Multielectron C-O bond activation mediated by a family of reduced uranium complexes.

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp(P) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type Cp(X)UI2((Mes)PDI(Me)) (1-Cp(X); X = * or P), Cp(X)UI((Mes)PDI(Me)) (2-Cp(X)), and Cp(X)U((Mes)PDI(Me))(THF)n (3-Cp(X); *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of (Mes)PDI(Me) reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp(X), which has a doubly reduced (Mes)PDI(Me), with furfural results in radical coupling between the substrate and (Mes)PDI(Me), leading to C-C bond formation to form Cp(X)UI((Mes)PDI(Me)-CHOC4H3O) (4-Cp(X)). Exposure of 3-Cp* and 3-Cp(P), which contain a triply reduced (Mes)PDI(Me) ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)((Mes)PDI(Me)) (5-Cp*) and Cp(P)U(O2C2Ph4)((Mes)PDI(Me)) (5-Cp(P)). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1-5 have been characterized by (1)H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [(Mes)PDI(Me)](-) ligand in 1-Cp(P) and 5-Cp(P). Structural parameters of complexes 1-Cp(P), 2-Cp(X), 4-Cp*, and 5-Cp(X) have been elucidated by X-ray crystallography.