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

Abstract

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.

Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes

“…Two resonances (18H each) for the o-and p-tertbutyl groups appear at −5.38 and 37.66 ppm, respectively, along with a smaller resonance (2H) shifted drastically to −248.69 ppm for the aryl−CH protons. This extreme upfield shift has been noted previously for uranium species bearing radical ligands, 26,28,29 leading to the assignment of the green product as (DOPO sq )UI 2 , with a semiquinone radical ligand by analogy to ( R isq)UI 3 (THF) 2 .…”

“…The solid state structure shows the THF rings are bent away from the Cp* ring, presumably relieving steric interactions. The U−Cp* cent distance of 2.493 Å is on the order of those for other U(IV)−Cp* complexes 26,31,40. As in the case for 2, the U−O distances (2.237(3) and 2.242(3) Å) point to anionic uranium−oxygen bonds, as would be expected for a reduced ligand framework.…”

Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2014

“…The entries in the series differed in the extent of pyridine(diimine) ligand reduction, which was made possible by treatment with potassium graphite. [149] M a n u s c r i p t For the larger Cp * derivative, benzophenone was too bulky to react, but analogous coupling chemistry was possible with benzaldehyde. In the latter case, an unusual M a n u s c r i p t 125 electronic structure for the monoanionic [ Mes PDI Me ] -ligand was noted and formulated based on the metrical parameters.…”

“…In the latter case, an unusual M a n u s c r i p t 125 electronic structure for the monoanionic [ Mes PDI Me ] -ligand was noted and formulated based on the metrical parameters. [149] It has been reported that the synthetic efficacy of thorium tetrachloride bis(dimethoxyethane), ThCl 4 (DME) 2 , is improved by adding 1,2-dimethoxyethane (DME)…”

“…With the sterically smaller Cp P derivative, a new pinacolate compound was formed by benzophenone coupling, whereas reactivity with aldehydes resulted in decomposition (Scheme 146) [149]. …”