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.