Synthesis and characterization of a uranium(II) monoarene complex supported by δ backbonding

The low-temperature (<-35 C) reduction of the trivalent uranium monoarene complex [{(^Ad,MeArO)₃mes}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)][((^Ad,MeArO)₃mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature ¹H 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 5f⁴ electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc⁺ is principally metal-centered. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)