Synthesis and Characterization of a Uranium(II) Monoarene Complex Supported by δ Backbonding

The low-temperature (< -35 degrees C) reduction of the trivalent uranium monoarene complex [{((ArO)-Ar-Ad,Me)(3)mes} U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)]-[(((ArO)-Ar-Ad,Me)(3)mes) U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature H-1 NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f (4) electronic configuration supported by delta backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc(+) is principally metal-centered.