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

We
report the synthesis and characterization of uranium­(IV) and
thorium­(IV) mesoionic carbene complexes [An­{N­(SiMe3)2}2(CH2SiMe2NSiMe3)­{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(SiMe3)2}3(MIC)] (1) where computational
studies suggested that the 5f3 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(SiMe3)2}2(CH2SiMe2NSiMe3)] (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 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.