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

[Image: see text] We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe(3))(2)}(2)(CH(2)SiMe(2)NSiMe(3)){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(3))(2)}(3)(MIC)] (1) where computational studies suggested that the 5f(3) 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(3))(2)}(2)(CH(2)SiMe(2)NSiMe(3))] (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 ΔG(298.15K) = 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.