Synthesis and Characterization of N-Heterocyclic Carbene Complexes of Uranium(III)

Reaction of [((AdArO)3tacn)UIII] (1) or [((Me3Si)2N)3UIII] (3) with tetramethylimidazol-2-ylidene (Me4IMC:) yields novel N-heterocyclic carbene complexes [((AdArO)3tacn)UIII(Me4IMC:)] (2) and [((Me3Si)2N)3UIII(Me4IMC:)] (4). Uranium complexes 2 and 4 represent the first examples of compounds with an N-heterocyclic carbene ligand coordinated to a low-valent uranium center. The paramagnetic complexes 1, 2, and 4 were characterized by 1H NMR, UV−vis−NIR, and EPR spectroscopy as well as SQUID magnetization measurements and X-ray diffraction analyses. DFT studies indicate a significant degree of π-bonding in the U(III)−carbene entity.