Terminal Uranium Research Articles

A Comprehensive Study Concerning the Synthesis, Structure, and Reactivity of Terminal Uranium Oxido, Sulfido, and Selenido Metallocenes.

Terminal uranium oxido, sulfido, and selenido metallocenes were synthesized, and their reactivity was comprehensively studied. Heating of an equimolar mixture of [η5-1,2,4-(Me3Si)3C5H2]2UMe2 (2) and [η5-1,2,4-(Me3Si)3C5H2]2U(NH-p-tolyl)2 (3) in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene forms [η5-1,2,4-(Me3Si)3C5H2]2U═N(p-tolyl)(dmap) (4), which is a useful precursor for the preparation of the terminal uranium oxido, sulfido, and selenido metallocenes [η5-1,2,4-(Me3Si)3C5H2]2U═E(dmap) (E = O (5), S (6), Se (7)) employing a cycloaddition-elimination methodology with Ph2C═E (E = O, S) or (p-MeOPh)2CSe, respectively. Metallocenes 5-7 are inert toward alkynes, but they act as nucleophiles in the presence of alkylsilyl halides. The oxido and sulfido metallocenes 5 and 6 undergo [2 + 2] cycloadditions with isothiocyanate PhNCS or CS2, while the selenido derivative 7 does not. The experimental studies are complemented by density functional theory (DFT) computations.

Uranium nitride triple bond is surprisingly covalent

An unusual uranium nitride complex contains a highly covalent triple bond that pushes the boundaries of actinide bonding ( Nat. Commun. 2021, DOI: 10.1038/s41467-021-25863-2 ). Covalency describes the degree of electron sharing between two atoms. In general, the earlier actinides tend to engage in ionic or polarized covalent bonding. This bonding character falls between the lanthanides, which prefer ionic bonding, and transition-metal complexes, which can have much more covalent bonds. But in 2013, a team led by Stephen T. Liddle, now at the University of Manchester, created a terminal uranium(VI)-nitride complex (shown) that was computationally predicted to have a remarkably covalent U≡N bond. Liddle’s team has now experimentally confirmed this with 15 N nuclear magnetic resonance spectroscopy, after synthesizing a version of the complex containing the isotope. NMR data showed that the U≡N bond is even more covalent than analogous transition metal–nitride complexes. “We were really surprised,” Liddle says. “We

Exceptional uranium(VI)-nitride triple bond covalency from 15N nuclear magnetic resonance spectroscopy and quantum chemical analysis

Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.

Influence of the 1,3-Bis(trimethylsilyl)cyclopentadienyl Ligand on the Reactivity of the Uranium Phosphinidene [η5-1,3-(Me3Si)2C5H3]2U(═P-2,4,6-iPr3C6H2)(OPPh3)

At ambient temperature [η5-1,3-(Me3Si)2C5H3]2U(Cl)Me (3) reacts with 2,4,6-iPr3C6H2PHK in toluene in the presence of Ph3PO to yield the Lewis base supported terminal uranium phosphinidene metallocene [η5-1,3-(Me3Si)2C5H3]2U(═P-2,4,6-iPr3C6H2)(OPPh3) (4), whose structure and reactivity were probed. When it is treated with conjugated alkynes and diazabutadienes, compound 4 acts as a masked synthon for the divalent uranium fragment [η5-1,3-(Me3Si)2C5H3]2UII to give [η5-1,3-(Me3Si)2C5H3][η5-1-(CH2Me2Si)-3-(Me3Si)C5H3]U[(1-(2,4,6-iPr3C6H2)-2,5-Ph2C4HP] (7) and [η5-1,3-(Me3Si)2C5H3]2U[N(p-tolyl)CH(P-2,4,6-iPr3C6H2)CHN(p-tolyl)] (8), respectively. Nevertheless, in the presence of internal alkynes and various heterounsaturated molecules such as imines, carbodiimides, isothiocyanates, aldehydes, nitriles, isonitriles, and organic azides complex 4 affords metallaheterocycles, sulfido, oxido, and imido complexes in good yields. Furthermore, when it is exposed to the diazene PhN═NPh, complex 4 converts to the four-membered heterocycle [η5-1,3-(Me3Si)2C5H3]2U[N(Ph)P(2,4,6-iPr3C6H2)N(Ph)] (12) in good yield.

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Reactivity studies involving a Lewis base supported terminal uranium phosphinidene metallocene [η5-1,3-(Me3C)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3).

The Lewis base supported terminal uranium phosphinidene metallocene [η5-1,3-(Me3C)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (2) could be isolated from a salt metathesis reaction in toluene at ambient temperature between [η5-1,3-(Me3C)2C5H3]2U(Cl)Me (1) and 2,4,6-iPr3C6H2PHK in the presence of Me3PO, and its structure and reactivity were probed in detail. No reaction of 2 with internal alkynes was observed, but it reacts in the presence of various heterounsaturated molecules such as CS2, isothiocyanates, aldehydes, imines, diazenes, carbodiimides, nitriles, isonitriles, diazoalkane, and organic azides, forming carbodithioates, sulfidos, oxidos, metallaheterocycles, and imido complexes, in good yields. Moreover, on reaction with the diazoalkane derivative Me3SiCHN2 the pseudophosphinimido uranium(iii) complex [η5-1,3-(Me3C)2C5H3]2U(N[double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (20) can be isolated in good yield.

Experimental and Computational Studies on a Base‐Free Terminal Uranium Phosphinidene Metallocene

The first stable base‐free terminal uranium phosphinidene metallocene is presented; and its structure and reactivity have been studied in detail and compared to that of the corresponding thorium derivative. Salt metathesis reaction of the methyl iodide uranium metallocene Cp’’’2U(I)Me (2, Cp’’’=η 5‐1,2,4‐(Me3C)3C5H2) with Mes*PHK (Mes*=2,4,6‐(Me3C)3C6H2) in THF yields the base‐free terminal uranium phosphinidene metallocene, Cp’’’2U=PMes* (3). In addition, density functional theory (DFT) studies suggest substantial 5f orbital contributions to the bonding within the uranium phosphinidene [U]=PAr moiety, which results in a more covalent bonding between the [Cp’’’2U]2+ and [Mes*P]2− fragments than that for the related thorium derivative. This difference in bonding besides steric reasons causes different reactivity patterns for both molecules. Therefore, the uranium derivative 3 may act as a Cp’’’2U(II) synthon releasing the phosphinidene moiety (Mes*P:) when treated with alkynes or a variety of hetero‐unsaturated molecules such as imines, thiazoles, ketazines, bipy, organic azides, diazene derivatives, ketones, and carbodiimides.

Photochemical Synthesis of a Stable Terminal Uranium(VI) Nitride.

Terminal uranium nitrides have so far proven impossible to isolate by photolysis of azides. Here we report the second ever example of an isolated terminal uranium(VI) nitride. We show that the terminal nitride [NBu4][U(OSi(OtBu)3)4(N)], 3, can be prepared upon photolysis with UV light of the U(IV) azide analogue. This is achieved by careful tailoring of the azide precursor and of the reaction conditions. Complex 3 is stable under ambient conditions but reacts readily with electrophiles (H+ and CO).

A Lewis Base Supported Terminal Uranium Phosphinidene Metallocene.

A Lewis base supported terminal uranium phosphinidene, [η5-1,3-(Me3C)2C5H3]2U(═P-2,4,6-tBu3C6H2)(OPMe3) (5), is isolated from the reaction of the uranium methyl chloride [η5-1,3-(Me3C)2C5H3]2U(Cl)Me (4) with 2,4,6-(Me3C)3C6H2PHK in toluene in the presence of Me3PO. Moreover, the reactivity of uranium phospinidene 5 toward a series of small molecules was comprehensively explored. While no reactivity of 5 with internal alkynes is observed attributed to steric hindrance, it readily reacts in good yields with various small molecules including isothiocyanates, aldehydes, imines, diazenes, carbodiimides, nitriles, isonitriles, and organic azides, yielding uranium sulfidos, oxidos, metallaheterocycles, and imido complexes.

Terminal uranium(V)-nitride hydrogenations involving direct addition or Frustrated Lewis Pair mechanisms

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium(III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium(VI)-nitride is inert to hydrogenolysis, suggesting the 5f1 electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.

Thorium- and uranium-azide reductions: a transient dithorium-nitride versus isolable diuranium-nitrides.

Molecular uranium-nitrides are now well known, but there are no isolable molecular thorium-nitrides outside of cryogenic matrix isolation experiments. We report that treatment of [M(TrenDMBS)(I)] (M = U, 1; Th, 2; TrenDMBS = {N(CH2CH2NSiMe2Bu t )3}3-) with NaN3 or KN3, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(TrenDMBS)(μ-N3)} n ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(TrenDMBS)}2(μ-N)][K(THF)6] (5) and [{U(TrenDMBS)}2(μ-N)] (6), whereas photolysis produces exclusively 6. Complexes 5 and 6 can be reversibly inter-converted by oxidation and reduction, respectively, showing that these UNU cores are robust with no evidence for any C-H bond activations being observed. In contrast, reductions of 4 in arene or ethereal solvents gives [{Th(TrenDMBS)}2(μ-NH)] (7) or [{Th(TrenDMBS)}{Th(N[CH2CH2NSiMe2Bu t ]2CH2CH2NSi[μ-CH2]MeBu t )}(μ-NH)][K(DME)4] (8), respectively, providing evidence unprecedented outside of matrix isolation for a transient dithorium-nitride. This suggests that thorium-nitrides are intrinsically much more reactive than uranium-nitrides, since they consistently activate C-H bonds to form rare examples of Th-N(H)-Th linkages with alkyl by-products. The conversion here of a bridging thorium(iv)-nitride to imido-alkyl combination by 1,2-addition parallels the reactivity of transient terminal uranium(iv)-nitrides, but contrasts to terminal uranium(vi)-nitrides that produce alkyl-amides by 1,1-insertion, suggesting a systematic general pattern of C-H activation chemistry for metal(iv)- vs. metal(vi)-nitrides. Surprisingly, computational studies reveal a σ > π energy ordering for all these bridging nitride bonds, a phenomenon for actinides only observed before in terminal uranium nitrides and uranyl with very short U-N or U-O distances.

Terminal Uranium(V/VI) Nitride Activation of Carbon Dioxide and Carbon Disulfide: Factors Governing Diverse and Well-Defined Cleavage and Redox Reactions.

The reactivity of terminal uranium(V/VI) nitrides with CE2 (E=O, S) is presented. Well-defined C=E cleavage followed by zero-, one-, and two-electron redox events is observed. The uranium(V) nitride [U(TrenTIPS )(N)][K(B15C5)2 ] (1, TrenTIPS =N(CH2 CH2 NSiiPr3 )3 ; B15C5=benzo-15-crown-5) reacts with CO2 to give [U(TrenTIPS )(O)(NCO)][K(B15C5)2 ] (3), whereas the uranium(VI) nitride [U(TrenTIPS )(N)] (2) reacts with CO2 to give isolable [U(TrenTIPS )(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(TrenTIPS )(O)] (5) with concomitant formation of N2 and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS2 to give [U(TrenTIPS )(κ2 -CS3 )][K(B15C5)2 ] (6), 2, and [K(B15C5)2 ][NCS] (7), whereas 2 reacts with CS2 to give [U(TrenTIPS )(NCS)] (8) and "S", with the latter trapped as Ph3 PS. Calculated reaction profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE2 follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one-electron reactions and a less-common two-electron redox event. Caution, we suggest, is warranted when utilising CS2 as a reactivity surrogate for CO2 .

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond.

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (Ad,MeArO)3tacn3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((Ad,MeArO)3tacn)U-SH] (2) is obtained from the reaction of H2S with the uranium(iii) starting material [((Ad,MeArO)3tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.

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

The reactions of the tetrasiloxide U(iii) complexes [U(OSi(O t Bu)3)4K] and [U(OSi(O t Bu)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 t Bu)3)4K2]2, 1, and [{SU(OSi(O t Bu)3)4K2}2(μ-18c6)], 2, with concomitant formation of the U(iv) complex [U(OSi(O t Bu)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 t Bu)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 t Bu)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 t Bu)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.

Two‐Electron Reductive Carbonylation of Terminal Uranium(V) and Uranium(VI) Nitrides to Cyanate by Carbon Monoxide

AbstractTwo‐electron reductive carbonylation of the uranium(VI) nitride [U(TrenTIPS)(N)] (2, TrenTIPS=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(TrenTIPS)(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(TrenTIPS)] (4) and KNCO, or cyanate retention in [U(TrenTIPS)(NCO)][K(B15C5)2] (5, B15C5=benzo‐15‐crown‐5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(TrenTIPS)(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(TrenTIPS)(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f‐block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol−1 for uranium(VI) and uranium(V), respectively. A remarkably simple two‐step, two‐electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.

Two-electron reductive carbonylation of terminal uranium(V) and uranium(VI) nitrides to cyanate by carbon monoxide.

Two-electron reductive carbonylation of the uranium(VI) nitride [U(TrenTIPS)(N)] (2, TrenTIPS=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(TrenTIPS)(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(TrenTIPS)] (4) and KNCO, or cyanate retention in [U(TrenTIPS)(NCO)][K(B15C5)2] (5, B15C5=benzo-15-crown-5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(TrenTIPS)(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(TrenTIPS)(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f-block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol−1 for uranium(VI) and uranium(V), respectively. A remarkably simple two-step, two-electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.

ChemInform Abstract: Progress in Molecular Uranium‐Nitride Chemistry

AbstractReview: 121 refs.

Infrared spectra and electronic structure calculations for NN complexes with U, UN, and NUN in solid argon, neon, and nitrogen.

Reactions of laser-ablated U atoms with N2 molecules upon codeposition in excess argon or neon at 4 K gave intense NUN and weak UN absorptions. Annealing produced progressions of new absorptions for the UN2(N2)1,2,3,4,5 and UN(N2)1,2,3,4,5,6 complexes. The neon-to-argon matrix shift decreases with increasing NN ligation and therefore the number of noble gas atoms left in the primary coordination sphere around the NUN molecule. Small matrix shifts are observed when the secondary coordination layers around the primary UN2(N2)1,2,3,4,5 and UN(N2)1,2,3,4,5,6 complexes are changed from neon-to-argon to nitrogen. Electronic structure, energy, and frequency calculations provide support for the identification of these complexes and the characterization of the N≡U≡N and U≡N core molecules as terminal uranium nitrides. Codeposition of U with pure nitrogen produced the saturated U(NN)7 complex, which UV irradiation converted to the NUN(NN)5 complex with slightly lower frequencies than found in solid argon.

Synthesis of Terminal Uranium(IV) Disulfido and Diselenido Compounds by Activation of Elemental Sulfur and Selenium

Rare stakes: Terminal uranium(IV) disulfido and diselenido compounds, Tp*2U(E2) (E=S, Se), were synthesized by the activation of elemental chalcogens. Structural, spectroscopic, computational and magnetic studies of these species establish their tetravalency and highly polarized U-E bonds.

Infrared Spectra and Electronic Structure Calculations for the NUN(NN)1–5 and NU(NN)1–6 Complexes in Solid Argon

Reactions of laser-ablated U atoms with N2 molecules in excess argon during co-deposition at 4 K gave intense NUN and weaker UN absorptions. Annealing increased progressions of new absorptions for the NUN(NN)1,2,3,4,5 and NU(NN)1,2,3,4,5,6 uranium nitride complexes. Small matrix shifts are observed when the secondary coordination layers around the primary NUN(NN)1,2,3,4,5 and NU(NN)1,2,3,4,5,6 complexes are changed from argon to nitrogen. Electronic structure and energy and frequency calculations provide support for the identification of these complexes and further characterization of the N≡U≡N and U≡N core molecules as terminal uranium nitrides with full triple bonds.