Mesoionic Carbene Complexes of Uranium(IV) and Thorium(IV)

John A. Seed, Lisa Vondung, Ralph W. Adams, Ashley J. Wooles, Erli Lu, Stephen T. Liddle

Research output: Contribution to journalArticlepeer-review

Abstract

We report the synthesis and characterization of the uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe3)2}2(CH2SiMe2NSiMe3){MIC}] (An = U, 4U; Th, 4Th; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C-H activation intramolecular cyclometallation reaction of actinide-halides, with concomitant formal 1,4-proton migration of an N-heterocyclic olefin (NHO). Quantum chemical calculations suggest the An-carbene bond is comprised of only a σ-component, in contrast to the uranium(III) analogue, [U{N(SiMe3)2}3(MIC)] (1) where computational studies suggested that the 5f3 uranium(III) ion engages in a weak one-electron π-back-bond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe3)2}2(CH2SiMe2NSiMe3)] (6Th) and free NHO (3). The thermodynamic parameters of this equilibrium were probed using variabletemperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of ΔG298.15K = 0.89 kcal mol-1. Energy-decomposition analysis (EDA-NOCV) of the actinide-carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes.
Original languageEnglish
JournalOrganometallics
Publication statusAccepted/In press - 28 Apr 2022

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