Search NASA⌕ Search

Engineering topics

Bergman, Robert G.

Publications and source records attributed to Bergman, Robert G..

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Covalency-Driven Differences in the Hydrogenation Chemistry of Lanthanide- and Actinide-Based Frustrated Lewis Pairs

The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. As a result, consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (C 5 H 4 SiMe 3 ) 3 U prove to be key to the reversible formation of (C 5 H 4 SiMe 3 ) 3 U–H 2 (complex 1), and the paramagnetism of the f 3 center facilitates the detection of complex 1 by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (C 5 H 4 SiMe 3 ) 3 U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid–base complex. Here, the spectroscopic and computational results are compared to those reported for lanthanide metallocenes to yield insight into the nature of–and future possibilities for–f-element dihydrogen complexation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Source of Rate Acceleration for Carbocation Cyclization in Biomimetic Supramolecular Cages

The results of quantum chemical and molecular dynamics calculations reveal that polyanionic gallium-based cages accelerate cyclization reactions of pentadienyl alcohols as a result of substrate cage interactions, preferential binding of reactive conformations of substrate/H 3 O + pairs, and increased substrate basicity. However, the increase in basicity dominates. Here, experimental structure–activity relationship studies in which the metal vertices and overall charge of the cage are varied confirm the model derived via calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed. The enantioselectivities were correlated to the flexibility of the supramolecular host scaffold as measured by the kinetics of exchange of a model cationic guest. Finally, this correlation led to the development of a best-in-class catalyst by substituting the gallium(III)-based host with one based on indium(III), which generated the most flexible and selective catalyst.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Spectroscopic, Magnetic, and Computational Investigations on a Series of Rhenium(III) Cyclopentadienide β-diketiminate Halide and Pseudohalide Complexes

Here the low-valent rhenium(I) salt Na[Re(η 5 -Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) was shown to react with various halide and chalcogenolate reagents, leading to the isolation of a series of rhenium(III)-halide and -pseudohalide complexes: Re(X)(η 5 -Cp)(BDI) (1-X, X = F, Cl, Br, I; 2, X = OTf) and Re(ER)(η 5 -Cp)(BDI) (3-ER, ER = SBn, SeBn, TePh). The 1 H NMR spectra of these complexes displayed sharp resonances shifted several ppm from typical diamagnetic regions, as well as distinct chemical shift trends down both the halide (with the exception of F) and chalcogenolate series, with both Cp and BDI backbone peaks moving downfield with increasing atomic number. Subsequent magnetic susceptibility measurements of the rhenium(III) halides 1-Cl, 1-Br, and 1-I indicated that these complexes display temperature-independent paramagnetism (TIP), with χ TIP values of 7.41(44) × 10 -4 to 1.50(51) × 10 -3 cm 3 mol -1 . Multireference complete active space self-consistent field (CASSCF) computations incorporating spin-orbit state mixing revealed small energy separations (1.7-3.0 kcal/mol) between thermally isolated ground states and the first excited spin-orbit states for the rhenium halides, confirming that low-lying excited states are responsible for the observed TIP behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chemoselective and Site-Selective Reductions Catalyzed by a Supramolecular Host and a Pyridine–Borane Cofactor

Supramolecular catalysts emulate the mechanism of enzymes to achieve large rate accelerations and precise selectivity under mild and aqueous conditions. While significant strides have been made in the supramolecular host-promoted synthesis of small molecules, applications of this reactivity to chemoselective and site-selective modification of complex biomolecules remain virtually unexplored. We report here a supramolecular system where coencapsulation of pyridine-borane with a variety of molecules including enones, ketones, aldehydes, oximes, hydrazones, and imines effects efficient reductions under basic aqueous conditions. Furthermore, upon subjecting unprotected lysine to the host-mediated reductive amination conditions, we observed excellent ε-selectivity, indicating that differential guest binding within the same molecule is possible without sacrificing reactivity. Inspired by the post-translational modification of complex biomolecules by enzymatic systems, we then applied this supramolecular reaction to the site-selective labeling of a single lysine residue in an 11-amino acid peptide chain and human insulin.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

An isolated water droplet in the aqueous solution of a supramolecular tetrahedral cage

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid–solid phase transitions, but with structural and dynamical signatures that map onto known regions of water’s phase diagram. Using terahertz (THz) absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a dynamically distinct network of 9 ± 1 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the Ga 4 L 6 12– host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water. A further inference is that the release of the highly unusual encapsulated water droplet creates a strong thermodynamic driver for the high-affinity binding of guests in aqueous solution for the Ga 4 L 6 12– supramolecular construct.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Advances in supramolecular host-mediated reactivity

Since the trailblazing discoveries of Lehn, Cram and Pedersen, supramolecular chemistry has established itself as a cornerstone of organic chemistry. Supramolecular hosts offer defined microenvironments that mimic the active sites of enzymes, utilizing specific host–guest interactions to enable remarkable rate enhancements and product selectivity. The development of a diverse array of self-assembled hosts, coupled with the increased demand for shorter and greener synthetic routes, have spurred significant progress in the field of supramolecular catalysis. This Review Article covers recent advances in the field, ranging from novel organic reactivity aided by supramolecular hosts to catalytic cooperation between hosts and organometallic compounds or metal nanoparticles. Furthermore, strides have also been made in the synthetic application of these hosts in site-selective substrate modifications and challenging photochemical reactions. These efforts have enabled the incorporation of non-covalent macromolecular catalysis in natural product syntheses, evidencing their unique advantages as a synthetic tool, and their powerful potential for practical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗