Synthesis and Characterization of an Oxo-Centered Homotrimetallic Uranium(IV)–Cyclobutadienyl Dianion Complex

Reaction of [Li2{C4(SiMe3)4}­(THF)2] (1) with [U­(η5-C5Me5)­I2(THF)] (2) produced the oxo-centered homotrimetallic uranium–pentamethylcyclopentadienyl complex [{U­(η5-C5Me5)­(μ-I)2}3{μ3-O}­{Li­(THF)3}0.5]2[Li­(THF)4] (3) as the only isolable product in a very low yield. In contrast, reaction of 2 with [Mg­{C4(SiMe3)4}­(THF)3] (4) produced the oxo-centered homotrimetallic uranium­(IV)–cyclobutadienyl complex [{U­(C4[SiMe3]4)­(μ-I)2}3{μ3-O}]­[Mg­(THF)6] (5). The solid state structure of 5 reveals average U–C and U–C4 centroid distances of 2.574(7) and 2.355(7) Å, respectively, and displacements of the silicon atoms from the C4 plane ranging from 0.478(13) to 0.6528(12) Å. Variable-temperature magnetic susceptibility measurements on powdered 5 confirm the exclusively uranium­(IV) formulation with no evidence found for any uranium···uranium magnetic coupling. Quantum chemical calculations suggest polarized uranium–cyclobutadienyl bonding interactions but underscore the essentially exclusive π-bonding nature of these linkages with no δ-bonding component. This π-bonding also accounts for the displacements of the silyl groups from the C4 plane, which maximizes U–C4 orbital overlap. The compounds reported here have been variously characterized by single-crystal X-ray diffraction, ATR-IR spectroscopy, elemental analysis, SQUID magnetometry, and quantum chemical calculations.