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, (PDIMe)-P-Mes ((PDIMe)-P-Mes = 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 (CpUI2)-U-X((PDIMe)-P-Mes) (1-Cp-X; X = * or P), (CpUI)-U-X((PDIMe)-P-Mes) (2-Cp-X), and (CpU)-U-X((PDIMe)-P-Mes)(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 (PDIMe)-P-Mes reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp-X, which has a doubly reduced (PDIMe)-P-Mes, with furfural results in radical coupling between the substrate and (PDIMe)-P-Mes, leading to C-C bond formation to form (CpUI)-U-X((PDIMe)-P-Mes-CHOC4H3O) (4-Cp-X). Exposure of 3-Cp* and 3-Cp-P, which contain a triply reduced (PDIMe)-P-Mes ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)((PDIMe)-P-Mes) (5-Cp*) and (CpU)-U-P(O2C2Ph4)((PDIMe)-P-Mes) (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 H-1 NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [(PDIMe)-P-Mes] 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.