Brian S. Newell

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, (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.

Synthesis of terminal uranium(IV) disulfido and diselenido compounds by activation of elemental sulfur and selenium.

Rare stakes: Terminal uranium(IV) disulfido and diselenido compounds, Tp*2U(E2) (E=S, Se), were synthesized by the activation of elemental chalcogens. Structural, spectroscopic, computational and magnetic studies of these species establish their tetravalency and highly polarized U-E bonds.

Correlating Reactivity and Selectivity to Cyclopentadienyl Ligand Properties in Rh(III)-Catalyzed C–H Activation Reactions: An Experimental and Computational Study

CpXRh(III)-catalyzed C-H functionalization reactions are a proven method for the efficient assembly of small molecules. However, rationalization of the effects of cyclopentadienyl (CpX) ligand structure on reaction rate and selectivity has been viewed as a black box, and a truly systematic study is lacking. Consequently, predicting the outcomes of these reactions is challenging because subtle variations in ligand structure can cause notable changes in reaction behavior. A predictive tool is, nonetheless, of considerable value to the community as it would greatly accelerate reaction development. Designing a data set in which the steric and electronic properties of the CpXRh(III) catalysts were systematically varied allowed us to apply multivariate linear regression algorithms to establish correlations between these catalyst-based descriptors and the regio-, diastereoselectivity, and rate of model reactions. This, in turn, led to the development of quantitative predictive models that describe catalyst performance. Our newly described cone angles and Sterimol parameters for CpX ligands served as highly correlative steric descriptors in the regression models. Through rational design of training and validation sets, key diastereoselectivity outliers were identified. Computations reveal the origins of the outstanding stereoinduction displayed by these outliers. The results are consistent with partial η5-η3 ligand slippage that occurs in the transition state of the selectivity-determining step. In addition to the instructive value of our study, we believe that the insights gained are transposable to other group 9 transition metals and pave the way toward rational design of C-H functionalization catalysts.

Imidazolium-containing, hydrophobic-ionic-hydrophilic ABC triblock copolymers: synthesis, ordered phase-separation, and supported membrane fabrication

Novel ABC triblock copolymers containing hydrophobic, imidazolium ionic liquid (IL)-based ionic, and non-charged hydrophilic blocks were synthesized by direct sequential, ring-opening metathesis polymerization (ROMP) of three chemically immiscible norborene monomers. The resulting ABC triblock copolymers were found by small-angle X-ray scattering to phase-separate into different nanostructures in their pure melt states, depending on their block sequence and compositions. Supported composite membranes of these triblock copolymers were successfully fabricated with defect-free, ≤20 microns thick top coatings. Preliminary CO2/light gas transport studies demonstrated the potential of this new type of IL-based block copolymer material for gas separation applications.

Trivalent Uranium Phenylchalcogenide Complexes: Exploring the Bonding and Reactivity with CS2 in the Tp*2UEPh Series (E = O, S, Se, Te)

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U-E bond. All compounds have been characterized by (1)H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry. Computational studies establish Mulliken spin densities for the uranium centers ranging from 3.005 to 3.027 (B3LYP), consistent for uranium-chalcogenide bonds that are primarily ionic in nature, with a small covalent contribution. The reactivity of 2-4 toward carbon disulfide was also investigated and showed reversible CS2 insertion into the U(III)-E bond, forming Tp*2U(κ(2)-S2CEPh) (E = S (5), Se (6), Te (7)). Compound 5 was characterized crystallographically.

Synthesis and Investigation of [B20H17O(CH2)5]3–, a Novel Solvent Complex of the [B20H18]4– Ion

Reaction of the [trans-B(20)H(18)](2-) ion with the n-butoxide ion, formed in situ from reaction of n-butanol and NaH, in tetrahydropyran (THP) produces in good yield an unexpected and isolable solvent-coordinated polyhedral borane anion, [ae-B(20)H(17)O(CH(2))(5)](3-). The anticipated product of nucleophilic attack, [ae-B(20)H(17)On-Bu](4-), is not observed under the reaction conditions. The solvent-coordinated product is also formed in the presence of either ethoxide or carbamate ion but is not observed if the ethoxide or carbamate ion is not present in stoichiometric amounts. In the presence of the n-butanethiol anion, the coordinated THP ring undergoes a ring-opening reaction, yielding the [ae-B(20)H(17)O(CH(2))(5)Sn-Bu](4-) anion. Ring opening is also observed in the presence of the ethoxide ion in refluxing THP. Isolation of the previously proposed analogous solvent-coordinated tetrahydrofuran (THF) product, [ae-B(20)H(17)O(CH(2))(4)](3-), was unsuccessful; however, the product resulting from ring opening of THF by the n-butanethiol anion is reported.