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(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 Δ G 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.