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

Authors :: Seed JA
Vondung L
Adams RW
Wooles AJ
Lu E
Liddle ST
Source :: Organometallics [Organometallics] 2022 Jun 13; Vol. 41 (11), pp. 1353-1363. Date of Electronic Publication: 2022 May 18.
Publication Year :: 2022
Abstract: We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe <subscript>3</subscript> ) <subscript>2</subscript> } <subscript>2</subscript> (CH <subscript>2</subscript> SiMe <subscript>2</subscript> NSiMe <subscript>3</subscript> ){MIC}] (An = U, 4U and 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 that the An-carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe <subscript>3</subscript> ) <subscript>2</subscript> } <subscript>3</subscript> (MIC)] ( 1 ) where computational studies suggested that the 5f <superscript>3</superscript> uranium(III) ion engages in a weak one-electron π-backbond 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(SiMe <subscript>3</subscript> ) <subscript>2</subscript> } <subscript>2</subscript> (CH <subscript>2</subscript> SiMe <subscript>2</subscript> NSiMe <subscript>3</subscript> )] ( 6Th ) and free NHO ( 3 ). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of Δ G <subscript>298.15K</subscript> = 0.89 kcal mol <superscript>-1</superscript> . 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.<br />Competing Interests: The authors declare no competing financial interest.<br /> (© 2022 The Authors. Published by American Chemical Society.)