From Chemical Curiosities and Trophy Molecules to Uranium-Based Catalysis: Developments for Uranium Catalysis as a New Facet in Molecular Uranium Chemistry

Abstract

Catalysis remains one of the final frontiers in molecular uranium chemistry. Depleted uranium is mildly radioactive, continuously generated in large quantities from the production and consumption of nuclear fuels and accessible through the regeneration of "uranium waste". Organometallic complexes of uranium possess a number of properties that are appealing for applications in homogeneous catalysis. Uranium exists in a wide range of oxidation states, and its large ionic radii support chelating ligands with high coordination numbers resulting in increased complex stability. Its position within the actinide series allows it to involve its f-orbitals in partial covalent bonding; yet, the U-L bonds remain highly polarized. This causes these bonds to be reactive and, with few exceptions, relatively weak, allowing for high substrate on/off rates. Thus, it is reasonable that uranium could be considered as a source of metal catalysts. Accordingly, uranium complexes in oxidation states +4, +5, and +6 have been studied extensively as catalysts in sigma-bond metathesis reactions, with a body of literature spanning the past 40 years. High-valent species have been documented to perform a wide variety of reactions, including oligomerization, hydrogenation, and hydrosilylation. Concurrently, electron-rich uranium complexes in oxidation states +2 and +3 have been proven capable of performing reductive small molecule activation of N₂, CO₂, CO, and H₂O. Hence, uranium's ability to activate small molecules of biological and industrial relevance is particularly pertinent when looking toward a sustainable future, especially due to its promising ability to generate ammonia, molecular hydrogen, and liquid hydrocarbons, though the advance of catalysis in these areas is in the early stages of development. In this Perspective, we will look at the challenges associated with the advance of new uranium catalysts, the tools produced to combat these challenges, the triumphs in achieving uranium catalysis, and our future outlook on the topic.

Figure 1

(A) Scott’s reported side-on-bound N₂-bridged diuranium complex; adapted from ref (2). (B) Cummins’ example of the first end-on-bound N₂-bridged heteronuclear U/Mo complex; adapted from ref (3). (C) Evans’ demonstration of a terminally bound uranium dinitrogen complex; adapted from ref (5). (D) Liddle’s report of the first example of a terminally bound uranium nitride; adapted from ref (6). (E) Mazzanti’s uranium “cluster” showing the activation of molecular dinitrogen in a uranium-based system capable of evolving ammonia; adapted from ref (8).

Figure 2

(A) Anderson’s terminally bound carbon monoxide complex; adapted from ref (10). (B) Meyer’s end-on-bound carbon dioxide complex; adapted from ref (14). (C) Cloke’s reductive CO cyclotrimerization; adapted from ref (12).

Figure 3

(A) Generalization of radical behavior displayed by [UO₂]²⁺ upon irradiation. (B) Sorenson’s radical fluorination using uranyl nitrate as a catalyst; adapted from ref (18). (C) Ravelli’s example of radical C–C bond formation using uranyl nitrate as a catalyst; adapted from ref (19). (D) Jiang’s oxidation of organic sulfides; adapted from ref (20). (E) Arnold’s photocatalytic oxidation of C–H bonds; adapted from ref (21).

Figure 4

(A) General process for oxidative addition and reductive elimination through a two-electron pathway. (B) General process for oxidative addition and reductive elimination through two single-electron processes either concerted or stepwise. (C) General mechanism for catalytic cross coupling reactions.

Figure 5

(A) Seyam’s example of reductive elimination from a uranium complex; adapted from ref (25). (B) Bart’s illustration of induced reductive elimination caused by introduction of a redox-active ligand; adapted from ref (26). (C) Liddle’s case of the reversible bonding of a diazo-complex, demonstrating a rare net four-electron reductive elimination; adapted from ref (27).

Figure 6

(A) Marks’ model of catalytic olefin hydrogenation, which is thought to function through a sigma-bond metathesis mechanism; adapted from ref (28). (B) Cloke’s example of alkene hydrogenation using uranium(III) complex 3; adapted from ref (29).

Figure 7

(A) Eisen’s linear polymerization of ε-caprolactone; adapted from ref (30). (B) Baker’s cyclic polymerization of ε-caprolactone using a uranyl complex; adapted from ref (31).

Figure 8

(A) Catalytic mechanism for alkyne dimer, trimer and cyclotrimerization reactions using uranium(IV) catalysts. (B) Distribution of products for each catalyst indicating different mechanisms for these reactions.

Figure 9

(A) Eisen’s example of catalytic hydroamination of alkynes; adapted from ref (34). (B) Marks’ blueprint for intramolecular hydroamination of tethered amines; adapted from ref (35). (C) Eisen’s work on catalytic reactions with various heteroallenes.

Figure 10

(A) Eisen’s hydrosilylation of alkynes; adapted from ref (39). (B) Cantat’s display of the importance of the steric bulk of silanes on the selectivity of hydrosilylation versus homocoupling of ethers; adapted from ref (40). (C) Eisen’s demonstration of the hydroboration of carbodiimides using catalyst 10; adapted from ref (41). (D) Eisen’s hydroboration of internal ketones using catalyst 13; adapted from ref (42).

Figure 11

Our proposed electrocatalytic cycle for the production of H₂ via water reduction, using the arene-supported uranium(III) catalyst [U^III((^Ad,MeArO)₃mes)] (14). Crucial steps include the oxidative addition of an O–H bond from H₂O and the cleavage of the U–OH bond with elimination of OH^–; adapted from ref (44).

Figure 12

Proposed photocatalytic cycle for Pal’s uranium-catalyzed generation of hydrogen; adapted from ref (45).

Figure 13

Arnold’s dinuclear [U(mTP)]₂ complex 15 capable of the production of ammonia; adapted from ref (46).

Figure 14

Closed-synthetic-cycle, demonstrating the synthesis of carbodiimides using the uranium(III) species [U((OAr^Ad,tBu)₃tacn)] 17; adapted from ref (48).

Figure 15

Cloke’s synthetic cycle for the production of methoxytrimethylsilane from CO and H₂ gas; adapted from ref (49).

Figure 16

Our example of CO₂ reduction to CO and CO₃^2–using a tris-aryloxide uranium(III) complex; adapted from ref (51).

Figure 17

Liddle’s demonstration of CO coupling and transformation to 2(5H)-furanone using the [U(tren^DMSB)]-system; adapted from ref (52).