David M. King

Synthesis and structure of a terminal uranium nitride complex.

UN Coordination Uranium is best known for its radioactivity. From the standpoint of lower-energy chemistry, uranium is also intriguing for its bonding motifs, which involve trinodal f orbitals. King et al. (p. 717, published online 28 June; see the Perspective by Sattelberger and Johnson) synthesized and isolated a molecule bearing a uranium-nitrogen triple bond. Theoretical calculations allowed the mapping of the orbital interactions, distinguishing it from similar motifs in compounds of lighter metals. The preparation required use of a rigid, bulky ligand framework to keep the reactive uranium nitride group from binding to another molecule nearby, a pathway that has plagued prior attempts to prepare this class of compounds. A uranium triple bond to nitrogen makes use of the heavy element’s f orbitals. 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(TrenTIPS)][Na(12-crown-4)2] {in which TrenTIPS = [N(CH2CH2NSiPri3)3]3– and Pri = CH(CH3)2} by reaction of the uranium(III) complex [U(TrenTIPS)] 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 15N-isotopic labeling, electronic absorption spectroscopy, magnetometry, electronic structure calculations, elemental analyses, and liberation of ammonia after treatment with water.

Isolation and characterization of a uranium(VI)–nitride triple bond

The nature and extent of covalency in uranium bonding is still unclear compared with that of transition metals, and there is great interest in studying uranium-ligand multiple bonds. Although U=O and U=NR double bonds (where R is an alkyl group) are well-known analogues to transition-metal oxo and imido complexes, the uranium(VI)-nitride triple bond has long remained a synthetic target in actinide chemistry. Here, we report the preparation of a uranium(VI)-nitride triple bond. We highlight the importance of (1) ancillary ligand design, (2) employing mild redox reactions instead of harsh photochemical methods that decompose transiently formed uranium(VI) nitrides, (3) an electrostatically stabilizing sodium ion during nitride installation, (4) selecting the right sodium sequestering reagent, (5) inner versus outer sphere oxidation and (6) stability with respect to the uranium oxidation state. Computational analyses suggest covalent contributions to U≡N triple bonds that are surprisingly comparable to those of their group 6 transition-metal nitride counterparts.

Single-Molecule Magnetism in a Single-Ion Triamidoamine Uranium(V) Terminal Mono-Oxo Complex†

Single-molecule magnets (SMMs) are defined as molecules that exhibit slow relaxation of magnetization of purely molecular origin. SMMs are intensively researched not only for their important fundamental physics, but also because of potential applications in high-density data storage, quantum information processing, and spintronics. These applications are conceivable because SMMs generally possess well-isolated high-spin ground states in which spin– orbit coupling results in zero-field splitting of the (2S + 1)fold degenerate ground multiplet. 6] This phenomenon creates a thermal barrier to the relaxation of the magnetization which gives rise to slow magnetic relaxation and magnetic bistability. Great advances have been made with lanthanide SMMs arising from the huge magnetic anisotropies that can result from the crystal field splitting of the total angular momentum (J) ground states. Very recently, there has been great interest in actinide and especially uranium SMMs. This stems from the same phenomena, but with the potential advantage that uranium can engage in covalent bonding which can enable stronger magnetic interactions. However, only a handful of uranium-based SMMs have been isolated, and the ground rules for maximizing their blocking temperatures are not clear. Furthermore, when we initiated this study all uranium SMMs exploited the highly anisotropic (5f) Kramers ion uranium(III) which has an I9/2 ground state. We reasoned that a highly anisotropic, strongly axially coordinated ligand environment at a uranium(V) Kramers ion should engender increased magnetic anisotropy and thus SMM behavior, despite the smaller total angular momentum (F5/2) of uranium(V) compared to uranium(III). Here we report the first monometallic uranium(V), 5f SMM, where the physical properties result from imposing a strongly axial ligand field with C3v symmetry. Recently, a uranyl(V)/manganese(II) (UO2)12Mn6 N,N’ethylenebis(salicylimine) cluster was demonstrated to exhibit SMM behavior. In this cluster the authors speculate that there is significant exchange coupling between the uranyl(V) and Mn ions, hence the relative importance of the ligand field and exchange coupling to the SMM behavior was unclear. We now report a triamidoamine uranium(V) terminal-oxo complex which is a SMM. This monometallic uranium(V) SMM provides the first unambiguous confirmation that uranium(V) complexes can indeed exhibit SMM behavior. We previously reported the trivalent uranium complex [U(Tren)] [1, Tren = {N(CH2CH2NSiiPr3)3} 3 ] and its use in the preparation of a terminal uranium(V)-nitride by a two-electron oxidation with sodium azide and the abstraction of the sodium ion with [12]crown-4 ether. Similarly, two-electron oxidation of 1 with trimethyl-N-oxide in toluene afforded the terminal mono-oxo uranium(V) complex [U(O)(Tren)] (2 ; Scheme 1 and Figure 1) as red crystals in 52% yield following work-up and recrystallization from hexane.

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.

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.

Molecular and electronic structure of terminal and alkali metal-capped uranium(V) nitride complexes

Determining the electronic structure of actinide complexes is intrinsically challenging because inter-electronic repulsion, crystal field, and spin–orbit coupling effects can be of similar magnitude. Moreover, such efforts have been hampered by the lack of structurally analogous families of complexes to study. Here we report an improved method to U≡N triple bonds, and assemble a family of uranium(V) nitrides. Along with an isoelectronic oxo, we quantify the electronic structure of this 5f1 family by magnetometry, optical and electron paramagnetic resonance (EPR) spectroscopies and modelling. Thus, we define the relative importance of the spin–orbit and crystal field interactions, and explain the experimentally observed different ground states. We find optical absorption linewidths give a potential tool to identify spin–orbit coupled states, and show measurement of UV···UV super-exchange coupling in dimers by EPR. We show that observed slow magnetic relaxation occurs via two-phonon processes, with no obvious correlation to the crystal field.

Uranium halide complexes stabilized by a new sterically demanding tripodal tris(N-adamantylamidodimethylsilyl)methane ligand

Abstract The new tris(N-adamantylamine-dimethylsilyl)methane HC{SiMe2N(H)Ad}3 (TsAdH3, 1) and its trilithium salt [HC{SiMe2N(Li)Ad}3] (TsAdLi3, 2) were synthesized and characterized by multinuclear NMR and IR spectroscopy, elemental microanalysis, and single-crystal X-ray diffraction (XRD). The utility of 2 as a ligand transfer reagent for uranium was examined by targeting TsAd-uranium(IV) and (III) complexes via its reaction with UCl4 and UI3(THF)4; however, the crystalline material isolated, [U(TsAd)(Cl)(μ-Cl){Li(THF)3}] (4) and [U(TsAd)U(I)] (5), are products of lithium chloride inclusion and uranium disproportionation/ligand redistribution, respectively, and hints at the wide scope of reactivity accessible to TsAd-uranium complexes. Complex 5 was also independently synthesized stepwise in good yield from 2 via treatment with UCl4 and, subsequently, Me3SiI. The uranium complexes were characterized by a combination of NMR and IR spectroscopy, elemental microanalysis, magnetometric methods, and single-crystal XRD.