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Multielectron C–O Bond Activation Mediated by a Family of Reduced Uranium Complexes

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journal contribution
posted on 07.04.2014, 00:00 by John J. Kiernicki, Brian S. Newell, Ellen M. Matson, Nickolas H. Anderson, Phillip E. Fanwick, Matthew P. Shores, Suzanne C. Bart
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 15 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.