The importance of a single methyl group in determining the reaction chemistry of pentamethylcyclopentadienyl cyclooctatetraenyl uranium metallocenes

The steric factors that allow trivalent (C(5)Me(5))(3)U] (1) to function as a three-electron reductant with C(8)H(8) to form tetravalent {(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(8)H(8))] (2) have been explored by examining the synthesis and reactivity of the intermediate, "(C(5)Me(5))(2)(C(8)H(8))U]" (3), and the slightly less crowded analogues, (C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] and (C(5)Me(4)H)(2)(C(8)H(8))U], that have, successively one less methyl group. The reaction of {(C(5)Me(5))(C(8)H(8))U(μ-OTf)}(2)] (4; OTf=OSO(2) CF(3)) with two equivalents of KC(5)Me(5) in THF gave ring-opening to "(C(5)Me(5))(C(8)H(8))U{O(CH(2))(4)(C(5) Me(5))}]" consistent with in situ formation of 3. Reaction of 4 with two and four equivalents of KC(5)Me(4)H generates two equivalents of (C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] (5) and (C(5)Me(4)H)(2)(C(8)H(8))U] (6), respectively, which in contrast to 3 were isolable. Tetravalent 5 reduces phenazine and PhEEPh (E=S, Se, and Te) to form the tetravalent uranium reduction products, {(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(12)H(8)N(2))] (7), {(C(5)Me(5))(C(8)H(8))U}(2)(μ-SPh)(2)] (8), {(C(5)Me(5))(C(8)H(8))U}(2)(μ-SePh)(2)] (9), and {(C(5)Me(5))(C(8)H(8))U}(2)(μ-TePh)(2)] (10), consistent with sterically induced reduction. In contrast, the less sterically crowded 6 does not react with these substrates.