in 2-Ce(PN*) as compared to 2-Tb(PN*).

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework.

We report the structural properties of ultra-small ThO2 and UO2 nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO2 and UO2 NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO2 and UO2 materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th-O and U-O bond distances despite retaining average bond lengths that are characteristic of bulk ThO2 and UO2. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

Design, Isolation, and Spectroscopic Analysis of a Tetravalent Terbium Complex.

Synthetic strategies to yield molecular complexes of high-valent lanthanides, other than the ubiquitous Ce4+ ion, are exceptionally rare, and thorough, detailed characterization in these systems is limited by complex lifetime and reaction and isolation conditions. The synthesis of high-symmetry complexes in high purity with significant lifetimes in solution and the solid state is essential for determining the role of ligand-field splitting, multiconfigurational behavior, and covalency in governing the reactivity and physical properties of these potentially technologically transformative tetravalent ions. We report the synthesis and physical characterization of an S4 symmetric, four-coordinate tetravalent terbium complex, [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] (where Et is ethyl and tBu is tert-butyl). The ligand field in this complex is weak and the metal-ligand bonds sufficiently covalent so that the tetravalent terbium ion is stable and accessible via a mild oxidant from the anionic, trivalent, terbium precursor, [(Et2O)K][Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4]. The significant stability of the tetravalent complex enables its thorough characterization. The stepwise development of the supporting ligand points to key ligand control elements for further extending the known tetravalent lanthanide ions in molecular complexes. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), and density functional theory studies indicate a 4f7 ground state for [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] with considerable zero-field splitting, demonstrating that magnetic, tetravalent lanthanide ions engage in covalent metal-ligand bonds. This result has significant implications for the use of tetravalent lanthanide ions in magnetic applications since the observed zero-field splitting is intermediate between that observed for the trivalent lanthanides and for the transition metals. The similarity of the multiconfigurational behavior in the ground state of [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] (measured by Tb L3-edge XAS) to that observed in TbO2 implicates ligand control of multiconfigurational behavior as a key component of the stability of the complex.

Diethyl ether adducts of trivalent lanthanide iodides.

The synthesis and structural characterization of the first molecular complexes of lanthanide iodides supported by the weak-base, diethyl ether, are reported. Single-crystal diffraction studies reveal a conserved [LnI3(mer-Et2O)3] structure (Ln = Ce, Pr, Nd, Sm, Gd, Tb, and Tm). These precursors are prepared from lanthanide metal and iodine in diethyl ether for all lanthanides La to Tm (excluding Pm and Eu) and provide the THF adducts in good to excellent (60-91%) yield in a two-step process.

Homoleptic Imidophosphorane Stabilization of Tetravalent Cerium.

The homoleptic complexes of cerium with the tris(piperidinyl)imidophosphorane ligand, [NP(pip)3]-, present the most negative Ce3+/4+ redox couple known (<-2.64 V vs Fc/Fc+). This dramatic stabilization of the cerium tetravalent oxidation state [>4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV-vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal-ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.