Reductive silylation of uranyl mediated by iminosemiquinone ligands

Distinguishing Desirable and Undesirable Reactions in Multicomponent Systems for Redox Activation of the Uranyl Ion

Although it has been established that covalent functionalization of the U-O bonds in the uranyl dication (UO22+) generally requires use of strong reductants and electrophiles, little work has examined how interactions between the individual reaction components could affect final outcomes in solution. Here, the patterns of such reactivity have been studied in a UO22+-containing model system supported by a workhorse pentadentate ligand, 2,2'-[(methylimino)bis(2,1-ethanediylnitrilomethylidyne)]bis-phenol. Oxo activation and functionalization have been tested with (i) electrochemical and chemical reduction, and (ii) coordinating and noncoordinating solvents. In acetonitrile, uranyl reduction was achieved cleanly, but treatment of the reduced species with tris(pentafluorophenyl)borane (BCF) resulted in a mixture of products arising from direct electron transfer to BCF. In dichloromethane (CH2Cl2), electrochemical reduction of uranyl was achieved cleanly, but clean chemical reactivity was inaccessible. Despite these challenges, one trinuclear and oxo-deficient uranium-containing product was crystallized from CH2Cl2 solution and characterized; thus, desirable electrophilic reactivity can proceed to some degree in CH2Cl2 with BCF. Computational studies were used to investigate the properties of the trinuclear uranium product and the changes that could be inducible by further reduction. Taken together, the reactivity patterns identified here could inform design of improved systems for actinyl oxo functionalization.

Elucidating the Impact of Rare Earth or Transition Metal Identity on the Physical and Electronic Structural Properties of a Series of Redox-Active Tris(amido) Complexes

The use of redox-active ligands with the f-block elements has been employed to promote unique chemical transformations and explore their unique emergent electronic properties for a myriad of applications. In this study, we report eight new tris(amido) metal complexes: 1-Ln (Ln = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+), 1-La, and 1-Ti (an early transition metal analogue). The one-electron oxidation of the tris(amido) ligand was conducted to generate semi-iminato complexes 2-Ln, 2-La, and 2-Ti, and these complexes were studied using EPR. Tris(amido) complexes 1-Ln, 1-La, and 1-Ti were fully characterized using a range of spectroscopic (NMR and UV-vis/NIR) and physical techniques (X-ray diffraction and cyclic voltammetry, with the exception of 1-La). Computational methods were employed to further elucidate the electronic structures of these complexes. Lastly, complexes 1-Ln, 1-La, and 1-Ti were probed as catalysts for alkyl-alkyl cross-coupling, and the initial rate of the reaction was measured to explore the influence of the metal ion.

Coordination of Uranyl to the Redox-Active Calix[4]pyrrole Ligand

Reaction of [Li(THF)]₄[L] (L = Me₈-calix[4]pyrrole]) with 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]₂[L^Δ] (1), concomitant with formation of reduced uranium oxide byproducts. Complex 1 can also be generated by reaction of [Li(THF)]₄[L] with 1 equiv of I₂. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]₂[cis-U^VIO₂(calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ or 1 equiv of [U^VIO₂(OTf)₂(THF)₃], which affords the isostructural uranyl complexes, [Li(THF)][U^VIO₂(L^Δ)Cl(THF)] (2) and [Li(THF)][U^VIO₂(L^Δ)(OTf)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-η⁵-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, 2 and 3 exhibit some of the smallest O-U-O angles reported to date (2: 162.0(7) and 162.7(7)°; 3: 164.5(5)°). Importantly, the O-U-O bending observed in these complexes suggests that the oxidation of [Li(THF)]₄[L] does indeed occur via an unobserved cis-uranyl intermediate.