Publications

Design, Synthesis, and Biological Evaluation of a Robust, Biodegradable Dendrimer

PEGylated dendrimers are attractive for biological applications due to their tunable pharmacokinetics and ability to carry multiple copies of bioactive molecules. The rapid and efficient synthesis of a robust and biodegradable PEGylated dendrimer based on a polyester-polyamide hybrid core is described. The architecture is designed to avoid destructive side reactions during dendrimer preparation while maintaining biodegradability. Therefore, a dendrimer functionalized with doxorubicin (Dox) was prepared from commercial starting materials in nine, high-yielding linear steps. Both the dendrimer and Doxil were evaluated in parallel using equimolar dosage in the treatment of C26 murine colon carcinoma, leading to statistically equivalent results with most mice tumor-free at the end of the 60 day experiment. The attractive features of this dendritic drug carrier are its simple synthesis, biodegradability, and versatility for application to a variety of drug payloads with high drug loadings.

β-Phosphinoethylboranes as Ambiphilic Ligands in Nickel−Methyl Complexes

The ambiphilic β-phosphinoethylboranes Ph2PCH2CH2BR2 (BR2 = BCy2 (1a), BBN (1b)), which feature the ethano spacer CH2CH2 between the Lewis acidic boryl and Lewis basic phosphino groups, were synthesized in nearly quantitative yields via the hydroboration of vinyldiphenylphosphine. Compounds 1a,b were fully characterized by elemental analysis and by NMR and IR spectroscopy. X-ray crystallographic studies of compound 1b revealed infinite helical chains of the molecules connected through P···B donor–acceptor interactions. The ability of these ambiphilic ligands to concurrently act as donors and acceptors was highlighted by their reactions with (dmpe)NiMe2. The zwitterionic complexes (dmpe)NiMe(Ph2PCH2CH2BCy2Me) (2a) and (dmpe)NiMe(Ph2PCH2CH2[BBN]Me) (2b) were generated via the abstraction of one of the methyl groups, forming a borate, and intramolecular coordination of the phosphine moiety to the resulting cationic metal center. Compound 2b was characterized by X-ray crystallography. Furthermore, B(C6F5)3 abstracts the methyl group of a coordinated borate ligand to generate a free, three-coordinate borane center in [(dmpe)NiMe(1a)]+[MeB(C6F5)3]– (3).

Cobalt-Mediated [3 + 2]-Annulation Reaction of Alkenes with α,β-Unsaturated Ketones and Imines

The utility of cobalt dinitrosyl complexes for the [3 + 2] annulation of alkenes with unsaturated enones and ketimines has been demonstrated. Reaction of a series of cobalt dinitrosyl/alkene adducts with conjugate acceptors in the presence of Sc(OTf)(3)/LHMDS formed two new C-C bonds at the carbons alpha to the nitrosyl groups of the substrate, leading to unusual tri- and tetracycles. Retrocycloaddition of these products in the presence of norbornadiene yielded functionalized tetrasubstituted bicyclic olefins.

CCDC 1827473: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

CCDC 922544: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Functionalized Silicone Nanospheres: Synthesis, Transition Metal Immobilization, and Catalytic Applications

Silicone nanospheres containing a variety of functional groups (pyridines, phosphines, thiols, amines, etc.) have been prepared by emulsion copolymerization of methyltrimethoxysilane, MeSi(OMe)3, and the functionalized monomer of interest, RSi(OMe)3. This procedure provides a reproducible synthesis of spherical particles in the 12−28 nm size regime as determined by transmission electron microscopy (TEM). The presence of the functional groups is supported by a combination of spectroscopic methods including DRUV−vis, DRIFTS, and NMR spectroscopy. Comonomer dispersity within the nanospheres was probed using elemental mapping techniques, and these support a homogeneous distribution of functional groups within the particles. Palladium(0) immobilization on phosphine-substituted nanospheres also results in a random distribution of the transition metal throughout the particles. Nanospheres containing multiple acid/base functionalities were also prepared, and these demonstrate functional group cooperativity based on enhanced conversions in the base-catalyzed Henry reaction, relative to nanosphere catalysts containing only basic groups. The diversity of functional groups that may be incorporated into the spheres suggests that these materials hold considerable promise as ligand supports and catalysts.

Topological insulators beyond the Brillouin zone via Chern parity

The topological insulator is an electronic phase stabilized by spin-orbit coupling that supports propagating edge states and is not adiabatically connected to the ordinary insulator. In several ways it is a spin-orbit-induced analogue in time-reversal-invariant systems of the integer quantum Hall effect (IQHE). This paper studies the topological insulator phase in disordered two-dimensional systems, using a model graphene Hamiltonian introduced by Kane and Mele as an example. The nonperturbative definition of a topological insulator given here is distinct from previous efforts in that it involves boundary phase twists that couple only to charge, does not refer to edge states, and can be measured by pumping cycles of ordinary charge. In this definition, the phase of a Slater determinant of electronic states is determined by a Chern parity analogous to Chern number in the IQHE case. Numerically we find, in agreement with recent network model studies, that the direct transition between ordinary and topological insulators that occurs in band structures is a consequence of the perfect crystalline lattice. Generically these two phases are separated by a metallic phase, which is allowed in two dimensions when spin-orbit coupling is present. The same approach can be used to study three-dimensional topological insulators.

Photon extraction: the key physics for approaching solar cell efficiency limits

Theoretical efficiency limits are useful primarily because they provide a means for selecting which technologies to pursue, and they are a driving force for further progress. Yet implicit in such a process is the assumption that the upper limit provides a realistic estimate of potential performance. Real systems will never be perfect, but small deviations in material quality or optical design should yield only small deviations in performance. Shockley-Queisser efficiencies are not robust to small deviations. Although they provide a simple calculational tool, they obscure important internal dynamics. We examine these dynamics, resulting in a surprising conclusion: instead of considering external emission as a loss mechanism, it should actually be designed for. A solar cell must have almost perfect photon extraction, or it will fall far short of the Shockley-Queisser efficiency limit.

Kinetic Pathways Templated by Low-Temperature Intermediates during Solid-State Synthesis of Layered Oxides

Layered oxides have been the dominant cathodes in lithium-ion batteries, and among them, high-nickel (Ni) systems are attractive because of their high capacity. For practical use, synthetic control of stoichiometry and structural ordering is crucial but has been nontrivial due to the complexity inherent to synthesis reactions, which often proceed via nonequilibrium pathways. In this paper, we report here a combined in situ synchrotron X-ray diffraction and ab initio study of solid-state synthesis of layered oxides starting from acetate precursors for LiCoO<sub>2</sub> and LiNiO<sub>2</sub> and their solid solution LiNi<sub>0.8</sub>Co<sub>0.2</sub>O<sub>2</sub>. While all three systems ultimately evolve into the same thermodynamically stable layered phase ($R\\overline{3}m$), each chemistry involves distinct metastable intermediates. We explain the phase progressions using a structural template model, demonstrating that during the synthesis of LiCoO<sub>2</sub>, the formed metastable spinel polymorph (Li<sub>2</sub>Co<sub>2</sub>O<sub>4</sub>; $Fd\\overline{3}m$) is a kinetically facile lithiation product of spinel Co<sub>3</sub>O<sub>4</sub>—the low-temperature (LT) intermediate from the decomposed Co-acetate. Similarly, in the Ni-based systems, the acetate decomposition products, rocksalts (Ni,Co)O, topotactically template the kinetic pathways of forming disordered rocksalts (Li<sub>x</sub>(Ni,Co)<sub>2–x</sub>O<sub>2</sub>; $Fm\\overline{3}m$), consequently leading to off-stoichiometric Li<sub>x</sub>(Ni,Co)O<sub>2</sub> with undesired high Li/Ni mixing. These findings highlight new opportunities for engineering precursors to form LT intermediates that template the synthesis of target phases and structural properties.

CCDC 1428913: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Review of Contributions to the Workshop on SN1993J

At its peak, SN 1993J was one of the brightest supernovae in this century, and it is being studied more thoroughly than any supernova except SN 1987A. It is proving to be similar to the transition object SN 1987K, which metamorphosed from being a hydrogen-rich Type II near peak to having a hydrogen-deficient nebular phase. SN 1993J has been observed throughout the electromagnetic spectrum and with optical spectropolarimetry. It is interacting with a dense circumstellar nebula and is generating radio and X-ray flux, but it has probably not been detected in gamma rays. The photometric and spectral evolution are consistent with a star of original mass ∼ 15 M ʘ that lost appreciable mass to a binary companion leaving an extended, helium-rich hydrogen envelope of ≲ 0.5 M ʘ and a helium core of ∼ 4 M ʘ . The spectral evolution will put strong constraints on the mixing of 66 Ni and other species.

MicroED Elucidation of Diverse Solid-State Packing in a Family of Electron-Deficient Expanded Helicenes

Solid-state packing plays a defining role in the properties of a molecular organic material, but it is difficult to elucidate in the absence of single crystals that are suitable for X-ray diffraction. Here, we demonstrate the coupling of divergent synthesis with microcrystal electron diffraction (MicroED) for rapid assessment of solid-state packing motifs, using a class of chiral nanocarbons – expanded helicenes – as a proof of concept. Two highly selective oxidative dearomatizations of a readily-accessible helicene provided a divergent route to four electron-deficient analogues containing quinone or quinoxaline units. Crystallization efforts consistently yielded microcrystals that were unsuitable for single crystal X-ray diffraction, but ideal for MicroED. This technique facilitated the elucidation of solid-state structures of all five compounds with <1.1 Å resolution. The otherwise-inaccessible data revealed a range of notable packing behavior, including four different space groups, homochirality in a crystal for a helicene with an extremely low enantiomerization barrier, and nanometer scale cavities. The results of this study suggest that MicroED will soon become an indispensable tool for high-throughput investigations in pursuit of next-generation organic materials.

Faculty Opinions recommendation of Sensitivity of water and carbon fluxes to climate changes from 1960 to 2100 in European forest ecosystems.

An Observable Silene/Silylene Rearrangement in a Cationic Iridium Complex

The iridium complex Cp*(PMe3)Ir(Me)(SiMe2OTf) (2) reacts with LiB(C6F5)4(Et2O)2 to form a mixture of the silene complex [Cp*(PMe3)Ir(η2-CH2SiMe2)(H)][B(C6F5)4] (3) and the base-stabilized silylene complex [Cp*(PMe3)Ir(Me)(SiMe2(Et2O))][B(C6F5)4] (4). The silene complex 3 has been crystallographically characterized. Addition of pyridine to this mixture affords Cp*(PMe3)Ir(Me)(SiMe2(L)][B(C6F5)4] (5a, L = pyridine) in good yield, whereas addition of CO or ethylene results in clean formation of Cp*(PMe3)Ir(L)(SiMe3)][B(C6F5)4] (8a, L = CO; 8b, L = C2H4).

High-Cycle Fatigue and Time-Dependent Failure in Metallic Alloys for Propulsion Systems

The objective of the AFaSR-MURI High-Cycle Fatigue program is to characterize and model the limiting damage states at the onset of high-cycle fatigue to facilitate mechanistic understanding and to develop a basis for life prediction. Efforts have been focused on the influence of foreign object damage (FAD) and fretting on a Ti-6A1-4V blade alloy and on a polycrystalline Ni-base disk alloy. Notable highlights during the fourth year include the characterization and quantitative modeling of fretting and FaD and the definition of the role of mixed-mode loading on HCF thresholds in Ti-6A1-4V. Accomplishments are outlined below: Worst-case fatigue threshold stress intensities have been measured in STOA Ti-6A1-4V using large (> 5 mm) cracks under representative HCF conditions (R > 0.95, 1000 Hz). Values provide a practical, frequency-independent (20 - 20,000 Hz) lower-bound for the growth of naturally-initiated, physically-small (> 40 gm) cracks.

Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

Design rules for high capacity disordered rocksalt Li-ion cathodes applied to the Li–Mn–V–O–F system.

Micrometeorology and flux partitioning between forest understorey and overstorey: a synthesis based on FLUXNET data

International audience

ChemInform Abstract: A Model to Compute Phase Diagrams in Oxides with Empirical or First‐ Principles Energy Methods and Application to the Solubility Limits in the CaO‐MgO System.

Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.