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

Area of Science:

• Organometallic Chemistry
• Actinide Chemistry
• Coordination Chemistry

Background:

• Mesoionic carbene (MIC) ligands are increasingly utilized in coordination chemistry.
• Actinide complexes with novel ligand systems are crucial for understanding fundamental bonding and reactivity.
• Thorium-carbene complexes are notably rare in the literature.

Purpose of the Study:

• To synthesize and characterize novel uranium(IV) and thorium(IV) mesoionic carbene complexes.
• To investigate the nature of actinide-carbene bonding through experimental and computational methods.
• To explore the solution-phase behavior and thermodynamic stability of the thorium complex.

Main Methods:

• Synthesis of uranium(IV) and thorium(IV) complexes via C-H activation and cyclometallation.
• Characterization using spectroscopic techniques (e.g., NMR) and quantum chemical calculations.
• Thermodynamic analysis of solution equilibria using variable-temperature NMR spectroscopy.

Main Results:

• Successful synthesis of uranium(IV) and thorium(IV) mesoionic carbene complexes.
• Computational studies indicate a predominantly sigma-bond for An-carbene in U(IV)/Th(IV) complexes, contrasting with U(III).
• The thorium(IV) complex exhibits an equilibrium with its metallacycle and free N-heterocyclic olefin, which is entropically favored but enthalpically endothermic.

Conclusions:

• The bonding in actinide-carbene complexes varies significantly with the actinide's oxidation state.
• The uranium(IV) complex exhibits a stronger and more covalent actinide-carbon bond compared to the thorium(IV) analogue.
• This work expands the scope of actinide carbene chemistry and provides insights into their electronic structure and stability.