Stephen T. Liddle

A delocalized arene-bridged diuranium single-molecule magnet

Single-molecule magnets (SMMs) are compounds that, below a blocking temperature, exhibit stable magnetization purely of molecular origin, and not caused by long-range ordering of magnetic moments in the bulk. They thus show promise for applications such as data storage of ultra-high density. The stability of the magnetization increases with increasing ground-state spin and magnetic anisotropy. Transition-metal SMMs typically possess high-spin ground states, but insufficient magnetic anisotropies. Lanthanide SMMs exhibit large magnetic anisotropies, but building high-spin ground states is difficult because they tend to form ionic bonds that limit magnetic exchange coupling. In contrast, the significant covalent bonding and large spin-orbit contributions associated with uranium are particularly attractive for the development of improved SMMs. Here we report a delocalized arene-bridged diuranium SMM. This study demonstrates that arene-bridged polyuranium clusters can exhibit SMM behaviour without relying on the superexchange coupling of spins. This approach may lead to increased blocking temperatures.

Improving f-element single molecule magnets

Ever since the discovery that certain manganese clusters retain their magnetisation for months at low temperatures, there has been intense interest in molecular nanomagnets because of potential applications in data storage, spintronics, quantum computing, and magnetocaloric cooling. In this Tutorial Review, we summarise some key historical developments, and centre our discussion principally on the increasing trend to exploit the large magnetic moments and anisotropies of f-element ions. We focus on the important theme of strategies to improve these systems with the ultimate aim of developing materials for ultra-high-density data storage devices. We present a critical discussion of key parameters to be optimised, as well as of experimental and theoretical techniques to be used to this end.

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.

The Renaissance of Non-Aqueous Uranium Chemistry.

Prior to the year 2000, non-aqueous uranium chemistry mainly involved metallocene and classical alkyl, amide, or alkoxide compounds as well as established carbene, imido, and oxo derivatives. Since then, there has been a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-molecule activation, and magnetism have been reported. This Review 1) introduces the reader to some of the specialist theories of the area, 2) covers all-important starting materials, 3) surveys contemporary ligand classes installed at uranium, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compounds, 4) describes advances in the area of single-molecule magnetism, and 5) summarizes the coordination and activation of small molecules, including carbon monoxide, carbon dioxide, nitric oxide, dinitrogen, white phosphorus, and alkanes.

Synthesis of a Uranium(VI)-Carbene: Reductive Formation of Uranyl(V)-Methanides, Oxidative Preparation of a [R2C═U═O]2+ Analogue of the [O═U═O]2+ Uranyl Ion (R = Ph2PNSiMe3), and Comparison of the Nature of UIV═C, UV═C, and UVI═C Double Bonds

We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.

Homologation and functionalization of carbon monoxide by a recyclable uranium complex

Carbon monoxide (CO) is in principle an excellent resource from which to produce industrial hydrocarbon feedstocks as alternatives to crude oil; however, CO has proven remarkably resistant to selective homologation, and the few complexes that can effect this transformation cannot be recycled because liberation of the homologated product destroys the complexes or they are substitutionally inert. Here, we show that under mild conditions a simple triamidoamine uranium(III) complex can reductively homologate CO and be recycled for reuse. Following treatment with organosilyl halides, bis(organosiloxy)acetylenes, which readily convert to furanones, are produced, and this was confirmed by the use of isotopically 13C-labeled CO. The precursor to the triamido uranium(III) complex is formed concomitantly. These findings establish that, under appropriate conditions, uranium(III) can mediate a complete synthetic cycle for the homologation of CO to higher derivatives. This work may prove useful in spurring wider efforts in CO homologation, and the simplicity of this system suggests that catalytic CO functionalization may soon be within reach.

Regioselective C-H activation and sequential C-C and C-O bond formation reactions of aryl ketones promoted by an yttrium carbene.

Rare earth carbenes exclusively exhibit Wittig-type reactivity with carbonyl compounds to afford alkenes. Here, we report that yttrium carbenes can effect regioselective ortho-C-H activation and sequential C-C and C-O bond formation reactions of aryl ketones to give iso-benzofurans and hydroxymethylbenzophenones. With MeCOPh, cyclotetramerization occurs giving a substituted cyclohexene. This demonstrates new rare earth carbene reactivity which complements existing Wittig-type reactivity.

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.

The Inherent Single-Molecule Magnet Character of Trivalent Uranium†

SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell–Saunders multiplet engenders large magnetic anisotropies. [5–11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7, 12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13, 14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14, 16–20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15, 21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0o f 238 U cannot induce tunneling of the