Recent developments in synthesis and structural chemistry of nonaqueous actinide complexes

Organometallic Neptunium Chemistry

Fifty years have passed since the foundation of organometallic neptunium chemistry, and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and separations of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a critical assessment of the successes and difficulties in their analysis and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorganic chemistry laboratories, the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochemical synthesis of one organometallic compound), and the available anhydrous starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chemical behavior between Np and its closest neighbors, uranium and plutonium, are found.

Crystalline Diuranium Phosphinidiide and μ-Phosphido Complexes with Symmetric and Asymmetric UPU Cores

Reaction of [U(TrenTIPS )(PH2 )] (1, TrenTIPS =N(CH2 CH2 NSiPri3 )3 ) with C6 H5 CH2 K and [U(TrenTIPS )(THF)][BPh4 ] (2) afforded a rare diuranium parent phosphinidiide complex [{U(TrenTIPS )}2 (μ-PH)] (3). Treatment of 3 with C6 H5 CH2 K and two equivalents of benzo-15-crown-5 ether (B15C5) gave the diuranium μ-phosphido complex [{U(TrenTIPS )}2 (μ-P)][K(B15C5)2 ] (4). Alternatively, reaction of [U(TrenTIPS )(PH)][Na(12C4)2 ] (5, 12C4=12-crown-4 ether) with [U{N(CH2 CH2 NSiMe2 But )2 CH2 CH2 NSi(Me)(CH2 )(But )}] (6) produced the diuranium μ-phosphido complex [{U(TrenTIPS )}(μ-P){U(TrenDMBS )}][Na(12C4)2 ] [7, TrenDMBS =N(CH2 CH2 NSiMe2 But )3 ]. Compounds 4 and 7 are unprecedented examples of uranium phosphido complexes outside of matrix isolation studies, and they rapidly decompose in solution underscoring the paucity of uranium phosphido complexes. Interestingly, 4 and 7 feature symmetric and asymmetric UPU cores, respectively, reflecting their differing steric profiles.

Metathesis of a UV imido complex: a route to a terminal UV sulfide

The metathesis reaction of a U^V imido complex supported by sterically demanding tris(tert-butoxy)siloxide ligands with CS₂ afforded a terminal U^V thiocarbonate but metathesis with H₂S afforded the first example of a terminal U^V sulfide.

Herein, we report the synthesis and characterisation of the first terminal uranium(v) sulfide and a related U^V trithiocarbonate complex supported by sterically demanding tris(tert-butoxy)siloxide ligands. The reaction of the potassium-bound U^V imido complex, [U(NAd){OSi(OtBu)₃}₄K] (4), with CS₂ led to the isolation of perthiodicarbonate [K(18c6)]₂[C₂S₆] (6), with concomitant formation of the U^IV complex, [U{OSi(OtBu)₃}₄], and S Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CNAd. In contrast, the reaction of the U^V imido complex, [K(2.2.2-cryptand)][U(NAd){OSi(OtBu)₃}₄] (5), with one or two equivalents of CS₂ afforded the trithiocarbonate complex, [K(2.2.2-cryptand)][U(CS₃){OSi(OtBu)₃}₄] (7), which was isolated in 57% yield, with concomitant elimination of the admantyl thiocyanate product, SCNAd. Complex 7 is likely formed by fast nucleophilic addition of a U^V terminal sulfide intermediate, resulting from the slow metathesis reaction of the imido complex with CS₂, to a second CS₂ molecule. The addition of a solution of H₂S in thf (1.3 eq.) to 4 afforded the first isolable U^V terminal sulfide complex, [K(2.2.2-cryptand)][US{OSi(OtBu)₃}₄] (8), in 41% yield. Based on DFT calculations, triple-bond character with a strong covalent interaction is suggested for the U–S bond in complex 7.

Probing Chemical Bonding and Electronic Structures in ThO- by Anion Photoelectron Imaging and Theoretical Calculations

Because of renewed research on thorium-based molten salt reactors, there is growing demand and interest in enhancing the knowledge of thorium chemistry both experimentally and theoretically. Compared with uranium, thorium has few chemical studies reported up to the present. Here we report the vibrationally resolved photoelectron imaging of the thorium monoxide anion. The electron affinity of ThO is first reported to be 0.707 ± 0.020 eV. Vibrational frequencies of the ThO molecule and its anion are determined from Franck-Condon simulation. Spectroscopic evidence is obtained for the two-electron transition in ThO^-, indicating the strong electron correlation among the (7s_σ)²(6d_δ)¹ electrons in ThO^- and the (7s_σ)² electrons in ThO. These findings are explained by using quantum-chemical calculations including spin-orbit coupling, and the chemical bonding of gaseous ThO molecules is analyzed. The present work will enrich our understanding of bonding capacities with the 6d valence shell.

Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {PuII[C5H3(SiMe3)2]3}<sup/>

Over 70 years of chemical investigations have shown that plutonium exhibits some of the most complicated chemistry in the periodic table. Six Pu oxidation states have been unambiguously confirmed (0 and +3 to +7), and four different oxidation states can exist simultaneously in solution. We report a new formal oxidation state for plutonium, namely Pu²⁺ in [K(2.2.2-cryptand)][Pu^IICp″₃], Cp″ = C₅H₃(SiMe₃)₂. The synthetic precursor Pu^IIICp″₃ is also reported, comprising the first structural characterization of a Pu-C bond. Absorption spectroscopy and DFT calculations indicate that the Pu²⁺ ion has predominantly a 5f⁶ electron configuration with some 6d mixing.

Triamidoamine thorium-arsenic complexes with parent arsenide, arsinidiide and arsenido structural motifs

Despite a major expansion of uranium-ligand multiple bond chemistry in recent years, analogous complexes involving other actinides (An) remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, phosphorus and chalcogenides are reported, and none to arsenic are known; indeed only two complexes with thorium-arsenic single bonds have been structurally authenticated, reflecting the challenges of stabilizing polar linkages at the large thorium ion. Here, we report thorium parent-arsenide (ThAsH₂), -arsinidiides (ThAs(H)K and ThAs(H)Th) and arsenido (ThAsTh) linkages stabilized by a bulky triamidoamine ligand. The ThAs(H)K and ThAsTh linkages exhibit polarized-covalent thorium-arsenic multiple bonding interactions, hitherto restricted to cryogenic matrix isolation experiments, and the AnAs(H)An and AnAsAn linkages reported here have no precedent in f-block chemistry. 7s, 6d and 5f orbital contributions to the Th-As bonds are suggested by quantum chemical calculations, and their compositions unexpectedly appear to be tensioned differently compared to phosphorus congeners.

Two uranyl heterocyclic carboxyl compounds with fluorescent properties as high sensitivity and selectivity optical detectors for nitroaromatics

Uranyl skeleton compounds showed high sensitivity and selectivity for the detection of nitroaromatics.

Organoactinides in catalytic transformations: scope, mechanisms and Quo Vadis

The last decade has witnessed brilliant and remarkable advances in the chemistry of the early actinides in stoichiometric and in challenging catalytic processes. This canvas of knowledge allows the design of chemical reactivities reaching a high level of sophistication. This review highlights the latest results obtained since 2008 on those catalytic processes.

Synthesis, structure and bonding of actinide disulphide dications in the gas phase

CASPT2 computations reveal that gas-phase AnS₂²⁺ ions have ground states with triangular geometries and linear thio-actinyl structures are higher in energy, with a difference that increases upon moving from U to Pu.

Facile syntheses of pure uranium halides: UCl4, UBr4 and UI4

Herein we describe convenient lab scale syntheses of several uranium(iv) halides of high purity by reaction of AlX₃ (X = Cl, Br and I) with UO₂, which is readily available by reduction of uranyl salts like UO₂(NO₃)₂·6H₂O.

Thorium-phosphorus triamidoamine complexes containing Th-P single- and multiple-bond interactions

Despite the burgeoning field of uranium-ligand multiple bonds, analogous complexes involving other actinides remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, oxygen, sulfur, selenium and tellurium are reported, and no multiple bonds to phosphorus are known, reflecting a general paucity of synthetic methodologies and also problems associated with stabilising these linkages at the large thorium ion. Here we report structurally authenticated examples of a parent thorium(IV)-phosphanide (Th-PH₂), a terminal thorium(IV)-phosphinidene (Th=PH), a parent dithorium(IV)-phosphinidiide (Th-P(H)-Th) and a discrete actinide-phosphido complex under ambient conditions (Th=P=Th). Although thorium is traditionally considered to have dominant 6d-orbital contributions to its bonding, contrasting to majority 5f-orbital character for uranium, computational analyses suggests that the bonding of thorium can be more nuanced, in terms of 5f- versus 6d-orbital composition and also significant involvement of the 7s-orbital and how this affects the balance of 5f- versus 6d-orbital bonding character.

An Unprecedented Two-Fold Nested Super-Polyrotaxane: Sulfate-Directed Hierarchical Polythreading Assembly of Uranyl Polyrotaxane Moieties

The hierarchical assembly of well-organized submoieties could lead to more complicated superstructures with intriguing properties. We describe herein an unprecedented polyrotaxane polythreading framework containing a two-fold nested super-polyrotaxane substructure, which was synthesized through a uranyl-directed hierarchical polythreading assembly of one-dimensional polyrotaxane chains and two-dimensional polyrotaxane networks. This special assembly mode actually affords a new way of supramolecular chemistry instead of covalently linked bulky stoppers to construct stable interlocked rotaxane moieties. An investigation of the synthesis condition shows that sulfate can assume a vital role in mediating the formation of different uranyl species, especially the unique trinuclear uranyl moiety [(UO2 )3 O(OH)2 ](2+) , involving a notable bent [O=U=O] bond with a bond angle of 172.0(9)°. Detailed analysis of the coordination features, the thermal stability as well as a fluorescence, and electrochemical characterization demonstrate that the uniqueness of this super-polyrotaxane structure is mainly closely related to the trinuclear uranyl moiety, which is confirmed by quantum chemical calculations.

Synthesis and reactivity of a terminal uranium(iv) sulfide supported by siloxide ligands

The S-transfer reaction from Ph₃PS to the tetrasiloxide U(iii) complex [U(OSi(O^tBu)₃)₄K] affords a stable U(iv) triply bonded terminal sulfide that can be protonated to yield a U(iv) doubly bonded terminal hydrosulfide.

The reactions of the tetrasiloxide U(iii) complexes [U(OSi(O^tBu)₃)₄K] and [U(OSi(O^tBu)₃)₄][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(iv) sulfide complexes [SU(OSi(O^tBu)₃)₄K₂]₂, 1, and [{SU(OSi(O^tBu)₃)₄K₂}₂(μ-18c6)], 2, with concomitant formation of the U(iv) complex [U(OSi(O^tBu)₃)₄]. Addition of 1 equiv. of 2.2.2-cryptand to complex 1 resulted in the isolation of a terminal sulfide complex, [SU(OSi(O^tBu)₃)₄K][Kcryptand], 3. The crucial role of the K⁺ Lewis acid in these reductive sulfur transfer reactions was confirmed, since the formation of complex 3 from the reaction of the U(iii) complex [U(OSi(O^tBu)₃)₄][Kcryptand] and 0.5 equiv. of PPh₃S was not possible. Reactivity studies of the U(iv) sulfide complexes showed that the sulfide is easily transferred to CO₂ and CS₂ to afford S-functionalized products. Moreover, we have found that the sulfide provides a convenient precursor for the synthesis of the corresponding U(iv) hydrosulfide, {[(SH)U(OSi(O^tBu)₃)₄][K18c6]}, 5, after protonation with PyHCl. Finally, DFT calculations were performed to investigate the nature of the U–S bond in complexes 1, 3 and 5. Based on various analyses, triple-bond character was suggested for the U–S bond in complexes 1 and 3, while double-bond character was determined for the U–SH bond in complex 5.

Emergence of comparable covalency in isostructural cerium(iv)- and uranium(iv)-carbon multiple bonds

We report comparable levels of covalency in cerium- and uranium-carbon multiple bonds in the iso-structural carbene complexes [M(BIPMTMS)(ODipp)2] [M = Ce (1), U (2), Th (3); BIPMTMS = C(PPh2NSiMe3)2; Dipp = C6H3-2,6-iPr2] whereas for M = Th the M[double bond, length as m-dash]C bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1-3 are confirmed formally as bona fide metal(iv) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1-3 via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to covalency. For these complexes similar levels of covalency are found for cerium(iv) and uranium(iv), whereas thorium(iv) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in covalency of these systems of Ce ∼ U > Th are also reproduced in experimental exchange reactions of 1-3 with MCl4 salts where 1 and 2 do not exchange with ThCl4, but 3 does exchange with MCl4 (M = Ce, U) and 1 and 2 react with UCl4 and CeCl4, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide-ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent.

Use of (77)Se and (125)Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3](-) (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners

Reaction of [Th(I)(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K(18-crown-6)]2[Se4] or [K(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(η(2)-E2)(NR2)3] (E = Se, 2; E = Te, 3), respectively. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K(18-crown-6)][Th(E)(NR2)3] (E = Se, 4; E = Te, 5), respectively. Both 4 and 5 were characterized by X-ray crystallography and were found to feature the shortest known Th-Se and Th-Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with (77)Se and (125)Te NMR spectroscopy accompanied by detailed quantum-chemical analysis. We also recorded the (77)Se NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)3](-) (δ((77)Se) = 4905 ppm), which features the highest frequency (77)Se NMR shift yet reported, and expands the known (77)Se chemical shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both (77)Se and (125)Te NMR chemical shifts of given chalcogenide ligands were identified as quantitative measures of the An-E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide-ligand bonding for uranium(VI) complexes in contrast to those involving thorium(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and density functional theory (DFT) calculations provide convincing evidence for the actinide-chalcogen multiple bonding in the title complexes. Larger An-E covalency is observed in the [U(O)(E)(NR2)3](-) series, which decreases as the chalcogen atom becomes heavier.

Theoretical investigation of low-valent uranium and transuranium complexes of a flexible small-cavity macrocycle: structural, formation reaction and redox properties

Size matching of a flexible macrocycle with low-valent actinide(III/IV) ions as well as their bonding determines different coordination modes.

Extraction and coordination studies of a carbonyl–phosphine oxide scorpionate ligand with uranyl and lanthanide(iii) nitrates: structural, spectroscopic and DFT characterization of the complexes

A scorpionate organophosphorus ligand exhibits promising extraction properties and variable denticity in f-block element complexes in the solid state and solution.