Structure, Reactivity, and Density Functional Theory Analysis of the Six-Electron Reductant, (C5Me5)2U2(μ-η6:η6-C6H6), Synthesized via a New Mode of (C5Me5)3M Reactivity

The sterically crowded (C(5)Me(5))(3)U complex reacts with KC(8) or K/(18-crown-6) in benzene to form (C(5)Me(5))(2)U(2)(mu-eta(6):eta(6)-C(6)H(6)), 1, and KC(5)Me(5). These reactions suggested that (C(5)Me(5))(3)U could be susceptible to (C(5)Me(5))(1-) substitution by benzene anions via ionic salt metathesis. To test this idea in the synthesis of a more conventional product, (C(5)Me(5))(3)U was treated with KN(SiMe(3))(2) to form (C(5)Me(5))(2)UN(SiMe(3))(2) and KC(5)Me(5). 1 has long U-C(C(5)Me(5)) bond distances comparable to (C(5)Me(5))(3)U, and it too is susceptible to (C(5)Me(5))(1-) substitution via ionic metathesis: 1 reacts with KN(SiMe(3))(2) to make its amide-substituted analogue [(Me(3)Si)(2)N(C(5)Me(5))U](2)(mu-eta(6):eta(6)-C(6)H(6)), 2. Complexes 1 and 2 have nonplanar C(6)H(6)-derived ligands sandwiched between the two uranium ions. 1 and 2 were examined by reactivity studies, electronic absorption spectroscopy, and density functional theory calculations. (C(5)Me(5))(2)U(2)(mu-eta(6):eta(6)-C(6)H(6)) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form (C(5)Me(5))(C(8)H(8))U(2)(mu-eta(3):eta(3)-C(8)H(8)), (C(5)Me(5))(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 (C(5)Me(5))(1-) ligands, and two electrons from a bridging (C(6)H(6))(2-) moiety.