Advancing actinide redox, hydride, and alkyl chemistry through sterically demanding heteroleptic aryloxide metallocene frameworks

Abstract

Actinide hydride and alkyl chemistry is governed by the unique electronic structure of f-element centers, wherein pronounced oxophilicity and a d⁰/fⁿ configuration favor polar, non-redox pathways over classical oxidative addition. As a result, actinide–hydrogen and actinide–carbon bonds exhibit reactivity dominated by insertion and σ-bond metathesis processes. Contemporary f-block organometallic chemistry is driven by the unique structural and electronic properties of actinide ions, which enable reactivity distinct from transition metals. Their large ionic radii support high coordination numbers and flexible geometries, leading to less saturated metal centers that readily engage in substrate binding and ligand exchange. The involvement of 5f and 6d orbitals imparts greater electrostatic character and electronic flexibility, enhancing metal electrophilicity and facilitating diverse bonding interactions. Historically, however, actinide organometallic chemistry was limited to early efforts focused on volatile species for nuclear applications, with the development of cyclopentadienyl frameworks emerging only later. As a result, key areas such as actinide hydride chemistry and redox reactivity have remained comparatively underexplored. These features position actinide hydride complexes as promising platforms for probing An–H bonding, f-element covalency, and non-traditional reactivity, with potential to expand the scope of organometallic transformations beyond established transition metal paradigms. This dissertation explores the synthesis, structure, and reactivity of actinide–hydride (An–H) and actinide–alkyl bonds, with an emphasis on their bonding characteristics and reactivity patterns relative to Group 4 transition metal analogues. Particular focus is placed on understanding the nature of metal–ligand covalency in f-element systems and how this influences fundamental reactivity, including interactions with small molecules and heterocyclic substrates. The first two chapters describe the synthesis of a rare terminal uranium(IV) hydride complex, [(C₅Me₅)₂(2,6-tBu₂-4-MeC₆H₂O)U(H)], prepared via hydrogenation of a uranium(IV) metallocene hydrocarbyl precursor. The reactivity of this complex was investigated toward a range of substrates to probe σ-bond metathesis (e.g., PhEEPh; E = S, Se, Te) and insertion reactions (e.g., CO₂, ethylene). Modification of the supporting ligand environment enabled the isolation of an ethenediolate-bridged uranium complex, providing insight into CO homologation pathways. Complementary studies involving isostructural hydride complexes of Hf, Th, and U, [(C₅Me₅)₂(2,6-iPr₂C₆H₃O)M(H)], allowed direct comparison of metal–hydride bonding across d- and f-block elements, with experimental and computational NMR analyses offering a benchmark for evaluating covalency. The last chapter addresses the challenge of C–H bond activation in heterocyclic substrates, in this case thiophenes, where transition metal systems are known to promote hydrogenation and C–S bond cleavage, but analogous f-element reactivity remains underdeveloped. In this context, a redox-inactive thorium–alkyl metallocene framework is shown to mediate an unusual sequence of ring opening, C–H activation, and C–C bond formation in thiophene derivatives. This reactivity contrasts sharply with Group 4 analogues and highlights the ability of actinide complexes to access complex bond reorganization pathways through σ-bond metathesis and thermodynamically driven, non-redox mechanisms. Collectively, these studies advance the understanding of An–H and An–C bonding, provide new insight into f-element covalency, and expand the scope of actinide-mediated transformations, particularly in the activation of challenging hydrocarbon and heterocyclic substrates.