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Our understanding of the bonding, reactivity and electronic structure of actinides, though it has both fundamental and practical importance, lags behind that of the rest of the periodic table. A collection of articles in this Focus highlights recent developments in this area, in particular featuring uranium(VI) dianions bearing four U–N multiple bonds, berkelium(IV) stabilized in aqueous solution and a plutonium material showing evidence for the delocalization of 5f electrons.
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.
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.