Structure, Reactivity, and Density Functional Theory Analysis of the Six-Electron Reductant, [(C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>U]<sub>2</sub>(μ<i>-</i>η<sup>6</sup>:η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>), Synthesized via a New Mode of (C<sub>5</sub>Me<sub>5</sub>)<sub>3</sub>M Reactivity
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The sterically crowded (C5Me5)3U complex reacts with KC8 or K/(18-crown-6) in benzene to form [(C5Me5)2U]2(μ-η6:η6-C6H6), 1, and KC5Me5. These reactions suggested that (C5Me5)3U could be susceptible to (C5Me5)1- substitution by benzene anions via ionic salt metathesis. To test this idea in the synthesis of a more conventional product, (C5Me5)3U was treated with KN(SiMe3)2 to form (C5Me5)2U[N(SiMe3)2] and KC5Me5. 1 has long U−C(C5Me5) bond distances comparable to (C5Me5)3U, and it too is susceptible to (C5Me5)1- substitution via ionic metathesis: 1 reacts with KN(SiMe3)2 to make its amide-substituted analogue {[(Me3Si)2N](C5Me5)U}2(μ-η6:η6-C6H6), 2. Complexes 1 and 2 have nonplanar C6H6-derived ligands sandwiched between the two uranium ions. 1 and 2 were examined by reactivity studies, electronic absorption spectroscopy, and density functional theory calculations. [(C5Me5)2U]2(μ-η6:η6-C6H6) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form [(C5Me5)(C8H8)U]2(μ-η3:η3-C8H8), (C5Me5)2, and benzene. This multielectron transformation can be formally attributed to three different sources: two electrons from two U(III) centers, two electrons from sterically induced reduction by two (C5Me5)1- ligands, and two electrons from a bridging (C6H6)2- moiety.
