The forefront of research into the complexes of uranium reveals chemical transformations that challenge and expand our view of this unique element. Certain ligands form multiple bonds to uranium, and small, inert molecules such as nitrogen and carbon dioxide become reactive when in complex with the metal. Such complexes provide clues to the catalytic future of uranium, in which the applications of the element extend far beyond the nuclear industry. Most excitingly, the ability of uranium to use its outermost f electrons for binding ligands might enable the element to catalyse reactions that are impossible with conventional, transition-metal catalysts.

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Towards uranium catalysts.

The electron-rich, six-coordinate tris-aryloxide uranium(III) complex [((AdArO)3tacn)UIII] [where (AdArOH)3tacn = 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane] reacts rapidly with CO2 to yield [((AdArO)3tacn)UIV(CO2)], a complex in which the CO2 ligand is linearly coordinated to the metal through its oxygen atom (η1-OCO). The latter complex has been crystallographically and spectroscopically characterized. The inequivalent O–C–O bond lengths [1.122 angstroms (A) for the O–C bond adjacent to uranium and 1.277 A for the other], considered together with magnetization data and electronic and vibrational spectra, support the following bonding model: UIV=O=C⚫–O– ↔ UIV–OC–O–. In these charge-separated resonance structures, the uranium center is oxidized to uranium(IV) and the CO2 ligand reduced by one electron.

https://science.sciencemag.org/content/sci/305/5691/1757.full.pdf

A Linear, O-Coordinated η1-CO2 Bound to Uranium

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.

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The Renaissance of Non-Aqueous Uranium Chemistry

Arene-Bridged Diuranium Complexes: Inverted Sandwiches Supported by δ Backbonding

This review describes recent computational investigations into the electronic and geometric structures of molecular actinide compounds. Following brief introductions to (i) the effects of relativity in chemistry and (ii) ab initio and density functional quantum chemical methods, four areas of contemporary research are discussed. These are π backbonding in uranium complexes, the geometric structures of bis benzene actinide compounds, the valence electronic structure of the uranyl ion, and the inverse trans influence in pseudo-octahedral [AnOX5]n−. Comparisons are made with experimental studies, and similarities and differences between d- and f-block chemistry are highlighted.

Recent developments in computational actinide chemistry.

Migratory insertion of an anionic group onto coordinate carbon monoxide or an isocyanide is an important mechanistic postulate in organoactinide chemistry. In contrast to transition metals, where carbon monoxide complexes abound, only three examples of carbon monoxide coordination to uranium have been observed in matrix isolation studies at cyrogenic temperatures. These studies showed that U(CO)/sub 6/ can exist below ca. 20 K and that nu/sub CO/ of 1961 cm/sup -1/ is similar to that found for W(CO)/sub 6/, nu/sub CO/ is 1987 cm/sup -1/ under similar conditions. The nu/sub CO/ is lowered substantially from gaseous CO (nu = 2145 cm/sup -1/) which implies that uranium metal is a ..pi..-donor, though the bonds are either kinetically labile, thermodynamically weak, or both. In UF/sub 4/(CO) the nu/sub CO/ of 2182 cm/sup -1/ at 20 K/sup 2c/ shows that the tetravalent compound does not engage in ..pi..-back-bonding to CO. In another study, UO/sub 2/ has been shown to absorb CO at temperatures below 20 K; the CO stretching frequency was not measured. In this paper, evidence is given for (Me/sub 3/SiC/sub 5/H/sub 4/)/sub 3/UCO, the first molecular actinide complex of carbon monoxide in solution and solid phase. 8 references.

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Preparation of the First Molecular Carbon Monoxide Complex of Uranium, (Me3SiC5H4)3UCO

Reactions of a Uranium(IV) Tertiary Alkyl Bond: Facile Ligand-Assisted Reduction and Insertion of Ethylene and Carbon Monoxide

Exposure of a solution of the metallocene (I) to CO at 1 atm and 20°C yields the title complex (II), which may be stored for at least 2 years at - 80°C. (I) also reversibly absorbs CO in the solid state.

John G. Brennan

Papers

Preparation of the First Molecular Carbon Monoxide Complex of Uranium, (Me3SiC5H4)3UCO

Reactions of a Uranium(IV) Tertiary Alkyl Bond: Facile Ligand-Assisted Reduction and Insertion of Ethylene and Carbon Monoxide

Preparation of the First Molecular Carbon Monoxide Complex of Uranium, (Me3SiC5H4)3UCO.