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 N 2 , CO 2 , CO, and H 2 O. 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.

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

“…Examples of catalytic reactions mediated by uranium complexes have appeared in recent decades. [68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83] For example, Meyer and coworkers reported an example of uranium-mediated electrocatalytic production of H 2 from H 2 O. 76,77 Recently, Arnold and coworkers demonstrated that a dinuclear uranium complex could bind N 2 , mediating its reduction and functionalization to NH 3 .…”

Heterometallic Clusters with Uranium–Metal Bonds Supported by Double-Layer Nitrogen–Phosphorus Ligands

“…54 These results represent the first example of alkyne hydroboration catalyzed by a multimetallic U−Co species. 55…”

[AnI3(THF)4] (An = Np, Pu) Preparation Bypassing An0 Metal Precursors: Access to Np3+/Pu3+ Nonaqueous and Organometallic Complexes

“…Progress has ranged from the discovery of new oxidation states (An 2+ ; An = Th, U, Np, Pu) 13,[17][18][19]23,24 to an array of non-actinyl multiple-bond chemistry, 1−3,25−32 to covalency studies 4,9,26,33−36 and small molecule reactivity. 15,37,38 An expanding and diverse synthetic "toolbox" of nonaqueous, organic solvent soluble Th/U starting materials has been key in facilitating the explosion of molecular chemistry studies for these early actinide elements. Addressing the documented limitation of U 3+ starting materials, 39 in the late 1980s and mid-1990s, facile routes were reported to [UI 3 (THF) 4 ] (THF = tetrahydrofuran), [UI 3 (DME) 2 ] (DME = 1,2-dimethoxyethane), [UI 3 (py) 4 ] (py = pyridine), and [UBr 3 (THF) 4 ], through oxidation of amalgamated uranium metal turnings with I 2 or Br 2 in Lewis base solvents.…”

“…The scientific understanding of the role that f-electrons play in the bonding, electronic structure, − redox chemistry, − and physics of the actinides (An) is continually evolving. ,, Widespread advances over the last 30 years have remapped the known boundaries of molecular Th and, in particular, U molecular chemistry. Progress has ranged from the discovery of new oxidation states (An 2+ ; An = Th, U, Np, Pu) ,− ,, to an array of non-actinyl multiple-bond chemistry, − ,− to covalency studies ,,,− and small molecule reactivity. ,, An expanding and diverse synthetic “toolbox” of nonaqueous, organic solvent soluble Th/U starting materials has been key in facilitating the explosion of molecular chemistry studies for these early actinide elements. Addressing the documented limitation of U 3+ starting materials, in the late 1980s and mid-1990s, facile routes were reported to [UI 3 (THF) 4 ] (THF = tetrahydrofuran), [UI 3 (DME) 2 ] (DME = 1,2-dimethoxyethane), [UI 3 (py) 4 ] (py = pyridine), and [UBr 3 (THF) 4 ], through oxidation of amalgamated uranium metal turnings with I 2 or Br 2 in Lewis base solvents. , This route to [UI 3 (THF) 4 ] (and other Lewis base adducts) as well as the synthesis of [U­(N″) 3 ] (N″ = {N­(SiMe 3 ) 2 }) generated from these halide complexes (which was more convenient than prior routes to [U­(N″) 3 ], which comprised the in situ reduction of UCl 4 with Na/(C 10 H 8 ) and then reaction with NaN″), has become a staple of U chemistry. , Building upon those contributions, additional precursors and routes to U 3+ and U 4+ synthons have been established using U 0 metal as the source material and also metallic-phase-free routes.…”