Terminal Uranium Nitride Research Articles

A terminal neptunium(V)-mono(oxo) complex.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σu > πu energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium-mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal-ligand bond can be stabilized and studied in isolation.

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).

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.

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 and Characterization of an f-Block Terminal Parent Imido [U═NH] Complex: A Masked Uranium(IV) Nitride

Deprotonation of [U(TrenTIPS)(NH2)] (1) [TrenTIPS = N(CH2CH2NSiPri3)3] with organoalkali metal reagents MR (M = Li, R = But; M = Na–Cs, R = CH2C6H5) afforded the imido-bridged dimers [{U(TrenTIPS)(μ-N[H]M)}2] [M = Li–Cs (2a–e)]. Treatment of 2c (M = K) with 2 equiv of 15-crown-5 ether (15C5) afforded the uranium terminal parent imido complex [U(TrenTIPS)(NH)][K(15C5)2] (3c), which can also be viewed as a masked uranium(IV) nitride. The uranium–imido linkage was found to be essentially linear, and theoretical calculations suggested σ2π4 polarized U–N multiple bonding. Attempts to oxidize 3c to afford the neutral uranium terminal parent imido complex [U(TrenTIPS)(NH)] (4) resulted in spontaneous disproportionation to give 1 and the uranium–nitride complex [U(TrenTIPS)(N)] (5); this reaction is a new way to prepare the terminal uranium–nitride linkage and was calculated to be exothermic by −3.25 kcal mol–1.

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.

Recent developments in actinide–ligand multiple bonding

In the last few years, considerable progress has been made in the synthesis and characterization of complexes containing actinide-ligand multiple bonds, especially in regards to isolation of terminal oxo, nitrido and carbene-containing complexes. This review summarizes the synthesis, structure, and reactivity of these complexes, from 2010 until present. These complexes are of interest for a variety of reasons, including their potential use in novel catalytic transformations and their ability to engage the 5f orbitals in metal-ligand bonding. Of particular note are the recent syntheses of the first isolable complex containing the nitride-substituted uranyl ion, [NUO](+), and the first report of an isolable terminal uranium nitride. Considerable progress has also been made toward the syntheses of actinide carbenes and thorium complexes containing metal-ligand multiple bonds.

Synthesis and Structure of a Terminal Uranium Nitride Complex

The terminal uranium nitride linkage is a fundamental target in the study of f-orbital participation in metal-ligand multiple bonding but has previously eluded characterization in an isolable molecule. Here, we report the preparation of the terminal uranium(V) nitride complex [UN(Tren(TIPS))][Na(12-crown-4)(2)] {in which Tren(TIPS) = [N(CH(2)CH(2)NSiPr(i)(3))(3)](3-) and Pr(i) = CH(CH(3))(2)} by reaction of the uranium(III) complex [U(Tren(TIPS))] with sodium azide followed by abstraction and encapsulation of the sodium cation by the polydentate crown ether 12-crown-4. Single-crystal x-ray diffraction reveals a uranium-terminal nitride bond length of 1.825(15) angstroms (where 15 is the standard uncertainty). The structural assignment is supported by means of (15)N-isotopic labeling, electronic absorption spectroscopy, magnetometry, electronic structure calculations, elemental analyses, and liberation of ammonia after treatment with water.

Combined Triple and Double Bonds to Uranium: The N≡U═N−H Uranimine Nitride Molecule Prepared in Solid Argon

Reactions of laser-ablated U atoms with N(2) and H(2) mixtures upon codeposition in excess argon at 5 K gave strong NUN and weak UN infrared absorptions and new bands at 3349.7, 966.9, 752.4, and 433.0 cm(-1) for the unusual new U(V) molecule N≡U=N-H, uranimine nitride, containing both triple and double bonds. This identification is based on D and (15)N isotopic substitution and comparison with frequencies computed by density functional theory for the (2)Δ ground state NUNH. Calculated bond lengths are compared to those of the (1)Σ(g)(+) ground state of U(VI) uranium dinitride N≡U≡N, the (2)Φ ground state of the isoelectronic nitride oxide N≡U=O, and the (3)A ground state of the U(IV) uranimine dihydride HN=UH(2) molecule, which have all been prepared in solid argon matrices. Mulliken bond orders based on the CASSCF orbitals for N≡U=N-H are 2.91, 2.19, and 1.05, respectively. Here, the terminal nitride is effectively a triple bond, just as found for N≡U≡N. The solid argon matrix is a convenient medium to isolate reactive terminal uranium nitrides for examination of their spectroscopic properties.

Uranium azide photolysis results in C–H bond activation and provides evidence for a terminal uranium nitride

Uranium nitride [U[triple bond]N](x) is an alternative nuclear fuel that has great potential in the expanding future of nuclear power; however, very little is known about the U[triple bond]N functionality. We show, for the first time, that a terminal uranium nitride complex can be generated by photolysis of an azide (U-N=N=N) precursor. The transient U[triple bond]N fragment is reactive and undergoes insertion into a ligand C-H bond to generate new N-H and N-C bonds. The mechanism of this unprecedented reaction has been evaluated through computational and spectroscopic studies, which reveal that the photochemical azide activation pathway can be shut down through coordination of the terminal azide ligand to the Lewis acid B(C(6)F(5))(3). These studies demonstrate that photochemistry can be a powerful tool for inducing redox transformations for organometallic actinide complexes, and that the terminal uranium nitride fragment is reactive, cleaving strong C-H bonds.