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, MesPDIMe (MesPDIMe = 2,6-((Mes)­NCMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized.
Using either Cp* or CpP (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide,
CpP = 1-(7,7-dimethylbenzyl)­cyclopentadienide), uranium
complexes of the type CpXUI2(MesPDIMe) (1-CpX; X = * or P),
CpXUI­(MesPDIMe) (2-CpX), and CpXU­(MesPDIMe)­(THF)n (3-CpX; *, n = 1; P, n = 0) were isolated and characterized. The series was generated via
ligand centered reduction events; thus the extent of MesPDIMe reduction varies in each case, but the uranium­(IV)
oxidation state is maintained. Treating 2-CpX, which has a doubly reduced MesPDIMe, with furfural results in radical coupling between the substrate
and MesPDIMe, leading to C–C bond formation
to form CpXUI­(MesPDIMe-CHOC4H3O) (4-CpX). Exposure
of 3-Cp* and 3-CpP, which contain a triply reduced MesPDIMe ligand,
to benzaldehyde and benzophenone, respectively, results in the corresponding
pinacolate complexes Cp*U­(O2C2Ph2H2)­(MesPDIMe) (5-Cp*) and CpPU­(O2C2Ph4)­(MesPDIMe) (5-CpP). 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 1H NMR and electronic absorption spectroscopies, and
SQUID magnetometry was employed to confirm the mono­(anionic) [MesPDIMe]− ligand in 1-CpP and 5-CpP. Structural parameters of complexes 1-CpP, 2-CpX, 4-Cp*, and 5-CpX have
been elucidated by X-ray crystallography.