Actinide chemistry

Our understanding of actinide chemistry lags behind that of the rest of the periodic table. A collection of articles in this issue highlights recent progress featuring uranium(VI) dianions bearing four U–N multiple bonds, berkelium(IV) stabilized in solution and delocalization of 5f electrons in a plutonium material.

The field of high-valent uranium chemistry has been dominated by the linear uranyl moiety [UO₂]²⁺ and its imido analogues. A family of tetrakis(imido)uranate dianions has now been developed that displays four uranium–nitrogen multiple bonds. Their geometry is dictated by cation coordination and steric factors rather than electronic ones.

Berkelium is the only transplutonium element predicted to be able to exhibit both +III and +IV oxidation states in solution. Bk(IV) has now been stabilized through chelation with a siderophore derivative. The resulting neutral coordination compound was characterized and compared with the negatively charged species obtained by chelation of neighbouring trivalent actinides.

Unlike in the d block, intervalence charge transfer is rare in the 5f block owing to localized valence electrons and poor overlap between metal and ligand orbitals. Delocalization of 5f electrons has now been observed in a Pu(III)/Pu(IV)–pyridinedicarboxylate solid-state compound. It occurs through metal-to-ligand charge transfer with both plutonium centres.

Suzanne Bart from Purdue University talks to Nature Chemistry about her investigations into the chemistry of actinides, and why she finds them both challenging and rewarding.

The first new element produced after the Second World War has led a rather peaceful life since entering the period table — until it became the target of those producing superheavy elements, as Andreas Trabesinger describes.

Covalency in actinide–­ligand bonding is poorly understood compared to that in other parts of the periodic table due to the lack of experimental data. Here, pulsed electron paramagnetic resonance methods are used to directly measure the electron spin densities at coordinated ligands in molecular thorium and uranium complexes.

Introducing C–F bonds into organic molecules is a challenging task, particularly through C–H activation methods. Now, a uranium-based photocatalyst turns traditional selectivity rules on their heads and fluorinates unfunctionalized alkane Csp³–H bonds, even in the presence of C–H bonds that are typically more reactive.

Probing the chemistry of transuranic elements is notoriously challenging. Now, three neptunium(III) organometallic sandwich complexes have been prepared using a flexible macrocycle as ligand, and their molecular and electronic structures characterized, adding to our understanding of the behaviour of f-elements and suggesting that the lower oxidation state Np(II) may be chemically accessible.

The nature of actinide–ligand bonding is attracting attention, in particular in the context of nuclear waste separations. Structurally authenticated one-, two- and threefold uranium–arsenic bonding interactions are now reported. Computational analysis suggests the presence of polarized σ², σ²π², and σ²π⁴ in the arsenide, terminal arsinidene, and arsenido complexes, respectively.

Multi-electron redox chemistry is important in transition-metal-mediated processes, but is rarely observed with uranium due to its propensity to undergo single-electron reactions. Now, uranium can use its electrons, coupled with those stored in redox-active ligands, to perform multi electron reduction of organoazides and form uranium tris(imido) derivatives.

Actinides generally form ionic compounds, however, when electron-rich ligands with large hyperpolarizabilities are used, partially covalent bonds can also form. Now a rare californium borate is shown to exhibit significant differences from other f-elements in its structure and bonding. Quantum mechanical calculations support Cf and ligand orbital interactions, also indicating partial covalent bonding.

A terminal uranium(VI)–nitride has been shown to be accessible and isolable by a redox strategy whereas a photochemical approach resulted in decomposition. Computational analyses suggest that the U≡N triple bonds are surprisingly comparable to analogous group 6 transition metal nitrides, with a covalent character dominated by 5f rather than 6d contributions.

A complex featuring a uranium(VI) terminal nitride functional group has been isolated through mild oxidation, and shown to be highly reactive. Under photolysis, it converts into a compound that is capable of C–H bond activation.

The oxo groups in the common trans-uranyl ion — present in the majority of known uranium compounds — are linear and inert. Now, a new reduced binuclear uranium–dioxo compound with very strong metal coupling and remarkable air stability has been formed through oxo migration and silylation.

Simple uranium complexes, UX₃, are shown to disproportionate in the presence of a reducing agent under mild conditions, cooperatively binding and reducing arenes. This enables arene C–H bond activation and borylation, and the trapping of reactive substituted arenes in inverse sandwich complexes.

A {U₁₂Mn₆} wheel-shaped cluster that has been assembled through cation–cation interactions exhibits single-molecule-magnet behaviour. Single-molecule magnets are promising for magnetic storage devices at the nanoscale, and the observation of magnetic bistability with an open hysteresis loop and high relaxation barrier in this 5f–3d complex suggests that uranium-based compounds could be useful components.

Uranium and manganese cations have been combined in a wheel-shaped supramolecular assembly that retains its magnetic spin state after the external field is removed, with a high barrier to its relaxation. This cluster supports recent predictions of the usefulness of the actinides in single-molecule magnetic devices.

Single-molecule magnets (SMMs) are multinuclear clusters whose behaviour typically relies on intramolecular spin-coupling interactions between neighbouring metal ions. A diuranium–arene complex has now been prepared that shows behaviour characteristic of an SMM without relying on this type of superexchange mechanism. This may enable the construction of SMMs that maintain their magnetism at higher temperatures.

A diuranium compound featuring an arene bridge shows single-molecule-magnet behaviour, which could arise from a mechanism different from the traditional 'super-exchange' spin coupling.

Uranium oxo groups are very inert, in contrast with many transition metal oxo compounds that can carry out reactions that are difficult to achieve with other reagents. Now, the controlled lithiation of a ‘Pacman’ complex is shown to activate the uranium oxo group towards functionalization and single electron transfer.

The chemistry of the U≡N species is little known, even though solid uranium nitride has been proposed for use as a nuclear fuel. Now, photolysis of a uranium azide complex has been shown to release N₂ and generate a transient U≡N fragment that can activate C–H bonds.

The first thing that comes to mind about uranium is certainly not its similarity to iron. Chemists have now shown that the two have a lot in common.

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.