Structure, reactivity, and density functional theory analysis of the six-electron reductant, [(C5Me5)2U]2(μ-η6:η6-C6H6), synthesized via a new mode of (C5Me5)3M reactivity

The sterically crowded (C5Me5)(3)U Complex reacts with KC8 or K/(18-crown-6) in benzene to form [(C5Me5)(2)U](2)(mu-eta(6):eta(6)-C6H6), 1, and KC5Me5. These reactions suggested that (C5Me5)(3)U 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)(3)U was treated with KN(SiMe3)(2) to form (C5Me5)(2)U[N(SiMe3)(2)] and KC5Me5. 1 has long U-C(C5Me5) bond distances comparable to (C5Me5)(3)U, 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)(2)N](C5Me5)U}(2)(mu-eta(6):eta(6)-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)(2)U](2)(mu-eta(6):eta(6)-C6H6) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form [(C5Me5)(C8H8)U](2)(mu-eta(3):eta(3)-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.