Reduction reactions involving more than two electrons are not common for metal complexes containing just one metal. In f-element chemistry, multielectron reductions from monometallic species are particularly rare since the common lowvalent ions Th III , U III , Eu II , Yb II , Sm II , and Tm II typically react as one-electron An IV /An III or Ln III /Ln II redox couples. [3Β±5] Recently however, we have shown that there is a new way of accomplishing reductive chemistry with f-element complexes by steric crowding [6, 7] and we report here how this method can be coupled with a traditional one-electron redox couple to accomplish multielectron reduction with a monometallic uranium complex.
It has recently been discovered that the sterically crowded [(C 5 Me 5 ) 3 Ln] complexes (Ln Sm, [6] Nd [7] ) can act as oneelectron reductants as shown in Equation ( ). The reductive reactivity of the [(C 5 Me 5 ) 3 Ln] complexes can be rationalized by the fact that the C 5 Me 5 Γ ions are not well-stabilized electrostatically in these compounds due to the long LnΓC(C 5 Me 5 ) distances caused by steric congestion. If the redox reaction in Equation ( ) is coupled to a one-electron metal-based reduction, for example, the U IV /U III couple of uranium, then a complex such as [(C 5 Me 5 ) 3 U] could be a two-electron reducing agent. This concept was tested by reacting [(C 5 Me 5 ) 3 U] with 1,3,5,7-cyclooctatetraene (C 8 H 8 ). It had been shown that U III complexes would act as one-electron reductants with cyclooctatetraene. [10] Our efforts to couple that reactivity with sterically induced reduction are reported here.
Reaction of [(C 5 Me 5 ) 3 U] with C 8 H 8 in a 1:1 stoichiometry formed (C 5 Me 5 ) 2 and a new product, 1, but residual [(C 5 Me 5 ) 3 U] remained at the end of the reaction. Subsequently, it was determined that a 2:3 [(C 5 Me 5 ) 3 U]:C 8 H 8 1: In a THF-free glovebox, 1,3,5,7-C 8 H 8 (24 mg, 0.23 mmol) was added to a brown solution of [(C 5 Me 5 ) 3 U] (100 mg, 0.155 mmol) in toluene (10 mL). After the brown solution was stirred at room temperature for 4 d, the solvent was removed by rotary evaporation. The gummy brown solid was washed with hexanes and dried under reduced pressure to afford 1 as a dark brown powder ( mg, 85 %). IR (KBr): n Γ 3022 m, 2903 s, 2851 m, 1435 w, 1373 w, 1020 w, 902 w, 797 w, 729 sh, 673 sh, 568 w cm Γ1 ; Elemental analysis (U 2 C 44 H 54 ): calcd: C 49.91, H 5.14; found: C 49.64, H 4.92; 1 H NMR (400 MHz, [D 6 ]benzene, 25 8C): d 5.5 (s, C 5 Me 5 ; Dn 1/2 10 Hz, 30 H), Γ 41.7 (s, C 8 H 8 ; Dn 1/2 14 Hz,16 H), Γ 42.2 (s, C 8 H 8 ; Dn 1/2 20 Hz, 8 H); 1 H NMR (400 MHz, [D 8 ]toluene, Γ 100 8C): d 14.5 (s, C 5 Me 5 ), Γ 80.8 (s, C 8 H 8 , 16 H), Γ 87.5 (s, C 8 H 8 , 8 H); 13 C NMR (100 MHz, [D 6 ]benzene, 25 8C): d 395, 280, 279, Γ 26.