Terminal Uranium Research Articles

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.

Reductive Cleavage of Nitrite to Form Terminal Uranium Mono-Oxo Complexes

Uranium terminal mono-oxo complexes are prepared with a unique activation of nitrite following reductive cleavage of an N-O bond with loss of nitric oxide. The thermodynamic driving force of U═O bond formation differentiates this reactivity from known mechanisms of nitrite reduction, which are typically mediated by proton transfer. Mechanistic details are explored by DFT supporting a simple homolytic cleavage pathway from a κ(1)-ONO bound intermediate. Complexes of the formula U(VI)OX[N(SiMe(3))(2)](3) are formed providing a trigonal bipyramidal framework into which ligands trans to the U═O bond may be installed.

A Complete Family of Terminal Uranium Chalcogenides, [U(E)(N{SiMe3}2)3]− (E = O, S, Se, Te)

Addition of 1 equiv of E (E = 0.125 S(8), Se, Te) to U(H(2)C═PPh(3))(NR(2))(3) (R = SiMe(3)) (1) in Et(2)O results in generation of the terminal chalcogenide complexes, [Ph(3)PCH(3)][U(E)(NR(2))(3)] (E = S, 2; Se, 3; Te, 4; R = SiMe(3)), in modest yield. Complexes 2-4 represent extremely rare examples of terminal uranium monochalcogenides. Synthesis of the oxo analogue, [Cp*(2)Co][U(O)(NR(2))(3)] (5), was achieved by reduction of [U(O)(NR(2))(3)] with Cp*(2)Co. All complexes were fully characterized, including analysis by X-ray crystallography. In the solid state, complexes 2-5 feature short U-E bond lengths, suggestive of actinide-ligand multiple bonding.

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.

Synthesis of Uranium(VI) Terminal Oxo Complexes: Molecular Geometry Driven by the Inverse Trans-Influence

Oxidation of our previously reported uranium(V) oxo complexes, supported by the chelating ((R)ArO)(3)tacn(3-) ligand system (R = tert-butyl (t-Bu), 1-t-Bu; R = 1-adamantyl (Ad), 1-Ad), yields terminal uranium(VI) oxo complexes [(((R)ArO)(3)tacn)U(VI)(O)]SbF(6) (R = t-Bu, 2-t-Bu; R = Ad, 2-Ad). These complexes differ in their molecular geometry in that 2-t-Bu possesses pseudo-C(s) symmetry in solution and solid state as the terminal oxo ligand lies in the equatorial plane (as defined by the three aryloxide arms of the ligand) in order to accommodate the thermodynamic preference of high-valent uranium oxo complexes to have a σ- and π-donating ligand trans to the oxo (vis-à-vis the ubiquity of the linear UO(2)(2+) moiety). The distortion of the ligand--which stands in contrast to all other complexes of uranium supported by the ((R)ArO)(3)tacn(3-) ligand, including 2-Ad--is most clearly seen in the structures of 2-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)]SbF(6), and 3-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF(3))(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O(ArO) bond length is shortened by 0.1 Å in comparison to the cis U-O(ArO) bonds and the trans U-O-C(ipso) angle is linearized (157.67° versus 147.85° and 130.03°). Remarkably, the minor modification of the ligand to have Ad groups at the ortho positions of the aryloxide arms is sufficient to stabilize a C(3v)-symmetric terminal uranium(VI) oxo complex (2-Ad) without a ligand trans to the oxo. These experimental results were reproduced in DFT calculations and allow the qualitative bracketing of the relative thermodynamic stabilization afforded by the inverse trans-influence as ∼6 kcal mol(-1).

Synthesis of U(iv) imidos from Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by extrusion of bibenzyl

Addition of organic azides, N(3)R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(iv) imido derivatives, Tp*(2)U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both (1)H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ(3)-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ(2)-coordination of the pyrazoles. This results in a uranium-hydrogen interaction with the Tp* B-H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*(2)U(O), and the corresponding imine.

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.

Now U=C it

A terminal uranium–carbon multiple bond has long been sought-after in actinide chemistry. Now, a complex featuring a dianionic carbon atom as part of a multidentate ligand brings actinide carbenes a little nearer.